The standard view of singlet exciton fission in organic semiconductor is that one photon creates a singlet exciton which subsequently decays into a correlated triplet pair state (TT) multiexciton states. The triplet pair state then splits to form two free triplets. Although the theoretical description of (TT) is well developed since 1970, it has so far proved difficult to determine the role and nature of the (TT) state in solid films from experiment directly. Here, using a combination of highly sensitive broadband transient absorption and photoluminescence spectroscopies on a range of polyacene films, we demonstrate that the (TT) multiexciton states is bound and energetically stabilised with respect to free triplets in even the most efficient singlet fission materials, such as TIPS-pentacene and pentacene. The (TT) multiexciton state is emissive, and we find that charge-transfer from one (TT) state to the neighboring electron acceptors has a yield of >100%, i.e. more than one charge is transferred per charge-transfer event. Our findings suggest that the formation of spin-correlated (TT) states emits as one particle and generates 2 charges in organic solar cells and thus open a range of fascinating questions regarding the potential to use entanglement to enhance organic photovoltaic efficiency and the application of organic materials in quantum information
Due to their ease of processing, organic semiconductors are promising candidates for applications in high performance flexible displays and fast organic electronic circuitry. Recently, a lot of advances have been made on organic semiconductors exhibiting surprisingly high performance and carrier mobilities exceeding those of amorphous silicon. However, there remain significant concerns about their operational and environmental stability, particularly in the context of applications that require a very high level of threshold voltage stability, such as active-matrix addressing of organic light-emitting diode (OLED) displays.
Here, we report a novel technique for dramatically improving the operational stress stability, performance and uniformity of high mobility polymer field-effect transistors by the addition of specific small molecule additives to the polymer semiconductor film. We demonstrate for the first time polymer FETs that exhibit stable threshold voltages with threshold voltage shifts of less than 1V when subjected to a constant current operational stress for 1 day under conditions that are representative for applications in OLED active matrix displays. The approach constitutes in our view a technological breakthrough; it also makes the device characteristics independent of the atmosphere in which it is operated, causes a significant reduction in contact resistance and significantly improves device uniformity. We will discuss in detail the microscopic mechanism by which the molecular additives lead to this significant improvement in device performance and stability.
Transport in organic semiconductors has traditionally been investigated using measurements of the temperature and gate voltage dependent mobility of charge carriers within the channel of organic field-effect transistors (OFETs). In such measurements, the behavior of charge carrier mobility with temperature and gate voltage, studied together with carrier activation energies, provide a metric to quantify the extent of disorder within these van der Waals bonded materials. In addition to the mobility and activation energy, another potent but often-overlooked transport coefficient useful in understanding disorder is the Seebeck coefficient (also known as thermoelectric power). Fundamentally, the Seebeck coefficient represents the entropy per charge carrier in the solid state, and thus proves powerful in distinguishing materials in which charge carriers move freely from those where a high degree of disorder causes the induced carriers to remain trapped. This paper briefly covers the recent highlights in the field of organic thermoelectrics, showing how significant strides have been made both from an applied standpoint as well as from a viewpoint of fundamental thermoelectric transport physics. It shall be illustrated how thermoelectric transport parameters in organic semiconductors can be tuned over a significant range, and how this tunability facilitates an enhanced performance for heat-to-electricity conversion as well as quantifies energetic disorder and the nature of the density of states (DOS). The work of the authors shall be spotlighted in this context, illustrating how Seebeck coefficient measurements in the polymer indacenodithiophene-co-benzothiadiazole (IDTBT) known for its ultra-low degree of torsion within the polymer backbone, has a trend consistent with low disorder. <sup>1</sup> Finally, using examples of the small molecules C8-BTBT and C10-DNTT, it shall be discussed how the Seebeck coefficient can aid the estimation of the density and distribution of trap states within these materials. <sup>2, 3</sup>
Stability of organic photovoltaic devices is a limiting factor for their commercialization and still remains a major challenge whilst power conversion efficiencies are now reaching the minimum requirements. The inverted organic solar cell architecture shows promising potential for improving significantly the cells working lifetime however, often when solution processed ZnO is used as electron extraction layer (EEL), a light soaking step is required before the device reaches a non-permanent maximum performance. Here we show that by doping ZnO with Sr or Ba using sol-gel processing the light-soaking step is circumvented. In a model poly [3-hexylthiophene] (P3HT): [6, 6]-Phenyl C60 butyl acid methyl ester (PCBM) system we obtain EQE 55% before UV exposure for ZnSrO or ZnBaO EELs as compared to 10% for undoped ZnO EEL. We have investigated the origin of this improvement by comparing the response to UV light of doped and undoped ZnO. Characterization includes electrical conductivity and x-ray photoemission spectroscopy studies on thin films, current-voltage experiments and electroabsorption (EA) spectroscopy to probe the built-in field in the devices. We will discuss how the results obtained and in particular the higher effective built-in field in doped ZnO devices (1.5V) compared to a ZnO device (0.5V) can help interpret the mechanism behind the device performance improvement with Sr and Ba doping of ZnO.
We have fabricated high-mobility ambipolar polymer transistors and have integrated multiple transistors to demonstrate
their implementation into CMOS-like logic circuitry. The performance of a selenophene-based polymer semiconductor
PSeDPPBT is initially screened using standard long-channel field-effect transistors. The polymer exhibits high and
balanced hole and electron mobilities of ∼ 0.5 cm<sup>2</sup>/Vs and ∼ 1.0 cm<sup>2</sup>/Vs, respectively. Next, exploiting the beneficial
electronic properties of PSeDPPBT, we have fabricated ambipolar inverters, ring oscillators and logic NOR gates.
Ambipolar inverters are shown to exhibit voltage inversion with proper noise margins and no voltage loss over multiple
stages. The potential speed of ambipolar logic is demonstrated by the realization of ambipolar ring oscillators with
unprecedented performance. The feasibility to perform logic operations is demonstrated by the fabrication of ambipolar
NOR gates. The combined results, (i) no loss in voltage over multiple inverters, (ii) the unprecedented speed, and (iii) the
accomplishment of a functionally complete logic operation, demonstrate the feasibility of ambipolar logic as a reliable
substitute for complementary-based logic in order to realize cost-efficient electronics.
Recently, there has been growing interest in the field of organic spintronics, where the research on organic semiconductors (OSCs) has extended from the complex aspects of charge carrier transport to the study of the spin transport properties of those anisotropic and partly localized systems.<sup>1</sup> Furthermore, solution-processed OSCs are not only interesting due to their technological applications, but it has recently been shown in 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-pentacene) thin film transistors that they can exhibit a negative temperature coefficient of the mobility due to localized transport limited by thermal lattice fluctuations.<sup>2</sup> Here, spin injection and transport in solution-processed TIPS-pentacene are investigated exploiting vertical CoPt/TIPSpentacene/AlO<sub>x</sub>/Co spin valve architectures.<sup>3</sup> The antiparallel magnetization state of the relative orientation of CoPt and Co is achieved due to their different coercive fields. A spin valve effect is detected from <i>T</i> = 175 K up to room temperature, where the resistance of the device is lower for the antiparallel magnetization state. The first observation of the scaling of the magnetoresistance (MR) with the bulk mobility of the OSC as a function of temperature, together with the dependence of the MR on the interlayer thickness, clearly indicates spin injection and transport in TIPS-pentacene. From OSC-spacer thickness-dependent MR measurements, a spin relaxation length of TIPS-pentacene of (24±6) nm and a spin relaxation time of approximately 3.5 μs at room temperature are estimated, taking the measured bulk mobility of holes into account.
Ambipolar organic field-effect transistors (FET) are interesting as building blocks for low power complementary circuits in organic electronics. Another intriguing feature of ambipolar FETs is the recombination of holes and electrons within the channel, which leads to the formation of excitons that can relax radiatively and thus emit light. We have recently demonstrated that ambipolar charge transport is a generic feature in a wide range of polymer semiconductors when appropriate injection electrodes and trapfree dielectrics are used. Among these materials are those that are generally used in light-emitting diodes and thus show high photoluminescence efficiencies.
Here we demonstrate ambipolar light-emitting field-effect transistors based on the conjugated polymer OC<sub>1</sub>C<sub>10-</sub>PPV (poly(2-methoxy-5-(3,7-dimethyloctoxy)-p-phenylenevinylene)) as the semiconducting and emissive layer. OC<sub>1</sub>C<sub>10-</sub> PPV shows efficient electron and hole transport with field-effect mobilities of 3⋅10<sup>-3</sup> cm<sup>2</sup>/Vs and 6⋅10<sup>-4</sup> cm<sup>2</sup>/Vs, respectively. Electrons and holes are injected from calcium and gold source and drain electrodes, respectively, and recombine radiatively within the transistor channel leading to visible light emission. We can actively control the position of the recombination zone through the applied gate and source-drain bias in both constant and variable current mode and thus move the emission zone from the source through the channel to the drain electrode and vice versa. The intensity of light emitted from the channel is proportional to the drain current with efficiencies comparable to those of LEDs based on OC<sub>1</sub>C<sub>10-</sub>PPV.
Well-characterized F8T2 polyfluorene (Dow Chemical) has been prepared with weight average molecular weights (Mw) ranging from about 20,000 to 120,000. This semiconducting polymer has been used by Plastic Logic to fabricate arrays of 4,800 thin film transistors (TFTs) with 50 dpi, to be used as backplanes for active matrix displays. In this paper, the effects that molecular weight and thermal treatment have on the electrical characteristics of F8T2-based TFTs are
reported. First, transistor performance improves with increasing molecular weight, with maximum values of TFT mobility approaching 1x 10<sup>-2</sup> cm<sup>2</sup> /V-s. Consistently higher mobilities are obtained when the F8T2 semiconductor makes contact with PEDOT/PSS versus gold electrodes. Alignment of F8T2 on a rubbed polyimide substrate is maintained after quenching, as determined by measurement of the dichroic ratios. Early-stage results on the development of inks
based on F8T2 polyfluorene are also reported.
All-polymer thin film transistors and circuits have been fabricated by inkjet printing. Source, drain and gate electrodes were printed with a solution of conducting conjugated polymer, poly-ethylenedioxythiophene (PEDOT), and semiconductor and gate dielectric were spin-coated from solutions of conjugated polymer and insulator polymer, respectively. The transistors printed in air show comparable performances to the reference samples with gold electrodes. In order to overcome the resolution limit of inkjet printing, water-based PEDOT solution has been deposited onto a pre-patterned substrate which defines a channel by wettability contrast between hydrophilic and hydrophobic surface regions. Polymer transistors with a channel length of 5 microns have been achieved by this approach. In order to improve carrier mobility, main chains of the polymer semiconductor were self-aligned along the channel direction, and a mobility of 0.02 cm2/V+s has been achieved in the printed transistor. We demonstrate simple printed circuits (inverters) with via-holes and load resistors formed by inkjet technology.
Organic field effect transistors FET have been fabricated with active semiconducting organic thin films that are only a few monolayers thick. The motivation of this study has been to establish a direct correlation between transistor performance and the polymer microstructure in the ultrathin accumulation layer of the transistor. Monolayer thick films of a block copolymer and several model oligomers consisting of a rigid conjugated sexithiophene (6T) block and a flexible polyethyleneoxide (PEO) block have been deposited onto the surface of e.g. SiO<SUB>2</SUB> gate dielectrics functionalized with a self-assembled monolayer. Block copolymer phase behavior and surface morphology has been studied as a function of chain length, solvent and film thickness. Operational transistors have been demonstrated with film thicknesses of only one or two monolayers. Typical device characteristics show a carrier mobility on the order of 10<SUP>-2</SUP> - 10<SUP>-3</SUP> cm<SUP>2</SUP>/Vs and ON-OFF current ratio of 10<SUP>3</SUP> - 10<SUP>5</SUP>. Film microstructure, orientation of micro-crystallites and film thickness have been studied by atomic force microscopy (AFM), UV-Vis absorption spectroscopy and X-ray diffraction.
The high luminescence efficiencies and significant blue shift of the 2,3-disubstituted poly(1,4-phenylene vinylene) polymer 4 have prompted further investigation, and in this paper the synthesis and characteristics of the homopolymer 9 and copolymers 10 and 12 are described. Semiempirical calculations and single x-ray crystallography offer further insight into the explanation of the properties of this class of polymers. A promising organic semiconductor 15 has been prepared and used as the active layer in a thin film transistor. This material exhibited excellent device characteristics, including a field effect mobility of 0.02- 0.05 cm<SUP>2</SUP> V<SUP>-1</SUP>s<SUP>-1</SUP> and a high On/Off ratio.
A solution to the thin film silicon transistor gate metallization problem in active matrix liquid crystal displays is demonstrated in the form of a self-passivation process for copper. Bottom-level copper (Cu) lines are passivated by a self-aligned chromium oxide encapsulation formed by surface segregation of chromium (Cr) from dilute Cu<SUB>1-x</SUB>Cr<SUB>x</SUB> alloys at 400 degrees C. The encapsulation is an efficient barrier for Cu diffusion into the SiN<SUB>x</SUB> gate insulator during the plasma deposition and transistor processing, and solves the problems of oxidation and adhesion to the glass substrate without introducing additional mask steps into the manufacturing process. Gate line resistivities of 4.5 (mu) (Omega) cm are obtained. The performance of self-passivated Cu-gate thin film transistors is comparable to that of transistors with refractory metal gates.