Device, Interface, Process and Electrode Engineering Towards Low Cost and High Efficiency Polymer Solar Cells in Inverted Structure
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As a promising technology for economically viable alternative energy source, polymer solar cells (PSCs) have attracted substantial interests and made significant progress in the past few years, due the advantages of being potentially easily solution processed into large areas, flexible, light weight, and have the versatility of material design. In this dissertation, an integrated approach is taken to improve the overall performance of polymer solar cells by the development of new polymer materials, device architectures, interface engineering of the contacts between layers, and new transparent electrodes. First, several new classes of polymers are explored as potential light harvesting materials for solar cells. Processing has been optimized and efficiency as high as 6.24% has been demonstrated. Then, with the development of inverted device structure, which has better air stability by utilizing more air stable, high work function metals, newly developed high efficiency polymers have been integrated into inverted structure device with integrated engineering approach. A comprehensive characterization and optical modeling based on conventional and inverted devices have been performed to understand the effect of device geometry on photovoltaic performance based on a newly developed high performance polymer poly(indacenodithiophene-co-phananthrene-quinoxaline) (PIDT-PhanQ). By modifying anode with a bilayer combining graphene oxide (GO) and poly(3,4-ethylenedioxylenethiophene):poly(styrenesulfonic acid) (PEDOT:PSS) as hole transporter/electron blocker, it further improved device performance of inverted structured to 6.38%. A novel processing method of sequentially bilayer deposition for active layer has been conducted based on a low band-gap polymer poly[2, 6-(4, 4-bis-(2-ethylhexyl)-4H-cyclopenta [2,1-b;3,4-b′] dithiophene)-alt-4,7-(2, 1, 3- fluorobenzothiadiazole)] (PCPDT-FBT). Inverted structure devices processed from bilayer deposition shows even higher performance than bulk-heterojunction. Polymer and fullerene distribute uniformly throughout the layer in vertical direction. Better electron mobility and better crystallinity in inverted bilayer film bas been observed, which can contribute to the higher IQE in inverted bilayer device. Metal grid/conducting polymer hybrid transparent electrode has been proved can be an alternative to ITO in inverted structure with similar device performance. Further, a novel protocol to fabricate highly transparent ultra thin silver films as transparent electrode on both glass and plastic substrates also has been demonstrated, based on ZnO/Ag/ZnO tri-layer structure and self-assembled monolayer interfacial modification. Sophisticated interfacial engineering method is applied at necessary interfaces to functioning the ultra thin silver film as a platform for polymer solar cells, and superior device performance that even exceeds using ITO has been achieved.