Construction And Performance Study Of Graphene Nanowalls/silicon Schottky Junction Photovoltaic Devices

Graphene,a new type of two-dimensional crystal material,is formed of sp2 hybridization of carbon atoms.It is suitable for application in heterojuntion solar cells due to its excellent mechanical,thermal,optical and electrical properties.The fabrication process of graphene/silicon Schottky junction solar cells is very simple,which is expected to be a new generation of low-cost and high-performance solar cells.However,currently graphene/silicon solar cells have poor photovoltaic performance.Moreover,the mechanical stropping method,the silicon carbide epitaxy method,the oxidation-reduction method,and the chemical vapor deposition method(CVD)are difficult to realize the growth of large-scale,low-cost,and high-quality graphene.Comparing to CVD method,plasma-enhanced chemical vapor deposition(PECVD)method has many advantages including low temperature growth,rapid deposition,low energy consumption,etc.Therefore,in this thesis,we employed customized PECVD system to rapidly grow high-quality graphene nanowalls(GNWs)and investigated the application of GNWs in silicon-based heterojunction solar cells.In addition,we have also systematically studied how to improve the photovoltaic performance of the graphene nanowalls/silicon(GNWs/Si)solar cells.The main contents are summarized as follows:(1)We developed a simple customized PECVD system to directly grow graphene nanowalls on planar silicon and fabricated GNWs/Si Schottky junction solar cells.The method of GNWs directly grown on silicon not only can simplify the solar cell manufacturing process,but also can avoid the damage of graphene caused by transfer process.The effects of different growth parameters(including methane/argon flow ratio,discharge power,growth temperature and deposition time)on graphene nanowalls/silicon solar cells were investigated by Raman spectroscopy,scanning electron microscope(SEM),Hall effect test and photovoltaic characteristic test.The analysis results showed that the optimal growth parameters were CHa/Ar flow rate ratio of 10 sccm/40 sccm,discharge power of 880 W,growth temperature of 850?and deposition time of 120 s.We achieved a photovaltaic conversion efficiency(PCE)of 3.36%.In addition,the plasma groups of graphene nanowalls grown on silicon substrates were diagnosed by optical emission spectroscopy.The reaction mechanism of graphene nanowalls was studied by Raman spectroscopy,SEM and optical emission spectroscopy analysis.(2)We used the pyramid and inverted pyramid silicon with excellent trapping structures as the absorption layer of GNWs/Si solar cells and addressed the problem of the significant light loss caused by reflection of the planar silicon.According to the optimal growth parameters of GNWs,we employed PECVD method to directly deposite GNWs on the pyramid and inverted pyramid silicon and fabricated solar cells.The PCE increased from 3.36%to 3.87%and 4.05%,respectively.In addition,we investigated the effects of different doping materials and titanium dioxide(TiO2)as an antireflective layer on the graphene nanowalls/inverted pyramid silicon solar cells.After experimental optimization,the PCE of 7.20%,6.80%,6.42%and 6.34%were obtained through the use of HNO3,SOC12,HCl,H2O2 as a doping material and TiO2 as an antireflective layer,respectively.(3)The optimal growth parameters of GNWs grown on copper were investigated by customized PECVD system,which were growth temperature of 850°C,discharge power of 880 W,CH4/Ar of 10 sccm/40 sccm and growth time of 60 s.The copper-based graphene nanowalls were successfully applied to the silicon-based heterojunction solar cells by wet-transfer method,and the PCE of 3.20%was obtained.On this basis,a layer of P3HT was introduced as the interface layer of the graphene nanowalls/silicon solar cells.P3HT has an ideal band structure,which can effectively block electron transport,suppress carrier recombination at the interface of the device and increase the Schottky barrier.The performance of solar cells was also optimized by adjusting the thickness of P3HT layer.When the thickness of P3HT layer was 12 nm,the performance of solar cells was excellent,and the PCE increased from 3.20%to 6.65%.After using PMMA as an antireflective layer on the optimized device,we achieved the PCE of 8.26%,which was the highest PCE of GNWs/silicon solar cells prepared by PECVD system without chemical doping.By using PMMA as an antireflective layer,the stability of the device was also improved,and the PCE of the device still remained 66%after 28 days.