Short Excited-State Lifetimes Mediate Charge-Recombination Losses in Organic Solar Cell Blends with Low Charge-Transfer Driving Force

Research output: Contribution to journalResearch articleContributedpeer-review

Contributors

Abstract

A blend of a low-optical-gap diketopyrrolopyrrole polymer and a fullerene derivative, with near-zero driving force for electron transfer, is investigated. Using femtosecond transient absorption and electroabsorption spectroscopy, the charge transfer (CT) and recombination dynamics as well as the early-time transport are quantified. Electron transfer is ultrafast, consistent with a Marcus–Levich–Jortner description. However, significant charge recombination and unusually short excited (S1) and CT state lifetimes (≈14 ps) are observed. At low S1–CT offset, a short S1 lifetime mediates charge recombination because: i) back-transfer from the CT to the S1 state followed by S1 recombination occurs and ii) additional S1–CT hybridization decreases the CT lifetime. Both effects are confirmed by density functional theory calculations. In addition, relatively slow (tens of picoseconds) dissociation of charges from the CT state is observed, due to low local charge mobility. Simulations using a four-state kinetic model entailing the effects of energetic disorder reveal that the free charge yield can be increased from the observed 12% to 60% by increasing the S1 and CT lifetimes to 150 ps. Alternatively, decreasing the interfacial CT state disorder while increasing bulk disorder of free charges enhances the yield to 65% in spite of the short lifetimes.

Details

Original languageEnglish
Article number2101784
JournalAdvanced materials
Volume34
Issue number22
Publication statusPublished - 2 Jun 2022
Peer-reviewedYes

External IDs

WOS 000685435800001
ORCID /0000-0002-8487-0972/work/142247510

Keywords

DFG Classification of Subject Areas according to Review Boards

Subject groups, research areas, subject areas according to Destatis

Keywords

  • charge transfer, energy materials, organic solar cells, photophysics, ultrafast spectroscopy, Charge transfer, Organic solar cells, Photophysics, Ultrafast spectroscopy, Energy materials