Understanding the Role of Triplet‐Triplet Annihilation in Non‐Fullerene Acceptor Organic Solar Cells

• Published in: Advanced energy materials 2023-09, Vol.13 (36), p.n/a
• Main Authors: Hart, Lucy J. F., Grüne, Jeannine, Liu, Wei, Lau, Tsz‐ki, Luke, Joel, Chin, Yi‐Chun, Jiang, Xinyu, Zhang, Huotian, Sowood, Daniel J. C., Unson, Darcy M. L., Kim, Ji‐Seon, Lu, Xinhui, Zou, Yingping, Gao, Feng, Sperlich, Andreas, Dyakonov, Vladimir, Yuan, Jun, Gillett, Alexander J.
• Format: Article
• Language: English
• Subjects: Circuits; Dipole moments; Electric potential; Energy conversion efficiency; Excitons; Fullerenes; non‐radiative voltage losses; organic solar cells; photoluminescence‐detected magnetic resonance; Photovoltaic cells; Radiative recombination; Solar cells; transient absorption; triplet excitons; triplet‐triplet annihilation; Voltage
• ISSN: 1614-6832; 1614-6840; 1614-6840
• DOI: 10.1002/aenm.202301357

Non‐fullerene acceptors (NFAs) have enabled power conversion efficiencies exceeding 19% in organic solar cells (OSCs). However, the open‐circuit voltage of OSCs remains low relative to their optical gap due to excessive non‐radiative recombination, and this now limits performance. Here, an important aspect of OSC design is considered, namely management of the triplet exciton population formed after non‐geminate charge recombination. By comparing the blends PM6:Y11 and PM6:Y6, it is shown that the greater crystallinity of the NFA domains in PM6:Y11 leads to a higher rate of triplet‐triplet annihilation (TTA). This is attributed to the four times larger ground state dipole moment of Y11 versus Y6, which improves the long range NFA out‐of‐plane ordering. Since TTA converts a fraction of the non‐emissive triplet states into bright singlet states, it has the potential to reduce non‐radiative voltage losses. Through a kinetic analysis of the recombination processes under 1‐Sun illumination, a framework is provided for determining the conditions under which TTA may improve OSC performance. If these could be satisfied, TTA has the potential to reduce non‐radiative voltage losses by up to several tens of millivolts. In this work, the role of triplet‐triplet annihilation in organic solar cells is examined. It is shown that improving the crystallinity of the non‐fullerene acceptor domains can enhance the rate of triplet‐triplet annihilation. The potential for triplet‐triplet annihilation to improve the performance of organic solar cells is then considered where, in certain scenarios, it can improve the open circuit voltage by several tens of millivolts.