High-efficiency Cu2ZnSn(S,Se)4 solar cells are reported by applying In2S3/CdS double emitters. This new structure offers a high doping concentration within the Cu2ZnSn(S,Se)4 solar cells, resulting in a substantial enhancement in open-circuit voltage. The 12.4% device is obtained with a record open-circuit voltage deficit of 593 mV.

of 8.4% has a V OC,def of 789 mV. [ 3 ] The V OC,def issue in CZTSSe solar cells has been investigated extensively and various intrinsic factors have been proposed: low dielectric constant, [ 4,5 ] fl uctuating potential of conduction and valence bands, [5][6][7] high defect density, [ 8 ] deep defects, and nonradiative channels in the CZTSSe absorbers. [ 9 ] Given the signifi cant increase of V OC,def with S/(S + Se) ratio, there could be additional extrinsic factors related to S-content to explain the high V OC,def in CZTSSe with high S/(S + Se) ratio (e.g., formation of secondary phases).The second pressing issue in kesterite technology is the relatively low device fi ll factor (FF). [ 10 ] For example, the current champion CZTSSe device has FF = 69.8% [ 1 ] compared to FF = 79.4% in the champion Cu(In,Ga)Se 2 solar cell with the same E g of 1.13 eV. [ 11 ] In general, there are several mechanisms of FF loss in solar cells (in approximate order of their negative impact in kesterite devices): (1) V OC and ideality factor ( n ) loss; [ 12 ] (2) series resistance ( R S ) loss; [ 12 ] (3) voltage-dependent collection effi ciency (VDCE) loss [ 13 ] and (4) shunt conductance ( G S ) loss. [ 12 ] The nature of the V OC defi cit problem has been discussed in refs. [ 5 ] and [ 6 ] , and therefore we focus on the second factor ( R S loss) in this work. R S is usually extracted from the light-JV (LJV) curve, and thus denoted by R SL . The R SL values of the champion CZTSe, CZTSSe, and CZTS solar cells are 0.5, [ 2 ] 0.7, [ 1 ] and 1.4 Ω cm 2 , [ 3 ] respectively, while other literature also report a high R SL for CZTS. [14][15][16] The extraordinarily high R SL could be due to the formation of (high E g ) interfacial secondary phases, which act as a charge blocking

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On page 7427, J. Kim, H. Hiroi, T. K. Todorov, D. B. Mitzi, and co‐workers report high‐efficiency Cu2ZnSn(S,Se)4 solar cells by applying In2S3/CdS double emitters. This new structure offers a high doping concentration within the Cu2ZnSn(S,Se)4 solar cells, resulting in substantial enhancement in the open‐circuit voltage. The 12.4% device is obtained with a record open‐circuit voltage deficit of 593 mV.