High-performance organic phototransistors (OPTs) based on single-crystalline n-channel organic nanowires could miniaturize electronic and optoelectronic devices, yielding higher light sensitivity than their bulk counterparts. Phototransistors are semiconductor devices in which the incident light intensity can modulate the charge-carrier density in the channel. Compared with conventional photodiodes, phototransistors enable easier control of light-detection sensitivity without the noise increment. However, to date, most research has focused on thin-film OPTs; nanoscale OPTs have scarcely been reported. Now, researchers Joon Hak Oh and Hojeong Yu of Ulsan National Institute of Science and Technology (UNIST) and Zhenan Bao of Stanford University in California have developed n-channel single-crystalline nanowire organic phototransistors (NW-OPTs) that provide enhanced charge-carrier mobility. “The development of OPTs based on n-channel single-crystalline organic semiconducting NWs/MWs [nanowires/microwires] is highly desirable for the bottom-up fabrication of complementary metal oxide semiconductor (CMOS)-like photoelectronic circuits, which provides various advantages such as high operational stability, easy control of photoswitching voltages, high photosensitivity and responsivity,” said Oh, an assistant professor at UNIST’s School of Nano-Bioscience & Chemical Engineering. Single-crystalline NWs/MWs based on organic semiconductors have drawn the interest of scientists because of their promise as building blocks for various electronic and optoelectronic applications. In particular, OPTs based on single-crystalline NWs/MWs may yield higher light sensitivity than their bulk counterparts. In addition, their one-dimensional, intrinsically defect-free and highly ordered nature could provide a better understanding of the fundamental mechanisms of charge generation and transport in OPTs, while enabling a bottom-up fabrication of optoelectronic nanodevices. The photoelectronic characteristics of the single-crystalline NW-OPTs, such as the photoresponsivity, the photoswitching ratio and the photoconductive gain, were analyzed from the I-V characteristics coupled with light irradiation and compared with those of vacuum-deposited thin-film devices. The external quantum efficiencies also were investigated for the NW-OPTs and thin-film OPTs. The investigators also calculated the charge accumulation and release rates from deep traps and examined the effects of incident light intensity on their photoelectronic properties. A mobility enhancement was observed, the scientists say, when the incident optical power density increased and the wavelength of the light source matched the light-absorption rate of the photoactive material. They also discovered that the photoswitching ratio is strongly dependent on the incident optical power density, whereas the photoresponsivity is more dependent on matching the light-source wavelength with the maximum absorption range of the photoactive material. “Our approach to charge-accumulation/release-rate calculations could provide a fundamental understanding about charge-carrier-density variations under light irradiation, which subsequently enables in-depth study of OPTs,” Oh said. “Hence, organic single-crystalline NW-OPTs are a highly promising alternative to conventional thin-film-type photodiodes and can effectively pave the way for optoelectronic device miniaturization.” The research — supported by a National Research Foundation of Korea grant funded by the Ministry of Education, Science, and Technology, and the Global Frontier Research Center for Advanced Soft Electronics — was published in Advanced Functional Materials (doi: 10.1002/adfm.201201848). For more information, visit: www.unist.ac.kr