Study Of The Structure, Manufacturing Process And Performance Of Nanowire-based Solar Cells

The structure design and efficiency of solar cell have significantly been improved during the half century. How to increase the efficiency and reduce the cost of solar cells has always been research hotspot in this research area. Because of the large specific surface area and quantum effect, low dimension nanomaterials, such as nanowire,exhibit unique physical and chemical properties, and has extensive potential application in high-performance transistors, ultrahigh sensitivity chemical/biological sensors, high performance-price-ratio photovoltaic devices, low-cost thermoelectric devices, and high-capacity lithium-ion rechargeable batteries and so on. In this study, two types of nanowire-based solar cells(Ag/AZO/n-Si:H/α-Si/c-Si(P)NW/Al, based on silicon nanowire arrays(SiNW); Ag/n-Si:H/i-Si:H/p-Si:H/ZnO nanowire array/FTO/glass,based on ZnO nanowire arrays) were prepared, and related scientific issues were systematically studied. The main contents of the study and innovations have been presented as follows:1. Large area silicon nanowire arrays were prepared by metal-catalyzed electroless etching(MCEE) technology on surface of mono-crystalline silicon wafers. The influences of etching time on the morphology and reflectivity were studied. As the etching time increased, nanowires length increased nonlinearly, however, the diameter of the nanowires reduced, which led to breakage and collecting togather of the head of nanowires. SiNW arrays possessed good light trapping effect. The reflectivity of SiNW with 1.6 μm lengthin visible light was reduced to 2.5%. ZnO nanowire arrays were prepared by hydrothermal method. The thickness and surface propertied of ZnO seeding layer had important effect on the morphology of ZnO nanowire array. With the preparation time increasing, the length of nanowire became large, and the verticality was impoved.2. The SiNW arrays were covered by silicon film through Plasma Enhanced Chemical Vapor Deposition. Three-dimensional growth mechanism of the film was simulated using FLUENT software. It was found that the velocity of gas flow in the topof SiNW was greater than that in the bottom, which reflected that the supply of reactive groups on the top of nanowire was more than that in the bottom. So the silicon film on the top of silicon nanowire was thicker than that in the bottom. The longer the silicon nanowire was and the faster the deposition rate was, the more obvious the nonuniformity of silicon film covered on silicon nanowire became.3. Boron doped ZnO was studied by first-principles method. B2H6 plasma treatment was employed to achieve n-type doping via B atoms substitution for Zn atoms.the effects of B2H6 content and RF power on the properties of ZnO nanowire arrays were studied. It is found that the shallow donor energy level was formed in bottom of conduction band of ZnO after B doping, leading to the reduced mobility, increased carrier concentration, and enhanced absorption strength in the visible region.4. The silicon nanowire arrays were passivated using α-Si:H film and the effect of the thickness of α-Si:H film on the performance of solar cell was studied. As the thickness of α-Si:H film increased, the open circuit voltage and fill factor of solar cell became large. However, when the thickness of α-Si:H film was too large, the conversion efficiency of solar cell would become deteriorative. Taking advantage of electronic tunneling effect of ultrathin Al2O3 film, the n-Si:H films in silicon based nanowire solar cells were passivated by ultrathin Al2O3 film prepared by atomic layer deposition. The effect of the thickness of the Al2O3 film on the performances of solar cell was studied to obtain the relationship between passivation effect and tunneling current value.5. n-Si:H films were deposited using Plasma Enhanced Chemical Vapor Deposition method(PECVD). The effects of deposition process on properties of films and performance of solar cell were studied. The influence of the RF power of deposition of n-Si:H film on the open circuit voltage was not obviously. However, it had a significant effect on the short circuit current density of solar cell. With the RF power of deposition of n-Si:H film increasing, the conversion efficiency of solar cell became large. However,the uniformity of n-Si:H film covering on silicon nanowire got worse if the RF power was too big, which led to the reducing of the conversion efficiency of the solar cell.Meanwhile, the conversion efficiency would reduce with the increasing of the thickness of the n-Si:H film.6. Using ZnO nanowire arrays as the light trapping structure and charge transporting layers, ZnO based nanowire solar cells with Ag/n-Si:H/i-Si:H/p-Si:H/ZnO nanowire array/FTO/glass structure were fabricated. The effect of the length of ZnO nanowires, the thickness of the i-Si:H layer and the B-doped ZnO on the performance of solar cell was investigated. Results indicated that using B-doped ZnO nanowire arrays,the performances of solar cell were obviously improved, and the conversion efficiency was around 5.19%.