Improved carrier collection efficiency in CZTS solar cells by Li-enhanced liquid-phase-assisted grain growth

Abstract

The liquid-phase-assisted grain growth (LGG) process is a promising strategy to fabricate large-grain pure sulfide Cu₂ZnSnS₄ (CZTS) layers that span the absorber thickness and improve the carrier collection efficiency in photovoltaic devices. Li doping is an effective route to promote such LGG process of Cu₂ZnSn(S,Se)₄ (CZTSSe) as it can provide liquid Li-Se phase facilitating the growth of large-grain CZTSSe. However, the detailed function of the added Li in grain growth has rarely been investigated in both CZTS and CZTSSe, as the reported in situ, and pre-deposition doping strategies usually suffer from substantial Li losses during the spin-coating process and/or the hightemperature sulfurization process. Herein, by monitoring the temperaturedependent Li loss during the sulfurization process, we demonstrate that a small proportion of the added Li can remain at the CZTS film from the early sulfurization stage and provide Li-S flux to promote the LGG process. An encouraging efficiency of 10.53%, with a remarkably high short-circuit current density of 22.6 mA/cm² and open-circuit voltage of 0.744 V, is achieved by a significantly enlarged grain size of 3 μm with Li addition. This work could enhance the knowledge of employing Li-S as flux for growing large-grain chalcogenide absorbers for high performance devices with better carrier transport.

Keywords

alkaline element / carrier collection / Cd-free / grain growth / kesterite solar cells

References

[1] Wang A, He M, Green MA, Sun K, Hao X. A critical review on the progress of kesterite solar cells: current strategies and insights. Adv Energy Mater. 2023;13(2):2203046.

[2] Pal K, Singh P, Bhaduri A, Thapa KB. Current challenges and future prospects for a highly efficient (>20%) kesterite CZTS solar cell: a review. Sol Energy Mater Sol Cells. 2019;196:138-156.

[3] Ito K. Copper Zinc Tin Sulfide-Based Thin-Film Solar Cells. John Wiley &Sons;2015.

[4] Giraldo S, Jehl Z, Placidi M, Izquierdo-Roca V, Pérez-Rodríguez A, Saucedo E. Progress and perspectives of thin film kesterite photovoltaic Technology: a critical review. Adv Mater. 2019;31(16).

[5] Green MA, Dunlop ED, Hohl-Ebinger J, Yoshita M, Kopidakis N, Hao X. Solar cell efficiency tables (version 62). Prog Photovoltaics Res Appl. 2023;31(7):1-13.

[6] Siebentritt S. Why are kesterite solar cells not 20% efficient? Thin Solid Films. 2013;535:1-4.

[7] Gong Y, Qiu R, Niu C, et al. Ag Incorporation with controlled grain growth enables 12.5% efficient Kesterite solar cell with open circuit voltage reached 64.2% Shockley–Queisser limit. Adv Funct Mater. 2021;31(24):2101927.

[8] Huang W-C, Wei S-Y, Cai C-H, Ho W-H, Lai C-H. The role of Ag in aqueous solution processed (Ag, Cu)₂ZnSn(S, Se)₄ kesterite solar cells: antisite defect elimination and importance of Na passivation. J Mater Chem A. 2018;6(31):15170-15181.

[9] Grenet L, Suzon MAA, Emieux F, Roux F. Analysis of failure modes in kesterite solar cells. ACS Appl Energy Mater. 2018;1(5):2103-2113.

[10] Li J, Huang J, Ma F, et al. Unveiling microscopic carrier loss mechanisms in 12% efficient Cu₂ZnSnSe₄ solar cells. Nat Energy. 2022;7:754-764.

[11] German RM, Suri P, Park SJ. Liquid phase sintering. J Mater Sci. 2009;44:1-39.

[12] Sutter-Fella CM, Stückelberger JA, Hagendorfer H, et al. Sodium assisted sintering of chalcogenides and its application to solution processed Cu₂ZnSn(S, Se)₄ thin film solar cells. Chem Mater. 2014;26(3):1420-1425.

[13] Chen H, Fu S, Wu S, Tsai T, Wu H, Shih C. Impact of SnS buffer layer at Mo/Cu₂ZnSnS₄ interface. J Am Ceram Soc. 2016;99(5):1808-1814.

[14] Kim S, Kim KM, Tampo H, Shibata H, Matsubara K, Niki S. Ge-incorporated Cu₂ZnSnSe₄ thin-film solar cells with efficiency greater than 10. Sol Energy Mater Sol Cells. 2016;144:488-492.

[15] Giraldo S, Neuschitzer M, Placidi M, Pistor P, Perez-Rodriguez A, Saucedo E. Cu₂ZnSnSe₄-based solar cells with efficiency exceeding 10% by adding a superficial Ge nanolayer: the interaction between Ge and Na. IEEE J Photovoltaics. 2016;6(3):754-759.

[16] Sun R, Zhao M, Zhuang D, et al. Cu₂ZnSnS₄ ceramic target: determination of sintering temperature by TG–DSC. Ceram Int. 2016;42(8):9630-9635.

[17] Yuan X, Li J, Huang J, et al. 10.3% efficient green Cd-free Cu₂ZnSnSe₄ solar cells enabled by liquid-phase promoted grain growth. Small. 2022;18(50).

[18] Hsieh YT, Han Q, Jiang C, et al. Efficiency enhancement of Cu₂ZnSn(S, Se)₄ solar cells via alkali metals doping. Adv Energy Mater. 2016;6(7):1-6.

[19] Ehira H, Jimbo K, Katagiri H. Impact of Na-addition effect into CZTS thin film solar cells. In: AIP Conf. Proc. AIP Publishing LLC;2017.50004.

[20] Gershon T, Shin B, Bojarczuk N, Hopstaken M, Mitzi DB, Guha S. The role of sodium as a surfactant and suppressor of non-radiative recombination at internal surfaces in Cu₂ZnSnSe₄. Adv Energy Mater. 2015;5(2):1400849.

[21] Nagaoka A, Miyake H, Taniyama T, et al. Effects of sodium on electrical properties in Cu₂ZnSnSe₄ single crystal. Appl Phys Lett. 2014;104(15):152101.

[22] Yang Y, Huang L, Pan D. New insight of Li-doped Cu₂ZnSn(S, Se)₄ thin films: Li-induced Na diffusion from soda-lime glass by a cation-exchange reaction. ACS Appl Mater Interfaces. 2017;9(28):23878-23883.

[23] Cabas-Vidani A, Haass SG, Andres C, et al. High-efficiency (Li_xCu_1–x)₂ZnSn(S, Se)₄ kesterite solar cells with lithium alloying. Adv Energy Mater. 2018;8(34):1-8.

[24] Diagram E. The Li-S (Lithium-Sulfur) system. J Phase Equil. 1995;16(1):94-97.

[25] Zhou J, Xu X, Duan B, et al. Regulating crystal growth via organic lithium salt additive for efficient Kesterite solar cells. Nano Energy. 2021;89:106405.

[26] He M, Zhang X, Huang J, et al. High efficiency Cu₂ZnSn(S, Se)₄ solar cells with shallow LiZn acceptor defects enabled by solution-based Li post-deposition treatment. Adv Energy Mater. 2021;11(13):2003783.

[27] Xin H, Vorpahl SM, Collord AD, et al. Lithium-doping inverts the nanoscale electric field at the grain boundaries in Cu₂ZnSn (S, Se)₄ and increases photovoltaic efficiency. Phys Chem Chem Phys. 2015;17(37):23859-23866.

[28] Haass SG, Andres C, Figi R, et al. Complex interplay between absorber composition and alkali doping in high-efficiency kesterite solar cells. Adv Energy Mater. 2018;8(4):1701760.

[29] Guo H, Wang G, Meng R, et al. An efficient Li⁺-doping strategy to optimize the band alignment of a Cu₂ZnSn(S, Se)₄/CdS interface by a Se&LiF co-selenization process. J Mater Chem A. 2020;8(42):22065-22074.

[30] Properties of Substance: Lithium Chloride, (n.d.). http://chemister.ru/Database/properties-en.php?dbid=1&id=614

[31] Properties of Substance: Lithium Sulfide, (n.d.). http://chemister.ru/Database/properties-en.php?dbid=1&id=6015

[32] Zhang J, Liao J, Shao L-X, Xue S-W, Wang Z-G. Lithium/silver-doped Cu₂ZnSnSe₄ with tunable band gaps and phase structures: a first-principles study. Chin Phys Lett. 2018;35(8):83101.

[33] Maeda T, Kawabata A, Wada T. First-principles study on alkali-metal effect of Li, Na, and K in Cu₂ZnSnSe₄ and Cu₂ZnSnSe₄. Phys Status Solidi. 2015;12(6):631-637.

[34] Abou-Ras D, Kirchartz T, Rau U. Advanced Characterization Techniques for Thin Film Solar Cells. Wiley Online Library;2011.

[35] Issue S. Thin-Film Solar Cells: Device Measurements and Analysis. 2004;176:155-176.

[36] Cui X, Sun K, Huang J, et al. Enhanced heterojunction interface quality to achieve 9.3% efficient Cd-free Cu₂ZnSnSe₄ solar cells using atomic layer deposition ZnSnO buffer layer. Chem Mater. 2018;30(21):7860-7871.

RIGHTS & PERMISSIONS

2024 2024 The Authors. EcoEnergy published by John Wiley & Sons Australia, Ltd on behalf of China Chemical Safety Association.