Excitation Intensity Dependent Photoluminescence Research Articles

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.

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.

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.

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.

Photoluminescence study of solution-deposited Cu2BaSnS4 thin films

To experimentally identify the character of radiative transitions in trigonal Cu2BaSnS4, we conduct temperature and excitation intensity dependent photoluminescence (PL) measurements in the temperature range of 15–300 K. The low-temperature near band edge PL spectrum is interpreted as the free exciton at 2.11 eV and the bound exciton at 2.08 eV, coupled with associated phonon-assisted transitions. In the low energy region, we assign the dominant defect emission at 1.96 eV to donor–acceptor-pair recombination and the weak broad emission at 1.6 eV to the free-to-bound transition. The activation energies and temperature shift for the radiative transitions are determined and discussed. Above 90 K, the free exciton recombination becomes the dominant radiative transition, with its energy shift mainly governed by the contribution of optical phonons.

Latent Photoinduced Oxygen Doping Revealed from Emission Saturation of Aggregated Domains in Conjugated Polymer Nanofibers

AbstractPhotoinduced oxidation (doping) of conjugated polymers by complexation with oxygen can have a significant impact on electronic properties and performance in device environments. Nanofiber model forms of poly(3‐hexylthiophene) (P3HT) are investigated using single molecule spectroscopy that possess similar morphological qualities as their bulk thin film counterparts yet, heterogeneity is confined to the spatial dimensions of these particles. Specifically, P3HT nanofibers assembled in anisole solutions contain both aggregated and nonaggregated (amorphous) chains with distinct electronic properties. Excitation intensity dependent photoluminescence (PL) emission imaging is then used to expose differences in oxygen affinity and reactivity upon photoexcitation. Nanofiber regions with low PL yields tend to show faster PL intensity saturation that also degrade much faster following periods of high excitation intensity soaking. Conversely, other regions show gains in PL intensity and virtually no saturation. These PL “gainer” and “loser” behaviors are assigned as originating from amorphous and aggregated P3HT chains, respectively. The apparent propensity of aggregated chains to undergo latent oxygen doping indicates a greater affinity probably due to a larger extent of electronic delocalization in these structures. The results shed new light on degradation factors studied frequently at the bulk material level, which often lacks sufficient sensitivity to specific structural forms.

Photoluminescence properties of ZnO/ZnMgO multiple quantum wells under high excitation

Abstract ZnO/ZnMgO multiple quantum wells (MQWs) were grown on c-plane sapphire substrates by plasma-assisted molecular beam epitaxy. Optical properties of the MQWs under high excitation were investigated by temperature- and excitation intensity-dependent photoluminescence spectroscopy. For weak confinement, increasing the excitation intensity leads to escape of excitons from the MQWs and appearance of ZnMgO barrier emission. While for better confinement, the ZnMgO barrier emission is almost independent on temperature and excitation intensity. The internal quantum efficiency of the MQWs increases monotonously with excitation intensity, reaching a highest value of 47% at room temperature, which suggests high quality of the MQWs.

Origin and tunability of dual color emission in highly stable Mn doped CsPbCl3 nanocrystals grown by a solid-state process

Herein, we report on a facile, bulk scale room temperature solid-state synthesis of highly stable and luminescent Mn-doped CsPbCl3 nanocrystal (NCs) with high Mn substitution, and investigate the origin and tunability of its dual-color emission. The structural and compositional analyses confirm the successful doping of Mn (1–40%) ion in CsPbCl3 NCs. The Mn-doped CsPbCl3 NCs exhibit distinct excitonic photoluminescence (PL) emission at ~412 nm (Iex) and Mn related emission at ~590 nm (IMn) with high PL quantum yield (up to 35%). Our study reveals that the intensity ratio IMn/Iex could be widely tuned by a factor of 64 and 23 by changing the Mn concentration and the measurement temperature, respectively. The stronger Mn emission at higher temperature is attributed to the higher rate of transfer of photoexcited carriers from the excitonic state of CsPbCl3 host to Mn related state. Further, excitation intensity-dependent PL along with time-resolved PL studies revealed important insights into the origin and tunability of Mn related orange-red PL in the dual color emitting Mn-doped CsPbCl3 NCs. The PL line shape analysis revealed additional PL peak in the range ~634–666 nm, which grows stronger with higher doping concentration and it is attributed, for the first time, to structural defects in the NCs. Remarkably, the Mn-doped CsPbCl3 NCs synthesized by our method exhibit about 10 times higher storage stability than those reported earlier due to the high Mn doping efficiencies achieved due to the strain induced enhanced doping process. Finally, white light emitting diodes (LEDs) were fabricated by employing blue-emitting CsPbCl1.5Br1.5 NCs, green-emitting CsPbBr3 NCs and orange-red emitting 10% Mn-doped CsPbCl3 NCs all grown by the same method. Our work demonstrates a low-cost synthesis strategy for high Mn doping in CsPbCl3 NCs with superior ambient stability and highly tunable emission.

A PL and PLE Study of High Cu Content Cu2ZnSnSe4 Films on Mo/Glass and Solar Cells

Cu2ZnSnSe4 (CZTSe) is amongst leading candidates for the absorber layer in sustainable solar cells. We examine CZTSe thin films with [Cu]/[Zn + Sn] of 0.99 and [Zn]/[Sn] of 1.07, deposited on Mo/glass substrates, and solar cells fabricated from these films. The bandgap (Eg) of the as deposited films and solar cells was examined by photoluminescence excitation (PLE) whereas the temperature and excitation intensity dependence of photoluminescence (PL) spectra was used to examine the nature of radiative recombination. The 6 K PL spectra of CZTSe/Mo exhibit an intense broad and asymmetrical band P1 at 0.822 eV and a lower intensity band P2 at 0.93 eV. The shape of this band, high rates of blue shift with excitation intensity rise ( j-shift) j(P1) = 14 meV and j(P2) = 8 meV per decade, and red shifts of both bands with increasing temperature suggest that both bands are associated with valence band tails due to potential fluctuations caused by high populations of charged defects. The mean depth of such fluctuation γ of 24 meV was estimated from the low energy side of P1. Device processing increased Eg, blue shifted P1, decreased its width, j-shift and the mean depth of potential fluctuations. These can be due to the annealing and/or can partly be related to KCN etching and the chemical effect of Cd, from CdS replacing copper at the CdS–CZTSe interface layer. Processing induced a new broad band P3 at 1.3 eV (quenching with Ea = 200 meV). We attributed P3 to defects in the CdS layer.

Al doping in ZnO nanowires enhances ultraviolet emission and suppresses broad defect emission

Abstract The effect of Al doping on the nature of ultraviolet (UV) near band edge emission and broadband “green” visible emission from deep defects in ZnO nanowires is explored by temperature- and excitation intensity-dependent photoluminescence (PL) spectroscopy. Unlike comparably fabricated undoped ZnO nanowires, whose PL spectra is dominated by green emission from oxygen vacancies and whose UV emission broadens and redshifts with increasing excitation intensity, Al-doped ZnO nanowires grown by chemical vapor deposition are smaller and have PL spectra dominated by UV emission that neither broadens nor redshifts significantly with increasing excitation intensity. The excitation intensity-dependent manner in which Al doping enhances UV emission at the expense of green emission indicates that the doping process creates many new donor sites that prevent excitons localized there from activating these green emitting defects.

Interface modification in type-II ZnCdSe/Zn(Cd)Te QDs for high efficiency intermediate band solar cells

A new growth process for type-II ZnCdSe/ZnCdTe quantum dots (QDs) is developed to avoid formation of a deleterious strain-inducing ZnSe interfacial layer (IL) that forms during the migration enhanced epitaxy growth process used to form the QDs. This new growth sequence allows for improved control of the interfacial composition and simplifies the fabrication of the intermediate band solar cell device structure based on these QDs, since additional strain balancing schemes are no longer required to grow stress-free structures. In contrast to previous results, lattice-matched QD superlattices were obtained using near lattice-matched ZnCdSe barrier layers. X-ray diffraction and excitation intensity dependent photoluminescence studies were used to support such a conclusion. Our findings have applications for the growth of other heterointerfaces in which an undesirable IL may form due to lack of a common anion, and to desorption and incorporation of competing elements.

A PL and PLE study of high Cu content Cu-=SUB=-2-=/SUB=-ZnSnSe-=SUB=-4-=/SUB=- films on Mo/glass and solar cells -=SUP=-*-=/SUP=-

AbstractCu_2ZnSnSe_4 (CZTSe) is amongst leading candidates for the absorber layer in sustainable solar cells. We examine CZTSe thin films with [Cu]/[Zn + Sn] of 0.99 and [Zn]/[Sn] of 1.07, deposited on Mo/glass substrates, and solar cells fabricated from these films. The bandgap ( E _ g ) of the as deposited films and solar cells was examined by photoluminescence excitation (PLE) whereas the temperature and excitation intensity dependence of photoluminescence (PL) spectra was used to examine the nature of radiative recombination. The 6 K PL spectra of CZTSe/Mo exhibit an intense broad and asymmetrical band P1 at 0.822 eV and a lower intensity band P2 at 0.93 eV. The shape of this band, high rates of blue shift with excitation intensity rise ( j -shift) j (P1) = 14 meV and j (P2) = 8 meV per decade, and red shifts of both bands with increasing temperature suggest that both bands are associated with valence band tails due to potential fluctuations caused by high populations of charged defects. The mean depth of such fluctuation γ of 24 meV was estimated from the low energy side of P1. Device processing increased E _ g , blue shifted P1, decreased its width, j -shift and the mean depth of potential fluctuations. These can be due to the annealing and/or can partly be related to KCN etching and the chemical effect of Cd, from CdS replacing copper at the CdS–CZTSe interface layer. Processing induced a new broad band P3 at 1.3 eV (quenching with E _ a = 200 meV). We attributed P3 to defects in the CdS layer.

Tailoring of defect luminescence in CVD grown monolayer [formula omitted] film

Optical properties of strictly monolayer MoS2 films, which are grown on c-sapphire substrates using microcavity based CVD route, have been investigated by temperature dependent absorption and temperature as well as excitation intensity dependent photoluminescence (PL) spectroscopy. Our study reveals a high intensity broad luminescence band appearing at ∼1.7 eV along with the usual free exciton/trion peak (∼1.86 eV). The investigation furthermore attributes this broad transition to excitons bound to two types of defects, whose binding energies are found to be ∼11 and ∼118 meV. Integrated intensity of this feature decreases with the increase in the impinging sulfur flux during growth and by post-growth annealing in sulfur atmosphere suggesting that these defects must be related to sulfur deficiency. Interestingly, exciton-phonon coupling for these defect bound excitons is found to be much stronger than that is associated with free excitons/trions.

Changes of the absorption cross section of Si nanocrystals with temperature and distance.

The absorption cross section (ACS) of silicon nanocrystals (Si NCs) in single-layer and multilayer structures with variable thickness of oxide barriers is determined via a photoluminescence (PL) modulation technique that is based on the analysis of excitation intensity-dependent PL kinetics under modulated pumping. We clearly demonstrate that roughly doubling the barrier thickness (from ca. 1 to 2.2 nm) induces a decrease of the ACS by a factor of 1.5. An optimum separation barrier thickness of ca. 1.6 nm is calculated to maximize the PL intensity yield. This large variation of ACS values with barrier thickness is attributed to a modulation of either defect population states or of the efficiency of energy transfer between confined NC layers. An exponential decrease of the ACS with decreasing temperature down to 120 K can be explained by smaller occupation number of phonons and expansion of the band gap of Si NCs at low temperatures. This study clearly shows that the ACS of Si NCs cannot be considered as independent on experimental conditions and sample parameters.

Chemical composition dependence of photoluminescence from Cu2ZnSnS4 thin films with potential fluctuations

The temperature and excitation intensity dependence of photoluminescence (PL) spectra from Cu2ZnSnS4 (CZTS) thin films with various chemical compositions was investigated. Three types of CZTS thin films were examined: Zn‐rich (group A), Cu‐ and Zn‐poor (group B), and Cu‐rich and Zn‐poor (group C). Groups A, B, and C gave PL spectra with peaks at around 1.2, 1.30, and 1.45 eV, respectively. These spectra exhibited large peak energy shifts to higher energies as the excitation intensity increased by more than 10 meV/decade due to potential fluctuations. The PL spectra of groups A and B can be attributed to band‐to‐impurity luminescence, and the origin of the spectra was found to be CuSn and VZn or [VZn + SnZn], respectively. The potential fluctuations were estimated from the lower energy side of the PL spectra, and the Cu‐poor samples exhibited smaller potential fluctuations.

Site selective doping of Zn for the p‐type Cu(In,Ga)Se2 thin film for solar cell application

Selective doping of a Zn impurity at the group III site in a Cu(In, Ga)Se2 (CIGS) film was performed by the doping of Zn at the first stage of the three‐stage method. The p‐type CIGS:Zn film was obtained, which is in contrast to the n‐type CIGS:Zn film obtained by the Zn impurity doping at the second and third‐stages. Based on excitation intensity dependence of photoluminescence (PL) at low‐temperature, the change in the acceptor level was observed. The enhancement of carrier concentration as a result of Zn‐doping in the p‐type CIGS:Zn film was observed. The CIGS:Zn solar cells exhibited η of 14.5% and Voc of 0.658 V, which are higher than that of the corresponding solar cells using the undoped CIGS films

Excitation intensity dependent photoluminescence of annealed two-dimensional MoS2 grown by chemical vapor deposition

Here, we present detailed results of Raman and photoluminescence (PL) characterization of monolayers of MoS2 grown by chemical vapor deposition (CVD) on SiO2/Si substrates after thermal annealing at 150 °C, 200 °C, and 250 °C in an argon atmosphere. In comparison to the as-grown monolayers, annealing in the temperature range of 150–250 °C brings about significant changes in the band edge luminescence. It is observed that annealing at 150 °C gives rise to a 100-fold increase in the PL intensity and produces a strong band at 1.852 eV attributed to a free-to-bound transition that dominates over the band edge excitonic luminescence. This band disappears for the higher annealing temperatures. The improvement in PL after the 200 °C anneal is reduced in comparison to that obtained after the 150 °C anneal; this is suggested to arise from a decrease in the non-radiative lifetime caused by the creation of sulfur di-vacancies. Annealing at 250 °C degrades the PL in comparison to the as-grown sample because of the onset of disorder/decomposition of the sample. It is clear that the PL features of the CVD-grown MoS2 monolayer are profoundly affected by thermal annealing in Ar atmosphere. However, further detailed studies are needed to identify, unambiguously, the role of native defects and/or adsorbed species in defining the radiative channels in annealed samples so that the beneficial effect of improvement in the optical efficiency of the MoS2 monolayers can be leveraged for various device applications.

Saturable Two-Step Photocurrent Generation in Intermediate-Band Solar Cells Including InAs Quantum Dots Embedded in Al0.3Ga0.7/GaAs Quantum Wells

We studied in detail the photocurrent generation process in two-step photon absorption in intermediate-band solar cells, including InAs quantum dots embedded in Al $_{0.3}$ Ga $_{0.7}$ As/GaAs quantum wells at room temperature. The photocurrent generated by the two-step photon absorption exhibited saturation as the interband excitation intensity increased in strength. On the other hand, as the intersubband excitation intensity increased, the two-step photoexcitation current deviated from a power law. Furthermore, the two-step photoexcitation current exhibiting saturation and deviation strongly depended on both the interband and intersubband excitation intensities. To interpret these phenomena, we performed a theoretical simulation of the two-step photoexcitation current. The results suggest that the photocurrent saturation and deviation were caused by filling of the intermediate states with electrons. Furthermore, our calculated results indicate that the electron-recombination lifetime in the intermediate states is extremely long. The results of the temperature dependence of the two-step photoexcitation current and the excitation intensity dependence of photoluminescence suggest that efficient electron–hole separation extends electron lifetime.

Trap States and Their Dynamics in Organometal Halide Perovskite Nanoparticles and Bulk Crystals

Organometal halide perovskites have attracted tremendous attention for optoelectronic applications. Charge carrier trapping is one of the dominant processes often deteriorating the performance of devices. Here, we investigate the details of trap behavior in colloidal nanoparticles (NPs) of CH3NH3PbBr3 perovskites with mean size of 8 nm and the corresponding bulk crystals (BCs). We use excitation intensity dependence of photoluminescence (PL) dynamics together with comprehensive simulation of charge carrier trapping and the trap-state dynamics. In the bulk at very low excitation intensities the PL is quenched by trapping. A considerable fraction of the traps become filled if excitation fluence is increased. We identified two different traps, one exhibiting ultralong lifetime (∼70 μs) which leads to efficient accumulation of trap filling even at relatively low excitation intensities. In colloidal NPs, the average number of surface traps is estimated to be 0.7 per NP. It means about 30% excitation would unde...

Excitation intensity dependence of photoluminescence from monolayers of MoS2 and WS2/MoS2 heterostructures

A detailed study of the excitation dependence of the photoluminescence (PL) from monolayers of MoS2 and WS2/MoS2 heterostructures grown by chemical vapor deposition on Si substrates has revealed that the luminescence from band edge excitons from MoS2 monolayers shows a linear dependence on excitation intensity for both above band gap and resonant excitation conditions. In particular, a band separated by ∼55 meV from the A exciton, referred to as the C band, shows the same linear dependence on excitation intensity as the band edge excitons. A band similar to the C band has been previously ascribed to a trion, a charged, three-particle exciton. However, in our study the C band does not show the 3/2 power dependence on excitation intensity as would be expected for a three-particle exciton. Further, the PL from the MoS2 monolayer in a bilayer WS2/MoS2 heterostructure, under resonant excitation conditions where only the MoS2 absorbs the laser energy, also revealed a linear dependence on excitation intensity for the C band, confirming that its origin is not due to a trion but instead a bound exciton, presumably of an unintentional impurity or a native point defect such as a sulfur vacancy. The PL from the WS2/MoS2 heterostructure, under resonant excitation conditions also showed additional features which are suggested to arise from the interface states at the heteroboundary. Further studies are required to clearly identify the origin of these features.