Long‐Range Resonant Energy Transfer for Enhanced Exciton Harvesting for Organic Solar Cells

Abstract

Exciton harvesting is of fundamental importance for the efficient operation of organic photovoltaic devices. The quantum efficiencies of many organic and hybrid organic-inorganic devices are still limited by low exciton harvesting efficiencies. This problem is most apparent in planar heterostructures that suffer from a direct tradeoff between light absorption and exciton harvesting. The bulk heterojunction concept [1,2] was designed to alleviate the problem of limited exciton migration by intimately blending the donor and acceptor phases on the nanometer length scale. In some polymer/fullerene systems, such as poly(2-methoxy,5-(3,7-dimethyloctyloxy)-1,4phenylenevinylene) (MDMO-PPV)/(6,6)-phenyl C61-butyric acid methyl ester (PCBM), time resolved spectroscopy shows the photoinduced formation of the radical anions and cations on femtosecond timescales. [3] This ultrafast formation of the polaron signature is only possible if every exciton is formed on a polymer chain segment that is immediately adjacent to one or more fullerene molecules. However, in other systems, very large domains prevail and, consequently, exciton harvesting is inefficient. [4] This has made the fabrication of efficient devices incorporating new materials difficult because each new material leads to a new morphology with its own characteristic length scale. The ordered bulk heterojunction architecture is intended to alleviate these issues by using a pre-patterned nanostructured scaffold [5–7] that has been engineered to have both straight pathways to the electrodes to ensure efficient carrier collection, and controlled domain size to ensure efficient exciton harvesting. Recently, chain alignment has been shown to be promoted for regioregular poly(3-hexyl thiophene) (RR-P3HT) in straight nanopores of anodic alumina leading to a 20-fold increase in hole-mobility in these structures. [8] For solar cell applications, higher mobilities reduce the effects of space charge [9] and increase the probability of separating the geminate pair formed immediately after exciton dissociation. [10–13] While the ordered bulk heterojunction architecture shows promise, existing structures have domains too large [14,15] for efficient exciton harvesting with singlet diffusion lengths only ca. 3–8 nm. [16–18] At this time, few alternatives other than nanostructuring have been proposed to increase exciton harvesting. Triplet excitons have been shown to have large diffusion lengths due to their long lifetimes, [18–20] but except in a few examples [21,22] this usually comes at the expense of a loss in energy of 0.4–0.8 eV associated with intersystem crossing between the photoexcited singlet to the first excited triplet. [23–25]