Construction And Optoelectronic Properties Of High-performance Photovoltaic Devices Based On Graphene And Silicon Nanostructures

Graphene is an attractive candidate for the application as transparent electrode infuture optoelectronic devices due to its extraordinary optical and electrical properties.Silicon (Si) nanostructures, especially Si nanoarray structures, are of tremendousinterests in new-generation solar cell applications, because of their outstandinglight-trapping and excellent carrier transport abilities. Construction of high efficiencygraphene/Si solar cells with low cost and stable performance has attracted significantattention recently.In this thesis, we conducted a systematic study on the high-performance photovoltaicdevices based on graphene and Si nanostructures. The main investigations are focusedon the synthesis of high-quality graphene and various Si nano (micro) structures withexcellent light-trapping capability, modification and passivation of graphene and Si, aswell as construction of high-performance graphene/Si solar cells by using of electronblocking layer. Additionally, the heterojunction photovoltaic devices based on Sinanowire (SiNW) array were also fabricated and their photovoltaic and photoresponseproperties were studied. The main results are summarized as follows:1) High-quality graphene was synthesized by chemical vapor deposition (CVD)method, and HNO3and AuCl3doping were employed to tune its conductivity and workfunction. SiNWs and various Si nanoarrays (microarray) with excellent light-trappingabilities were prepared by using CVD, metal-assisted chemical etching and reactive ionetching methods. The carrier recombination activity at Si surface was effectivelysupressed by using a passivation method including CH3and Pt nanoparticlesmodification.2) We fabricated monolayer graphene/SiNW array Schottky junction solar cells andextensively studied the effect of device configuration on the photovoltaic characteristics.A maximum PCE of2.15%was achieved through filling the interspace of SiNW arraywith graphene suspension. Furthermore, solar cells based on monolayer graphenenanoribbon/multiple SiNWs were also constructed. By enhancing the doping level ofSiNWs, an optimal efficiency of1.47%was attained.3) We proposed a strategy to increase the effective junction area between grapheneand Si by using Si nanohole (SiNH) array. The effects of Si passivation, graphene layernumber, as well as HNO3doping time on the devices photovoltaic performance were studied. The PCEs of6.85%and7.65%were achieved for SiNW array/graphene andSiNH array/graphene devices, respectively, by optimizing the device architectures.What is more, the PCEs were substantially enhanced to8.71%and10.30%, respectively,through inserting an organic electron blocking layer between graphene and Si, whichcan lower the probability of electron diffusion from Si to graphene. Furthermore,photovoltaic devices based on graphene/Si micro-hole array were fabricated. The holesdepth and corresponding light-trapping ability were studied in order to increase thedevice photovoltaic performance. The maximum efficiency was enhanced to as high as10.40%with excellent air stability through doping graphene with AuCl3.4) Three-dimensional core-shell heterojunctions based on carbon quantum dots(CQDs) and SiNW array were first prepared, and some key parameters of theheterojunctions including rectification ratio, turn-on voltage, ideality factor as well asbarrier height were studied. The maximum PCE of9.10%was achieved by optimizingthe CQDs layer number. In addition, the heterojuncitons can function as self-poweredvisible light photodetectors with high-sensitivity and high-speed. Some key parametersrelated to photodetector were also investigated.5) We successfully fabricated p-CdTe nanoribbon/n-SiNW array heteojunctionoptoelectronic devices and investigated some key parameters of the heteojunction. Dueto their matched energy bandgap and outstanding light-trapping ability of SiNW array,the heterojunctions can effectively absorb most incident light ranging from visible tonear-infrared (NIR), giving rise to an PCE of2.3%, much higher than that of planar Sibased device. What is more, the heterojunctions can serve as high-speed self-poweredphotodetectors operated in the visible to NIR range with good stability.The main innovations of this thesis are:(1) The high-performance graphene/Si solarcells were achieved through tuning the conductivity and work function of graphene,reducing the surface carrier recombination velocity of Si, enhancing the light-trappingability of Si nano (micro) structures, increasing the effective junction area ofgraphene/Si as well as lowering the probability of electron diffusion from Si tographene.(2) The high-performance heteojunction optoelectronic devices based onSiNW array were attained by taking advantage of the excellent light-trapping ability ofSiNW array, the matched energy bandgap of heterojunction and the unique chargetransport property of three-dimensional core-shell structure.