Molecular understanding of organic solar cells: The challenges

A key feature of π-conjugated organic semiconductors that has impacted the design and geometry of organic photovoltaic devices for the past decades is the excitonic character of their optical properties. While optical absorption in a conventional inorganic semiconductor results in the immediate creation of free charge carriers, it leads in an organic semiconductor to the formation of a spatially localized electron-hole pair, i.e., an exciton, which is electrically neutral. In order to generate an electrical current, the exciton must first dissociate; this is the reason why a critical component in the architecture of organic solar cells is the design of the heterojunction between an electron-donor (D) material and an electron-acceptor (A) material. In this presentation, we describe some of the electronic and optical processes that take place during the operation of a bulk-heterojunction organic solar cell with a focus on the D/A interface, discuss recent theoretical advances, and highlight a number of theoretical challenges that still need to be met in order to gain a comprehensive understanding of organic solar cells at the molecular level.