CONTACT AND MORPHOLOGY EFFECTS IN ORGANIC SOLAR CELLS

Abstract

The power conversion efficiency (PCE) of organic solar cells is highly dependent on the efficiency of the generation, transport and extraction of charge carriers out of the photoactive layer (PAL). There has been rapid progress in the development of new donor and acceptor materials that give better absorption and carrier mobilities in the PAL, which have since led to large improvements in device performance. However, the device physics of organic solar cells is still not well understood despite numerous studies. For example, it has not been possible until now to systematically explore the role of contacts in the good-cell regime, and the effect of morphology in controlled donor-acceptor networks. The work in this thesis seeks to explore these questions. I formulated a drift-diffusion-generation model that takes into account built-in potential of the cell to study the effects of contact carrier density on cell performance, which has received little attention in the literature. Using different hole-collection layers with tuneable workfunctions from 4.4 to 5.2 eV in ultrafine steps and model poly(3-hexylthiophene): phenyl-C61-butyric acid methyl ester (PCBM) as the PAL, I demonstrate a strong dependence of contact resistivity on electrode workfunction in organic solar cells, and the need to drive the electrode workfunction beyond Fermi-level pinning to achieve ohmic transition. Finally, using a crosslinked high mobility state-of-the-art donor polymer network and PCBM as acceptor, I demonstrate a systematic variation in the fill factor (FF) and diffusion current slope which reveals a breakup of the PCBM phase with increasing thickness of these devices despite the imposed contiguous donor polymer network.