Influence Of Cu Doping On The Physical Properties Of In₂S₃ Thin Films And Investigation Of CZTS/In₂S₃ Heterojunctions

Solar energy is a kind of clean and sustainable energy,and solar cell is a device which can transfer light into electric power.Due to the easy installation and no pollutions in the energy conversion,solar energy has attracted many reseach interests.Being little pollution,high absorbance,suitable band gap,rich material composition and high theoretical conversion efficiency up to 32.2%,CZTS-based solar cells have wide developing prospect.CdS is often used as a buffer layer in CZTS-based solar cells,which,however,is a toxic material.Being different from CdS,In2S3 is a non-toxic buffer layer material.Unfortrunely,the performance of a CZTS-based solar cell with an In2S3 buffer layer is not very good,which limites its application.This work is carried out under the background mentioned above.Besides,we also prepare CZTS-based solar cells with a buffer layer of In2S3.The main contents and results of this study are as follows:1.The electrical and optical properties of intrinsic In2S3 films,which are grown by thermal evaporation method and then annealed under different temperatures,have been investigated.We found that the band gap of In2S3 films investigated in this study is indirect.With the optical,electrical and structural properties taken into account,the best annealing condition is under Ar atmosphere at a temperature of 300? for 1 hour.2.In2S3 thin films with different concentrations of Cu incorporation have been grown by vacuum thermal evaporation method.The influences of the Cu incorporation on the structural,optical and electrical properties of In2S3 thin films have been investigated by Hall,X-ray diffraction(XRD),Raman,scanning electron microscope(SEM)and spectroscopic ellipsometry(SE).Electrical measurements indicated that the incorporation of Cu will lead to n-type doping and a decrease of resistivity of the thin films.X-ray photoelectron spectroscopy(XPS)study suggested the incorporated Cu will exist as Cu+ or Cu0.XRD analyses revealed that Cu doping will neither change the structure of In2S3 nor lead to any formation of new crystalline compounds.The refractive index n of the In2S3 thin films which was extracted from spectroscopic ellipsometry measurements showed a slight reduction in the long-wavelength region and a little enhancement in the short-wavelength region after Cu doping.It was also found that the band gap of the thin films was indirect and slightly increased from 1.90(7 at.%Cu doping)to about 2.02 eV(10 at.%Cu doping)after Cu doping.3.XPS was used to investigate the influence of growth temperatures of the buffer layer In2S3 on the band offset of the CZTS/In2S3 heterojunctions.The valence band offsets(VBO)of the In2S3 layer grown under room temperature,100?,150? and 200? were determined to be 0.28?0.28?0.34 and 0.42 eV,respectively,and the corresponding conduction band offsets(CBO)were measured to be 0.3?0.41?0.22 and 0.01 eV.Besides,we also investigated the influence of growth temperatures of buffer layer on the performance of CZTS-based solar cells.It was found that best solar cell performace with a efficiency of 0.26%can be achieved when the buffer layer was grown under 150?.When the buffer layers were grown at room temperature,100?and 200?,respectively,the efficiencies of the corresponding solar cells were 0.11%,0.008%and 0.03%.These different device performances may be attributed to the different CBO and VBO resulted from different growth temperatures.When the growth temperature of the buffer layer was up to 200?,the diffusion of Cu element may reduce the quality of CZTS/In2S3 heterojunction and led to bad device performances.