Fabrication And Interfacial Modification Of Broad Band Response GaAs/Graphene Schottky Junction Solar Cell

Solar cells changed the way we harvest energy from the environment.Up to date,the most widely-used solar cell is the Si thin film solar cells.However,Si is a semiconductor with indirect band gap,low adsorption efficiency and carrier mobility,which limits the further improvement of Si thin film solar cell.Compared with Si,GaAs has a direct band gap of 1.42 eV,and high electron mobility up to 8000 cm²/?V?s?which make it a good candidate for high performance solar cells.Unfortunately,the high cost of the fabrication and growth of GaAs thin film solar cell limits the wide utilization of it.As one of the most promising candidate for the next generation low-cost high-efficient solar cells,GaAs/graphene Schottky junction solar cell has been attracted people's attentions in recent years.Although the device performance is greatly improved during the past several years,the efficiency and stability of the GaAs-graphene Schottky junction solar cell is still lower than GaAs thin film solar cell.This can be attributed to the following reasons:first,the carrier collection and transport in single layer graphene is poor.Second,interfacial carrier recombination at the GaAs/graphene interface is severe.Third,the spectrum response for GaAs/graphene solar cell is rather limited.This thesis aims at developing GaAs-graphene Schottky junction solar cells with high efficiency and stability by optimizing the fabrication process,the GaAs-graphene interface and the band structure of GaAs.The main achievements are as follows:First,the multilayer graphene?MLG?was applied in the fabrication of GaAs/graphene solar cells by a one-step transfer process,which greatly improve the carrier collection and transport properties of graphene.Moreover,a MoO_3-x/Zr_x O_y anti-reflection coating?ARC?was utilized to enhance the light absorption of the device.It was found that the layer number played a key role in the performance enhancement of GaAs/MLG solar cells.On the one hand,with the increase of the layer number of graphene,the contact resistance between GaAs and graphene is decreased,which lead to the highly-efficient carrier separation and transport in graphene and resulted in the improvement of fill factor and open-circuit voltage(V_oc)of the devices.On the other hand,when layer number of MLG was increased,the light absorbtion in graphene was increased,which caused the degradation of short circuit current density(J_sc).Under the optimized conditions,the power conversion efficiency?PCE?of device fabricated by 3-layer graphene can be greatly improved to 8.2%.The PCE of device can be further improved to 10.9%by applying a MoO_3-x/Zr_xO_y anti-reflection coating.It was revealed that the utilization of a 5nm-thick MoO_3-x buffer layer can effectively suppressed the direct bombardment of the high-energetic ions,leading to a great enhancement of J_sc from 18.2 mA/cm² to 25.9 mA/cm².Second,a metal-insulator-semiconductor solar cell was fabricated by sandwiching a monolayer assembled from alkyl thiol molecular?SAM?at the interface of GaAs/graphene.The effects of the chain length and the passivation time of the thiol molecular were systematically investigated.It was found that the hydrophilic carbon chains of the thiol molecular can effectively prevent the GaAs from oxidation caused by the wet transfer process of graphene.Additionally,the dense monolayer that assembled by thiol molecular can suppressed the interfacial carrier recombination and the reverse current caused by the emission of majority carriers.Based on the experimental results,a modified model that considering the carrier transport process in MIS solar cell was applied to further reveal the reason why the performance of the device is improved with the increase of the carbon chain length.The GaAs/SAM/graphene solar cells were stable and highly-efficient,and a PCE of 11.1%can been achieved,which made a new record for GaAs/graphene solar cells without doping and ARC.Moreover,GaAs/SAM/graphene solar cells showed superior performance at large device areas.Third,an intermediate band GaAs/graphene Schottky junction solar cell was proposed to further improve the device performance.InAs quantum dots with high size uniformity was epitaxially grown on GaAs substrate by molecular beam epitaxy.It was found that by controlling the growth rates,the Oswald's ripening process can be greatly suppressed and thus led to the improved size uniformity of the quantum dots.Based on this point,5-layer InAs/GaAs quantum dots with low atomic intermixing was achieved by utilizing a two-step space layer growth procedure.Then,the intermediate band GaAs/graphene Schottky junction solar cell is fabricated with the 5-layer quantum dots.According to the external quantum efficiency measurements,the spectrum response of the device was broadened from 870 nm to 970 nm.Due to the wide spectrum response range,the short current density of the device was improved from 15.7 mA/cm² to 16.0 mA/cm².Although the current was improved,V_oc was deteriorated caused by the introduction of the quantum dots,which together resulted in a degradation of PCE from 8.7%to 6.3%.More efforts were needed to pay in this promising solar cell.In this thesis,the effects of MLG,interfacial modification and energy band engineering on the performances of GaAs/graphene Schottky junction solar cells were studied in detail.GaAs/graphene solar cell with high PCE and wide spectrum response was achieved.This work displayed the potential of GaAs-graphene solar cell which will provide guidance for next generation solar cells with low cost and high performance.