Chemical-diping-free Graphene-silicon Heterojunction Solar Cells

The fascinating properties of graphene (Gr), such as high carrier mobility, high optical transparency and tunable work function, make it an attractive candidate for application in photovoltaic devices. Recently, graphene-silicon (Gr/Si) heterojunction solar cell based onSchottky junction has attracted much attention as one of promising candidate for low-cost photovoltaic applications since the Gr/Si solar cell can be fabricated through a low cost process by simply transferring Gr film onto Si substrateat room temperature. However, the power conversion efficiency of the Gr/Si solar cells is greatly limited by thelow Gr-Si Schottky barrier height, the serious carrier recombination at Gr-Si interface and the high sheet resistance of CVD-grown Gr film.Chemical doping has been recognized as the most effective approach to enhance the performance of Gr/Si solar cells since it could increase the Gr-Si Schottky barrier height and decrease the Gr sheet resistance through modulating the Femi level of the Gr film. However, it should be noted that the chemical doping effect of Gr film is generally not stable and therefore the chemically-dopped Gr/Si solar cells suffer severe efficiency degradation, which is not suitable for practical application. Excluding the chemical doping, the efficiencies of pristine monolayer Gr/Si solar cells reported by various research groups are actually not higher than 4%.This dissertation aims at developing chemical-doping-free Gr/Si solar cells with high efficiency and improved stability through optimizing the Gr-Si interface and improving the electricalproperty of the Gr film. The important results achieved in this dissertation are listed as follows:(1) We have reported a new structure of Gr/Si solar cells by introducing a graphene oxide (GO) interlayer to engineer the Gr/Si interface for improving the device performance. It is found that the Voc of Gr/GO/Si solar cells strongly depend on the thickness of GO interlayer, which reaches a maximum value at an optimum thickness. The GO interlayer can modify the electronic states at the Gr/Si interface from the deep levels to shallow levels, and meanwhile reduce the density of interface states. Thus, the power conversion efficiency of pristine Gr/Si solar cells can be increased by about four times, with a maximum value of 6.18%. Besides, it is clarified that our chemical-doping-free Gr/GO/Si solar cell performs much more stable than the previously reported chemically doped Gr/Si solar cells.(2) We have introduced an alumina oxide (Al2O3) interlayer to optimize the Gr-Si interface and improved the performance of the Gr/Si solar cells. It is found that the performance of the Gr/Al2O3/Si solar cells is strongly dependent on the resistivity of the Si substrate and the Gr/Al2O3/Si solar cells fabricated with 1-3Ω·cm Si substrate actually work basing on p-n junction. The p-n junction in the Gr/Al2O3/Si solar cell is formed between the n type Si substrate and the p type strong inversion layer generated near the Si surface because of the fixed negative charge existing at the Al2O3-Si interface. As a result, the dark current of the solar cell is largely reduced and the Voc of the solar cell is effectively enhanced. Meanwhile, the field passivation effect caused by the strong inversion suppresses the interface recombination of the minority carriers and enhances the short circuit current density (Jsc) and fill factor (FF) of the solar cell. Hence, the performances of the solar cells are largely improved and a maximum efficiency of 8.44% has been achieved for the Gr/Al2O3/Si solar cell with a device-active area of 9 mm2.(3) We have demonstrated that electric field doping can be used to tune the work function of the Gr film and improve the photovoltaic performance of the Gr/Si solar cell. The electric field can be applied though connecting the Gr/Si solar cell to an external power supply or by polarizing a vinylidene fluoride-tetrafluoroethylene copolymer P(VDF-TrFE) ferroelectric layer integrated in the Gr/Si solar cell. It is found that the efficiency of the Gr/Si can bedoubled through utilizing this strategy.(4) We have developed a new transparent conductive electrode by combing Gr film with silver nanowires (AgNWs) network soldered by graphene oxide (GO) flakes. The resultant Gr-GO-AgNWs hybrid film exhibits excellent optoelectronic property, which has a sheet resistance of 9Ω/□ and a direct-current to optical conductivity ratio of 168. Furthermore, we have used this Gr-GO-AgNWs hybrid film to fabricate a novel structural Gr/Si solar cell with the AgNWs network acting as the buried contacts for carrier collection. An efficiency of 8.68% has been achieved for the Gr-AgNWs-Si solar cell with a device-active area of 16 mm2, which is the highest efficiency for chemically-doped-free Gr/Si solar cell reported by now.