Recent developments in organic solar cells (OSCs) have achieved over 17% power conversion efficiency (PCE) under AM1.5G conditions. However, OSCs still suffers from voltage loss and instability compare to inorganic solar cells as well as perovskite solar cell counterparts. Recently, an organic solar cell with a small driving force has been reported. The small driving force minimizes the voltage loss and achieves high open-circuit voltage (Voc) ~1.1V. However, the physics behind the OSCs still not clear. In this thesis, I will focus on studying the charge transport mechanism of small driving force OSCs, the impact of charge transport interlayers on device performance under indoor conditions, and the origin of the good performance of thick film non-fullerene-based OSCs under indoo...[ Read more ]

Recent developments in organic solar cells (OSCs) have achieved over 17% power conversion efficiency (PCE) under AM1.5G conditions. However, OSCs still suffers from voltage loss and instability compare to inorganic solar cells as well as perovskite solar cell counterparts. Recently, an organic solar cell with a small driving force has been reported. The small driving force minimizes the voltage loss and achieves high open-circuit voltage (Voc) ~1.1V. However, the physics behind the OSCs still not clear. In this thesis, I will focus on studying the charge transport mechanism of small driving force OSCs, the impact of charge transport interlayers on device performance under indoor conditions, and the origin of the good performance of thick film non-fullerene-based OSCs under indoor conditions.

In chapter 2, the charge transport property of two small driving force system bulk heterojunction (BHJ) OSCs is studied. In our study, we observed a surprisingly large increase (up to 30x) of electron mobility in an OSC blend when introducing polymer donor into small molecular acceptor. By ruling out the morphology reasons, we show that the donor polymer can assist the electron transport by providing “bridges” or “shortcut” for electron transport across the domains of small molecular acceptors and thus facilitates the overall electron transport in the blend. This can happen because for these systems the LUMO offset is small. Our study shows the benefits of donor assisting electron transport in BHJ systems with small energetic offsets.

Chapter 3 and 4 concentrate on the performance of OSCs under indoor conditions. In chapter 3, the small driving OSCs and a new BHJ system are studied under AM 1.5G and indoor conditions. We successfully demonstrated a high-efficiency non-fullerene organic photovoltaic (OPV) cells with over 30% power conversion efficiency (PCE) under indoor conditions. Our results show that the choice of electron transporting layer (ETL) is critically important to enable such performance. The use of an ETL (named PDI-NO) with a deep highest occupied molecular orbital (HOMO) level can effectively suppress leakage current and reduce trap assisted recombination of the devices. When applying this ETL to a bulk-heterojunction (BHJ) blend based on a low-bandgap, acceptor molecule, we achieve a PCE of 31%, which is the best indoor OPV efficiency reported to date. We also show that this ETL can be applied to another BHJ system based on a wide bandgap PDI acceptor, which also leads to efficient indoor OPV devices with a PCE of 26% and a high open-circuit voltage (V_oc) of over 1V. Our study paves the way towards high-performance indoor OPV devices for powering IoT electronics. Chapter 4 is based on the findings in Chapter 2 and 3. In this chapter, a non-fullerene based OSC under indoor conditions can achieve over 20% of PCE even increase the active layer thickness to 200nm. We find that it is attributed to 1.) the small LUMO offset BHJ systems (chapter 2), where the donor polymer can assist electrons to hop back and forth between the donor and acceptor domains, 2.) the deep HOMO level ETL, which can suppress the leakage current, and 3.) low energetic disorder and Urbach energy. Therefore, the devices can still maintain good performance under the indoor conditions (chapter 3).