Jean-Christophe Bolsée

Poly(3-alkylthiophene) nanofibers for optoelectronic devices

Semiconducting poly(3-alkylthiophene) nanofibers show remarkable optical and electrical properties, and because of their high aspect ratios they are perfectly suited to serve as organic quasi one-dimensional charge carriers. Hence, they offer interesting perspectives for next generation printable optoelectronic applications. This feature article provides an overview of the current state of the art regarding the preparation and characterization of poly(3-alkylthiophene) nanofibers, and a discussion on nanofiber-based optoelectronic applications, i.e. organic field-effect transistors and bulk heterojunction organic solar cells. In addition, current shortcomings and points of attention for future development are identified.

Organic phototransistors using poly(3-hexylthiophene) nanofibres

Here we report the fabrication of nanofibre-based organic phototransistors (OPTs) using preformed poly(3-hexylthiophene) (P3HT) nanofibres. OPT performance is analysed based on two important parameters: photoresponsivity R and photosensitivity P. Before testing the devices as OPTs, the normal organic field-effect transistor (OFET) operation is characterized, revealing a surface-coverage-dependent performance. With R reaching 250 A W(-1) in the on-state (V(GS) = -40 V) and P reaching 6.8 × 10(3) in the off-state (V(GS) = 10 V) under white light illumination (I(inc) = 0.91 mW cm(-2)), the best nanofibre-based OPTs outperform the OPTs fabricated from a solution of P3HT in chlorobenzene, in which no preformed fibres are present. The better performance is attributed to an increase in active layer crystallinity, a better layer connectivity and an improved edge-on orientation of the thiophene rings along the polymer backbone, resulting in a longer exciton diffusion length and enhanced charge carrier mobility, linked to a decreased interchain coupling energy. In addition, the increased order in the active layer crystallinity induces a better spectral overlap between the white light emission spectrum and the active layer absorption spectrum, and the absorption of incident light is maximised by the favourable parallel orientation of the polymer chains with respect to the OPT substrate. Combining both leads to an increase in the overall light absorption. In comparison with previously reported solution-processed organic OPTs, it is shown here that no special dielectric surface treatment or post-deposition treatment of the active device layer is needed to obtain high OPT performance. Finally, it is also shown that, inherent to an intrinsic gate-tuneable gain mechanism, changing the gate potential results in a variation of R over at least five orders of magnitude. As such, it is shown that R can be adjusted according to the incident light intensity.