The role of buffer/kesterite interface recombination and minority carrier lifetime on kesterite thin film solar cells
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This paper presents for the first time a theoretical study of the impact of kesterite/buffer interface recombination and kesterite minority carrier lifetime on both CZTS and CZTSe solar cells. It demonstrates that only an 11% efficiency can be reached in CZTS solar cells by improving absorber crystalline quality, pointing out the need for an improved CdS/CZTS interface. It further demonstrates that a CZTS solar cell efficiency enhancement of up to 18%, with an open-circuit voltage value of up to 918 mV, can be achieved depending on CZTS minority carrier lifetime and CdS/CZTS interface recombination speed values. Moreover, this paper shows that by improving CZTSe crystalline quality, a record efficiency value of 17% could be achieved without focusing on improving CdS/CZTSe interface quality. Consequently, CZTSe is presented as a better candidate for solar cell applications. Conditions under which CdS/kesterite interface recombination and trap-assisted tunneling recombination become dominant are provided. In particular, we find that CdS/CZTS interface recombination is the dominant transport mechanism for CZTS minority carrier lifetime values higher than 5 ns, while for CZTSe minority carrier lifetime values lower than 0.1 μs, CdS/CZTSe interface losses are negligible.Keywords:
Kesterite
Carrier lifetime
In this work, first principle calculations of Cu$_2$ZnSnS$_4$ (CZTS), Cu$_2$ZnGeS$_4$ (CZGS) and Cu$_2$ZnSiS$_4$ (CZSS) are performed to highlight the impact of the cationic substitution on the structural, electronic and optical properties of kesterite compounds. Direct bandgaps are reported with values of 1.32, 1.89 and 3.06 eV respectively for CZTS, CZGS and CZSS. In addition, absorption coefficient values of the order of $10^4$ cm$^{-1}$ are obtained, indicating the applicability of these materials as absorber layer for solar cell applications. In the second part of this study, ab initio results are used as input data to model the electrical power conversion efficiency of kesterite-based solar cell. In that perspective, we used an improved version of the Shockley-Queisser theoretical model including non-radiative recombination via an external parameter defined as the internal quantum efficiency. Based on predicted optimal absorber layer thicknesses, the variation of the solar cell maximal efficiency is studied as a function of the non-radiative recombination rate. Maximal efficiencies of 25.88, 19.94 and 3.11% are reported respectively for CZTS, CZGS and CZSS for vanishing non-radiative recombination rate. Using an internal quantum efficiency providing $V_{OC}$ values comparable to experimental measurements, solar cell efficiencies of 15.88, 14.98 and 2.66% are reported respectively for CZTS, CZGS and CZSS (for an optimal thickness of 1.15 $\mu$m). With this methodology, we confirm the suitability of CZTS in single junction solar cells, with a possible efficiency improvement of 10% enabled through the reduction of the non-radiative recombination rate. In addition, CZGS appears to be an interesting candidate as top cell absorber layer for tandem approaches whereas CZSS might be interesting for transparent PV windows.
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Abstract In this work, first-principles calculations of Cu 2 ZnSnS 4 , Cu 2 ZnGeS 4 and Cu 2 ZnSiS 4 are performed to highlight the impact of the cationic substitution on the structural, electronic and optical properties of kesterite compounds. Direct bandgaps are reported with values of 1.32, 1.89 and 3.06 eV respectively for Cu 2 ZnSnS 4 , Cu 2 ZnGeS 4 and Cu 2 ZnSiS 4 and absorption coefficients of the order of 10 4 cm −1 are obtained, indicating the applicability of these materials as absorber layer for solar cell applications. In the second part of this study, ab initio results are used as input data to model the electrical power conversion efficiency of kesterite-based solar cells. In that perspective, we used an improved version of the Shockley–Queisser model including non-radiative recombination via an external parameter defined as the internal quantum efficiency. Based on predicted optimal absorber layer thicknesses, the variation of the solar cell maximal efficiency is studied as a function of the non-radiative recombination rate. Maximal efficiencies of 25.71%, 19.85% and 3.10% are reported respectively for Cu 2 ZnSnS 4 , Cu 2 ZnGeS 4 and Cu 2 ZnSiS 4 for vanishing non-radiative recombination rate. Using an internal quantum efficiency value providing experimentally comparable V O C values, cell efficiencies of 15.88%, 14.98% and 2.66% are reported respectively for Cu 2 ZnSnS 4 , Cu 2 ZnGeS 4 and Cu 2 ZnSiS 4 . We confirm the suitability of Cu 2 ZnSnS 4 in single junction solar cells, with a possible efficiency improvement of nearly 10% enabled through the reduction of the non-radiative recombination rate. In addition, Cu 2 ZnGeS 4 appears to be an interesting candidate as top cell absorber layer for tandem approaches whereas Cu 2 ZnSiS 4 might be interesting for transparent photovoltaic windows.
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The kesterite Cu 2 ZnSnS 4 (CZTS) nanocrystals (NCs) were successfully synthesized using a relatively simple and one‐step hydrothermal route. The structural, compositional, and optical properties of the kesterite CZTS NCs have been studied in detail. The pH‐dependent CZTS phase formation has been elucidated for the first time. The X‐ray diffraction and Raman spectroscopy confirmed the formation of a main phase kesterite CZTS structure only at pH 7. However, for pH values (4.3, 5, and 9), the formation of CZTS alongwith few secondary phases like Cu 2 SnS 3 (CTS), Cu 2− x S, and SnS 2 /Sn 2 S 3 have been detected. CZTS NCs of size 10–100 nm were obtained at 200 °C and pH 7. The synthesized NCs showed a pH‐dependent variation in optical band gap values from 1.15 to 1.44 eV, which is near optimum value for low cost thin film solar cells.
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This paper presents for the first time a theoretical study of the impact of kesterite/buffer interface recombination and kesterite minority carrier lifetime on both CZTS and CZTSe solar cells. It demonstrates that only an 11% efficiency can be reached in CZTS solar cells by improving absorber crystalline quality, pointing out the need for an improved CdS/CZTS interface. It further demonstrates that a CZTS solar cell efficiency enhancement of up to 18%, with an open-circuit voltage value of up to 918 mV, can be achieved depending on CZTS minority carrier lifetime and CdS/CZTS interface recombination speed values. Moreover, this paper shows that by improving CZTSe crystalline quality, a record efficiency value of 17% could be achieved without focusing on improving CdS/CZTSe interface quality. Consequently, CZTSe is presented as a better candidate for solar cell applications. Conditions under which CdS/kesterite interface recombination and trap-assisted tunneling recombination become dominant are provided. In particular, we find that CdS/CZTS interface recombination is the dominant transport mechanism for CZTS minority carrier lifetime values higher than 5 ns, while for CZTSe minority carrier lifetime values lower than 0.1 μs, CdS/CZTSe interface losses are negligible.
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Kesterite Cu2ZnSnS4-xSex (CZTS) is a promising thin film photovoltaic (PV) material with low cost and nontoxic constitute as well as decent PV properties, being regarded as a PV technology that is truly compatible with terawatt deployment. The kesterite CZTS thin film solar cell has experienced impressive development since its first report in 1996 with power conversion efficiencies (PCEs) of only 0.66% to current highest value of 13.0%, while the understanding of the material, device physics, and loss mechanism is increasingly demanded. This chapter will review the development history of kesterite technology, present the basic material properties, and summarize the loss mechanism and strategies to tackle these problems to date. This chapter will help researchers have brief background knowledge of kesterite CZTS technology and understand the future direction to further propel this new technology forward.
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Cu2ZnSnS4–xSex (CZTS) is an important semiconductor with significant potential for applications in the next generation of solar cells. CZTS has an optimal band gap (∼1.5 eV) and contains no expensive or toxic elements. However, CZTS-based solar cells suffer from low efficiency because of poor crystal quality, which is partly caused by secondary phase formation during synthesis. We use density functional theory+U calculations to systematically investigate the stabilities of three CZTS phases: kesterite, stannite, and wurtzite. In agreement with previous experiment and theory, we confirm that these three phases have very similar formation energies. This finding is consistent with the known difficulties in synthesizing pure kesterite CZTS, the phase that is desirable for photovoltaic applications. To overcome this problem, we characterize surfaces and interfaces of CZTS and are able to identify certain "beneficial surfaces" that could be exploited to potentially provide extra stability for the kesterite phase. We propose the zinc blende ZnS (001) surface as a substrate to induce formation of these beneficial surfaces and to stabilize the kesterite phase, thereby serving as an effective crystallization template for the fabrication of high-performance CZTS solar cells.
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