over 30% detailed balance limiting efficiency, as well as to its earth-abundant and environment-benign constituents. [1-3] The increase in power conversion efficiency to a record of 12.6% in the last decade has demonstrated the huge potential of these materials. [4,5] However, as one of the most complicated compound semiconductors, kesterite has much more intricate defect chemistry than its counterparts, Cu(In,Ga)Se 2 (CIGS) and CdTe, [6-8] making the control of intrinsic defects a major challenge. Deep intrinsic defects like Sn Zn antisites and related [Cu Zn +Sn Zn ] clusters act as deep recombination centers, leading to the short carrier lifetime. [7,9,10] Additionally, the large population of defect clusters like [2Cu Zn +Sn Zn ] introduces considerable potential (i.e., band or electrostatic) fluctuation. [11] Consequently, the performance of CZTSSe solar cells are currently stagnated by the large open-circuit voltage (V OC) deficit. [12,13] To address the detrimental intrinsic defects and defect clusters in CZTSSe absorber, multiple strategies have been employed. As suggested by the first-principle calculations, the formation energy of intrinsic defects and Kesterite-based Cu 2 ZnSn(S,Se) 4 semiconductors are emerging as promising materials for low-cost, environment-benign, and high-efficiency thin-film photo voltaics. However, the current state-of-the-art Cu 2 ZnSn(S,Se) 4 devices suffer from cation-disordering defects and defect clusters, which generally result in severe potential fluctuation, low minority carrier lifetime, and ultimately unsatisfactory performance. Herein, critical growth conditions are reported for obtaining high-quality Cu 2 ZnSnSe 4 absorber layers with the formation of detrimental intrinsic defects largely suppressed. By controlling the oxidation states of cations and modifying the local chemical composition, the local chemical environment is essentially modified during the synthesis of kesterite phase, thereby effectively suppressing detrimental intrinsic defects and activating desirable shallow acceptor Cu vacancies. Consequently, a confirmed 12.5% efficiency is demonstrated with a high V OC of 491 mV, which is the new record efficiency of pure-selenide Cu 2 ZnSnSe 4 cells with lowest V OC deficit in the kesterite family by E g /q-Voc. These encouraging results demonstrate an essential route to overcome the long-standing challenge of defect control in kesterite semiconductors, which may also be generally applicable to other multinary compound semiconductors.
Pure-sulfide kesterite Cu 2 ZnSnS 4 (CZTS)-based thin-film solar cell has been emerging as a promising cost-effective thin-film photovoltaic (PV) technology, enjoying its Earth-abundant and ecofriendly constituents, thermodynamically stable structure, combined with the ideal bandgap perfectly matching with solar spectrum, and the compatibility with both rigid and flexible substrates. [1][2][3][4][5][6] These compelling features endow this PV technology huge potential for application of various scenes in the future, including wearable and portable PV power sources, building-integrated PVs (BIPVs) at curved building surfaces, and sustainable power sources for internet of things (IOT). [7,8] Moreover, pure-sulfide CZTS is also one of the most promising candidates as the top cell for silicon-based tandem solar cells, potentially triggering further technological evolution for largescale deployment of PV technologies. [9][10][11] Nevertheless, the current status of CZTS thin-film solar cells suffers from a much more open-circuit voltage (V OC ) loss than low-bandgap Cu 2 ZnSnS,Se 4 (CZTSSe) solar cells. [3,12,13] Besides the more severe bandgap/potential fluctuation and shorter photoluminescence (PL) decay time (related to real minority carrier lifetime), [14][15][16] the unfavorable "cliff"-like conduction band offset (CBO) at CZTS/CdS heterojunction interface is well believed to be a serious limiting factor to the V OC of CZTS solar cells. [17][18][19][20] To address this issue, alternative buffer materials with wide bandgap and a suitable conduction band edge have been screened, among which (Zn,Cd)S and (Zn,Sn)O are the most successful materials, allowing a great V OC boost up to 100 mV. [18,19] Nevertheless, the V OC of CZTS solar cells with a bandgap of 1.5 eV is still limited to be below 750 mV, far lower than that of the moderate CdTe solar cells with a similar recombination bandgap (1.45 eV) and low minority carrier lifetime (1À2 ns), let alone the "cliff"-like CBO at the CdTe/CdS interface. [21][22][23][24] Results of SunsÀV OC measurements for these cells configured with (Zn,Sn)O or (Zn,Cd)S buffer layer have revealed that J 02 (representing nonradiative recombination at the heterojunction region) is still 5 orders of magnitude larger than J 01 (representing nonradiative recombination in the quasineutral bulk region) (10 À7 A cm À2 for J 02 vs. 10 À12 A cm À2 for J 01 ). [18,19] This verifies that the V OC of pure-sulfide CZTS solar cells is still currently limited by nonradiative recombination in the heterojunction interface region, even though the unfavorable "cliff-like" CBO has been avoided, indicating that other important interface recombination mechanisms may persist and are yet to be properly recognized.
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