Impact of Marginal Exciton-Charge-Transfer State Offset on Charge Generation and Recombination in Polymer: Fullerene Solar Cells

Michelle S. Vezie, Mohammed Azzouzi, Andrew M. Telford, Thomas R. Hopper, Alexander B. Sieval, Jan C. Hummelen, Kealan Fallon, Hugo Bronstein, Thomas Kirchartz, Artem A. Bakulin, Tracey M. Clarke, Jenny Nelson*

*Corresponding author for this work

Research output: Contribution to journalArticleAcademicpeer-review

7 Citations (Scopus)


The energetic offset between the initial photo excited state and charge-transfer (CT) state in organic heterojunction solar cells influences both charge generation and open-circuit voltage (V-oc). Here, we use time-resolved spectroscopy and voltage loss measurements to analyze the effect of the exciton-CT state offset on charge transfer, separation, and recombination processes in blends of a low-band-gap polymer (INDT-S) with fullerene derivatives of different electron affinity (PCBM and KL). For the lower exciton-CT state offset blend (INDT-S:PCBM), both photocurrent generation and non-radiative voltage losses are lower. The INDT-S:PCBM blend shows different excited-state dynamics depending on whether the donor or acceptor is photoexcited. Surprisingly, the charge recombination dynamics in INDT-S:PCBM are distinctly faster than those in INDT-S:KL upon excitation of the donor. We reconcile these observations using a kinetic model and by considering hybridization between the lowest excitonic and CT states. The modeling results show that this hybridization can significantly reduce V-oc losses while still allowing reasonable charge generation efficiency.

Original languageEnglish
Pages (from-to)2096-2103
Number of pages15
JournalACS Energy Letters
Issue number9
Publication statusPublished - Sep-2019


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