Preparation And Performance Optimization Of SnS Thin Film Solar Cells By RTE

Compared with traditional crystalline silicon solar cells,thin-film solar cells have the advantages of low preparation cost,low material usage,and lightweight and flexible products,thus broadening the application fields of solar cells.Among them,tin sulfide（Sn S）as a non-toxic,stable,high abundance of new thin film light-absorbing materials,with a large absorption coefficient（>10⁵ cm）,suitable band width（1.1-1.3 e V）,low melting point（882℃）and other advantages,has great potential for development.Due to its lower melting point,Sn S is well suited for film preparation by rapid thermal evaporation with low energy consumption and large area continuous production.However,the current rapid thermal evaporation still has the following problems:limited distance between the evaporation source and the substrate,serious temperature crosstalk,and easy back-evaporation.Therefore,this will lead to a limited film growth time,which restricts the grain growth of Sn S films;high vacuum deposition promotes the gas phase decomposition and back-evaporation of Sn S,making the films S-poor and thinning.S-poor leads to the generation of deep energy level defects such as vacancies（V_S）,which will promote carrier compounding.In addition,the higher conduction band position of Sn S leads to lower device V_oc.The preparation of high quality Sn S films and the selection of n-type layers with matching energy bands are key to the preparation of high performance solar cells.To address the two problems mentioned above,this thesis focuses on the following two aspects:（1）We first designed a suitable rapid thermal evaporation（RTE）apparatus for preparing Sn S thin films and explored the process for preparing high-quality Sn S thin films.First,we prepared dense,flat and well-crystallized Sn S films by adjusting the high vacuum rapid evaporation deposition and subsequent pressurized in-situ annealing time.The optimal process conditions:deposition temperature of 550°C and optimal holding and annealing time of 200 s.The Sn S films were characterized by X-ray diffraction（XRD）,Raman and X-ray photoelectron spectroscopy（XPS）to confirm the preparation of pure-phase,well-oriented Sn S films with a complex stoichiometric ratio.We constructed Sn S thin film solar cells using common Cd S as the n-type buffer layer and obtained photovoltaic devices with an open circuit voltage(V_oc)=0.31 V,a short circuit current(J_sc)of 21.79 m A/cm²,and a fill factor（FF）of 41.47%.By comparing the material,optical and electrical properties of Sn S and Cd S,we analyzed that the reason for the low performance of the devices is that the Sn S material easily forms a bad"cliff"type energy band structure at the interface with Cd S,which causes electrons and holes to compound at the interface and seriously affects the V_oc enhancement.Therefore,our next work focuses on the selection of suitable n-type layers.（2）Zn Mg O films are non-toxic,with large forbidden band widths and adjustable energy bands are common PV n-type materials.We optimize the Zn Mg O energy band structure and photovoltaic properties by adjusting the Mg/（Zn+Mg）ratio.Hall test data of different Zn Mg O thin film samples show that the resistivity increases and the carrier concentration decreases with increasing Mg/（Zn+Mg）doping concentration.When the ratio of Mg/（Zn+Mg）in the Zn Mg O buffer layer is 0.24,the carrier concentration is 1.68E+18 cm^-3 and the valence band position is-3.62 e V,which forms a"spike"energy band structure with a CBO value of 0.18 with Sn S at-3.80 e V.The ZMO-2 device achieves the highest efficiency of 4.43%with J_sc,V_oc and FF of22.13 m A/cm²,0.390 V and 51.32%,respectively,by suppressing the complexation at the Sn S/Cd S interface.The main reasons for the improved performance of the device are,first,the wider band gap of the ZMO-2 buffer layer,which results in a reduced parasitic absorption of photons in the short-wave region and thus an improved response to light;and second,the enhanced conduction band offset of the Zn Mg O buffer layer relative to Cd S,which better matches the energy band structure of Sn S,the"spike"type CBO,which greatly enhances the open-circuit voltage and fill factor of the device,while achieving a low compound of light-generating carriers at the Zn Mg O/Sn S buffer layer heterojunction interface.