Singlet Fission Copolymer Design: An Approach toward building Third Generation Solar Cells

Singlet Fission (SF) is a process, in which a singlet excited state is converted into two triplet excited states within a molecular system ^[1] . From an application point of view, SF in the molecular semiconductor is known to generate triplet excitons that are energetically matched to the bandgap of silicon or perovskite ^[2] . When the triplet excitons generated by SF process are transferred to semiconductors, they create additional electron–hole pairs, which made SF one of the promising ways to surpass the power conversion efficiency limit of conventional single-junction solar cells (Shockley-Quisser limit)^[3] . Recently, materials with the potential to undergo SF processes have become a hot topic in designing third-generation solar cells.

One of the challenges of realizing SF in the real world is that structures of organic materials capable of SF have been limited. A more widely applicable route to the SF would be based on the intramolecular SF process (iSF), where the SF efficiency relies on the intrinsic property of the molecule. This process provides insight into utilize SF molecules as building blocks in designing SF polymers with tunable chemical structures. Starting with different designing strategies, SF alternating copolymer ^[4] and SF branched copolymer ^[5] have been synthesized and evaluated as competitive candidates for SF devices.

[1] Casillas, R., Papadopoulos, I., Ullrich, T., Thiel, D., Kunzmann, A., & Guldi, D. M. (2020). Molecular insights and concepts to engineer singlet fission energy conversion devices. Energy & Environmental Science, 13(9), 2741– 2804. https://doi.org/10.1039/d0ee00495b

[2] Einzinger, M., Wu, T., Kompalla, J. F., Smith, H. L., Perkinson, C. F., Nienhaus, L., Wieghold, S., Congreve, D. N., Kahn, A., Bawendi, M. G., & Baldo, M. A. (2019). Sensitization of silicon by singlet exciton fission in tetracene. Nature, 571(7763), 90–94. https://doi.org/10.1038/s41586-019- 1339-4

[3] Xia, J., Sanders, S. N., Cheng, W., Low, J. Z., Liu, J., Campos, L. M., & Sun, T. (2017). Singlet Fission: Progress and Prospects in Solar Cells. Advanced Materials, 29(20). https://doi.org/10.1002/adma.201601652

[4] Busby, E., Xia, J., Wu, Q., Low, J. Z., Song, R., Miller, J. R., Zhu, X. Y., Campos, L. M., & Sfeir, M. Y. (2015). A design strategy for intramolecular singlet fission mediated by charge-transfer states in donor-acceptor organic materials. Nature Materials, 14(4), 426–433. https://doi.org/10.1038/nmat4175

[5] Yablon, L. M., Sanders, S. N., Li, H., Parenti, K. R., Kumarasamy, E., Fallon, K. J., Hore, M. J. A., Cacciuto, A., Sfeir, M. Y., & Campos, L. M. (2019). Persistent Multiexcitons from Polymers with Pendent Pentacenes. Journal of the American Chemical Society, 141(24), 9564–9569. https://doi.org/10.1021/jacs.9b02241