Enhancing Light Harvesting In Photovoltaic Cells By Semiconductor Nanomaterials And Nanostructures

For high-efficiency solar cells, harvesting more light is always among the mostimportant issues as light absorption directly influences the solar cell efficiency. Forcommercial solar cells, two methods are mainly used for improving light harvesting.One is surface texture and the other is SiNx antireflection layer. However, the surfacereflectance is still high, and more needs to be done in order to achieve anomni-directional low reflection surface. Semiconductor nanowires can provide verylow surface reflection, but so far it is hard to produce a high efficiency nanowire cellwith large-area. In our study, we try to combine nanostructures with high efficiencybulk solar cells, and nanomaterials and nanostructures have been designed forimproving light harvesting in solar cells.We applied ZnO nanorod arrays (ZNAs) on as-produced commercial Si solarcells for further surface antireflection, and the requirements of nanorod arrays forcells with different textured morphology have been studied. The ZNAs weresynthesized through a simple low-temperature hydrothermal method on bothmulticrystalline silicon (mc-Si) and crystalline silicon (c-Si) solar cells, which is anondestructive producing process for the original cells. We have studied the affectionof preparation conditions on the size and morphology of ZNAs. The antireflectioneffects and mechanisms have also been investigated. For both mc-Si and c-Si solarcells, the surface reflection have been greatly suppressed after, and the weightedreflectance has been reduced by more than50%. Consequently, the short-circuitcurrent desity (Jsc) of cells have been largely improved, and for mc-Si cells, the Jsccan be enhanced by3.64%. For bare Si cells with no SiNx antireflection layer, theenhancement is much more significant, which can be increased by8.76%from33.55to36.49mA/cm2. ZnO and ZnO:Al (AZO) thin films deposited on the texturedsurface of bare silicon cells can provide a comparable antireflection effect as SiNxlayer。AZO can provide a Jsc of37mA/cm2, which is higher than SiNx antireflectionlayer under the same conditions. The feasibility of applying such antireflection layeron large-area solar cells has been discussed. In addition, our investigations haveshown that the textured surfaces influences the requirements of nanorod arrays. Forpyramidal c-Si surfaces, thinner and shorter nanorods are better for antireflection,while thicker and longer nanorods are need for grooved mc-Si surfaces. Additionally, hybrid structures have been applied for antireflection. Wedeveloped a simple and controllable one-pot hydrothermal method for synthesizingnanospheres@nanorod hybrid arrays. The hybrid arrays can be grown on anysubstrates and erbium has been incorporated into nanospheres, while nanorodscontains only ZnO. The preparation parameters such as reaction temperature and time,precursor concentration, Zn and Er precursor ratio have been investigated forcontrollable synthesize of hybrid arrays. We have also studied the growth mechanismof such hybrid arrays which helps us for synthesizing the structures we need. Thehybrid arrays can provide omni-directional antireflection, and the absolute reflectionof the surface is less than1%. Our investigation revealed that the size andmorphology of both nanospheres and nanorods will influence the antireflectionproperty of the hybrid structure.As the ZNAs have a significant absorption of the light with wavelength shorterthan380nm, the external quantum efficiency (EQE) of the ZNAs decorated cell isalmost zero in such wavelength range. We use a simple sulfuration conversionmethod to change ZnO into ZnS. The sulfuration conditions have been studied. Aftersulfuration, the absorption of antireflection layer blue-shifted and the EQE of cells inthe range of330-380nm has been greatly improved (i.e. EQE increased from0%to19%at375nm) while the EQE for other part has not been influenced. Additionally,we applied ZnSe:Mn quantum dots (QDs) for down conversion of ultraviolet (UV)light into visible light to improve the usage of solar light. The EQE of cells withsulfurized ZNAs coatings in the range of300-370nm have been enhanced to over20%, which proves that down conversion materials can offer an alternative methodfor improving the usage of UV light.Finally, surface passivation have been studied to lower the surface recombinationthus increase the photon utilization of solar cells. Different from other passivationexperiment on flat silicon, we have studied the surface passivation of surface texturedsilicon. We use atomic layer deposition (ALD) for the layer-by-layer deposition ofAl2O3passivation films. The influence of the thickness of Al2O3thin film andannealing temperature on the passivation effect have been investigated. The lifetimeof minority carriers increased about300times once a proper passivation condition hadbeen offered. Our study showed that3-5nm of Al2O3with annealing under500-550oC for15min can provide an excellent passivation. The EQE of passivated emitter andrear cell demonstrates an obvious improvement in the range from400-1200nm in comparison with a standard cell, indicating that surface passivation is an efficient wayof enhancing the light conversion efficiency.