Performance Enhancment Of CZTSSe Thin Film Solar Cells By Optimizating The Back Contact Interface

As deteriorating of environmental pollution and exhausting of fossil energy sources,it is imperative to search for clean and renewable energy source.Sustainable and environmentally benign thin-film solar cells is one of the key aspects in the development of clean renewable energy.The kesterite material Cu₂ Zn Sn?S,Se?₄?CZTSSe?promise a new generation of thin-film solar cells with the most potential for the industrialization based on adjustable band gap,high light absorption coefficient,high stability,and abundant raw material sources.It is well known that a detrimental reaction can occur between the interface of CZTSSe absorber and Mo back contact which results in MoS₂ interface layer and secondary phases.The over-thick MoS₂ layer is currently a serious and prevalent problem in CZTSSe solar cells fabricated by post annealing approaches.However,the interlayer remains intact after selenized,which may block holes from transferring to the Mo back contact,thereby deteriorating the performance of the CZTSSe solar cell.In addition,the recent current literatures also reported that the band alignment of CZTS/MoS₂ interface is actually harmful while the band alignment of CZTSe/MoSe₂ interface is beneficial for carrier extractions.Obviously,eliminating interlayer to reduce the thickness of MoS₂ and optimizing the interfacial band alignment are critical for the high-efficiency CZTSSe thin-film solar cells.There are two mainly problems at the back contact of CZTSSe thin-film solar cells:?1?The interfacial layer which introduced to reduce the thickness of MoS₂ was still found after selenized,which leads to the reduce of device performance;?2?Interfacial MoS₂ layer alters the band alignment and creates an undesirable potential barrier at the back contact interface,and ultimately deteriorates the device's photoelectric conversion efficiency.To address the above problems,we presented corresponding strategies: First,the MoO₃ thin film was deposited as intermediate layer on the back contact to prevent detrimental reactions and effectively reduce the thickness of MoS₂.Second,a layer of MoSe₂ film was prepared by in-situ growth method to optimize the energy band alignment of the CZTSSe/ MoS₂ interface.Finally,by applying this new band profile,FF and VOC increased,thereby improving the photovoltaic performance of the device.The specific work of this paper is as follows:1.Tailoring thickness of the MoS₂ interlayer and enhancing device efficiency: we prepared the MoO₃ thin film on the back contact of CZTSSe thin film cells by thermal evaporation,and the effect of MoO₃ layer with different thickness on the device performance was discussed in detail.The results demonstrated that the presence of the MoO₃ layer effectively suppresses the reaction between Mo and CZTSSe in the annealing process,and the thickness of MoS₂ is largely reduced from 209 nm to 84 nm.Furthermore,through the optimization of thickness of MoO₃ films,the kesterite CZTSSe photoelectric device with 11.37 % conversion efficiency has been achieved.2.Optimizating band alignment of the back contact interface and improving majority carriers transport: the MoSe₂ thin film is pre-fabricated in a rapid thermal processer?RTP?furnace,and the influence of MoSe₂ layer with different pre-fabricated film temperatures on the quality of the back contact and device performance were investigated in detail.The results shown that the interface layer evolved from MoS₂ to Mo S?e?₂ at the Mo back contact of CZTSSe thin film cells.The upward of valence band has rencently been achieved,leading to a new band alignment at the CZTSSe/Mo S?e?₂ interface.In consequence,the carrier separation and transmission efficiency are improved significantly,thereby improving the performance of CZTSSe solar cells,from 10.28 % to 11.46 % when the optimized pre-fabricated temperature is 350 ?.