Radiative Recombination Channels Research Articles

Impact of blue-shifted effective joint density of electronic states on the photoluminescence of nanostructured silicon

Light emission from nanostructured silicon has triggered tremendous research interest for three decades. Yet, the exact mechanism of photoluminescence from silicon-based nano-systems is still not completely understood. It is generally believed that quantum confinement and surface chemistry play a combined role in determining the luminescence characteristics of nano silicon. In this work, we show that the evolution of electron-hole effective joint density of states, resulting from the relaxation of k-selection rule, can account for the much-observed temperature dependent shift of the luminescence spectra of nanostructured silicon. Deconvolution of the emission characteristics reveals three distinct radiative recombination channels those can be attributed to band-to-band, band-to-trap and trap-to-trap transitions. At temperatures below ∼200 K, the peak due to band-to-band transition exhibit a nearly linear blue shift with increasing temperature while at higher temperatures, this trend is reversed. The rate of variation of emission peak energy with temperature is also found to be dependent on the effective crystallite size. These results are explained in terms of shift in the effective joint density of state function of photogenerated electron-hole pairs. The shift provides experimental evidence of the pseudo-direct transitions in the quantum-confined nanostructure of silicon.

Cu(In,Ga)Se_2-based solar cells for space applications: proton irradiation and annealing recovery

In this work, we present an experimental study of a Cu(In,Ga)Se_2 (CIGS)-based solar cell (SC), irradiated with protons of energy 80 and 180 keV and with fluences of 10^{12}, 10^{13}, and 10^{14} cm^{-2}, as well as a strategy to recover the induced damage. The possible modifications of the structural, electrical, and optical properties, induced by the proton irradiation, were investigated. Although the irradiation did not promote any major modification in the crystalline structure, it did induce the creation of defects responsible for changes in the electronic structure which caused a partial PL quenching and significant changes in the PL spectral shape, as well as a reduction of the power conversion efficiency and open-circuit voltage of up to 30% as revealed by J–V measurements. The photoluminescence results showed a broadening, redshift and decrease in the signal-to-noise ratio. The recovery of damage induced by irradiation in several SCs was tested through annealing steps performed at different temperatures and time intervals. It was found that the best recovery strategy for the investigated irradiation parameters was carrying out several isothermal annealing at 200 °C for 30 min. This strategy is compatible with the intermittent variation of the temperature in space and allowed to recover a power conversion efficiency comparable to that of the as grown cell. In particular, it must be highlighted that keeping the SC at room temperature in ambient atmosphere and in the dark, did not promote significant recovery in contradiction with some previous reports. This recovery methodology was applied in parallel for non-irradiated SCs and no increase in power conversion efficiency was found, but rather a slight decrease. The dominant radiative recombination channel was, apparently, unchanged with the irradiation and the subsequent recovery process. Nonetheless, changes in the concentration of defects of different types cannot be excluded, which is in line with a significant influence of fluctuating potentials in both as grown and after recovery stages of the solar cell. This work constitutes a first systematic study that simultaneously encompasses the influence of proton irradiation on the optical and electrical properties of CIGS SCs and a damage recovery methodology with a high potential to be explored in space applications. Additionally, it contributes to reinforcing the high potential of CIGS technology in the context of creating constellations of small satellites that are being developed by different entities, particularly private ones.

High-resolution photoluminescence study on donor-acceptor pair (DAP) recombination in silicon crystals co-doped with phosphorous and gallium

Before lower purity, lower cost silicon (Si) materials, such as compensated Si, can play a role in the terawatt-level (TW) capacity of photovoltaics, a better understanding of the fundamental properties of impurities in compensated Si is essential. In this work, high-resolution photoluminescence (PL) has been used to study the charge carrier radiative recombination through Donor-Acceptor pairs (DAPs) in phosphorus (P) and gallium (Ga) co-doped Si material grown for solar cell applications. The high spectral resolution of our PL system, 0.06 meV, enables us to overcome hitherto prior issues of overlapping spectral lines, giving access to extremely fine structures associated with DA pair (DAP) recombination. Our results confirm the presence of three broad bands and a discrete line structure related to DAP luminescence. The comparison of the discrete line structure due to DAPs recombination in the PL spectra with the theoretically predicted one allows the accurate determination of the Ga ionization energy. Temperature-dependent PL is then used to understand the thermally-induced changes in the DAP luminescence. In particular, we observe that the radiative recombination channel remains active for distant DAPs up to ∼40 K, unlike that for close-range DAPs for which the radiative channel is quenched after only slight increases in the temperature range 10–25 K. Furthermore, the analysis of the temperature dependent changes in the PL intensity of the broad DAP bands up to ∼200 K is used to derive the ionization energy of P donors in compensated Si material. In light of this important information, the significance of using high resolution PL to analyse spectral features in compensated Si is demonstrated.

Direct Arylation of Silicon Nanocrystals with Hexadehydro‐Diels–Alder‐Derived Benzynes

AbstractColloidal silicon nanocrystals (SiNCs) have garnered significant interest in optoelectronics and biomedical applications. Direct arylation provides pathways to enhance the solution processability of particles and manipulate the photophysical and electronic properties of SiNCs. Unfortunately, existing methods employed to prepare aryl‐functionalized SiNCs are based on organometallic coupling or transition‐metal‐catalyzed strategies, which require metal‐based reagents for preactivation or the precursors and complicated post‐treatment processes for product purification. Herein, we demonstrate a metal‐free method that directly functionalizes SiNCs with aryl‐based ligands. We design a series of benzyne derivatives formed from the thermal cyclization of predesigned alkynes, allowing efficient arylation on hydride‐terminated silicon surfaces under mild conditions. These aryl‐functionalized SiNCs exhibit strong blue emissions with nanosecond‐scaled decay, suggesting the formation of a new radiative recombination channel on SiNC surfaces.

Direct Arylation of Silicon Nanocrystals with Hexadehydro-Diels-Alder-Derived Benzynes.

Colloidal silicon nanocrystals (SiNCs) have garnered significant interest in optoelectronics and biomedical applications. Direct arylation provides pathways to enhance the solution processability of particles and manipulate the photophysical and electronic properties of SiNCs. Unfortunately, existing methods employed to prepare aryl-functionalized SiNCs are based on organometallic coupling or transition-metal-catalyzed strategies, which require metal-based reagents for preactivation or the precursors and complicated post-treatment processes for product purification. Herein, we demonstrate a metal-free method that directly functionalizes SiNCs with aryl-based ligands. We design a series of benzyne derivatives formed from the thermal cyclization of predesigned alkynes, allowing efficient arylation on hydride-terminated silicon surfaces under mild conditions. These aryl-functionalized SiNCs exhibit strong blue emissions with nanosecond-scaled decay, suggesting the formation of a new radiative recombination channel on SiNC surfaces.

Vitamins as Active Agents for Highly Emissive and Stable Nanostructured Halide Perovskite Inks and 3D Composites Fabricated by Additive Manufacturing

AbstractThe use of non‐toxic and low‐cost vitamins like α‐tocopherol (α‐TCP, vitamin E) to improve the photophysical properties and stability of perovskite nanocrystals (PNCs), through post‐synthetic ligand surface passivation, is demonstrated for the first time. Especially interesting is its effect on CsPbI3 the most unstable inorganic PNC. Adding α‐TCP produces that the photoluminescence quantum yield (PLQY) of freshly prepared and aged PNCs achieves values of ≈98% and 100%, respectively. After storing 2 months under ambient air and 60% relative humidity, PLQY is maintained at 85% and 67%, respectively. α‐TCP restores the PL features of aged CsPbI3 PNCs, and mediates the radiative recombination channels by reducing surface defects. In addition, the combination of α‐TCP and PNCs facilitates the chemical formulation to prepare PNCs‐acrylic polymer composites processable by additive manufacturing. This enables the development of complex shaped parts with improved luminescent features and long‐term stability for 4 months, which is not possible for non‐modified PNCs. A PLQY ≈92% is reached in the 3D printed polymer/PNC composite, the highest value obtained for a red‐emitting composite solid until now as far as it is known. The passivation shell provided by α‐TCP makes that PNCs inks do not suffer any degradation process avoiding the contact with the environment and preserve their properties after reacting with polar monomers during composite polymerization.

Wurtzite Gallium Phosphide via Chemical Beam Epitaxy: Impurity-Related Luminescence vs Growth Conditions.

The metastable wurtzite crystal phase in gallium phosphide (WZ GaP) is a relatively new structure with little available information about its emission properties compared to the most stable zinc-blend phase. Here, the effect of growth conditions of WZ GaP nano- and microstructures obtained via chemical beam epitaxy on the optical properties was studied using power- and temperature-dependent photoluminescence (PL). We showed that the PL spectra are dominated by two strong broad emission bands at 1.68 and 1.88 eV and two relatively narrow peaks at 2.04 and 2.09 eV. The broad emissions are associated with the presence of carbon and a small number of extended crystal defects, respectively. For the sharp emissions, two main radiative recombination channels were observed with ionization energies estimated in the range of 50-80 meV and lower than 10 meV. No variation of the low-temperature PL spectra was observed for samples grown at different P precursor flows, while increasing Ga content enhanced the dominant broad emission at around 1.68 eV, suggesting that the group III organometallic precursor is the main source of impurities. Finally, Be-doped samples were grown, and their characteristic optical emission at 2.03 eV was identified. These results contribute to the understanding of impurity-related luminescence in hexagonal GaP, being useful for further crystal growth optimization required for the fabrication of optoelectronic devices.

Photoluminescence related to Ca in GaN

The ${\mathrm{Ca}}_{\mathrm{Ga}}$ acceptor in GaN was studied using photoluminescence (PL) experiments and first-principles calculations. The experimentally found $\text{--}/0$ transition level of the ${\mathrm{Ca}}_{\mathrm{Ga}}$ at 0.50 \ifmmode\pm\else\textpm\fi{} 0.02 eV above the valence band reasonably agrees with the value calculated using the Heyd-Scuseria-Ernzerhof functional tuned to fulfill the generalized Koopmans condition (0.70 eV). The ${\mathrm{Ca}}_{\mathrm{Ga}}$ acceptor is responsible for the green luminescence band (labeled ${\mathrm{GL}}_{\mathrm{Ca}}$) with a maximum at 2.5 eV. The electron- and hole-capture coefficients for the ${\mathrm{Ca}}_{\mathrm{Ga}}$ acceptor are obtained from PL experiments: ${C}_{n}=9\ifmmode\times\else\texttimes\fi{}{10}^{\text{--}14}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{3}/\mathrm{s}$ and ${C}_{p}=6\ifmmode\times\else\texttimes\fi{}{10}^{\text{--}7}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{3}/\mathrm{s}$. The ${\mathrm{GL}}_{\mathrm{Ca}}$ band is an efficient radiative recombination channel in GaN samples implanted with Ca and in unintentionally doped GaN grown by molecular-beam epitaxy at relatively low temperatures, where Ca is a common contaminant.

Fine‐Tuning Single‐Source White‐Light Emission from All‐Inorganic Corrugated 2D Antimony‐Halide Perovskite

AbstractCorrugated 2D antimony‐halide perovskites such as Cs3Sb2Cl9 (CSC) are promising candidates for single‐source white‐light emission due to their ultra‐broadband spectra. However, CSC has a serious luminescence quenching phenomenon due to inadequate confinement of excitons. By the homovalent substitution of trivalent antimony cation Sb3+ by a small amount of trivalent rare earth (RE) cations RE3+, the photoluminescence intensities from high‐quality Cs3(Sb1−xREx)2Cl9 (CSRC) (RE = Ce, Sm, Nd, Y, Er, etc.) films at room temperature (RT) are over two orders of magnitude higher than that of CSC film. Especially, the photoluminescence quantum yield (PLQY) for the Cs3(Sb0.995Er0.005)2Cl9 film is 9.5% at RT, which is much higher than A3B2X9 perovskites previously reported for single‐source white‐light lighting. Furthermore, the Cs3(Sb0.995Er0.005)2Cl9 film exhibits an ultra‐broadband emission with the full width at half maximum reaching 554 meV at RT, resulting in a “warm” white‐light with the CIE coordinate (0.33, 0.46) and the correlated color temperature of 5450 K. The PLQY enhancement can be considered as the fact that a high activation energy by bandgap widening effect and Type‐I‐like “straddling” band alignment between Cs3Sb2Cl9 and Cs3Er2Cl9 lead to reducing nonradiative losses and increasing radiative recombination channels. Meanwhile, the spectral broadening can be considered to be attributed to strong effect of electron–phonon interaction.

Photoluminescence of Cis-Polyacetylene Semiconductor Material

Photoluminescence (PL) is one of the key experimental characterizations of optoelectronic materials, including conjugated polymers (CPs). In this study, a simplified model of an undoped cis-polyacetylene (cis-PA) oligomer was selected and used to explain the mechanism of photoluminescence (PL) of the CPs. Using a combination of the ab initio electronic structure and a time-dependent density matrix methodology, the photo-induced time-dependent excited state dynamics were computed. We explored the phonon-induced relaxation of the photoexcited state for a single oligomer of cis-PA. Here, the dissipative Redfield equation of the motion was used to compute the dissipative excited state dynamics of electronic degrees of freedom. This equation used the nonadiabatic couplings as parameters. The computed excited state dynamics showed that the relaxation rate of the electron is faster than the relaxation rate of the hole. The dissipative excited-state dynamics were combined with radiative recombination channels to predict the PL spectrum. The simulated results showed that the absorption and emission spectra both have a similar transition. The main result is that the computed PL spectrum demonstrates two mechanisms of light emission originating from (i) the inter-band transitions, corresponding to the same range of transition energies as the absorption spectrum and (ii) intra-band transitions not available in the absorption spectra. However, the dissipative Redfield equation of the motion was used to compute the electronic degrees of freedom of the nonadiabatic couplings, which helped to process the time propagation of the excited dynamic state. This excited dynamic state shows that the relaxation rate of the electron is faster than the relaxation rate of the hole, which can be used for improving organic semiconductor materials for photovoltaic and LED applications.

Стимулированное излучение в гетероструктурах InAs/InAsSb/InAsSbP с асимметричным электронным ограничением

The electroluminescent characteristics of the InAs/InAs1−ySby /InAsSbP asymmetric light-emitting diode heterostructures with high InSb mole fraction in the active region (y > 0.09) in the temperature range 4.2−300 K have been studied. Stimulated emission was achieved in the wavelength range 4.1−4.2 μm at low temperatures (T < 30 K). It was found that the electroluminescence spectra were formed as a result of the superposition of contributions from different channels of radiative recombination of charge carriers near the type II heterointerface. The effect of the quality of the type II InAsSb/InAsSbP heterojunction on the radiative interface transitions with an increase in the content of InSb in the ternary solid solution is considered.

Stimulated emission in the InAs/InAsSb/InAsSbP heterostructures with asymmetric electronic confinement

The electroluminescent characteristics of the InAs/InAs1-ySby/InAsSbP asymmetric light-emitting diode heterostructures with high InSb mole fraction in the active region (y>0.09) in the temperature range 4.2-300 K have been studied. Stimulated emission was achieved in the wavelength range 4.1-4.2 μm at low temperatures (T<30 K). It was found that the electroluminescence spectra were formed as a result of the superposition of contributions from different channels of radiative recombination of charge carriers near the type II heterointerface. The effect of the quality of the type II InAsSb/InAsSbP heterojunction on the radiative interface transitions with an increase in the content of InSb in the ternary solid solution is considered. Keywords: heterojunctions, InAs, antimonides, electroluminescence, light-emitting diodes.

In Situ Phase-Transition Crystallization of All-Inorganic Water-Resistant Exciton-Radiative Heteroepitaxial CsPbBr3–CsPb2Br5 Core–Shell Perovskite Nanocrystals

The instability of the metal halide perovskites upon exposure to moisture or heat strongly hampers their applications in optoelectronic devices. Here, we report the large-yield synthesis of highly water-resistant total-inorganic green luminescent CsPbBr3/CsPb2Br5 core/shell heteronanocrystals (HNCs) by developing an in situ phase-transition approach. It is implemented via the water-driven phase transition of the original monoclinic CsPbBr3 nanocrystal and the resultant tetragonal CsPb2Br5 nanoshell has a small lattice mismatch with the CsPbBr3 core, which ensures formation of an epitaxial interface for the synthesised CsPbBr3/CsPb2Br5 HNCs. These HNCs maintain nearly 100% of the original luminescence intensity after immersion in water for 11 months and the luminescence intensity drops only to 81.3% at 100 °C. Transient luminescence spectroscopy and density functional theory calculation reveal that there are double radiative recombination channels in the core CsPbBr3 nanocrystal, and the electron potential barrier provided by the CsPb2Br5 nanoshell significantly improves the exciton recombination rate. A prototype quasi-white light-emitting device based on these robust CsPbBr3/CsPb2Br5 HNCs is realized, showing their strong competence in solid-state lighting and wide color-gamut displays.

Oxygen annealing induced changes in defects within β-Ga2O3 epitaxial films measured using photoluminescence

In this work, we use photoluminescence spectroscopy (PL) to monitor changes in the UV, blue, and green emission bands from n-type (010) Ga2O3 films grown by metalorganic vapor phase epitaxy induced by annealing at different temperatures under O2 ambient. Annealing at successively higher temperatures decreases the overall PL yield and UV intensity at nearly the same rates, indicating the increase in the formation of at least one non-radiative defect type. Simultaneously, the PL yield ratios of blue/UV and green/UV increase, suggesting that defects associated with these emissions increase in concentration with O2 annealing. Utilizing the different absorption coefficients of 240 and 266 nm polarization-dependent excitation, we find activation energy for the generation of non-radiative defects of 1.34 eV in the bulk but 2.53 eV near the surface. We also deduce activation energies for the green emission-related defects of 1.20 eV near the surface and 2.21 eV at low temperatures and 0.74 eV at high temperatures through the films, whereas the blue-related defects have activation energy in the range 0.72–0.77 eV for all depths. Lastly, we observe hillock surface morphologies and Cr diffusion from the substrate into the film for temperatures above 1050 °C. These observations are consistent with the formation and diffusion of V Ga and its complexes as a dominant process during O2 annealing, but further work will be necessary to determine which defects and complexes provide radiative and non-radiative recombination channels and the detailed kinetic processes occurring at surfaces and in bulk amongst defect populations.

Investigation of Radiation Recombination Channels in Long-Wavelength InAs/InAsSb/InAsSbP LED Heterostructures

This work presents the results of the investigation of optical properties of long-wavelength (~5 µm at 300 K) InAs/InAsSb/InAsSbP LED heterostructures. These heterostructures are used in various applications in mid-wavelength infrared range, such as environmental monitoring, etc. Electroluminescence was used to study the optical characteristics of the structures in the temperature range 4.2–300 K. Various radiative recombination channels in LED heterostructures were considered, including those associated with the InAs substrate and those related to the active layer, the latter competing depending on the temperature. The obtained results can be useful when designing optoelectronic devices with weak temperature dependence of the emission wavelength.

Charge‐Carrier Trapping and Radiative Recombination in Metal Halide Perovskite Semiconductors

AbstractTrap‐related charge‐carrier recombination fundamentally limits the performance of perovskite solar cells and other optoelectronic devices. While improved fabrication and passivation techniques have reduced trap densities, the properties of trap states and their impact on the charge‐carrier dynamics in metal‐halide perovskites are still under debate. Here, a unified model is presented of the radiative and nonradiative recombination channels in a mixed formamidinium‐cesium lead iodide perovskite, including charge‐carrier trapping, de‐trapping and accumulation, as well as higher‐order recombination mechanisms. A fast initial photoluminescence (PL) decay component observed after pulsed photogeneration is demonstrated to result from rapid localization of free charge carriers in unoccupied trap states, which may be followed by de‐trapping, or nonradiative recombination with free carriers of opposite charge. Such initial decay components are shown to be highly sensitive to remnant charge carriers that accumulate in traps under pulsed‐laser excitation, with partial trap occupation masking the trap density actually present in the material. Finally, such modelling reveals a change in trap density at the phase transition, and disentangles the radiative and nonradiative charge recombination channels present in FA0.95Cs0.05PbI3, accurately predicting the experimentally recorded PL efficiencies between 50 and 295 K, and demonstrating that bimolecular recombination is a fully radiative process.

NATIVE DEFECTS IN CdZnS QUANTUM DOTS

Defects in quantum (QDs) not only can affect their optical properties, but also create additional channels of radiative and non-radiative recombination. We studied the structural and electronic characteristics of metal (cadmium, zinc) vacancies and interstitial sulfur as the most probable types of defects in Cd1-xZnxS nanocrystals by means the density functional theory (DFT). The total Sensor Electronics and and partial density of electronic states were calculated, the analysis of structural relaxation was carried out. Based on the obtained theoretical results and available experimental data, we concluded that among the all types of defects considered here, only cadmium vacancies can be the centers of radiative recombination in Cd1-xZnxS quantum dots

Two-dimensional electron bound hole photoluminescence in GaAs in perpendicular magnetic fields

The photoluminescence of quasi two-dimensional (2D) electrons in modulation doped AlxGa1 − xAs-GaAs single heterostructure recombining with bound holes was studied at liquid helium temperatures in magnetic fields up to 9 T. Luminescence contributions from the ground and a weakly populated first excited 2D subband were observed as well as bulk-like contributions from the GaAs buffer. The 2D subband and the bulk signals are linked and oscillate with the magnetic field strength. It is not the filling factor that rules but the alignment of the two subband Landau-ladders paired with intersubband scattering that drives an oscillatory behavior of the subband populations. Asymmetric Landau-level transition line shapes indicate a perturbed 2D electron system. We performed a detailed analyses of Landau-level dependent line strengths, positions and half widths and compared the experiment with calculations of subband energies and transition matrix elements. The total electron density is not fixed in our experiment but decreases with magnetic field strength. This might indicate that the 2D system exchanges electrons with a reservoir and/or that the correlation between radiative and nonradiative recombination channels vary strongly with the magnetic field strength.

Photoluminescence of compact GeSi quantum dot groups with increased probability of finding an electron in Ge

The photoluminescence (PL) of the combined Ge/Si structures representing a combination of large (200–250 nm) GeSi disk-like quantum dots (nanodisks) and four-layered stacks of compact groups of smaller (30 nm) quantum dots grown in the strain field of nanodisks was studied. The multiple increase in the PL intensity was achieved by the variation of parameters of vertically aligned quantum dot groups. The experimental results were analyzed on the basis of calculations of energy spectra, electron and hole wave functions. It was found that the quantum dot arrangement in compact groups provides the effective electron localization in Δx,y-valleys with an almost equal probability of finding an electron in the Si spacer and Ge barrier. As a result, the main channels of radiative recombination in the structures under study correspond to spatially direct optical transitions.

Microwave Synthesis of Silver Sulfide and Silver Nanoparticles: Light and Time Influence.

Silver sulfide (Ag2S) is a low band gap material, which absorbs near-infrared light and is of great importance in areas such as nanotechnology and biomedicine. We report the influence of the starting reagents, synthesis time, and light radiation on the geometry and size of silver sulfide nanoparticles and on the fraction of metallic Ag obtained in a microwave reactor. The X-ray diffraction diffractograms confirmed that Ag2S is the main product if the reaction’s precursor contains silver in the oxidation state of +1 and mostly metallic silver (Ag°) when it is +2. Small nanoparticles (∼6 nm) of spherical geometry are present in the transmission electron microscopy images for the synthesis performed with the lamp light ON, while with the light switched OFF, wider and hundreds of nanometers longer particles are observed. This discriminative effect occurs with shorter synthesis time duration (<10 min) but when the time of reaction is extended, the particles coalesce for both light and dark conditions. Overall, it was observed by photoluminescence that crystalline Ag and Ag2S 4–8 nm nanoparticles obtained in 15 min and light irradiation during synthesis have a clear relative increase of the radiative recombination channels of the charged carriers, which are typical of materials characterized by the involvement of low density of states inside the band gap.