Title: Role of exchange excitons in organic optoelectronic devices

Abstract

Most organic and perovskite optoelectronic devices, such as organic solar cells (OSCs), perovskite solar cells (PSCs), organic light emitting diodes (OLEDs) and organic photo detectors (OPDs), involve either charge generation or light emission or both which occur at the interfaces between a donor (D) and an acceptor (A). It is this interface where the charge transfer (CT) excitons are generated in OSCs, PSCs and OPDs from the photo excited excitons and in OLEDs by recombination of injected charge carrier pairs. The charge transfer(CT) exciton states have been well identified as precursors for charge generation in OSCs and charge recombination in the thermally activated delayed fluorescence (TADF) based (third generation) OLEDs. There are two issues of very significant importance which have not yet been resolved: (i) Zhang et al. have reported 100% internal quantum efficiency from purely organic based TADF OLEDs, which means triplet excitons recombine radiatively without the aid of any heavy metal atoms and that means very weak spin-orbit coupling. It is very puzzling that without the significant strength of spin-orbit coupling how can triplet excitons recombine radiatively. (ii) Although the operation of OSCs and OLEDs is reversible, both the operations have not yet been achieved in a single device.