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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.

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.

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, 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.

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...

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.

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.

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.