| Summary: | 碩士 === 國立交通大學 === 光電工程研究所 === 102 === The price of crude oil climbs progressively, which winds the horn of energy crisis. Photovoltaics have been one of the critical roles in various green energies. In order to reduce raw material and fabrication cost, hybrid solar cells combining organic conductive polymers and inorganic semiconductors have become rising topic in recent years. Due to the properties of direct energy gap and high electron mobility, GaAs has high absorption coefficient in visible wavelengths and good carrier transport characteristics. In this work, we substitute GaAs for Si to fabricate organic/inorganic hybrid devices. According to the analysis of one-dimensional simulation, at the same surface and bulk defect level, the open-circuit voltage of GaAs-based cell is larger, but the short-circuit current density is lower because of proper band alignment and larger bandgap resulting in shorter cut-off wavelength of absorption spectrum respectively. It is expected that the cell performance would exceed Si-based one. In conventional solar cell design, base doping should be lower in order that the width of depletion region on base side would lengthen, which is beneficial for carrier collection. Generally, doping concentration of GaAs wafer is hard to be manufactured below 1017cm-3, therefore we grow buffer layer and low-doped(1016cm-3) absorber by MOCVD. All electrical properties are enhanced, and average efficiency achieves 6.6%, which is 203% of the one without these epilayers. The pattern of top contact is composed of one bar and several grids connected together, so the shading ratio is a significant parameter that dominates the amount of incident light and the condition of carrier transport. Optimized planar devices with shading ratio around 14% (60μm width of grid) can reach a high efficiency of 7.66%. In addition, we perform simulations for planar devices to analyze the internal electrical properties. For further cell performance improvement, we etch nanostructure on front surface to reduce reflectance and increase light absorption. Preferring anisotropic etching and precise morphology control, we choose dry etching method. By use of self-assembly polystyrene nanosphere lithography technique, monolayer of nanospheres is deposited on substrate, followed by two stages of reactive ion etching. First step is to shrink spheres, reserving the spacing among nanorods and controlling the upper part morphology of nanorods. Second step is GaAs etching, dominating the length of nanorods. Although damage removal etching is performed, residual defects and enlarged surface area slightly lower the Voc. However, compared to planar devices, the average Jsc enhances 33%, leading to the highest efficiency of 7.74%.
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