Perovskite/CIGS Spectral Splitting Double Junction Solar Cell with 28% Power Conversion Efficiency.

The highest theoretical efficiency of double junction solar cells is predicted for architectures with the bottom cell bandgap (E g ) of approximately 0.9-1.0 eV, which is lower than that of a typical Si cell (1.1 eV). Cu(In,Ga)(Se,S)2 (CIGS) solar cells exhibit a tunable E g depending on their elemental composition and depth profile. In this study, various CIGS solar cells with E g ranging from 1.02 to 1.14 eV are prepared and a spectrum splitting system is used to experimentally demonstrate the effect of using lower-E g cells as the bottom cell of two-junction solar cells. The four-terminal tandem cell configuration fabricated using a mixed-halide perovskite top cell (E g  = 1.59 eV; stand-alone efficiency = 21.0%) and CIGS bottom cell (E g  = 1.02 eV; stand-alone efficiency = 21.5%) with a 775-nm spectral splitting mirror exhibits an efficiency of 28.0% at the aperture area of 1 cm2.

Introduction

Improving the power conversion efficiency (PCE) of solar cells is essential for reducing the solar power cost (Peters et al., 2019). Solar cells with single-junction structures have reached the theoretical limit of PCE, and the possibility of further improvement of PCE is significantly low (Shockley and Queisser, 1961; Green et al., 2020). One of the approaches for overcoming this limitation is the introduction of tandem structures, where two or more solar cells with different bandgaps (E) are stacked while optimizing the wavelength absorption range. In this method, depending on the number of cells used, the theoretical PCE limit can be exceeded up to 50% (Meillaud et al., 2006; Alharbi and Kais, 2015). In fact, a PCE of approximately 40% was reported for III-V multi-junction cells (Green et al., 2020). However, the III-V semiconductor-based tandem devices are expensive, which limits their current applications to only a few markets, such as the space industry (Bosi and Pelosi, 2007).

For attaining a high PCE without an excessive cost, double junction devices that are constructed with perovskite (PVK) solar cells as top cells and Si (Chen et al., 2020; Uzu et al., 2015; Duong et al., 2020; Xu et al., 2020; Jaysankar et al., 2019b; Wang et al., 2020), Cu(In,Ga)(Se,S)2 (CIGS) (Kim et al., 2019; Gharibzadeh et al., 2020; Shen et al., 2018; Jiang et al., 2020; Jaysankar et al., 2019a; Al-Ashouri et al., 2019; Han et al., 2018), PVK (Tong et al., 2019; Lin et al., 2019; Palmstrom et al., 2019; Zhao et al., 2018; Yao et al., 2020; Abdollahi Nejand et al., 2020; Yang et al., 2019), and dye-sensitized (DS) (Kinoshita et al., 2015; Hosseinnezhad 2019) solar cells as bottom cells have garnered considerable attention recently. Although a higher PCE has been reported for the PVK/Si combination, the optimal E of the bottom cell in the double junction devices ranges from 0.9 to 1.0 eV (see Figure S1), which is lower than the typical E of crystalline Si solar cells (1.1 eV). However, depending on their elemental compositions and depth profiles, CIGS solar cells possess a tunable E, which can be as low as 1.0 eV (Nakamura et al., 2019), close to the optimal value of the bottom cell for both two-terminal (2-T) and four-terminal (4-T) tandem solar cells. In this study, we experimentally show that the PCE of the double junction solar cells is higher when the bottom cell E is closer to 1.0 eV, as predicted by theoretical studies. Subsequently, we conclude that CIGS solar cells are potentially the most superior among the choices as the bottom cell of Perovskite-based double junction solar cells.

Results and Discussion

The tandem solar cell with a spectral splitting system was constructed using a PVK top cell, CIGS bottom cell, and a dichroic mirror that was fixed at an angle of 45°, as schematically shown in Figure 1.

Figure 1

Schematics of Fabricated Tandem Solar Cells

(A) PVK top cell.

(B) CIGS bottom cell.

(C) Spectrum splitting system with a dichroic mirror.

The layered structure of the fabricated 0.995 cm2-aperture sized PVK cell was Fluorine-doped tin oxide (FTO)/c-TiO2/m-TiO2/PVK/Spiro-OMeTAD/Au, where the composition of PVK was selected as [K0.05(FA0.85MA0.15)0.95]Pb (I0.85Br0.15)3. The fabrication using mixed halide and alkali metal-doped mixed cation PVK has several advantages, the details of which are described elsewhere (Tang et al., 2017; Lu et al., 2020). However, the main reasons of using the PVK include an increase in the efficiency and reduction of the hysteresis in the current density-voltage (J-V) characteristics. The external quantum efficiency (EQE) spectrum and J-V curve of the fabricated PVK cell measured under the standard 1-sun condition (i.e., without using the dichroic mirror) are shown in Figures 2A and 2B, respectively. The Eg of the PVK cell was determined to be 1.59 eV using the peak position of the first order derivative of the EQE curve (-d(EQE)/dλ) (Krückemeier et al., 2020), which is an ideal value for the top cell of a two-junction tandem solar cell (see Figure S1). The efficiency of the fabricated 1 cm2-aperture sized PVK cell measured under 1,000 W/m2 in the reverse J-V scan was 21.0% with Jsc, Voc, and FF of 22.7 mA cm−2, 1,181.8 mV, and 78.1%, respectively. The efficiency of the cell in the forward J-V scan was measured to be 20.3% with Jsc, Voc, and FF of 22.6 mA cm−2, 1,173.8 mV, and 76.5%, respectively.

Figure 2

Device Characteristics of the PVK and CIGS Cells Measured Under the Standard 1-sun Irradiation Condition without Using a Dichroic Mirror

(A) Normalized EQE and -d(EQE)/dλ curve of PVK cell.

(B) Forward and reverse scanned J-V curves of PVK cell.

(C) EQE and -d(EQE)/dλ curves of CIGS cells.

(D) Forward scanned J-V curves of CIGS cells.

CIGS bottom cells with an aperture area of 1.04 cm2 were fabricated using the sulfurization after selenization (SAS) process, where a Copper-Gallium/Indium metal precursor on a glass/Molybdenum substrate was selenized with H2Se gas followed by sulfurization with H2S gas in a closed furnace. More than 100 CIGS cells with various Eg values were prepared by adjusting the elemental depth profile of the CIGS absorber layer via tuning some of the parameters in the SAS process, as described in a previous study (Nakamura et al., 2019). Among these cells, four CIGS cells with approximately the same efficiencies (21.5 ± 0.3%) and FFs (75 ± 2%) but with different Eg values ranging from 1.02 to 1.14 eV were used for constructing the tandem structure. The normalized EQE spectra and J-V curves of these four CIGS cells are shown in Figures 2C and 2D, respectively. Unlike PVK cells, CIGS cells generally showed negligible hysteresis; therefore, we displayed the J-V curves in the forward direction only. The device parameters are summarized in Table 1. The elemental depth profiles measured by glow discharge optical emission spectroscopy and calculated Eg depth profiles using an empirical equation are shown in Figure S2 (Bär et al., 2004).

Table 1

Parameters of the PVK and CIGS Solar Cells Extracted from the J-V Curves and EQE Spectra in Figure 2

	PCE (%)	Jsc (mA cm−2)	Voc (mV)	FF (%)
PVK-Rvs.	21.0	22.7	1,181.8	78.1
PVK-Frd.	20.3	22.6	1,173.8	76.5
CIGS (Eg = 1.14 eV)	21.6	39.1	728.8	75.7
CIGS (Eg = 1.11 eV)	21.8	39.8	715.1	76.5
CIGS (Eg = 1.08 eV)	21.5	41.0	708.9	74.1
CIGS (Eg = 1.02 eV)	21.5	43.6	659.0	74.8

The J-V curves and EQE spectra of the PVK top cell and the CIGS bottom cells measured using the dichroic mirror with splitting wavelength of 775 nm are shown in Figure 3, and the device parameters are summarized in Table 2. The splitting edge wavelength of the dichroic mirror was selected because 775 nm (or 1.60 eV) approximately matches the Eg of the PVK top cell (1.59 eV). The PCE of the PVK cell reduced from 21.0% to 19.1% because of the reduced Js and FF when the mirror was used. This reduction is reasonable considering that there are parasitic optical losses in the mirror as well as the slight mismatch between the splitting wavelengths and the Eg of the PVK cell. Nevertheless, retaining greater than 90% of the original PCE implies that the spectrum splitting system is well designed with the appropriate choice of wavelength. The PCE of all CIGS bottom cells with different Eg were almost the same when measured without the mirror, as shown in Table 1. Moreover, cells with a lower Eg showed a higher efficiency when measured with the mirror. Finally, the PCE difference of the cells with Eg of 1.02 and 1.14 eV reaches as high as 1.4%. This experimental value is in good agreement with the theoretical results, obtained from the detailed balanced calculations shown in Figure 4 (De Vos, 1980). The same trend was observed when the CIGS bottom cells were measured under dichroic mirrors with other splitting wavelengths (see Figure S3). To the best of our knowledge, this is the first report on the experimental procedure and evaluation of the advantage of using low-Eg CIGS as the bottom cell of a tandem device with PVK solar cells.

Figure 3

Device Characteristics of the PVK Top Cell and CIGS Bottom Cells Measured in Presence of the Dichroic Mirror with a Splitting Wavelength of 775 nm

(A) Forward and reverse scanned J-V curves of PVK top cell.

(B) J-V curves of CIGS cells with various bandgaps.

(C) EQE of the PVK top cell and CIGS bottom cell with Eg of 1.02 eV.

Table 2

Solar Cell Parameters of the PVK Top Cell and CIGS Bottom Cells Extracted from the J-V Curves and EQE Spectra in Figure 3. The highest 4-T tandem cell efficiency was obtained when the CIGS bottom cell with Eg of 1.02 eV was used.

	PCE (%)	Jsc (mA cm−2)	Voc (mV)	FF (%)
PVK-775 nm-Rev.	19.1	21.0	1,182.0	77.2
PVK-775 nm-Frd.	18.3	20.9	1,172.8	74.6
CIGS (Eg = 1.14 eV)	7.5	14.5	690.9	74.9
CIGS (Eg = 1.11 eV)	7.9	15.3	682.7	75.3
CIGS (Eg = 1.08 eV)	8.6	16.3	678.9	77.5
CIGS (Eg = 1.02 eV)	8.9	18.4	637.0	75.8
4-T tandem (best)	28.0	–	–	–

Figure 4

Relation between the Solar Cell Parameters and Bottom Cell Eg

(A) Experimental results for the CIGS bottom cells measured with the dichroic mirror in the spectrum splitting system.

(B) A detailed balanced calculation of the device performance of the bottom cell when the top cell Eg is fixed at 1.6 eV.

The overall PCE of the 4-T tandem cell with the spectral splitting system is 28%, comprising the PVK top cell with 19.1% PCE and the lowest-Eg CIGS bottom cell with 8.9% PCE. This is one of the highest reported PCE for the experimentally fabricated PVK-based tandem solar cells (see Table S1).

Conclusion

In this study, we experimentally demonstrated that the PCE of a double junction solar cell is higher when the Eg of the bottom cell is closer to 1.0 eV, as predicted theoretically. The device PCE was proven to be increased by an absolute 1.4% when the bottom cell Eg reduced from 1.14 to 1.02 eV. This result indicated the potential superiority of PVK/CIGS tandem solar cells over PVK/Si cells because a crystalline Si cell has a fixed Eg of approximately 1.1 eV. A high PCE of 28% was achieved in a PVK/CIGS 4-Terminal double junction structure solar cell comprising a PVK top cell and a CIGS bottom cell with Eg of 1.59 and 1.02 eV, respectively, combined in a spectrum splitting system equipped with a dichroic mirror with splitting wavelengths of 775 nm. As PVK and CIGS are both thin films, tandem devices using these materials can potentially exhibit high flexibilities, which cannot be achieved with Si cells. Spectrum splitting tandem cells may not be ideal for practical applications; however, our study is significantly important because we have shown experimentally that this PVK/CIGS tandem solar cell is not only mechanically superior but also exhibits a higher efficiency compared with Si-based tandem solar cell configurations. These observations will be utilized to design better tandem solar cells in future.

Limitations of the Study

In this study, we did not conduct MPP measurement; therefore, the steady-state efficiency may differ from the values presented in the paper to some extent. To compare the PCEs of the spectrum splitting tandem devices to other types of tandem devices may not be ideal as most of the optical losses are not taken into consideration for the former; however, the result of this research should provide a guideline for an optimal material design in the fabrication of tandem solar cells in the future.

Resource Availability

Lead Contact

Further information and requests for resources and reagents should be directed to and will be fulfilled by the Lead Contact, Hiroshi Segawa (csegawa@mail.ecc.u-tokyo.ac.jp).

Materials Availability

This study did not generate any new unique reagent.

Data and Code Availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Methods

All methods can be found in the accompanying Transparent Methods supplemental file.