Preparation Of High Quality Cu₂ZnSn（S,Se）₄ Film By Solution Method And Study Of The Photovoltaic Device Performance

Photovoltaic technology has attracted more and more attention in recent years.The development of cheap,safe,stable and efficient solar cells has been a hot topic.Thin film solar like Cu（In Ga）Se₂（CIGS）solar cell,which has comparable photoelectric conversion efficiency with crystalline silicon solar cell,is characterized by raw material saving,low cost and low pollution.It has a broad application in such aspects as flexible battery,photovoltaic building integration and wearable devices.However,as indium and gallium are rare elements,the large-scale application of CIGS solar cell is difficulty.As an alternative to CIGS,the kesterite material Cu₂Zn Sn（S,Se）₄（CZTSSe）not only has the advantages of high absorption coefficient,stability and low cost,but also has the additional advantages of high element abundance,which is a prominent aspect for large-scale application.The CZTSSe material mainly has two preparation methods:vacuum method and solution method.The solution method for preparing CZTSSe material has the advantages of low equipment cost,flexible and simple operation and is suitable for mass production.Therefore,it is of great significance to develop efficient and stable CZTSSe battery based on solution method.Based on this goal,we are committed to developing an environmentally friendly solution system suitable for large area preparation and systematically studying the photoelectric performance of CZTSSe film and related battery devices.The research results are as follows:（1）Two solvent systems were optimized for CZTSSe preparation:Ethanedithiol and ethylenediamine based system and DMSO based system.By adjusting the concentration of the solution,the film thickness and the parameters of the spin coating,a homogeneous precursor film was obtained.By optimizing the corresponding film parameters such as selenization time,temperature and heating rate,an absorber layer with good crystallization was obtained.The secondary phase and defects are effectively controlled by adjusting the proportion of metal elements.（2）in order to have a deeper understanding of the CZTSSe material,a self-healing phenomenon was systematically analyzed and studied.The efficiency improvement of the self-healing phenomenon is mainly manifested in the improvement of the open-circuit voltage and the filling factor.By storing in different atmosphere,we found the improvement is deriving from the device itself but not the external influence.Secondary ion mass spectrometry reveals the improvement is not caused by the diffusion of Na,Cd,Cu elements.The stable fluorescence,CV,DLCP show that the deep level and shallow level defects decreased after storage.The results of XPS analysis show that the bending of energy level increases after storage,which is consistent with the simulation prediction.The above analysis shows that the improvement of self-healing is caused by the defects decrease of the absorber layer,which reduces recombination and increases the carrier migration ability.This guides us to focus our following work on improving the quality of the absorber layer.（3）The mechanism of multilayer crystallization in DMSO solution system was revealed for the first time,and a simple and effective method to inhibit the formation of multilayer crystallization was developed.By introducing an isolating layer in Mo/CZTSSe interface,and the growth direction of the crystal was regulated which effectively eliminated the double layer crystallization.After regulating,a single layer large grains absorber layer was obtained.Through systematic experiments we found that the double layer crystal is related to Mo electrode.The reaction between Mo and CZTS in the precursor film introduce nucleation at the Mo/CZTSSe interface,so that crystallization occurred bi-directionally,resulting in double layer crystallization.By introducing an isolating layer to block the reaction,the crystallization on the back interface was effectively prevented.Raman,PL showed that the single layer with isolated layer had less second phase and smaller tail state.The characteristics of CV and DLCP,extended interface capacitance,transient photocurrent/photovoltage show that there are fewer defect states,less recombination,stronger charge collection ability,and wider depletion width in the regulated absorber layer.And the regulated solar cell achieved a 11.68%certified efficiency.（4）After obtaining high quality absorber layer preparation process,a preliminary attempt was made for the future industrialization:large area absorber layer films were prepared by electrostatic spraying.An independently designed automatic spraying device was built by ourselves.By adjusting the spray voltage,flow rate,distance between the nozzle and the film surface,a large area precursor film with uniform morphology and uniform thickness was obtained.XRD shows that there is no obvious phase difference of precursor film prepared by electrostatic spraying and the spin coating,corresponding absorber film also has good crystallization and no obvious secondary phase.The precursors fabricated by electrostatic spraying were used to prepare solar cell which has a photoelectric conversion efficiency of 10.2%,which showing the utilization potentiality of the electrostatic spraying method.