Nanowire phototransistors push device miniaturisation

A team of researchers from the Ulsan National Institute of Science and Technology (UNIST), South Korea has developed high-performance organic phototransistors (OPTs) based on single-crystalline n-channel organic nanowires. OPTs have many intrinsic advantages over their inorganic counterparts, such as the chemical tunability of optoelectronic properties by molecular design and high potential in low cost, light-weight, flexible applications, the team indicated.

Phototransistors are transistors 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 problems such as the noise increment. However, to date, the research has mostly focused on thin-film OPTs, and nanoscale OPTs have scarcely been reported.

Single-crystalline nano-/microwires (NWs/MWs) based on organic semiconductors have attracted great interest recently as they are promising 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 will allow a deeper understanding of the fundamental mechanisms of charge generation and transport in OPTs, while enabling a bottom-up fabrication of optoelectronic nanodevices.

Joon Hak Oh and Hojeong Yu, working at UNIST, together with Zhenan Bao, professor at Stanford University, have worked on n-channel single-crystalline nanowire organic phototransistors (NW-OPTs) and observed significant enhancement in the charge-carrier mobility of NW-OPTs.

Oh said, "The development of OPTs based on n-channel single-crystalline organic semiconducting NWs/MWs 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."

The photoelectronic characteristics of the single-crystalline NW-OPTs such as the photoresponsivity, the photo-switching ratio and the photoconductive gain, were analysed from the I-V characteristics coupled with light irradiation and compared with those of vacuum-deposited thin-film devices. The external quantum efficiencies (EQEs) were also investigated for the NW-OPTs and thin-film OPTs. In addition, they calculated the charge accumulation and release rates from deep traps, and investigated the effects of incident light intensity on their photoelectronic properties.

A mobility enhancement is observed when the incident optical power density increases and the wavelength of the light source matches the light-absorption range of the photoactive material. The photoswitching ratio is strongly dependent upon 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.

NW-OPTs based on n-channel semiconductor, N,N ?-bis(2-phenylethyl)-perylene-3,4:9,10-tetracarboxylic diimide (BPE-PTCDI), exhibited much higher external quantum efficiency (EQE) values (7900 times larger) than thin-film OPTs, with a maximum EQE of 263000 per cent. This phenomenon resulted from the intrinsically defect-free single-crystalline nature of the BPE-PTCDI NWs. In addition, an approach was devised to analyse the charge-transport behaviours using charge accumulation/release rates from deep traps under on/off switching of external light sources.

"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," said Oh. "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 miniaturisation."