Photoresponse of donor/acceptor blends in organic transistors: a tool for understanding field-assisted charge separation in small molecule bulk heterojunction solar cells

Photoresponse and ambipolar charge transport in organic bulk heterojunctions (BHJ) is investigated using field-effect transistors (FET) based on two donors, poly(3-hexylthiophene) (P3HT) and 3,6-bis(5-(benzofuran-2-yl)thiophen-2-yl)-2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4-dione (DPP(TBFu)2) blends with [6,6]-phenyl-C70-butyric acid methyl ester (PC70BM) acceptor. Upon 100 mW/cm2 AM 1.5 G illumination, P3HT:PC70BM shows an equivalent hole and electron current together with a largely enhanced photoresponse in the FET. The DPP(TBFu)2:PC70BM blends display an electron-dominating transport along with showing a relatively poor photoresponse in FETs upon irradiation. By comparing the two systems, it suggests that DPP(TBFu)2:PC70BM possesses a less-efficient charge separation assisted by electric fields after exciton dissociation. The FET results correlate well to the solar cell device performance and provide further understanding and optimizing of solution-processed DPP small molecule solar cells.

Polymeric Thin Films for Organic Electronics: Properties and Adaptive Structures

This review deals with the correlation between morphology, structure and performance of organic electronic devices including thin film transistors and solar cells. In particular, we report on solution processed devices going into the role of the 3D supramolecular organization in determining their electronic properties. A selection of case studies from recent literature are reviewed, relying on solution methods for organic thin-film deposition which allow fine control of the supramolecular aggregation of polymers confined at surfaces in nanoscopic layers. A special focus is given to issues exploiting morphological structures stemming from the intrinsic polymeric dynamic adaptation under non-equilibrium conditions.

Selecting speed-dependent pathways for a programmable nanoscale texture by wet interfaces

The realization of well-defined and ordered structures on the nanoscale is a main issue in nanoscience and nanotechnology, biotechnology and other related fields like plastic or organic electronics. Among the bottom-up approaches, to date, self-assembly (equilibrium aggregates) received a major attention. In spite of this, far from equilibrium conditions allow for the generation of a wider landscape of organized systems depending on the set of control parameters employed. Under an adaptation vision of the structures, here we report some case studies showing how it is possible to programme and control the nanoscale features of ordered super- or supra-aggregates at wet interfaces by modulating the dynamic parameters. In particular, speed is foreseen as a threshold factor for changing the aggregation mechanism along with the shape and degree of order of the structures as well as, within a specific aggregation path, their size and defectivity.

All-polymer bulk heterojunction solar cells with high fill factors based on blends of poly-3-hexylthiophene: poly(perylene diimide-alt-terthiophene)

The photovoltaic characteristics of all polymer bulk heterojunction solar cells made of P3HT and a perylene diimide-based copolymer (PEK3) have been studied. Thermal annealing is needed to improve the performances. Annealing optimization induces an enhancement of the power conversion efficiency from 0.06 to 1%, Jsc from 0.24 to 2.9 mA/cm2 and FF from 0.32 to 0.59. The origin of such improvements has been investigated by studying the P3HT:PEK3 blend morphology, by means of absorption and emission spectroscopy and charge transport, from single carrier measurements on P3HT:PEK3 diodes. Upon annealing we have observed an increase in phase segregation and a 100-fold enhancement of the hole and electron mobilities, that favor the dissociation of bound electron-hole pairs and their transport to the electrodes. This explains the high FF of the annealed P3HT:PEK3 solar cell.

Correlating the efficiency and nanomorphology of polymer blend solar cells utilizing resonant soft X-ray scattering

Enhanced scattering contrast afforded by resonant soft X-ray scattering (R-SoXS) is used to probe the nanomorphology of all-polymer solar cells based on blends of the donor polymer poly(3-hexylthiophene) (P3HT) with either the acceptor polymer poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-hexylthien-5-yl)-2,1,3-benzothiadiazole]-2',2"-diyl) (F8TBT) or poly([N,N'-bis(2-octyldodecyl)-11-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-12-bithiophene)) (P(NDI2OD-T2)). Both P3HT:F8TBT and P3HT:P(NDI2OD-T2) blends processed from chloroform with subsequent annealing exhibit complicated morphologies with a hierarchy of phase separation. A bimodal distribution of domain sizes is observed for P3HT:P(NDI2OD-T2) blends with small domains of size ~5-10 nm that evolve with annealing and larger domains of size ~100 nm that are insensitive to annealing. P3HT:F8TBT blends in contrast show a broader distribution of domain size but with the majority of this blend structured on the 10 nm length scale. For both P3HT:P(NDI2OD-T2) and P3HT:F8TBT blends, an evolution in device performance is observed that is correlated with a coarsening and purification of domains on the 5-10 nm length scale. Grazing-incidence wide-angle X-ray scattering (GI-WAXS) is also employed to probe material crystallinity, revealing P(NDI2OD-T2) crystallites 25-40 nm in thickness that are embedded in the larger domains observed by R-SoXS. A higher degree of P3HT crystallinity is also observed in blends with P(NDI2OD-T2) compared to F8TBT with the propensity of the polymers to crystallize in P3HT:P(NDI2OD-T2) blends hindering the structuring of morphology on the sub-10 nm length scale. This work also underscores the complementarity of R-SoXS and GI-WAXS, with R-SoXS measuring the size of compositionally distinguishable domains and GI-WAXS providing information regarding crystallinity and crystallite thickness.