In solution-processable small molecule semiconductors, the extent of charge carrier wavefunction localization induced by dynamic disorder can be probed spectroscopically as a function of temperature using charge modulation spectroscopy (CMS). Here, it is shown based on combined field-effect transistor and CMS measurements as a function of temperature that in certain molecular semiconductors, such as solution-processible pentacene, charge carriers become trapped at low temperatures in environments in which the charges become highly localized on individual molecules, while in some other molecules the charge carrier wavefunction can retain a degree of delocalization similar to what is present at room temperature. The experimental approach sheds new insight into the nature of shallow charge traps in these materials and allows identifying molecular systems in which intrinsic transport properties could, in principle, be observed at low temperatures if other transport bottlenecks associated with grain boundaries or contacts could be removed.

The electronic structure of low temperature, solution-processed indium–zinc oxide thin-film transistors is complex and remains insufficiently understood. As commonly observed, high device performance with mobility >1 cm² V⁻¹ s⁻¹ is achievable after annealing in air above typically 250 °C but performance decreases rapidly when annealing temperatures ≤200 °C are used. Here, the electronic structure of low temperature, solution-processed oxide thin films as a function of annealing temperature and environment using a combination of X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and photothermal deflection spectroscopy is investigated. The drop-off in performance at temperatures ≤200 °C to incomplete conversion of metal hydroxide species into the fully coordinated oxide is attributed. The effect of an additional vacuum annealing step, which is beneficial if performed for short times at low temperatures, but leads to catastrophic device failure if performed at too high temperatures or for too long is also investigated. Evidence is found that during vacuum annealing, the workfunction increases and a large concentration of sub-bandgap defect states (re)appears. These results demonstrate that good devices can only be achieved in low temperature, solution-processed oxides if a significant concentration of acceptor states below the conduction band minimum is compensated or passivated by shallow hydrogen and oxygen vacancy-induced donor levels.

Over the past 25 years, organic field-effect transistors (OFETs) have witnessed impressive improvements in materials performance by 3–4 orders of magnitude, and many of the key materials discoveries have been published in Advanced Materials. This includes some of the most recent demonstrations of organic field-effect transistors with performance that clearly exceeds that of benchmark amorphous silicon-based devices. In this article, state-of-the-art in OFETs are reviewed in light of requirements for demanding future applications, in particular active-matrix addressing for flexible organic light-emitting diode (OLED) displays. An overview is provided over both small molecule and conjugated polymer materials for which field-effect mobilities exceeding > 1 cm² V^–1 s^–1 have been reported. Current understanding is also reviewed of their charge transport physics that allows reaching such unexpectedly high mobilities in these weakly van der Waals bonded and structurally comparatively disordered materials with a view towards understanding the potential for further improvement in performance in the future.

The prospective of using direct-write printing techniques for the manufacture of organic photovoltaics (OPVs) has made these techniques highly attractive. OPVs have the potential to revolutionize small-scale portable electronic applications by directly providing electric power to the systems. However, no route is available for monolithically integrating the energy-harvesting units into a system in which other components, such as transistors, sensors, or displays, are already fabricated. Here, the fabrication and the measurement of inkjet-printed, air-processed organic solar cells is reported for the first time. Highly controlled conducting and semiconducting films using engineered inks for inkjet printing enable good efficiencies for printed OPVs between ≈2 and 5% power conversion efficiency. The results show that inkjet printing is an attractive digital printing technology for cost-effective, environmentally friendly integration of photovoltaic cells onto plastic substrates.

Stability of organic photovoltaic devices (OPVs) is a limiting factor for their commercialization and still remains a major challenge whilst power conversion efficiencies are now approaching minimum requirements. The inverted organic solar cell (iOSC) architecture shows promising potential for improving significantly the cell's working lifetime. However, when solution processed ZnO is used as electron extraction layer, an undesirable light-soaking step is commonly required before the device reaches a non-permanent maximum performance. This work investigates the use of Sr and Ba doped ZnO films, ZnSrO and ZnBaO, formed by sol-gel deposition using molecular alkoxide precursor solutions, as electron extraction layers in a model iOSCs with poly [3-hexylthiophene] (P3HT): [6, 6]-phenyl C60 butyl acid methyl ester (PCBM) as the active layer. We show that using these ternary oxides the light-soaking step can be circumvented by preventing a dipole forming between the oxide and the active organic layer as supported by electroabsorption spectroscopy measurements of the device built-in field. It is suggested that Sr or Ba doping results in suppression/reduction of the oxygen adsorption at mobile oxygen vacancy sites on the metal oxide surface. Like in thin film transistor (TFT) applications, where materials like InGaZnO are rapidly becoming an important technology, the use of amorphous, mixed metal oxides allows improving the performance and stability of interfacial charge extraction layers for organic solar cells.

A high electron mobility polymer, poly{[N,N’-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5’-(2,2’-bithiophene) (P(NDI2OD-T2)) is investigated for use as an electron acceptor in all-polymer blends. Despite the high bulk electron mobility, near-infrared absorption band and compatible energy levels, bulk heterojunction devices fabricated with poly(3-hexylthiophene) (P3HT) as the electron donor exhibit power conversion efficiencies of only 0.2%. In order to understand this disappointing photovoltaic performance, systematic investigations of the photophysics, device physics and morphology of this system are performed. Ultra-fast transient absorption spectroscopy reveals a two-stage decay process with an initial rapid loss of photoinduced polarons, followed by a second slower decay. This second slower decay is similar to what is observed for efficient P3HT:PCBM ([6,6]-phenyl C₆₁-butyric acid methyl ester) blends, however the initial fast decay that is absent in P3HT:PCBM blends suggests rapid, geminate recombination of charge pairs shortly after charge transfer. X-ray microscopy reveals coarse phase separation of P3HT:P(NDI2OD-T2) blends with domains of size 0.2 to 1 micrometer. P3HT photoluminescence, however, is still found to be efficiently quenched indicating intermixing within these mesoscale domains. This hierarchy of phase separation is consistent with the transient absorption, whereby localized confinement of charges on isolated chains in the matrix of the other polymer hinders the separation of interfacial electron-hole pairs. These results indicate that local, interfacial processes are the key factor determining the overall efficiency of this system and highlight the need for improved morphological control in order for the potential benefit of high-mobility electron accepting polymers to be realized.

Doping of organic semiconductors (OSCs) with transition metal oxides such as molybdenum trioxide (MoO₃) has been used as a powerful method to overcome common issues such as contact resistance and low conductivity, which are limiting factors in organic optoelectronic devices. In this study, the mechanism and efficiency of MoO₃-induced p-type doping in OSCs are investigated by means of simultaneous electrical and spectroscopic measurements on lateral diodes. It is demonstrated that energetic changes in the MoO₃ energy levels outside vacuum can limit charge-transfer doping and device performance. It is shown and investigated that these changes crucially depend on the OSC. The time evolution of important OSC parameters such as induced charge density, doping concentration and efficiency, conductivity and mobility, is deduced. Moreover, the energetic and chemical changes in MoO₃ are investigated via ultraviolet and x-ray photoemission spectroscopy. Combining these experiments, important conclusions are drawn on the time-dependence and stability of MoO₃-doping of OSCs, as well as on the processing conditions and device architectures suitable for high-performance devices.

Charge transport in the ribbon phase of poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT)—one of the most highly ordered, chain-extended crystalline microstructures available in a conjugated polymer semiconductor—is studied. Ribbon-phase PBTTT has previously been found not to exhibit high carrier mobilities, but it is shown here that field-effect mobilities depend strongly on the device architecture and active interface. When devices are constructed such that the ribbon-phase films are in contact with either a polymer gate dielectric or an SiO₂ gate dielectric modified by a hydrophobic, self-assembled monolayer, high mobilities of up to 0.4 cm² V⁻¹ s⁻¹ can be achieved, which is comparable to those observed previously in terrace-phase PBTTT. In uniaxially aligned, zone-cast films of ribbon-phase PBTTT the mobility anisotropy is measured for transport both parallel and perpendicular to the polymer chain direction. The mobility anisotropy is relatively small, with the mobility along the polymer chain direction being higher by a factor of 3–5, consistent with the grain size encountered in the two transport directions.

Field-effect transistors based on conjugated polymers are being developed for large-area electronic applications on flexible substrates, but they also provide a very useful tool to probe the charge transport physics of these complex materials. In this review we discuss recent progress in polymer semiconductor materials, which have brought the performance and mobility of polymer devices to levels comparable to that of small-molecule organic semiconductors. These new materials have also enabled deeper insight into the charge transport physics of high-mobility polymer semiconductors gained from experiments with high charge carrier concentration and better molecular-scale understanding of the electronic structure at the semiconductor/dielectric interface.

Solution processing of polymer semiconductors provides a new paradigm for large-area electronics manufacturing on flexible substrates, but it also severely restricts the realization of interesting advanced device architectures, such as lateral heterostructures with defined interfaces, which are easily accessible with inorganic materials using photolithography. This is because polymer semiconductors degrade, swell, or dissolve during conventional photoresist processing. Here a versatile, high-resolution photolithographic method is demonstrated for patterning of polymer semiconductors and exemplify this with high-performance p-type and n-type field-effect transistors (FETs) in both bottom- and top-gate architectures, as well as ambipolar light-emitting field-effect transistors (LEFETs), in which the recombination zone can be pinned at a photolithographically defined lateral heterojunction between two semiconducting polymers. The technique therefore enables the realization of a broad range of novel device architectures while retaining optimum materials performance.

Electron injection from the source–drain electrodes limits the performance of many n-type organic field-effect transistors (OFETs), particularly those based on organic semiconductors with electron affinities less than 3.5 eV. Here, it is shown that modification of gold source–drain electrodes with an overlying solution-deposited, patterned layer of an n-type metal oxide such as zinc oxide (ZnO) provides an efficient electron-injecting contact, which avoids the use of unstable low-work-function metals and is compatible with high-resolution patterning techniques such as photolithography. Ambipolar light-emitting field-effect transistors (LEFETs) based on green-light-emitting poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT) and blue-light-emitting poly(9,9-dioctylfluorene) (F8) with electron-injecting gold/ZnO and hole-injecting gold electrodes show significantly lower electron threshold voltages and several orders of magnitude higher ambipolar currents, and hence light emission intensities, than devices with bare gold electrodes. Moreover, different solution-deposited metal oxide injection layers are compared. By spin-coating ZnO from a low-temperature precursor, processing temperatures could be reduced to 150 °C. Ultraviolet photoemission spectroscopy (UPS) shows that the improvement in transistor performance is due to reduction of the electron injection barrier at the interface between the organic semiconductor and ZnO/Au compared to bare gold electrodes.

Organic field-effect transistor (FET) memory is an emerging technology with the potential to realize light-weight, low-cost, flexible charge storage media. Here, solution-processed poly[9,9-dioctylfluorenyl-2,7-diyl]-co-(bithiophene)] (F8T2) nano floating gate memory (NFGM) with a top-gate/bottom-contact device configuration is reported. A reversible shift in the threshold voltage (V_Th) and reliable memory characteristics was achieved by the incorporation of thin Au nanoparticles (NPs) as charge storage sites for negative charges (electrons) at the interface between polystyrene and cross-linked poly(4-vinylphenol). The F8T2 NFGM showed relatively high field-effect mobility (µ_FET) (0.02 cm² V⁻¹ s⁻¹) for an amorphous semiconducting polymer with a large memory window (ca. 30 V), a high on/off ratio (more than 10⁴) during writing and erasing with an operation voltage of 80 V of gate bias in a relatively short timescale (less than 1 s), and a retention time of a few hours. This top-gated polymer NFGM could be used as an organic transistor memory element for organic flash memory.

Controlling contact resistance in organic field-effect transistors (OFETs) is one of the major hurdles to achieve transistor scaling and dimensional reduction. In particular in the context of ambipolar and/or light-emitting OFETs it is a difficult challenge to obtain efficient injection of both electrons and holes from one injecting electrode such as gold since organic semiconductors have intrinsically large band gaps resulting in significant injection barrier heights for at least one type of carrier. Here, systematic control of electron and hole contact resistance in poly(9,9-di-n-octylfluorene-alt-benzothiadiazole) ambipolar OFETs using thiol-based self-assembled monolayers (SAMs) is demonstrated. In contrast to common believe, it is found that for a certain SAM the injection of both electrons and holes can be improved. This simultaneous enhancement of electron and hole injection cannot be explained by SAM-induced work-function modifications because the surface dipole induced by the SAM on the metal surface lowers the injection barrier only for one type of carrier, but increases it for the other. These investigations reveal that other key factors also affect contact resistance, including i) interfacial tunneling through the SAM, ii) SAM-induced modifications of interface morphology, and iii) the interface electronic structure. Of particular importance for top-gate OFET geometry is iv) the active polymer layer thickness that dominates the electrode/polymer contact resistance. Therefore, a consistent explanation of how SAM electrode modification is able to improve both electron and hole injection in ambipolar OFETs requires considering all mentioned factors.

Ambipolar light-emitting organic field-effect transistors (LEFETs) possess the ability to efficiently emit light due to charge recombination in the channel. Since the emission can be made to occur far from the metal electrodes, the LEFET structure has been proposed as a potential architecture for electrically pumped organic lasers. Here, a rib waveguide distributed feedback structure consisting of tantalum pentoxide (Ta₂O₅) integrated within the channel of a top gate/bottom contact LEFET based on poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT) is demonstrated. The emitted light is coupled efficiently into the resonant mode of the DFB waveguide when the recombination zone of the LEFET is placed directly above the waveguide ridge. This architecture provides strong mode confinement in two dimensions. Mode simulations are used to optimize the dielectric thickness and gate electrode material. It is shown that electrode absorption losses within the device can be eliminated and that the lasing threshold for optical pumping of the LEFET structure with all electrodes (4.5 µJ cm⁻²) is as low as that of reference devices without electrodes. These results enable quantitative judgement of the prospects for realizing an electrically pumped organic laser based on ambipolar LEFETs. The proposed device provides a powerful, low-loss architecture for integrating high-performance ambipolar organic semiconductor materials into electrically pumped lasing structures.

A scanning Kelvin probe microscopy (SKPM) study of the surface potential of vacuum sublimed pentacene transistors under bias stress and its correlation with the film morphology is presented. While for thicker films there are some trapping centers inhomogeneously distributed over the film, as previously reported by other authors, by decreasing the film thickness the effect of thin intergrain regions (IGRs) becomes clear and a very good correlation between the topography and the potential data is observed. It is shown that in the thick pentacene grains the potential is homogeneous and independent of the gate bias applied with negligible charge trapping, while in the thin IGRs the potential varies with the applied gate bias, indicating that only an incomplete accumulation layer can be formed. Clear evidence for preferential charge trapping in the thin IGRs is obtained.

A promising, general strategy for improving performance of optoelectronic devices based on conjugated polymer semiconductors is to make better use of the fast intrachain transport along the covalently bonded polymer backbone. Little is known, however, about how the recombination rate between electrons and holes would be affected in device structures in which current flow is primarily along the polymer chain. Here a light-emitting field effect transistor (LFET) structure with a uniaxially aligned semiconducting polymer is used to show that the width and shape of the recombination zone depend strongly on polymer alignment. For alignment of the polymer parallel to the current the emission zone is 5–10 times wider than for perpendicular alignment. 2D drift-diffusion modeling is used to show that such significant widening of the recombination zone in the case of parallel alignment implies that the recombination rate constant is more than 100 times lower than expected for standard Langevin recombination. On the basis of Monte Carlo modeling it is proposed that such unexpected weak recombination is a result of the significant mobility anisotropy of the aligned polymer. These results provide new fundamental insight into the recombination physics of polymer semiconductors.

Low operating voltage is an important requirement that must be met for industrial adoption of organic field-effect transistors (OFETs). We report here solution fabricated polymer brush gate insulators with good uniformity, low surface roughness and high capacitance. These ultra thin polymer films, synthesized by atom transfer radical polymerization (ATRP), were used to fabricate low voltage OFETs with both evaporated pentacene and solution deposited poly(3-hexylthiophene). The semiconductor-dielectric interfaces in these systems were studied with a variety of methods including scanning force microscopy, grazing incidence X-ray diffraction and neutron reflectometry. These studies highlighted key differences between the surfaces of brush and spun cast polymethyl methacrylate (PMMA) films.