UCLA

Photovoltaic (PV), which converts sunlight into electricity, is a promising solution to the energy and environmental crisis we are facing right now. In this dissertation, we are focusing on next generation semiconductors as the photoactive materials, i.e. organic and orgainic/inorganic hybrid perovskite semiconductors, to achieve cost effective and energy efficient solar cell technology.

Organic semiconductor always shows narrow absorption compared with the conventional inorganic semiconductors, which is one of the limiting factors of the organic photovoltaic (OPV). In the second chapter, we solve this problem by employing the multi-absorber bulk heterojunction (BHJ) device architecture, in which different polymer absorbers are blended together to cover panchromatic absorption. A comparison study reveals the working mechanism of this novel device, and the material selection rule is summarized. Eventually, the 8.7% power conversion efficiency is realized in ternary BHJ solar cell. Besides the narrow absorption, the organic BHJ layer is usually very thin (~100 nm) due to its limited carrier mobility, hence the absorption is insufficient in the photoactive layer. In the third chapter, we utilize the plasmonic effect of the metal nanoparticle to enhance the absorption of the OPVs. The addition of spectrally tuned SiO2-Au nanorods leads to improved photocurrent and device efficiency. On the other hand, the narrow absorption of organic materials also creates new device possibility that traditional semiconductor can't deliver. One of the good examples is the visibly semitransparent solar cells. In the fourth chapter, we develop this idea by engineering transparent top electrode and incorporate photonic distributed bragg reflector to further improve the device efficiency without compromising the visible transparency.

Organic/inorganic hybrid perovskite semiconductor attracts incredible attentions in the past few years. It originates from the excitonic semiconductor family but delivers superior photovoltaic device performance than the typical excitonic solar cell e.g. dye sensitized solar cells and organic solar cells. However, the photophysics of this type of material is still mysterious in many aspects. In the fifth chapter, we try to understand its photophysical property by some basic characterization methods. We discover that the photo excitations in this materials varies with different crystal size. Besides, we observe the photoluminescence lifetime and intensity improve dramatically after exposing to the moisture. The improved photophysical property eventually results in greatly enhanced photovoltaic device performance. In addition to the solar cell, we successfully demonstrates the ultral high photodetectivity based on the perovskite materials, the results are included in sixth chapter.