Atula S. D. Sandanayaka

Solution-Processed Organic–Inorganic Perovskite Field-Effect Transistors with High Hole Mobilities

A very high hole mobility of 15 cm2 V-1 s-1 along with negligible hysteresis are demonstrated in transistors with an organic-inorganic perovskite semiconductor. This high mobility results from the well-developed perovskite crystallites, improved conversion to perovskite, reduced hole trap density, and improved hole injection by employing a top-contact/top-gate structure with surface treatment and MoOx hole-injection layers.

Morphological control of organic–inorganic perovskite layers by hot isostatic pressing for efficient planar solar cells

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%).

Toward continuous-wave operation of organic semiconductor lasers

Organic semiconductor laser operating in the quasi-CW regime at 80 MHz and under 30 ms long pulse photoexcitation is demonstrated. The demonstration of continuous-wave lasing from organic semiconductor films is highly desirable for practical applications in the areas of spectroscopy, data communication, and sensing, but it still remains a challenging objective. We report low-threshold surface-emitting organic distributed feedback lasers operating in the quasi–continuous-wave regime at 80 MHz as well as under long-pulse photoexcitation of 30 ms. This outstanding performance was achieved using an organic semiconductor thin film with high optical gain, high photoluminescence quantum yield, and no triplet absorption losses at the lasing wavelength combined with a mixed-order distributed feedback grating to achieve a low lasing threshold. A simple encapsulation technique greatly reduced the laser-induced thermal degradation and suppressed the ablation of the gain medium otherwise taking place under intense continuous-wave photoexcitation. Overall, this study provides evidence that the development of a continuous-wave organic semiconductor laser technology is possible via the engineering of the gain medium and the device architecture.

N-channel field-effect transistors with an organic-inorganic layered perovskite semiconductor

Large electron injection barriers and electrode degradation are serious issues that need to be overcome to obtain n-channel operation in field-effect transistors with an organic-inorganic layered perovskite (C6H5C2H4NH3)2SnI4 semiconductor. By employing low-work-function Al source/drain electrodes and by inserting C60 layers between the perovskite semiconductor and the Al electrodes to reduce the injection barrier and to suppress the electrode degradation, we demonstrate n-channel perovskite transistors with electron mobilities of up to 2.1u2009cm2/V s, the highest value ever reported in spin-coated perovskite transistors. The n-channel transport properties of these transistors are relatively stable in vacuum but are very sensitive to oxygen, which works as electron traps in perovskite and C60 layers. In addition, grazing-incidence X-ray scattering and thermally stimulated current measurements revealed that crystallite size and electron traps largely affect the n-channel transport properties.

Low threshold amplified spontaneous emission and ambipolar charge transport in non-volatile liquid fluorene derivatives

Highly fluorescent non-volatile fluidic fluorene derivatives functionalized with siloxane chains were synthesized and used in monolithic solvent-free liquid organic semiconductor distributed feedback lasers. The photoluminescence quantum yield values, the amplified spontaneous emission thresholds and the ambipolar charge carrier mobilities demonstrate that this class of materials is extremely promising for organic fluidic light-emitting and lasing devices.

Extremely low amplified spontaneous emission threshold and blue electroluminescence from a spin-coated octafluorene neat film

We report on the photophysical, amplified spontaneous emission (ASE), and electroluminescence properties of a blue-emitting octafluorene derivative in spin-coated films. The neat film shows an extremely low ASE threshold of 90 nJ/cm2, which is related to its high photoluminescence quantum yield of 87% and its large radiative decay rate of 1.7 × 109 s−1. Low-threshold organic distributed feedback semiconductor lasers and fluorescent organic light-emitting diodes with a maximum external quantum efficiency as high as 4.4% are then demonstrated, providing evidence that this octafluorene derivative is a promising candidate for organic laser applications.

Improvement of the quasi-continuous-wave lasing properties in organic semiconductor lasers using oxygen as triplet quencher

We demonstrate quasi-continuous-wave lasing in solvent-free liquid norganic semiconductor ndistributed feedback lasers based on a blend containing a liquid 9-(2-ethylhexyl)carbazole host doped with a blue-emitting heptafluorene derivative. The liquid gain medium is bubbled with either oxygen or nitrogen in order to investigate the role of a triplet quencher such as molecular oxygen on the quasi-continuous-wave lasing properties of organic semiconductor lasers. The oxygenated laser device exhibits a low threshold of 2 μJ cm−2, which is lower than that measured in the nitrogenated device and is independent of the repetition rate in a range between 0.01 and 4u2009MHz.

Vacuum-and-solvent-free fabrication of organic semiconductor layers for field-effect transistors.

We demonstrate that cold and hot isostatic pressing (CIP and HIP) is a novel, alternative method for organic semiconductor layer fabrication, where organic powder is compressed into a layer shape directly on a substrate with 200u2009MPa pressure. Spatial gaps between powder particles and the other particles, substrates, or electrodes are crushed after CIP and HIP, making it possible to operate organic field-effect transistors (OFETs) containing the compressed powder as the semiconductor. The CIP-compressed powder of 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) had a hole mobility of (1.6u2009±u20090.4)u2009×u200910–2u2009cm2/Vs. HIP of C8-BTBT powder increased the hole mobility to an amorphous silicon-like value (0.22u2009±u20090.07u2009cm2/Vs) because of the growth of the C8-BTBT crystallites and the improved continuity between the powder particles. The vacuum and solution processes are not involved in our CIP and HIP techniques, offering a possibility of manufacturing OFETs at low cost.

Singlet-Triplet Exciton Annihilation Nearly Suppressed in Organic Semiconductor Laser Materials Using Oxygen as a Triplet Quencher

We report on the use of oxygen as triplet quencher to reduce singlet-triplet annihilation in light-emitting organic solid films and solvent-free molecular semiconducting liquids. For this purpose, a fluorescent heptafluorene derivative is dispersed either in a wide bandgap 4,4-Bis (N-carbazolyl)-1,1-biphenyl (CBP) host or in a solvent-free liquid matrix based on 9-(2-ethylhexyl)carbazole (EHCz). To introduce oxygen in the samples, a modified cold isostatic pressure technique is used in the case of spin-coated CBP blends while oxygen is bubbled in the liquids. The influence of the oxygenation on their photophysical and amplified spontaneous emission properties is examined. Both solid blend films and liquids showed before and after the introduction of oxygen a photoluminescence quantum yield higher than 80% and an amplified spontaneous emission threshold lower than 0.4 μJ/cm 2. While oxygen does not quench significantly the singlet excitons in these systems, singlet-triplet annihilation is strongly reduced in solid thin films and nearly suppressed in the liquid layers. This study demonstrates that liquid organic semiconductors are promising candidates for optically pumped continuous wave lasers and provides important insights for a more effective triplet management in organic semiconductor lasers.

Centrifugal-Coated Quasi-Two-Dimensional Perovskite CsPb2Br5 Films for Efficient and Stable Light-Emitting Diodes

The optical, structural, and electroluminescent (EL) characteristics of pure quasi-2D CsPb2Br5 were reported. We fabricated continuous, compact, well-crystallized CsPb2Br5 films by centrifugal coating from a colloidal solution containing CsPb2Br5 nanoparticles. The centrifugal-coated CsPb2Br5 films have a photoluminescence quantum yield (PLQY) of ∼35% because of its low-dimensional structure. Taking advantage of the high PLQY, we fabricated perovskite light-emitting diodes (PeLEDs) with a centrifugal-coated CsPb2Br5 emitting layer exhibiting bright green EL, a maximum luminance of 7317 cd m-2, an and external quantum efficiency of 1.1%. Additionally, the EL color could be changed easily from green to red using a halogen exchange method. The half lifetime of our CsPb2Br5 PeLEDs reached around 6 h under continuous operation at 10 mA cm-2.