Christopher R. McNeill

Critical Role of Alkyl Chain Branching of Organic Semiconductors in Enabling Solution-Processed N-Channel Organic Thin-Film Transistors with Mobility of up to 3.50 cm2 V–1 s–1

Substituted side chains are fundamental units in solution processable organic semiconductors in order to achieve a balance of close intermolecular stacking, high crystallinity, and good compatibility with different wet techniques. Based on four air-stable solution-processed naphthalene diimides fused with 2-(1,3-dithiol-2-ylidene)malononitrile groups (NDI-DTYM2) that bear branched alkyl chains with varied side-chain length and different branching position, we have carried out systematic studies on the relationship between film microstructure and charge transport in their organic thin-film transistors (OTFTs). In particular synchrotron measurements (grazing incidence X-ray diffraction and near-edge X-ray absorption fine structure) are combined with device optimization studies to probe the interplay between molecular structure, molecular packing, and OTFT mobility. It is found that the side-chain length has a moderate influence on thin-film microstructure but leads to only limited changes in OTFT performance. In contrast, the position of branching point results in subtle, yet critical changes in molecular packing and leads to dramatic differences in electron mobility ranging from ~0.001 to >3.0 cm(2) V(-1) s(-1). Incorporating a NDI-DTYM2 core with three-branched N-alkyl substituents of C(11,6) results in a dense in-plane molecular packing with an unit cell area of 127 Å(2), larger domain sizes of up to 1000 × 3000 nm(2), and an electron mobility of up to 3.50 cm(2) V(-1) s(-1), which is an unprecedented value for ambient stable n-channel solution-processed OTFTs reported to date. These results demonstrate that variation of the alkyl chain branching point is a powerful strategy for tuning of molecular packing to enable high charge transport mobilities.

Dual electron donor/electron acceptor character of a conjugated polymer in efficient photovoltaic diodes

The authors report efficient photovoltaic diodes which use poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-hexylthien-5-yl)-2,1,3-benzothiadiazole]-2′,2″-diyl) (F8TBT) both as electron acceptor, in blends with poly(3-hexylthiophene), and as hole acceptor, in blends with (6,6)-phenyl C61-butyric acid methyl ester. In both cases external quantum efficiencies of over 25% are achieved, with a power conversion efficiency of 1.8% under simulated sunlight for optimized F8TBT/poly(3-hexylthiophene) devices. The ambipolar nature of F8TBT is also demonstrated by the operation of light-emitting F8TBT transistors. The equivalent p- and n-type operation in this conjugated polymer represent an important extension of the range of useful n-type materials which may be developed.

Polarized X-ray scattering reveals non-crystalline orientational ordering in organic films

Molecular orientation critically influences the mechanical, chemical, optical and electronic properties of organic materials. So far, molecular-scale ordering in soft matter could be characterized with X-ray or electron microscopy techniques only if the sample exhibited sufficient crystallinity. Here, we show that the resonant scattering of polarized soft X-rays (P-SoXS) by molecular orbitals is not limited by crystallinity and that it can be used to probe molecular orientation down to size scales of 10 nm. We first apply the technique on highly crystalline small-molecule thin films and subsequently use its high sensitivity to probe the impact of liquid-crystalline ordering on charge mobility in polymeric transistors. P-SoXS also reveals scattering anisotropy in amorphous domains of all-polymer organic solar cells where interfacial interactions pattern orientational alignment in the matrix phase, which probably plays an important role in the photophysics. The energy and q-dependence of the scattering anisotropy allows the identification of the composition and the degree of orientational order in the domains.

Morphology of all-polymer solar cells

The blending of two semiconducting polymers with offset energy levels enables efficient charge generation in thin-film ‘all-polymer’ solar cells. A key requirement for efficient charge separation and collection is the formation of interconnected phase-separated domains structured on the sub-20 nm length-scale. This review provides an overview of recent advances in the characterisation of conjugated polymer blend nanostructure and developments in the linking of blend structure and device performance. This review also provides a general introduction to the polymer physics behind phase separation, experimental techniques used for characterising blend structure and novel ways to control nanomorphology.

Macroscopic and high-throughput printing of aligned nanostructured polymer semiconductors for MHz large-area electronics

High-mobility semiconducting polymers offer the opportunity to develop flexible and large-area electronics for several applications, including wearable, portable and distributed sensors, monitoring and actuating devices. An enabler of this technology is a scalable printing process achieving uniform electrical performances over large area. As opposed to the deposition of highly crystalline films, orientational alignment of polymer chains, albeit commonly achieved by non-scalable/slow bulk alignment schemes, is a more robust approach towards large-area electronics. By combining pre-aggregating solvents for formulating the semiconductor and by adopting a room temperature wired bar-coating technique, here we demonstrate the fast deposition of submonolayers and nanostructured films of a model electron-transporting polymer. Our approach enables directional self-assembling of polymer chains exhibiting large transport anisotropy and a mobility up to 6.4 cm2 V−1 s−1, allowing very simple device architectures to operate at 3.3 MHz. Thus, the proposed deposition strategy is exceptionally promising for mass manufacturing of high-performance polymer circuits.

Drift-diffusion modeling of photocurrent transients in bulk heterojunction solar cells

We utilize a time-dependent drift-diffusion model incorporating electron trapping and field-dependent charge separation to explore the device physics of organic bulk-heterojunction solar cells based on blends of poly(3-hexylthiophene) (P3HT) with a red polyfluorene copolymer. The model is used to reproduce experimental photocurrent transients measured in response to a step-function excitation of light of varied intensity. The experimental photocurrent transients are characterized by (i) a fast rise of order 1 μs followed by (ii) a slow rise of order 10–100 μs that evolves into a transient peak at high intensity, (iii) a fast decay component after turn-off and (iv) a long-lived tail with magnitude that does not scale linearly with light intensity or steady-state photocurrent. The fast rise and decay components are explained by the transport of mobile carriers while the slow rise and decay components are explained by slower electron trapping and detrapping processes. The transient photocurrent peak at high ...

Influence of backbone fluorination in regioregular poly(3-alkyl-4-fluoro)thiophenes.

We report two strategies toward the synthesis of 3-alkyl-4-fluorothiophenes containing straight (hexyl and octyl) and branched (2-ethylhexyl) alkyl groups. We demonstrate that treatment of the dibrominated monomer with 1 equiv of alkyl Grignard reagent leads to the formation of a single regioisomer as a result of the pronounced directing effect of the fluorine group. Polymerization of the resulting species affords highly regioregular poly(3-alkyl-4-fluoro)thiophenes. Comparison of their properties to those of the analogous non-fluorinated polymers shows that backbone fluorination leads to an increase in the polymer ionization potential without a significant change in optical band gap. Fluorination also results in an enhanced tendency to aggregate in solution, which is ascribed to a more co-planar backbone on the basis of Raman and DFT calculations. Average charge carrier mobilities in field-effect transistors are found to increase by up to a factor of 5 for the fluorinated polymers.

Performance, morphology and photophysics of high open-circuit voltage, low band gap all-polymer solar cells

The microstructure and photophysics of low-band gap, all-polymer photovoltaic blends are presented. Blends are based on the donor polymer BFS4 (a dithienyl-benzo[1,2-b:4,5-b]dithiophene/5-fluoro-2,1,3-benzothiadiazole co-polymer) paired with the naphthalene diimide-based acceptor polymer P(NDI2OD-T2). Efficiencies of over 4% are demonstrated, with an open circuit voltage of greater than 0.9 V achieved. Transmission electron microscopy reveals a relatively coarse phase-separated morphology, with elongated domains up to 200 nm in width. Near-edge X-ray absorption fine-structure (NEXAFS) spectroscopy and atomic force microscopy (AFM) measurements reveal that the top surface of BFS4:P(NDI2OD-T2) blends is covered with a pure BFS4 capping layer. Depth profiling measurements confirm this vertical phase separation with a surface-directed spinodal decomposition wave observed. Grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements confirm that BFS4 and P(NDI2OD-T2) are semicrystalline with both polymers retaining their semicrystalline nature when blended. Photoluminescence spectroscopy reveals incomplete photoluminescence quenching with as much as 30% of excitons failing to reach a donor/acceptor interface. Transient absorption spectroscopy measurements also find evidence for rapid geminate recombination.

Highly Efficient Single-Layer Polymer Ambipolar Light-Emitting Field-Effect Transistors

Single-layer polymer light-emitting field-effect transistors (LEFETs) that yield EQEs of >8% and luminance efficiencies >28 cd A(-1) are demonstrated. These values are the highest reported for LEFETs and amongst the highest values for fluorescent OLEDs. Due to the electrostatics of the ambipolar LEFET channel, LEFETs provide an inherent advantage over OLEDs in terms of minimizing exciton-polaron quenching.

Photocurrent transients in all-polymer solar cells: Trapping and detrapping effects

We have studied photocurrent transients in all-polymer bulk-heterojunction solar cells based on poly(3-hexylthiophene) and poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-hexylthien-5-yl)-2,1,3-benzothiadiazole]-2′,2″-diyl). By illuminating devices with square pulses of light of varying intensity, we reveal nonlinear photocurrent transients on the timescale of tens of microseconds. These microsecond photocurrent transients are attributed to the effects of trapping and detrapping of charges on this timescale, in particular, electrons. The buildup of trapped electrons results in the appearance of a peak in the photocurrent at high intensities at ∼10 μs after turn on. This trapped charge produces a local reduction in the strength of the internal electric field near the anode resulting in a net decrease in charge separation efficiency and an increase in the likelihood of bimolecular recombination due to increased and overlapping electron and hole densities. After turn off, a long photocurrent tail is obser...