Study Of Highly Efficient Solar Cells Based On Flexible Graphene / Si Heterojunctions

Since it was firstly reported in 2004, graphene has drawn a lot of attention from scientists and engineers becasuse of its fascinating physical properties. Due to its high conductivity, high transparency, mechanical flexibility and scalable growth by chemical vapor deposition(CVD), it has been considered as a promise material in many fileds, such as transparent conductive electrodes, flexible electronics and optoelectronics. In the solar cell sector, the flexible solar cells which possess light-weight, high-efficiency, wearable and low-cost are the trend of development in the future. But the conventional p-n junction based solar cells still dominates the large portions of commercial market because of its material abundance, mature techniques and steady performance. However, they are heavy and fragile due to the large thickness, which are not suitable for integrating with infrastructures of various shapes and sizes where lightweight and flexibility are important. Therefore,the flexible crystalline silicon(c-Si) based solar cell is rising and appealing. Given this reason, flexible graphene/silicon heterojunction solar cells were investigated in this thesis, which comprises of three parts: Introduction, Controllable preparation of materials and Fabrication and measurement of devices.Introduciton: the development of graphene/silicon heterojunciton solar cells and the flexible crystalline silicon based solar cells are reviewed, respectively.Controllable preparation of materials: graphene was grown by CVD method using catalyst Cu foils as substrates. Tuning the parameters of synthesis such as the ratio of CH4:H2, the annealing temperature, and the cooling rate, we finally found the right way to grow monolayer graphene, and then we took the characterization. Ultrathin c-Si substrates were fabricated by concentrated potassium hydroxide(KOH) solutions(50 wt%) at 90℃. By controlling etching time, the thin c-Si substrates with different thicknesses were obtained. We also investigated the macroscopic mechanical flexibility of as-fabricated thin c-Si substrates.Fabrication and measurement of devices: we constructed flexible heterojunction solar cells by simply depositing graphene onto thin flexible single crystalline silicon substrates. A polydimethylsiloxane(PDMS) layer with a diameter of 4 mm hole in the center was attached on the ultrathin c-Si substrate. In addition, a thin layer of poly(3-hexylthiophene)(P3HT) used as an electron blocking layer was inserted between graphene and ultrathin c-Si. The effect of c-Si thickness and graphene layer number on the device performance was investigated, respectively. With optimum layer number of graphene(3 layers) and effective surface passivation of ultra-thin c-Si surface, an optimum PCE of 8.42% has been achieved with thin Si substrate thickness of 40 μ m. The photovoltaic characteristics of the graphene/ultrathin silicon heterojunctions were further evaluated under bend conditions, revealing their excellent flexibility and durability. Given the simple device structure and low-cost solution processed capability, our work paves the way toward low-cost, high-performance, flexible graphene/silicon heterojunction solar cells.