In spite of the merits such as Earth abundance and high performance, Cu 2 ZnSn(S,Se) 4 (CZTSSe) solar cells suffer from unfavorable Sn Zn antisite defects and complexes, which act as nonradiative recombination centers and deteriorate the open-circuit voltage ( V OC ). Therefore, the management of Sn composition is the prerequisite for achieving high-efficiency CZTSSe photovoltaic devices. At present, the Sn-related composition and defect modifications at different selenization pressures remain unclear, which restrain the development of efficient kesterite solar cells. Herein, a facile yet effective strategy to accurately adjust the Sn content in CZTSSe films by simply optimizing the selenization pressure is demonstrated. Compared with the widely used atmospheric pressure, it is unveiled that the appropriate negative pressure (0.7 atm) can tailor the optimal Sn content in the absorber layer, influencing both the Sn-related defects and the microstructures. In contrast, a lower (0.4 atm) and a higher (1.3 atm) selenization pressure results in undesirable deep Cu Sn defects and a Sn(S,Se) 2 secondary phase, respectively. A champion device fabricated at this optimal selenization pressure (0.7 atm) exhibits a power conversion efficiency of 11.32% with a V OC of 0.496 V. This study paves the path toward highly efficient kesterite solar cells by tailoring the composition-dependent defects.
Polycrystalline SnSe has been widely explored due to its relatively low thermal conductivity. Yet its thermoelectric performance is limited due to its poor electrical conductivity. In this work, polycrystalline SnSe alloyed with different contents of CuAgSe were fabricated by a high-energy balling and melting method, combined with spark plasma sintering. It is revealed that substitution of Ag and Cu at Sn sites actually enhances the electrical conductivity, especially under Sn-deficient circumstances where the formation energy of AgSn is lowered. When there was excessive addition of CuAgSe, the hole carrier concentration was reduced due to the formation of Agi, which acts as an electron donor. Additionally, partial substitution of Ag and Cu into the Sn vacancies slightly improves the Hall mobility and the density of states effective mass. With increased electrical conductivity while maintaining a stable Seebeck coefficient, the Sn0.99Se sample alloyed with 0.75% CuAgSe attains a maximum power factor of 6.40 μW cm−1 K−2 at 847 K. In addition, a peak zT value of 0.97 at 847 K is achieved, combined with only a marginally decreased thermal conductivity, thereby exhibiting a prominent improvement over that of the pure SnSe sample. The present work opens up a new perspective for optimizing the thermoelectric performance of SnSe compounds with the strategy of tuning the content of Sn and alloying with CuAgSe.
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