Evaluation of the Thermoelectric Properties and Thermal Conductivity of CH3NH3PbI3-x Cl x Thin Films Prepared by Chemical Routes.

The thermoelectric properties and thermal conductivity of mixed-phase CH3NH3PbI3-x Cl x thin films have been reported as a function of temperature, ranging from room temperature (RT) to 388 K. Thermoelectric study confirms that CH3NH3PbI3-x Cl x is a p-type material and the charge carrier transport in CH3NH3PbI3-x Cl x is governed by polarons and the thermal scattering process. The Peltier function and power factor are found to decrease initially up to ∼325 K, after which they increase with increasing temperature. The position of (E F - E V) of all samples drops down sharply to zero level around 325 K. The avalanches of thermoelectric properties at ∼325 K indicate the existence of tetragonal-cubic phase transition in CH3NH3PbI3-x Cl x . The calculated thermal conductivity is very low, as desired for thermoelectric materials, due to strong anharmonic interactions. Both the figure of merit (ZT) and device efficiency increase with increasing temperature. However, ZT remains small with temperature. Despite the limitations on the operating temperature range due to phase complexity and small ZT, CH3NH3PbI3-x Cl x exhibits reasonable thermoelectric power and low thermal conductivity. This signifies the possibility of CH3NH3PbI3-x Cl x as a prospective thermoelectric material.

Introduction

The interesting feature of thermoelectric materials is the capability of converting heat energy into electrical energy directly, and the conversion process is known as the thermoelectric effect. The process of generating electric potential from a low temperature gradient is used in power generation, whereas a reverse process is used in heating or cooling applications. These thermal applications are considered as a promising solution for the generation of green or renewable energy. Therefore, thermoelectric materials have created a renewed interest in the scientific community in searching for an effective low-cost thermoelectric material for converting heat into electrical energy.

In recent years, organic–inorganic halide perovskite thin films have been attracting noteworthy attention of the scientific community. The typical perovskite structures are symmetric and can be represented by a chemical formula AMZ3, where “A” and “M” represent cations and “Z” is an anion that bonds to both “A” and “M”. Site “A” contains both organic and inorganic cations. The “M” cation is typically a divalent transition-metal ion such as Pb2+, Sn2+, or Ge2+, and the “Z” anions are negatively charged halides (Cl–, Br–, I–).[1−4] The organic–inorganic lead trihalide perovskites (CH3NH3PbI3) can be created when an inorganic “A” cation is replaced by small organic monovalent methylammonium (CH3NH3+) cations and accommodated with an inorganic metal ion (Pb) and iodine (I). From the application point of view, the choice of atoms or molecules in the organic–inorganic perovskite structure may exhibit an impressive array of thermoelectric, photovoltaic, photoluminescence, and optoelectronic properties, harmonic generation, and so on. In particular, when the charged element of halogens, such as chlorine (Cl), is substituted partially in the “Z” site, it forms AMZ3–Cl, which is known as an organic–inorganic mixed halide perovskite, for example, CH3NH3PbI3–Cl. Both CH3NH3PbI3 and CH3NH3PbI3–Cl thin films exhibit the same crystal structure at room temperature (RT). They usually show some similar characteristics, but the inclusion of Cl improves the photovoltaic and optoelectronic properties[5,6] of CH3NH3PbI3–Cl. Recently, the efficiency of solar cells made from CH3NH3PbI3–Cl has been improved from 3.8% (in 2009) to 25.5% and 29.1% for single-junction architecture and silicon-based tandem cells, respectively.[7,8] Nevertheless, low-cost, solution-processed CH3NH3PbI3–Cl thin films showed distinctive qualities as optoelectronic materials, such as multicolor excitonic emissions,[9,10] second-harmonic generation,[11] high absorption coefficient,[9,10] band gap tuning in the visible spectrum,[10] electron–hole diffusion lengths exceeding 1 μm,[5] high photoluminescence quantum yield,[12] etc. All of these properties make CH3NH3PbI3–Cl superior for the broader arena of photovoltaic, optoelectronic, and nanotechnology applications.

Furthermore, a few research groups have also suggested the possibility of using lead halide perovskite as a promising material in the area of thermoelectric generation beyond photovoltaic and optoelectronic applications. A good thermoelectric material should possess high thermopower, low thermal conductivity, and reasonably good electrical conduction. Pisoni et al.[13] was the first to report the ultralow thermal conductivity of CH3NH3PbI3. After this pioneering work, some significant theoretical and experimental investigations[14−23] have been carried out to understand the role of different organic–inorganic sites, thermal conductivity, and the thermal transport mechanism of single and polycrystalline CH3NH3PbI3. However, the thermoelectric properties of mixed halide perovskite of CH3NH3PbI3–Cl are yet to be reported. Since the presence of Cl can influence the physical properties of CH3NH3PbI3, it is an important task to explore the thermoelectric behavior of CH3NH3PbI3–Cl thin films for future thermal energy conversion research. It is noteworthy to say that the carrier dynamics of mixed halide perovskite is highly sensitive to temperature.[24] The physical properties of mixed halide perovskite suffer due to the orthorhombic–tetragonal–cubic phase complexity at 160 K and between 315 and 344 K, respectively.[24−26] Therefore, it is also necessary to explore the thermoelectric transport properties of mixed halide perovskite above room temperature.

Considering the above facts, we concentrate on understanding the temperature-dependent (RT-388 K) thermoelectric properties such as thermoelectric power and Peltier function of solution-processed nanocrystalline CH3NH3PbI3–Cl thin films prepared by the chemical dip-coating (CDC), spray pyrolysis (spray), and repeated dipping–withdrawing (dipping) methods. Furthermore, the effect of polarons on thermoelectric charge carrier transport, scattering factor, thermal activation energy, Fermi energy, and power factor has also been determined from thermoelectric measurements. For the evaluation of figure of merit (ZT) and efficiency (ξ) of thermoelectric power generation, the temperature-dependent thermal conductivity has been calculated. The lattice thermal conductivity has also been estimated theoretically by the Debye model using the Callaway method,[27] whereas, the electronic thermal conductivity was calculated from the Wiedemann–Franz law. The estimated thermal conductivity of CH3NH3PbI3–Cl was dominated by lattice and found to be remarkably low with a reasonably high thermopower, making CH3NH3PbI3–Cl a desirable thermoelectric material. However, small ZT limits thermoelectric applications. By improving the electrical conductivity of the mixed halide perovskite, a desirable ZT could be achieved for thermoelectric applications. Despite the limitations on the operating temperature range due to phase complexity and small ZT, we have presented a systematic study on the thermoelectric transport properties of CH3NH3PbI3–Cl thin films. We believe this study will offer a route for future thermoelectric-based perovskite research.

Results and Discussion

Structural Analysis

The crystal structure, lattice constant, crystallite size, dislocation density, strain, and unit cell volume of CDC-, spray-, and dipping-deposited CH3NH3PbI3–Cl films were determined by X-ray (XRD) analysis. Figure shows the XRD pattern of CH3NH3PbI3–Cl, which confirms the formation of the tetragonal CH3NH3PbI3–Cl phase for the CDC- and spray-deposited samples, whereas a combination of tetragonal CH3NH3PbI3–Cl and cubic CH3NH3PbCl3 phases was found for the dipping-deposited film. However, a monoclinic dihydrate (CH3NH3)4PbI6·2H2O phase at 2θ = 11.66° was found in the CDC-deposited sample, which confirms the formation of an isolated [PbI6]4– phase. The PbI2 phase is formed due to the decomposition of CH3NH3PbI3–Cl in air. The details of the crystal structure and phase formation can be found in earlier studies.[9−11]

Figure 1

XRD patterns showing different phases of CDC-, spray-, and dipping-deposited CH3NH3PbI3–Cl thin films.

The tetragonal lattice parameters “a” and “c” were calculated using a major diffraction peak and are tabulated in Table . The unit cell volume for tetragonal and cubic structures was calculated using the equations V = (√3/2)a2c and V = a3, respectively. The Debye–Scherrer crystallite size[28] ξ = (0.94λ/β cos θ), where β is the full width at half-maximum (FWHM) calculated for the major plane. The dislocation density (δ), defined as the length of dislocation lines per unit volume, has been calculated using[29] δ = 1/ξ2. The corresponding elastic strain (ϵ) and microstrain (ε) were calculated using equations[30,31] ϵ = (β tan θ/2) and ε = (β/4 tan θ), respectively. The different crystallographic parameters, viz., crystallite size, dislocation density, strain, and unit cell volume, are presented in Table . From Table , it is clearly seen that the crystallite size increases with the decrease of microstrain and dislocation density for all samples.

Table 1

Lattice Constant, Crystallite Size, Dislocation Density, Strain, and Unit Cell Volume of CH3NH3PbI3–Cl Thin Films for the Most Oriented Peaka

		lattice constant				strain (%)
sample	position 2θ (deg)	a (nm)	c (nm)	crystallite size ξ (nm)	dislocation density (lines/m2)	elastic × 10–4	micro × 10–2	volume V (nm)3
CDC	14.20	0.8812	1.278	24.25	1.69	3.58	1.15	0.859
spray	14.11	0.8868	1.260	22.23	2.02	3.88	1.26	0.853
dipping	14.08	0.8887	1.260	25.81	1.50	3.34	1.09	0.862
	15.52	0.5704	0.5704	24.29	1.69	3.92	1.05	0.185

Only for the dipping-deposited sample, all parameters have been calculated for the two most oriented peaks due to the coexistence of cubic and tetragonal phases.

The atomic % of chlorine (Cl) in CDC-, spray-, and dipping-deposited samples were 0.66, 0.69, and 1.24, respectively. The details of surface morphology and elemental analyses of CH3NH3PbI3–Cl thin films were reported in our previous publications.[9−11]

Thermoelectric Study

The thermoelectric power measurement of CDC-, spray-, and dipping-deposited CH3NH3PbI3–Cl thin films was carried out from 303 K to 388 K. The thermal e.m.f was found to be positive, which indicates that CH3NH3PbI3–Cl is a p-type semiconductor. Figure a shows that the e.m.f decreases up to a certain temperature (Tc) of 325 K, after which it increases up to the measured temperature range. Both theoretical and experimental studies suggest that the p-type nature of lead halide perovskite is mainly due to the presence of Pb and CH3NH3.[32,33] The p-type conductivity can be understood as follows: since the precursor solution was prepared by dissolving CH3NH3I and PbCl2 at a molar ratio of 3:1, the prepared CH3NH3PbI3–Cl thin films may comprise Pb, CH3NH3, I, and Cl vacancies. However, the lower formation energy of Pb and CH3NH3 vacancies compared to halogen vacancies may play a major role in p-type conductivity. The thermoelectric power or Seebeck coefficient for p-type semiconductor can be expressed asorwhere, EF – EV = E0 – γT, KB is the Boltzmann constant, γ is the coefficient of activation energy, E0 is the thermal activation energy, EF is the position of the Fermi energy, EV is the top of the valence band energy, and A is a constant that depends on the scattering processes involved in the system. The scattering factor A has been determined from the intercept on the y-axis of thermoelectric power vs inverse temperature (Figure b) at 1/T ≈ 0 using eq . The scattering index r has been calculated from the equation[34]r = 5/2 – A. The value of r is −0.5 for piezoelectric scattering, −1.5 for ionized impurity scattering, −1 for grain boundary scattering, and 1.5 for polar scattering of the optical phonons. In this study, r was found to be ≥2.5 in the lower-temperature region (303–325 K) and ∼−1.5 in the temperature range (326–388) K. These values are approximately the same for all samples (Table ). The calculated value of r at the lower-temperature region does not meet the criteria of the conventional scattering factor stated above.[34] Chen et al.[35] proposed a model that a charge carrier moving through an organic–inorganic mixed halide perovskite lattice may itself induce a local orientational rearrangement of the surrounding methylammonium dipoles. These dipoles tend to align to the direction of the electric field and produce a free dipolar polaron.[36−38] Such polarons in perovskites are termed as intermediate-coupling polarons.[37] Therefore, it is anticipated that polarons along with longitudinal optical phonon scattering govern the transport process in the low-temperature region. The value of r is around −1.5 in the (326–388) K region, which is a signature of ionized impurity scattering. Moreover, multiple scattering processes such as localized level hopping of charge carriers near the Fermi level and vacancy-assisted diffusion may be responsible for the carrier transport in the higher-temperature region.

Figure 2

Variation of (a) thermal e.m.f. with temperature, (b) thermoelectric power with inverse temperature, (c) Peltier function with temperature, and (d) fractional concentration of polarons with temperature of CH3NH3PbI3–Cl thin films.

Table 2

Scattering Factor (A), Scattering Index (r), Coefficient of Thermal Activation Energy (γ), and Thermal Activation Energy (E0) for CDC-, Spray-, and Dipping-Deposited CH3NH3PbI3–Cl Thin Films

	scattering factor A		scattering index r		γ × 10–4 (eV/K)		E0 (eV)
samples	(303–325) K	(326–388) K	(303–325) K	(326–388) K	(303–325) K	(326–388) K	(303–325) K	(326–388) K
CDC	–1.26	3.74	3.76	–1.24	–40.2	–7.16	1.40	–0.06
spray	–0.89	4.12	3.39	–1.62	–50.0	–8.89	1.75	–0.07
dipping	–0.18	4.33	2.68	–1.83	–24.3	–10.1	0.81	–0.19

The variation of thermoelectric power (Seebeck coefficient) with inverse temperature and the variation of Peltier function with temperature are shown in Figure b,c, respectively. The Peltier coefficient Π of all samples is calculated using the following equationThe value of γ is determined from the slope of Π vs T graph, and E0 is calculated from the intercept on y-axis at T ≈ 0. The values of γ and E0 are tabulated in Table .

Figure b,c shows that thermoelectric power and Peltier function initially decrease with increasing temperature up to ∼325 K due to the enhanced relative strength of polaronic effects. After this initial decrease, both thermoelectric power and Peltier function increase linearly with increasing temperature. The multiple scattering processes at the organic–inorganic interface develop a high-energy charge carrier transport and also increase the density of states with the increase of temperature, which enhanced the thermoelectric power and Peltier function of all samples.

From the thermoelectric power measurement, it is also observed that both thermoelectric power and Peltier function of the CDC- and spray-deposited samples are higher than those of the dipping-deposited sample. The presence of dual structure, confirmed by XRD, may suppress thermoelectric power in the dipping-deposited sample.

Polarons are fermionic quasi-particles that originate from the Coulomb interaction between an electric charge and its surrounding phonon cloud. The effect of polarons on thermoelectric charge carrier transportation proposed by Heikes and modified by Chaikin and Beniis is expressed by the following equation:[39,40]where pfc is the fractional concentration of a polaron and S is the thermoelectric power or Seebeck coefficient. The temperature dependence of the polaron fraction is shown in Figure d. It was observed that pfc initially increases in the temperature region (303–325) K and then decreases for the dipping-deposited sample but remains almost constant for the CDC- and spray-deposited samples. The molecular rotation of the organic parts in the perovskites strongly depends on temperature.[41,42] As the temperature increases, the polaron becomes deeply bound, which increases its effective mass up to a certain temperature (Tc = 325 K). The radius of the polaron starts to decrease after Tc due to charge carrier–phonon scattering, subsequently reducing the strength of the polaron.

The behavior of a thermoelectric material strongly depends on the position of (EF – EV), which is a key factor for p-type perovskites. Therefore, the variation of (EF – EV) with temperature is shown in Figure , from which it is clear that the value of (EF – EV) is much higher than KBT (0.026 eV). The position of (EF – EV) is found to drop down sharply to the vicinity of zero energy level at 325 K. Therefore, the alteration of thermoelectric behavior and the change of position of (EF – EV) at ∼325 K indicate the occurrence of a tetragonal-to-cubic phase transition in CH3NH3PbI3–Cl. A similar phase transition from tetragonal to cubic in the temperature range of (315–344) K was reported due to the combination of twisted inorganic PbI6 octahedral and an imbalance of organic CH3NH3+ rotation.[24]

Figure 3

Variation of (EF – EV) with temperature for CDC-, spray-, and dipping-deposited CH3NH3PbI3–Cl thin films.

The power factor (PF) of a sample is defined as the ratio of the square of thermoelectric power and resistivity of the material. The temperature dependence of the power factor for CH3NH3PbI3–Cl films is shown in Figure . It is found that the power factor of all samples decreases up to Tc because of enhanced polaronic effects and higher resistivity (Figure S1). After the initial decrease, the PF of all samples starts to increase remarkably after Tc with increasing temperature because of higher thermopower and reduced resistivity in this temperature range.

Figure 4

Temperature-dependent power factor of CDC-, spray-, and dipping-deposited CH3NH3PbI3–Cl thin films.

Thermal Conductivity of CH3NH3PbI3–Cl Thin Films

Thermal conductivity is an important thermophysical property of a material that measures the ability to conduct heat (energy transportation) due to a random molecular motion originated from temperature differences. In general, thermal conductivity is the combination of lattice (κlat) and electronic (κele) thermal conductivities of a material. The total thermal conductivity was evaluated by calculating the electronic thermal conductivity using the Wiedemann–Franz law and by estimating lattice thermal conductivity theoretically by the Debye model (see the Supporting Information), which is correlated with the phonon scattering rate governed by grain boundary, point defects, and Umklapp scattering sources. In this work, the crystallite size (ξ) and unit cell volume (V) of the prepared samples were used for estimating lattice thermal conductivity, whereas Debye temperature, Grüneisen parameter, point defect parameters, velocity of sound, and phonon frequency were taken from the literature for CH3NH3PbI3–Cl. The parameters used for calculating temperature-dependent thermal conductivity are tabulated in Table .

Table 3

Parameters Used for Calculating Temperature-Dependent Thermal Conductivity

samples	Debye temperature θD (K) [ref (16)]	Grüneisen parameter Γ [ref (16)]	fitted parameter Bpd× 10–4 [ref (16)]	velocity of sound υ (ms–1) [ref[20]]	phonon frequency ω (THz) [ref (43)]
CDC	120	3.6	1.6	1390	3.99
spray	120	3.6	1.6	1390	3.99
dipping	120	3.6	1.6	2194	6.75

The temperature dependence of total (κ), lattice (κlat), and electronic (κele) thermal conductivity for CDC-, spray-, and dipping-deposited samples is shown in Figure a,b, which clearly shows that the total thermal conductivity of all perovskite samples is overwhelmingly dominated (99.98%) by lattice thermal conductivity, whereas the contribution of electronic thermal conductivity to the total is insignificant (0.02%). The total thermal conductivities of CDC-, spray-, and dipping-deposited samples at 303 K are estimated to be 0.250, 0.249, and 0.374 Wm–1 K–1, respectively, which are consistent with the literature.[16,17,19] It is also noticed that the thermal conductivity of the dipping-deposited sample is higher than those of the CDC- and spray-deposited samples. From the EDS study, it was confirmed that the amount of Cl present in the CDC- and spray-deposited samples is lower than that in the dipping-deposited sample. The lower concentration of Cl offers low elastic stiffness[44] to CH3NH3PbI3–Cl thin films, causing lower thermal conductivities of the CDC- and spray-deposited samples. The low sound velocity of CH3NH3PbI3–Cl may also be responsible for the low thermal conductivity.[44,45] The variation of thermal conductivity with temperature may be caused by the strong anharmonic interaction of metastable states produced by the rotation, bending, or stretching of the organic part of the lattice.[46−48] The thermal conductivity of all samples shows T–1 variation rather than linear variation (Figure a), which indicates the occurrence of phonon scattering by the Umklapp process.[49]

Figure 5

Temperature-dependent (a) total, (b) lattice (left), and electronic (right) thermal conductivities of CDC-, spray-, and dipping-deposited CH3NH3PbI3–Cl thin films.

Figure of Merit and Device Efficiency of CH3NH3PbI3–Cl Thin Films

The performance of a thermoelectric material can be evaluated by the dimensionless figure of merit defined as ZT = (S2T/ρκ), where ρ is the resistivity. The temperature-dependent variation of the figure of merit for all perovskite samples is shown in Figure a, in which the ZT values primarily decrease before 325 K and then begin to increase slowly up to ∼360 K, followed by a sharp increase with increasing temperature. However, the calculated ZT is found to be low due to the low electrical conductivity. The maximum value of ZT ∼0.073 is estimated for the spray-deposited sample at 388 K. Low ZT values for inorganic or organic mixed halide perovskite are also reported, and it has been suggested that doping can maximize the ZT value.[14,50−52]

Figure 6

Temperature-dependent (a) ZT and (b) maximum efficiency of a thermoelectric generation device of CH3NH3PbI3–Cl thin films.

Using ZT values, the maximum efficiency ξ of a device of thermoelectric generation can be calculated using the equation , where Th and Tc are the temperatures of hot and cold junctions, respectively, and (ΔT/Th) and ΔT are the Carnot factor and temperature difference between the junctions, respectively. The variation of thermoelectric efficiency with temperature is shown in Figure b, which shows that thermoelectric efficiency increases with increasing temperature, tending to saturation at very high temperatures. The ZT and device efficiency of the CDC- and spray-deposited samples are found to be slightly higher due to higher thermopower and lower resistivity compared to the dipping-deposited sample.

Conclusions

Temperature-dependent (RT-388 K) thermoelectric properties, thermal conductivity, and thermoelectric device efficiency of CH3NH3PbI3–Cl thin films have been presented in this work. Thermoelectric study reveals that CH3NH3PbI3–Cl is a p-type semiconductor in which temperature-dependent multiple scattering is involved in the carrier transport process. The thermoelectric power as well as the Peltier functions and power factor of all samples initially decrease up to 325 K due to the enhanced relative strength of polaronic effects. The position of (EF – EV) is found to shift sharply to the vicinity of the zero energy level at 325 K, which indicates the occurrence of a tetragonal–cubic phase transition in CH3NH3PbI3–Cl. The low thermal conductivity of CH3NH3PbI3–Cl is due to a strong anharmonic interaction of metastable states produced by the rotation, bending, or stretching of the organic part of the lattice. The dimensionless ZT value and device efficiency are found to increase with increasing temperature. Overall, CH3NH3PbI3–Cl experiences a small ZT and phase separation at 325 K; however, it exhibits reasonable thermoelectric power and substantially low thermal conductivity, which justify that CH3NH3PbI3–Cl is a thermoelectric material.

Experimental Section

The solution-processed CH3NH3PbI3–Cl thin films were fabricated in ambient air on a glass substrate by three different cost-effective methods: chemical dip-coating (CDC), spray pyrolysis (spray), and repeated dipping–withdrawing (dipping). Details of the preparation of perovskite solutions and fabrication of CH3NH3PbI3–Cl thin films by the CDC, spray, and dipping methods have been described in our previous works.[9−11] A schematic of thermoelectric power measurement by an integral method[53] for CH3NH3PbI3–Cl is shown in the Supporting Information (Figure S2). For measuring thermoelectric power, one end of the specimen was kept in an ice bath (273 K) and the other end was heated from 303 K to 388 K. The temperature was varied by varying the heater voltage and monitored using a chromel–alumel thermocouple connected with a digital multimeter (KTI, KT 105). The thermal e.m.f. was measured using another digital multimeter. The lattice thermal conductivity (κlat) was investigated by a theoretical approach according to the Debye model using the Callaway method and is expressed aswhere and , θD is the Debye temperature, ωD is the Debye frequency, υ denotes the speed of sound, and τ is the phonon relaxation time. The details of the Debye–Callaway model are given in the Supporting Information.