Dependence Of Photoluminescence Intensity Research Articles

Grain boundaries are not the source of Urbach tails in Cu(In,Ga)Se2 absorbers

The presence of Urbach tails in Cu(In,Ga)Se2 (CIGSe) absorbers has been identified as a limiting factor for the performance of the CIGSe solar cells. The tail states contribute to both radiative and non-radiative recombination processes, ultimately leading to a reduction in the open-circuit voltage and, consequently, decreasing the overall efficiency of CIGSe devices. Urbach tails result from structural and thermal disorders. The Urbach tails can be characterized by the Urbach energy, which is associated with the magnitude of the tail states. Within polycrystalline CIGSe absorbers, grain boundaries can be considered as structural disorder and, therefore, can potentially contribute to the Urbach tails. In fact, it has been proposed that the band bending at grain boundaries contribute significantly to the tail states. This study focuses on examining the correlation between Urbach tails and the band bending at the grain boundaries. The Urbach energies of the CIGSe samples are extracted from photoluminescence (PL) measurements, which reveal that the introduction of Sodium (Na) into the material can lead to a reduction in the Urbach energy, and an even further decrease can be achieved through the RbF post-deposition treatment. The band bending at the grain boundaries is investigated by Kelvin probe force microscopy measurements. A thorough statistical analysis of more than 340 grain boundaries does not show any correlation between Urbach tails and grain boundaries. We measure small band bending values at the grain boundaries, in the range of the thermal energy (26 meV at room temperature). Furthermore, our intensity dependent PL measurements indicate that Urbach tails are, at least in part, a result of electrostatic potential fluctuations. This supports the model that the introduction of alkali elements mainly decreases the magnitude of electrostatic potential fluctuations, resulting in a subsequent reduction in the Urbach energy.

Photoexcited Carrier Dynamics in Iodine-Doped CH3NH3PbBr3 Single Crystals.

Iodine-doped bromide perovskite single crystals (IBPSCs) have important applications in optoelectronic fields, such as in solar cells. Currently, much research has aimed to study the phase separation phenomenon and device performance improvements in IBPSCs. However, important intrinsic photoexcited carrier dynamics are often overlooked in IBPSCs. Here, we explored the photoexcited carrier dynamics in typical iodine-doped MAPbBr3 single crystals using the excitation intensity-dependent steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL) technique. We found that the trap state density changes with an increase in the amount of doped iodine. Further, we noticed that there is an influence of carrier diffusion on the photoexcited carrier dynamics, and then, we evaluated the carrier diffusion coefficients and recombination constants via numerical simulations of the PL kinetics. Consequently, we found that the electron shallow trap-related carrier behaviors substantially impacted the PL kinetics. Our results greatly facilitate a deeper understanding of the fundamental characteristics of mixed halide perovskite material.

Effect of confinement on photoluminescence of MeLPPP/SBA-15 nanocomposite

Photoluminescence spectra and kinetics of methyl-substituted ladder-type poly(para-phenylene) (MeLPPP)/SBA-15 mesoporous silica (SBA-15) nanocomposite have been studied and compared with photoluminescence properties of the MELPPP film and solution. Relaxation processes of excited singlet states in the closed geometry of the nanocomposite and in the diluted solution compared to the film based on the temperature dependence of photoluminescence (5–220 K) were studied. It was shown that the nanocomposite has twice the photoluminescence lifetime and three times weaker temperature dependence of photoluminescence intensity in comparison with the film. These peculiarities are due to a decrease in the number of polymer chains in a low-dimensional structure and a reduced role of exciton diffusion.

Two- and three-photon absorption in bulk CuI

We report on photoluminescence emission in copper iodide bulk single crystals induced by two- and three-photon absorption around 1.525 eV. These non-linear optical processes are investigated utilizing density-dependent, steady-state, as well as time-resolved photoluminescence spectroscopy as a function of the excitation energy. Using an excitation energy that corresponds to half of the bandgap energy, the observed photoluminescence intensity dependence on the excitation power shows an almost parabolic behavior. By further reduction of the photon energy, a cubic contribution is observable, which increases with decreasing excitation energy. The experimentally observed behavior can be well described by taking into account two- and three-photon absorption. By a simultaneous analysis of the intensity behavior for all used excitation energies, we determined a ratio between the two- and three-photon absorption cross section on the order of σ0(3)/σ0(2)≈10−28 cm2s.

Lasing properties and carrier dynamics of CsPbBr3 perovskite nanocrystal vertical-cavity surface-emitting laser

Abstract All-inorganic lead halide perovskite nanocrystals (NCs) have been widely investigated as highly promising optical gain materials due to their compelling electrical and optical properties. Although many efforts have been carried out, a deep understanding of perovskite NC vertical-cavity surface-emitting lasers (VCSELs) is elusive, which is very important in the development of photoelectronic integrated circuits. Along these lines, in this work, a low lasing threshold (22 μJ/cm2) single-mode VCSEL consisting of CsPbBr3 NCs film and two distributed Bragg reflectors was successfully constructed. The CsPbBr3 NCs were synthesized by using the supersaturated recrystallization method. Interestingly, benefiting from the strong coupling between the active layer and the optical field in the cavity, a single-mode lasing at 527 nm was demonstrated under femtosecond optical pumping. The carrier dynamics of the perovskite NC VCSEL was also thoroughly investigated by performing pump intensity-dependent time-resolved photoluminescence measurements. The typical gain-switching phenomenon was observed with an ultrafast decay of the laser pulse of ∼10 ps. Our work provides valuable insights for the implementation of the CsPbBr3 NC VCSEL for various optoelectronic applications.

Strong energy transfer and color tunable emission in Eu2+ and Mn2+ co-doped Na2BaSr(PO4)2 phosphor

The new dual-emitting and color tunable single phase of Na2BaSr(PO4)2:0.03Eu2+, xMn2+ (NBSP:0.03Eu2+, xMn2+) phosphor are successful prepared in this work. The phase purity and photoluminescence properties are investigated. Regarding to the coordination, valence and cation radius, the Eu2+ ions occupy the Sr2 and Sr3 site and Mn2+ ions occupy the Sr3 site. According to the photoluminescence (PL) spectra of NBSP:0.03Eu2+, xMn2+ (x = 0–0.09), the strong energy transfer (ET) between Eu2+ and Mn2+ exists in these phosphors. The strong ET efficiency from Eu2+ to Mn2+ is as high as 92%, while the ET mechanism of Eu2+ and Mn2+ is quadrupole-quadrupole interaction. Moreover, the CIE coordinate of Eu2+ and Mn2+ co-doped Na2BaSr(PO4)2 phosphors can realize the color tunable emission from blue to cold white. The temperature dependent photoluminescence intensity of NBSP:0.03Eu2+, 0.05Mn2+ phosphor still keeps 75.8% of that initial intensity when the temperature is 150 °C. In summary, the NBSP:0.03Eu2+, xMn2+ phosphors have a promising application in white light LED.

Impact of acetic acid exposure on metal contact degradation of different crystalline silicon solar cell technologies

Degradation due to acetic acid in photovoltaic (PV) modules has been a commonly observed phenomenon for both damp-heat exposure and outdoor operations. Acetic acid is a degradation byproduct of ethylene-vinyl acetate (EVA), a common module encapsulant. To address this issue, robust metallization pastes and cell technologies are being developed. However, it is important to assess how these technologies perform in an acetic acid environment and withstand degradation before they are implemented in the solar market. In this work, we investigate the impact of acetic acid exposure on four different cell groups: monofacial passivated emitter and rear contact (PERC) cells with advanced telluride-based front contact pastes, bifacial PERC cells with novel aluminum rear contact pastes, bifacial tunnel oxide passivated contacts (TOPCon) cells, and silicon heterojunction (SHJ) cells. These cells were exposed to acetic acid for different time increments. The recombination losses were characterized by Suns- V OC , and multi-variate regression analysis of intensity-dependent photoluminescence (PL) images with Griddler AI. Resistive losses were tracked with the transmission line method (TLM). Samples showing severe performance degradation were selected for further materials characterization to understand the root cause. Top-down and cross-sectional scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS) were performed to investigate the change in materials properties. Our study shows that the front contacts of the bifacial TOPCon cells and monofacial PERC cells were significantly affected by acetic acid exposure. The SHJ cells were found to be the most stable. • Effect of acetic acid on metallization pastes and cell technologies was investigated. • Front Ag-Al contacts for Bifacial TOPCon cells degrade the most due to acid exposure. • Monofacial PERC cells with Pb-free Te-O paste is the second group that is affected. • Bifacial PERC cells were stable with Al back contacts showing no signs degradation. • SHJ cells were the most stable group in this experiment.

Photoluminescence and Raman Spectra of One-Dimensional Lead-free Perovskite CsCu2I3 Single-Crystal Wires.

Lead-free highly luminescent CsCu2I3 perovskite has attracted much attention recently, but agreements on basic optical properties have remained unsettled. By correlating X-ray diffraction with the photoluminescence (PL) of CsCu2I3 single-crystal wires, we first show that blue PL at 420 nm originates from CuI. We then exclude defect states as a source for the broadband emission centered at 570 nm from the lack of defect absorption, PL under sub-bandgap photoexcitation, observations of a linear dependence of PL intensity on excitation laser power, and a strong spectral blueshift under mild hydrostatic pressure. Finally, using a model of the self-trapped exciton and the associated coordinate configuration diagram, we explain pressure evolutions of PL energy, intensity, and lifetime. Single-crystal wires also enable us to obtain polarization-dependent Raman spectra down to 10 cm-1 and confirm their respective ambient crystal structure of orthorhombic Cmcm and phase transition to Pbnm at ∼5 GPa.

Polyvinylpyrrolidone as a Stabilizer in Synthesis of AgInS2 Quantum Dots.

A synthesis protocol of polyvinylpyrrolidone-capped AgInS2 quantum dots in aqueous solution is reported. Nanoparticle morphology and chemical composition were studied by means of TEM, XRD, XPS, and FTIR. The obtained quantum dots were luminescent in the visible range. The photoluminescence intensity dependence on the polyvinylpyrrolidone amount was demonstrated. The wavelength of the emission maximum varied with changing the [Ag]:[In] molar ratio. The temperature dependence of the photoluminescence intensity of the polyvinylpyrrolidone-capped AgInS2 quantum dots was investigated within the temperature range of 11–294 K.

The Effect of Annealing Technique on ZnO Film Properties

ZnO films deposited on glass substrates by ultrasonically chemical spray pyrolysis technique have been investigated to establish the effect of the annealing technique on film properties. For this purpose, films have been exposed to standard thermal annealing and microwave annealing in an air atmosphere. It has been determined that the structural, morphological, compositional, and optical properties of the ZnO films correlate with each other. X-ray diffraction analyses have been revealed the highest crystallization level and changing preferred orientations for microwave annealed films. It has been identified that the optical band gap values of the films decreased from 3.27 eV to 3.23 eV and 3.21 eV after the standard and microwave thermal annealing, respectively. Scanning electron microscope images have been revealed homogeneous morphology at plan-view images of all films. Also, it has been determined that the root-like morphology from the higher magnification scanning electron microscopy images. Thicknesses of the films have been also determined from cross-sectional scanning electron microscope images as 1.04m, 0.92m, and 0.92m for ZnO, standard thermal annealed and microwave annealed ZnO films, respectively. Also, the O/Zn ratio revealed improved stoichiometry for annealed films according to as-deposited film for investigated regions. The dependence of photoluminescence intensity on annealing technique has also been investigated in the current study.

Carrier localization effect in the photoluminescence of In composition engineered InAlAs random alloy

Detailed spectral structures in the low-temperature photoluminescence (3.5 K) of an InAlAs random alloy grown by molecular beam epitaxy on an InP substrate were studied, with three clearly distinguished emission peaks. Based on the temperature-dependent photoluminescence measurements, the two peaks at 1.5404 and 1.5229 eV are localized state emissions, and the one at 1.4916 eV is likely caused by longitudinal optical (LO) phonon replica. The peak assignments are also supported by excitation intensity-dependent photoluminescence spectrum and Raman spectrum measurements. The peak at 1.5229 eV shows a strongly localized behavior evidenced by the typical inverted S-shaped variation of the peak position with respect to increased temperatures which can be well described by the localized-state ensemble (LSE) model. A localization energy of around 19.8 meV is determined for the 1.5229 eV emission at 3.5 K. The temporal behaviors of these emissions at 3.5 K were also discussed based on the time-resolved photoluminescence spectra. The findings here unveil the characteristics of the localized states in the InAlAs alloy and may be useful for developing optoelectronic devices in the near IR region.

Revealing the doping density in perovskite solar cells and its impact on device performance

Traditional inorganic semiconductors can be electronically doped with high precision. Conversely, there is still conjecture regarding the assessment of the electronic doping density in metal-halide perovskites, not to mention of a control thereof. This paper presents a multifaceted approach to determine the electronic doping density for a range of different lead-halide perovskite systems. Optical and electrical characterization techniques, comprising intensity-dependent and transient photoluminescence, AC Hall effect, transfer-length-methods, and charge extraction measurements were instrumental in quantifying an upper limit for the doping density. The obtained values are subsequently compared to the electrode charge per cell volume under short-circuit conditions (CUbi/eV), which amounts to roughly 1016 cm−3. This figure of merit represents the critical limit below which doping-induced charges do not influence the device performance. The experimental results consistently demonstrate that the doping density is below this critical threshold (∼1012 cm−3, which means ≪ CUbi/eV) for all common lead-based metal-halide perovskites. Nevertheless, although the density of doping-induced charges is too low to redistribute the built-in voltage in the perovskite active layer, mobile ions are present in sufficient quantities to create space-charge-regions in the active layer, reminiscent of doped pn-junctions. These results are well supported by drift–diffusion simulations, which confirm that the device performance is not affected by such low doping densities.

A Microfluid Fiber Device for Trace Detection of Aggregation Induced Emission Molecules

An ultra-sensitive microfluid device for detecting aggregation of induced emission molecules is proposed and demonstrated. The minimum detectable concentration is greatly reduced by use of a micromachined fiber tip and a liquid core waveguide. The fiber tip is a tapered-microlens structure with optimized shape parameters. The liquid waveguide is formed with low refractive index Teflon capillary. The dependence of photoluminescence intensity on the concentration change is tested with the device. The limit of detection is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.316 \boldsymbol {\mu }\text{M}$ </tex-math></inline-formula> , the volume of the sample solution used is only <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.649 \boldsymbol {\mu }\text{L}$ </tex-math></inline-formula> and the minimum detectable concentration is down to 100 nM. It is a valuable result obtained with fiber spectrometer (a CCD-type machine), better than that obtained with traditional fluorospectro photometer (a photomultiplier tubes-type one). The reversibility and stability are also testified through experimental method. Together with the compact size, small sample volume and high sensitivity, the durable microfluidic sensing device has great potential for biosensing applications.

Plasmon enhanced optical properties of Ag nanowire decorated conjugated polymer

The effect of Ag nanowires (NWs) on the optical properties of optoelectronic polymer poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) have been systemically investigated. The decoration of Ag NWs with an average diameter of 160 nm leads to improved absorption and photoluminescence (PL) ability due to the coexistence of localized surface plasmons and propagating surface plasmons. Results of steady-state spectra exhibit 180% enhancement of absorption and 190% enhancement at ∼630 nm of PL. Temperature and excitation intensity dependent PL measurements demonstrate that Ag NWs can effectively suppress non-radiative recombination and enhance radiative recombination. Time-resolved PL measurement reveals the interaction between plasmons and interchain species plays a dominant role in the enhanced PL. These results will provide an effective way for further improving the optical properties of optoelectronic devices based on conjugated polymers.

Photoluminescence spectroscopy of Mn2+ in zero-thermal expansion Zn4B6O13 green-emitting phosphor

Investigating temperature dependence of phosphor properties is not only of scientific interest, but also technological importance. Here, we investigate the temperature dependence of photoluminescence intensity, IPL, for the Zn4B6O13:Mn2+ green-emitting phosphor. Interestingly, Zn4B6O13 crystal has been reported to exhibit zero-thermal expansion (zero-TE) behavior along with its unique phosphor properties, called zero-thermal quenching behavior of IPL, up to T = 523 K. It has been shown that such unique temperature dependence of IPL is not related to the zero-TE behavior of Zn4B6O13, but is to be due to an increased phonon number that leads to an increased parity-forbidden 4T1 → 6A1 transition probability with increasing T. No or a very little change in the excited-state zero-phonon line transition energy, E(4T1)ZPL, of the Zn4B6O13:Mn2+ phosphor from T = 20–523 K can be well understood by considering the Tanabe−Sugano energy-level diagram with taking into account the zero-TE behavior, i.e., no large change of the metal−ligand distance RMn−O (or, equivalently, no large change of the crystal-field splitting energy Dq) against T in such zero-TE phosphor of Zn4B6O13:Mn2+.

Photoluminescence Spectroscopy of the InAsSb-Based p-i-n Heterostructure.

Photoluminescence in a double heterostructure based on a ternary InAsSb solid solution was observed in the mid-infrared range of 2.5–4 μm. A range of compositions of the InAs1−ySby ternary solid solution has been established, where the energy resonance between the band gap and the splitting-off band in the valence band of the semiconductor can be achieved. Due to the impact of nonradiative Auger recombination processes, different temperature dependence of photoluminescence intensity was found for the barrier layer and the narrow-gap active region, respectively. It was shown that efficient high-temperature photoluminescence can be achieved by suppressing the nonradiative Auger recombination (CHHS) process. Increased temperature, for which the energy gap is lower than the split-off band energy, leads to violation of the resonance condition in narrow gap antimonide compounds, which explains the observed phenomenon. This finding might influence future application of the investigated material systems in mid-infrared emitters used for, e.g., optical gas sensing.

Structural, electrical, and luminescence properties of (0001) ZnO epitaxial layers grown on c-GaN/sapphire templates by pulsed laser deposition technique

A systematic study of growth, structural, electrical, and luminescence properties of zinc oxide (ZnO) layers grown on c-oriented GaN/sapphire templates by the pulsed laser deposition technique is carried out. A thorough high-resolution x-ray diffraction study reveals that c-ZnO films with high crystalline quality can be grown under certain growth conditions. Screw and edge dislocation densities in these films are found to be as low as 7×108 and 3×1010cm−2, respectively. All layers are found to be unintentionally n-type with ∼1019cm−3 electron concentration and mobility as high as ∼50 cm2 V−1 s−1. Temperature and excitation intensity dependent photoluminescence (PL) studies as functions of the growth conditions are carried out to identify the transition processes behind various luminescence features found in these samples. At low temperatures, PL spectra are marked by sharp neutral donor bound excitonic transitions, their phonon replicas, and two broad luminescence bands at 2.2 and 2.9 eV. These broad bands are attributed to transitions from the (2+/0) oxygen vacancy (VO) and (2+/+/0) zinc-interstitial (Zni) levels, respectively, to the valence band. Thermal energy needed to depopulate these defects is found to be 11 and 385 meV, respectively, for the (2+/0) VO and (2+/+/0) Zni levels. Low temperature PL spectra for the samples grown with relatively high oxygen pressures are featured by the Zn-vacancy (VZn) related neutral acceptor bound excitonic transition, its phonon replicas, and a broad band at 2.75 eV. This band diminishes with increasing temperature and, instead, another broad feature appears at ∼2.1 eV. Our study attributes the 2.75 eV band to transition from the conduction band to (0/−) VZn levels and the 2.1 eV feature to the transition between (−/2−) VZn levels and the valence band. It has been found that all the defect related features can be minimized by adjusting the growth conditions.

Determination of the types of optical transitions and concentrations of donors and acceptors in GaN by the dependence of photoluminescence intensity on the excitation power

The results of the calculated and experimental depen-dence of the photoluminescence intensity on the excitation power density for silicon-doped GaN layers grown by molecular beam epitaxy are presented. A model was constructed for transitions in a compensated semiconductor upon interband generation of electron-hole pairs. It is shown that the dependence of the photoluminescence intensity on the excitation power density can be used to determine the recombination mechanism and concentrations of donors and acceptors in semiconductor. Keywords: GaN, ammonia-MBE, photoluminescence, heterostructures, point defects.

Improved two-step photon absorption current by Cl-doping in ZnTeO-based intermediate band solar cells with n-ZnS layer

We report the improved photocurrent induced by two-step photon absorption (TSPA) in the intermediate band solar cells (IBSCs) with a n-ZnS/ZnTe/ZnTeO/ZnTe/p-ZnTe structure using Cl-doped ZnTeO. In the IBSC with an undoped ZnTeO, the TSPA current is not observed at room temperature but it is significantly improved by using Cl-doped ZnTeO. Temperature and excitation intensity dependent photoluminescence (PL) spectra reveal the high activation energy of Cl-donor, and the observed activation temperature by PL measurements is consistent with the temperature where the TSPA current starts to increase. The temperature dependence of the open circuit voltage of the IBSC using Cl-doped ZnTeO also supports that the Cl doping in the ZnTeO layer introduces electrons into the intermediate band.

Simultaneous measurement of photocurrent and recombination emission in organic solar cell

Charge transfer (CT) state is a key intermediate to understand recombination processes in organic solar cells (OSCs). In this study, we measured the recombination emission from the CT state under different applied voltages in OSCs and a photocurrent density flowing on the circuit simultaneously. We proposed a “photoluminescence (PL)–voltage (V) plot” that is the voltage dependence of PL intensity of the CT state. The PL–V plot includes information only from the CT state recombination at the donor/acceptor interface and is complementary to the “current density (J)–V plot” that is the most important information for evaluating OSCs. The results demonstrated that the fill factor (FF) of the PL–V plot is higher than that of the J–V plot, predicting the ideal FF of the device. Our result demonstrated that the simultaneous measurement of photocurrent and recombination emission could be a strong tool for evaluating photoconversion characteristics in OSCs.