Kesterite Solar Cells: Insights into Current Strategies and Challenges

Abstract

Earth‐abundant and environmentally benign kesterite Cu2ZnSn(S,Se)4 (CZTSSe) is a promising alternative to its cousin chalcopyrite Cu(In,Ga)(S,Se)2 (CIGS) for photovoltaic applications. However, the power conversion efficiency of CZTSSe solar cells has been stagnant at 12.6% for years, still far lower than that of CIGS (23.35%). In this report, insights into the latest cutting‐edge strategies for further advance in the performance of kesterite solar cells is provided, particularly focusing on the postdeposition thermal treatment (for bare absorber, heterojunction, and completed device), alkali doping, and bandgap grading by engineering graded cation and/or anion alloying. These strategies, which have led to the step‐change improvements in the power conversion efficiency of the counterpart CIGS solar cells, are also the most promising ones to achieve further efficiency breakthroughs for kesterite solar cells. Herein, the recent advances in kesterite solar cells along these pathways are reviewed, and more importantly, a comprehensive understanding of the underlying mechanisms is provided, and promising directions for the ongoing development of kesterite solar cells are proposed.

Large‐Grain Spanning Monolayer Cu2ZnSnSe4Thin‐Film Solar Cells Grown from Metal Precursor

“…[ 1–5 ] Nevertheless, after two decades development, their power conversion efficiency (PCE) still largely lags behind that of chalcogenide counterparts Cu(In,Ga)(S,Se) 2 and Cd(Te,Se), still much lower than the commercially viable level. [ 6–10 ] Besides the abundant cation‐disordering defects that lead to short minority carrier lifetime and prevalent potential fluctuations, [ 2,11–13 ] the performance of CZTSSe solar cells is also plagued by the unfavorable morphology of the CZTSSe absorber with presence of small grains, voids, and secondary phases at the bottom of the absorber layer. [ 14,15 ] These undesirable structures are believed to be responsible for the poor carrier transport near the back contact, leading to considerable nonradiative recombination in this region and a significant increase of series resistance ( R S ) due to the enhanced scattering of hole carriers.…”

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less, after two decades development, their power conversion efficiency (PCE) still largely lags behind that of chalcogenide counterparts Cu(In,Ga)(S,Se) 2 and Cd(Te,Se), still much lower than the commercially viable level. [6][7][8][9][10] Besides the abundant cation-disordering defects that lead to short minority carrier lifetime and prevalent potential fluctuations, [2,[11][12][13] the performance of CZTSSe solar cells is also plagued by the unfavorable morphology of the CZTSSe absorber with presence of small grains, voids, and secondary phasesThe ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/smll.202105044.

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A Thin In2S3 Interfacial Layer for Reducing Defects and Roughness of Cu2ZnSn(S,Se)4 Thin‐Film Solar Cells

“…[1][2][3][4] CZTSSe exhibits excellent optical and electrical properties, including a tunable direct band gap (1.0-1.5 eV) that varies with the S/Se ratio and a high absorption coefficient (> 10 4 cm À 1 ). [5][6][7] It has also attracted interest as the bottom cell of tandem solar cells, [8][9][10] resulting in low power generation cost and environmental impact in combination with promising perovskite solar cells. [11] The highest certified and reported efficiencies of CZTSSe thin-film solar cells are 13.0 % (NREL efficiency chart) and 12.6 %, [3,12] respectively; however, this value is much lower than the theoretical efficiency of CZTS-based single junction cells [i. e., 32.4 % with short-circuit current density (J SC ) = 29.6 mA • cm À 2 , open-circuit voltage (V OC ) = 1.21 V, and fill factor (FF) = 89.9 %] calculated using the Shockley-Queisser efficiency limit.…”

“…Cu 2 ZnSn(S,Se) 4 (CZTSSe) thin‐film solar cell has drawn considerable attention as an alternative to the Cu(In,Ga)Se 2 and CdTe solar cells because of its use of earth‐abundant and non‐toxic materials (i. e., Zn and Sn) rather than expensive and toxic elements (i. e., In and Cd) [1–4] . CZTSSe exhibits excellent optical and electrical properties, including a tunable direct band gap (1.0–1.5 eV) that varies with the S/Se ratio and a high absorption coefficient (>10 4 cm −1 ) [5–7] . It has also attracted interest as the bottom cell of tandem solar cells, [8–10] resulting in low power generation cost and environmental impact in combination with promising perovskite solar cells [11] .…”

Unveiling microscopic carrier loss mechanisms in 12% efficient Cu2ZnSnSe4 solar cells

“…For CZTSSe, due to the multiple competitive secondary phases and complex intrinsic defect system, 21 most efforts have been paid to the bulk and heterojunction interfaces, through modifications of the chemical composition and growth conditions, [22][23][24][25] extrinsic cation/alkali doping or alloying, 7,26,27 alternative buffer materials, 28,29 or interface passivation. [30][31][32] Although some properties of the GIs and GBs such as intragrain crystallinity defects and band bending at the GBs have been investigated using high-resolution structural and electrical analysis respectively, 33,34 detailed loss mechanisms in these microscopic regions especially GB recombination and grain interior carrier lifetime, and their impact on the device performance remain unknown. This, as a critical gap between the understanding of CZTSSe and its mature cousins CdTe and CIGSSe, can be one of the key origins of the efficiency stagnation of CZTSSe in recent years, thus requiring comprehensive investigation urgently.…”