Preparation Of Plasmonic Nanostructures And Their Applications On Solar Cells

Clean energies, especially solar energy, have attracted great attentions due to energy crisis. Solar cells can directly transform light to electricity using photovoltaic effect. Thin-film solar cells with thicknesses in the range of 1-2μm can effectively reduce the cost associated with expensive semiconductor materials. However, thin-film solar cells usually have lower energy conversion efficiency in comparion with wafer-based solar cells because of the relatively poor light absorption efficiency. As a result, developing techniques that can effectively increase optical absorption of thin film solar cells is very important.However, traditional methods, which use surface textures that are typically around 10 microns in size, cannot be used. While submicron surface texturing can be used to achieve light trapping, it also increases minority carrier recombination in the surface and junction regions. Another method is depositing anti-reflection films, which usually use SiOx and SiNx. However the cost of depositing these films is expensive, which increases the fabrication costs.SPRs are the photon-induced collective oscillation of valence electrons,. SPRs can enhance the electromagnetic fields in the vicinity of the particles, the forward scattering, thus increasing the optical path length in solar cells and the light absorbance. Wavelength and intensity of the SPR depend not only on the nature of the metal, but also on the size and shape of metallic nanostructures. In this thesis, we adopt a novel approach which uses the surface plasmon resonances (SPRs) of metal nanoparticles for increasing optical absorption of GaInP thin film solar cells. The technique is simple, cheap, and compatible to present manufacturing line and suitable to large area fabrication.In order to couple SPRs and solar cells, we first should fabricate metal NPs on the surface of thin film solar cells. Four chapters are included in this thesis. They are Chapter-1 Introduction, Chapter-2 Fabrication Ag NPs using sputtering and light induced, Chapter-3 Fabricating Ag NPs by spontaneous galvanic displacement process on the surface of solar cells and improving energy conversion efficiency of GaInP thin films solar cells. Chapter-4 Conclusions and prospects. The detailed research contents are as follows:(1) We first prepared Ag NPs by sputtering on the surface of GaInP solar cells. The morphologies and densities of the Ag NPs were adjusted by changing the sputtering time and the annealing temperatures. We find that annealing at low-temperature, e.g. at 250℃, shows little effects on the size of the Ag NPs. However, the densities increase by increasing the sputtering time. In contrast, we find that annealing at high-temperature, such as 450℃, results in the formation of larger Ag NPs, which could benefit light absorption of solar cells.(2) We then explored the possibility of optical excitations on the synthesis of Ag NPs of a variety of shapes and sizes by using light emitting diodes (LEDs) of different wavelengths. First, photoinitiator 1173 is excitated in solution by UV irradiation to form weak reductive free radicals by photolysis reaction, which can reduce Ag+ to Ag0. The Ag atoms then form spherical seeds with an average diameter of-10 nm in the presence of the stabilizer, sodium citrate. The Ag seeds were irradiated by single colored or two colored LEDs of different wavelengths and the growth kinetics of the Ag NPs were studied by UV-Visible absorption spectroscopy. After irradiation, we found that the spherical Ag seeds can transfer to decahedral, triangular prism and nano rings structures, and the colors of the solutions also changed.(3) Ag NPs were deposited on the surface of the GaInP thin film solar cells spontaneous galvanic displacement process. As the n-type AlInP layer contacts the 10 mM AgNO3 solution, the conduction band electrons can be captured by the Ag+ in solution, resulting in their reduction to Ag0. The Ag atoms then nucleated on the surface, followed by aggregation and crystallization to form Ag NPs. Adjusting the reaction time can change the sizes and densities of Ag NPs.(4) We further demonstrated that Ag plasmonic NPs can effectively improve energy conversion efficiency of GaInP thin film solar cells. The Ag NPs were fabricated by spontaneous galvanic displacement process. It was confirmed that localized surface plasmon resonance (LSPs) can increase forward light scattering to improve the light absorption and power conversion efficiency (PCE). Our experiments show that the short circuit current density (Jsc) increases from 14.9 to 17.8 mA/cm2 by 19.5%, the power conversion efficiency (PCE) increases from 15.2% to 18.3% by 20.4%, and the open-circuit voltage(Voc) almost keep the same.In conclusion, we prepared Ag plasmonic NPs by three types of methods, which are 1) a sputtering/sintering process,2) a light assisted growth method by using LEDs of different wavelengths, and 3) the spontaneous galvanic displacement process on the surface of the GaInP thin film solar cells. We then studied the effects of plasmonic NPs on the performances of GaInP solar cells. It was found that the Ag NPs supports SPRs, and thus can greatly increase light absorbance and photocurrent responses of the solar cells by scattering and re-reflection of the incident light.