UC Santa Barbara

Organic photovoltaic devices (OPVs) have been progressively developed as a promising green energy source. The solar-to-electrical power conversion efficiency (PCE) of the OPVs has been steadily enhanced in the last few decades by advanced chemical designs and device structures/architectures. A state-of-the-art class of photoactive materials recently contributed to a drastic increase in PCEs, reaching almost 20%. Compared to the other PVs such as inorganic-based and perovskite PVs, the OPVs have unique merits: thinness, lightweight, flexibility, and ease to tune color and transparency. These characteristics have attracted the commercialization of the OPVs in various fields (e.g. incorporating with clothing and aesthetic windows). However, in contrast to the remarkable PCE improvements, the OPVs are still suffering from short lifetime under environmental operating conditions (e.g. intense sunlight, high temperature, etc.), which is a hurdle to encourage wide-ranging commercial products of the OPVs. The lack of understanding of fundamental degradation processes and mechanisms within an entire OPV has delayed the development of long-term stable OPVs. In the aspect of chemical synthesis, many efforts have introduced durable photoactive materials (thermally stable and/or photostable chemicals), but synthetic steps often require complicated processes and high cost, and provide low yield and toxicity which is another potential environmental issue. This dissertation demonstrates a comprehensive understanding of possible degradation mechanisms within an OPV device, and suggests a facile method to improve the lifetime of the OPVs based on existing materials. In the first study, we discussed both the physical and chemical degradation processes of layers, especially, at interfaces between adjacent layers within an OPV. Molecular-level insights are provided into the impact of different metal top electrodes (the most commonly used aluminum (Al) and silver (Ag)) on the interfacial morphology and stability of photoactive layers in PM6:Y6 bulk-heterojunction (BHJ) OPVs. OPVs with an Al top electrode exhibit inferior stability compared to Ag electrode devices upon thermal aging, whereby thermal stress induces the diffusion of both Al and Ag atoms to the PM6:Y6 BHJ layer. Multiscale characterizations (X-ray photoelectron, solid-state nuclear magnetic resonance, and electron paramagnetic resonance spectroscopy) suggest the different local chemical environments of PM6 and Y6 moieties in PM6:Y6/Al-contact. By comparison, the Ag atoms do not adversely affect PM6:Y6 BHJ morphology and the associated device physics. Next, a common cross-linking method is applied for the organic BHJ photoactive to address a fundamental morphological stability issue with high performing small molecule non-fullerene acceptors (NFAs). The molecular diffusion and/or aggregation of the NFAs due to thermal energy alter optimum BHJ morphology which is required to yield high PCE, leading to undesirable morphology and hence reducing PCE. Herein, a photochemically activated multi-bridged azide cross-linker (6Bx) restricts molecular aggregation and crystallization of the Y6, a representative high performing NFA, in solid state. Solid-state magnetic resonance and infrared spectroscopy analyses demonstrate that the 6Bx molecules closely interact with Y6 moieties within the PM6:Y6 blend, facilitating molecularly cross-linked Y6 regions. Consequently, 0.05 wt% 6Bx cross-linked PM6:Y6 BHJ OPVs retain 93.4% of initial power conversion efficiency upon thermal aging at 85 °C for 1680 h. This dissertation comprehensively examines physical and chemical reactions/interactions within an entire OPV device by multiscale characterizations and device physics, therefore will provide guidance to develop stable and efficient organic electronics.