Unsymmetrical Small Molecules for Broad-Band Photoresponse and Efficient Charge Transport in Organic Phototransistors

Organic photosensitizers have been investigated as effective light-sensing elements that can promote strong absorption with high field-effect mobility in organic phototransistors (OPTs). In this study, a novel organic photosensitizer is synthesized to demonstrate broad-band photoresponse with enhanced electrical performance. An unsymmetrical small molecule of a solubilizing donor (Dsol)-acceptor (A)-dye donor (Ddye) type connected with a twisted conjugation system is designed for broad-band detection (ranging from 250 to 700 nm). This molecule has high solubility, thereby facilitating the formation of uniformly dispersed nanoparticles in an insulating polymer matrix, which is deposited on top of OPT semiconductors by a simple solution process. The broad-band photodetection shown by the organic photosensitizer is realized with improved mobility close to an order of magnitude and high on/off current ratio (∼105) of the organic semiconductor. Furthermore, p-type charge transport behavior in the channel of the OPT is enhanced through the intrinsic electron-accepting ability of the organic photosensitizer caused by the unique molecular configuration. These structural properties of organic photosensitizers contribute to an improvement in broad-band photosensing systems with new optoelectronic properties and functionalities.