Lead-Free Hybrid Perovskite Absorbers for Viable Application: Can We Eat the Cake and Have It too?

Many years since the booming of research on perovskite solar cells (PSCs), the hybrid perovskite materials developed for photovoltaic application form three main categories since 2009: (i) high-performance unstable lead-containing perovskites, (ii) low-performance lead-free perovskites, and (iii) moderate performance and stable lead-containing perovskites. The search for alternative materials to replace lead leads to the second group of perovskite materials. To date, a number of these compounds have been synthesized and applied in photovoltaic devices. Here, lead-free hybrid light absorbers used in PV devices are focused and their recent developments in related solar cell applications are reviewed comprehensively. In the first part, group 14 metals (Sn and Ge)-based perovskites are introduced with more emphasis on the optimization of Sn-based PSCs. Then concerns on halide hybrids of group 15 metals (Bi and Sb) are raised, which are mainly perovskite derivatives. At the same time, transition metal Cu-based perovskites are also referred. In the end, an outlook is given on the design strategy of lead-free halide hybrid absorbers for photovoltaic applications. It is believed that this timely review can represent our unique view of the field and shed some light on the direction of development of such promising materials.

Introduction

Owing to the increasing consumption of fossil energy and the deterioration of air pollution, there is an imperative need of clean and renewable energy resources for humanity. Among the new energy solutions, photovoltaic (PV) technology, which converts solar energy into electricity directly, is a promising approach to get sustainable clean energy safely. As one of the third‐generation PV technologies, hybrid halide perovskite solar cells (PSCs) emerged since Miyasaka and co‐workers1 incorporated MAPbX3 (X = I, Br) as sensitizers into dye‐sensitized solar cells (DSSCs), achieving a power conversion efficiency (PCE) of 3.8% in 2009. After numerous research endeavors in the past eight years, the PCEs of PSCs rapidly improved to 22.1%.2 Originally, hybrid halide perovskites with the general formula of ABX3 are structural analogs of the natural mineral CaTiO3, while A is a monovalent, organic or alkali metal cation, M is a divalent p‐block metal (typically Pb, Sn, and Ge), and X is a halide anion.3 Depending on the demand of the researchers, hybrid halide perovskites and their derivatives for the photovoltaic application can be classified into three main categories since 2009 (Figure ).

Figure 1

Classification of hybrid halide perovskites for photovoltaic application.

The first group of halide perovskites is dedicated to demonstrating the potential of PSCs in achieving PCE up to a theoretical upper limit of 31%.4 They are the most studied hybrid halide perovskites with the formula APblM1−lX3 (A = CH3NH3, HC(NH2)2, Cs, Rb, or their mixture; M = Sn (II),5, 6, 7, 8 Ge (II),9, 10 Mn (II),11 Co (II),12 In (III),13 Al (III),14 or Sb (III),15 etc. or their mixture; X = Cl, Br, I, or their mixture) featuring the containing of Pb as the main metal cation and 3D network of corner‐sharing (Pb1−lMl)X6 4− octahedrons with the monovalent cation occupying the cuboctahedral cavity. The unique electronic configuration of Pb2+ endows 3D lead perovskites with excellent optoelectronic properties. For example, MAPbI3, the archetypal hybrid halide perovskite, possesses many ideal properties as a solar absorber: a direct bandgap (E g) of 1.53 eV,16 small exciton binding energies (37 or 45 meV),17, 18, 19 long charge carrier diffusion lengths over 3.5 µm,20, 21 and excellent charge carrier mobilities.22, 23, 24 Inherited all the merits of tribasic MAPbI3, the compositionally engineered polybasic 3D lead halide perovskite‐based PSCs exhibited a skyrocketing certified PCE from initial 14.9% to state‐of‐the‐art 22.1%25 within three years.

Aforementioned 3D lead halide perovskites realized the highest‐performing solution‐processed solar cell on record, rivaling commercial crystalline silicon solar cells in efficiency.26 However, the toxicity issue of the lead urged some researchers to seek alternatives to lead‐based perovskites. We classify these alternatives as the second group of perovskites, which features less toxic lead‐free hybrid halide light absorbers. Lead‐free hybrid halide light absorbers mainly include group 14 metals like tin (Sn) and germanium (Ge), group 15 posttransition metals like bismuth (Bi) and antimony (Sb), and transition metal copper (Cu) as the metal cations.27 In this case, a variety of crystallographic polymorphs appeared: Sn‐ and Ge‐based compounds with 3D perovskite framework; Bi‐ and Sb‐based “pseudoperovskite” without corner‐shared MX6 octahedra structure; Cu‐based typical 2D layered perovskites. As light absorbers used in solar cells, Sn‐based perovskites achieved the highest efficiency so far of 8.12% among all lead‐free hybrid halide compounds.28

In the group of high‐efficiency lead halide perovskites, there is another problem of insufficient long‐term stability for the application of the devices. One solution to this problem is mixing 3D perovskites with 2D perovskites,29, 30, 31, 32 which can be categorized as the third group of perovskites with the mission of realizing high efficiency and stability simultaneously. The mixed dimensional (MD) perovskites have a general chemical formula30 of (A)2(CH3NH3) −1MX3 +1 (n is an integer), where A is a primary aliphatic or aromatic alkylammonium cation, M is a divalent metal, and X is a halide anion. In the MD perovskites, the large organic cations (A) defragment the 3D structure and isolate certain number (n) of inorganic perovskite layers of corner‐sharing [MX6]4− octahedrons.33 This configuration was found to prevent moisture from attacking the perovskite and therefore improve the stability of perovskite film. Additionally, the wide variety of “A and n” brings MD perovskites abundant tunability and flexibility to control the physical properties, as well as balanced stability versus optoelectronic performance of corresponding devices. So far, the quasi‐2D PEA2(CH3NH3) −1PbI3 +1 32 and 2D perovskite (BA)2(MA)3Pb4I13 34 displayed good efficiency over 12% through optimizing stoichiometry of materials and showed much improved stability than intrinsic 3D perovskites.

The three groups of hybrid perovskites attracted attention to different extents. In this series review, we focus on the second group of perovskites or lead‐free hybrid light absorbers used in photovoltaic devices. Recent development in structures, optoelectronic properties, and the related solar cell applications of these types of hybrid light absorbers are summarized. In the first part, group 14 metals (Sn and Ge)‐based perovskites are introduced with more emphasis on the optimization of Sn‐based PSCs. Then we put concerns on halide hybrids of group 15 metals (Bi and Sb), which are mainly perovskite derivatives. At the same time, transition metal Cu‐based perovskites are also referred. In the end, we give an outlook on the design strategy of lead‐free halide hybrid absorbers for photovoltaic applications (Figure ). An almost complete summary of the state‐of‐the‐art development of the lead‐free halide hybrid absorbers in PV devices is listed in Table .

Figure 2

Scope of the lead‐free hybrid absorbers discussed in this review (Sb: Adapted with permission.35 Copyright 2016, ACS; Bi: Adapted with permission.36 Copyright 2015, ACS; Adapted with permission.37 Copyright 2017, RSC; Adapted with permission.38 Copyright 2016, ACS; Cu: Adapted with permission.39 Copyright 2016, ACS; Ge: Adapted with permission.40 Copyright 2015, RSC; Sn: Adapted with permission.41 Copyright 2017, ACS).

Table 1

Lead‐free hybrid absorbers comparing to the lead‐containing counterparts

Metal Cations		Absorber	PCE (%)	V OC (V)	J SC (mA cm−2)	FF (%)	E g (eV)	Architecture	Fabrication method	Additive	Ref.
Pb		MAPbI3	20.4	1.11	23.7	77.3	1.55	FTO/c‐TiO2/mp‐TiO2/absorber/spiro‐OMeTAD/Au	Spin coating+ solvent‐engineering	No	42
		FAPbI3	18.1	1.04	23.2	74.9	1.45	FTO/c‐TiO2/mp‐TiO2/absorber/spiro‐OMeTAD/Au	Spin coating‐solvent bathing	No	43
		FA1− xMAxPbI3− yBry	22.1	1.1	25.0	80.3	1.50	FTO/c‐TiO2/mp‐TiO2/absorber/PTAA/Au	Spin coating	I2	44
		Rb0.05[Cs0.05(MA0.17FA0.83)0.95]0.95Pb(I0.83Br0.17)3	21.8	1.18	22.8	81	1.63	FTO/c‐TiO2/mp‐TiO2/absorber/spiro‐OMeTAD/Au	Spin coating + solvent‐engineering	No	45
Sn(II)		MASnI3	6.4	0.88	16.8	42	1.2	FTO/c‐TiO2/mp‐TiO2/absorber/spiro‐OMeTAD/Au	Spin coating	No	46
		MASnI3	5.44	0.716	15.18	50.1	1.3	FTO/c‐TiO2/mp‐TiO2/absorber/spiro‐OMeTAD/Au	Spin coating	No	5
		MASnI3	3.89	0.38	19.9	51.7	1.3	FTO/c‐TiO2/mp‐TiO2/absorber/PTTA/Au	Spin coating (hydrazine vapor)	SnF2	41
		MASnI3	3.15	0.32	21.4	46	1.3	FTO/c‐TiO2/mp‐TiO2/absorber/Au	solvent‐engineering	SnF2	47
		MASnI3	2.14	0.45	11.8	40	–	ITO/PEDOT:PSS/absorber/C60/BCP/Ag	Spin coating‐solvent bathing	SnF2	48
		MASnI3	1.86	0.27	17.4	39.1	1.26	FTO/c‐TiO2/mp‐TiO2/absorber/PTTA/Au	VASP	No	49
		MASnI3	1.7	0.38	12.1	36.6	1.3	ITO/PEDOT:PSS/Poly‐TPD/absorber/ C60/BCP/Ag	thermal co‐evaporation	No	50
		MASnI3	–	–	–	–	1.23	–	vapor deposition + solution‐spinning
		MASnI3− xBrx	5.73	0.82	12.30	57	1.75	FTO/c‐TiO2/mp‐TiO2/absorber/spiro‐OMeTAD/Au	Spin coating	No	51
		MASnBr3 MA0.9Cs0.1SnI3	1.12	0.50	4.27	49.1	2.2	FTO/c‐TiO2/mp‐TiO2/absorber/P3HT/Au	sequential evaporation	No	52
			0.51	0.49	2.24	46.4	2.13	FTO/c‐TiO2/mp‐TiO2/absorber/PTTA/Au	(VASP)	No	53
			0.3	0.20	4.5	36	1.41	ITO/PEDOT:PSS/absorber/PCBM/Bis‐C60/Ag	Spin coating solvent‐engineering (Tol)	No	54
		{en}MASnI3	6.63	0.43	24.3	63.7	1.4	FTO/c‐TiO2/mp‐TiO2/absorber/PTTA/Au	Spin coating (hydrazine vapor)	SnF2	55
		FASnI3	6.6	0.48	21.3	64.6	1.36	ITO/PEDOT:PSS/absorber/C60/BCP/Ag	Spin coating solvent‐engineering (CB)	SnF2	28
		FASnI3	6.22	0.47	22.1	60.7	1.4	ITO/PEDOT:PSS/absorber/C60/BCP/Ag.	Spin coating solvent‐engineering (DEE)	SnF2	56
		FASnI3	5.27	0.38	23.1	60.0	1.4	FTO/c‐TiO2/mp‐TiO2/ZnS/absorber/ PTAA/Au	Spin coating solvent‐engineering (DEE)	SnF2	57
		FASnI3	4.8	0.32	23.7	63	1.4	FTO/c‐TiO2/mp‐TiO2/absorber/spiro‐OMeTAD/Au	Spin coating	SnF2‐pyrazine	58
		FASnI3	2.45	0.31	17.17	46.0	1.4	–	–	No
		FASnI3	2.1	0.24	24.5	36	1.41	FTO/c‐TiO2/mp‐TiO2/absorber/spiro‐OMeTAD/Au	Spin coating	SnF2	59
		FASnI2Br	1.72	0.47	6.82	54.3	1.68	ITO/PEDOT:PSS/absorber/C60/Ca/Al	Spin coating solvent‐engineering (CB)	No	60
		FASnBr3	–	–	–	–	2.4	–	–	No	61
		FA1− xMAxSnBr3	–	–	–	–	2.4–1.9	–	–	No
		FA0.25MA0.75SnI3	4.49	0.48	20.7	45.2	1.28	ITO/PEDOT:PSS/absorber/C60/BCP/Ag	Spin coating solvent‐engineering (CB)	SnF2	28
		FA0.50MA0.50SnI3	5.92	0.53	21.3	52.4	1.3	ITO/PEDOT:PSS/absorber/C60/BCP/Ag	Spin coating solvent‐engineering (CB)	SnF2	28
		FA0.75MA0.25SnI3	8.12	0.61	21.2	62.	1.33	ITO/PEDOT:PSS/absorber/C60/BCP/Ag	Spin coating solvent‐engineering (CB)	SnF2	28
		FA0.8Cs0.2SnI3	1.4	0.24	16.1	36	–	ITO/PEDOT:PSS/absorber/PCBM/Bis‐C60/Ag	Spin coating solvent‐engineering (Tol)	No	54
		{en}FASnI3	7.14	0.48	22.5	66.0	1.5	FTO/c‐TiO2/mp‐TiO2/absorber/PTTA/Au	Spin coating	SnF2	62
		(BA)2(MA)3Sn4I13	2.53	0.229	24.1	45.7	1.42	FTO/c‐TiO2/mp‐TiO2/absorber/PTTA/Au	Spin coating	(TEP)SnF2	63
		(PEA)2(FA)8Sn9I28	5.94	0.59	14.4	69	1.789	ITO/NiOx/absorber/PCBM/Al	Spin coating solvent‐engineering (Tol)	SnF2	64
		CsSnI3	4.81	0.38	25.71	49.1	1.3	FTO/c‐TiO2/mp‐TiO2/absorber/PTTA/Au (TPFB)	Spin coating	SnI2	65
		CsSnI3	3.56	0.50	9.89	68	1.3	ITO/absorber/PC61BM/BCP/Al	Spin coating	SnCl2	66
		CsSnI3	3.31	0.52	10.2	62.5	1.3	ITO/NiOx/absorber/PCBM/Al	Spin coating; coarse‐Grained	‐	67
		CsSnI3	2.76	0.43	12.3	39.5	1.3	ITO/CuI/absorber/ICBA/BCP/Al	Spin coating	SnI2	68
		CsSnI3	2.0	0.24	22.7	37	1.3	FTO/c‐TiO2/mp‐TiO2/absorber/m‐MTDATA/Au	Spin coating	SnF2	69
		CsSnI3	1.83	0.17	30.8	34.9	1.25	FTO/c‐TiO2/mp‐TiO2/absorber/PTTA/Au (TPFB)	Spin coating (hydrazine vapor)	SnF2	41
		CsSnI3	1.66	0.20	27.7	29	1.27	FTO/TiO2/mp‐TiO2/absorber/spiro‐OMeTAD/Au	Spin coating	SnF2	70
		CsSnI3	0.88	0.42	4.8	22	1.3	ITO/absorber/Au/Ti	sequential thermal evaporation	No	71
		CsSnI3 a)	8.51	–	–	–	–	HTM in DSSCs	‐	SnF2	72
		CsSnI2Br	1.67	0.29	15.1	38	1.37	FTO/TiO2/mp‐TiO2/absorber/spiro‐OMeTAD/Au	Spin coating	SnF2	70
		CsSnIBr2	3.2	0.31	17.4	57	1.63	FTO/c‐TiO2/Al2O3/absorber/C	Spin coating	HPA‐ SnF2	73
		CsSnIBr2	1.56	0.31	11.6	43	1.65	FTO/TiO2/mp‐TiO2/absorber/spiro‐OMeTAD/Au	Spin coating	SnF2	70
		CsSnBr3	3.04	0.37	14.0	59.4	1.79	FTO/c‐TiO2/mp‐TiO2/absorber/PTTA/Au (TPFB)	Spin coating (hydrazine vapor)	SnF2	41
		CsSnBr3	2.17	0.42	9.1	57	1.75	FTO/c‐TiO2/mp‐TiO2/absorber/spiro‐OMeTAD/Au	Spin coating	SnF2	74
		CsSnBr3	0.95	0.41	3.99	58	1.75	FTO/TiO2/mp‐TiO2/absorber/spiro‐OMeTAD/Au	Spin coating	SnF2	70
		CsSnBr3	0.55	0.45	2.4	55	1.8	ITO/MoO3/absorber/C60/BCP/Ag	All vapor‐deposited	SnF2	75
Sn(IV)		Cs2SnI6	0.96	0.51	5.41	35	1.48	FTO/TiO2/absorber/P3HT/Ag	sequential evaporation	No	76
		Cs2SnI6	0.86	0.52	3.20	51.5	1.48	FTO/seed layer/ZnO nanorods/absorber/ P3HT/Ag	Spin coating	No	77
		Cs2SnI6	–	–	–	–	1.3	–	–	–	78
		Cs2SnI6	–	–	–	–	1.6	–	–	–	79
		Cs2SnI6	1.47	0.37	6.75	59.5	1.30	FTO/bl‐TiO2/2wt% Sn‐TiO2/Cs2SnI6− xBrx/ Cs2SnI6 HTM/LPAH/FTO	–	No	80
		Cs2SnI5Br	1.60	0.44	6.58	55.0	1.38
		Cs2SnI4Br2	2.03	0.56	6.23	57.7	1.40
		Cs2SnI2Br4	1.08	0.58	3.41	54.8	1.63
		Cs2SnIBr5	0.002	0.57	0.01	37.2	2.36
		Cs2SnIBr6	non	non	non	non	2.85
		Cs2SnBr6	–	–	–	–	2.7	–	–	–	81
		Cs2SnCl6	–	–	–	–	3.9	–	–	–
Ge		MAGeI3	0.2	0.15	4.0	30	2.0	FTO/c‐TiO2/mp‐TiO2/absorber/spiro‐OMeTAD/Au	Spin coating	–	–
		FAGeI3	–	–	–	–	2.35	‐
		CsGeI3	0.01	0.07	5.7	27	1.63	FTO/c‐TiO2/mp‐TiO2/absorber/spiro‐OMeTAD/Au
Bi	0D	MA3Bi2I9	0.42	0.67	1.00	62.5	2.1	ITO/TiO2/mp‐TiO2/absorber/spiro‐OMeTAD/MoO3 /Ag	Spin coating	–	82
		MA3Bi2I9	0.39	0.83	1.39	34	2.22	ITO/PEDOT: PSS/absorber/C60/BCP/Ag	Two‐step evaporation–spin‐coating	–	83
		MA3Bi2I9	0.36	0.65	1.10	0.50	2.1	FTO/TiO2/mp‐TiO2/absorber/spiro‐OMeTAD/Au	Solvent‐engineering (chlorobenzene)		84
		MA3Bi2I9	0.31	0.51	0.94	0.61	–	FTO/TiO2/mp‐TiO2/absorber/spiro‐OMeTAD/Au	Spin coating	NMP	85
		MA3Bi2I9	0.26	0.56	0.83	49	–	FTO/TiO2/mp‐TiO2/absorber/spiro‐OMeTAD/Au	Spin coating	–	86
		MA3Bi2I9	0.19	0.35	1.16	46.4	2.11	FTO/TiO2/mp‐TiO2/absorber/P3HT/Au	Spin coating	–	87
		MA3Bi2I9	0.12	0.68	0.52	33	2.1	FTO/TiO2/mp‐TiO2/absorber/spiro‐OMeTAD/Ag	Spin coating	–	88
		MA3Bi2I9	0.11	0.72	0.49	31.8	2.26	FTO/TiO2/absorber/spiro‐MeOTAD/Au	solvent‐engineering (chlorobenzene)	–	89
		MA3Bi2I9	0.08	0.69	0.37	32	2.1	FTO/TiO2/absorber/spiro‐MeOTAD/Ag	Spin coating, gas‐assisted	–	90
		MA3Bi2I9	0.07	0.66	0.22	49	2.9	ITO/PEDOT:PSS/absorber/PCBM/Ca/Al	Spin coating	–	91
		MA3Bi2I9	–	–	–	–	2.04	–	vapor‐assisted conversion	–	92
		MA3Bi2I9Clx	0.003	0.04	0.18	38	2.4	FTO/TiO2/mp‐TiO2/absorber/spiro‐OMeTAD/Ag	Spin coating	–	88
		MA3Bi2I9Sx	–	–	–	–	1.45	–	In situ, thermal	–	93
		(MA3Bi2I9)0.2(BiI3)0.8	0.08	0.57	0.27	50	‐	FTO/TiO2/mp‐TiO2/absorber/PTAA/PIDT‐DFBT/Ag	Spin coating	–	94
		FA3Bi2I9	–	–	–	–	2.0	–	–	–	95
		(C3H5N2)3Bi2I9	–	–	–	–	–	–	–	–	96
		(C6H14N)3Bi2I9	–	–	–	–	–	–	–	–	97
	1D	MA3Bi2Cl9	–	–	–	–	–	–	–	–	98
		C5H6NBiI4	0.9	0.62	2.71	0.54	1.98	FTO/c‐TiO2/mp‐TiO2/absorber/ZrO2/C	Spin coating	–	99
		C6H8NBiI4	–	–	–	–	2.17
		(H3NC6H12NH3)BiI5	0.03	0.40	0.12	43	2.1	FTO/c‐TiO2/mp‐TiO2/absorber/spiro‐OMeTAD/Au	Spin coating	–	100
		(TMP)BiX5 (X = Cl, Br, I)	–	–	–	–	2.02‐ 3.21	–	–	–	101
	2D	MA3Bi2Br9	–	–	–	–	2.5	–	–	–	102
		(NH4)3Bi2I9	–	–	–	–	2.04	–	–	–	103
		(TMP)1.5Bi2I7Cl2	–	–	–	–	2.1	–	–	–	101
	3D	MA2KBiCl6	–	–	–	–	3.04	–	–	–	104
		MA2TlBiBr6	–	–	–	–	2.16(direct)	–	–	–	105
		MA2AgBiBr6	–	–	–	–	2.02	–	–	–	106
	0D	Cs3Bi2I9	0.02	0.02	0.18	37	2.03	FTO/TiO2/mp‐TiO2/absorber/P3HT/Ag	Spin coating	–	100
		Cs3Bi2I9	1.09	0.85	2.15	60	2.2	FTO/TiO2/mp‐TiO2/absorber/spiro‐OMeTAD/Ag	Spin coating	–	88
		Cs3Bi2I9	–	–	–	–	1.9	–	–		36
		CsBi3I10	0.40	0.31	3.4	38	1.77	FTO/TiO2/mp‐TiO2/absorber/P3HT/Ag	Spin coating	–	100
	2D	K3Bi2I9	–	–	–	–	2.1	–	–	–	36
		Rb3Bi2I9	–	–	–	–		–	–	–
		Cs3Bi2Br9	–	–	–	–	2.71	–	–	–	107
	3D	Cs2AgBiBr6	–	–	–	–	1.9	–	–	–	108
		Cs2AgBiBr6	–	–	–	–	1.95		–	–	38
		Cs2AgBiBr6	–	–	–	–	2.19	–	–	–	109
		Cs2AgBiBr6	2.43	0.98	3.93	63	2.21	FTO/c‐TiO2/mp‐TiO2/absorber/spiro‐OMeTAD/Au	Spin coating	–	110
		Cs2(Ag1− aBi1− b)TlxBr6	–	–	–	–	1.40 1.57(direct)	–	–	–	111
		Cs2Ag(Bi0.625Sb0.375)Br6	–	–	–	–	1.86 2.15(direct)	–	–	–	112
		Cs2AgBiCl6	–	–	–	–	2.2	–	–	–	108
		Cs2AgBiCl6	–	–	–	–	2.77	–	–	–	109
		Cs2InAgCl6 (non‐Bi)	–	–	–	–	3.3(direct)	–	–	–	113
		AgBi2I7	1.22	0.56	3.30	67.4	1.87	FTO/TiO2/mp‐TiO2/absorber/P3HT/Ag	Spin coating	–	114
		Ag2BiI5 (R3¯ m AgI/BiI3 = 2: 1)	2.1	0.49	6.8	63	1.85	FTO/TiO2/mp‐TiO2/absorber/P3HT/Au	Spin coating	–	115
		AgBi2I7 (Fd3¯ m AgI/BiI3 = 1: 2)	0.4	0.46	1.6	56	1.78
Sb		(NH4)3Sb2IxBr9− x	0.51	1.03	1.15	42.9	2.27‐ 2.78	ITO/PEDOT:PSS/absorber/PC61BM/Al	Spin coating	–	116
		MA3Sb2I9	0.49	0.90	1.0	55	2.14	ITO/PEDOT:PSS/absorber/PC61BM/nano‐ZnO/Al	Spin coating; solvent‐engineering (Tol)	–	117
		MA3Sb2I9	2.04	0.62	5.41	60.8	1.95	ITO/PEDOT:PSS/absorber/PC61BM/ C60/BCP/Al	Spin coating	HI	118
		Rb3Sb2I9	0.66	0.55	2.11	57	2.1	FTO/TiO2/mp‐TiO2/absorber/Poly‐TPD/ Au	SbI3 in toluene treated (Tol)	SbI3	35
		Cs3Sb2I9	<1.0	0.31	<0.1	–	2.05	FTO/c‐TiO2/ absorber/PTAA/Au	Co‐evaporation / vapor‐assisted conversion	SbI3 vapor	119
		Cs3Sb2I9	0.84	0.6	2.91	48.1	2.0	ITO/PEDOT:PSS/absorber/PC61BM/ C60/BCP/Al	Spin coating	HI	118
		[CH3SC(NH2)2]2SbA5	–	–	–	–	2.41–3.34	–	–	–	120
		Cs4CuSb2Cl12	–	–	–	–	1.0	–	–	–	121
Cu		(CH3(CH2)3NH3)2CuBr4	0.63	0.88	1.78	40	1.76	FTO/c‐TiO2/mp‐TiO2/absorber/spiro‐OMeTAD/Ag	Spin coating	–	122
		(p‐F‐C6H5C2H4‐NH3)2‐CuBr4	0.51	0.87	1.46	40	1.74
		MA2CuCl2Br2	0.02	0.26	0.22	32	2.12	FTO/c‐TiO2/mp‐TiO2/absorber/spiro‐OMeTAD/Au	Spin coating	–	39
		MA2CuCl0.5Br3.5	0.002	0.29	0.021	28	1.8
		C6H4NH2CuBr2I	0.46	0.20	6.20	46	1.64	FTO/c‐TiO2/mp‐TiO2/absorber/ZrO2/C	Drop casting	–	123

Used as an HTM in the solid‐state dye‐sensitized solar cell.

Lead‐Free Halide Hybrid Perovskite and Related Absorbers

Although we have achieved high efficiency beyond 22% based on lead halide perovskites, which is comparable to commercial crystalline silicon solar cells, the existence of Pb is an urgent problem against the final application of PSCs. The toxicity of lead is documented to disturb the functioning of the blood, kidneys, liver, testes, brain, and nervous system.124, 125, 126 The toxicity of lead is due, in general, to its binding affinity to thiol and cellular phosphate groups of numerous enzymes, proteins, and cell membranes.127 Lead is toxic to the central nervous system, especially in children.128 Thus, there come the questions that: can we eat the cake and have it too? Meaning can we have high‐efficiency PSCs without being poisoned? The urge to explore authentic high‐efficiency lead‐free metal halide absorbers led to an abundance of works which will be shown in the following sections.

The Group 14 Metals (Sn and Ge) Based Absorbers

Tin‐Based Absorbers

Tin (Sn) as a member of group 14 congeners and less‐toxic metal (than Pb) is expected to have comparable properties with its Pb analogs. Owing to their direct and narrow bandgaps (1.2–1.4 eV), low exciton binding energies (18 meV), and super charge‐carrier mobilities,129, 130, 131, 132 Sn‐based perovskites ASnX3 (A = MA, FA or Cs, X = halide) were the most investigated nonlead hybrid perovskite absorbers. Depending on the types of A cations and the solid structure, we classified them into following groups and made discussions accordingly.

MASnI3

The crystal structure of MASnI3 belongs to the cubic m (no. 221) space group at room temperature (Figure a). MASnI3 possesses an optical bandgap of 1.20–1.35 eV (Figure 3b), displaying remarkably high carrier mobility (electron mobility ≈2320 cm2 V−1 s−1, hole carrier mobility of ≈322 cm2 V−1 s−1),132, 133 and long carrier diffusion lengths exceeding 500 nm.134 In 2014, MASnI3 46 and MASn(I1− Br)3 51 were reported as the first completely lead‐free solar absorbers in PSCs based on a traditional mesoscopic device structure of FTO/c‐TiO2/mp‐TiO2/absorber/spiro‐OMeTAD/Au. PCEs of 6.4% and 5.73% were reported, respectively, and after three years they are still the record efficiencies among MASnX3‐based PSCs (Figure 3c). This dramatic different situation from the lead‐based perovskite can be understood by the fact that Sn‐based perovskites are prone to self‐doping in ambient air resulting in instability and poor reproducibility.46, 47, 48, 51, 52, 53, 135

Figure 3

a) Ball‐and‐stick diagrams of crystal structures and the {SnI6} octahedral structure units in the MASnI3 and FASnI3 single crystals. Reproduced with permission.133 b) The spectra of MASnI3 and FASnI3 prepared with the solution method as compared with other perovskites. Reproduced with permission.132 Copyright 2013, ACS. c) J–V curve of MASnI3 on TiO2 and bandgap of the material determined using the Tauc plot. Reproduced with permission.46 Copyright 2014, RSC. d) Cross‐sectional SEM image of the entire device with FASnI3 and 10 mol% SnF2 additive; e) J–V characteristics of FASnI3‐based devices under 100 mW cm−2 AM1.5G illumination under reverse and forward voltage scans. Reproduced with permission.56

FASnI3

Similar to the case of lead halide perovskite, FA is another candidate that can be employed as A site cation for Sn halide perovskites. FASnI3 takes the cubic m (no. 221) space group structure at room temperature and has only one single stable phase up to 200 °C (Figure 3a). This is different from its lead analogue‐FAPbI3, which has two competing δ‐phase and α‐phase structure.132, 136, 137 Another unique point is that due to the symmetry reduction of the 3D [SnI3]− framework with larger cations group like FA,132 FASnI3 has a larger band gap (1.41 eV) value than that of MASnI3.59 In 2015, Koh et al. first took FASnI3 as a light absorber in solar cell applications and realized a high short‐circuit current density (J SC) of 24.45 mA cm−2 and PCE of merely 2.1%.59 It was reported that FASnI3 is more stable than MASnI3 due to the reduced extent of Sn oxidation. 133, 138 Accordingly, FASnI3‐based PSCs exhibited much better reproducibility as compared to MASnI3‐based devices.138 In 2016, Liao et al.56 reported a PCE of 6.22% based on FASnI3 PSCs with high reproducibility, which is one of the best efficiencies among Sn‐based PSCs (Figure 3d,e). Very recently, Shi et al.139 studied the phenomenon theoretically and found that the antibonding coupling between Sn‐s and I‐p is weaker in FASnI3 than in MASnI3 due to the larger ionic size of FA, leading to higher formation energies of Sn vacancies in FASnI3. Subsequently, the conductivity of FASnI3 can be tuned from p‐type to intrinsic by varying the growth conditions of the perovskite semiconductor. In contrast, MASnI3 shows unipolar high p‐type conductivity independent of the growth conditions. Ion mixing is also one important approach for Sn perovskites. Ferrara et al.61 reported the first mixed A‐cation compositions of tin perovskites FA1‐ MASnBr3, with cubic structures and the band gaps ranging from 2.4 eV (x = 0) to ≈1.92 eV (x = 0.82). However, no device performance was reported. Very recently, Zhao et al.28 reported another mixed‐organic‐cation perovskite absorber (FA)(MA)1− SnI3. The optimized ratio of FA and MA cations is 0.75 versus 0.25 and the resulted (FA)0.75(MA)0.25SnI3 has an ideal bandgap of 1.33 eV. PSCs with an inverted structure based on (FA)0.75(MA)0.25SnI3 showed an improved V OC of 0.61 V and the best ever PCE of 8.12%, which is the highest efficiency among the Sn‐based PSCs (Figure ).

Figure 4

a) Schematic illustration of the device structure. b) Band alignment diagram. c) J–V curves of the champion device measured using both forward and reverse scan mode at a scan rate of 300 mV s−1 under the simulation of AM 1.5G, 100 mW cm−2. Reproduced with permission.28

CsSnI3

Compared to the organic–inorganic hybrid perovskite materials, the all‐inorganic halide perovskites exhibited much higher thermal stability.16, 70, 140 Replacing the organic cations (MA+ or FA+) with the inorganic Cs+, CsSnI3 perovskite shows a melting point of 435 °C,72, 141 suggesting superior intrinsic thermal stability. In contrast, hybrid perovskites MASnI3 and FASnI3 start to decompose at ≈200 °C. CsSnI3 adopts a black orthorhombic perovskite phase structure, possesses a direct band gap of 1.3 eV, and has a high hole mobility of ≈585 cm2 V−1 s−1 at room temperature (Figure a). In 2012, due to its high hole mobility, it was first taken as a hole transport material (HTM) in DSSCs.72 Very recently, the further potential of CsSnI3 for solar cell applications was uncovered by Wu et al.142 They reported that the melt‐synthesized CsSnI3 ingots contain high‐quality large single crystal (SC) grains, which bear superior properties like high bulk carrier lifetimes (>6.6 ns), low doping concentrations of ≈4.5 × 1017 cm−3, and long minority‐carrier diffusion lengths approaching 1 µm. In this regard, they predicted a PCE of ≈23% for optimized CsSnI3 SC solar cells. As a light absorber, CsSnI3 was firstly used in a Schottky contact type PSC with a simple layer structure of ITO/CsSnI3/Au/Ti in 2012, showing a PCE of 0.9%.71 Four years later, Marshall et al.66 designed an HTM‐free CsSnI3 PSC with exceptionally high fill factor up to 0.69, and a PCE up to 3.56%. In 2017, Song et al.65 assembled CsSnI3‐based PSCs with reducing atmosphere‐assisted dispersible additive. The new technique produced a PCE of up to 4.81%, which is the highest efficiency among all CsSnI3 PSCs so far (Figure 5a). Meanwhile, benefiting from the superior thermal stability of CsSnX3 perovskite, Li et al.73 fabricated an all‐inorganic CsSnIBr2 mesoscopic PSC with thermal stability up to 200 °C, achieving an average PCE of 3.0% and long‐term stability without efficiency‐loss over 77 d (Figure 5c).

Figure 5

a) Crystal structures of B‐Y‐CsSnI3, Y‐CsSnI3, and Cs2SnI6 and their phase transformation routes. Reproduced with permission.143 Copyright 2017, ACS. b) J–V characteristic of the 0.4‐CsI/SnI2‐based device. Reproduced with permission.65 Copyright 2017, ACS. c) Bandgap variation with Br concentration. Reproduced with permission.70 Copyright 2015, ACS. d) Distorted 3D structure of Cs2SnI6 at room temperature. Reproduced with permission.78 Copyright 2014, ACS. e) J–V curves of a series of cells with different composition of Cs2SnI6 Br. The inset shows the IPCE values. f) The stability curves for 50 d are shown here for Cs2SnI6 (black), and the Cs2SnBr2I4 (green)‐based solar cells. Reproduced with permission.80 Copyright 2017, RSC.

Cs2SnI6

The SnII in CsSnI3 and M(F)ASnI3 has the tendency to be oxidized to SnIV leading to the spontaneous change to Cs2SnI6 and M(F)A2SnI6 respectively in ambient atmosphere. Cs2SnI6 with a molecular salt structure is not bona fide perovskite but crystallizes into a face‐centered cubic m space group and the lattice parameter of which is 11.63 Å132, 144 (Figure 5d). The direct bandgap (E g) of Cs2SnI6 is ≈1.3–1.6 eV76, 79, 132 depending on the preparation methods at room temperature with absorption coefficient over 105 cm−1 from 1.7 eV. Computational work based on Cs2SnI6 145 shows that the iodine vacancies and interstitial Sn are the dominant defects that give intrinsic n‐type behavior, unlike the p‐type behavior in CsSnI3. The carrier concentration (electrons) of Cs2SnI6 is on the order of ≈1 × 1014 cm−3 measured by Hall effect measurements at room temperature, with a high electron mobility of 310 cm2 V−1 s−1. Moreover, due to the full oxidation state of Sn4+, Cs2SnI6 exhibits better stability in the air with moisture than CsSnI3. In 2014, Lee et al. used Cs2SnI6 as a hole transporter in solid‐state DSSCs and fabricated devices in the air with an efficiency close to 8%.78 The study of Cs2SnI6 as a light absorber in the solar cells is rare. Until 2016, Cs2SnI6 was first studied by Qiu et al.76, 77 as a light harvester, demonstrating a PCE of ≈1%. Very recently, Lee et al.80 further improved the PCE up to 1.47%. They designed a series of compounds with the general formula of Cs2SnI6− Br. With the increase of Br composition (x), the bandgaps can be tuned from ≈1.3 to ≈2.9 eV and the color of the films was changing from dark brown to brown/red, then to light yellow. The Cs2SnI6− Br films were fabricated with a two‐step solution process: the crystal structure of CsI was optimized in Step‐1 by postannealing at 300 °C for 30 min after electrospraying deposition and in Step‐2 the CsI film was reacted with a SnI4 solution at 110 °C for 20 min (Figure 5e,f). After that, a stoichiometric, smooth, uniform, and thick active layer was obtained. The freshly made films were constructed into the typical “sandwich” type device structure of FTO/bl‐TiO2/2wt% Sn‐TiO2/Cs2SnI6‐ Br/Cs2SnI6 HTM/large‐effective‐surface‐area polyaromatic hydrocarbon (LPAH)/FTO. The best‐achieved efficiency was ≈2.03% when x = 2. It is worth noting that their device fabrication process was carried out in the air and did not use any additive to protect the active material. The Cs2SnI6− Br films and corresponding devices showed excellent stability in the air for 50 d (see Figure 5f). Thus, being a molecular salt (0D), Cs2SnI6 has the bandgap similar to that of the 3D perovskites (CsSnI3 and MAPbI3), high absorption coefficient, and high carrier mobility. Coupled with its intrinsic ambient stability, such Sn‐perovskite variants can be explored with more effort in the future for achieving more efficient and stable Sn‐based PSCs.

(PEA)2(FA)8Sn9I28

In the attempt to improve the environmental stability, Cao et al.63 decrease the dimensionality of the perovskite materials by mixing the CH3(CH2)3NH3 + (BA+) and CH3NH3 + (MA+). The obtained 2D Ruddlesden−Popper (CH3(CH2)3NH3)2(CH3NH3) −1SnI3 +1 perovskites possessed optimal optical bandgaps of 1.50 and 1.42 eV for solar cells when n = 3 and n = 4, respectively. The 2D tin perovskite outperformed its 3D analogs for higher moisture stability with an encouraging PCE of 2.5% (from n = 4). More importantly, incorporating 20% phenyl ethyl ammonium (PEA) into FA‐based Sn iodide perovskites yielded low dimensional (PEA)2(FA)8Sn9I28 64 which exhibited markedly enhanced air stability in comparison with their 3D counterparts FASnI3. The inverted structure‐based devices with (PEA)2(FA)8Sn9I28 perovskite exhibited the best PCE up to 5.94% and showed super stability over 100 h without encapsulation.

Germanium‐Based Absorbers

Germanium, another group IV metal, with ns2 electronic configuration, has the same valent state with the lead. Due to the 4s lone pairs of Ge is more active than the Pb 6s lone pair, Ge2+ is easier to be oxidized leading to metallic conductivity in the hybrid materials and short‐circuit behavior in the photovoltaic devices similar to the case of Sn‐based perovskite.40, 135, 141 However, germanium has demonstrated much less toxicity compared to lead146 and is expected to be a promising candidate in the search for Pb‐free perovskite materials. To prove the concept, Sun et al.147 investigated the structural and electronic properties of MAGeX3 (X = Cl, Br, I) by density functional theory (DFT) methods and showed that MAGeI3 is a good absorber for applications in PSCs. Based on DFT calculation, Ming et al.148 also proposed that CsGeI3 might be a good HTM in solar cells. Krishnamoorthy40 studied the solid structure of three AGeI3 (A = Cs, MA, or FA) halide perovskites and revealed trigonal phase (with R3m space group symmetry) in contrast to Pb and Sn‐based perovskites with a tetragonal phase (I4/mcm)147 at room temperature. All compounds are remarkably stable up to 150 °C and show no phase transition in the range of device working temperatures (r.t. to 150 °C). With increasing size of the A+ cation, the band gaps of AGeI3 are 1.63, 2.0, and 2.35 eV for CsGeI3, MAGeI3, and FAGeI3, respectively (Figure a). The values of the valence bands of CsGeI3, MAGeI3, and FAGeI3 perovskites measured by the PESA are −5.10, −5.2, and −5.5 eV, respectively (Figure 6b). However, unfortunately, the solar cells based on AGeI3 were fabricated with a mesoscopic structure, achieving 0.2% PCE for MAGeI3, 0.11% PCE for CsGeI3 and no photocurrent for FAGeI3 (Figure 6c). The low performance of AGeI3‐based solar cells was attributed to the low concentration of precursor solutions, poor quality of perovskite films, and the oxidation sensitivity of the materials. Recently, theorists studied mixed tin and germanium perovskites and predicted that RbSn0.5Ge0.5I3 possesses not only a direct bandgap within the optimal range of 0.9–1.6 eV but also a desirable optical absorption spectrum that is comparable to those of the state‐of‐the‐art MAPbI3 perovskites. It has favorable effective masses for high carrier mobility as well as a greater resistance to water penetration than the prototypical inorganic–organic lead‐containing halide perovskite.149

Figure 6

a) Optical absorption spectrum of CsGeI3, MAGeI3, and FAGeI3, in comparison with CsSnI3; b) Schematic energy level diagram of CsGeI3, MAGeI3, and FAGeI3; c) J–V curves of photovoltaic devices fabricated with different germanium halide perovskites. Reproduced with permission.40 Copyright 2015, RSC.

Devices Engineering Efforts Toward High Efficiency and Stable Sn2+‐Based PSCs

Thanks to the great endeavor of the researchers, Sn‐based PSCs have obtained relative stable PCE of ≈8%.28 The improvement in performance of PSCs is not only due to the material itself but also the evolution in device engineering. In this small section, we will introduce various devices preparation approaches towards efficient and stable Sn2+‐based PSCs over the recent years.

Efforts to Suppress Sn Vacancy Defects and Improve Oxidation Stability:

One of the major problems of Sn‐based PSCs is their poor stability originating from the facile formation of Sn vacancy associated with oxidation of Sn2+ to Sn4+ when exposed in air,46, 141, 150 which leads to poor reproducibility and deteriorates the devices rapidly.46, 51 For example, Xu et al.151 studied the defect properties of CsSnI3 perovskite and depicted the influence of defects and synthesis conditions on the photovoltaic performance. They found that due to the strong Sn 4s‐I 5p antibonding coupling, Sn vacancies have very low formation energies in CsSnI3, leading to a very high concentration of Sn vacancies and therefore high p‐type conductivity regardless of the growth conditions. To solve the problem, reductive or sacrificial additives like Sn(II) halide salt etc., were added to cure the problem.

SnF2

Before tin halide perovskite served as a light‐absorber in PSCs, Chung et al.72 demonstrated that when CsSnI3 with SnF2 additive (though the formulation of CsSnI2.95F0.05 is not correct152) was used as HTM in DSSC, enhancements of 29% and 21% in J SC and η were achieved, respectively. The magic effect of SnF2 attracted researcher to investigate its working mechanism. In 2014, Kumar et al.69 demonstrated for the first time that the introduction of SnF2 into CsSnI3 can reduce Sn vacancies and background carrier densities. Therefore, high J SC of more than 22 mA cm−2 and a PCE of 2.02% were achieved in contrast to the nonfunctioning devices without SnF2. One year later, the same group59 further investigated the doping effects of SnF2 in FASnI3. The X‐ray photoelectron spectroscopy (XPS) data show that only Sn2+ but no Sn4+ appears in the FASnI3: 20% SnF2 sample, indicating that no oxidation side reaction happened inside the doped sample (Figure a–d). Moreover, they claimed that the addition of SnF2 improved the environmental stability, as they found that the color of the FASnI3 films with SnF2 remained yellow when left overnight, but change into orange without SnF2 (Figure 7e). The resulting photovoltaic devices gave maximum PCE of 2.10%, with high J SC of 24.45 mA cm−2. Based on the same hypothesis, SnF2 dopant was also introduced to reduce Sn vacancies for efficient and stable CsSnBr3 based PSCs.74, 75 The author has concluded that the SnF2 cannot only improve interfacial energy alignment but also increase stability to electron beam damage. Nowadays, most Sn‐based perovskites films prepared by solution deposition need additional SnF2 to get good performance,47, 48, 57, 58, 73 including the recent (FA)0.75(MA)0.25SnI3 based PSCs with the highest PCE of 8.12%.56 Additionally, Ma et al.134 claimed that SnF2 as the inhibitor of Sn2+ oxidation in the MASnI3 film could increase the fluorescence lifetime up to 10 times and give longer carrier diffusion lengths >500 nm, compared with pristine MASnI3 films.

Figure 7

a,b) Top‐view FESEM images of FASnI3 and FASnI3:20%SnF2 perovskite films deposited on the mesoporous TiO2 layer. c) Expanded view of the nanoplatelet‐like structure found on the surface of FASnI3:30%SnF2. d) Cross‐sectional FESEM image of the full device showing individual layers as follows: FTO/TiO2 + FASnI3/spiro‐OMeTAD/Au. e) Color change in FASnI3 with and without SnF2 addition and XPS measurement. Reproduced with permission.59 Copyright 2015, RSC.

SnX2 (X = Cl, Br, I)

Apart from the tin fluoride additives, excess tin halide salts like SnI2 were also believed to improve device stability towards air oxidation. In 2015, Marshall et al.68 prepared the CsSnI3 films with an excess of SnI2 during CsSnI3 synthesis from CsI and SnI2. They believed that the excess of SnI2 occupies some of the space between adjacent CsSnI3 crystals, hindering the ingress of the oxygen and water so that the barrier of the transformation of CsSnI3 into Cs2CsI6 at the site of a Sn vacancy is increased. The fabricated devices have inverted planar structure of ITO/CuI/CsSnI3 (with or without the excess of SnI2)/C60/BCP/Al. The devices with the excess amount of SnI2 exhibit up to 30% increase in J SC and V OC as compared to the control devices. The devices' stability is also enhanced in that the PCE of devices with 10 mol% excess SnI2 exhibited only 10% reduction after 10 d period of storage, contrary to the 70% loss with control devices. In 2016, they further demonstrated that excess SnI2 is beneficial to improve both efficiency and stability of CsSnI3‐based PSCs.66 Therefore, their results suggest that the excess amount of SnI2 in the precursor is also an effective strategy to improve the performance of CsSnI3‐based PSCs. Similarly, an excess of SnBr2 75 was also found to reduce the density of defect states or Sn vacancies in all‐thermal‐vapor deposited CsSnBr3 PSCs by preventing the oxidation of Sn2+ to Sn4+ in ambient air. The resulted devices obtained a higher V OC of 0.41 V than the previously reported 0.19 V.23

It was not until 2016 that a systematic study was made about the effect of different Tin halide salts in resolving the problem of tin vacancies in tin halide perovskites66 (Figure a–f). Focusing on the low fill factor (FF) problem of Sn‐based PSCs, Marshall et al. evaluated SnF2, SnCl2 and SnBr2 additives in CsSnI3‐based PSCs. Simplified device architecture (ITO/CsSnI3/PC61BM/BCP/Al) was fabricated without HTM and the performance of SnCl2 doped CsSnI3 PSC devices was tested without encapsulation in ambient air at a humidity of ≈25% under constant 1 sun simulated solar illumination. They showed that the champion stability was exhibited only by devices with a tin halide additive: 11 h with no additive; 16 h with 10 mol% SnCl2; and 22 h for SnI2 (Figure 8g,h). Moreover, SnCl2 as additive offers the advantage of the highest η (3.56%, due to reduced sensitivity of device parameters to pin holes while SnBr2 and SnF2 gave η ≤ 0.4%. For the same reason, SnCl2‐doped CsSnI3 also showed FF up to 0.69, which is the highest FF among reported tin halide PSCs. They attributed SnCl2 being the best of the tin halide additives to the interplay of three factors. First, the added SnCl2 is distributed toward the surface of the crystallites to form a tin‐rich top layer for the perovskites. Second, the greater covalence offers SnCl2 better solubility in common solvents (including DMF) than SnF2. Last, tin chloride diffusion into fullerene (ETL) is easier than other halide analogs. Therefore, SnCl2 can be used as a promising additive to improve the performance of CsSnI3 PSCs and probably any other Sn‐based PSCs. Currently, only one report on SnCl2 modified FASnI3 as an inhibitor of Sn4+, but poor PCE was achieved.153

Figure 8

SEM images of CsSnI3 films on ITO glass prepared with different tin halide additives. SEM images with no a) tin halide additive, b) 10 mol% added SnI2, c) 10 mol% added SnBr2, d) 10 mol% added SnF2, and e) 10 mol% added SnCl2. f) Schematic diagram of the proposed film structure in case e: CsSnI3 crystallites capped with a thin SnCl2 layer. Reproduced with permission.66 Copyright 2016, Nature Publishing Group. g) The effect of constant illumination on PCE of devices (100 mW cm−2 of AM 1.5) for 245 min. Reproduced with permission.68 Copyright 2015, RSC. h) J–V characteristics of different CsSnI3 devices after 60 d storage under nitrogen. Reproduced with permission.66 Copyright 2016, Nature Publishing Group.

SnO and Sn(OH)2

Except for the tin halide additives, the presence of SnO and Sn(OH)2 53 in the MASnI3 film was also found to be beneficial to reduce hole carrier concentration, leading to an improved air stability of the Sn‐based perovskite devices. This finding suggests that not only the commonly used tin halide additives but also other divalent Sn compounds could serve as Sn vacancy suppressors assisting the realization of efficient and stable tin‐based PSCs.

SnX2/Organic Reduce Agents

The primary cause of the instability of tin‐based PSCs is the oxidation of the Sn(II) to Sn(IV). Therefore, it is reasonable to add reductive agents besides divalent tin additives, to further suppress Sn vacancy for more stable lead‐free PSCs. As an example, hypophosphorous acid (HPA)73 (Figure a) was introduced into fabricating all‐inorganic CsSnIBr2‐based mesoscopic PSCs to reduce the concentration of Sn vacancies, achieving a higher PCE of 3.0% than 1.7% of without HPA addition. Moreover, Song et al.41 introduced hydrazine vapor (reducing vapor atmosphere) in the presence of SnF2 additive into fabrication process of Sn‐based PSCs (Figure 9b). The results showed that the additional hydrazine vapor process led to significantly suppressed carrier recombination with more than 20% reduction of Sn4+/Sn2+ ratios. And by tuning amounts of hydrazine vapor properly, the PCEs of MASnI3 and CsSnI3 devices were improved from an average of ≈0.02% to 3.40% and ≈0.16% to 1.50%, respectively. Four months later, the same group reported65 more efficient CsSnI3 cells with PCE up to 4.81% using the same hydrazine vapor treatment with an excess of SnI2. In this research, they also studied the optimum ratios of monovalence cations (MA+, FA+ , or Cs+) to SnI2 in the MASnI3, FASnI3 and CsSnI3 perovskite materials, which were 0.4–0.6, 0.6–0.8, and 0.4, respectively. Furthermore, they prepared ASnI3 devices in a pure N2 atmosphere for comparison. The results showed that only FASnI3 could work with V OC > 0.15 V, while the other two devices behaved short‐circuit even at the optimum AI/SnI2 (A: MA+, FA+ , or Cs+) ratio that worked in a weak hydrazine vapor atmosphere. This result showed the importance of using reducing agents like hydrazine to suppress SnI2 from forming Sn4+ and ensure Sn2+‐rich environment to compensate the Sn2+ vacancies for obtaining efficient devices. On the other hand, the phenomenon that only FASnI3 ‐based devices could survive without additional hydrazine consists with aforementioned results133, 138 where FASnI3 displayed alleviated self‐doping effect possibly due to the competitive formation the hydrogen bonding between H2O and FA+ (Figure 9c–e).

Figure 9

a) PL time decay trace on CsSnIBr2 perovskite thin films on glass attached Al2O3 mesoporous layer. Reproduced with permission.73 Copyright 2016, RSC. b) Proposed possible mechanism of hydrazine vapor reaction with Sn‐based perovskite materials. Reduction process: 2SnI6 2− + N2H4 → 2SnI4 2− + N2 + 4HI. The best J–V curves of c) MASnI3, d) CsSnI3, and e) CsSnBr3. Reproduced with permission.41 Copyright 2017, ACS.

5‐Ammonium Valeric Acid Iodide and Ascorbic Acid

In the end, beyond above‐mentioned Sn(II) containing panaceas, Hoshi et al.154 claimed that the addition of 5‐AVAI in the preparation process could significantly improve the oxidation stability of the MASnI3 films in the air, due possibly to the formation of (5‐AVAI)(CH3NH3)1− PbI3. In addition, ascorbic acid (AA),155 as a common antioxidant, was also introduced as an effective additive to retard the oxidation of Sn‐containing precursor solutions for making Pb/Sn mixed perovskites. It is of great necessity to examine these effects on the pure Sn‐based perovskites.

Methods to Control the Morphology of the Sn‐Based Perovskite Layers:

Apart from the divalent tin additives, the film quality of the fabricated perovskite layer is another parameter that controls the final performance of the tin‐based PSCs. The film quality mainly refers to the morphology of the perovskite layer, such as homogeneity and coverage, which are strongly influenced by the crystallization process of perovskite film. The conventional one‐step film deposition method of tin perovskites often engenders randomly oriented film growth accompanied by forming large crystals platelets and poor surface coverage with micron‐sized pinholes.46 The flawed films further led to the poorly performing devices.69 Moreover, due to the reaction kinetics between organic/inorganic halide and tin halide salts is faster than its lead analogs,19, 46, 156 the control of tin perovskite crystallization during the deposition process is more challenging. Therefore, developing novel preparation methods to achieve high‐quality tin perovskite film is of great importance to boost both efficiency and stability of Sn‐based PSCs. In lead‐based perovskites, the methods for morphology engineering include addition of additives,157, 158, 159 solvent engineering,160, 161 vacuum engineering,162, 163 thermal annealing,164 self‐healing,165 vapor deposition,166 and so on. Herein, the reported methods used for preparing high‐quality tin perovskite layers are introduced.

Additives for the Morphology Control

Additives such as methyl ammonium chloride (MACl),157 1,8‐diiodooctane (DIO),158 and butyl phosphonic acid 4‐ammonium chloride (4‐ABPACl)159 have been successfully used to obtain high‐quality perovskite films for high‐efficiency lead‐based PSCs. In the case of tin perovskite, the popularly used SnF2 additive for the elimination of the Sn vacancy could result in poor film morphology and bad device performance due to the phase separation of SnF2 within the perovskite film.6, 135 In this regard, Lee et al.58 introduced pyrazine into FASnI3 perovskite precursors in conjunction with SnF2 in the form of the SnF2‐pyrazine complex (Figure a,b). The complex is believed to assist a uniform distribution SnF2 in the perovskite film, thereby substantially improving the morphology of FASnI3 perovskite. Finally, the resulted FASnI3 PSCs achieved a maximum PCE of 4.8% with high reproducibility (Figure 10c). Besides being a reducing agent, hypophosphorous acid (HPA) can also act as a morphology controller73 in the fabrication process of all‐inorganic CsSnIBr2 mesoscopic PSCs. HPA has the P—O bond that could strongly coordinate with Sn2+, producing HPA‐CsSnIBr2 clusters via Sn—O—P—O—Sn coordination bonds.167 The formed clusters in precursor solution promoted the growth of perovskite crystals and expelled the redundant SnF2 to the surface of the film. Due to the suppressed SnF2 phase separation in the CsSnIBr2 thin films, the highest reported PCE of ≈3.2% for the all‐inorganic Sn‐based PSCs was achieved. Likewise, hydrazine41, 65 was used not only as reducing agent to reduce the oxidization of Sn2+ to Sn4+, but also as modifier asserted by Song et al.41 to achieve better film morphologies for enhanced device efficiencies. Unfortunately, there was no explanation of why hydrazine could do a good job in ameliorating perovskite film morphologies. Moreover, a hydrazine atmosphere can help the disperse of SnI2 65 to improve the quality of perovskite film, as indicated by no observable agglomeration of SnI2 from the SEM/EDS characterization (Figure 10d–i). The corresponding device displayed the best PCE (4.81%) of CsSnI3‐based PSCs, which is in the meantime much higher than the SnF2 41 modified devices (1.83%). Additionally, triethyl phosphine (TEP)63 as a soft Lewis base can form intermediate complexes with Sn2+ species via weak coordinating interaction, which could slow down the perovskite crystallization process and improve the film morphology as well as the device performance. The devices with TEP showed increased FF in average (from 42.0% to 53.7%), and an average PCE from 1.15% to 1.75%, with the champion device reaching ≈2% efficiency (Figure j–n).

Figure 10

SEM images a) in the absence of pyrazine and b) in the presence of pyrazine and c) J–V curves of FASnI3 perovskite film in the presence of SnF2 with and without pyrazine. Reproduced with permission.58 Copyright 2016, ACS. d–i) SEM images of the CsSnI3 perovskite films grown with various CsI/SnI2 molar ratios, together with a neat SnI2 film. Reproduced with permission.65 Copyright 2017, ACS. Top surface SEM images of j) pristine 2D Sn3I10 film, k) (Sn3I10 + SnF2) film, and l) (Sn3I10 + SnF2 + TEP) film. Cross‐sectional SEM images of m) (Sn3I10 + SnF2) film and n) (Sn3I10 + SnF2 + TEP) film. Reproduced with permission.63 Copyright 2017, ACS.

Solvent Engineering

Solvent engineering technique has been proved many times as the most effective method for preparing high‐quality lead perovskite films to achieve high‐performance PSCs.160, 161, 168, 169, 170 The work on solvent engineering of Sn‐based perovskite was first reported by Hao et al.,47 where they investigated the solvent effects on the crystallization of the MASnI3 perovskite films. They found that highly uniform, pinhole‐free perovskite films can be obtained by using a dimethyl sulfoxide (DMSO) as solvents in perovskite precursor. And the transitional SnI2·3DMSO intermediate phase was very important in achieving a high‐quality perovskite film. The heterojunction depleted hole‐transporting layer‐free solar cells based on mesoporous TiO2 showed a high photocurrent up to 21 mA cm−2 and a PCE of 3.15%. Furthermore, Lee et al.58 used mixed solvents of N,N‐dimethylformamide (DMF) and DMSO in FASnI3 precursor solution followed by toluene drop‐casting, which led to uniform and dense FASnI3 perovskite layers. The role of DMSO was to retard the crystallization of FAI and SnI2 during spin‐coating process (Figure a–d). The realized smooth and dense perovskite layer enables a maximum efficiency up to 4.8% for FASnI3‐based PSCs and the encapsulated devices kept stable for over 100 d. The same method was also applied in low‐dimensional tin halide perovskites (PEA)2(FA)8Sn9I28 to obtain compact and smooth perovskite surface morphology.64 Chlorobenzene,161 as a common antisolvent applied in lead‐based PSCs, was adopted by Zhang et al.60 as antisolvent to achieve a dense FASnI2Br film giving a device PCE of 1.72%. As an antisolvent, diethyl ether seems to work better than chlorobenzene in improving the morphology of Sn‐based perovskites.56, 57 For example, recently, Liao et al.56 used diethyl ether as an antisolvent in solvent engineering process to synthesize uniform and pinhole‐free FASnI3 perovskite thin films. The fresh spin‐coated films showed a reddish intermediate state after dripping with diethyl ether, which might be crucial for the good film morphology, in contrast to chlorobenzene and toluene‐based antisolvents, which led to black films immediately (Figure 11e–h). Champion PCE of 6.22% and high reproducibility were achieved based on FASnI3 PSCs, showing good stability for 30 d (maintaining 85% of its initial efficiency stored in dark and glove box) with encapsulated cells. Later, Ke et al.57 also used diethyl ether as antisolvent to prepare uniform FASnI3 perovskite films with high surface coverage on the neat TiO2 and TiO2–ZnS substrates. They pointed out that the use of diethyl ether as an antisolvent in fabrication procedure was advantageous to inhibiting phase separation caused by excess SnF2. They proved again that the film prepared with chlorobenzene as antisolvent exhibited poor film coverage (Figure 11i,j). However, Zhao et al.28 used chlorobenzene as antisolvent to obtain (FA)0.75(MA)0.25SnI3 and FASnI3 film with complete coverage and no phase separation. The exceptionally good effect of chlorobenzene may be due to the use of different solution solvent. This conjecture needs to be proved by more studies.

Figure 11

SEM images of the as‐obtained MASnI3 perovskite layer on mesoporous TiO2 from different solvents, a) DMF, b) NMP, and c,d) DMSO with different magnifications. Reproduced with permission.47 Copyright 2015, ACS. FASnI3 films deposited on PEDOT: PSS by different anti‐solvent drippings: e) no dripping, f) chlorobenzene, g) toluene, and h) diethyl ether. Reproduced with permission.56 SEM images of FASnI3 with i) diethyl ether and j) chlorobenzene as antisolvent. Reproduced with permission.57 Copyright 2016, ACS.

Solvent–solvent extraction technique is a derivative method of solvent engineering, which was first proposed by Zhou et al.168 for the fabrication of high‐quality lead perovskite thin films. The same method was applied by Milot et al.171 in Sn‐based perovskites with DMF and DMSO as mixed solvents for perovskite precursor solution. The wet film was immediately immersed into an antisolvent (anisole) after spin‐coating, producing a smooth, and continuous FASnI3 thin film. An appropriate crystallization speed is very important to the morphology of the perovskite films. Very recently, Fujihara et al.48 employed a mixture of toluene and hexane as the antisolvents and DMSO as the good solvent (Figure a). Depending on the ratio of antisolvents and temperature, they can control the crystallization speed of the MASnI3 perovskite films to achieve high surface coverage perovskite films on a planar PEDOT: PSS electrode (Figure 12b–e). Planar junction devices with the structure of glass/ITO/PEDOT: PSS/MASnI3/C60/BCP/Ag, exhibited an efficiency of 2.14 ± 0.35% with a high open circuit voltage of 0.45 ± 0.01 V and long lifetime of over 200 h under 1 Sun illumination conditions (AM1.5, 100 mW cm−2).

Figure 12

a) Concept illustration for controlling the crystallization speed using the technique of adding a cosolvent in solvent bathing; Scanning electron microscopy (SEM) images for the b) LT‐Mix, c) LT‐T, and d) LT‐H films. The X‐ray diffraction spectra for mixed solvent (red), toluene (blue), and hexane (green) are shown in (e). Reproduced with permission.48 Copyright 2017, RSC.

Vapor Deposition

Generally, vapor deposition process is known to provide higher controllability over perovskite films fabrication in terms of higher homogeneity, smoothness, and surface coverage than solution‐processed films.166, 172 Moreover, the vacuum condition adopted in vapor deposition process is especially beneficial to air‐sensitive tin perovskite. In 2015, Weiss et al.173 proposed a two‐step process combining vapor‐deposited SnI2 precursor films and solution deposited MAI for the preparation of MASnI3 perovskite films. The results showed homogeneous preformed SnI2 film and complete conversion of SnI2 to MASnI3. The final film showed complete surface coverage even with such short contact period (Figure a–f). In the same year, smooth MASnBr3 thin films were synthesized via sequential evaporation by Jung et al.52 (Figure 13g,h). The obtained planar structure device showed an efficiency of 1.12%. In addition, SnF2‐doped CsSnBr3 film with excellent ambient air stability was also prepared by sequential vapor deposition method.75 Later, a hybrid thermal evaporation method at room temperature for the fabrication of high‐quality MASnI3 perovskite thin film was reported by Yu et al.50 The as‐deposited MASnI3 thin films have excellent morphology, with smooth surfaces, high surface coverage, and strong crystallographic preferred orientation along the <100> direction. Inverted planar architecture solar cells devices were fabricated based on these films and gave an open‐circuit voltage up to 494 mV.

Figure 13

SEM images of a) 100 nm vapor‐deposited SnI2, nonannealed MASnI3 perovskite films prepared by spin‐coating of b) 6 mg mL−1, c) 10 mg mL−1, d) 20 mg mL−1, e) 40 mg mL−1 MAI solutions, and a f) MASnI3 perovskite film prepared by use of 20 mg mL−1 MAI and subsequent annealing at 80 °C for 10 min; Reproduced with permission.173 SEM image of coevaporated g) MASnBr3 (MABr:SnBr2 = 4:1) Reproduced with permission.52 Copyright 2015, RSC and h) MASnI3. Reproduced with permission.50 Copyright 2016, RSC.

Vapor‐Assisted Solution Process

The so‐called vapor‐assisted solution process (VASP) was first reported to construct a high‐quality MAPbI3 film by Chen et al.174 in 2013. To improve tin perovskite surface coverage, Yokoyama et al.135 developed low‐temperature vapor‐assisted solution process (LT‐VASP), a kinetically control gas‐solid reaction method, to prepare lead‐free MASnI3 thin films. They pointed out that the substrate temperature (60−80 °C window) of preformed solid SnI2 is very important for achieving homogeneous and high surface‐coverage perovskite films. The acquired high‐quality MASnI3 films were fabricated in solar cells with an efficiency of 1.86% with good reproducibility. It is important to point out that LT‐VASP method is a pioneer work that explored alternative suitable fabrication methods for tin perovskite films (Figure a–c).

Figure 14

a) One‐step MASnI3; b) VASP–MASnI3; c) LT–VASP‐MASnI3; Reproduced with permission.135 Copyright 2016, ACS. SEM micrographs of the B‐g‐CsSnI3 thin films annealed at different temperatures for 2 min. d) As‐deposited, e) 100 °C, f) 150 °C, g) 200 °C, h) 250 °C, and i) 300 °C. j) Average grain size of the B‐g‐CsSnI3 thin films with increasing annealing temperature. Reproduced with permission.67

Thermal Annealing

Thermal annealing is a critical step in most of the perovskite film deposition steps. It can influence the film formation of perovskites significantly by driving solid‐state coarsening of perovskite grains. An interesting work was done by Wang et al.67 involving an all‐inorganic and thermally stable B‐γ‐CsSnI3 films deposited by spin‐coating. The films were postannealed at different temperatures over a range between 100 and 300 °C for 2 min to coarsen the grains. The B‐γ−CsSnI3 thin films annealed at 150 °C displayed large grain size, high film smoothness, and moderate Sn vacancy (V Sn) generation, which are responsible for the best performing PSC devices. The B‐γ‐CsSnI3 film after 150 °C annealing was applied in an inverted planar device architecture with nickel oxide (NiO) as the photocathode. They achieved a PCE of 3.31% without the use of any additive. This work demonstrated that proper thermal annealing is another efficient method for preparing high‐performance Sn‐based PSCs (Figure 14d–j).

Summary

In summary, as two less toxic family members of Pb, Sn and Ge are deemed as the redeemer to the toxic Pb element and tremendous efforts have been put in optimizing the materials and devices. So far, Sn‐based perovskites with lower bandgaps than lead analogs, have obtained a “stable” efficiency up to 6%. In this case, the issue of Sn2+ oxidization has been partly overcome by adding divalent tin halide additives and some reductive reagents. Additionally, the poor morphology of the tin halide perovskite layer has been improved by the various fabrication methods. Recently, great progress has been made in Sn‐based PSCs with inverted device architecture,28, 56, 64, 66, 67 due to the omission of doped HTMs. Unlike Pb‐based PSCs where the high efficiencies are usually achieved with doped HTMs, the use of dopants will accelerate the deterioration of tin perovskites. Hence, exploring high‐performance and dopant‐free HTMs is very important to get efficient and stable Sn‐based PSCs. And it is necessary to further study tin‐based perovskite material fundamentally (such as the mechanisms of self‐doping) and explore novel device structures with different charge selective contact materials toward more efficient and stable Sn‐PSCs. For germanium perovskites, the study of these compounds is very rare in the photovoltaic application so far. In this case, due to the relatively small ionic radius of Ge2+, the octahedra [GeX6]3− network is heavily distorted, which leads to wide band gap (>1.6 eV). In addition, the poor solubility of these compounds in polar solvents causes terrible morphology with low efficiency of only 0.2% from solution process. It is urgent to find new preparation methods of germanium perovskites for more efficient Ge‐based PSCs. Moreover, the easy oxidation of Ge2+ and Sn2+ will be definitely a challenge.

The Group 15 Metals Bi and Sb‐Based Absorbers

Beyond group 14 elements, two of group 15 metals in the periodic table, bismuth (Bi) and antimony (Sb) have been also studied for replacing lead in the solar energy absorbing materials. In this section, a brief introduction of typical A3Bi2X9, ABi3X10, A3Sb2X9 (A = MA+, FA+, Cs+; X = I−, Br−, Cl−) polymorphs and other derivatives will be given. Compared with Sn‐based perovskites, they form more diverse dimensionality in terms of the connection type of BiX6 3− (SbX6 3−) octahedron.101 Here we divided them into 0D, 1D, 2D, and 3D structures.

Bismuth‐Based Absorbers

Being adjacent to Pb2+ in the periodic table, Bi3+ has the similar 6s26p0 electronic configuration, which endows MAPbX3 with the strong light absorption and long carrier lifetimes.176 More importantly, it is much less toxic than Pb,177, 178 and has been used in organic synthesis and medicines.179, 180, 181 Hence, Bi‐based perovskite or hybrid materials are attractive options to replace lead perovskites.

0D Hybrid Materials:

MA: Among all the reported bismuth‐based absorbers, organic–inorganic hybrid bismuth halide MA3Bi2I9 is the most studied polymorph type. Owing to the tervalence state of Bi3+, the solid structure of MA3Bi2I9 features two face‐sharing 0D perovskite structure,88, 182, 183 which is constructed by the MA+ surrounded binuclear octahedral (Bi2I9)3− clusters, contrasting to the 3D MAPbI3 perovskite (Figure a).

Figure 15

a) Crystal structure of (CH3NH3)3Bi2I9 (MBI) (left) local structure of the Bi2I9 3− anion and (right) cation and anion positions in the unit cell. Reproduced with permission.183 Copyright 2016, RSC. Air stability of MBI, measured from mid‐July to mid‐August in Cambridge, MA, USA. b) Photographs of MBI and MAPbI3 on quartz over time in the ambient air. c,d) Normalized XRD patterns of MBI over time with air exposure. e) The relative change in the normalized intensity of the diffraction peaks of MBI (day 25 vs. day 1). Reproduced with permission.92

The single crystal of MA3Bi2I9 normally displays color like red wine89 and has regular hexagonal shape with a diameter ranging from 100 to 200 µm,87 or even up to 4–5 mm,89 adopting P63/mmc space group.88, 182, 184 The optical bandgap of MA3Bi2I9 was reported to be ≈1.94–2.26 eV87, 88, 89 with absorption coefficient up to ≈1 × 105 cm−1, the same order of magnitude with MAPbI3.185 The valence band maximum (VBM) of the MA3Bi2I9 was measured to be 5.63 eV by photoelectron spectroscopy in the air (PESA)89 and 5.9–6.0 eV by ultraviolet photoelectron spectroscopy (UPS)87, 89 in vacuum, respectively, which are all well aligned with the conduction band of TiO2. The electron mobility of MA3Bi2I9 single crystal is 29.7 cm2 V−1 s−1 as estimated by the space charge limited conduction (SCLC),89 which is comparable to that of MAPbI3 (38 cm2 V−1 s−1).186 The same carrier mobility was estimated to be 1 cm2 V−1 s−1 by Hall Effect.87 The phase‐pure and compact MA3Bi2I9 film showed long PL decay over 0.76 ns, with the bulk lifetime approach to 5.6 ns. And this film exhibited robust air stability than MAPbI3 after 25 d of continuous air exposure with 61% relative humidity (Figure 15b,c). It was speculated that the high air stability is owing to the formation of Bi2O3 or BiOI from BiI3, which could serve as a protective layer to prohibit the ingress of water and oxygen into bulk materials.92 Therefore, on account of its good optoelectronic properties and excellent stability, MA3Bi2I9 is the prevailing candidate materials for replacing lead perovskites in the photovoltaic application.

The first report on MA3Bi2I9 as light absorbers used in solar cells was by Park et al.88 They studied the morphology of MA3Bi2I9 and Cl‐doped (MA3Bi2I9Cl) films on TiO2 substrate by SEM. The results showed that MA3Bi2I9 had interconnected layers compared to the more particle‐like structure of MA3Bi2I9Cl. Meanwhile, they pointed out that the excellent surface morphology of MA3Bi2I9 could be propitious to form excitons with lower energy. In traditional mesoscopic solar cells, the best device based on MA3Bi2I9 showed J SC = 0.52 mA cm−2, V OC = 0.68 V, FF = 0.33, and η = 0.12%. By contrast, MA3Bi2I9Cl showed J SC = 0.18 mA cm−2, V OC = 0.04 V, FF = 0.38, and η = 0.003%. The extreme low V OC of MA3Bi2I9Cl was attributed to the poor morphology with perovskite particles surrounded by amorphous BiCl3. Two months later, Lyu et al.87 employed poly(3‐hexylthiophene‐2,5‐diyl) (P3HT) as the HTM to replace spiro‐OMeTAD in the MA3Bi2I9‐based solar cells with a mesoscopic structure, which showed a PCE of 0.19% (Figure b).

Figure 16

a) Large 70 mm thick single crystal of (CH3NH3)3Bi2I9 grown on the ITO substrate. Reproduced with permission.89 Copyright 2016, RSC. J–V curve of b) the P3HT based n–i–p device. Reproduced with permission.87 Copyright 2016, Springer. And c,d) p–i–n devices. c) Reproduced with permission.91 Copyright 2016, Elsevier. d) Reproduced with permission.83 Copyright 2017, ACS.

It became a comment sense that the performance of MA3Bi2I9‐based PSCs was improving due to the better morphology of the absorber layer. Singh et al.86 deposited uniform MA3Bi2I9 layers atop mesoporous anatase TiO2 and exhibited best PCE of 0.2%, as well as 10 weeks stability of the device in ambient condition. Zhang et al.187 used smooth indium tin oxide (ITO)/glass substrate to achieve a dense MA3Bi2I9 thin film, which gave a maximum PCE of 0.42% and high FF up to 0.64. This is one of the highest performances among MA3Bi2I9‐based lead‐free PSCs.

More efforts have been paid to modulate the morphology of MA3Bi2I9 perovskite. For example, 1‐methyl‐2‐pyrrolidinone (NMP)85 as morphology controller was added into the MA3Bi2I9–DMF precursor solution, producing a homogeneous film of MA3Bi2I9. The device yielded highly reproducible PCE of 0.31% and kept stable for 30 d in an ambient atmosphere (relative humidity of 50–60%). On the other hand, antisolvent assisted crystallization (ASAC) method also was used to improve MA3Bi2I9 thin films like the cases in the lead‐ and tin‐based PSCs. Abulikemu et al.89 first used this method, but gave a PCE of only 0.11%. Very recently, Mali et al.84 achieved cuboid‐shaped crystals of MA3Bi2I9 on the surface of mesoporous TiO2 by this method. The obtained thin film had excellent air stability with almost no color change even after two month exposure to air. The best solar cell devices made from this kind of films exhibited a PCE of 0.36%, with no substantial efficiency loss after 60 d. Gas‐assisted deposition method was first reported by Huang et al.188 to create uniform and dense lead perovskite thin films. Naturally, this method was used90 to improve the quality of MA3Bi2I9 films. Ultimately a PCE of merely 0.08% was obtained, which is 17% higher compared with the conventional one‐step method. Besides the common n–i–p‐type devices, the first p–i–n planar heterojunction device of MA3Bi2I9 perovskite was reported by Öz et al.91 with PCE of ≈0.1%. To achieve a smooth, uniform, and compact MA3Bi2I9 film in the p–i–n device structure, Ran et al.83 reported a two‐step (evaporation and spin‐coating) process of MA3Bi2I9 and obtained a PCE of 0.39% and V OC as high as 0.83 V, which is the highest V OC among MA3Bi2I9‐based solar cells so far (Figure 16d).

Sulfur‐Doped MA: Sulfur‐doped MA3Bi2I9 was developed93 to reduce bandgap of MA3Bi2I9 (2.1 eV), which is a relatively higher for the ideal single junction solar cell.129 Sulfur‐doped bismuth perovskites were obtained by in situ sulfur doping of MA3Bi2I9 through the thermal decomposition of Bi(xt)3 (xt = ethyl xanthate) precursor. The color of obtained perovskite films changed from orange to black when annealed from 80 to 150 °C, and there was a notably red shift in the optical absorption edge (Figure a). The bandgap of sulfur‐doped bismuth perovskite was measured to be 1.45 eV, which is even lower than the prototype MAPbI3.189 Moreover, sulfur‐doped MA3Bi2I9 exhibited a high carrier mobility of 2.28 cm2 V−1 s−1, about twice than that of pristine MA3Bi2I9. This work showed that doping could reduce the bandgap of MA3Bi2I9 while improving the charge transport, and further might enhance the performance of MA3Bi2I9‐based solar cells. However, unfortunately, there was no device prepared based on such kinds of material.

Figure 17

a) XRD patterns and color for sulfur‐doped MBI at different postheating temperatures. Reproduced with permission.93 Copyright 2016, ACS. b) Color of (BiI3)1− (MBI) films prepared by the solution method. Reproduced with permission.94 Copyright 2017, Elsevier. c) J–V curves MA3BiI2I9Cl, MA3Bi2I9, and Cs3Bi2I9‐based solar cell devices. Reproduced with permission.88

(BiI: One of the problems with MA3Bi2I9 is that its bandgap is even wider than that of BiI3 (1.8 eV). Therefore, to improve light absorption of MA3Bi2I9, Lan et al.94 designed active composite layers taking advantages of optoelectronic properties of BiI3 190, 191, 192, 193 and suitable energy level alignment of MA3Bi2I9 with TiO2 (Figure 17b). When 20% of MA3Bi2I9 perovskite was introduced into the active layers, the (BiI3)0.8(MA3Bi2I9)0.2 solar cells displayed improved V OC from 0.44 to 0.57 V, with a PCE of 0.08%.

A: Replacing MA with formamidinium (FA), the FA3Bi2I9 95 exhibits the same structure as MA3Bi2I9, with a bandgap of 2.0 eV. Additionally, when more bulky cations like imidazolium and cyclohexyl ammonium are used as the A cation, 0D perovskite‐like structure are formed. The synthesized (C3H5N2)3[Bi2I9]96 has two temperature induced solid–solid structural phase transition, and (C6H14N)3Bi2I9 97 has red emissions at room temperature. All‐inorganic bismuth halide compounds have also been studied to replace lead perovskites in PSCs. Cs3Bi2I9 as a light harvester was first studied by Park et al.88 with face‐sharing octahedra dimer ((Bi2I9)3−, P63/mmc space group, 0D structure) similar to MA3Bi2I9. It possesses a bandgap of 2.2 eV close to MA3Bi2I9 (2.1 eV). A detailed study of its band gap structure was shown by Zhang et al.194 Compared with MA3Bi2I9 and MA3Bi2I9Cl, Cs3Bi2I9 displays a relatively high PL yield, suggesting low losses in nonradiative recombination. Consequently, the best Cs3Bi2I9‐based solar cell shows a PCE of 1.09% (V OC = 0.85 V, J SC = 2.15 mA cm−2, and FF = 0.6) (Figure 17c).

1D Hybrid Materials:

MA: MA3Bi2Cl9 98 is a 1D organic–inorganic hybrid bismuth compound with zig‐zag double chains of distorted BiCl6 3−‐octahedra structure. The past studies of MA3Bi2Cl9 focused mainly on structural phase transition, which revealed two‐phase transitions195 at T c1 = 349 K and T c2 = 247 K, respectively.

(H: 1,6‐hexane diammonium bismuth halide (H3NC6H12NH3)BiI5 (HDABiI5) showing 1D chains structure has early been reported by Mousdis et al.196 and Mitzi et al.,197 respectively (Figure a,b). Owning an optical bandgap of ≈2.0 eV, Fabian and Ardo198 first applied HDABiI5 as a light absorber in PSCs. HDABiI5 layer had near‐complete surface coverage on mesoporous TiO2. The low volatility of organic 1,6‐hexane diamine group endowed HDABiI5 with good thermal stability up to 200 °C. However, the final PCE of the device was only 0.027%. TMP (N,N,N′,N′‐tetramethylpiperazine) can also be used as organic cation and yielded (TMP)BiX5 (X = Cl, Br, I) with 1D chains structure and optical band gaps of 2.02–3.21 eV.101

Figure 18

Top‐view SEM image of a) HDABiI5 and b) MAPbI3 deposited on mTiO2/cTiO2/FTO. Reproduced with permission.198 Copyright 2016, RSC. Crystallographic packing diagrams of c) [py][BiI4] and d) [mepy][BiI4]; e) Boltzmann‐fitted J–V curves of the [py][BiI4] champion cell. Reproduced with permission.37 Copyright 2017, RSC.

C: Another two 1D chained organic–inorganic iodobismuthates, C5H6NBiI4 ([py][BiI4]) (py: pyridinium) and C6H8NBiI4 ([mepy][BiI4]), (mepy: methyl pyridinium) were prepared by Li et al.37 and applied in HTM‐free PSCs with mesoscopic anode and carbon counter electrode (Figure 18c,d). They pointed out that the protonated aromatic heterocycles play an active role in intermolecular interactions through the frontier orbitals, which endows them with pseudo‐3D charge transfer ability. [py][BiI4] and [mepy][BiI4] have bandgap of 1.98 and 2.17 eV, respectively. The best device efficiency of 0.9% was obtained using [py][BiI4], which is comparable with that of other reported Bi–iodide based devices83, 88, 187 (Figure 18e).

2D Hybrid Materials:

A: By replacing the Cs+ in typical 0D Cs3Bi2I9 perovskite with K+ and Rb+, K3Bi2I9 and Rb3Bi2I9 36 are achieved. The decreased size of cations induces 2D layered defect‐perovskite structure, with corrugated layers of Bi–I octahedra. K3Bi2I9 and Rb3Bi2I9 have direct bandgaps of 2.1 eV, while Cs3Bi2I9 has an indirect bandgap of 1.9 eV (Figure a). Lehner et al. pointed out that direct gaps coupled with the high DOS with a strong p‐character across the gap are criteria for effective light absorption. Besides, substituting MA cation for NH4 + cation in MA3Bi2I9, Sun et al.103 achieved a 2D layered perovskite‐like architecture (NH4)3Bi2I9, with a dark red color and a bandgap of 2.04 eV. But no device study was provided in these reports (Figure 19c).

Figure 19

2D layered crystal structures of a) Rb3Bi2I9 (P21/n); Reproduced with permission.36 Copyright 2015, ACS. b) MA3Bi2Br9; Reproduced with permission.102 c) (NH4)3Bi2l9; Reproduced with permission.103 Copyright 2016, American Institute of Physics. d) Cs3Bi2Br9; Reproduced with permission.199 Copyright 2017, ACS. e) Photo and SEM images of Cs3Bi2I9 (left) and CsBi3I10 (right). Reproduced with permission.100 Copyright 2016, ACS.

A: Besides the change of A cations, when the halide atom is changed to Br−, one can change the typical 0D Cs3Bi2I9 to 2D layered perovskites. For example, MA3Bi2Br9 102 crystallizes into trigonal symmetry ( m1) space group, forming corrugated layers of BX6 octahedra (Figure 19b). MA3Bi2Br9 possesses a direct bandgap of 2.5 eV102 and emits at 430 nm as quantum dots with photoluminescence quantum yield (PLQY) up to 12%. Another layered inorganic halide bismuth compound, Cs3Bi2Br9 was reported by Bass et al.199 It occupies the 2D structure, with corrugated layers of corner‐sharing BiBr6 3− octahedra, as illustrated in Figure 19d, different from iodine analogs with 0D structure. Cs3Bi2Br9 has a large exciton binding energy of 940 meV, which is indicative of a strongly localized character and resulted in highly structured emission. However, this large exciton binding energy will extremely limit its application in photovoltaic technologies, particularly as a light harvester.

CsBi: Johansson et al.100 reported another type of cesium bismuth iodine compound CsBi3I10. In contrast to previously reported Cs3Bi2I9, CsBi3I10 has a different orientation of crystal growth, which may explain a more uniform and smoother coverage on TiO2. CsBi3I10 film possesses a smaller bandgap of 1.77 eV and higher absorption coefficients up to 1.4 × 105 cm−1, which are advantageous to PV application, compared with the bandgap of Cs3Bi2I9 at 2.03 eV and absorption coefficients of 7 × 104 cm−1 (Figure 19e). For the same reason, this material was also used in a red‐light photodetector recently.200 The PV device with a structure of glass/FTO/compact TiO2/mesoporous TiO2/CsBi3I10/P3HT/Ag, showed a PCE of 0.4%, with a notable J SC of 3.4 mA cm−2. This work proved the possibility to further increase light absorption and photocurrents in bismuth halide absorbers based solar cells.

(TMP): It is interesting that (TMP)[Bi2I5] with one kind of halide species show 1D chain structure, while the addition of Cl leads to the formation of (TMP)1.5[Bi2I7Cl2] with a 2D structure.101 It has an optical bandgap of 2.10 eV and improved electrical conductivity of 2.37 × 10−6 S cm−1. Moreover, it displayed efficient photoconductivity response and very high stability either in humid air or long‐time irradiation in a simple device.

3D Hybrid Materials (Double Perovskites):

The organic–inorganic hybrid bismuth perovskites mentioned above are all low dimensional structures with PSCs efficiency of only ≈1%. To realize the 3D perovskite architecture which has demonstrated advantages for high efficiency in lead perovskites, double perovskite with 3D structure was developed. Incorporating a monovalent metal into bismuth perovskites could yield a 3D double perovskite with the chemical formula of AI 2BIBiIIIX6. Double perovskites usually exhibits a high tolerance to defects owing to the strong ionic nature of the constituents and 3D structure similar to organolead halide perovskites.201 But they usually have large bandgaps that prevent absorption of the whole solar spectrum.38, 109, 202, 203

MA: Early in 2015, Giorgi et al.175 proposed double‐perovskite structure MA2TlBiI6 computationally, by substituting Pb2+ with Tl+ and Bi3+ from parental MAPbI3, but stayed in theory because monovalent metal Tl is very toxic. As a compromise, the toxic thallium can be replaced by other monovalent metals like potassium. Therefore, organic–inorganic hybrid double‐perovskite MA2KBiCl6 was first synthesized by Wei et al.,104 which is solution processable but with a bandgap of 3.04 eV similar to that of the prototypical MAPbCl3 perovskite. Because its bandgap is too large to be used for the photovoltaic application. They105 finally turned to MA2TlBiBr6, which is isoelectronic with MAPbBr3. MA2TlBiBr6 adopts a space group of m like Cs2AgBiX6 (X = Cl, Br) (vide infra) and possesses a narrower bandgap of 2.16 eV than MA2KBiCl6 (Figure k). Very recently,106 they synthesized another new hybrid perovskite: MA2AgBiBr6 with a low bandgap of 2.02 eV and without toxic element. MA2AgBiBr6 also forms in cubic space group m, with better thermal stability (decomposition temperature up to 550 K) than MAPbBr3, and no obvious phase transition was detected from 120 to 360 K. Additionally, the crystal color changes from red to yellowish brown upon cooling (Figure 20j).

Figure 20

a) Single‐crystal structure of AgBi2I7 cubic structure six‐coordinated silver‐iodide octahedron sites. b) Tauc plot of AgBi2I7 from the UV/Vis spectroscopy to determine E g under the assumption of a direct bandgap. c)  J–V curves in the dark and illumination under 100 mW cm−2 AM 1.5 G. Reproduced with permission.114 d) Single‐crystal structure of Cs2AgBiBr6; Photograph of the single crystal; The Bi3+ face‐centered‐cubic sublattice, consisting of edge‐sharing tetrahedra. e) Absorbance spectrum of Cs2AgBiBr6 powder; f) Time‐resolved room‐temperature PL and fits for the PL decay time (t) in powder and single‐crystal samples. Reproduced with permission.38 Copyright 2016, ACS. g) Refined crystal structure of Cs2AgBiCl6; Diffuse reflectance spectra for h) Cs2AgBiBr6 and CH3NH3PbBr3 and i) Cs2AgBiCl6 and CH3NH3PbCl3; Reproduced with permission.109 Copyright 2016, Springer.Crystal structure of (j) (MA)2AgBiBr6; Reproduced with permission.106 Copyright 2017, Springer. And k) (MA)2TlBiBr6; l) Photographs of Cs2AgBiBr6 and Cs2(Ag1− Bi1− )TlBr6 (x = a + b = 0.075) single crystals and change of absorption onset. Reproduced with permission.111 Copyright 2017, ACS.

Cs: Like organic–inorganic hybrid bismuth compounds, inorganic ternary metal halides (A3Bi2X9) usually occupy low dimensional structures. However, before the report of organic–inorganic hybrid bismuth double‐perovskites, all‐inorganic halide bismuth double‐perovskite (3D) Cs2AgBiBr6 was already synthesized by Slavney et al.38 Cs2AgBiBr6 has an indirect bandgap of 1.95 eV, and the material shows a long PL decay of 660 ns at room temperature (Figure 20d–f). Additionally, Cs2AgBiBr6 shows higher stability with heat and moisture than MAPbI3. Almost the same time, McClure et al.109 reported Cs2AgBiBr6 with the chloride analogue Cs2AgBiCl6, with indirect bandgap of 2.19 and 2.77 eV, respectively. Both compounds adopt the cubic double‐perovskite structure with space group m. They are stable when exposed to air, but with the additional light, Cs2AgBiBr6 degrades over a period of weeks. In contrast, Cs2AgBiCl6 shows better stability with no apparent change observed. Volonakis et al.202 also designed and synthesized Cs2AgBiCl6 (Figure 20g–i). The bandgaps of double perovskites were between 1.95 and 3.04 eV, which were too wide to be used as absorbers in single junction photovoltaic cells. To lower bandgap, Slavney et al.111 incorporated Tl+ as a dilute impurity into Cs2AgBiBr6, achieving an opaque black octahedral perovskite crystals Cs2(Ag1− Bi1− )TlBr6 (0.003 < x = a + b < 0.075) with very stable structure. The Tl‐doped compounds Cs2(Ag1− Bi1− )TlBr6 displayed low bandgap down to 1.40 eV (indirect) and 1.57 eV (direct) when x = 0.075, which is competitive with that of MAPbI3.16 Moreover, time‐resolved photoconductivity measurements showed that the Tl‐doped materials had long‐lived carriers up to microsecond, though shorter than that of Cs2AgBiBr6 due to the extra doping of Tl. This study demonstrated the first double perovskite that has comparable band gap and carrier lifetime to those of MAPbI3, but regrettably, there was still toxic Tl in the compounds (Figure 20l). Additionally, through alloying of trivalence InIII/SbIII into Cs2AgBiBr6, the bandgap of double perovskite Cs2Ag(Bi1− M)Br6 112 (M = In or Sb) can be modulated. For example, when M is SbIII and x = 37.5%, Cs2Ag(Bi0.625Sb0.375)Br6 had a bandgap of 1.86 eV, which is 0.41 eV lower than the previous ternary compound.

To obtain an absorber with a direct bandgap, Volonakis et al.113 replaced Bi with In and calculated the band structure of Cs2AgInX6 (X = Cl, Br, I) by first‐principles calculations. The combined experiments identified that Cs2InAgCl6 has a direct bandgap of 3.3 eV. The potential of A2B′B″X6 type double perovskites for PV application was further studied by theoretical methods,105, 202, 204 and focus was put on A2In+Bi3+X6 perovskites. For example, Zhao et al. proposed Cs2InBiCl6 and Cs2InSbCl6 with low direct bandgaps ≈1 eV by HSE+SOC calculation.204 However, Xiao et al.205 used a combination of theoretical and experimental study to show that Cs2InBiCl6 and Cs2InSbCl6 were unstable due to spontaneous oxidation of In+ into In3+. Lately, Volonakis et al.206 proposed the rule of designing a useful double perovskite material: mimicking the electronic structure of MAPbI3. To stabilize the double perovskite structure of A2In+Bi3+X6, they suggested the use of mixed‐A‐site‐cation double perovskite (Cs/MA/FA)2InBiBr6 rather than all‐inorganic double perovskites. Although their attempts to synthesize MA2InBiBr6 and FA2InBiBr6 were failed, more efforts are needed to explore the suitable composition of the cations.

Among the reported double‐perovskites, Cs2AgBiBr6 is the only one that was applied in a working device. Very recently, Greul et al.110 prepared phase pure Cs2AgBiBr6 films with a optimal post‐annealing temperature of 285 °C. The corresponding mesoscopic devices displayed an incredible maximum PCE of 2.43%, with JSC = 3.93 mA cm‐2, VOC = 0.98 V and FF = 0.63. Moreover, stability of Cs2AgBiBr6‐based devices was tested under constant illumination at ambient conditions during 100 min. This work suggested the potential of double‐perovskites as lead‐free alternatives to MAPbI3.

AgBi: Besides double‐perovskites, Kim et al.114 synthesized AgBi2I7 with cubic‐phases composed of vacancy‐free corner‐sharing bismuth iodide hexahedra and silver iodide octahedra. (Figure 20a–c) They fabricated dense, smooth, and pinhole‐free AgBi2I7 thin films with 200–800 nm large grains after annealing at 150 °C. The AgBi2I7 film absorbs light across the range from 350 to 750 nm, with an E g value of 1.87 eV. They applied it in solar cells and the best AgBi2I7‐based device had a PCE of 1.22 % and showed good stability with only 8% PCE reduction over 10 d under ambient conditions. From solutions with different ratios of AgI and BiI3 (AgI/BiI3 = 2:1), Zhu et al. got a new crystal structure of Ag2BiI5 with a space group of m.115 The Ag2BiI5‐based devices showed a maximum IPCE of 45% and a promising PCE above 2%. The results show the potential of finding new lead‐free absorbers and the possibility to tune the properties of bismuth halides by adding a different ratio of precursors.

Antimony‐Based absorbers

Antimony (Sb) is on the top right‐hand corner of lead in the periodic table, and its trivalent cation possesses a similar electronic configuration with divalent Pb2+. Antimonial compounds have been studied and used as therapeutic agents for human leishmaniasis and demonstrated low toxicity with appropriate regulations.207, 208 Hence, Sb is expected to be a nontoxic alternative to lead as well. Due to the high oxidation state (+3), Sb3+‐based halides have crystal structures of low dimensionality with the typical chemical structure A3Sb2X9, forming in dimer structure or layered structures.119, 209

Cs: Depending on the synthesis conditions, Cs3Sb2I9 forms completely different solid structure. From solution preparation, Cs3Sb2I9 preferentially forms 0D structure with isolated dimers of face sharing octahedrons (space group P63/mmc, no. 194) similar to that of Cs3Bi2I9. While from solid‐state or gas‐phase reactions, it forms 2D layered structure ( m1, no. 164) (Figure a,b). In 2015, Saparov et al.119 reported preparation and characterization of Cs3Sb2I9 thin films, and the first solar cell using Cs3Sb2I9 as light absorbers. The prepared Cs3Sb2I9 derivative from two‐step deposition approach has a layered structure and shows large grains above 1 µm. The layered Cs3Sb2I9 film shows red color as opposed to the orange color of the 0D Cs3Sb2I9. The film has a bandgap of 2.05 eV, high absorption coefficients up to 105 cm−1, an ionization energy of 5.6 eV, and better stability in ambient air than MAPbI3 films (Figure 21c). Unfortunately, the PV device with an architecture of glass/FTO/c‐TiO2/Cs3Sb2I9/PTAA/Au showed PCE below 1% and a low open‐circuit voltage between 0.25 and 0.3 V. They ascribed the low performance of Cs3Sb2I9 to the deep defects that promote nonradiative recombination, which is substantially suppressed in MAPbI3 with shallow defects. Later, 0 D structure was also synthetized to fabricate solar cell devices and displays champion efficiency of 0.84% with improved Voc of 0.60 V.118

Figure 21

a) Removal of every third Sb layer along the 〈111〉 direction of a) the perovskite structure results in b) the 2D layered modification of Cs3Sb2I9; c) Bandgap of the layered modification of Cs3Sb2I9 (inset shows a thin film) using the Tauc relation. Reproduced with permission.119 Copyright 2015, Springer. d) Schematic showing the influence of A cation size on the structure of A3Sb2I9; e) J–V curve under forward and reverse scans of the best device with the energy levels of Rb3Sb2I9 shown in inset; Reproduced with permission.35 Copyright 2016, Springer. f) Crystal structure of (CH3NH3)3Sb2I9; g) Comparison of the absorption coefficient of various Bi‐based perovskites and (CH3NH3)3Sb2I9 determined by PDS measurements; h) J–V curve of (CH3NH3)3Sb2I9 solar cell measured with “up” and “down” sweep with a rate of 0.1 V s−1. Reproduced with permission.117 Copyright 2016, ACS.

Rb: As mentioned above, Cs3Sb2I9 is inclined to form dimer type 0D structure when obtained via a solution process. However, in 2016, Harikesh et al.35 found that when Cs is replaced by Rb, the resulting compound can easily form layered structures via solution processing (Figure 21d). They compared the formation energies of the dimer and layered forms of A3Sb2I9 with A = Cs and Rb by DFT calculations. The results showed that Rb3Sb2I9 had higher preference for the layered phase, which is linked to the smaller ionic radius of Rb (1.72 Å), as compared to that of Cs (1.88 Å). Moreover, Rb3Sb2I9 is thermally stable up to 250 °C, and no phase transition between −40 to 200 °C, which is beneficial for operating in a solar cell. Additionally, they obtained a near ideal stoichiometry Rb3Sb2I9 film with a single 3+ oxidation state of Sb by an excess SbI3 treatment method. The perovskite films with the SbI3 treatment showed better coverage compared to the pristine films, with an absorption coefficient of >1 × 105 cm−1 and an indirect bandgap of 2.1 eV. The solar cells based on Rb3Sb2I9 with poly‐TPD as HTM exhibited J SC = 2.11 mA cm−2, V OC = 0.55 V and a PCE of 0.66% (Figure 21e).

MA: Unlike inorganic Rb3Sb2I9 and Cs3Sb2I9, which tend to form two different structures (layered or dimer) depending on the crystallization conditions, when organic cation MA+ is used, the hybrid antimony‐based perovskites MA3Sb2I9 only forms 0D dimer structure, with octahedral anionic metal halide units (Sb2I9)3− surrounded by (MA)+ cations210 (Figure 20f). MA3Sb2I9 used in the photovoltaic application was first reported by Hebig et al.117 They prepared flat and homogeneous thin films of MA3Sb2I9 by a two‐step spin‐coating process followed by toluene treatment. The MA3Sb2I9 thin film shows a peak absorption coefficient (α) above 105 cm−1 and an optical bandgap of 2.14 eV. Additionally, they found that the Sb‐perovskite showed no exciton peak in its absorption spectrum contrasting with the related Bi compound. The Urbach tail energy of this amorphous compound is close to 62 meV, indicating a high degree of energetic disorder, which will bring additional sources of nonradiative recombination engendering low open‐circuit voltages.211, 212, 213 They fabricated a planar heterojunction solar cell with the architecture of ITO/PEDOT: PSS/absorber/PC61BM/ZnO‐NP/Al, which showed a PCE of η ≈ 0.5%, V OC of 0.89 V, and extremely low photocurrent densities (Figure 20h). Very recently, Boopathi et al.118 reported one‐step prepared MA3Sb2I9 through optimization of the precursor solution and HI concentrations. The highly crystalline MA3Sb2I9 film along with improved surface morphology contributed to the record PCE of 2.04% based on a planar architecture devices.

(NH: An even larger A cation was used by Zuo and Ding to synthesize a family of layered perovskite type light absorbers (NH4)3Sb2IBr9− (0 ethanol, variable absorption onset from 558 to 453 nm and high carrier mobility (hole of 4.8 cm2 V−1 s−1 and electron of 12.3 cm2 V−1 s−1). (NH4)3Sb2I9 solar cells gave an extraordinarily high V OC of 1.03 V and a PCE of 0.51%.

Cs: Aiming for high‐performance lead‐free metal‐halide perovskite, Vargas et al.121 incorporated Cu2+ into α‐Cs3Sb2Cl9 and yielded layered perovskite Cs4CuSb2Cl12, which has a direct bandgap of 1.0 eV and better conductivity than that of MAPbI3. Additionally, Cs4CuSb2Cl12 displayed high thermal‐stability, photostability, and resistance to humidity. These properties show that Cs4CuSb2Cl12 is a promising material for photovoltaic applications.

Aforementioned Bi3+ and Sb3+‐based perovskites and related absorbers represent the efforts of the scientist to find alternative lead‐free active materials in PSCs. Bi3+‐based compounds are much less toxic, even than divalent Sn2+ and Ge2+ ions, while displaying admirable air stability among all metal‐halide hybrid absorbers. However, the trivalence oxidation state of Bi makes the related compounds usually form low dimensional phases, with large or indirect band gap (≈2 eV), high exciton binding energy (70–300 meV),88 and relatively low charge transport ability. On account of the disadvantages, the highest PCE of Bi‐based PSCs reported so far is only 2.1%. Though the heterovalent substitution with monovalent metal could yield a 3D double perovskites, they usually display wide bandgap or need to involve toxic elements. Actually, there is no real application of these 3D double perovskites in solar cells to date. A recent theoretical study indicates that these structural 3D double perovskites do not have a 3D electronic structure.203 Another problem is the poor morphology of Bi‐based perovskite, which might originate from the preferred tendency to form regular hexagonal crystalline phase. Thus, two main strategies may be used by the community to construct ideal high‐efficiency Bi‐based PSCs: (i) compositionally engineered bismuth perovskite with 3D electronic structure and therefore low bandgap and (ii) new film fabrication methods which are suitable for Bi‐based perovskite. At the same time, the development of Sb3+‐based perovskites is still in its infancy. Owing to the trivalence oxidation state similar with Bi3+, Sb3+‐based perovskites also have wide bandgap and low dimensional structure with low efficiency. Sb3+‐based perovskite has deep level defects (versus shallow levels in MAPbI3 214), which is extremely detrimental to solar cells performance. Besides, Sb3+‐based perovskites (Cs3Sb2I9, Rb3Sb2I9) are prone to form in 0D dimer structure with poor charge transport when they are prepared via a solution process, which is also disadvantageous to high‐efficiency solar cells. Though these research results on the photovoltaic performance are unsatisfactory, they paved the way to lead‐free halide hybrid absorbers for photovoltaic applications. The dimensionality variation of bismuth and antimony‐based absorbers is shown in Table .

Table 2

Dimensionality variation of bismuth and antimony‐based absorbers

B Cation	0D	1D	2D	3D
Bismuth	Cs3Bi2I9 88 MA3Bi2I9 88 FA3Bi2I9 95 (C3H5N2)3Bi2I9 96 (C6H14N)3Bi2I9 97	MA3Bi2Cl9 98 (H3NC6H12NH3)BiI5 100 C5H6NBiI4 99 C6H8NBiI4 99 (TMP)BiX5 (X = Cl, Br, I)101	K3Bi2I9 36 Rb3Bi2I9 36 Cs3Bi2Br9 107 CsBi3I10 100 (NH4)3Bi2I9 103 MA3Bi2Br9 102 (TMP)1.5Bi2I7Cl2 101	Cs2AgBiCl6 108 Cs2AgBiBr6 38 Cs2(Ag1− aBi1− b)TlxBr6 111 Cs2Ag(Bi0.625Sb0.375)Br6 112 Cs2InAgCl6 (non‐Bi)113 MA2TlBiBr6 105 MA2TlBiI6 175 MA2KBiCl6 104 MA2AgBiBr6 104 AgBi2I7 114
Antimony	Cs3Sb2I9 119 Rb3Sb2I9 35 MA3Sb2I9 117	[CH3SC(NH2)2]2SbA5 120	Cs3Sb2I9 a)  119 Rb3Sb2I9 a)  35 (NH4)3Sb2IxBr9− x 116 Cs4CuSb2Cl12 121

Transformed.

Copper‐Based Perovskites

Copper (Cu), one of the first‐row transition metals, is essential in the human body as part of enzymes, and also could be applied in medical research and clinical practice for radiotherapy of cancer cell.215 Hence Cu is relatively environmentally friendly, which is also used as a plain conductor in our daily life. Unlike conventional Pb‐based perovskites with 3D structure, Cu‐based perovskites usually form 2D layered structure, due to its smaller ionic radii. Their general formula is (RNH3)2CuX4, where R‐NH3 + is aliphatic or aromatic ammonium cation and X is a halogen.122 Cu2+ with an electronic configuration of 3d9 (t2g 6 eg 3), is more stable in the air than other two divalent Sn2+ and Ge2+ (Figure a). In 2015, Cui et al.122 synthesized two cupric bromide hybrid perovskites, (p–F–C6H5C2H4–NH3)2CuBr4, and (CH3(CH2)3NH3)2CuBr4, with absorption from 300 to 750 nm and studied their photovoltaic performance. This is the first report on Cu‐based PSCs and showed PCEs of 0.51% and PCE of 0.63%, respectively. Both devices exhibited good air stability with less than 5% decrease of the efficiencies after 1 d in the air with humidity of 50% without encapsulation. Later, Cortecchia et al.39 reported Cl‐doped MA2CuBr4 perovskites, and found that the Cl was essential for stabilizing MA2CuClBr4− perovskites against copper reduction and enhancing the perovskite crystallization. By tuning the Br/Cl ratio, the optical absorption can be adjusted and extended to the near‐infrared. Further optimizing the infiltration of mesoporous TiO2 by 2D copper perovskites yielded a PCE of 0.017% using MA2CuCl2Br2 as the light harvester (Figure 22b). Moreover, Li et al.123 claimed the syntheses of a highly stable C6H4NH2CuBr2I compound by equimolar reaction of hydrophobic C6H4NH2I (2‐indoaniline) with low‐toxic CuBr2. The XRD patterns of the C6H4NH2CuBr2I thin film showed almost no change after 4 h of immersion in water, and the printable mesoscopic solar cell based on carbon back‐contact achieved the best PCE of 0.46% (Figure 22c). The low device performance is due to low absorption coefficient (<105 cm−1),39 anisotropic charge transport in low‐dimensional structure and heavy mass of the holes. Moreover, the existence of Cu2+ reduction could introduce higher trap density, which is unfavorable to photovoltaic performance.

Figure 22

a) Crystal structure of MA2CuCl2Br2; b) Absorption coefficient for perovskites of the series MA2CuClBr4− showing strong CT bands below 650 nm and broad d–d transitions between 700 and 900 nm (inset); c) J–V curve of solar cells sensitized with MA2CuCl2Br2 (red) and MA2CuCl0.5Br3.5 (brown) under 1 sun of light illumination. Reproduced with permission.39 Copyright 2016, ACS.

Conclusion and Outlook

We have thoroughly reviewed a series of lead‐free halide hybrid absorbers with various metallic cations including Sn, Ge, Bi, Sb, and Cu etc. in the context of solar cells application. It has been proved that the variation of halide elements in the X position of a typical perovskite material will change the E g significantly and the trend follows the electronic negativity of the halide ions.3 Similarly, the variation of A cations also influences the E g in various ways depending on the type of central metal ion3 (Figure a). In the case of lead halide perovskite, the E g is decreasing with the increase of radii of A cations. However, in the case of Ge halide perovskite, the trend is reversed with CsGeI3 has the smallest E g. For Sb and Bi‐based absorbers, the lowest E g appears when Cs+ is used as the A cation. The E g will increase no matter the radii of A cation are larger or smaller than Cs+. This phenomenon may be due to the change in the dimensionality of the material. In the case of Sn‐based material, MASnI3 has the lowest E g. Among all the metal cations under study, Sn perovskite has the lowest E g 132 while Ge perovskite gives the highest E g.95 The differences in E g reflect directly in the short current density (J SC) of the corresponding devices. As can be seen from Figure 22b, Sn‐based perovskites provide the highest J SC, while materials based on Bi and Sb yield the lowest J SC due to the low dimensionality and wide E g. A comparison of the device performance between different lead‐free absorbers is visualized in Figure 23c. It is clear that there is still a huge gap in PCEs between them and Pb‐based perovskite. Hence, a wise decision should be made while efforts should be put in the most plausible direction.

Figure 23

a) E g versus the type of metal cation; b) J SC versus E g of the absorbers with different metal cation; c) Comparison of the performance of the device between different lead‐free absorbers (data were analyzed based on Table 1).

As the most studied lead‐free perovskites, Sn‐based absorbers with the retaining 3D framework like Pb analogs hold excellent optoelectronic properties, especially the narrow bandgaps and high carrier mobilities. However, the notorious “self‐doping” effect impedes their further development. To suppress “self‐doping” effect, various tin halide additives and organic reducing agents were introduced into the active layers. As usual, the bad morphology of perovskite films is detrimental to the device performance. Thus, strategies containing the use of additives, solvent engineering, vacuum process, vapor‐assisted solution process (VASP), and thermal annealing were adopted to improve the quality of tin perovskites film. Recently, great progress was made in an inverted device architecture. The combination of SnF2 additive and antisolvent treatment with chlorobenzene gave a new record efficiency of 8.12% with good reproducibility from (FA)0.75(MA)0.25SnI3‐based PSCs. More importantly, 80% of PCE retained after 400 h. Among the various types of tin‐based perovskites (different A cations), different cations showed different advantages. First, Sn‐based perovskites with FA cation showed higher efficiency and stability than MA and Cs counterparts. It has been speculated that FA cation can endow the perovskite with higher formation energies of Sn vacancies139 and more resistance against oxidation.133, 138 On the other hand, FA‐based hybrid perovskites have better solubility than all‐inorganic perovskites. According to the summary in Table 1, no spin coating with solvent‐engineering was used in Cs‐based perovskites due to their poor solubility, but in the case of FA counterparts, most good results28, 56, 57 originated from solvent‐engineering resulted in excellent morphology. Second, mixing organic cations at A position seems an effective method to improve devices performance. FA–MA mixed tin perovskite showed V OC up to 0.61 eV, while FA(MA)–BA(PEA) mixed Sn perovskite yielded a 2D structure with improved air stability. Thirdly, Cs‐based perovskites have the best thermal stability up to 200 °C. Due to the high valence Sn4+ cation, Cs2SnI6− Br showed the highest environment stability, the PSCs based on which were processed in the air without using any additives.

Despite that the Sn‐based absorbers attained a promising efficiency of ≈8%, it is still far from the best Pb‐based perovskites. “Self‐doping” effect will still be a challenge to all the researchers working on Sn‐based perovskite. A deep insight into the mechanisms of “self‐doping” effect is crucial for achieving efficiency up to 15% or higher. A more suitable procedure only employs “intermediate agent” that could be removed in the final stage or even without using any sacrificial additives. Successful compositional engineering could help to give efficient and stable compounds similar to Pb‐based perovskites. Additionally, high‐performance and dopant‐free HTMs could also assist in achieving more efficient and stable Sn‐PSCs.

For the absorbers based on metals beyond the group 14, owing to the low dimensionality and wide band gaps, the photovoltaic performance of Bi, Sb and Cu‐based devices are still unsatisfactory with efficiency ≈2%. More efforts are needed at two possible directions to open up the avenues toward high‐performance Bi‐based absorbers. The first one is new double perovskites, which may involve a lot of theoretical calculation and the corresponding experimental study. So far, researchers have used Cs2AgBiBr6 to make device with PCE up to 2.43%. The other one is the ferroelectric perovskites represented by Bi‐based compounds. As far back as 1956, photovoltaic (PV) effect has been found in oxide perovskite BaTiO3 which has no lead elements.216 Afterward, more lead‐free oxide perovskites were studied on PV effect, such as BiFeO3,217, 218 BiMnO3,219 [KNbO3]1− [BaNi1/2Nb1/2O3− ] 220 and Bi2FeCrO6,221 etc. So far, the highest PCE among all oxide perovskites was obtained as 8.1% by using double‐perovskite Bi2FeCrO6.221 The ferroelectric oxide perovskites often showed exceptionally high photovoltages, which are normally much larger than their bandgaps.

For the transition metal Cu‐based absorbers, like 2D lead perovskites, their wide compositional tunability, and increased environmental stability are their intrinsic advantages. For example, one can introduce optoelectronically active organic cations to increase optical absorption cross‐section and improve vertical charge transport. Another approach is to make multidimensional (MD) perovskites by mixing the 2D and 3D materials.

Among all the above‐mentioned lead‐free absorbers, Sn‐based perovskites possess the most efficient PCE up to 8.12% while other absorbers showed only below 2.1%, although Bi‐based absorbers showed the highest air stability. Moreover, theoretical calculation222, 223 indicated that efficiencies above 15% could be obtained from MASnI3 PSCs. Thus, we argue that the Sn‐based absorbers are the most promising surrogate for Pb in PSCs. However, we have bear in mind that Sn element is still harmful to the human body in their practical utilization,224, 225 while Bi, Sb, and Cu are more environmentally friendly. Finally, we should think about our initial question: can we get the clean power output from the new hybrid absorbers without the danger of environmental contamination?226 The answer may need to be found in the future development of new lead‐free hybrid light harvesting materials.

During the revision of our manuscript, we noticed that there are two important Sn‐based and one bismuth‐based absorbers reported recently. Firstly, ethylenediammonium(en) cations were incorporated into FASnI3,62 MASnI3 and CsSnI3,55 and then form so‐called “hollow” {en}ASnI3 perovskite. For the sake of incorparation of appropriate amount of en cations, the 3D structure of perovskite is retained while perovskite film morphology is significantly improved. Finally, the best‐performing solar cells display a high efficiency of 6.63% of {en}MASnI3 and 7.14% of {en}ASnI3, respectively. These results are presented in table 1. Furthermore, Zhang et al. report a novel two‐step vacuum deposition procedure to get homogeneous transformation of BiI3 to MA3Bi2I9 for highly compact, pinhole‐free, large‐grained films. The solar cells realized a record PCE of 1.64% and also a high EQE approaching 60%.227

Conflict of Interest

The authors declare no conflict of interest.