Molecular Understanding of Organic Solar Cells: The Challenges

TLDR

The account reviews the optical and electronic processes that occur in solid‑state organic solar cells, establishing a conceptual framework for the field. Its objective is to (i) survey these processes, (ii) identify the theoretical challenges at the molecular level, and (iii) highlight recent advances in interface energetics and dynamics to guide material optimization. The authors analyze the sequence of optical absorption, exciton formation, migration to the donor–acceptor interface, dissociation into charge carriers, carrier mobility, and electrode collection, and describe the theoretical challenges for each step. Recent theoretical studies on interface energetics and dynamics show that improving photovoltaic performance requires balancing material parameters that often exert opposing effects.

Abstract

Our objective in this Account is 3-fold. First, we provide an overview of the optical and electronic processes that take place in a solid-state organic solar cell, which we define as a cell in which the semiconducting materials between the electrodes are organic, be them polymers, oligomers, or small molecules; this discussion is also meant to set the conceptual framework in which many of the contributions to this Special Issue on Photovoltaics can be viewed. We successively turn our attention to (i) optical absorption and exciton formation, (ii) exciton migration to the donor-acceptor interface, (iii) exciton dissociation into charge carriers, resulting in the appearance of holes in the donor and electrons in the acceptor, (iv) charge-carrier mobility, and (v) charge collection at the electrodes. For each of these processes, we also describe the theoretical challenges that need to be overcome to gain a comprehensive understanding at the molecular level. Finally, we highlight recent theoretical advances, in particular regarding the determination of the energetics and dynamics at organic-organic interfaces, and underline that the right balance needs to be found for the optimization of material parameters that often result in opposite effects on the photovoltaic performance.