Study Of The Device Physics And Performance Of Dye-sensitized Solar Cells

With the rapidly increasing of world population, the limited supply of fossil fuelsis hardly expected to cope with the human’s energy consumption. Exploitation ofrenewable energy sources has been a heated topic throughout the world. Among allthe renewable energy sources in our planet, solar energy, which can be transferred toelectricity, is expected to play a vital role as a future energy source. In the past decade,widely viewed as a potential third generation photovoltaic (PV) technology,dye-sensitized solar cells (DSC) have drawn much attention due to their low cost andhigh energy conversion efficiency. The performance of the DSCs is not yet as high asthe existing crystalline silicon based photovoltaic devices. Therefore, the exploitationof the real potential of DSC is of great importance. Nowadays, a great number ofresearchers focus on materials and device constructure to improve the performance ofDSCs. However, the seldom-studied inner physic mechanisms of DSCs are essentialto the DSCs’ performance. Based on the research of DSCs’ physic mechanisms, weproposed an accurate and efficient method to analyze the inner physic mechanismsand measure the inner physic parameters of DSCs. Through these studies, we haveexplored effective ways to improve the device performance.Firstly, the key factors, which relate to the photo-electronic efficiency of DSCs,were studied by the equivalent circuit model. Employed a single-diode model, GeneticAlgorithm (GA), Particle Swarm Optimization (PSO）, and Differential Evolution (DE）are used respectively to analyze and extract the equivalent circuit parameters of DSCs.Through the comparisons of parameter accuracy, anti-noise capacity and calculationefficiency, PSO has shown excellent performance and was proved to be the bestmethod for parameter extraction. Through the accurate extraction of circuitparameters, we found that the DSCs’ internal series resistance Rshas a very importantinfluence on device performance. Therefore, the research of the ways to decrease Rsmust be conducted in order to achieve higher photo-electronic conversion efficiency.By this study, the research direction of DSCs was demonstrated.Secondly, the performance-improved DSCs were achieved by slowing downrecombination reaction, and their physical mechanisms were analyzed. In DSCs, thereis a direct correlation between the energy conversion efficiency and the recombinationreaction rate. Slowing down the recombination reaction can significantly increase theeffective electron concentration, improve the diffusion current density and enhance the capacity of electron transport in the photoanode, which will result in the reduction of Rs in the battery. To research the corresponding relation of recombination reaction and the electron concentration quantitatively, we studied and measured the free electrons’lifetime τn, which is the most important parameter to evaluate recombination reaction accurately. The conventional measurement and evaluation of zn is much complex. In this study, based on the transient photo-electronic measurement and Savitzky-Golay filtering technology, we firstly proposed a rank-variable differential smooth method to analyze and calculate the electrons’ lifetime. By this method, an accurate and rapid measurement of electrons’lifetime τn was realized, which provide us with a reliable way to study the complex recombination reaction in DSCs.Thirdly, we proposed a new Zinc-oxide semiconductor material doped with rare-earth element, and its electrical and optical performance have been studied. Based on the first principles calculation technique, we found the electron density of this new material increases clealy and it shows a metallic property, which can enhance effectively the electron transfer capability and collection efficiency, thus lower the resistance of the devices’photoanode. At the same time, the bandgap of this new La-doped semiconductor has been broadened, which means the absorption offside has been blue-shifted and the transmission spectral range has been broaden. The photoanode’s reflectivity and absorption are also decreased. All these factors contributed to the improvement of incident photons in light absorption layer, and the improvement of the output current and energy conversion efficiency. The study not only revealed an effective way to improve the DSC’s photoelectric conversion, but also showed the ZnO as DSC photoanode material of potential application.Finally, the optic management method was proposed to improve DSCs’ performance by modifying the interface of photoanode. Silver-coated TiO2electrodes were prepared by photochemical catalysis, and the amount of nano-particles covered on the TiO2electrodes were adjusted by controlling the illuminating durations. The Ag nano-particles with the ability of scattering light can lengthen the optical path in the electrodes and improve the utilization rate of solar radiation, which will increase the device’s short circuit density. The decrease of the surface state density of TiO2nano-particles can slow down the recombination rate between the photo electrons and the oxide in electrolyte, and accelerate the electrons in optic-anode, which will decrease the inner series resistance Rs. By employing silver-coated TiO2electrode, the conversion efficiency of DSCs was improved from5.97%to6.86%, which demonstrates the effectiveness of proposed interface modification in improving theDSCs’ photoelectric conversion efficiency. Thus, the method is considered to be ofgreat importance to guide the design of DSCs.