ИСТИНА

Thiophene-phenelyne co-oligomers (TPCO) single crystals are promising materials for organic light emitting transistors (OLETs) and injection lasers as they combine high luminescence yield with efficient charge transport.i However, the nature of excellent luminescent properties of TPCO single crystals have not yet been properly understood. In this work, we demonstrate that luminescent properties of 4-5-ring TPCO can be dominated by impurities that are longer TPCO unintentionally appeared during the synthesis. Trying different synthetic routes to TPCO having the conjugated core PTTP and PTPTP (P and T are phenylene and thiophene rings, respectively) with various end groups (H, F, CF3, trimethylsilyl), we have identified the presence of longer TPCOs at a concentration less than 1%. The TPCO crystals were grown from the vapor phase and from solution. We have found that the photoluminescence (PL) spectrum and quantum efficiency (QE) of the TPCO single crystals are strongly varied with the concentration of longer TPCO, as they have a lower bandgap energy. These dopants act as an excitation energy funnel (Fig.1a) usually increasing the PL QE in some cases even higher than that of the diluted solution. Surprisingly, the unintentional doping did not show any noticeable effect on the hole mobility so that is beneficial for OLETs. The crystal growth via physical vapor phase transport (PVT) technique (Fig. 1b) was found to decrease the impurity concentration in the TPCO crystals. Fig.1c shows that the crystals distributed along the growth zones have significantly different PL spectrum and QE. The solution growth methods resulted in TPCO crystals containing the amount of impurities similar to the initial material (powder). Fig. 1. a) Schematic of energy funneling from host TPCO to dopants; b) PVT growth setup, 1,2,3 denote growth zones; c) PL spectra of crystals grown in zones 1,2,3. We conclude that TPCO doping by longer co-oligomers is a promising route to more efficient optoelectronic materials and devices converting electricity to light. This work was supported by RFBR (project № 15-02-09375), Russian Science Foundation (project № 15-12-30031) and Lomonosov MSU Program of Development.