Fundamental studies of charge transport for optoelectronic processes : nano-structure electrodes for perovskite solar cells and electrochromic devices
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Charge transport at interfaces and in bulk phases governs the efficiency and performance of various heterojunction optoelectronic devices such as solar cells and electrochromic devices. In this dissertation, research on charge transport processes in two different optoelectronic devices (nanostructured heterojunction photovoltaic cells and electrochromic devices) is presented.;Due to the increment of dark current and electron transport resistance in thicker layer, current lead halide perovskite solar cells have a photoactive layer no more than 600 nm thick. In order to develop a thicker photoactive layer, TiO2 nanowires (NWs) have been synthesized. These rutile NWs arrays with different length have been used as photoanodes in perovskite solar cells, and we achieved 11.7% efficiency, which is 2% (absolute value) higher than the best values reported in the literatures for perovskite solar cells using nanowires as a photoanode. In order to maximize the attainable photovoltage, perovskite solar cells use as the back cathode low chemical potential, high work-function (ϕ) precious metals, such as gold (ϕ=5.1 eV). Herein, we report a set of perovskite-type solar cells with nickel (ϕ=5.04 eV), an earth abundant element and non-precious metal, as back electrode. These cells achieve nearly the same open circuit voltage as gold, and an efficiency of 10.4%. This opens a low-cost way towards high efficient perovskite solar cells.;Lots of solar cell research has been done to increase efficiency and replace lead with non-toxic tin, organic hole transport material (HTM) with cheaper inorganic HTM, and noble metal Au with cheaper materials. Decomposition of perovskite materials in moist air is the main barrier for application of perovskite solar cells. Only a few studies have been performed to improve the stability of perovskite materials for this type of solar cell, such as using FAPbI3 instead of MAPbI3 and changing the morphology of perovskite materials. It is known that pseudohalides have similar chemical properties and behaviors as halogens. Here, a new perovskite material for this type of solar cell with Pb(SCN)2 as lead source was synthesized. This new material has shown more than 4 hours of stability in the accelerated experiment under air with 95% humidity, compared to less than 2 hours with CH3NH3PbI3 perovskite material.;In order to obtain a better response time and low driving voltage, a solid-state electrochromic (EC) device based on 3-D conductive fluorinated tin oxides (FTO) nanobeads electrodes with over 500 roughness factor was developed. The 3-D conductive FTO nano-beads electrode reduced the driving voltage to less than 1.2 V and the response time to 272ms, in comparison to 2.5 V and 2 s for a 2-D flat FTO film as the electrode.