A phototransistor is a light-sensitive transistor that have integrated function of photodiode and transistor. They convert light signals, which are very low intensity or wavelength range of limited human vision, into perceivable electrical signals. Such phototransistors have attracted a great deal of research attentions because they show various applications such as optical communication, remote control systems, and biological health monitoring and imaging.
In most studies on phototransistors, organic semiconductors have been intensively considered as promising materials owing to their flexibility, low-temperature and solution processability. Photo-OFETs (photo-organic field effect transistors) generally use a single semiconductor layer that can offer both light-sensing and amplification. However, it can be a trade-off in terms of material selection because photo-induced charge excitation and charge-carrier transportation appear within the same channel. An approach for efficient electrical modulation and light sensing is to employ a sensitizer, providing high absorption and heterogeneous integration with the FET channel. Moreover, the concept of the isolated sensitizer on the channel layer has been proposed for enhanced absorption, ideally without interfering with the electrical modulation.
In this presentation, we report synthesis of novel unsymmetrical small molecule as a sensitizer of the photo-OFETs. The cross-conjugation system as a strategy of enhancing the intramolecular charge transfer (ICT) interactions and extending the absorption range is proposed for the molecular design of donor-acceptor-donor type, TFP (triphenlyamine-fluorinated thiadiazole-pyrene), with π-conjugated spacer. The TFP presents wide absorption coverage (250 nm to 700 nm), attributed to the structural property of donor-acceptor-donor type, and has high solubility in various solvent caused by a tilted unsymmetrical an end group. The well-dissolved TFP in orthogonal solvents of the semiconducting materials enable to form a thin film through solution processing without damage to the underlying semiconducting layer. Moreover, in sensitized photo-OFET devices, it exhibits broadband photoresponse with enhanced charge carrier mobility and On/Off ratio. The various properties of TFP are analyzed by a combination of Differential Scanning Calorimetry (DSC), Thermogravimetric analysis (TGA), UV-Vis absorption, cyclic voltammetry and atomic force microscopy (AFM). The results indicate that the molecular design of organic sensitizer in the proposed system is a new approach not only for improving the photosensitivity, but also for ideal FET performance.