From the Mechanism to the Device in Polymer-Assisted Rubrene Crystallization

The rational design of organic crystalline materials is exceedingly challenging owing to the insufficient understanding of crystallization mechanisms. Here, we used a polymer (polystyrene) as a crystallization medium for an organic semiconductor (rubrene). This enabled a slow crystallization process whose mechanism was elucidated via direct transmission electron microscopy imaging and further employed to control crystallization and fabricate devices based on rubrene crystals. The elucidated mechanism involved (1) the initial formation of amorphous aggregates; (2) nucleation within the aggregates; (3) crystal growth; (4) break up into single nanocrystals; and (5) oriented attachment of nanocrystals to form platelet-like crystals. This mechanistic insight indicated that uniform nonclassical nucleation can be realized using a precise temperature control, leading to high-quality rubrene crystals. By employing this methodology, we fabricated solution-processed organic field-effect transistors (OFETs) and organic phototransistors (OPTs) that exhibited high mobility, reproducibility, and environmental stability. The devices showed an average mobility of 1 ± 0.8 cm2 V-1 s-1, a threshold voltage of -10 ± 6 V, and an on/off ratio of up to 106. Under white light irradiation, rubrene OPTs exhibited strong photoresponse with a photo/dark current ratio of P ≈ 105. Our work demonstrated that mechanistic information can be employed to fabricate high-quality OFETs, having implications for the rational design of crystalline organic electronic materials.