Charge Transfer in Ternary Solar Cells Employing Two Fullerene Derivatives: Where do Electrons Go?

TL;DR

This study reveals how electrons redistribute in ternary organic solar cells with mixed fullerene acceptors, showing reduced energy differences and explaining voltage shifts, which supports the use of multi-acceptor blends for improved solar cell performance.

Contribution

It provides direct experimental evidence of electron redistribution and LUMO hybridization in ternary OSCs with mixed fullerene acceptors, clarifying charge transfer mechanisms.

Findings

Electrons redistribute over acceptors according to stoichiometry and LUMO levels.

Reduced delta LUMO explains open circuit voltage shifts.

Multi-acceptor blends are effective for solar cells and photodetectors.

Abstract

Earlier reports demonstrated that ternary organic solar cells (OSC) made of donor polymers (D) blended with different mixtures of fullerene acceptors (A:A) performed very similarly. This finding is surprising, as the corresponding fullerene LUMO levels are slightly different, which might result in decisive differences in the charge transfer step. We investigate ternary OSC (D:A:A) made of the donor polymer P3HT with stoichiometric mixtures of different fullerene derivatives, PC60BM:PC70BM and PC70BM:IC60BA, respectively. Using quantitative electron paramagnetic resonance (EPR) we can distinguish between positive and negative polarons, localized on the specific molecules. We found that after the initial charge transfer step, the electrons are re-distributed over two nearby acceptors in agreement with their stoichiometry and their relative LUMO energy difference. Remarkably, the measured delta LUMO differences in fullerene mixtures are reduced by an order of magnitude compared to that of the pristine materials, i.e., below 1 meV for PC60BM:PC70BM and (20 +/- 5) meV for PC70BM:IC60BA. Furthermore, we found that this reduced delta LUMO explains the shift in open circuit voltage for D:A:A organic solar cells. We attribute these findings to hybridization, leading to an effective fullerene LUMO. Consequently, multi-acceptor blends are indeed a viable option for photodetectors and solar cells, as they combine the best electron acceptor and light absorbing properties.