Organic Photovoltaics Using Multiple Exciton Effects

Abstract

Multiple exciton effects in organic semiconductors have the potential to improve photon utilization in solar cells. When a semiconductor absorbs light, photons with energy below the bandgap are transmitted, while photoexcitations with energy above the bandgap are rapidly thermalized to the band edge. Consequently, large swaths of the solar spectrum remain unabsorbed while at the same time, the energy of most absorbed photons are not harvested in full. Together, sub-bandgap transmission losses and above-bandgap thermalization losses account for the vast majority of fundamental energy losses in a solar cell, and any strategies to circumvent these mechanisms have great potential to bring solar cells beyond the Shockley-Queisser limit.

Unlike inorganic semiconductors, organic semiconductors are excitonic. Notably, they possess triplet states that are distinct from (and lower in energy than) the optically excitable singlet state. This means that while the singlet state defines the absorption threshold of the material, there are mid-gap states that can be populated and exploited for novel purposes. Singlet fission is a process whereby a high energy singlet exciton is divided into two triplet excitons with approximately half of the singlet energy. Triplet-triplet annihilation is the reverse process, whereby two triplets pool their energies together to form a higher energy singlet.

In the first part of this work, we investigate organic solar cells containing singlet fission absorbers and find that the donor-acceptor charge transfer state energy is highly dependent on the interfacial morphology. In particular, interfaces that exhibit a higher degree of structural order are correlated with stabilized charge transfer states that are more efficient at dissociating triplets generated by the singlet fission absorber.

In the second part of this work, we present a proof-of-concept solid-state organic intermediate band solar cell that is able to harvest sub-bandgap photons by taking advantage of triplet-triplet annihilation upconversion. Next, we outline directions for using low-dimensional metal-halide perovskite materials as triplet sensitizers. Finally, we identify several simple and intuitive design principles governing charge transfer at interfaces between two-dimensional perovskite and organic semiconductors, with the goal of extending these principles to that of triplet-triplet energy transfer in future work.