Graphene/Silicon Heterojunction Based High-performance Solar Cells

Graphene has aroused great attention due to its fascinating optical and electrical properties since it was firstly reported in 2004. The high optical transmittance, high conductivity, and controllable fabrication method made it potential as the new generation transparent conductive film, replacing traditional indium tin oxide(ITO). In photovoltaics, graphene/silicon heterojunction solar cells not only inherit the advantages of traditional Si based solar cells, with potential high efficiency realization probability, but also simplify the fabrication processes, avoiding the high temperature diffusion and ion implantation etc., which cost lower. In addition, based on the excellent mechanical flexibility of graphene, the flexible graphene/silicon heterojunction solar cells will also be the research focus for light, portable, and wearable use in the future. Given the reasons mentioned above, the photovoltaic devices based on graphene/silicon heterojunction were studied in this thesis, and the main research content includes:(1) High-quality and large-area graphene was successfully fabricated by chemical vapor deposition(CVD) method, with optimizing the parameters during the graphene growth process;Layer-by-layer transferring method was adopted for the obtainment of few-layer graphene sheets, which were employed in the graphene/silicon heterojunction solar cells.(2) From the viewpoint of interface engineering, the transition metal oxide(MoO3) with high work function property was introduced as the interfacial layer in the graphene/silicon heterojunction solar cells. Surface charge transfer doping(SCTD) happens at the interface between MoO3 and n-type silicon due to the large work function differences. Large amounts of holes were injected from MoO3 to n-type silicon, resulting in the formation of p-type inversion layer at the surface of n-type silicon. The formation of inversion layer leads the enhancement of effective barrier height, which reduces the recombination of photo-generated carriers and improves the device performance. Based on SCTD method, in combination with additional device optimization by graphene doping and by a polymer anti-reflection layer, a high power conversion efficiency of 12.2% was finally obtained.(3) Flexible graphene/silicon heterojunction solar cells were fabricated based on thin silicon films, which was obtained by the alkali solution etching of bulk silicon, accompanied by the reactive ion etching(RIE) method. However, with the thickness decreases, the optical absorption loss becomes obvious, especially in the long wavelength region, due to the low absorption coefficient of silicon. Therefore, silicon nanowires(SiNWs) were fabricated on the thin silicon film to enhance the light absorption, which enhanced the device performance.