Study On Photovoltaic Properties Of Micro/Nano CU-ZN-SN-S Compounds And Novel Thin-Film Solar Cell

Tandem solar cell which can convert broad solar spectrum with high-efficient has much higher efficiency than that of single-junction solar cell. Nowadays, the efficiency of the 3-junction GaInP/GaAs/Ge compound thin-film solar cell has reached 41.6% under concentrated sunlight. However, the applied photovoltaic materials are almost GaAs-based, which makes the cost of the tandem solar cell very high and hence limits its large-scale application in ground. Therefore, searching for new low-cost alternative compound materials has been an important topic for tandem solar cell.Some achievements have been made on the photovoltaic materials of nitrogen-doped, In(Ga)N and quantum dots, but the prices of these materials are still too high. In this paper, we try to synthesize and study the photovoltaic properties of several new compounds from the Cu-Zn-Sn-S system free of scarce elements for the first time. In addition, the compound thin-films for studying the photovoltaic properties and for the tandem solar cell application are usually prepared by vacuum processes, which are rather low-efficient, complex, expensive and difficult to scale-up for commercial production. In this study, the Cu-Zn-Sn-S compounds were synthesized by non-vacuun mechanochemical process with high-efficient for the first time. The compound thin-films were prepared by printing technology and their photovoltaic properties were studied.Cu₂ZnSnS₄ (CZTS) has been proved as a material with excellent photovoltaic properties and bandgap of about 1.5 eV, thus has a great potential for un-concentrated single and 3-junction solar cell application. However, the CZTS films are traditionally prepared by vacuum methods. In this study, the photovoltaic properties of the micro/nano compound CZTS were studied by the combination of mechanochemical-printing processes for the first time. The characteristics of the combination process have been summarized, which may offer a useful guide for large-scale commercial production of solar cells. To test the photoelectric properties of the compound and cut down the total cost of solar cell, a novel thin-film solar cell with all of its function thin-films can be prepared by non-vacuum technologies has been successfully designed.Besides, micro/nano compound Cu₂SnS₃ with the bandgap value of 0.83 eV and excellent photovoltaic performance has been successfully synthesized for the first time, which may suitable for un-concentrated 3-, 4-junction and concentrated 2-junction solar cell application. In addition, a method that can enhance the performance of the solar cell by creating a buried p-n junction is demonstrated with around doubled Jsc. The increased Jsc could better meet the requirement of current-match in the tandem solar cell, and also further illustrates the great potential of Cu₂SnS₃ for tandem solar cell application.Furthermore, we have successfully synthesized Cu-rich and Zn-poor micro/nano compound Cu₄SnS₄ with the bandgap value of 0.55 eV and excellent photovoltaic performance for the first time, which could offer a very promising candidate for concentrated 3- and 4-junction tandem solar cell application. We have also taken out a comparative study on the photovoltaic properties of CZTS, Cu₂SnS₃ and Cu₄SnS₄. It is found that, with the increased value of Cu/Sn & Zn/Sn: (1) the microstructures of the compounds significantly change, and the sensitivity of annealing temperature become higher; (2) the absorption edges of the absorption spectra become unclear gradually and the bandgap values gradually decrease; (3) the interface contact characteristics of the p-n junctions in the fabricated solar cells were improved and the efficiencies of the solar cells increase from 0.60% to 1.92%, and final 2.59%. These findings may provide important guidance for developing more Cu-Zn-Sn-S system new materials for tandem solar cell.