Understanding the Photogeneration Process in Organic Photovoltaics: From the Bulk to the Edges

Abstract

The field of organic semiconductors have been brought under great attention from both the academia and the public when high efficiency organic light emitting diodes (OLEDs) quickly grew in to a huge market with a $40 billion annual revenue. However, the performance of the organic photovoltaics (OPVs) has yet to meet the criteria as a player in the solar industry, popping a big question: what is missing for OPVs? In this thesis, the whole photogeneration process of OPVs is reviewed from nanoscopic mechanisms to macroscopic device performance, in a hope to provide guidelines towards high performance OPVs that can be commercially adopted. The first part of this thesis is focused on the charge transfer (CT) states in organic bulk heterojunctions (BHJs). We use a combination of experimental, theoretical and computational methods to explore the optical and electrical properties of CT states and their relationship to molecular and morphological structures. In the second part, we move from the BHJ to the edges of the OPV device and study the interfacial energy loss between the BHJ and peripheral layers. By studying the energy landscape near the interface, the interfacial energy losses are quantitatively characterized. A counterintuitive way to reduce the interfacial energy loss by up to 30% and improve the device open-circuit voltage is provided by growing a thin layer of acceptors on the anode side of the BHJ. With the understanding of the interfacial voltage losses, we proceed to study the internal distribution of voltages on various layers of the OPV, which leads to the development of an analysis method that can separate the properties of the BHJ of and OPV from the peripheral structures by calculating its “bulk quantum efficiency” (BQE). The third part of this thesis presents studies of the photo- and thermal stability of OPVs. A major part of the effort is focused on non-fullerene acceptor (NFA) based OPVs which currently provide the highest efficiencies. With the help of various experimental tools and the BQE analysis, we characterize various degrading mechanisms that limit NFA-based OPV stabilities. In Chapter 5, we show that photocatalytic reaction between BHJ and ZnO is a major cause to device photodegradation. A self-assembled monolayer between the BHJ and ZnO can effectively suppress this reaction. We also show that UV-induced damage to the BHJ is another source of photodegradation and a low-cost UV-absorbing coating is proposed to suppress it. As a result, a projected operational lifetime over 30 years is achieved. In Chapter 6, we show that thermally activated Cl redistribution in BHJ driven by a redox reaction at the BHJ/MoOx interface leads to a check-mark shaped thermal degradation pattern. A thin layer of C70 inserted between BHJ and MoOx can effectively suppress this mechanism, significantly improving the device T80 lifetime at 80oC from 20 min to 800 h. With these results, we achieve an OPV structure with excellent photo- and thermal stability, making one big step toward the commercial adoption of OPV. In the last part, several unanswered questions are introduced with preliminary experimental results and proposals are made for future studies around understanding and improving OPV performance. In the end, an outlook for OPV devices is presented.