Triplet‐Charge Annihilation in a Small Molecule Donor: Acceptor Blend as a Major Loss Mechanism in Organic Photovoltaics

Jose M Marin‐Beloqui,Naitik A Panjwani,Daniel T W Toolan,Tracey M Clarke,Ji‐Seon Kim,Saurav Limbu

doi:10.1002/aenm.202100539

Abstract

AbstractOrganic photovoltaics (OPV) are close to reaching a landmark 20% device efficiency. One of the proposed reasons that OPVs have yet to attain this milestone is their propensity toward triplet formation. Herein, a small molecule donor, DRCN5T, is studied using a variety of morphology and spectroscopy techniques, and blended with both fullerene and non‐fullerene acceptors. Specifically, grazing incidence wide‐angle X‐ray scattering and transient absorption, Raman, and electron paramagnetic resonance spectroscopies are focused on. It is shown that despite DRCN5T's ability to achieve OPV efficiencies of over 10%, it generates an unusually high population of triplets. These triplets are primarily formed in amorphous regions via back recombination from a charge transfer state, and also undergo triplet‐charge annihilation. As such, triplets have a dual role in DRCN5T device efficiency suppression: they both hinder free charge carrier formation and annihilate those free charges that do form. Using microsecond transient absorption spectroscopy under oxygen conditions, this triplet‐charge annihilation (TCA) is directly observed as a general phenomenon in a variety of DRCN5T: fullerene and non‐fullerene blends. Since TCA is usually inferred rather than directly observed, it is demonstrated that this technique is a reliable method to establish the presence of TCA.

Highlights

Triplets can be generated via several different pathways in organic solar cells.[8,9] First, the photogenerated singlet excitonOrganic photovoltaics (OPV) have improved notably in effi- can undergo intersystem crossing (ISC)
The blend of DRCN5T with PC70BM causes a 0.14 eV blue shift of the absorbance maximum to 550 nm. This blue-shift may indicate that the introduction of the fullerene causes a distortion in the crystal lattice of the DRCN5T domains, similar to that reported in other materials.[45,46,47]
To restore the crystallinity of the blend film, thermal annealing was performed for 20 min at 120 °C: these conditions are known to enhance OPV efficiency in DRCN5T:PC70BM blends.[31,32]

Summary

Introduction

Triplets can be generated via several different pathways in organic solar cells.[8,9] First, the photogenerated singlet excitonOrganic photovoltaics (OPV) have improved notably in effi- can undergo intersystem crossing (ISC). Triplets can be generated via several different pathways in organic solar cells.[8,9] First, the photogenerated singlet exciton. Triplets can ciency in the last few years, reaching over 18% in single junction form via charge transfer (CT) states at the donor/acceptor devices.[1,2] Despite this rapid enhancement, there are still sev- interface. These CT states can undergo rapid spin-mixing due eral aspects in these devices’ functions that are not completely to the low exchange energy between 3CT and 1CT states, often.

Results

Discussion

Conclusion