Electrodeposition Preparation And Performance Regulation Of CZGSe-based Thin Film Solar Cells

Energy crisis and environmental pollution are one of the most concerned problems in the 21st century.Compared with other energy sources,solar energy is environmentally friendly,directly accessible and inexhaustible,which is one of the important ways to solve the current energy crisis.In order to reduce material cost and applications in large area,thin films are the inevitable choice for solar cell development.Among many thin film solar cell materials,the direct band gap semiconductor material Cu₂Zn Ge Se₄（CZGSe）with kesterite structure has attracted much attention due to its high theoretical photoelectric conversion efficiency（33.3%）,ideal optical band gap（1.3-1.5 e V）and high light absorption coefficient.Compared with Sn-based kesterite solar materials（CZTSe）,CZGSe has the following significant advantages:（1）Inhibition of various defects and secondary phases induced by variable valence Sn;（2）The theoretical Shockley–Queisser（SQ）limit value of CZGSe is higher than that of CZTSe（CZGSe:33.3%,CZTSe:31.6%）;（3）Cu _Zn defect was partially inhibited,and the recombination rate induced by Ge _Zn was lower than that induced by Sn_Zn.However,CZGSe thin film solar cells have a champion efficiency of 8.5%,which is lower than Sn-based kesterite solar cells（13%）and far from its theoretical efficiency limit.This is mainly because:（1）although the Sn-related defects are inhibited,the defects in CZGSe are still very complex due to the similar oxidation-reducing properties of Ge and Sn,especially the Ge dominated defects(such as Cu _Zn-Ge _Zn,etc.),resulting in a high non-radiative recombination rate of CZGSe.（2）The introduction of Ge raises the position of the conduction band,so that the band alignment between CZGSe and CdS changes from the original"spike-type"to"cliff-type",thus reducing the interface quality and increasing the open voltage deficit.In addition,there are few preparation schemes of CZGSe,and only a few thermal evaporation,magnetron sputtering and solution methods have been reported.Therefore,the exploration of new methods and the suppression of undesirable defects are the key to the preparation of high quality CZGSe absorption layers.Aiming at the key problems existing in CZGSe thin film solar cells,a series of solutions are proposed.Firstly,in view of the poor solubility of Ge-based salts and high requirements for electrodeposition process,the green electrodeposition process was proposed to prepare the Cu-Zn mental layer,and then the CZGSe absorption layer was prepared by means of thermal evaporation Ge and subsequent selenization.The combination of Ge and Se was used to simplify the experimental process and optimize the experimental scheme to improve the quality of CZGSe absorber layer.Finally,to address the interface issue between CZGSe and CdS,Zn_xCd_1-xS buffer layer was selected to replace the CdS buffer layer and optimize the p-n heterogeneous interface.The specific research contents are as follows:1.CZGSe thin film solar cells prepared by electrodeposition processCu-Zn metal was first fabricated by stacked electrodeposition process with a three-electrode system and selenized at high temperature to prepare high quality CZGSe absorber layer.First,the Ge was incorporated by thermal evaporation of the Ge Se₂ layer at high temperature,and then selenized in an atmosphere containing Se and a small amount of Ge Se₂.The influence of selenization temperature on the composition,morphology,crystallinity and other aspects of CZGSe absorber layer film was investigated to reveal the influence of the device performance,and finally the CZGSe film solar cell with photoelectric conversion efficiency of 2.14%was obtained to fill the blank of electrodeposition processed CZGSe solar cells.2.The prepared optimization of electrodeposition processed CZGSe solar cellsIn order to further simplify the experimental process and optimize the experimental scheme,Ge Se with lower melting point was used to replace Ge Se₂ to realize the synchronization of germanization and selenization,improve the quality of CZGSe absorption layer and improve its photovoltaic performance.The effects of different selenization temperatures on the composition,morphology and crystallinity of the absorption layer were investigated to reveal the relationship between temperature parameters and photovoltaic properties.Compared with the series of characterization results,the advantages of using Ge Se and its effect on the bulk phase and interface of the absorption layer were analyzed,and the breakthrough of the maximum efficiency of electrodeposited CZGSe thin film solar cells was achieved with 3.69%.3.Interface optimization of CZGSe thin film solar cells through buffer layer substitutionTo solve the interface problem between CZGSe and CdS,further improve the photovoltaic performance.Zn_xCd_1-xS was proposed to replace the traditional CdS buffer layer,increase the conduction band position of the buffer layer,realize the band alignment transition of CZGSe and CdS from"cliff-type"to"spike-type",optimize interface quality and improve device performance.The optical performance of Zn_xCd_1-xS buffer layer with different Cd/Zn ratio and its influence on the performance of photovoltaic devices were investigated.The results show that when the atom ratio of Zn_xCd_1-xS buffer layer is Zn_0.18Cd_0.82S,the interface quality of p-n junction is the best,and the device efficiency is 3.71%,which achieves a new breakthrough of the highest efficiency for electrodeposition processed CZGSe solar cells.