Darko Ljubic

The effect of azobenzene derivatives on UV-responsive organic thin-film transistors with a 2,7-dipentylbenzo[b]benzo[4,5]thieno[2,3-d]thiophene semiconductor

We have studied a UV responsive phototransistor and how the addition of various azobenzene derivatives alters the rise and relaxation times when exposed to and removed from UV light, respectively. A three-component semiconductor system was studied consisting of a UV responsive material C5-BTBT, a polymer binder PMMA, and 1 of 5 different azobenzene materials for UV response enhancement. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) were determined experimentally and found from DFT theory. Azobenzene units with a pendent nitro group have lower HOMO and LUMO levels than the semiconductor C5-BTBT. This combined with their electron withdrawing nature allows them to stabilize excited electrons, extending the lifetime of excitons, and keeping the system at a high current longer. Using a bi-exponential model, we see the relaxation rate constant τ increase from 278 to 578 s when nitro-azobenzene is used. Meanwhile, when azobenzene contains the electron donating unit −NH2, the HOMO of the material is found to be higher than that of C5-BTBT. This allowed another pathway for excited electrons to decay to their ground state, causing hole pair recombination, reducing IDS. The relaxation curves when UV light is removed demonstrate a clear increase in decay rate over the control system, showing that the charge donating amino-azobenzene assists in charge recombination.

Effect of Polymer Binders on UV-Responsive Organic Thin-Film Phototransistors with Benzothienobenzothiophene Semiconductor

The influence of polymer binders on the UV response of organic thin-film phototransistors (OTF-PTs) is reported. The active channel of the OTF-PTs was fabricated by blending a UV responsive 2,7-dipenty-[1]benzothieno[2,3-b][1]benzothiophene (C5-BTBT) as small molecule semiconductor and a branched unsaturated polyester (B-upe) as dielectric binder (ratio 1:1). To understand the influence of the polymer composition on the photoelectrical properties and UV response of C5-BTBT, control blends were prepared using common dielectric polymers, namely, poly(vinyl acetate) (PVAc), polycarbonate (PC), and polystyrene (PS), for comparison. Thin-film morphology and nanostructure of the C5-BTBT/polymer blends were investigated by means of optical and atomic force microscopy, and powder X-ray diffraction, respectively. Electrical and photoelectrical characteristics of the studied OTF-PTs were evaluated in the dark and under UV illumination with a constant light intensity (P = 3 mW cm(-2), λ = 365 nm), respectively, using two- and three-terminal I-V measurements. Results revealed that the purposely chosen B-upe polymer binder strongly affected the UV response of OTF-PTs. A photocurrent increase of more than 5 orders of magnitude in the subthreshold region was observed with a responsivity as high as 9.7 AW(-1), at VG = 0 V. The photocurrent increase and dramatic shift of VTh,average (∼86 V) were justified by the high number of photogenerated charge carriers upon the high trap density in bulk 8.0 × 10(12) cm(-2) eV(-1) generated by highly dispersed C5-BTBT in B-upe binder. Compared with other devices, the B-upe OTF-PTs had the fastest UV response times (τr1/τr2 = 0.5/6.0) reaching the highest saturated photocurrent (>10(6)), at VG = -5 V and VSD = -60 V. The enhanced UV sensing properties of B-upe based OTF-PTs were attributed to a self-induced thin-film morphology. The enlarged interface facilitated the electron withdrawing/donating functional groups in the polymer chains in influencing the photocharge separation, trapping and recombination.

Organoclay-polymer nanocomposites

The properties of polymer nanocomposites exceed the properties of common composite materials due to the nanoscale size and morphology of the fillers used. Particulate fillersare commonly used in polymers forimproved mechanical and thermal properties, as well as modified electrical properties and cost reduction. Organically modified layered clays, such asmontmorillonite, are among the most widely used fillers for the improvement of polymer matrices. Presented in this review are some of the most studied clay nanocomposites including clay-polyolefin, clay-polyester and clay-thermoplastic polyurethanenanocomposites. Additionally, the properties of clay-biopolymers nanocomposites will also be discussed.