Improved Performance Of Cu（In,Ga）（S,Se）₂ Solar Cells By Improving Heterojunction Interface Based On In-doped CdS

Copper indium gallium sulfide selenide[Cu（In,Ga）（S,Se）₂,CIGSSe]is a direct bandgap semiconductor material.It has attracted much attention due to its adjustable bandgap,high absorption coefficient,and good stability.Significant progress has been made in the photoelectric conversion efficiency of CIGSSe solar cells through engineering the passivation of surface defects in the CIGSSe absorber and the engineering the band structure of heterojunction interface.Hence,optimizating of the heterojunction interface is still an urgent need for high performance CIGSSe solar cells.The heterojunction interface of CIGSSe solar cells is mainly composed of a CIGSSe absorber and buffer layer.At present,researchers mainly focus on the regulation of the CIGSSe absorber layer,and the buffer layer,as an important part of the heterojunction interface,regulates the buffer layer.It is also important to improve the quality of the heterojunction interface.It is worth noting that the world record efficiency of 23.35%was also obtained due to the optimization of the buffer layer by the researchers.Thus,further optimize the band alignment of the CdS and CIGSSe,and improve the carrier separation capability at the CdS/CIGSSe interface are expected to achieve breakthroughs in the efficiency of CIGSSe devices.Doping CdS with In is endorsed as an attractive approach because it can simultaneously modify the energy levels and modulate the CdS carrier density.In addition,In-doped CdS is easy to obtain because In and Cdhave a similar ionic radius and the In_Cd formation energy is low.However,how to achieve uniform doping of In and avoid the impurity generated during the In doping process is currently a challenge.Based on the above analysis,this paper mainly carries out the following work:The first part of the work:A thin layer of In2S3 was deposited on the surface of CdS by thermal evaporation,and then In was diffused into CdS by thermal annealing（two-step method）.We researched the In₂S₃ thickness,annealing temperature and annealing time on the CIGSSe device performace,and determined the optimal processing conditions to be 15 nm/120℃/5 min.X-ray（XRD）and sweeping electron microscopy（SEM）results show that the In element successfully entered into the CdS,and the surface morphology of CdS did not change significantly after doping.In order to explore the reason why indium doping improves the device efficiency,the transmission spectrum analysis of In-doped thin films was carried out.The test results show that the In-doped CdS film has better transmittance,and the optical band gap of the film is improved,which can better reduce the light absorption of the buffer layer in the short wavelength region.The In-doped film has a wider depletion region and a larger built-in potential,which is conducive to carrier diffusion and collection,and reduces the recombination at the heterojunction interface.The PCE was increased from 12.5%（un-doped CdS device）to 15.2%（In-doped CdS device）.The efficiency of the In-doped device was increased by about 22%compared with the undoped device.In the first part of the work,it is proved that the In-doped CdS buffer layer is effective in improving the device performance.However,heat treatment cannot precisely control the doping amount of In.And at the same time,it will lead to the existence of complex phase compositions（including:CdS,In₂S₃,In:CdS）at the interface.In addition,the thermal evaporation deposition of In₂S₃ requires high cost and complicated preparation process operations.The second part of the work:we demonstrated a novel sequential CBD strategy,in which the In source is sequentially slowly added in the reaction solution during the CdS deposition process.In this condition,only one drop of In source is involved in the real-time deposition and the concentration of In ions involved in the real-time deposition is dramatically reduced.Thus,the In₂S₃ precipitation reaction is significantly inhibited.By utilizing this strategy,a uniform and dense In:CdS buffer layer with highly increased In content is successfully prepared.The approximate flat-band CBO between CIGSSe and CdS（0.02 e V）changes to a moderate spike CBO（0.29 e V）after In doping.It is more efficient to reduce the CIGSSe/CdS interface recombination and is considered as the more favorable band alignment at the CIGSSe/CdS interface.The interfacial activation energy before and after In doping decreases from 196me V to 118 me V.It indicates that the doping of In reduces the number of interfacial recombination centers and inhibits the interfacial recombination of the heterojunction.As a result,an In:CdS-based CIGSSe device yields a champion efficiency of 16.4%,with a V_OC of 670 m V and an FF of 75.3%.Both the PCE and V_OC are among the top values in nonhydrazine solution-processed CIGSSe solar cells,indicating the In doping as a promising strategy to improve CIGSSe devices.