No abstract

We report the first successful application of an ordered bicontinuous gyroid semiconducting network in a hybrid bulk heterojunction solar cell. The freestanding gyroid network is fabricated by electrochemical deposition into the 10 nm wide voided channels of a self-assembled, selectively degradable block copolymer film. The highly ordered pore structure is ideal for uniform infiltration of an organic hole transporting material, and solid-state dye-sensitized solar cells only 400 nm thick exhibit up to 1.7% power conversion efficiency. This patterning technique can be readily extended to other promising heterojunction systems and is a major step toward realizing the full potential of self-assembly in the next generation of device technologies.

Highly oriented films of an electron accepting polymer semiconductor, poly{[N,N'-bis(2-octyldodecyl)-1,4,5,8-naphthalenedicarboximide-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)} (PNDI2OD-T2), are obtained by two different methods, namely directional epitaxial crystallization (DEC) on 1,3,5-trichlorobenzene (TCB) and epitaxy on friction transferred poly(tetrafluoroethylene) (PTFE) substrates. Two distinct polymorphs with unprecedented intrachain resolution are identified by high-resolution transmission electron microscopy (HR-TEM). Form I is obtained by DEC on TCB, whereas highly oriented films of form II are obtained on PTFE substrates after melting at T = 300 °C and cooling at 0.5 K/min. In form I, both electron diffraction and HR-TEM indicate a segregated stacking of bithiophene (T2) and naphthalene diimide (NDI) units forming separate columns. In form II, a ∼c/2 shift between successive π-stacked chains leads to mixed π-overlaps of T2 and NDI. Form I can be transformed into form II by annealing at T > 250 °C. The different π-stacking of NDI and T2 in the two polymorphs have characteristic signatures in the UV-vis spectra, especially in the charge transfer band around 750 nm which is also observed in spin-coated films.

cells today. While the majority of donoracceptor polymers are hole-conducting (p-type), [3][4][5] important progress has been achieved in the development of high performance, n-type polymeric semiconductors in recent years. [6][7][8] In 2009, Facchetti and co-workers introduced a novel n-type, donor-acceptor polymer, poly{[N,N′-bis(2-octyldodecyl)-1,4,5,8-naphthalenedicarboximide-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} P(NDI2OD-T2), which exhibits excellent electron mobilities as high as 0.85 cm 2 V −1 s −1 in top gate transistor devices under ambient conditions; [ 6 ] bulk-mobilities were found in the range of 5 × 10 −3 cm 2 V −1 s −1 for timeof-fl ight and electron-only current measurements. [ 9 ] Promising results have been further shown for all-polymer solar cells based on poly(3-hexylthiophene)s (P3HT) and P(NDI2OD-T2) as donor and acceptor, respectively, reaching power conversion effi ciencies of 1.4%. [ 10 ] Publications on P(NDI2OD-T2) initially focused on the chemistry, [ 7 ] charge transport and injection in multiple devices, [ 9,11,12 ] whereas little was reported about the structure of the semiconductor layer. While at fi rst it was assumed that P(NDI2OD-T2) forms mainly amorphous layers, [ 6 ] X-ray diffraction analysis and transmission electron microscopy (TEM) revealed the semicrystalline character of P(NDI2OD-T2) thin fi lms in recent years. [13][14][15][16][17] Rivnay et al. were the fi rst to show a remarkable degree of in-plane order in as-cast fi lms with an unconventional face-on texture in the bulk. [ 13 ] A striking texture change was observed upon melt-annealing, when the polymer chains undergo a transition from mainly face-on to edge-on. [ 15,18 ] A very recent study by Schuettfort et al. reports on a preferential edge-on texture at the top surface both for as-cast and melt-annealed layers. [ 16 ] Using mainly spectroscopic measurements, Steyrleuthner et al. demonstrated the strong tendency for aggregation not only in thin fi lms but also in solution, thereby identifying two different kinds of aggregates. [ 19 ] The precise stacking mode of the naphthalene diimide (NDI) and bithiophene (T2) units within the crystalline lattice was investigated via TEM by Brinkmann and coworkers [ 17 ] and Heeger and coworkers. [ 20 ] Highly oriented thin fi lms of P(NDI2OD-T2) were prepared by directional epitaxial crystallization (DEC) on 1,3,5-trichlorobenzene (TCB) and epitaxy on aligned fi lms of poly(tetrafl uoroethylene) (PTFE). Two polymorphs were identifi ed: i) in form I, the NDI and T2 which is up to 10 times higher than those perpendicular to the polymer chain.

No abstract

Highly oriented films of regioregular poly(3‐hexylthiophene) (P3HT) are prepared by two methods: mechanical rubbing and directional epitaxial crystallization. The structure, nanomorphology, and optical and charge‐transport properties of the oriented films are investigated by electron diffraction, high resolution transmission electron microscopy (HR‐TEM), absorption spectroscopy, and transistor field‐effect measurements. In rubbed films, P3HT chains align parallel to the rubbing direction and the crystalline domains orientation changes from preferential edge‐on to flat‐on orientation. The maximum in‐plane orientation probed by absorption spectroscopy is a function of the polymer molecular weight Mw; the lower the Mw, the higher the in‐plane orientation induced by rubbing. The anisotropy of field‐effect mobility measured parallel and perpendicular to the rubbing shows the same trend as the absorption. The Mw‐dependence of the orientation is explained in terms of chain folding and entanglement that prevent the reorientation and reorganization of the π‐stacked chains, especially when Mw ≥ 50 kDa. For comparison, P3HT films are oriented by directional epitaxial crystallization using a zone‐melting technique. Electron diffraction and HR‐TEM show that epitaxial and rubbed films differ in terms of intralamellar order within layers of π‐stacked chains. Comparison of UV‐vis absorption spectra for the different samples suggests that the vibronic structure is sensitive to intralamellar disorder.

The contact resistance in organic thin-film transistors (TFTs) is the limiting factor in the development of high-frequency organic TFTs. In devices fabricated in the inverted (bottom-gate) device architecture, staggered (top-contact) organic TFTs have usually shown or are predicted to show lower contact resistance than coplanar (bottom-contact) organic TFTs. However, through comparison of organic TFTs with different gate-dielectric thicknesses based on the small-molecule organic semiconductor 2,9-diphenyl-dinaphtho[2,3- b :2’,3’- f ]thieno[3,2- b ]thiophene, we show the potential for bottom-contact TFTs to have lower contact resistance than top-contact TFTs, provided the gate dielectric is sufficiently thin and an interface layer such as pentafluorobenzenethiol is used to treat the surface of the source and drain contacts. We demonstrate bottom-contact TFTs fabricated on flexible plastic substrates with record-low contact resistance (29 Ωcm), record subthreshold swing (62 mV/decade), and signal-propagation delays in 11-stage unipolar ring oscillators as short as 138 ns per stage, all at operating voltages of about 3 V.

solution processing with interesting optoelectronic properties such as high charge transport mobility. Many of the best performing conjugated polymers, including the poly(alkylthiophene)s, derive their high charge mobility from an ability to crystallize. [ 1 , 2 ] While charge transport along an individual conjugated chain is predicted to be extremely rapid, [ 3 ] over longer distances charge must also pass between chains. [ 4 ] Crystallization aids interchain charge transfer by bringing planarized chains together in regular, more intimate contact. The importance of crystalline morphology has been established by previous studies reporting the sensitivity of measured charge transport to fi lm formation parameters that impose the kinetics of crystallization. [5][6][7] However, systematic study of transport-limiting morphological features and how one might optimize crystalline structure remain extremely challenging because of the diffi culty of incremental control over crystallization. In particular, typical solution casting conditions (even from high boiling point carrier solvents [ 5 ] ) lead to extremely high nucleation density, such that macroscopic charge transport probes average over an enormous number of randomly oriented grain boundaries whose density is neither well-known or easily adjusted. [ 8 , 9 ] Methodologies, such as self seeding, adapted from studies of classical semicrystalline polymers, exist that permit systematic control of important morphological characteristics such as nucleation density and lamellar width. We show how controlled solvent swelling and deswelling of a precast poly(3-hexylthiophene) (P3HT) fi lm is an extremely effective method for controlling crystalline morphology (independent of fi lm formation) by fully incremental control of nucleation density over many orders of magnitude.In P3HT and many other main chain conjugated polymers, crystallization is dominated by strong π − π interactions perpendicular to the thiophene ring, which drive a highly anisotropic growth of stacked aggregates. When confi ned to a thin fi lm, the π -stacking [010] direction lies in-plane, with the molecules adopting an edge-on orientation ([100] alkyl side chains aligned perpendicular to the substrate). Long crystalline lamellae separated by amorphous regions containing chain folds and ends provide effi cient in-plane transport channels along the π -stacking direction. Device transport characteristics, While molecular ordering via crystallization is responsible for many of the impressive optoelectronic properties of thin-fi lm semiconducting polymer devices, crystalline morphology and its crucial infl uence on performance remains poorly controlled and is usually studied as a passive result of the conditions imposed by fi lm deposition parameters. A method for systematic control over crystalline morphology in conjugated polymer thin fi lms by very precise control of nucleation density and crystal growth conditions is presented. A precast poly(3-hexylthiophene) fi lm is fi rst swollen into a solution-lik...