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Thermally activated delayed fluorescence (TADF) materials have shown great potential for highly efficient organic light-emitting diodes (OLEDs). While the current molecular design of TADF materials primarily focuses on combining donor and acceptor units, we present a novel system based on the use of excited-state intramolecular proton transfer (ESIPT) to achieve efficient TADF without relying on the well-established donor–acceptor scheme. In an appropriately designed acridone-based compound with intramolecular hydrogen bonding, ESIPT leads to separation of the highest occupied and lowest unoccupied molecular orbitals, resulting in TADF emission with a photoluminescence quantum yield of nearly 60%. High external electroluminescence quantum efficiencies of up to 14% in OLEDs using this emitter prove that efficient triplet harvesting is possible with ESIPT-based TADF materials. This work will expand and accelerate the development of a wide variety of TADF materials for high performance OLEDs., Excited-state intramolecular proton transfer opens a new route for efficient organic light-emitting diodes emitters based on thermally activated delayed fluorescence without relying on a donor−acceptor scheme.

Perovskite light-emitting diodes are promising for next-generation lighting and displays because of their high colour purity and performance1. Although the management of singlet and triplet excitons is fundamental to the design of efficient organic light-emitting diodes, the nature of how excitons affect performance is still not clear in perovskite2–4 and quasi-two-dimensional (2D) perovskite-based devices5–9. Here, we show that triplet excitons are key to efficient emission in green quasi-2D perovskite devices and that quenching of triplets by the organic cation is a major loss path. Employing an organic cation with a high triplet energy level (phenylethylammonium) in a quasi-2D perovskite based on formamidinium lead bromide yields efficient harvesting of triplets. Furthermore, we show that upconversion of triplets to singlets can occur, making 100% harvesting of electrically generated excitons potentially possible. The external quantum and current efficiencies of our green (527 nm) devices reached 12.4% and 52.1 cd A−1, respectively. Careful harvesting of triplet excitons allows the realization of efficient green-emitting quasi-2D perovskite LEDs.

Firstly, multi-fold subphthalocyanine (SubPc) homo-tandem cells were fabricated. When complementary absorbing SubPc and chloroaluminum phthalocyanine (ClAlPc) were used to prepare tandem cells, both short circuit current ( J SC ) and fill factor (FF) are significantly improved relative to the SubPc double tandem cell. 3,4,9,10-perylenetetracarboxylic bisbenzimidazole (PTCBI) as electron transporting layer (ETL) in the charge recombination zone (CRZ) achieves a much higher FF than bathocuproine (BCP) thus higher power conversion efficiency ( η PCE ) in both the normal and reverse tandem cells, ascribed to the matched energy levels, very smooth film surface, and ohmic contact with Ag interlayer. The effect of Ag interlayer thickness was also investigated. Ultrathin Ag layer with isolated clusters is helpful for obtaining higher photocurrent in both PTCBI and BCP based CRZs, originating from a flatter interface, less optical loss, and a plasmonic effect induced absorption enhancement of C 60 in bottom subcell. By optical modeling for current matching, the performance of normal tandem cell is improved, exhibiting a high open circuit voltage of 1.80 V and an overall η PCE of 3.49%.

While the majority of research on organic thermoelectric generators has focused on individual devices with organic films having thicknesses of several hundred nanometers (nano-films), films with micrometer-scale thicknesses (micro-films) provide a longer thermal conduction path that results in a larger temperature gradient and higher thermoelectric voltages in modules. In this study, the properties of solution-processed nano- and micro-films of the p-type semiconductor P3HT doped with two different dopants, F 4 -TCNQ and Fe 3+ -tos 3 ·6H 2 O, were investigated. While doping with F 4 -TCNQ resulted in high electrical conductivity only in nano-films, doping with Fe 3+ -tos 3 ·6H 2 O from a 25 mM solution yielded power factors of up to ∼30 μWm −1 K −2 with a conductivity of 55.4 Scm −1 in micro-films. Changes in the molecular packing were compared based on X-ray diffraction, and the best operational stability in air was found for the Fe 3+ -tos 3 ·6H 2 O-doped micro-films. Using Fe 3+ -tos 3 ·6H 2 O as dopant, flexible thermoelectric modules with solution-processed micro-films patterned by a photo-etching technique that does not require alignment and assembly of individual devices were demonstrated, exhibiting a maximum power output of 1.94 nWK −2 for a uni-leg module with 48 elements. Analysis of the flexible module performance showed that the performance is limited by the contact resistance, which must be taken into consideration when optimizing module structure.

Recent progress in conducting polymer-based organic thermoelectric generators (OTEGs) has resulted in high performance due to high Seebeck coefficient, high electrical conductivity (σ), and low thermal conductivity obtained by chemically controlling the materials's redox levels. In addition to improving the properties of individual OTEGs to obtain high performance, the development of solution processes for the fabrication of OTEG modules is necessary to realize large thermoelectric voltage and low-cost mass production. However, the scarcity of good candidates for soluble organic n-type materials limits the use of π-leg module structures consisting of complementary elements of p- and n-type materials because of unbalanced transport coefficients that lead to power losses. In particular, the extremely low σ of n-type materials compared with that of p-type materials is a serious challenge. In this study, poly(pyridinium phenylene) (P(PymPh)) was tested as an n-type semiconductor in solution-processed OTEGs, and the carrier density was controlled by a solution-based chemical doping process using the dopant sodium naphthalenide, a well-known reductant. The electronic structures and doping mechanism of P(PymPh) were explored based on the changes in UV-Vis-IR absorption, ultraviolet photoelectron, and X-ray photoelectron spectra. By controlling the dopant concentration, we demonstrate a maximum n-type power factor of 0.81 μW m−1 K−2 with high σ, and at higher doping concentrations, a switch from n-type to p-type TE operation. This is one of the first cases of a switch in polarity just by increasing the concentration of the reductant and may open a new route for simplified fabrication of complementary organic layers.

Degradation mechanisms of CH3 NH3 PbI3 -based planar perovskite solar cells (PSCs) are investigated using a thermally stimulated current technique. Hole traps lying above the valence-band edge of the CH3 NH3 PbI3 are detected in PSCs degraded by continuous simulated solar illumination. One source of the hole traps is the photodegradation of CH3 NH3 PbI3 in the presence of water.

In this work, various interlayers were inserted between the donor and acceptor in a dinaphthothienothiophene (DNTT)/C 60 planar heterojunction (PHJ) to sensitize the performance. The power conversion efficiency is enhanced from 1.16% for the DNTT/C 60 PHJ cell to 2.23% by inserting a 5-nm-thick tetraphenyldibenzoperiflanthene (DBP) interlayer because of the greatly improved photocurrent and open-circuit voltage ( V OC ). To achieve high V OC , fullerene-free PHJs of DNTT/boron subphthalocyanine chloride (SubPc) (donor/acceptor) were fabricated, and V OC is further improved by doping various fractions of DBP into the SubPc layer. The V OC clearly increases from 0.82 V to 1.24 V by 70 wt%-DBP doping and is accompanied by a slight increase in photocurrent. The bipolar transfer characteristics of SubPc and DBP are investigated by field-effect transistors and show that both can transport electrons, indicating their potential as acceptors in photovoltaic devices. When another 5-nm-thick SubPc layer was included in the fullerene-free DNTT/SubPc:DBP PHJ cell, the power conversion efficiency further increases to 1.32%. These results indicate that DBP is a promising sensitizer for enhancing the performance of DNTT-based photovoltaics.

Morphological control of organic–inorganic perovskite layers is crucial for efficient planar solar cells. In this study, we show that hot isostatic pressing (HIP) of perovskite layers using a pressure of 200 MPa in 90 °C water is very effective for improving the perovskite film morphology. After HIP treatment, undesirable pin holes and spatial gaps between crystals in the perovskite layers were significantly reduced. Improved crystallinity and enhanced diffusion lengths for both electrons and holes were also confirmed in the HIP-treated perovskite layers. Solar cells containing the perovskite layers as light absorbers were fabricated and characterized under simulated solar light (AM1.5G, 100 mW cm−2). The HIP treatment induced a marked enhancement of short-circuit current density, open-circuit voltage, fill factor, and power conversion efficiency because of the improved morphology and crystallinity and enhanced carrier diffusion. The HIP-treated solar cells achieved efficiencies of 10.6 ± 0.7%, which are about 1.5 times higher than those of the untreated solar cells (7.20 ± 0.59%).

To understand why performance degradation is reduced for sputtered cathodes on organic devices when the electron transport layer (ETL) is doped with Li, we analyze electron-only devices using the thermally stimulated current (TSC) technique and modeling of temperature-dependent current–voltage characteristics with a trapped-charge-limited current (TCLC) model. The combined results suggest that the trap density measured by TSC might also include a portion of the density of the hopping sites in the lowest unoccupied molecular orbital levels, which contributes to charge transport. Compared to undoped devices, doped devices maintain a high density of hopping sites even when the Al is sputtered. We propose that the reduced effect of sputtering on electron injection and transport properties is because radical anions of Alq 3 might still be formed by the strong reducer Li even if the organic material is partially damaged. An additional TSC peak and increased driving voltage for doped tris(8-hydroxyquinoline)aluminum (Alq 3 ) as an ETL with a sputtered cathode suggests the formation of new traps possibly because of damage even though the transport is better compared to the undoped device. Such traps are not found in doped bathophenanthroline (Bphen) as an ETL, which shows no change in driving voltage.

The influence of the host molecules on the photoluminescent (PL) and electroluminescent (EL) properties of organic light-emitting diode (OLED) emitters showing efficient thermally-activated delayed fluorescence (TADF) has yet to be investigated in detail. Here we demonstrate that the choice of host can cause large variations in the PL quantum yield ( Φ PL ∼15–70%) and delayed PL transient decay ( τ del ∼2–70 ms) of a spiro-acridine-based TADF guest. We show that the effect of exciplex formation on Φ PL must be considered even at low concentrations of the TADF guest. Using the same TADF guest but changing the host layer, we are able to greatly vary the PL transient decay time from ∼4 to ∼70 ms while maintaining a high Φ PL ∼70%, which can lead to new applications. Detailed spectral characterization during PL decay reveals a gradually increasing singlet–triplet energy gap (Δ E ST ) as the origin of these observations. The time-varying Δ E ST is explained based on dipole interactions between the host and guest molecules. Finally, PL and electrical considerations for host selection are discussed based on the performance of OLED devices.

Comparative studies of the effects of a series of polycrystalline donors on the performance of 95 wt.%-C 70 -based bulk-heterojunction (BHJ) photovoltaics were conducted. A BHJ based on the wide band-gap molecule dinaphthothienothiophene (DNTT) shows power conversion efficiency ( η PCE ) of up to 4.28%. The photovoltaic parameters are superior to those of devices using the similar molecule pentacene (PEN) or polycrystalline copper phthalocyanine (CuPc) for donor concentrations from 5 to 30 wt.%. The low-lying DNTT ionization potential and the high μ h in the DNTT blend support the excellent DNTT device performance. The low performance of BHJs with 5 wt.% PEN and 5 wt.% CuPc may stem from strong exciplex recombination in the PEN:C 70 blend and limited hole mobility combined with geminate polaron-pair recombination in the CuPc:C 70 blend. The zero-field hole mobility of the blends with 5 wt.% donor has a positive correlation with the corresponding device performance. The η PCE of a 5 wt.%-DNTT BHJ cell was improved to 4.92% by optimizing the cathode buffer layer.

In this study, we investigate the lasing properties of 4,4′-bis[(N-carbazole)styryl]biphenyl thin films under electrical pumping. The electroluminescent devices incorporate a mixed-order distributed feedback SiO2 grating into an organic light-emitting diode structure and emit blue lasing. The results provide an indication of lasing by direct injection of current into an organic thin film through selection of a high-gain organic semiconductor showing clear separation of the lasing wavelength from significant triplet and polaron absorption and design of a proper feedback structure with low losses at high current densities. This study represents an important advance toward a future organic laser diode technology.

In this study, the overall barrier performance of multilayer thin-films and the shelf lifetime of encapsulated organic solar cells were correlated through the total amount of water vapor that permeated into the solar cell. Effective water vapor transmission rates were measured in both the transient and steady-state transport regimes for multilayer barrier films consisting of SiNx and parylene. The efficiency of pentacene/C60-based solar cells encapsulated with one or two pairs of SiNx/parylene dropped to 50% after permeation of about 1.63 g/m2 of water vapor regardless of effective transmission rate of the barrier. From these calculations, cells encapsulated with three dyads were predicted to maintain performance for at least 13,500 h while experiments up to 7500 h showed less than 10% degradation in performance.

Highly stable, solution-processed, small molecule-polymer blend organic field-effect transistors (OFETs) with a top-gate geometry were demonstrated on a flexible polyethersulfone (PES) substrate. The top-gate dielectric was a bi-layer comprised of CYTOP and a high-k Al2O3 layer grown by atomic layer deposition (ALD). A solution processed 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) and poly(triarylamine) (PTAA) blend was used as the organic semiconductor. TIPS-pentacene and PTAA blend OFETs with the CYTOP/Al2O3 bi-layer top gate dielectric showed an averaged saturation mobility value of 0.24 ± 0.08 cm2/Vs at operation voltages below 8 V. A constant direct-current bias stress test was carried out to examine their operational stability for 2 h. Under bias stress, neither significant change in mobility nor shift in the threshold voltage has been observed in these OFETs. To evaluate the real potential of these OFETs towards the development of circuit components commonly used in electronic applications, a resistive-load inverter was implemented by connecting an OFET to an external load resistor. Excellent stability of the transistor led to electrically stable inverters with negligible variations of the voltage transfer characteristics before and after bias stress. After the operational stability test, these OFETs were exposed to air and then were subjected to bending experiments. Even after exposure to air for 4 months and bending for 30 min, no significant changes in performance were observed in either a single transistor device or in a resistive-load inverter.

We report on vertically stacked complementary inverters implemented with a solution-processed [6,6]-phenyl c 61 butyric acid methyl ester (PCBM) n -channel thin-film transistor (TFT) fabricated on top of a 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) and poly(triarylamine) (PTAA) blend p -channel TFT. With a shared common gate electrode positioned between two dielectric layers, bottom-contact p - and top-contact n -channel TFTs showed saturation mobility values of 0.25 and 0.004 cm 2 /V s and threshold voltages of −3.9, and 0.3 V, respectively. The inverter yielded a gain value of −24 V/V with a switching threshold voltage value of 3.3 V at a supply voltage of 7 V. This demonstration of the use of solution-processed semiconductors in a vertically stacked complementary inverter geometry is a step forward towards the development of low-cost complementary electronics.

Over the past several years, great progress has been made in the development of organic fi eld-effect transistors (OFETs). Prototypes of electronic devices such as drivers for fl at-panel displays, [ 1 ] complementary circuits, [ 2 , 3 ] radio-frequency identifi cation tags, [ 4 ] and chemical or biological sensors [ 5 , 6 ] have already been demonstrated. While charge-carrier mobility values have improved [ 2 , 3 , 7–9 ] with comparable values for both n and p -channel transistors, long-term environmental and operational stability remain two major issues that need to be resolved before OFETs can realize their full commercial potential. Recently, much effort has been devoted to improve the stability of OFETs. [ 10–18 ] For instance, to improve the environmental stability of OFETs, air-stable organic semiconductors have been synthesized [ 10 , 11 ] or encapsulation layers have been developed. [ 12 , 13 ] On the other hand, achieving operational stability is still a major challenge faced by OFETs as well as other fi eld-effect transistor (FET) technologies, such as those based on a -Si:H, poly-Si, and metal-oxide semiconductors. The operational stability of a FET is in general related to dipolar orientation and charge trapping/de-trapping events at all its critical interfaces and in the bulk of the semiconductor and gate dielectric. [ 14–18 ] The degradation of the performance of a FET during operation is refl ected by changes of its current-voltage characteristics that result from changes of mobility ( μ ), of threshold voltage ( V th ), or variations of the capacitance density ( C in ) of the gate dielectric. The dynamics of the physical and/or chemical mechanisms producing these changes, intrinsic or extrinsic, affect the performance of a FET on different time scales. [ 14 ] The stability of a FET is determined by the total effects produced by several physical and/or chemical processes, but in general, one tends to dominate over the others. This has caused current approaches to improve the stability to focus on mitigating individual processes. [ 15–18 ] Furthermore, the stability of OFETs has been primarily evaluated in devices with a bottom-gate geometry. OFETs with a top-gate geometry are relatively rare because the choice of gate dielectric material is limited since its deposition can potentially damage the organic semiconductor layer underneath. The use of an amorphous fl uoropolymer, CYTOP

Hybrid organic–inorganic complementary inverters were demonstrated on a flexible polyethersulfone (PES) substrate with vertically stacked p-channel pentacene and n-channel amorphous indium gallium zinc oxide thin-film transistors (TFT). Al2O3 layers grown by atomic layer deposition were used as top- and bottom-gate dielectric layers. Common-gate top-contact p- and bottom-contact n-channel TFTs showed saturation mobility values of 0.3 ± 0.02 and 5.3 ± 0.2 cm2/Vs and low threshold voltage values. Complementary inverters yielded high gain values of 61 V/V with high and balanced noise margins at a low supply voltage of 5 V. The independent control of the thickness of the gate dielectric layer used for each transistor in this proposed vertically stacked geometry, allows for the realization of high-density low-power complementary circuits with high gain and balanced noise margins.

We report on the photovoltaic properties of inverted polymer solar cells where the transparent electron-collecting electrode is formed by a ZnO-modified indium−tin oxide (ITO) electrode. The ZnO layers were deposited by atomic layer deposition (ALD) with varying thicknesses from 0.1 to 100 nm. The work function, surface roughness, and morphology of ITO/ZnO were found to be independent of the ZnO thickness. However, the device performance was found to be strongly dependent on a critical ZnO thickness, around 10 nm. Below the critical thickness the device performance was degraded because of the appearance of a “kink” in the current−voltage characteristics. The kink features became less pronounced after ultraviolet (UV) exposure. This was attributed to oxygen desorption, leading to an increased conductivity of the ZnO layer. At and above this critical thickness, the device performance significantly improved and no longer depended strongly on the thickness of the ZnO layer, in agreement with optical simulatio...

Harvesting of both triplets and singlets yields electroluminescence quantum efficiencies of nearly 100% in organic light-emitting diodes (OLEDs), but the production efficiency of excitons that can undergo radiative decay is theoretically limited to 100% of the electron-hole pairs. Here, breaking of this limit by exploiting singlet fission in an OLED is reported. Based on the dependence of electroluminescence intensity on an applied magnetic field, it is confirmed that triplets produced by singlet fission in a rubrene host matrix are emitted as near-infrared (NIR) electroluminescence by erbium(III) tris(8-hydroxyquinoline) (ErQ3 ) after excitonic energy transfer from the "dark" triplet state of rubrene to an "emissive" state of ErQ3 , leading to NIR electroluminescence with an overall exciton production efficiency of 100.8%. This demonstration clearly indicates that the harvesting of triplets produced by singlet fission as electroluminescence is possible even under electrical excitation, leading to an enhancement of the quantum efficiency of the OLEDs. Electroluminescence employing singlet fission provides a route toward developing high-intensity NIR light sources, which are of particular interest for sensing, optical communications, and medical applications.

The contact resistance in pentacene organic field-effect transistors (OFETs) is found to be significantly reduced by selectively doping the organic semiconductor region beneath the source/drain electrodes. A 10 nm co-evaporated (1:1 ratio) layer of molybdenum tris-[1,2-bis(trifluoromethyl)ethane-1,2-dithiolene] and pentacene was deposited under the metal electrodes for this purpose. The width-normalized contact resistance (varying channel lengths of 25–200 μm used for the study) in contact-doped devices was lowered significantly (0.5 kΩ-cm) in comparison to reference devices (3.4 kΩ-cm) in the accumulation regime (VGS = −30 V). Doping of the contacts did not affect the stability of the devices under continuous bias stress significantly.

A series of highly soluble donor–acceptor (D–A) copolymers containing N-(3,4,5-tri-n-decyloxyphenyl)-dithieno[3,2-b:2′,3′-d]pyrrole (DTP) or N-(2-decyltetradecyl)-dithieno[3,2-b:2′,3′-d]pyrrole (DTP′) as donor and three different acceptors, 4,7-dithien-2-yl-[2,1,3]-benzothiadiazole, 4,9-dithien-2-yl-6,7-di-n-hexyl-[1,2,5]thiadiazolo[3,4-g]quinoxaline and 4,8-dithien-2-yl-2λ4δ2-benzo[1,2-c;4,5-c′]bis[1,2,5]thiadiazole (BThX, X = BTD, TQHx2, BBT, respectively) were synthesized by Stille coupling polymerizations. The optical and electrochemical properties of these copolymers were investigated, along with their use in field-effect transistors and photovoltaic devices. The band gaps (eV) estimated from UV-vis-NIR spectra and electrochemical measurements of the copolymers varied from ca. 1.5–0.5 eV, and were consistent with quantum-chemical estimates extrapolated using density functional theory. Oxidative and reductive spectroelectrochemistry of the copolymers indicated they can be both p-doped and n-doped, and three to four differently colored redox states of the polymers can be accessed through electrochemical oxidation or reduction. The DTP-BThBTD and DTP-BThTQHx2 copolymers exhibited average field-effect hole mobilities of 1.2 × 10−4 and 2.2 × 10−3 cm2/(Vs), respectively. DTP-BThBBT exhibited ambipolar field-effect characteristics and showed hole and electron mobilities of 1.2 × 10−3 and 5.8 × 10−4 cm2/(Vs), respectively. Bulk heterojunction photovoltaic devices made from blends of the copolymers with 3′-phenyl-3′H-cyclopropa[1,9](C60-Ih)[5,6]fullerene-3′-butanoic acid methyl ester (PCBM) (1:3 weight ratio) exhibited average power conversion efficiencies as high as 1.3% under simulated irradiance of 75 mW/cm2.

Organic photovoltaics, which convert sunlight into electricity with thin films of organic semiconductors, have been the subject of active research over the past 20 years. The global energy challenge has greatly increased interest in this technology in recent years. Low-temperature processing of organic small molecules from the vapor phase or of polymers from solution can confer organic semiconductors with a critical advantage over inorganic photovoltaic materials since the high-temperature processing requirements of the latter limit the range of substrates on which they can be deposited. Unfortunately, despite significant advances, the power conversion efficiency of organic solar cells remains low, with maximum values in the range of 6%. A better understanding of the physical processes that determine the efficiency of organic photovoltaic cells is crucial to enhancing their competitiveness with other thin-film technologies. Maximum values for the photocurrent can be estimated from the light-harvesting capability of the individual molecules or polymers in the device. However, a better understanding of the materials-level processes, particularly those in layer-to-layer interfaces, that determine the open-circuit voltage (V(OC)) in organic solar cells is critical and remains the subject of active research. The conventional wisdom is to use organic semiconductors with smaller band gaps to harvest a larger portion of the solar spectrum. This method is not always an effective prescription for increasing efficiency: it ignores the fact that the value of V(OC) is generally decreased in devices employing materials with smaller band gaps, as is the case with inorganic semiconductors. In this Account, we discuss the influence of the different interfaces formed in organic multilayer photovoltaic devices on the value of V(OC); we use pentacene-C(60) solar cells as a model. In particular, we use top and bottom electrodes with different work function values, finding that V(OC) is nearly invariant. In contrast, studies on devices incorporating hole-transport layers with different ionization potentials confirm that the value of V(OC) depends largely on the relative energy levels of the donor and acceptor species that form the essential heterojunction. An analysis of the properties of solar cells using equivalent-circuit methods reveals that V(OC) is proportional to the logarithm of the ratio of the photocurrent density J(ph) divided by the reverse saturation current density J(0). Hence, an understanding of the physical origin of J(0) directly yields information on what limits V(OC). We assign the physical origin of J(0) to the thermal excitation of carriers from the donor to the acceptor materials that form the organic heterojunction. Finally, we show that the solution to achieving higher power conversion efficiency in organic solar cells will be to control simultaneously the energetics and the electronic coupling between the donor and acceptor materials, in both the ground and excited state.

We report on the photovoltaic properties of organic solar cells based on pentacene and C 60 thin films with a focus on their spectral responses and the effect of thermal annealing. Spectra of external quantum efficiency (EQE) are measured and analyzed with a one-dimensional exciton diffusion model dependent upon the complex optical functions of pentacene films, which are measured by spectroscopic ellipsometry. An improvement in EQE is observed when the thickness of the bathocuproine (BCP) layer is decreased from 12 nm to 6 nm. Detailed analysis of the EQE spectra indicates that large exciton diffusion lengths in the pentacene films are responsible for the overall high EQE values near wavelengths of 668 nm. Analysis also shows that improvement in the EQE of devices with the thinner BCP layer can be attributed to a net gain in optical field distribution and improvement in carrier collection efficiency. An improvement in open-circuit voltage ( V OC ) is also achieved through a thermal annealing process, leading to a net increase in power conversion efficiency. Integration of the EQE spectrum with an AM1.5 G spectrum yields a predicted power conversion efficiency of 1.8 ± 0.2%. The increase in V OC is attributed to a significant reduction in the diode reverse saturation current upon annealing.

Currently, organic light-emitting diodes (OLEDs) have reached the stage of commercialization, and there has been an intense drive to use them in various applications from small- and medium-sized mobile devices to illumination equipment and large television screens. In particular, room-temperature phosphorescent materials have become core OLED components as alternatives to conventionally used fluorescent materials because devices made with phosphorescent materials exhibit excellent light-emitting performance with internal electroluminescence efficiencies (η int) of nearly 100 %. However, phosphorescent materials have several intrinsic problems, such as being limited to metal–organic compounds containing rare metals, for example, Ir, Pt, Au, and Os, and difficulty in realizing stable blue light emission. As a result, researchers have attempted to develop new materials for use as emissive dopants in OLEDs that overcome these limitations. In this chapter, first we briefly review the progress of OLED materials and device architectures mainly based on fluorescent (first-generation) and phosphorescent (second-generation) emitters. Then, we discuss third-generation OLEDs that use a new light-emitting mechanism called thermally activated delayed fluorescence (TADF). Recently, highly efficient TADF, which had been difficult to realize with conventional molecular design, has been achieved by very sophisticated molecular structures, allowing access to the unlimited freedom of molecular design using carbon-based materials. This has led to the production of ultimate OLEDs that are made of common organic compounds without precious metals and can convert electricity to light at η int of nearly 100 %.

Red fluorescent molecules suffer from large, non-radiative internal conversion rates (k(IC)) governed by the energy gap law. To design efficient red thermally activated delayed fluorescence (TADF) emitters for organic light-emitting diodes (OLEDs), a large fluorescence rate (k(F)) as well as a small energy difference between the lowest singlet and triplet excited states (ΔE(ST)) is necessary. Herein, we demonstrated that increasing the distance between donor (D) and acceptor (A) in intramolecular-charge-transfer molecules is a promising strategy for simultaneously achieving small ΔE(ST) and large k(F). Four D-Ph-A-Ph-D-type molecules with an anthraquinone acceptor, phenyl (Ph) bridge, and various donors were designed, synthesized, and compared with corresponding D-A-D-type molecules. Yellow to red TADF was observed from all of them. The k(F) and ΔE(ST) values determined from the measurements of quantum yield and lifetime of the fluorescence and TADF components are in good agreement with those predicted by corrected time-dependent density functional theory and are approximatively proportional to the square of the cosine of the theoretical twisting angles between each subunit. However, the introduction of a Ph-bridge was found to enhance k(F) without increasing ΔE(ST). Molecular simulation revealed a twisting and stretching motion of the N-C bond in the D-A-type molecules, which is thought to lower ΔE(ST) and k(F) but raise k(IC), that was experimentally confirmed in both solution and doped film. OLEDs containing D-Ph-A-Ph-D-type molecules with diphenylamine and bis(4-biphenyl)amine donors demonstrated maximum external quantum efficiencies of 12.5% and 9.0% with emission peaks at 624 and 637 nm, respectively.

We report on large-area pentacene / C60 organic solar cells in which indium-tin-oxide (ITO) is replaced with a conductive polymer electrode and a 5 μm-thick metal grid is used to reduce resistive power losses. The performance of cells with the polymer electrode was compared with that of pentacene / C60 devices using ITO as the transparent electrode. For large-area devices (7.3 cm2) on glass substrates with an integrated metal grid, the performance of a device with the polymer electrode is comparable to that of a device with an ITO electrode combined with a grid.

In this talk we will discuss the modeling of large-area organic solar cells. Degradation of the performance with increased area is observed and analyzed in terms of the power loss density concept. The equivalent circuit model is used to verify that a change in power loss density (or RSA) can have a strong influence on device performance. The limited sheet resistance of ITO is found to be one of the major limiting factors when the area of the cell is increased. The effective series resistance of the ITO film can be minimized by integrating metal grids and the improvement is analyzed using a power loss density analysis. By integrating metal grids onto ITO, the series resistance could be reduced significantly yielding improved performance.

Organic photovoltaics, which convert sunlight into electricity with thin films of organic semiconductors, have been the subject of active research over the past 20 years. The global energy challenge has greatly increased interest in this technology in recent years. Low-temperature processing of organic small molecules from the vapor phase or of polymers from solution can confer organic semiconductors with a critical advantage over inorganic photovoltaic materials since the high-temperature processing requirements of the latter limit the range of substrates on which they can be deposited. Unfortunately, despite significant advances, the power conversion efficiency of organic solar cells remains low, with maximum values in the range of 6%. A better understanding of the physical processes that determine the efficiency of organic photovoltaic cells is crucial to enhancing their competitiveness with other thin-film technologies. Maximum values for the photocurrent can be estimated from the light-harvesting cap...

Methods for scalable output voltage and encapsulation of organic photovoltaic cells are addressed in this paper. To obtain scalable output voltages, integrated photovoltaic modules comprised of a bulk heterojunction of poly(3- hexylthiophene) (P3HT) and a soluble C 70 derivative, [6,6]-phenyl C 71 butyric acid methyl ester (PCBM-70), were fabricated. Power conversion efficiency of individual P3HT/PCBM-70 cells was estimated to be 4.1 % for AM1.5 G illumination. Modules of one to four cells connected in series produced open-circuit voltages V OC that linearly depend on the number of cells N as V OC = N × 0.621 V with a nearly constant short-circuit current of 1.4 ± 0.1 mA. Separately, shelf lifetimes of more than one year were achieved for pentacene/C 60 solar cells by encapsulation with a 200-nm-thick layer of Al 2 O 3 deposited by atomic layer deposition (ALD). In addition, the ALD process improved the open-circuit voltage and power conversion efficiency of the solar cells by thermal annealing that occurs during the process.

Efficient organic photovoltaic (OPV) modules and cells have been fabricated based on a blend of poly(3-hexylthiophene) (P3HT) and a soluble C70 derivative, [6,6]-phenyl C71 butyric acid methyl ester (PCBM-70). Power conversion efficiency of 4.1 % under AM 1.5 G were obtained in individual P3HT/PCBM-70 cells. Integrated photovoltaic modules with up to four cells connected in series were demonstrated with a maximum open-circuit voltage of 2.48 V.

Successful commercialization of flexible organic electronic devices is largely dependent on proper encapsulation that protects them from permeation of oxygen and water vapor. At present, low permeation encapsulation materials generally consist of multilayer films of organic/inorganic materials which can be deposited by plasma enhanced chemical vapor deposition (PECVD), atomic layer deposition (ALD), and vapor phase deposition. For this study, we report the effective water vapor transmission rates (WVTR) for multilayer thin films consisting of low temperature PECVD deposited SiNx and SiOx combined with a parylene organic layer. The effective WVTR was measured as a function of the number of bilayer pairs using Ca corrosion tests. The effective WVTR at 20 °C and 50% relative humidity [RH] for three bilayer pairs of SiOx/parylene ranged between 4.4–8.0 × 10−4 g/m2/day while SiNx/parylene had a transmission rate 1.3×10−4 g/m2/day. In general, additional layers were found to decrease the permeation rates to as low as 3.9×10−5 g/m2/day, while the SiNx/parylene coatings performed the best overall.

Efficient bulk heterojunction (BHJ) photovoltaic cells (PVs) based on 5 wt. % donors and C70 were fabricated. Tris[4-(5-phenylthiophen-2-yl)phenyl]-amine (TPTPA)-based BHJ PVs show higher power conversion efficiency (ηPCE) than aluminum phthalocyanine chloride-based BHJ PVs. Although the absorption of AlPcCl is complementary to that of C70, TPTPA's high hole mobility and symmetrical molecular structure are likely to be crucial contributing factors to the higher ηPCE. Phase separation occurs in the 5%-TPTPA blend. The device was optimized via replacement of the bathocuproine buffer by a combination of 3,4,9,10-perylenetetracarboxylic bis-benzimidazole and bathocuproine. ηPCE of 5.96% is achieved because of the decreased series resistance.

Highly efficient photovoltaic cells based on a bulk heterojunction configuration composed of C70 with various donor materials at 5 wt. % donor concentration were fabricated. The tetraphenyldibenzoperiflanthene (DBP) donor achieved the highest power conversion efficiency (ηPCE) of 6.4% for the optimized cell. The improved performance with DBP arises from a combination of a higher absorption coefficient than 1,1-bis-(4-bis(4-methyl-phenyl)-amino-phenyl)-cyclohexane and a symmetrical molecular structure. The high ηPCE with only 5 wt. % donor is attributed to a sufficient donor concentration for enhanced Frenkel exciton dissociation in C70, while efficiency and electron mobility decrease at higher donor concentrations.

Organic solar cells (OSCs) based on chloroaluminum phthalocyanine (ClAlPc) as donor and fullerene C60 as acceptor with a multi-tandem structure were fabricated. We demonstrated very high open-circuit voltage (VOC) and enhanced power conversion efficiency (PCE) for the multi-tandem OSCs. Using a fivefold structure, we obtained PCE of 2.49% with a VOC of 3.50 V, in comparison with PCE of ∼2% and VOC of 0.72–0.81 V for the single device. We also fabricated a tenfold-stacked OSC showing an extremely high VOC of 5.89 V. The multi-tandem OSCs with very high VOC have great potential for applications in limited-area low-power electronics.

We report on the operational stability of low-voltage hybrid organic-inorganic complementary inverters with a top-gate bottom source-drain geometry. The inverters are comprised of p-channel pentacene and n-channel amorphous InGaZnO thin-film transistors (TFTs) with bi-layer gate dielectrics formed from an amorphous layer of a fluoropolymer (CYTOP) and a high-k layer of Al2O3. The p- and n- channel TFTs show saturation mobility values of 0.1 ± 0.01 and 5.0 ± 0.5 cm2/Vs, respectively. The individual transistors show high electrical stability with less than 6% drain-to-source current variations after 1 h direct current (DC) bias stress. Complementary inverters yield hysteresis-free voltage transfer characteristics for forward and reverse input biases with static DC gain values larger than 45 V/V at 8 V before and after being subjected to different conditions of electrical stress. Small and reversible variations of the switching threshold voltage of the inverters during these stress tests are compatible with the observed stability of the individual TFTs.

Perylene-3,4,9,10-tetracarboxylic diimides (PDI) have been linked to norbornene through either an imide or a “bay” position. These PDI-functionalised norbornenes are readily polymerised using a ruthenium ring-opening metathesis polymerisation (ROMP) initiator. In one case the polymerisation is shown to be well controlled with a linear dependence of molecular weight on the monomer/initiator ratio. For the polymers with PDI–norbornene linkages through the imide nitrogen, UV-vis spectroscopy suggests significant PDI aggregation both in dilute solution and the solid state, a powder X-ray diffraction peak attributable to π-stacking is seen, and weak n-channel field-effect transistor behavior (electron mobilities

We report on a systematic study of solvent and polymer matrix effects on the phase segregation behavior of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) blends incorporated into two different amorphous polymer matrices, poly (α-methyl styrene) and poly (triarylamine), and using two solvents, chlorobenzene and tetralin. Optical microscopy, X-ray diffraction analyses, and optical absorption measurements are used to evaluate the film morphology, crystallinity, and optical density, respectively. These analyses are correlated with the extent of vertical segregation of TIPS-pentacene, as observed for the blended films by depth-profile XPS analyses. The microstructure and vertical phase segregation of TIPS-pentacene in blend films are found to be strongly influenced by the choice of solvent. Tetralin, a solvent with a high boiling temperature, was found to be more desirable for achieving distinct phase segregation/crystallization of TIPS-pentacene in blend films and best performance in OFETs with a dual-gate geometry. The electrical properties of top and bottom channels were consistent with the morphological characterization and OFETs processed from tetralin showed higher mobility values than those from chlorobenzene. Further modification of the annealing conditions in the TIPS-pentacene/PTAA/tetralin ternary system led to top-gate OFETs with mobility values up to 2.82 cm2/Vs.

We report on indium tin oxide (ITO)-free and metal-free semitransparent organic solar cells with a high-conductivity poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) (PH1000) as both the bottom and the top electrodes. The PH1000 film showed a conductivity of 680±50 S/cm. A ZnO layer was used as an interlayer to produce an electron-selective electrode. The semitransparent devices with a structure of glass/PH1000/ZnO/poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester/PEDOT:PSS (CPP 105 D)/PH1000 exhibited an average power conversion efficiency of 1.8% estimated for 100 mW/cm2 air mass 1.5 global illumination. This geometry alleviates the need of vacuum deposition of a top electrode.

Organic photovoltaics is an active research area because thin-film solar cells can be processed directly onto large-area substrates and patterned into modules, thus lowering manufacturing costs.

High performance solution-processed n-channel organic field-effect transistors based on [6,6]-phenyl C61 butyric acid methyl ester with low operating voltages (3 V) are demonstrated using a high-k hafnium dioxide gate dielectric grown by atomic layer deposition. Devices exhibit excellent n-channel performance with electron mobility values up to 0.14 cm2/V s, threshold voltages of ∼0.3 V, current on/off ratios >105, and very low values of subthreshold slope (∼140 mV/decade).

We report on the performance of organic solar cells based on pentacene/C60 heterojunctions as a function of active area. Devices with areas of 0.13 and 7 cm2 were fabricated on indium-tin-oxide (ITO) coated glass. Degradation of the performance with increased area is observed and analyzed in terms of the power loss density concept. The various power loss contributions to the total series resistance (RSA) are measured independently and compared to the values of the series resistance extracted from the current-voltage characteristics using a Shockley equivalent circuit model. The limited sheet resistance of ITO is found to be one of the major limiting factors when the area of the cell is increased. To reduce the effects of series resistance, thick, electroplated, metal grid electrodes were integrated with ITO in large-area cells. The metal grids were fabricated directly onto ITO and passivated with an insulator to prevent electrical shorts during the deposition of the top Al electrode. By integrating metal ...

Solution processed n-channel organic field-effect transistors based on [6,6]-phenyl C61 butyric acid methyl ester with high mobility and low contact resistance are reported. Ca, Au, or Ca capped with Au (Ca/Au) was used as the top source/drain electrodes. The devices with Ca electrodes exhibit excellent n-channel behavior with electron mobility values of 0.12 cm2/V s, low threshold voltages (∼2.2 V), high current on/off ratios (105–106) and subthreshold slopes of 0.7 V/decade. By varying the channel lengths (25–200 μm) in devices with different metal/semiconductor interfaces, the effect of channel length scaling on mobility is studied and the contact resistance is extracted. The width-normalized contact resistance (RCW) for Au (12 kΩ cm) is high in comparison to Ca (7.2 kΩ cm) or Ca/Au (7.5 kΩ cm) electrodes at low gate voltage (VGS=10 V). However, in the strong accumulation regime at high gate voltage (VGS=30 V), its value is nearly independent of the choice of metal electrodes and in a range of 2.2–2.6 kΩ cm. These devices show stable electrical behavior under multiple scans and low threshold voltage instability under electrical bias stress (VDS=VGS=30 V, 1 h) in N2 atmosphere.

We report a thin-film encapsulation method for organic electronics that combines the deposition of a layer of SiOx or SiNx (100 nm) by plasma enhanced chemical vapor deposition followed by a layer of Al2O3 (10–50 nm) by atomic layer deposition and a 1-μm-thick layer of parylene by chemical vapor deposition. The effective water vapor transmission rates of the encapsulation was (2±1)×10−5 g/m2 day at 20 °C and 50% relative humidity (RH). The encapsulation was integrated with pentacene/C60 solar cells, which showed no decrease in conversion efficiency after 5800 h of exposure to air demonstrating the effectiveness of the encapsulation methodology.

We report on low-voltage, high-performance amorphous indium gallium zinc oxide n-channel thin-film transistors fabricated using 100-nm-thick Al2O3 grown by atomic layer deposition as the gate dielectric layer. The Al2O3 gate dielectric shows very small current densities and has a capacitance density of 81±1 nF/cm2. Due to a very small contact resistance, transistors with channel lengths ranging from 100 μm down to 5 μm yield a channel-independent, field-effect mobility of 8±1 cm2/V s, subthreshold slopes of 0.1±0.01 V/decade, low threshold voltages of 0.4±0.1 V, and high on-off current ratios up to 6×107 (W/L=400/5 μm) at 5 V.

We report on flexible organic complementary inverters using pentacene and C60 as active semiconductors fabricated on a plastic substrate. Individual transistors as well as inverters show good operational stability with negligible hysteresis. The threshold voltages are comparable for p-channel pentacene and n-channel C60 organic field-effect transistors, and noise margins larger than 80% of the maximum theoretical values were obtained at a supply voltage VDD as low as 3 V. A high dc gain of 180 was achieved at VDD=5 V. The inverters demonstrated good mechanical stability when tested after bending under both tensile and compressive stress.

We investigate the effects of surface modification of indium tin oxide (ITO) on the performance of organic multilayer molecular photovoltaic devices based on pentacene/C60 bi-layer heterojunctions. Values of the open-circuit voltage (Voc), short circuit current density (Jsc), fill factor (FF), and power conversion efficiency (η) are found invariant of the work function and surface hydrophobicity of ITO. Insensitivity of these parameters to variations of work function in the range of 4.50 to 5.40 eV achieved through the use of surface modifiers are correlated with an invariance of the barrier height (∼0.6 eV) due to Fermi level pinning at the ITO/pentacene interface. Energy barrier heights are extracted independently from the analysis of the electrical characteristics of single-layer diodes based on modified ITO and pentacene using an equivalent circuit model.