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N,N-di(4-methoxyphenyl)amino-9,9′-spirobifluorene (spiro-OMeTAD) is a widely used hole transport layer (HTL) material for emerging lead halide hybrid perovskite solar cells (PSCs). Finding alternative HTL materials is required for the future development of more efficient and stable PSCs. Here, we developed a new HTL material of N2,N2,N,7N7-tetrakis(4-methoxyphenyl)benzo[b]benzo[4,5]thieno[2,3-d]thiophene-2,7-diamine (TBTD). Chemically doped TBTD provided better hole extraction in PSCs than chemically doped spiro-OMeTAD due to higher electrical conductivity and a more suitable hole transport energy level. Thus, PSCs with the TBTD HTL showed higher power conversion efficiency (∼21%) than spiro-OMeTAD HTL (∼19%). The operational stability of TBTD-based PSCs was similar to that of spiro-OMeTAD-based PSCs. The findings discussed in this study will aid in the future development of high-performance PSCs.

Continuous-wave (CW) lasing is still difficult to realize in organic laser dyes, one reason being the thermal degradation caused by intense photoexcitation. A decrease in laser threshold suppresses...

Quasi-two dimensional perovskites demonstrate unique excitonic properties due to their multilayer structure making them attractive for various optoelectronic applications. However, the thickness of individual perovskite sheets in wet cast quasi-2D layers tends to randomly fluctuate giving rise to a specific type of disorder, which impacts on the carrier dynamics and is rather complex and remains understudied. Here, we present a study of carrier transport in Ruddlesden–Popper type (PEA)2FAn−1PbnBr3n+1 layers of order n from one to four, and in the bulk FAPbBr3 layer. We use a light induced transient grating technique to measure the carrier diffusion coefficient directly, and the transient absorption via photoluminescence to investigate the energy relaxation pathways. We observe two distinct energy transfer processes on different time scales. Fast energy funnelling in thicker (n ≥ 3) layers is observed up to 10 ps after excitation; we attribute this to short-distance transfer of excitons to neighbouring perovskite sheets of higher order. On the longer timescale of hundreds of picoseconds, carrier in-plane transport is governed by exciton diffusion in n = 1 and 2 layers and by free carrier plasma in thicker ones. Within the carrier density range of (0.5–4) × 1019 cm−3, the exciton diffusion coefficient in n = 1, 2 increases slowly from 1 to 2.8 cm2 s−1, whereas in thicker layers the dependence is much stronger and the diffusivity grows from 0.09 to 1.9 cm2 s−1. We explain these dependencies by a higher structural order in the thinner samples and the stronger localization of carriers in thicker ones. Also, amplified spontaneous emission (ASE) is observed in thicker (n ≥ 3) layers in electron–hole plasma, as evidenced by the typical ASE line redshift upon excitation.

Metal halide perovskites are promising as the emitter of efficient light-emitting diodes (LEDs). External quantum efficiencies (EQEs) of perovskite LEDs (Pe-LEDs) have already surpassed 20%, which ...

Organic–inorganic lead halide quasi-two-dimensional (2D) perovskites are promising gain media for lasing applications because of their low cost, tunable colour, excellent stability and solution processability1–3. Optically pumped continuous-wave (CW) lasing is highly desired for practical applications in high-density integrated optoelectronics devices and constitutes a key step towards electrically pumped lasers4–6. However, CW lasing has not yet been realized at room temperature because of the ‘lasing death’ phenomenon (the abrupt termination of lasing under CW optical pumping), the cause of which remains unknown. Here we study lead halide-based quasi-2D perovskite films with different organic cations and observe that long-lived triplet excitons considerably impede population inversion during amplified spontaneous emission and optically pumped pulsed and CW lasing. Our results indicate that singlet–triplet exciton annihilation is a possible intrinsic mechanism causing lasing death. By using a distributed-feedback cavity with a high quality factor and applying triplet management strategies, we achieve stable green quasi-2D perovskite lasers under CW optical pumping in air at room temperature. We expect that our findings will pave the way to the realization of future current-injection perovskite lasers. Lead halide-based quasi-two-dimensional perovskite films with different organic cations are used to create stable green lasers under continuous-wave optical pumping in air at room temperature.

Quasi-2D perovskites have attracted wide attention as the emitter of light-emitting diodes (LEDs) in recent years because of the ease of obtaining high external quantum efficiencies (EQEs). However, the quick degradation under continuous operation and significant EQE roll-off at high current densities are issues that need to be overcome for future practical applications using quasi-2D perovskite LEDs (PeLEDs). In this context, we discuss the mechanism of the degradation and EQE roll-off on the basis of ion migration. The migration of ligand cations though domain boundaries of quasi-2D perovskite films induces the gradual loss of defect passivation at the boundaries, which results in the reversible PeLED degradation and severe EQE roll-off. When the device operation time is long, the mobile cations enter and interact with the electron transport layer, leading to the stage of irreversible PeLED degradation. The device degradation mechanisms we discovered here are constructive for developing quasi-2D PeLEDs with better operational durability.

Halide perovskites are currently intensively being investigated for applications in light-emitting diodes for next-generation lighting and display technology. A recent report shows that the control...

In organic light-emitting diodes (OLEDs) based on materials that show thermally activated delayed fluorescence (TADF), the internal quantum efficiency of 100 % can be obtained without using phosphorescence-based organometallics that contain rare metals. Therefore, with TADF-based emitters, it is possible to fabricate high-performing OLEDs at a lower cost. However, compared with fluorescence- and phosphorescence-based OLEDs, an understanding of degradation mechanisms in TADF-based OLEDs is still insufficient for future commercialization. In particular, it is widely recognized that the development of electron transport materials is crucial for improving OLED characteristics, especially driving voltages and operational durability. In this study, it was demonstrated that the operational durability of TADF-based OLEDs was greatly improved by introducing a triazine-based material of 2,4,6-tris(1,1'-biphenyl-4-yl)-[1,3,5]triazine (pT2T) as a hole-blocking layer (HBL) compared with a conventional HBL material of 2,4,6-tris(biphenyl-3-yl)-[1,3,5]triazine (T2T). Several experiments were carried out to make the reasons of the improved durability clearer, and attributed the improved durability to the shift of a carrier recombination zone from the emitting layer/HBL interface and the suppressed formation of excited-state quenchers in the pT2T HBL, because of the higher electron mobility of pT2T and the better stability of its radical anion state.

Organic light-emitting diode (OLED) technology is promising for applications in next-generation displays and lighting. However, it is difficult-especially in large-area mass production-to cover a large substrate uniformly with organic layers, and variations in thickness cause the formation of shunting paths between electrodes1,2, thereby lowering device production yield. To overcome this issue, thicker organic transport layers are desirable because they can cover particles and residue on substrates, but increasing their thickness increases the driving voltage because of the intrinsically low charge-carrier mobilities of organics. Chemical doping of organic layers increases their electrical conductivity and enables fabrication of thicker OLEDs3,4, but additional absorption bands originating from charge transfer appear5, reducing electroluminescence efficiency because of light absorption. Thick OLEDs made with organic single crystals have been demonstrated6, but are not practical for mass production. Therefore, an alternative method of fabricating thicker OLEDs is needed. Here we show that extraordinarily thick OLEDs can be fabricated by using the organic-inorganic perovskite methylammonium lead chloride, CH3NH3PbCl3 (MAPbCl3), instead of organics as the transport layers. Because MAPbCl3 films have high carrier mobilities and are transparent to visible light, we were able to increase the total thickness of MAPbCl3 transport layers to 2,000 nanometres-more than ten times the thickness of standard OLEDs-without requiring high voltage or reducing either internal electroluminescence quantum efficiency or operational durability. These findings will contribute towards a higher production yield of high-quality OLEDs, which may be used for other organic devices, such as lasers, solar cells, memory devices and sensors.

Tin iodide perovskite CH3NH3SnI3 is often considered as a replacement for toxic lead halide perovskites. Tin iodide is not only suitable for production of solar cells, but also it emits in the near...

Heat management of optoelectronic devices is of critical significance in lead halide perovskites due to the intrinsically low thermal conductivity of this material. Despite its importance, thermal conductivity remains understudied, particularly in polycrystalline perovskite layers with different halides. Here, we employ a novel method for investigation of thermal properties in perovskite layers, which is based on a light-induced transient diffraction grating technique. We demonstrate the applicability of thermal grating technique by determining in an all-optical way the thermo-optic coefficient, speed of sound, and thermal conductivity in vapor-deposited polycrystalline layers of MAPbX3 (X = Cl, Br, I), MAPbBr2I, and MAPbCl2Br perovskites. We reveal the spatial anisotropy of thermal conductivity, which is noticeably lower in the direction along the layer surface (0.2–0.5 W/(m K)) if compared to that across the layer (0.3–1.1 W/(m K)). Finally, we demonstrate that for both directions the thermal conductivi...

We investigated the dependence of electrical properties on vacuum-deposition rate for films of N,N՛-diphenyl-N,N՛-bis(1-naphthyl)-1,1՛-biphenyl-4,4՛-diamine (α-NPD) and tris-(8-hydroxyquinoline)aluminium (Alq3), hole- and electron-transport materials widely used in organic light-emitting diodes (OLEDs), respectively. Hole-only devices (HODs) of α-NPD showed an increase of hole current when α-NPD layers were fabricated at high deposition rates, while the tendency was opposite for electron-only devices (EODs) of Alq3. We found that the increased hole current at high deposition rates for HODs was caused by a horizontal orientation of α-NPD molecules relative to a substrate plane which facilitates hole transport through the films. On the other hand, the decreased electron current at high deposition rates for EODs could be ascribed to the increased surface roughness of Alq3 films which decreases electron injection. Additionally, we demonstrated that long-term operational stability was enhanced for HODs fabricated at high deposition rates and operated at a constant current. Use of a high deposition rate means a reduction of deposition time which is helpful in OLED manufacturing. The improved device performance when high deposition rates were used and the detailed understanding of its origins as we demonstrated here would lead to fabrication of high-performance OLEDs at a low cost in the future.

This work investigates the feasibility of using low-dimensional perovskites for electrically driven lasers given the current status of perovskite light-emitting diodes and optically pumped lasers. ...

The current status of electrochemical impedance spectroscopy (EIS) and related analysis on perovskite solar cells (PSC) is still unsatisfactory. The provided models are still vague and not really helpful for guiding the efforts to develop more efficient and stable devices. Due to the slow and complex dynamics of these devices, the obtained spectra need to be validated, which is hardly ever done. This study may be the first to provide fully validated impedance spectra and presents reproducible EIS time series at open circuit voltage (VOC) for more than 20 hours, with a total of 140 analysed spectra. We conclude that the observed changes stem from a temporary reduction of the electronically active area of the devices, as can be deduced from the inverse behaviour of resistance and capacitance. The changes in these values are almost 100% reversible if the devices are kept in the dark for only one day, while the time constant of the high-frequency process remains unchanged throughout the whole characterization procedure. The tested devices are full PSC devices that have proven to be stable over more than 500 hours, and the non-steady impedance measurements shine a critical light on previously published EIS data. With the results of this study, it can be rationalized that the high-frequency semicircle can serve as a good indicator for ionic migration by monitoring its consequences. The results presented here are helpful to quantify ionic migration on the device level in order to derive new stability criteria and countermeasures against degradation.

Heptacene (1) has been produced via a monoketone precursor, 2, which was prepared from 1,2,4,5-tetrabromobenzene in nine steps in a total yield of 10 %. Compound 2 was converted to 1 quantitatively by heating at 202 °C. Heptacene exhibited high thermal stability in the solid state without any observable change over two months. To investigate the potential value of 1 as a material for p-type organic field-effect transistors (OFETs), top-contact OFET devices were fabricated by vacuum deposition of 1 onto a hexamethyldisilazane (HMDS)/SiO2 /Si substrate. The best hole mobility performance was 2.2 cm2 V-1 s-1 . This is the first report of stable heptacene being used in an effective device and examined for its charge carrier properties.

Heptacene (1) has been produced via a monoketone precursor, 2, which was prepared from 1,2,4,5-tetrabromobenzene in nine steps in a total yield of 10 %. Compound 2 was converted to 1 quantitatively by heating at 202 °C. Heptacene exhibited high thermal stability in the solid state without any observable change over two months. To investigate the potential value of 1 as a material for p-type organic field-effect transistors (OFETs), top-contact OFET devices were fabricated by vacuum deposition of 1 onto a hexamethyldisilazane (HMDS)/SiO

Large external quantum efficiency rolloff at high current densities in organic light-emitting diodes (OLEDs) is frequently caused by the quenching of radiative singlet excitons by long-lived triplet excitons [singlet–triplet annihilation (STA)]. In this study, we adopted a triplet scavenging strategy to overcome the aforementioned STA issue. To construct a model system for the triplet scavenging, we selected 2,6-dicyano-1,1-diphenyl-λ5σ4-phosphinine (DCNP) as the emitter and 4,4′-bis[(N-carbazole)styryl]biphenyl (BSBCz) as the host material by considering their singlet and triplet energy levels. In this system, the DCNP’s triplets are effectively scavenged by BSBCz while the DCNP’s singlets are intact, resulting in the suppressed STA under electrical excitation. Therefore, OLEDs with a 1 wt.%-DCNP-doped BSBCz emitting layer demonstrated the greatly suppressed efficiency rolloff even at higher current densities. This finding favourably provides the advanced light-emitting performance for OLEDs and organic semiconductor laser diodes from the aspect of the suppressed efficiency rolloff., Though organic light-emitting diodes are highly desirable for display and lighting applications, efficiency roll-off at high current densities limits its applicability. Here, the authors report a triplet scavenging strategy for improved triplet exciton management and reduced efficiency roll-off.

Although metal halide perovskites are very promising for use in solar cells, other applications using perovskites are possible. In Kyushu University, we obtained a four-fold increase in external quantum efficiency of perovskite LEDs by choosing appropriate organic cations for the incorporation into perovskite structures. We showed that perovskites can work as the excellent host or transport layers of organic LEDs. We demonstrated hole and electron mobilities of up to 26 and 4.6 cm2 V−1 s−1 in solution-processed perovskite field-effect transistors. Our transistors fabricated by the lamination with perovskite single crystals revealed hole and electron mobilities ranging from 50 to 100 cm2 V−1 s−1, along with excellent stability even in air.

Triplet exciton accumulation and singlet-triplet annihilation (STA) are major issues, which makes continuous-wave (CW) operation difficult, in organic lasers. We synthesized an organic laser dye, 2,6-dicyano-1,1-diphenyl-λ5σ4-phosphinine (DCNP), having a small singlet-triplet energy gap. In DCNP-doped 4-4΄-bis[(N-carbazole)styryl] biphenyl (BSBCz) films, the triplets formed on DCNP are easily transferred to BSBCz, resulting in the suppressed STA and triplet accumulation. With this system, we demonstrated true-CW laser operation, with a low threshold and high photostability. Laser emission was further characterized by plane-polarized light output, far-field beam interference, exciton decay lifetime, and visible beam observed on an illuminance paper.

A novel solution processable organic semiconductor dye, 9,9'-(((1E,1'E)-(9,9-dihexyl-9H-fluorene-2,7-diyl)bis(ethene-2,1-diyl))bis(4,1-phenylene))bis(9H-carbazole) (BSFCz) was synthesized and investigated in terms of its amplified spontaneous emission (ASE) and surface emission laser properties under optical excitation. Films of BSFCz, which were blended with tris(4-carbazoyl-9-ylphenyl)amine (TCTA) at 6 wt.% with solution processing, had a low ASE threshold of 1.1 μJ cm−2 and surface emission laser threshold of 0.9 μJ cm−2 with second-order distributed feedback structure. Moreover, our BSFCz-based devices exhibited quasi-continuous-wave lasing with good stability up to 10 ms. To the best of our knowledge, this is the first ever demonstration of solution-processed organic lasers, which can work under the quasi-continuous wave regime.

Organic-inorganic lead halide quasi-two-dimensional (2D) perovskites are promising gain media for lasing applications because of their low cost, tunable colour, excellent stability and solution processability

We show that a chlorine-based metal halide perovskite with high transparency in the visible region and high hole and electron mobilities can be used as the excellent host or carrier transport layers in organic light-emitting diodes.

New deposition methods of halide perovskites are being developed with the aim of improving solar cell power conversion efficiency by controlling the physiochemical properties of the perovskite film. In the case of methylammonium lead iodide (MAPbI3), deep level traps limit efficiency by participating in charge carrier recombination. Prior work has shown that the solar cell efficiency of MAPbI3 solar cells varied significantly with deposition method; specifically, efficiencies of 13.5 and 17.7% were observed for MAPbI3 processed with a one- and two-step method, respectively. However, the origin of the difference in efficiency remains unclear. In this study, we analyze the interplay between deep level traps and efficiency by simulating the photoexcited charge carrier pathway across solar cells processed via the one- and two-step method using finite-element drift-diffusion modeling. We determined that in the case of one-step processing, the traps propagate throughout the bulk, while for two-step, the traps congregate at the interface where the MAPbI3 was grown (mesoporous TiO2). Composition and structural analysis are used to propose a plausible explanation as to why the difference in processing changes the spatial distribution of the traps.

Carrier mobility is one of the crucial parameters determining the electronic device performance. We apply the light-induced transient grating technique to measure independently the carrier diffusion coefficient and lifetime, and to reveal the impact of additives on carrier transport properties in wet-cast CH3NH3PbI3 (MAPbI3) perovskite films. We use the high excitation regime, where diffusion length of carriers is controlled purely by carrier diffusion and not by the lifetime. We demonstrate a four-fold increase in diffusion coefficient due to the reduction of localization center density by additives; however, the density dependence analysis shows the dominance of localization-limited diffusion regime. The presented approach allows us to estimate the limits of technological improvement—carrier diffusion coefficient in wet-cast layers can be expected to be enhanced by up to one order of magnitude.

In this study, we investigate the nanoscale effects of photochemically active additives of benzoquinone (BQ), hydroquinone (HQ), and tetracyanoquinodimethane (TCNQ) on grain boundaries in CH3NH3PbI3 solar cells. We employ scanning probe microscopy under light illumination, in particular Kelvin probe force microscopy, to study surface potential changes under laser light illumination. The recently found improvement in the efficiency of BQ added solar cells can be clearly seen in vanishing contact potential differences at grain boundaries under illumination, rendering the material more uniform under solar cell operating conditions. These effects are observed for BQ, but not for HQ and TCNQ. Our findings shed light onto halide perovskite materials and the functional additive design for improved solar cell performance.

A halide-substituted higher acene, 2-bromohexacene, and its precursor with a carbonyl bridge moiety were synthesized. The precursor was synthesized through 7 steps in a total yield of 2.5%. The structure of precursor and thermally converted 2-bromohexacene were characterized by solid state NMR, IR, and absorption spectra, as well as by DFT computation analysis. It exhibited high stability in the solid state over 3 months, therefore can be utilized in the fabrication of opto-electronic devices. The organic thin-film transistors (OFETs) were fabricated by using 2-bromohexacene and parent hexacene through vaccum deposition method. The best film mobility of 2-bromohexacene was observed at 0.83 cm2 V−1 s−1 with an on/off ratio of 5.0 × 104 and a threshold of −52 V, while the best film mobility of hexacene was observed at 0.076 cm2 V−1 s−1 with an on/off ratio of 2.4 × 102 and a threshold of −21 V. AFM measurement of 2-bromohexacene showed smooth film formation. The averaged mobility of 2-bromohexacene is 8 fold higher than the non-substituted hexacene.

Organo-metal-halide perovskites are a promising set of materials for optoelectronic applications such as solar cells, light emitting diodes and lasers. Perovskite thin films have demonstrated amplified spontaneous emission thresholds as low as 1.6 μJ cm-2 and lasing thresholds as low as 0.2 μJ cm-2. Recently the performance of perovskite light emitting diodes has rapidly risen due to the formation of quasi 2D films using bulky ligands such as phenylethylammonium. Despite the high photoluminescent yield and external quantum efficiency of quasi 2D perovskites, few reports exist on amplified spontaneous emission. We show within this report that the threshold for amplified spontaneous emission of quasi 2D perovskite films increases with the concentration of phenylethylammonium. We attribute this increasing threshold to a charge transfer state at the PEA interface that competes for excitons with the ASE process. Additionally, the comparatively slow inter-grain charge transfer process cannot significantly contribute to the fast radiative recombination in amplified spontaneous emission. These results suggest that relatively low order PEA based perovskite films that are suitable for LED applications are not well suited for lasing applications. However high order films were able to maintain their low threshold values and may still benefit from improved stability.

The influences of film density and molecular orientation on the carrier conduction and air stability of vacuum-deposited amorphous organic films of N,N'-di(1-naphthyl)-N,N'-diphenyl-(1,1'-biphenyl)-4,4'-diamine (α-NPD) were investigated. The substrate temperature (Tsub) during vacuum deposition had different effects on the film density and molecular orientation of α-NPD. Film density was a concave function of Tsub; maximum density was attained at Tsub = 270-300 K. α-NPD molecules were randomly oriented at Tsub = 342 K, and their horizontal orientation on the substrate became dominant as Tsub decreased. Hole current and air stability were clearly raised by increasing the film density by 1 to 2%; these effects were, respectively, attributed to enhanced carrier hopping between neighboring α-NPD molecules and suppressed penetration of oxygen and water. These results imply that increasing film density is more effective to enhance the electrical performance of organic thin-film devices with α-NPD films than control of molecular orientation.

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.

Metal halide perovskites are attractive materials for the realization of cheap and effective solar cells, thin film transistors, and light emitters. Carrier diffusion at high excitations, however, is poorly addressed in perovskites, even though it governs the diffusion length and determines the efficiency of photonic devices. To fully understand the dependence of diffusion length on carrier density, we performed direct and independent measurements of the carrier diffusion coefficient and recombination rate in several methylammonium lead-halide perovskite layers by applying the light-induced transient grating technique. We demonstrate the existence of two distinct carrier diffusion regimes within the density range of 1018–1020 cm–3. In the perovskite films of high compositional quality, diffusion is governed by a bandlike transport of free carriers. The diffusivity is high (0.28–0.7 cm2/s) in these samples, even at low carrier density, and further increases with excitation due to carrier degeneracy. The op...

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.