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

Rishi Shivhare,Stefan C B Mannsfeld,Anton Kiriy,Mike Hambsch,Andreas Hofacker,Natalie Banerji,Yufei Zhong,Frank Ortmann,Tim Erdmann,Gareth John Moore,Sebastian Hutsch

doi:10.1002/adma.202101784

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 (≈14ps) 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 150ps. 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.

Highlights

Using femtosecond transient absorption and electroabsorption spectroscopy, is important for commercial realizathe charge transfer (CT) and recombination dynamics as well as the early-time transport are quantified
To probe the S1 excited state lifetime and electron transfer dynamics, we carried out TA spectroscopy on the pristine polymer and the polymer:fullerene blend films
We have investigated a blend of a DPP polymer and the PC[70]BM fullerene derivative with a near-zero (≈50–70 meV) driving force for the electron transfer process

Summary

Results and Discussion

To probe the S1 excited state lifetime and electron transfer dynamics, we carried out TA spectroscopy on the pristine polymer and the polymer:fullerene blend films. The significant electron back-transfer to the S1 state observed here explains the relatively high photoluminescence that was reported earlier for this blend system.[14] Another key finding is that the average CT dissociation rate (CT → F) is about 65 ps, which is orders of magnitude slower than the ultrafast (≈100 fs) free charge generation reported for typical polymer:fullerene blends.[27,39,40,41] Such slow CT state dissociation has been observed for other systems with low driving force for charge transfer.[42,43] This suggests that an energetic S1–CT offset is necessary for excitons to directly couple into delocalized states of fullerene clusters and enable prompt separation of electron–hole pairs. The effect is modified by the relative broadness of the S1 and F manifolds: A large S1 width enhances back-transfer and recombination, while a large F width enhances dissociation

Conclusion

Experimental Section

Data Availability Statement