Suppression Of Surface And Bulk Defects In CZTSSe Absorber By Air Annealing Treatment

Cu₂Zn Sn（S,Se）₄（CZTSSe）is an ideal light-absorbing layer material for thin-film solar cells,with the advantages of abundant constituent elements and high absorption coefficient.However,the current efficiency record of CZTSSe thin-film solar cells is only 13.8%,which is much lower than the conversion efficiency record of Cu（In,Ga）Se₂（CIGS）（23.35%）with similar solar cell structure.The main reason hindering the efficiency improvement of CZTSSe solar cells is the open-circuit voltage(V_OC)loss.The V_OCloss can be evaluated by the Shockley-Queisser theoretical limit.The current V_OC of CZTSSe solar cells is only about 60%of the Shockley-Queisser limit,while their short-circuit current density(J_SC)and fill factor（FF）have reached nearly 80%of the theoretical limit.Therefore,reducing the V_OC loss of CZTSSe solar cells is the key to improve their photoelectric conversion performance.Comparisons between CIGS and CZTSSe show that,the CIGS absorber layer usually has a monolayer grain structure from bottom to top,which is conducive to reduce the amount of grain boundaries thus to improve the carrier transport efficiency and further the photovoltaic performance.In contrast,the absorber layer of most CZTSSe solar cells prepared by solution and vacuum methods is usually a bilayer structure with large grains on the top and fine grains and voids at the bottom.There are a large number of grain boundaries in this structure,which cause severe carrier combination,reducing the V_OC of the device.On the other hand,the severe V_OC loss of CZTSSe solar cells is directly related to the defects in the bulk and surface of the absorber.The interface between absorber and buffer layer is an important part for electron and hole pair separation and collection,and the defects on the absorber surface will cause carrier bonding,while poor crystallinity of bulk will bring more grain boundaries as carrier traps or effective compound centers,which has serious limitations on carrier lifetime and carrier mobility.Aiming at suppressing the defects in the absorber and improving the bulk crystallinity,the following two parts of work are carried out in this dissertation:（1）Air-annealing on CZTSSe absorber for interface and grain boundary passivation:In this part, the absorber samples were annealed in air under different annealing conditions to investigate the degree of oxidation to produce a suitable oxygen passivation layer on the absorber surface.The effects of air-annealing on the absorber were analyzed by structural and electrical characterization.Furthermore,the effects on the energy band alignment at the heterojunction interface and the photovoltaic performance of CZTSSe solar cell devices before and after air-annealing were investigated.and a deep-reason analysis was performed in terms of carrier collection as well as the change in defect concentration.It is found that the potential difference between the grain boundary and the grain interior of absorber increases,the defect concentration decreases after air-annealing,the carrier collection efficiency increases in the upper absorber with suppressed interface recombination,and the energy band between the absorber layer and the buffer layer matches better.Compared with the photoelectric conversion efficiency of 9.74%for the untreated device,the photoelectric conversion efficiency of the optimized air-annealed device is increased to 12.04%.（2）CZTSSe precursor air-annealing aiming to improving absorber crystallinity:Although the previous work effectively improved the V_OC and device efficiency,the characterization results revealed that the improvement was mainly in the upper half of the absorber.Therefore,there is still a large number of grain boundaries still existing in the absorber.In this part of our work,we optimized the crystallinity of CZTSSe absorber to reduce the amount of grain boundaries and defects in the absorber to suppress the recombination of carrier collection.The specific process is to anneal the precursor films in air,explore different annealing conditions,as well as the effects of the treatment on the changes on precursor morphology,absorber layer crystallinity,and final device performance.It is found that the annealed precursor films become more porous.Such a structure is more favorable for the selenium vapor to enter the precursor films during the selenization process,resulting in more adequate grain growth and thus the better crystalized absorber layer.This is more favorable for the photo generated carrier collection.The V_OC of CZTSSe devices fabricated under the optimized conditions has been increased from 459.57 m V to 492.72 m V,and the highest photovoltaic conversion efficiency of 11.83%was achieved.