Abstract
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 language | English |
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Pages (from-to) | 2096-2103 |
Number of pages | 15 |
Journal | ACS Energy Letters |
Volume | 4 |
Issue number | 9 |
DOIs | |
Publication status | Published - Sep-2019 |
Keywords
- QUANTUM EFFICIENCY
- ABSORPTION
- SEPARATION
- ENERGY
- GAP
- DISSOCIATION
- DYNAMICS