Charge Transport in Organic Semiconducting Crystals Exhibiting TADF: Insight from Quantum Chemical Calculations

Luminophores featuring thermally activated delayed fluorescence (TADF) are the workhorses of the third- and fourth-generation OLEDs. While these compounds have usually been used as dopants embedded in the host, non-doped TADF OLEDs have recently shown significant progress as well and have attained performances comparable to those of the host-dopant systems. For efficient operation of non-doped OLEDs, the charge transport in neat films and single crystals of TADF luminophores is important; however, this issue was nearly unexplored theoretically. In the current study, we calculated the charge-carrier mobilities in four single crystals of TADF luminophores that have different molecular packing motifs. Specifically, in one of them both the donor and acceptor moieties form uniform π-stacks, while in the others the donors (acceptors) show alternating lateral shifts along the stacks; the difference in the molecular packing resulted in the difference in the transfer integrals between the molecules. The reorganization energies differed as well by up to four times for the studied crystals. As a result, the charge mobilities varied from 0.001 to ~0.3 cm2/(V∙s), with the largest being predicted for the crystal of the luminophore that consisted of a rigid donor and acceptor. We anticipate that the results obtained will be useful in the design of TADF luminophores for non-doped OLEDs, OLETs, and other organic light-emitting devices

Intrachain Aggregation of Charge-Transfer Complexes in Conjugated Polymer:Acceptor Blends from Photoluminescence Quenching

Recent studies of conjugated polymer donor–acceptor blends show that the donor and acceptor can form a weak charge-transfer complex (CTC) in the electronic ground state, and these CTCs can significantly change the photophysics in the blend. In this work, we study photoluminescence quenching in model polymer acceptor blends of poly­(methoxy,5-(2′-ethyl-hexyloxy-1,4-phenylene-vinylene)) (MEH-PPV) with TNF (2,4,7-trinitrofluorenone) in solution. Our experimental data show that the observed strong increase in the CTC concentration with acceptor content results only in a moderate quenching enhancement. We propose an extended Stern–Volmer relation to model photoluminescence quenching in conjugated polymers with statistically homogeneous distribution of CTCs over polymer chains. We compare the experimental data with the model and conclude that the CTCs are not randomly distributed within a chain but form intrachain CTC clusters. These findings imply that the CTCs can influence the morphology of donor–acceptor blends, which is of paramount importance for the performance of organic solar cells

Dual Optoelectronic Organic Field-Effect Device: Combination of Electroluminescence and Photosensitivity

Merging the functionality of an organic field-effect transistor (OFET) with either a light emission or a photoelectric effect can increase the efficiency of displays or photosensing devices. In this work, we show that an organic semiconductor enables a multifunctional OFET combining electroluminescence (EL) and a photoelectric effect. Specifically, our computational and experimental investigations of a six-ring thiophene-phenylene co-oligomer (TPCO) revealed that this material is promising for OFETs, light-emitting, and photoelectric devices because of the large oscillator strength of the lowest-energy singlet transition, efficient luminescence, pronounced delocalization of the excited state, and balanced charge transport. The fabricated OFETs showed a photoelectric response for wavelengths shorter than 530 nm and simultaneously EL in the transistor channel, with a maximum at ~570 nm. The devices demonstrated an EL external quantum efficiency (EQE) of ~1.4% and a photoelectric responsivity of ~0.7 A W–1, which are among the best values reported for state-of-the-art organic light-emitting transistors and phototransistors, respectively. We anticipate that our results will stimulate the design of efficient materials for multifunctional organic optoelectronic devices and expand the potential applications of organic (opto)electronics

Structure-based rational design of an enhanced fluorogen-activating protein for fluorogens based on GFP chromophore

Structural analyses guide the rational design of mutant fluorogen-activating proteins with enhanced fluorescence