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The wettability issue associated with the Me-4PACz hole-selective monolayer is solved by the introduction of the second component to the precursor solution. This results in a similar performance while simultaneously significantly improving the yield of the devices.

Photoluminescence of composites containing carbon nanostructures is critical for many modern applications. Of particular significance are metamaterials capable of generating white light photoluminescence within a single structure, as the white luminescence usually needs separate red-green-blue emitters. This work provides an insight into the photoluminescent properties of a promising family of hierarchical metamaterials made of carbon nanocones in a silicon oxide matrix in view of their use as efficient single-layer white light emitters. The composites were prepared using a facile plasma-enhanced technology, followed by a thermal treatment. ATR-FTIR, Raman, SEM, and AFM microscopy and profilometry characterization confirmed the presence of silica and carbon nanocones. The advanced comprehensive photoluminescence studies conducted using solid-state Yb:KGW laser revealed a significant difference in photoluminescent properties for the composites containing sharp nanocones of similar parameters. A comprehensive morphological analysis performed using several analytical techniques including the 2D fast Fourier transform spectra, Hough distributions, spectral density function, and Minkowski functionals revealed the Minkowski boundary functional, ordering, and connectivity to be the most important morphological descriptors for the photoluminescent response. This study suggests that these morphological parameters play a critical role in defining the key properties of advanced metamaterials via the overlap of ψ functions and may therefore be targeted for informed material design and intelligent fabrication.

To attain commercial viability, perovskite solar cells (PSCs) have to be reasonably priced, highly efficient, and stable for a long period of time. Although a new record of a certified power conversion efficiency (PCE) value over 25% was achieved, PSC performance is limited by the lack of hole-transporting materials (HTMs), which extract positive charges from the light-absorbing perovskite layer and carry them to the electrode. Here, we report spirobifluorene-based HTMs with finely tuned energy levels, high glass-transition temperature, and excellent charge mobility and conductivity enabled by molecularly engineered enamine arms. HTMs are synthesized using simple condensation chemistry, which does not require costly catalysts, inert reaction conditions, and time-consuming product purification procedures. Enamine-derived HTMs allow the fabrication of PSCs reaching a maximum PCE of 19.2% and stability comparable to spiro-OMeTAD. This work demonstrates that simple enamine condensation reactions could be used as a universal path to obtain HTMs for highly efficient and stable PSCs.

Electronic doping of organic semiconductors is essential for their usage in highly efficient optoelectronic devices. Although molecular and metal complex-based dopants have already enabled significant progress of devices based on organic semiconductors, there remains a need for clean, efficient and low-cost dopants if a widespread transition towards larger-area organic electronic devices is to occur. Here we report dimethyl sulfoxide adducts as p-dopants that fulfil these conditions for a range of organic semiconductors. These adduct-based dopants are compatible with both solution and vapour-phase processing. We explore the doping mechanism and use the knowledge we gain to 'decouple' the dopants from the choice of counterion. We demonstrate that asymmetric p-doping is possible using solution processing routes, and demonstrate its use in metal halide perovskite solar cells, organic thin-film transistors and organic light-emitting diodes, which showcases the versatility of this doping approach.

In this work, we report on the fabrication of a GaN/AlGaN waveguiding structure dedicated to modal phase matching, where GaN waveguide has planar polarity inversion. First, we optimized the growth conditions for the AlGaN epilayer. Second, on top of the AlGaN epilayer, we fabricated the waveguiding structure starting with the growth of the Ga-polar GaN epilayer followed by atomic layer deposition (ALD) of an Al $$_{2}$$ O $$_{3}$$ layer, then, continuing with the growth of N-polar GaN epilayer. We tested several layer thicknesses, but with 20 nm we managed to inverse the GaN polarity from Ga to N. To confirm the N-polarity, we etched the GaN epilayer surface in aqueous KOH solution. We performed out-of-plane (0002) and in-plane (11–20) X-ray diffraction and rocking curve measurements to estimate the crystalline quality of the AlGaN epilayer, Ga- and N-polar GaN epilayer. Atomic force microscopy measurement lets us evaluate the epilayer surface morphology and roughness. Optical and scanning electron microscopy inspection revealed characteristic hexagonal N-polar GaN epilayer surface. We used high-resolution transmission electron microscopy to investigate the crystallinity and orientation of the Ga- and N-polar GaN epilayer, also the Al $$_{2}$$ O $$_{3}$$ ALD layer, the interface quality of the waveguide structure.

Bismuth films with thicknesses between 6 and ∼30 nm were grown on Si (111) substrate by molecular beam epitaxy (MBE). Two main phases of bismuth — α-Bi and β-Bi — were identified from high-resolution X-ray diffraction (XRD) measurements. The crystal structure dependencies on the layer thicknesses of these films were analyzed. β-Bi layers were epitaxial and homogenous in lateral regions that are greater than 200 nm despite the layer thickness. Further, an increase in in-plane 2θ values showed the biaxial compressive strain. For comparison, α-Bi layers are misoriented in six in-plane directions and have β-Bi inserts in thicker layers. That leads to smaller (about 60 nm) lateral crystallites which are compressively strained in all three directions. Raman measurement confirmed the XRD results. The blue-sift of Raman signals compared with bulk Bi crystals occurs due to the phonon confinement effect, which is larger in the thinnest α-Bi layers due to higher compression.

Triphenylamine (TPA) compounds are versatile charge transport materials, demonstrating high emission efficiency and interesting interactions in solid state. Their spectroscopic emission parameters highly depend on surrounding medium owning to strong CT character in solution. Although TPA compounds demonstrate good charge transport capabilities, further optimization via molecular design is required. In this work, various TPA molecular systems with varying number of additional dimethylamine (dMA) and phenylethenyl (PE) substituents were thoroughly investigated. Additionally, influence of quaternarization of tertiary amine groups in the triphenylamine backbone on emission and electronic properties was analyzed. TPA-dMA compounds demonstrated aggregation induced emission enhancement with high emission efficiency and excellent (μh = 4.7·10−3 cm2V−1s−1) charge transport in the solid state. Quaternarization of tertiary amine groups in the triphenylamine backbone has led to emission and electronic properties in the wide 400–700 nm spectral region.

A group of near infrared (NIR)-absorbing dimeric polymethine dyes have been developed and successfully applied in photodetectors based on focus-induced photoresponse technology. The devices are based upon simple and inexpensive solid-state dye-sensitized solar cell architecture. The investigated molecules, designed and synthesized specifically for focus-induced photoresponse application, demonstrate relatively narrow and intense absorption in the NIR region of the spectrum up to 900 nm. Detectors containing the investigated dyes function in the near-infrared region of the spectrum, and suitable photocurrent and signal-to-noise ratio can be obtained at both 730 nm and 850 nm wavelengths. Additionally, these photodetectors can also be used in applications requiring simultaneous tracking of multiple light spots, as devices maintain appropriate characteristics at light modulation frequencies between 1 Hz and 1000 Hz.

Thin films of InN were grown on sapphire by metalorganic chemical vapor deposition using multi-step growth technique (thermal cleaning and nitridation of sapphire substrate, deposition and annealing of nucleation layer, and growth of the active InN layer). The effects of different growth steps on the structural quality and luminescence properties of the samples grown for the study were analyzed by X-ray structural analysis (ω-2Θ scans, measurement of rocking curves) and photoluminescence spectroscopy with sub-micrometer spatial resolution enabled by applying confocal microscopy. The influence of nitridation duration, variation of thickness and annealing conditions of nucleation layer, and the temperature of deposition of the active InN layer were studied and discussed.

Recent progress of vapor-deposited perovskite solar cells (PSCs) has proved the feasibility of this deposition method in achieving promising photovoltaic devices. For the first time, it is probed the versatility of the co-evaporation process in creating perovskite layers customizable for different device architectures. A gradient of composition is created within the perovskite films by tuning the background chamber pressure during the growth process. This method leads to co-evaporated MAPbI3 film with graded Fermi levels across the thickness. Here it is proved that this growth process is beneficial for p-i-n PSCs as it can guarantee a favorable energy alignment at the charge selective interfaces. Co-evaporated p-i-n PSCs, with different hole transporting layers, consistently achieve power conversion efficiency (PCE) over 20% with a champion value of 20.6%, one of the highest reported to date. The scaled-up p-i-n PSCs, with active areas of 1 and 1.96 cm2, achieved the record PCEs of 19.1% and 17.2%, respectively, while the flexible PSCs reached a PCE of 19.3%. Unencapsulated PSCs demonstrate remarkable long-term stability, retaining ≈90% of their initial PCE when stored in ambient for 1000 h. These PSCs also preserve over 80% of their initial PCE after 500 h of thermal aging at 85 °C. © 2021 Wiley-VCH GmbH

Perovskite solar cells deliver high efficiencies, but are often made from high-cost bespoke chemicals, such as the archetypical hole-conductor, 2,2′,7,7′-tetrakis(N,N-di-p-methoxy-phenylamine)-9-9′-spirobifluorene (spiro-OMeTAD). Herein, new charge-transporting carbazole-based enamine molecules are reported. The new hole conductors do not require chemical oxidation to reach high power conversion efficiencies (PCEs) when employed in n-type-intrinsic-p-type perovskite solar cells; thus, reducing the risk of moisture degrading the perovskite layer through the hydrophilicity of oxidizing additives that are typically used with conventional hole conductors. Devices made with these new undoped carbazole-based enamines achieve comparable PCEs to those employing doped spiro-OMeTAD, and greatly enhanced stability under 85 °C thermal aging; maintaining 83% of their peak efficiency after 1000 h, compared with spiro-OMeTAD-based devices that degrade to 26% of the peak PCE within 24 h. Furthermore, the carbazole-based enamines can be synthesized without the use of organometallic catalysts and complicated purification techniques, lowering the material cost by one order of magnitude compared with spiro-OMeTAD. As a result, we calculate that the overall manufacturing costs of future photovoltaic (PV) modules are reduced, making the levelized cost of electricity competitive with silicon PV modules.

ZnO materials have been at the centre of many studies for decades. Doping of ZnO by lithium atoms is a prospective approach for compensation of n-type conductivity in the unintentionally doped ZnO aimed at obtaining p-type semiconductor. In this study, we have synthesized ZnO rod-like powders doped with lithium ions (0–0.65 atom%) by the new microwave-assisted solvothermal method in order to obtain greater photoluminescence intensity of ZnO emissions in the UV region. The obtained powders contain nanoparticles from 20 nm up to 250 nm depending on Li content. X-ray diffractometry and Raman spectroscopy were employed to characterise the structure of ZnO powders, scanning electron microscopy was used to determine the morphology, and most importantly the photoluminescence technique was used to investigate the optical properties of the obtained materials. As a result, the photoluminescence intensity has been increased up to an order of magnitude with the Li content in the ZnO crystals. The increase of the excitation laser power has led to an increased carrier lifetime and photoluminescence response intensity that is observed for all the samples.

In a short period of time, the rapid development of perovskite solar cells attracted a lot of attention in the science community with the record for power conversion efficiency being broken every year. Despite the fast progress in power conversion efficiency there are still many issues that need to be solved before starting large scale commercial applications, such as, among others, the difficult and costly synthesis and usage of toxic solvents for the deposition of hole transport materials (HTMs). We herein report new enamine-based charge transport materials obtained via a simple one step synthesis procedure, from commercially available precursors and without the use of expensive organometallic catalysts. The developed materials demonstrated rapid loss of mass during thermogravimetry analysis suggesting that they could be processed not only using solution processing but also via vacuum deposition. Furthermore, all HTMs demonstrated high charge carrier mobility with H2 possessing the highest mobility of 2.5 × 10−2 cm2 V−1 s−1 under strong electric fields. The investigated materials were employed in vacuum-deposited p–i–n perovskite solar cells and champion devices with enamine H2 demonstrate a PCE of 18.4%.

Novel bipolar molecules based on naphthalene- (V555) and perylene- (V565) tetracarboxydiimide core, bearing 1-phenyl-1,2,3,4-tetrahydroquinoline end capping hole transporting groups, were designed and synthesized. Electro-optical and thermal measurements, such as UV/VIS, charge mobility, ionization potential, thermogravimetric analysis, and differential scanning calorimetry, were carried out to characterize the new materials. V555 and V565 showed bipolar behavior with both hole and electron mobilities under ambient conditions without any protection from oxygen or moisture. The gelation of V565 was observed that provides the ability to tune the charge transport properties.

Cerium-doped mixed garnet-type single crystals (GdxY1-x)3Al2Ga3O12 with different yttrium content have been fabricated and studied as a prospective scintillating material enabling improvement of scintillation properties by tuning the composition of the matrix-building crystal. The influence of the matrix composition on the emitting Ce ion is studied using linear and time-resolved nonlinear optical absorption and time-resolved photoluminescence spectroscopy. The study of photoluminescence at resonant excitation revealed a composite origin of Ce3+ emission band. This behavior is interpreted by the contribution of Ce3+ ions located in inequivalent positions expected due to the disorder caused in the garnet-type lattice of a mixed crystal by compositional fluctuations. The substitution of gadolinium by yttrium in the lattice results not only in an emission blue shift and in decreased splitting of the lowest doublet 5d state of Ce3+ but also in an increased separation between the lowest doublet level 5d1 and the lowest triplet 5d3 level, as well as in changing the rates of intracenter and extracenter energy relaxation.

In this work, for the first time, reactive radical-cation species present in hole-transporting materials were shown to react with tert-butylpyridine additive, routinely used in hole transporting layer composition. As a result, new pyridinated products were isolated and characterized by NMR and MS analysis. Additionally, their optical and photophysical properties (i.e., solid-state ionization potentials (Ip), cyclic voltammetry (CV), UV/vis characteristics, and conductivities) were determined. Formation of the pyridinated products was confirmed in the aged perovskite solar cells by means of mass spectrometry, and shown to have negative influence on the overall device performance. We believe that these findings will help improve the stability of perovskite devices by either molecular engineering of hole-transporting materials or utilization of less-reactive or sterically hindered pyridine derivatives.

Capabilities of repeated deposition of ultrathin layers by pulsed metalorganic chemical vapor deposition (MOCVD) for improvement of structural and luminescence properties of InN thin films on GaN/sapphire templates were studied by varying the growth temperature and the durations of pulse and pause in the delivery of In precursor. X-ray diffraction, atomic force microscopy, and spatially-resolved photoluminescence (PL) spectroscopy were exploited to characterize the structural quality, surface morphology and luminescence properties. Better structural quality is achieved by using longer trimethylindium pulses. However, it is shown that the luminescence properties of InN epilayers correlate with the pause and pulse ratio rather than with their absolute lengths, and the deposition of 1.5–2 monolayers of InN during one growth cycle is optimal to achieve the highest PL intensity. Moreover, the use of temperature ramping enabled achieving the highest PL intensity and the smallest blue shift of the PL band. The luminescence parameters are linked with the structural properties, and domain-like patterns of InN layers are revealed.

The GaN was grown on Si(100) with Er 2 O 3 (110) buffer by metal organic chemical vapor deposition (MOCVD). The results showed the polycrystalline nature of the layers with dominant non-polar (11–20) and semi-polar (10–13) orientations. GaN was formed in a two step process regime: low temperature growth was followed by high temperature growth regime. At a low temperature GaN tends to crystalize with dominant non-polar (11–20) orientation, while high temperature leads to preferential semi-polar (10–13) orientation growth. Additionally, the effects of parameters such as growth temperature, V/III-ratio and type of carrier gases on structural, morphological and optical properties of GaN was discussed in the current work.

Three Troger's base scaffold-based metal-free sensitizers TD1, TD2 and TD3 with triphenylamine donor and rhodanine-3-acetic acid as an acceptor/anchoring group were designed and synthesized for dye-sensitized solar cells. The sensitizer series was designed to investigate the influence of the poly[n]enic (from n = 0 to 2) backbones and their anchoring effect in DSSCs. Optical, electrochemical and photovoltaic properties were compared with analogues dyes D1-D3 possessing one anchoring group. It has been proven that the extended polymethine chains ensure flexibility of these units and inspire the interaction between two chromophores promoting aggregate formation in these Troger's base-based dyes.

Efficiency from hole-selective contacts Perovskite/silicon tandem solar cells must stabilize a perovskite material with a wide bandgap and also maintain efficient charge carrier transport. Al-Ashouri et al. stabilized a perovskite with a 1.68–electron volt bandgap with a self-assembled monolayer that acted as an efficient hole-selective contact that minimizes nonradiative carrier recombination. In air without encapsulation, a tandem silicon cell retained 95% of its initial power conversion efficiency of 29% after 300 hours of operation. Science , this issue p. 1300

We report a novel quasi-phase-matching method based on coupling compensation for second harmonic generation in GaN/AlGaN waveguides on sapphire substrates. The proposed method can overcome limitations of periodically oriented GaN waveguides because of spatial periodic modulation of the light intensity along the propagation direction, rather than the conventional spatial periodic modulation of the nonlinear optical coefficients of GaN. The method can be realized by using a dual-planar or dual-ridge waveguide frequency doubler structure. This concept has the potential to open a new range of applications for broadband on-chip nonlinear frequency conversion.

Carrier recombination and scattering at the semiconductor boundaries can substantially limit the device efficiency. However, surface and interface recombination is generally neglected in the nitride-based devices. Here, we study carrier recombination and diffusivity in AlGaN/GaN/sapphire heterointerfaces with AlGaN barriers of different quality. We employ the light induced transient grating and time-resolved photoluminescence spectroscopy techniques to extract carrier lifetime in different depths of the GaN buffer as well as in the AlGaN barrier, and to evaluate the carrier diffusion coefficient in the buffer. Moreover, we assess interface recombination velocity, Shockley-Read-Hall and radiative recombination rates. We reveal the adverse barrier influence on carrier dynamics in the underlying buffer: AlGaN barrier accelerates the nonradiative carrier recombination in the GaN buffer. The interface recombination velocity in the GaN buffer increases with decreasing AlGaN barrier quality, and the dominating recombination mechanism switches from Shockley-Read-Hall to interface recombination. These phenomena are governed by a cumulative effect of various interface-deteriorating barrier defects. Meanwhile, the carrier diffusivity in the GaN buffer is not affected by the AlGaN barrier. We conclude that barrier-accelerated interface recombination can become a major carrier loss mechanism in AlGaN/GaN interface, and may substantially limit the efficiency in nitride-based UV LEDs.

In this work we present a study of spatial and temperature dependencies of carrier lifetime and photoluminescence in pseudomorphic Ge0.95Sn0.05 layer on silicon by applying contactless time-resolved differential absorption and photoluminescence techniques. The observed small lifetime and photoluminescence intensity spatial variations (~10%) indicate for good material homogeneity. Differential transmission kinetics exhibit fast bleaching and slow absorption components related to thermalization (few hundreds of picoseconds) and surface recombination (few tens of nanoseconds) processes. Temperature dependent photo-luminescence measurements indicate activation of carriers from L to Γ valley, governing dominant direct photoluminescence emission band at 1850 nm at 300 K. These direct transitions explain more efficient and blue-shifted by 150 nm photoluminescence emission at room temperature. At low temperatures direct emission is related to non-thermalized carriers, while indirect one is caused by depopulation of light hole band with temperature.

Photo-detection in the near-infrared is commonly performed by Ge or InGaAs-based photodetectors. Further extension of the detection range to the mid-infrared region can be performed by germanium-tin (GeSn) material which shows promising characteristics and is fully compatible with silicon electronics as can be directly grown on silicon substrates. In this study, we focused on the optoelectronic properties of the photodiodes prepared by using 200 nm thick Ge0.95Sn0.05 epitaxial layers on Ge/n-Si substrate with aluminium contacts. Photodiodes were formed on non-irradiated and Nd:YAG laser irradiated Ge0.95Sn0.05 layers. The samples were irradiated by pulsed Nd:YAG laser with 61.5–259.2 MW/cm2 intensity. The photodiodes were characterized by using short laser pulses with the wavelength in 2.0–2.6 μm range. The laser-irradiated diode was found more sensitive in the long wavelength range due to laser induced Sn atoms redistribution providing formation of graded bandgap structure. Sub-millisecond photocurrent relaxation in the diodes revealed their suitability for image sensors. Our findings open the perspective for improving the photo-sensitivity of GeSn alloys in the mid-infrared by pulsed laser processing.

Indium nitride epilayers were grown by metalorganic chemical vapour deposition (MOCVD) on graphene/SiC substrates with different terrace widths. Photoluminescence (PL) properties in the epilayers of different thickness and along the epilayer growth direction were studied with spatial resolution using confocal microscopy. The PL properties have been linked with the structure of the epilayers studied by X-ray diffraction, transmission electron microscopy, atomic force microscopy and Raman scattering. It is revealed that InN on graphene/SiC layers with a thickness of up to ~2 μm consists of chaotically oriented nanocrystals of 20 nm in diameter but tends to transform into a more homogeneous multicrystalline layer at longer deposition durations. The PL band in the top part of thick layers is peaked at 0.687 eV, closer to the band gap than in any InN epilayer grown by MOCVD before. This evidences a low density of equilibrium electrons usually deteriorating performance of InN epilayers. The PL band in thin samples or in the lower part of thick samples is shifted by 60 meV predominantly to quantum confinement in the nanocrystals.

Remote epitaxy via graphene has recently attracted significant attention, since it provides the possibility to lift-off the grown epitaxial layer, reuse the substrate, and produce flexible devices. However, extensive research is still necessary to fully understand the III-nitride formation on the van der Waals surface of a two-dimensional material and utilize remote epitaxy to its full potential. In this work, the growth of a GaN epilayer using a GaN/sapphire template covered with monolayer graphene is presented. Metalorganic vapor phase epitaxy is chosen to fabricate both the template and the nitride epilayer on top as a cost-effective approach toward GaN homoepitaxy. One-step and multi-step growth temperature protocols are demonstrated while paying particular attention to the graphene interface. GaN seed formation on graphene is analyzed to identify remote epitaxy. Crystalline quality improvement of the epilayer by adjusting the growth parameters is further discussed to provide useful insights into GaN growth on a GaN/sapphire template via monolayer graphene.

A simple carbazole-based conjugated enamine V950 was synthesized, fully characterized and incorporated into a perovskite solar cell, which displayed high power conversion efficiency close to 18%. The investigated hole transporting material was synthesized via an extremely simple route (one step, no expensive catalysts, no column chromatography or sublimation purification) from commercially available and relatively inexpensive starting reagents, resulting in more than one order of magnitude lower cost of the final product compared to the commercial 2,2′,7,7′-tetrakis(N,N-di-p-methoxy-phenylamine)-9-9′-spirobifluorene (spiro-OMeTAD). This material promises to be a viable p-type organic charge conductor to be employed in the scale-up and manufacturing of perovskite solar modules.

Triarylamine-based polymers with different functional groups were synthetized as hole-transport materials (HTMs) for perovskite solar cells (PSCs). The novel materials enabled efficient PSCs without the use of chemical doping (or additives) to enhance charge transport. Devices employing poly(triarylamine) with methylphenylethenyl functional groups (V873) showed a power conversion efficiency of 12.3 %, whereas widely used additive-free poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) demonstrated 10.8 %. Notably, devices with V873 enabled stable PSCs under 1 sun illumination at maximum power point tracking for approximately 40 h at room temperature, and in the dark under elevated temperature (85 °C) for more than 140 h. This is in stark contrast to additive-containing devices, which degrade significantly within the same time frame. The results present remarkable progress towards stable PSC under real working conditions and industrial stress tests.

The current state of the art in direct water splitting in photo-electrochemical cells (PECs) is presented together with: (i) a case study of water splitting using a simple solar cell with the most efficient water splitting electrodes and (ii) a detailed mechanism analysis. Detailed analysis of energy balance and efficiency of solar hydrogen production are presented. The role of hydrogen peroxide formation as an intermediate in oxygen evolution reaction is newly revealed and explains why an oxygen evolution is not taking place at the thermodynamically expected 1.23 V potential. Mechanism of cascading oxidation of water molecules via the peroxide stage is proposed and conforms with experimental observation. Solar hydrogen production with electrical-to-hydrogen conversion efficiency of 52% is demonstrated using a simple ∼0.7%-efficient n-Si/Ni Schottky solar cell connected to a water electrolysis cell. This case study shows that separation of the processes of solar harvesting and electrolysis avoids photo-electrode corrosion and utilizes optimal electrodes for hydrogen and oxygen evolution reactions and achieves ∼10% efficiency in light-to-hydrogen conversion with a standard 18% efficient household roof Si-solar cells. Strategy to increase efficiency above 15% for a single junction cell is outlined.

V-shaped Tröger's base core has been investigated as a central linking unit in the synthesis of new charge-transporting materials for optoelectronic applications. The studied molecules have been synthesized in two steps from relatively inexpensive starting materials, and demonstrate high glass transition temperatures, good stability of the amorphous state, and comparatively high hole drift mobility (up to 0.011 cm(2) V(-1) s(-1) ).

Room-temperature luminescence properties of a dense electron-hole plasma were studied in a set of GaN epilayers with thickness varying from 2 to 25 µm. The stimulated emission threshold was measured by photoluminescence and light-induced transient grating techniques under short-pulse excitation. Both techniques revealed the stimulated emission threshold increase with layer thickness despite the reduction of defect density. Numerical modeling of photoexcited carrier dynamics showed the different roles of carrier and photon populations. The stimulated emission threshold is mainly determined by the photon dynamics with weak influence of nonradiative carrier recombination. Increasing layer thickness results in the decreasing overlap of the gain layer with the optical mode, which reduces the transfer of energy from carriers to photons.