Fill factor of organic solar cells and applications of dilute donor devices

Abstract: Organic solar cells (OSCs) have attracted great attention due to their low cost, flexibility and solution-processibility. In recent years, the development of nonfullerene acceptors (NFAs) has truly promoted the efficiency of OSCs up to 19%, implying high potential for commercial applications. However, the stateof- the-art OSCs still lag behind the Shockley-Queisser limit, besides the intrinsic losses, understanding the extrinsic losses during charge generation, transport and extraction in devices is necessary.The short-circuit current (JSC) and open-circuit voltage (VOC) can be simultaneously optimized in OSCs by tuning the energy levels of NFAs. However, less attention has been paid to the fill factor (FF), a crucial parameter for device efficiency. The FF reflects how the output photocurrent changes for a solar cell with a load from zero to infinity, indicating the charge extraction capability. In this thesis, the roles of energy offset, electric field, disorder and morphology on charge carrier dynamics as well as how these factors influence FF and energy loss are introduced. It is observed that fast and field-insensitive charge extraction is essential for high FF, which can be enabled by balanced transport and reduced bimolecular recombination. Additionally, the correlation between FF and voltage loss are studied based on four NFA systems with different highest occupied molecular orbital (HOMO) offsets. Larger HOMO offset could suppress hole back transfer from donor to acceptor and then lead to a larger FF, but it also induces more voltage loss.The morphology of the active layer governs the charge dynamics and device performance. A comparative study based on all-polymer solar cells processed from chlorobenzene (CB) and o-Xylene has been performed. Film formation process and morphology characteristics demonstrate that CB-cast films exhibit better donor/acceptor miscibility and relatively ordered structure, yielding good device performance. Contrary, in o-Xylene cast devices, electron trapping leads to a smaller FF and more non-radiative recombination.The state-of-the-art OSCs usually require comparable donor/acceptor contents in bulk-heterojunctions. Herein, NFA’s contribution to hole transport is investigated in dilute donor solar cells (10 wt% PM6:Y6). Comparable hole mobilities of PM6 diluted in Y6 and insulators (PS &PMMA) indicate that the hole transport in dilute donor solar cells is still mainly via PM6 phases, although pristine Y6 can support ambipolar transport. Furthermore, impressive performance of the dilute donor solar cells motivate us to explore semitransparent OSCs for building-integrated photovoltaics (BIPV). Decent photovoltaic performance and acceptable visible transparency have been realized in dilute donor solar cells by decreasing visible-absorption and increasing near-infrared absorption.