Encapsulated Polymers for Organic Photovoltaics

In an organic solar cell (OSC), the interface between the electron donor and acceptor materials facilitates the key charge generation and recombination processes that directly govern the power conversion efficiency. Therefore, it is expected that OSC performance will be extremely sensitive to any disturbances in this critical region. Here, we challenge the accepted requirement that donor and acceptor molecules must have close intermolecular contacts for efficient OSC operation. This is achieved by ‘encapsulating’ the donor polymer by linking two parts of the backbone together using alkoxy bridging groups. Thus, encapsulation introduces bulk to the molecule and prevents the close approach of neighbouring species[1]<br/>We demonstrate this approach in a model system based on the benchmark PM6:Y6 blend[2]. We have synthesised an encapsulated PM6 derivative, ‘EP’, and fabricated OSC devices with the acceptor material Y6. Surprisingly, we find that despite significantly disrupting the interfacial morphology, the power conversion efficiency of our EP:Y6 device remains comparable to the reference PM6:Y6 blends. Transient absorption measurements indicate that while charge transfer is slightly slower in the encapsulated system, this process remains efficient despite the more distant donor-acceptor interactions. These results indicate that the donor-acceptor interface may not be as critical as previously suggested for obtaining high performance OSC blends. In addition, we find evidence that increasing the interfacial donor-acceptor separation through encapsulation also suppresses undesirable non-geminate recombination into the Y6 triplet exciton, providing a novel strategy for overcoming this critical loss pathway in OSCs[3].<br/><br/>[1] A. Leventis et al., <i>J. Am. Chem. Soc.</i>, vol. 140, pp. 1622–1626, 2018.<br/>[2] J. Yuan et al., <i>Joule</i>, vol. 3, pp. 1140–1151, 2019.<br/>[3] A. J. Gillett et al., <i>Nature</i>, vol. 597, pp. 666–671, 2021.