PL Decay Curves Research Articles

Photoluminescence properties of Mn2+-activated red-emitting phosphors with superior thermal stability for backlighting display applications

The red-emitting phosphors, Na2Mg1-x(PO3)4:xMn2+ with concentrations ranging from 0.01 to 0.11, have been successfully synthesized using a simple solid-state method. The as-synthesized phosphors are crystallized in an orthorhombic structure. The luminescence mechanism of Mn2+ ions in Na2Mg(PO3)4 lattice has been deeply explained using the Tanabe-Sugano (T-S) energy level diagram for the d5 electron configuration. The typical photoluminescent emission (PL) and photoluminescent excitation (PLE) spectra were measured. Several excitation peaks in the spectral range of 250–550 nm on the PLE spectrum and a strong emission peak at 613 nm indicate that the as-synthesized phosphors can be effectively excited using ultraviolet (UV) and near-UV LED chips. Several photometric characteristics were measured, such as the Commission International de'LEclarirage (CIE) chromaticity coordinate (x, y), color purity (CP), and correlated color temperature (CCT), corresponding to the concentration-dependent PL spectra. Furthermore, the thermal stability of the Na2Mg1-x(PO3)4:xMn2+ phosphors was investigated through temperature-dependent PL spectra and decay curves from 10 to 500 K. All results confirm that the as-synthesized red-emitting phosphors exhibit excellent thermal stability, which are attractive characteristics for white light emitting diodes (W-LEDs) applications.

Cool green-emissive Y2Si2O7:Tb3+ nanophosphor: auto-combustion synthesis and structural and photoluminescence characteristics with good thermal stability for lighting applications.

A cheap, versatile, sustainable and energy-efficient gel-combustion method was applied to develop a series of green-emitting down-converted Y2Si2O7:Tb3+ (YPS:Tb3+) nanophosphors. Employing XRD-based Rietveld refinement approach, the phase purity and crystallographic evaluation of the produced phosphor were conducted, revealing a triclinic crystal with P1̄ space group. EDX and TEM analyses were performed on the synthesized samples to determine their elemental composition and morphological properties. Diffuse reflectance spectra yielded 5.61 eV and 5.79 eV optical energy band gaps for the host and the optimized (0.04 mole of Tb3+) sample, respectively. UV light has the ability to excite the nanocrystalline phosphor in an efficient manner, leading to significant luminosity qualities attributed to the radiative relaxation of 5D4 → 7FJ (J = 6, 5, 4, 3). The bi-exponential decay function was derived by the PL decay curves. With an activation energy of 0.2206 eV, the Y1.96Si2O7:0.04Tb3+ phosphor exhibits good thermal quenching capabilities. Improved photometric attributes including CIE coordinates, CCT and color purity confirmed the green glow, indicating a strong competitor for cool-green emission in lighting applications.

A study on the photophysical properties of strong green-fluorescent N-doped carbon dots and application for pH sensing

Color tuneable effective pH sensor in both acidic and alkaline medium is important for environment and bioanalytical purpose. In this report, natural biomass derived N-doped fluorescent carbon dots are utilized as pH sensor. The fast microwave irradiation method was employed for synthesis using natural sweet lime juice extract. Ethylenediamine (EDA) was used for nitrogen doping with different amounts. The prepared N-doped carbon dots are comprehensively studied by various characterization methods to analyse the effect of dopant concentrations on photophysical and luminescence properties. Improved thermal stability from 900 °C to 1000 °C with EDA content is observed. The fluorescence spectra of prepared N-doped carbon dots exhibited stable green emission with high quantum yield 83 % under ultraviolet and 440 nm irradiation wavelengths for an optimal concentration of EDA. The color of emission confirmed by CIE plot and about 3.47 ns decay time is found by PL decay curve. The anti-photo bleaching ability and good photostability for 10 days are found. The prepared N-doped carbon dots exhibit high ionic stability and bio compatibility. Further, pH dependent color tuneable (green to yellow) emission spectra of prepared N-doped carbon dots displayed high fluorescence in acidic medium at pH -5. Although, emission was quenched and red shifted in alkaline medium. The effective pH-sensitivity, and good reversibility of fluorescent N-doped carbon dots samples were tested with real water samples having different pH values. The reasonable mechanism is discussed. It depicts the prepared N-doped carbon dots have potential to serve as pH sensor for physiochemical and bioanalytical measurements.

Photo and X-ray luminescence characteristics of CeF3 -doped SiO2 + B2O3 +AlF3 + NaF + CaF2 scintillating glasses

Fluoroborosilicate (SBFanc:SiO2 + B2O3 +AlF3 + NaF + CaF2) glasses with different CeF3 concentrations were fabricated by melt quenching technique to examine the scintillation properties of these glasses. The density and refractive index increase with an increase in CeF3 concentration. The structural properties of glasses were studied in the range of 800–4000 cm−1 using FT-IR transmittance spectroscopy. Luminescence characteristics of Ce3+:SBFanc glasses with an intensity peak around ∼345 nm (5d→4f) were studied using photo (λexc = 303 nm) and X-ray excited luminescence techniques. The energy transfer (ET) between Ce3+ -Ce3+ ion pairs taking place via dipole-dipole type interaction for the concentration quenching of Ce3+ emission on the basis of Dexter and Schulman theory. The PL decay curves can be fitted by a double exponential function with the decay times 64.28–99.87 ns and 15.78–21.29 ns for slow and fast decay components, respectively. The highest integrated light emission intensity of Ce3+:SBFanc glasses stimulated by X-ray is 35% of commercial Bi4Ge3O12 crystal and can be used for the scintillator materials.

Phosphonium-Based Ionic Thermally Activated Delayed Fluorescence Emitters for High-Performance Partially Solution-Processed Organic Light-Emitting Diodes

Phosphonium-Based Ionic Thermally Activated Delayed Fluorescence Emitters for High-Performance Partially Solution-Processed Organic Light-Emitting Diodes

Synthesis of the Red-Emitting (Ba, Ca)2ScAlO5:Eu3+ Phosphors with Photoluminescence Properties.

The light-emitting diodes (LED) are regarded as one of the most promising devices for inexpensive and widely used illumination; in particular, they are highly dependent on the development of red-emitting phosphors. Herein, we developed two types of red-emitting (Ba, Ca)2ScAlO5:Eu3+ multiple excitations phosphors (λex = 255-465 nm) via freeze-drying followed by calcination. Powder X-ray diffraction and NMR results point out that they have hexagonal space group P63/mmc (194), and the structural framework is composed of multi-coordinated Al3+-O2- polyhedron and Sc3+-O2- polyhedron. In addition, the valence state of europium (Eu3+) is confirmed by X-ray photoelectron spectroscopy characterization. Investigation on the photoluminescence properties showed that the photoluminescence process of (Ba, Ca)2ScAlO5:Eu3+ is attributable to the charge transfer band of Eu-O and abundant spectral terms of Eu3+. The α-(Ba, Ca)2ScAlO5:Eu3+ and β-(Ba, Ca)2ScAlO5:Eu3+ exhibited red emission under 465 and 395 nm excitation, respectively. The PL spectra and decay curves explain the intrinsic photoluminescence mechanism. The strongest emission peaks of the red-emitting α-(Ba, Ca)2ScAlO5:Eu3+ and β-(Ba, Ca)2ScAlO5:Eu3+ phosphors are at 615 and 619 nm, respectively, exhibiting a high fluorescence of 64 and 67% under the temperature of 423 K (150 °C). Further exploration of the red-emitting phosphors would provide a variety of choices for the design of red LEDs and white LEDs for the solid-state lighting system.

Charge separation and transfer activated by covalent bond in UiO-66-NH2/RGO heterostructure for CO2 photoreduction

Controlling charge dynamics in mixed photocatalytic materials by interface design is of great significance for achieving efficient photocatalytic reduction of CO2. Here, the design of covalent bond between UiO-66-NH2 and RGO successfully realized the efficient reduction of CO2. The presence of covalent bonds makes the CO yield up to 23.54 μmol·g−1 (UiO-66-NH2/RGO-3), which is significantly higher than that of photocatalysts without covalent bonds (14.22 μmol·g−1 for UiO-66/RGO-3). XPS and FTIR characterization proved that there is a covalent bond between UiO-66-NH2 and partially reduced graphene oxide. On the one hand, the UV–vis results show that UiO-66-NH2/RGO-3 is beneficial to generate more photogenerated electrons compared with UiO-66/RGO-3. On the other hand, the covalent bond strengthens the space charge separation state through the action of the internal electric field and the potential difference, and the photogenerated electrons can be quickly transferred to the RGO along the covalent bond. The combination of time-resolved PL decay curve and photoelectrochemical experiments clearly proves that covalent bonds can reduce the interface carrier transfer resistance and accelerate the charge transfer process. It is important that CO2-TPD and in-situ infrared indicate that the covalent bond has a stronger ability to adsorb and activate CO2, so that the photogenerated electrons quickly transferred to RGO can effectively participate in the reduction of CO2, thus speeding up the rate of CO2 reduction to CO. Therefore, this work provides inspiration for improving charge dynamics through interface design to promote photoreduction of CO2.

Z-scheme induced g-C3N4 /WS2 heterojunction photocatalyst with improved electron mobility for enhanced solar photocatalysis

This study reports the hybridization of g-C3N4/WS2 prepared through one-pot hydrothermal synthesis in the absence of a reducing agent. Heterojunction samples of varied WS2 compositions and fixed g-C3N4 concentrations (CNW-0.5, CNW-1.0 and CNW-1.5) were synthesized. All samples were physiochemically and electrochemically examined to understand their materials’ chemistry and performance. The particle size of the composite varied from 2.55 µm to 2.58 µm with a high crystalline structure. The Nyquist Plot demonstrated an improved charge mobility, and the transient photo current showed a prolonged lifetime of the charge carriers for the CNW-1.0 sample. The Mott Schottky plot proved the p-n junction formation of the composite with a potential energy barrier at the junction. The time-resolved transient PL decay curves demonstrated an average lifetime (τavg) of 8.51 μs for CNW-1.0, which was 2.6 times higher than that of WS2 alone. The highest solar photocatalytic degradation of 95.5% for Methylene Blue and 84.5% for Tetracycline were achieved with CNW-1.0 in 2 h, where (̇O2–) and (OḢ) radicals were dominant in the reaction. The obtained band positions suggested the Z-Scheme mechanism for the charge mobility across the heterojunction. The developed p-n heterojunction photocatalyst proved its ability in utilizing the solar energy for a robust photocatalysis reaction.

Tunable emission, energy transfer and thermal stability of Ce3+, Tb3+ co-doped Na2BaCa(PO4)2 phosphors

A series of single Ce3+ doped and Ce3+ and Tb3+ co-doped Na2BaCa(PO4)2 (NBCP) phosphors was synthesized by conventional solid-stated reaction method. The crystal structure, luminescence properties, thermal stability and energy transfer were carefully investigated. Ce3+ is inferred to substitute the Ba2+ site in NBCP lattice. The color-tunable emission from blue to green is observed by adjusting Tb3+ concentration among NBCP:0.03Ce3+,yTb3+ phosphors. The energy transfer behavior from Ce3+ to Tb3+ ions is both illustrated by co-doped PL spectra and decay curves. The energy transfer efficiency is as high as 91.5%. The mechanism of energy transfer is resonance type of dipole-dipole transition. In this work, the optimal phosphor exhibits the excellent thermal stability which keeps at 94.9% of that initial value at room temperature when temperature reaches to 150 °C. The Ce3+ and Tb3+ co-doped NBCP phosphor is a promising candidate for the application in the general lighting and display fields.

The effect of the electron-donor ability on the OLED efficiency of twisted donor-acceptor type emitters

Two twisted donor-acceptor (D-A) chemical structures, CCDMB and PCDMB, were developed as a new class of thermally activated delayed fluorescence (TADF) emitters for organic light-emitting diodes (OLEDs). Two emitters consist of 3-substituted carbazole as a first donor and trivalent boron as an electron acceptor in common, and carbazole and phenoxazine as second donors with different electron donor ability. While PCDMB with a strong phenoxazine donor decreased the lowest singlet excited state (S 1 ) level and thus showed a small singlet-triplet energy difference (ΔE ST ) value of 0.13 eV, resulting in effective reverse intersystem crossing (RISC), however, CCDMB with a weak donor showed a large ΔE ST value of 0.21 eV. Efficient triplet harvesting of PCDMB was confirmed by a delayed component in transient PL decay curves of 25 wt% PCDMB-doped bis[2-(diphenylphosphino)phenyl] ether oxide (DPEPO) films. OLED devices with a CCDMB emitter showed deep-blue emission with Commission Internationale de l’Éclairage (CIE) of (0.16, 0.12) but a low maximum EQE of 5.5%, indicative of insufficient triplet harvesting. PCDMB-based devices showed green emission with CIE of (0.21, 0.45) and a high maximum EQE of 22.3%. Our study revealed the effect of the electron donor ability of structurally similar emitters on ΔE ST values, triplet harvesting, and device efficiency. • We synthesized two thermally activated delayed fluorescence (TADF) emitters with difference only in electron-donor ability. • PCDMB has a smaller ΔE ST value than CCDMB owing to the stronger electron donating ability of phenoxazine than carbazole. • PCDMB -based devices showed a higher EQE compared to CCDMB -based devices. • This study revealed the effect of the electron donor ability of emitters on ΔE ST , triplet harvesting, and device efficiency.

Photoluminescence and energy transfer of efficient and thermally stable white-emitting Ca9La(PO4)7:Ce3+, Tb3+, Mn2+ phosphors

The development of novel single-component white-emitting phosphors with high thermal stability is essential for improving the illumination quality of white light-emitting diodes. In this work, we synthesized a series of Ce3+, Tb3+, Mn2+ single- and multiple-doped Ca9La(PO4)7 (CLPO) phosphors with β-Ca3(PO4)2-type structure by the simple high-temperature solid-state reaction. The crystallization behavior, crystal structure, surface morphology, photoluminescence performance, decay lifetime and thermal stability were systematically investigated. The PL spectra and decay curves have evidenced the efficient energy transfer from Ce3+ to Tb3+ and from Ce3+ to Mn2+ in the CLPO host, and corresponding energy transfer efficiency reaches 41.8% and 54.1%, respectively. The energy transfer process of Ce3+→Tb3+ and Ce3+→Mn2+ can be deduced to the resonant type via dipole-dipole and dipole-quadrupole interaction mechanism, and corresponding critical distance were determined to be 12.23 and 14.4 Å, respectively. Based on the efficient energy transfer, the white light emission can be successfully achieved in the single-component CLPO:0.15Ce3+, 0.10Tb3+, 0.04Mn2+ phosphor, which owns CIE chromaticity coordinates of (0.3245, 0.3347), CCT of 5878 K, internal and external quantum efficiency of 84.51% and 69.32%. Especially, compared with the emission intensity at 25 °C, it still remains 98.5% at 150 °C and 92.0% at 300 °C. Based on these results, the single-component white light emission phosphor CLPO:0.15Ce3+, 0.10Tb3+, 0.04Mn2+ is a potential candidate for UV-converted white LEDs.

Fabrication and opto-electronic analysis of down-converting blue-green emitting nanophosphor

A series of nano-scaled Tb3+-doped Ba3Y(PO4)3 (BYPO) nanophosphors were successfully fabricated utilizing a time-saving solution-combustion approach. The crystal structure and lattice parameters of the Ba3Y0.93Tb0.07(PO4)3 nanophosphor was examined by Reitveld refinement analysis. The incorporation of trivalent terbium ions into the cubic crystal lattice (I − 4 3 d (220) space group) did not bring any change in the crystal prototype. The photoluminescence (PL) result showed that the blue-to-green tunable emission can be achieved simply via varying the dopant concentration, with 7 mol% as the optimum concentration for standard CIE (Commission Internationale de I’Eclairage) coordinates of green emission. A comprehensive examination of PL-decay curves using Auzel’s model gives the intrinsic radiative lifetime (3.9585 ms) for the 5D4 state in Tb3+. The CIE coordinates values illustrate the blue region at lower concentrations and the green region at higher concentrations of the terbium ions, signifying the color-tunability in the system. The outcomes specify that this nanophosphor is a relevant candidate for phosphor converted white LEDs (PC-WLEDs) using near ultraviolet (UV) excitation.

Pure-Blue Fluorescence Molecule for Nondoped Electroluminescence with External Quantum Efficiency Approaching 13%

A pure-blue light-emitting material is one of the key components in the preparation of organic light-emitting diode (OLED) displays. Although high-efficiency blue OLEDs have been realized in therma...

Effect of samarium ions concentration on physical, optical and photoluminescence properties of Oxy-Fluoro Boro Tellurite glasses

A new series of samarium ions doped oxy-fluoro boro tellurite (OFBT) glasses have been prepared by using conventional melt quenching method. Details of functional groups and OH content present in the as prepared un-doped OFBT glass has been thoroughly investigated by using FT-IR spectrum. Thermal analysis of the glasses has been studied using TG-DSC to understand the thermal stability of the as prepared un-doped glass. Several physical properties, optical direct & indirect band gaps and urbach energies were measured to understand the glass structure using absorption spectral data. From the absorption spectra, the Judd-Ofelt (J-O) intensity parameters are evaluated to know the ligand field environment around the rare earth ion. Under 402 nm excitation, the photoluminescence spectra recorded for the as prepared glasses show intensified peak at 602 nm. By coupling the J-O parameters with PL spectral data, various radiative parameters were evaluated to optimize the best concentration suitable for laser applications. The experimental lifetimes estimated from the PL decay spectral features were coupled with radiative lifetimes to estimate quantum efficiency of the as prepared glasses. The PL decay curves were well fitted to single exponential at lower concentration and become non-exponential gradually as the concentration increases. The Inokutti-Hirayama (I–H) model applied to the PL decay spectral features reveals the underlying energy transfer mechanism responsible for the conversion of single exponential to non-exponential as dipole-dipole in nature. Reasonably higher values of stimulated emission cross-sections (σse) and quantum efficiencies (η) indicate the suitability of OFBTSm10 glass for lasers applications in orange region.

The Cavity-Effect in Site-Controlled GaN Nanocolumns with InGaN Insertions

We have studied optical properties of site-controlled GaN nanocolumns (NCs) with an insertion of InGaN QW grown on the micro-cone patterned substrate. Time-resolved photoluminescence spectroscopy reveals two bands emission. Comparison of these spectra with the emission of a separate nanocolumn, recorded by μ-CL with high spatial resolution, allows us to ascribe the higher energy band to emission from the InGaN QW situated on the top of NCs, while the lower-energy band originates from the QW in the rest area. By the modeling of the decay times and amplitudes of PL decay curves, we have exhibited a significant enhancement of signal from the QW inside the NCs. Investigation of the NCs as a Fabry-Perot cavity at eigenfrequencies around the optical transitions has revealed that the cavity modes can provide significant enhancement of the emission from the inserted QW owing to the Purcell factor.

High Thermal Stability Apatite Phosphors\xa0Ca2La8(SiO4)6O2:Dy3+/Sm3+ for White Light Emission: Synthesis, Structure, Luminescence Properties and Energy Transfer

What ideal w-LED phosphors always aim to do is to achieve a single phase near-sunlight emission phosphor simultaneously with both high luminescence efficiency and high thermal stability at operation temperature. And It is well known that apatite compound phosphors are one of the most promising optical materials to realize those above because of their unique structure enhanced luminescence properties and thermal stability. Here, we synthesized a co-doped single phase apatite phosphors Ca2La8(SiO4)6O2:Dy3+/Sm3+ (CLSO:Dy3+/Sm3+) for white light emission, which was provided with excellent thermal stability and of which luminescence intensity at 150 °C still was 92 percentage of that at room temperature. Moreover, X-ray diffraction technique, Fourier transform infrared spectroscopy, scanning electron microscope were employed to characterization of phase structure and morphology, and consequently pure apatite structure and gravel-like morphology of phosphors were proved. Analysis of photoluminescence spectra indicated that concentration quenching effect exist in single-doped CLSO:Dy3+ phosphors owing to dipole-dipole interaction between Dy3+ ions. It is revealed that maybe exist Dy3+ ↔ Sm3+ bilateral non-radiative energy transfer processes in Dy3+/Sm3+ co-doped CLSO system by PL spectra and decay curves. And variation of Sm3+ ion concentration can control color emission, namely CIE chromaticity coordinates and correlated color temperature, finally to achieve white light emission (0.309,0.309) with CCT 6848 K, able to be a potential candidate for commercial lighting applications.

Tunable emission with efficient energy transfer in Na2SrSi2O6:Ce3+, Tb3+ phosphor for near-UV LED

Two series of Na2SrSi2O6:Ce3+ and Na2SrSi2O6:Ce3+, Tb3+ phosphors were synthesized via high-temperature solid-state reaction. The obtained Na2SrSi2O6:Ce3+ phosphors exhibit an intense excitation peak at 284 and 360 nm, which was suited to the dominant emission band of a NUV light-emitting-diode (LED) chip. In the Ce3+ single doped system, the critical quench concentration of the phosphor was approximately 4 mol% and a dipole-dipole interaction was verified for the concentration quench mechanism. In addition, Tb3+ ions were introduced as a sensitizer, and the effects of introducing the Tb3+ ions are investigated by measuring the photoluminescence emission and excitation spectra, the PL decay curves, and the quantum efficiencies. Results confirm the occurrence of non-radiative energy transfer from Ce3+ to Tb3+ with an efficiency of over 60%, indicating that the developed phosphor can be used as a potential white-light-emitting phosphor for near-UV LED.

Near UV excitable warm white light emitting Zn doped γ-Ga2O3 nanoparticles for phosphor-converted white light emitting diode

Optical properties of Zn doped γ-Ga2O3 nanoparticles are investigated for the generation of white light, using near UV LED chip for phosphor-converted WLED (pc-WLED) application. Sol-gel combustion technique is adopted for the synthesis of Zn doped γ-Ga2O3 nanoparticles. The synthesized Ga2O3 nanoparticles are formed as metastable γ-phase with the face-centred cubic crystal structure. The structural stability is maintained up to 15 mol% doping of Zn ions. Formation of aggregated ultrafine particles of undoped and Zn doped γ-Ga2O3 with size less than 4 nm is found from TEM results. Doping of Zn ions has altered the optical band gap of γ-Ga2O3, and it varies from 4.13 to 3.97 eV. Broad visible emission in the wavelength range of 400–600 nm is observed and broadening of emission band increases after Zn doping. Formation of high defect concentration and smaller size of 10 mol% Zn doped γ-Ga2O3 nanoparticles resulted in higher emission intensity. Wide photoluminescence excitation band from 330 to 475 nm suggests that wide visible emission can be obtained from Zn doped γ-Ga2O3 nanoparticles with the excitation of both near UV and blue light. The bi-exponential behavior of PL decay curves of emission bands at the blue and yellow region indicated the complicate luminescence process of Zn doped γ-Ga2O3. The estimated lifetime of blue emission varies from 30 to 37 ns and of yellow emission varies from 54 to 62 ns. Under the excitation of 375 nm LED chip, warm white light emission with CIE color coordinates of (0.42, 0.33), color rendering index of ~ 87 and CCT of 2365 K is obtained for 10 mol% Zn doped γ-Ga2O3 nanoparticles which exemplifying its potential application in pc-WLED fabrication.

A facile route for highly efficient color-tunable Cu-Ga-Se/ZnSe quantum dots

I-III-VI group “green” quantum dots (QDs) are receiving increasing attention in photoelectronic conversion applications. In this work, a facile one-pot route directly dissolving copper iodide, gallium acetylacetonate and selenium powder as precursors into a mixed solution of dodecanethiol, oleylamine (OLA) and octadecene was developed to synthesize Cu-Ga-Se/ZnSe (CGSe/ZnSe) core/shell QDs. The resulting CGSe/ZnSe QDs with a relatively high photoluminescence quantum yield (PL QY) of 77.73% can be obtained and the particle size control from 4.20 to 5.73 nm can be realized via the variation of OLA dosage. The tunable emission from 485 to 630 nm can be acquired by introducing sulfur ion into CGSe host materials (anionic alloying) and varying the Cu/Ga precursor molar ratio. Furthermore, the PL recombination process in CGSe/ZnSe QDs is discussed using the PL decay curve. The proposed one-pot synthesis route for highly efficient color-tunable CGSe/ZnSe QDs is facile, rapid and environmentally-friendly. The as-prepared QDs are promising candidates for the exploration of novel and efficient light-emitting diodes.

Depth Profiling of Carrier Lifetime in Thick 4H-SiC Epilayers Using Two-Photon Absorption

Depth profiling of the ambipolar carrier lifetime was performed in n-type, 140mm thick silicon carbide (SiC) epilayer using excitation by two-photon absorption (TPA) with a pulsed 586nm laser, and confocal measurement of time resolved photoluminescence (TRPL) decay from the excited region. A depth resolution of ≈10mm was obtained. The PL decay curves were analyzed using a recently developed formalism that takes into account the TPA excitation, carrier diffusion and surface/interface recombination. The carrier lifetime decreases near the top surface of the epitaxial layer as well as near its interface with the substrate.