Stokes Shift And Its Application In Photovoltaic Devices Of Solar Energy

There exists Stokes shift for common luminescent species in their absorption-emission process, such as dyes and quantum dots, which results from the higher energy level of the absorption than lower energy level of the emission. When the absorption spectrum overlaps with emission spectrum partly, part fluorescence emitted by luminescent molecules will be reabsorbed by other luminescent molecules or by themselves in their conductive process, which is also called as self-absorption. So-called self-absorption loss for a luminescent waveguide is caused by the self-absorption of luminescent molecules during the transmission of fluorescence in the waveguide. Stokes shift of luminescent species will result in a loss of fluorescence efficiency and a shift of the wavelength shift when they are used in luminescent solar concentrator (LSC) and luminescent poly(ethylene vinyl acetate) film for encapsulation crystalline silicon PV modules.LSC employs the conductive process of fluorescence in waveguide for sunlight concentrating, and the Stokes shift of luminescent species in its waveguide is critical to its sunlight power conversion efficiency. Luminescent poly-ethylene vinyl acetate (EVA) film for encapsulation crystalline silicon photovoltaic (Si PV) modules is another kind typical of luminescent waveguide, which employs the luminescent down shifting (LDS) of luminescent species for energy conversion to enhance the power conversion efficiency of Si PV modules. In both applications, Stokes shift of luminescent species is the critical factor for their efficiency. In my thesis, the influence of Stokes shift of luminescent molecules on efficiency of LSC and LDS in EVA encapsulation crystalline silicon PV (Si PV) modules has been studied and main achievements are following:1. A new geometric structure of LSC, cylindrical coated luminescent solar concentrator (CC LSC), is proposed in this paper. It is fabricated by a cylinder with dye coating and its luminescence occurs close to the surface of the cylinder. The loss in emission of CC LSC is lower than that of cylindrical doped luminescent solar concentrator (CD LSC) whose luminescence occurs within the inner of the cylinder. Although the self-absorption probability rs of CC LSC is larger than that of CD LSC, the power conversion efficiency of CC LSC is 1.17-1.48times as much as that of CD LSC with equivalent absorbance and geometric gain when R6G is used as luminescent particles and the ratio of the coating thickness to the radius of the cylinder is0.01. This geometric structure could effectively improve the power conversion efficiency of cylindrical LSC and provide an available path to fabricate more effective luminescent solar concentrators.2. Preparation and characterization of luminescent solar concentrator with zero self-absorption loss. The EuTT LSC system has been prepared by use of rare earth complex EuTT with a large Stokes shift. The energy transfer process in EuTT molecule occurs between ligands and rare earth ions, resulting in a large Stokes shift, which is large enough to make no overlap between absorption and emission of EuTT, besides of large absorption section and tunable absorption spectrum. Therefore, LSC employing EuTT would present zero self-absorption loss. By studying Ⅰ-Ⅴ performances of LSC with different radiation area under AM1.5G and EQE changes of LSCs with different geometric gain, the zero self-absorption loss of EuTT LSC has been demonstrated and proved.3. Enhancement the Si PV modules by use of LDS EVA film doped with high efficient rare earth complex. Soaking method has been used in preparing EuTT EVA film and EuTD EVA film. By studying the effect of absorption spectra and self-absorption loss of EuTT EVA film and EuTD EVA film on their enhancements to mc-SiPV modules, ideal LDS specie in LDS technique has been explored. For mc-Si modules, ideal LDS specie should absorb wavelength less than390nm, emit wavelength more than450nm, has no self-absorption loss and near unity luminescent quantum efficiency(LQE). Thus, EuTT EVA film has improved spectra response of mc-Si effectively and an increase of0.42%absolute efficiency has been obtained because of its suitable absorption spectrum, higher LQE (0.73) and no self-absorption loss. According to cost estimation, it is found such an absolute efficiency increase can reduce power generation cost of mc-Si PV devices from US$1/Wp to US$0.98/Wp, which would bring about US$8/m2profit for manufacturers of mc-Si PV modules.