Since the pioneering work on thermally activated delayed fluorescence (TADF) emitters in the organic light-emitting diodes (OLEDs) for the triplet harvesting by Adachi, the external quantum efficiencies (EQEs) of TADF-OLEDs have seen a rapid increase in the past few years. In principle, the same concept can be implemented in organic photovoltaics (OPVs) to minimize the losses caused by the non-radiative decays through intersystem crossing to the triplet states. The absence of TADF dyes for efficient OPVs can be associated with numerous molecular design considerations and corresponding difficulties. There are indeed very limited examples of efficient low band gap TADF dyes and nearly all reported TADF compounds have very high LUMO levels (&gt; 3 eV), which make them unsuitable as either acceptor components for OPVs. Recently, Zheng et al. reported a record-breaking orange-red TADF-OLED based on a D-A small molecule, namely NAI-DMAC, featuring N-(4-tert-butylphenyl)-1,8-naphthalimide (NAI) as an electron acceptor unit and 9,9-dimethyl-9,10-dihydroacridine (DMAC) as donor unit. On account of highly rigid molecular architecture and strongly pre-twisted CT state between NAI and DMAC, this luminophore shows orange-red emission with relatively high photoluminescence quantum yield and small energy gap (ΔEST) between the lowest singlet (S1) and triplet (T1) excited states to ensure efficient TADF. Inspired by this work, herein we report on D-A or A-D-A type small molecules, featuring naphthalenediimide (NDI) and perylenediimide (PDI) as acceptor unit and DMAC as donor unit, as non-fullerene acceptor for OPV applications. NDI and PDI are chosen for their rigid and planar structure similar to NAI but with much stronger electron affinities. We show this new design maintains the TADF characteristics yet allowing a lower LUMO level to ensure efficient photoinduced electron transfer from common high-performance donor polymers.

Non-fullerene organic solar cells based on thermally activated delayed fluorescence active acceptors

ACS Fall 2020 Virtual Meeting

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