Sulfurization Of Cu₂ZnSnS₄ Thin Films,Germanium Doping And Interface Study In Solar Cells

Cu₂ZnSnS₄（CZTS）compounds with low preparation cost,rich composition and low toxicity which is a current research hotspot of high efficiency thin film solar cells.The experimental CZTS（Se）solar cell has achieved a photovoltaic conversion efficiency of 13.0%,which is far below the theoretical limit efficiency of 32.2%.Three major reasons for limiting the efficiency of CZTS solar cells are as follows:first,the poor crystalline quality of the absorber layer will lead to leakage and small short-circuit currents;second,the intrinsic antisite defects in the CZTS crystal structure lead to increased SRH recombination and decrease open circuit voltage;thirdly,the lattice mismatch and non-ideal band alignment at the front interface（CZTS/Cd S）of the absorber layer and the non-ohmic contact at the back interface（CZTS/Mo）lead to increased carrier recombination at the interface and then reduce the open circuit voltage.Investigations demonstrate that the sulfidation annealing process is the key process for the CZTS film formation,which directly affects its crystal quality and grain morphology;Ge doping replaces part of Sn in CZTS,which can reduce Sn-related deep level defects and improve the conduction band minimum,broadening the wide bandgap and absorption spectrum;the band engineering of the double interface in the Mo/CZTS/Cd S structure directly affects the interface state density and recombination mechanism.According to the above analysis,this thesis conducts specific research on the sulfide of the CZTS absorber layer,the substitution of part of Sn by Ge doping,and the Mo/CZTS/Cd S interface through experimental preparations and numerical calculations:（1）The CZTS films were fabricated by the sol-gel method to investigate the effects of vulcanisation process conditions on the crystalline properties,surface morphology,elemental composition and output parameters of CZTS films.The spin-coating-vulcanisation is performed in two cycles,reducing the number of spin-coating cycles before a single vulcanisation,which allows the partial pressure of sulphur to fully penetrate the entire precursor film during vulcanisation and promotes its grain growth.The experiment proves that this method effectively improves the grain surface flatness and grain size,and obtains a large grain layer absorber structure from the bottom through to the top.Finally,the conversion efficiency was increased from 1.66%to 3.60%by improving the sulphurisation conditions.（2）Construct a Cu2ZnGex Sn1-x S4（CZTGS）thin film solar cell model and analyse the effects of absorber layer thickness,carrier concentration,Ge/（Ge+Sn）ratio and the thickness of the Cd S buffer layer on the performance of the thin film solar cell under standard light conditions.The research found that:increasing the thickness or carrier concentration of CZTGS within a certain range can improve the solar cell performance,but over 2μm in thickness and 1×10^17 in carrier concentration will reduce the whole device performance;the CZTGS band gap value and electron affinity potential vary approximately linearly with the Ge/（Ge+Sn）ratio,when the Ge/（Ge+Sn）ratio is 0.2,i.e.CZTGS band gap value of 1.59e V,the cell conversion efficiency reaches a maximum of 15.08%.Additionally,a comparison of experimentally prepared CZTGS and CZTS thin film solar cells shows that Ge doping significantly improves device performance.（3）Construct a model of CZTS thin-film solar cells with Mo/CZTS/Cd S/Zn O stacked structure and investigate the effects of back electrode work function,CZTS/Cd S heterojunction energy band alignment and two optimized interface cell structures on cell performance.The study found that:the best efficiency was obtained for a back electrode work function of not less than 5.0 e V;a conduction band order（CBO）value of 0.1 e V at the CZTS/Cd S interface,with too large a magnitude hindering the interfacial transport of carriers.Two interface-optimized structures are proposed,introducing a backfield layer between CZTS/Mo and a low-doped interlayer between CZTS/Cd S.The Mo/BSF（p+-CZTS）/CZTS structure significantly improves the whole performance of the solar cell.The best device performance is achieved at a BSF layer thickness of 0.1μm and a carrier concentration of 10¹⁸ cm^-3,with an open circuit voltage of 0.85 V,a short circuit current of 27.96 m A/cm~2,a fill factor of 65.84%and an efficiency of 15.60%.The 5×10¹⁵ cm^-3low-doped interlayer on the CZTS/Cd S heterojunction provides effective suppression of interfacial recombination and significantly improves device performance.