Studies On The Bulk Regulation And Interface Modification Of Cu₂ZnSn（S,Se）₄ Solar Cell

Thin film solar cells take the advantages of low cost and wide application scenarios,thus become the important direction in the current photovoltaic research field.Among them,Cu₂Zn Sn（S,Se）₄（CZTSSe）thin film solar cells have attracted more and more attention due to their abundant raw material reserves,eco-friendly elements and good process compatibility.Currently,the maximum power conversion efficiency of CZTSSe solar cells reached 13.8%,which is still much lower than the theoretical limit efficiency of 32%.Serious bulk recombination and interface recombination are believed to be the key factors that influences the efficiency improvement of CZTSSe solar cells.In order to solve this problem,the research work of this thesis focuses on optimizing the back interface structure,modifying the preparation method of absorber,and passivating the front interface defects,and have achieved the following results:（1）The coordination molecular clusters of thioglycolic acid（TGA）and Sn are used to form a conductive porous carbon layer in situ on the back interface of the device,thereby improving the back contact and component tolerance of CZTSSe solar cells,and finally achieving a total area efficiency of 12.2%.It was found that,in the precursor solution,the coordination of Sn and TGA can form large metal-organic molecular clusters.And these molecular clusters will be converted into porous graphitic carbon layers during the subsequent high-temperature annealing process.The electrical characterization results prove that the conductive carbon layer can form an ohmic contact with CZTSSe at the back interface,improving the charge transport properties.Further studies on crystallization and chemical mechanism have shown that the porous carbon layer can drive the CZTSSe crystal into‘bottom to top’growth mode of heterogeneous thin-film system,and play the role of element storage and buffering to ensure the stability of the upper crystal composition.These results and the related material mechanism provide a new idea to regulate the crystal growth of CZTSSe thin films,thereby further improving the efficiency.（2）Basing on the dual-temperature zone selenization scheme,a solid-liquid and solid-gas（solid precursor and liquid/gas phase Se）synergistic reaction strategy has been developed to precisely regulate the selenization.Pre-deposited excess liquid Se provides high Se molecular concentration to drive a direct and fast formation of the CZTSSe phase,significantly reducing the amount of binary and ternary compounds within phase evolution.Liquid Se can assist the nucleation and growth of crystals and prevent the decomposition of CZTSSe surface.And organics removal can be accomplished via a synergistic optimization of Se condensation and subsequent volatilization.After systematical optimization,we achieve a high-performance CZTSSe solar cell with a remarkable PCE of 13.6%,and a large-area PCE of 12.0%（over 1cm²）.This strategy will provide a new idea for further improving efficiency of CZTSSe solar cells via phase evolution regulation,and also for other complicated multi-compound synthesis.（3）A low-temperature Rb passivation strategy has been proposed.In this strategy,Rb Cl is introduced into the chemical bath deposition（CBD）solution to assist the construction of CZTSSe/Cd S interface.First,Rb⁺can etch the surface of CZTSSe to generate more suitable cation vacancies to attract Cd²⁺,and eliminate Se-related deep defects.Further,Rb⁺can form weak coordination with thiourea（TU）during the chemical bath deposition process,inhibit the hydrolysis of TU,promote the heterogeneous growth of Cd S and reduce the loss of sulfur.These positive effects drive the growth of Cd S tending to in‘Ion by Ion’mode,which help to achieve a compact Cd S film with a good crystallinity.Finally,the flexible CZTSSe cell fabricated by this strategy achieved a high efficiency of 12.63%,with its open circuit voltage(V_OC)and fill factor（FF）reached 538 m V and 70.0%,respectively.This work enriches the interface passivation strategies and provides new ideas for further improving CZTSSe solar cells in the future.