Studies On Fabrication And Application Of Amorphous Silicon Germanium Thin Film Solar Cells

Since the beginning of 21st century,global energy issues have become increasingly prominent.Solar energy is the best choice for green energy with its clean and efficient characteristics.As one of the silicon-based alloy,amorphous silicon germanium（a-SiGe:H）material has received extensive attention in recent years due to its advantages of high absorption coefficient and flexibility to adjust the bandgap according to the needs of spectral absorption.In this thesis,plasma enhanced chemical vapor deposition（PECVD）was applied to systematically study the deposition processing of a-SiGe:H thin films.After obtaining high quality a-SiGe:H material,basic a-SiGe:H thin film solar cells were prepared with p-i-n configuration.The investigation of intrinsic absorber,doped layer,back reflector,interface between different materials and various device structures were studied in details to improve the power conversion efficiency（PCE）of a-SiGe:H thin film solar cells.Consequently,this thesis mainly carried out the following studies:First,the deposition parameters for a-SiGe:H material such as pressure,power,hydrogen dilution and germane ratio were optimized.The influences of these parameters on material characteristics such as deposition rate,activation energy,Ge content,bandgap,dark-conductivity,photo-conductivity and photosensitivity were investigated.After finishing these processing optimization studies,device-quality grade a-SiGe:H material was obtained and assembled as an absorption layer into thin film solar cells.Different device structures were designed from the perspective of absorption layer thickness,minimum bandgap,stepped bandgap and graded bandgap,the influences of these parameters on PCE were studied.Second,the back reflection layer and the interface layer of a-SiGe:H thin film solar cells were studied.The metal and metal composited materials such as aluminum（Al）,silver（Ag）,Ag/Al,silver/chromium/aluminum（Ag/Cr/Al）on solar cell performance were investigated,respectively.Hydrogenated indium oxide（IOH）was added between the n-layer and the above metal back reflection layers,the resistivity,mobility and carrier density of the IOH were adjusted by different water partial pressure.It indicated that the back reflection layer structure of IOH/Ag/Cr/Al can effectively improve the short-circuit current density(J_sc)and the fill factor（FF）of solar cell compared to other single material and structure.Meanwhile,amorphous silicon（a-Si:H）and silicon oxide（SiO_x）were used as buffer layers at the p/i and i/n interfaces.It was seen that the air exposure treated a-SiO_x performs better than the a-Si:H buffer layer material obtained by vacuum deposition to improve a-SiGe:H solar cell efficiency.Third,it was found that SiO_x material can be used as the p-and n-layers of a-SiGe:H thin film solar cell,with the method of adjusting the oxygen content in the SiO_x material to obtain excellent optoelectric properties,the PCE of solar cell can be remarkably improved.In this thesis,innovative designs for p-and n-layers was performed with 12 structures from the perspective of single layer,double layer,graded bandgap layer and their combinations,device fabrication and comparative analysis were conducted.It was found that the electrical properties of p-SiO_x materials can be tuned by adjusting the gas flow ratio of CO₂/SiH₄.The design of double layers with different bandgap p-SiO_x structure can significantly improve the open-circuit voltage(V_oc)and J_sc of the device.The V_oc and J_sc reached 750 mV and 17.75 mA/cm²,respectively.On this basis,n-SiO_x double layers structure with graded bandgap were fabricated and the performance of solar cell was further improved mainly due to the enhancement of J_sc and FF,finally high efficiency a-SiGe:H thin film solar cell was obtained with 9.35%.