Fabrication And Performance Optimization Of Graphene/GaAs Schottky Junction Solar Cells

Solar cell technology is one of the most promising technologies among various new-energy technologies.Nowadays,owing to considerable efforts and mature process,silicon-based solar cells have occupied 90%of the market share.However,high-cost and complex process of the silicon-based solar cells have become a big challenge.In recent years,a new type of solar cells based on two-dimensional materials has aroused people's attention.Graphene,as a new carbon material composed of single-layer carbon atoms,has attracted research interest around the world owing to its excellent photovoltaic performance.Graphene/semiconductor Schottky junction solar cells owing to their simple fabrication process are considered to be a promising device.This thesis is focused on the fabrication and performance optimization of the graphene/gallium arsenide solar cells.The main conclusion includes:(1)By choosing and optimizing the back electrode of the Gr/GaAs solar cell and controlling the layer number of graphene,the photovoltaic conversion efficiency of the Gr/GaAs solar cell gets increased.The annealed gold(Au)is a suitable back electrode for the Gr/GaAs solar cell in comparison with In-Ga and un-annealed Au.It is shown that employing Ge/Au as the back electrode can lead to a lower contact resistance and a higher open circuit voltage.As a result,the photovoltaic conversion efficiency of solar cells increases from 2.4%to 4.7%.Eventually,it has been demonstrated that adopting single-layer graphene to prepare the cell can get the best performance.(2)Inserting a 2,2',7,7'-Tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9'-spirobiflu-orene(Spiro-OMeTAD)film between graphene and GaAs as an electron-blocking/hole-transporting layer can lead to a higher efficiency by reducing the reverse saturation current and decreasing the recombination of the carrier.By optimizing the thickness of Spiro-OMeTAD,it has been demonstrated that too thin Spiro-OMeTAD film can hardly cover the surface of GaAs,which leads to a lower efficiency.When the thickness of the Spiro-OMeTAD film is too thick,serious adsorption of light and poor conductivity cause a worse performance.The cell efficiency reaches 7.7%when the thickness of the Spiro-OMeTAD film is controlled between 34 nm and 39 nm.(3)Graphene nanowalls(GrNWs)were prepared by plasma enhanced chemical vapor deposition(PECVD)system on the Cu substrate.Various characterization methods were adopted to explore the effect of deposition time on the growth of graphene nanowalls.Moreover,the graphene nanowalls/GaAs Schottky solar cells were fabricated and investigated.It is shown that the performance of the GrNWs/GaAs solar cells is best with an efficiency of 3.93%when the growth time is set at 60 s.In addition,the photovoltaic conversion efficiency of GrNWs/GaAs solar cells reaches 4.9%after chemical doping and employing TiO2 film as an antireflection layer.The efficiency decreases by 10%stored in the air for two weeks.