Recombination Of Free Carriers Research Articles

Stable Whispering Gallery Mode Lasing from Solution‐Processed Formamidinium Lead Bromide Perovskite Microdisks

AbstractFormamidinium (CH(NH2)2, FA) lead halide perovskites exhibit excellent optoelectronic properties for the applications of light‐emitting diodes and lasers. However, the reported formamidinium lead bromide (FAPbBr3) perovskite‐based lasers mainly depend on the external feedback or Fabry–Pérot mode cavities and suffer from moderate quality (Q) factors. Herein, solution‐processed square and quasi‐circular FAPbBr3 microdisks are developed to achieve room‐temperature naturally facet‐formed whispering gallery mode (WGM) lasers. These microdisks exhibit relatively high Q factor and tunable laser modes with threshold of 10–70 µJ cm−2 under 400 nm excitation. Although the synthesized circular microdisk is not regular enough, it still shows higher Q factor of 3455 than that of square microdisk due to the less boundary wave and pesudointegrable leakage. This simple and low‐cost solution processed method can tackle the complicated and expensive conventional preparation techniques of circular microdisks. Power dependent photoluminescence decay transients indicate that the band edge emission of the FAPbBr3 microdisks originates from free carrier recombination. In addition, these microdisks show high photostability and that room temperature lasing can sustain over 50 min. These results suggest that FAPbBr3 microdisks can serve as promising WGM lasers with excellent light emission capability and stability toward practical optoelectronic applications.

A new model on recombination dynamics of polar InGaN/GaN MQW LED and IQE measurement

Understanding the recombination nature in a polar InGaN/GaN multiple quantum well (MQW) light-emitting diode (LED) or similar device is critical for their further performance enhancements. This paper reports a new theoretical model for investigating the recombination dynamics in MQW LEDs, which more comprehensively takes both localized exciton recombination (LER) and free carrier recombination (FCR) into account. The obtained rates for LER, FCR and nonradiative recombination show a clear picture of recombination paths in a commercial blue MQW LED wafer. They can be also used to calculate the internal quantum efficiency without involving any extra measurements or prerequisites. This model may provide a universal solution to express the complicated recombination dynamics in various kinds of MQW LEDs.

Оптичні властивості CdTe, легованого Ca

Досліджено оптичне поглинання, відбивання і люмінесценцію CdTe:Ca. Встановлено, що отримані леговані Ca поверхневі шари характеризуються інтенсивною фотолюмінесценцією з η = 8-10% у крайовій області. Випромінювання формується внаслідок міжзонної рекомбінації вільних носіїв заряду і анігіляцією зв’язаних на ізовалентних домішках Ca екситонів. Зазначені складові спостерігаються у диференційних спектрах оптичного відбивання Ŕω у приповерхневому шарі отриманому при легування ізовалентною домішкою Ca підкладинок CdTe. Встановлено, що легування обумовлює утворення p-типу провідності.

Charge Carrier Dynamics and Broad Wavelength Tunable Amplified Spontaneous Emission in ZnxCd1-xSe Nanowires.

ZnxCd1-xSe is regarded as a promising semiconducting material for optoelectronic devices. However, the tunable amplified spontaneous emission (ASE) properties and corresponding charge carrier recombination dynamics in ZnxCd1-xSe (0 ≤ x ≤ 1) nanowires (NWs) remain poorly understood. Herein, the charge carrier dynamics and ASE properties in ZnxCd1-xSe NWs were systematically investigated. In these NWs, the one/two-photon pumped ASE wavelength across the entire visible spectrum (480-725 nm) can be easily tuned via compositional engineering. The ASE threshold is closely related to the absorption coefficient and PL lifetime. At room temperature, free-carrier recombination is dominated in the low fluence pumped PL process. The ASE behavior is determined by exciton recombination in the high pump fluence (>1018 cm-3) region. These findings uncover the origin of the tunable PL/ASE properties in ZnxCd1-xSe NWs and establish them as having practical application as a series of lasing gain materials.

Trade‐Off between Exciton Dissociation and Carrier Recombination and Dielectric Properties in Y6‐Sensitized Nonfullerene Ternary Organic Solar Cells

Organic photovoltaics (OPVs) have emerged as a promising renewable energy generation technology in past decades. However, the deep understanding of the details in exciton dissociation and carrier recombination in ternary organic solar cells (OSCs) is still lacking. Herein, a novel ternary OSC based on a PTB7‐Th:Y6:ITIC blend with a power conversion efficiency (PCE) enhancement of 29% is reported. A trade‐off is surprisingly found to exist between the exciton dissociation and carrier recombination process. The addition of nonfullerene acceptor Y6 in the ternary blend is found to create an efficient exciton dissociation process but accelerates the free carrier recombination process. Dielectric properties are also studied for ternary OSCs. The addition of Y6 into the binary blend is found to tune down the dielectric constant of the active layer and as a result accelerates the carrier recombination. The best performance is obtained for PTB7‐Th:Y6(5 wt%):ITIC(95 wt%)‐based ternary devices. In addition to its balanced charge carrier mobility and efficient charge extraction process, PTB7‐Th:Y6(5 wt%):ITIC(95 wt%)‐based ternary devices reach a balance in the trade‐off between the exciton dissociation and carrier recombination process and thus achieve the highest short‐circuit current density (Jsc) value.

Room-temperature stability of excitons and transverse-electric polarized deep-ultraviolet luminescence in atomically thin GaN quantum wells

We apply first-principles calculations to study the effects of extreme quantum confinement on the electronic, excitonic, and radiative properties of atomically thin (1–4 atomic monolayers) GaN quantum wells embedded in AlN. We determine the quasiparticle bandgaps, exciton energies and wave functions, radiative lifetimes, and Mott critical densities as a function of well and barrier thickness. Our results show that quantum confinement in GaN monolayers increases the bandgap up to 5.44 eV and the exciton binding energy up to 215 meV, indicating the thermal stability of excitons at room temperature. Exciton radiative lifetimes range from 1 to 3 ns at room temperature, while the Mott critical density for exciton dissociation is approximately 1013 cm−2. The luminescence is transverse-electric polarized, which facilitates light extraction from c-plane heterostructures. We also introduce a simple approximate model for calculating the exciton radiative lifetime based on the free-carrier bimolecular radiative recombination coefficient and the exciton radius, which agrees well with our results obtained with the Bethe–Salpeter equation predictions. Our results demonstrate that atomically thin GaN quantum wells exhibit stable excitons at room temperature for potential applications in efficient light emitters in the deep ultraviolet as well as room-temperature excitonic devices.

Ultrafast dynamics of pure many-body effect and its competition with bandgap widening via electron–phonon coupling in PbTe thin films

Bandgap renormalization (BGR), or bandgap narrowing effect, is one of the many-body effects predicted by many-body theory in semiconductor physics. However, pure BGR effect has never been observed experimentally although the mixed effect of BGR with lattice-heating induced bandgap narrowing was reported in GaAs and CdTe semiconductors. PbTe has a positive temperature coefficient of bandgap. Its lattice-heating induced bandgap narrowing can be eliminated so that the pure BGR effect is possibly observed. However, that has never been reported so far because the BGR effect is weak in PbTe due to its huge dielectric screening. We study the ultrafast dynamics of photo-excited carriers in PbTe films using near-infrared femtosecond-resolved transient differential transmission (TDT) spectroscopy. A complex oscillation-like damped decay dynamics is observed. A negative dip, a fingerprint signature of BGR effect, occurs immediately after the thermalization process of the photo-excited carriers. As such, we have observed a pure BGR effect in PbTe for the first time, strongly supporting the many-body theory. Meanwhile, we have also observed a dynamic transition from a transient absorption enhancement to absorption saturation, revealing the emergence of a slow lattice-heating induced bandgap widening via carrier-phonon couplings. Based on a two-temperature model, a simulation computation is carried out, supporting the lattice-heating induced bandgap widening leads to the dynamic transition. A phenomenological dynamic model is developed, including three dynamic components: the thermalization of the photo-excited carriers, recombination of the thermalized carriers and lattice-heating induced bandgap widening, and used to best fit the excitation-power dependent TDT dynamics. Time constants and strengths of the three dynamic components are obtained for different excitation powers. Time constant variation with the excitation power reveals different roles of the carrier–carrier and carrier-phonon scattering in the thermalizing process, as well as of free carrier and exciton recombination in the decaying process of hot carriers.

Decoding Charge Recombination through Charge Generation in Organic Solar Cells

The in‐depth understanding of charge carrier photogeneration and recombination mechanisms in organic solar cells is still an ongoing effort. In donor:acceptor (bulk) heterojunction organic solar cells, charge photogeneration and recombination are inter‐related via the kinetics of charge transfer states—being singlet or triplet states. Although high‐charge‐photogeneration quantum yields are achieved in many donor:acceptor systems, only very few systems show significantly reduced bimolecular recombination relative to the rate of free carrier encounters, in low‐mobility systems. This is a serious limitation for the industrialization of organic solar cells, in particular when aiming at thick active layers. Herein, a meta‐analysis of the device performance of numerous bulk heterojunction organic solar cells is presented for which field‐dependent photogeneration, charge carrier mobility, and fill factor are determined. Herein, a “spin‐related factor” that is dependent on the ratio of back electron transfer of the triplet charge transfer (CT) states to the decay rate of the singlet CT states is introduced. It is shown that this factor links the recombination reduction factor to charge‐generation efficiency. As a consequence, it is only in the systems with very efficient charge generation and very fast CT dissociation that free carrier recombination is strongly suppressed, regardless of the spin‐related factor.

Down-conversion luminescence of Ce-Yb ions in YF3

The single-phase Y1-x-yCexYbyF3 solid solution powders were prepared by melt crystallization technique. The 5d-4f interconfigurational luminescence of Ce3+ ions and 2F5/2-2F7/2 luminescence of Yb3+ ions in YF3 crystalline powder doped with Ce3+/Yb3+ ions pair upon 266 nm excitation were studied. The energy transfer from Ce3+ to Yb3+ after 266 nm excitation has complex character and deals with the UV excitation induced dynamic processes and color centers formation in YF3:Ce3+ powders. It was shown that UV light irradiation leads to in absorbing in 525–650 nm spectral range which is not observed for Ce3+-Yb3+ containing samples. This speaks for additional channels of Ce3+ and Yb3+ excitation as a result of recombination of free charge carriers originated from excited state absorption by Ce3+ ions and trapped by Yb3+ ions. The efficiency of energy transfer from Ce3+ to Yb3+ was 22,6% for YF3:Ce(0.05 mol. %):Yb(10.0 mol.%) solid solution with 0,91% down-conversion luminescence external quantum yield.

Effect of the Cu foam pretreatment in the growth and inhibition of copper oxide nanoneedles obtained by thermal oxidation and their evaluation as photocathodes

Cu2O-CuO layers were prepared in situ on copper foam substrates by thermal oxidation at 400 °C in air using different pretreatments with acetone, HCl and NaOH. The effect of the pretreatment in the shape and physicochemical properties of the Cu2O-CuO layers, as well as in the growth or inhibition of the copper oxide nanostructures was studied, and a growth mechanism is proposed. It was found that the pretreatment modulates the nucleation and growth of the copper oxide nanostructures, being the process with NaOH the most suitable to promote the formation of well-defined nanoneedles, while in the case of the samples pretreated with acetone and HCl, copper oxide layers with irregular shape microstructures were obtained. The composition, structural, morphological and optical properties of the copper oxide structures were determined by X-ray diffraction, scanning electron microscopy, UV-vis diffuse reflectance and photoluminescence spectroscopy. The results showed that in all cases, the presence of both copper oxides, Cu2O and CuO was observed, with an optical band gap of 1.0 and 1.3 eV. The copper oxide structures exhibited photoluminescence emission centered at 551 nm, related to the recombination of the electron-hole pairs in the samples. The materials prepared with a NaOH pretreatment showed the lower emission and recombination rate.Moreover, the 3D Cu-Cu2O-CuO based materials were evaluated as photocathodes in a 0.5 M Na2SO4 solution and under Xe lamp illumination. The photoelectrode where 1D nanostructures were grown, exhibited the lower resistance to the charge transference in the Nyquist plots, the highest current density in the linear voltammetry and the highest photoresponse in the on-off light experiments. The improved electrical and physicochemical properties of the samples pretreated with NaOH was related to the particular 1D nanoneedle morphology, which promoted higher conductivity and photoresponse, lower resistance to the charge transference and lower recombination of free charge carriers, demonstrating the potential use of these electrodes for photoelectrochemical applications. Finally, this work proved that it is possible to grow well-defined and highly crystalline CuO nanoneedles on copper foam porous substrates through a simple, fast and clean method.

Highly efficient nanocrystalline CsxMA1−xPbBrx perovskite layers for white light generation

Perovskite light converting layers optimization for cost-efficient white light emitting diodes (LED) was demonstrated. High excitation independent internal quantum efficiency (IQE) of 80% and weakly excitation dependent PL spectra suitable for white light generation were obtained in the mixed cation CsxMA1−xPbBr3 perovskite nanocrystal layers with optimal x = 0.3 being determined by effective surface passivation and phase mixing as revealed by x-ray diffraction. Enhancement of the PL homogeneity and the external quantum efficiency (EQE) were secured when using 2,2′,2″-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole (TPBi) additive in the layer preparation process. Excitation dependent PL intensity, decay time, and IQE revealed that the high emission efficiency of the layers originates from a dominant radiative localized exciton recombination (130 ns) weakly influenced by the nonradiative free carrier recombination (750 ns). Warm and cool white LEDs with correlated color temperature 3000 K and 5600 K, and color rendering index 82 and 74, respectively, were realized by using the optimized perovskite layers, poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) red emitter and a blue LED.

Determination of radiation hardness of silicon diodes

In this paper, we describe an experiment aimed to measure the physical observables, which can be used for the assessment of the radiation hardness of commercially available silicon photo diodes commonly used as nuclear detectors in particle accelerator laboratories. The experiment adopted the methodology developed during the International Atomic Energy Agency (IAEA) Coordinated Research Project (CRP No. F11016) “Utilization of Ion Accelerators for Studying and Modelling Ion Induced Radiation Defects in Semiconductors and Insulators”.This methodology is based on the selective irradiation of micrometer-sized regions with different fluences of MeV ions using an ion microbeam and on the measurement of the charge collection efficiency (CCE) degradation by Ion Beam Induced Charge (IBIC) microscopy performed in full depletion condition, using different probing ions.The IBIC results are analyzed through a theoretical approach based on the Shockley-Read-Hall model for the free carrier recombination in the presence of ion-induced deep traps. This interpretative model allows the evaluation of the material radiation hardness in terms of recombination parameters for both electrons and holes.The device under study in this experiment was a commercial p-i-n photodiode, which was initially characterized by i) standard electronic characterization techniques to determine its doping and ii) the Angle-Resolved IBIC to evaluate its effective entrance window. Nine regions of (100 × 100) µm2 were irradiated with 11.25 MeV He ions up to a maximum fluence of 3·1012 ions/cm2. The CCE degradation was measured by the IBIC technique using 11.25 MeV He and 1.4 MeV He as probing ions.The model presented here proved to be effective for fitting the experimental data. The fitting parameters correspond to the recombination coefficients, which are the key parameters for the characterization of the effects of radiation damage in semiconductors.

Effect of Light and Voltage on Electrochemical Impedance Spectroscopy of Perovskite Solar Cells: An Empirical Approach Based on Modified Randles Circuit

The voltage (V) and light-power (P) dependences of electrochemical impedance spectroscopy (EIS) curves of perovskite solar cells (PSCs) are investigated over the entire (V, P) space rather than limited to short-circuit or open-circuit conditions. For the first time, we report that under fixed V and increasing P, the EIS curves show complicated behaviors. Employing a modified Randles circuit, we successfully fitted the data using three newly proposed empirical equations. From the behaviors of the fitting parameters in the (V, P) space, we conclude that the dynamics inside PSCs consists of two parts: (1) the migration of mobile ions/vacancies and their capturing of the free carriers at the perovskite layer boundary and (2) a classical diode involving the migration and recombination of free carriers. Our calculated diffusivity and mobility agree with those of previous reports. We further found that the mobile ion density inside the perovskite layer is proportional to light power, which could explain the inve...

Photocatalytic activity of ZnO nanopowders: The role of production techniques in the formation of structural defects

The effect of the type of structural defect in zinc oxide on its photocatalytic properties was studied for phenol photodegradation under UV-irradiation. It was shown that the use of different types of precursors (zinc oxalate and zinc hydroxide) for the production of zinc oxide leads to the formation of a material with the same phase composition and equal energy of the forbidden band, but different photocatalytic activities. Simultaneously, the peculiarities of the luminescence and electron spin resonance spectra indicate the formation of different types of defects in the structure of the material, namely, oxygen vacancies (Vo) in the anionic and zinc vacancies (VZn) in the cationic sublattices of zinc oxide synthesised from the zinc oxalate and hydroxide, respectively. Also, the different characteristics of the luminescence decays reveal the different recombination paths for the free charge carriers in the systems synthesised from the different precursors. The different times of the luminescence decay also confirmed the different methods of recombination of free charge carriers in systems synthesised from different precursors. It was shown that the appearance of defects in the cationic sublattice leads to a decrease in the photocatalytic activity of the material relative to phenol degradation.

On using photoconductivity decay to determine Si free carrier recombination lifetime: possibilities and challenges

Free carrier recombination lifetime (τ) in Si single crystals is a parameter, that characterizes the quality of the material. The primary procedure for τ measurement in ingots is photoconductivity decay (PCD) analyses, including contactless μ-PCD and eddy current methods. One of the main problem in this methods is a surface recombination influence on the PCD curve. It is usually assumed that in ingots the influence of surface recombination can be neglected. Well-known dependence of effective lifetime (τeff) on τ, surface recombination velocity (S) and sample thickness (d) describes the maximum value of a lifetime. Based on the numerical calculations of the one dimensional continuity equation we have shown that if the sample thickness exceed 5LD, the maximum lifetime cannot be achieved before PCD signal decline to 5% of maximum, so the question arises what is the interval of the decay curve where the experimental value of the τeff has to be determined. We used the range from 45-5% of the maximum PCD signal following the recommended SEMI MF 1575 standard (τSEMI). The dependencies of τSEMI on τ, d and S were calculated for different versions of PCD measurements and measurement equipment. The recommended thickness for lifetime measurements of non-passivated samples is in the range (1-5)LD. The hardware capabilities of contactless methods are compared.

Photon-induced carrier recombination in the nonlayered-structured hybrid organic-inorganic perovskite nano-sheets.

The hybrid organic-inorganic perovskites (HOIPs) have attracted much attention recently due to their preeminent efficiency in solar cells. According to the difference on the crystalline structure, the HOIPs could be classified into layered and non-layered perovskites. Very recently, it has been realized that the non-layered HOIPs with common-vertex structure possess even better opto-electrical performance. Yet the carrier recombination mechanism in perovskite remains not very clear, and a clear understanding of this mechanism is essential to pinpoint the working mechanism of photovoltaic and electroluminescent materials. Here we report the optical studies on the hybrid perovskite crystalline nano-sheet of CH3NH3PbBr3 with common-vertex structure. It is shown that the non-layered perovskite crystalline nanosheets possess the exciton binding energy about two orders of magnitude smaller than that of the layered perovskite and the colloidal nanoplates, which is beneficial for the designing of the high-efficiency photovoltaic devices. By measuring the temperature-dependent photoluminescence (PL) spectra, the excitation-power-variant PL spectra, and the time-resolved PL spectra, we identify that both the free-carrier and the localized exciton recombination channels may coexist in the crystallites. Further, for the thin crystallite (∼60 nm), the free-carrier recombination channel dominates; whereas when the thickness increases beyond 200 nm, the localized exciton recombination channel plays the major role. We suggest these results are helpful to improve further the photovoltaic and electroluminescent performances of perovskite devices.

Light Emission by Nanoporous GaN Produced by a Top-Down, Nonlithographical Nanopatterning

Temperature-dependent photoluminescence (PL) spectroscopy is carried out to probe radiative recombination and related light emission processes in two-dimensional periodic close-packed nanopore arrays in gallium nitride (np-GaN). The arrays were produced by nonlithographic nanopatterning of wurtzite GaN followed by a dry etching. The results of Raman spectroscopy point to a small relaxation of the compressive stress of ~0.24 GPa in nanoporous vs. bulk GaN. At ~300 K, the PL emission is induced by excitons and not free-carrier interband radiative recombinations. An evolution of the emission spectra with T is confirmed to be mainly a result of a decay of nonexcitonic PL emission and less of spectral shifts of the underlying PL bands. A switching of excitonic PL regime observed experimentally was analyzed within the exciton recombination-generation framework. The study provides new insights into the behaviors and physical mechanisms regulating light emission processes in np-GaN, critical to the development of nano-opto-electronic devices based on mesoscopic GaN.

Interface engineering of G-PEDOT: PSS hole transport layer via interlayer chemical functionalization for enhanced efficiency of large-area hybrid solar cells and their charge transport investigation

In this study, in order to minimize the recombination current of free charge carriers in a large-area organic-inorganic hybrid solar cell (O-IHSCs), we improved the electrical conductivity of a graphene (G) and poly(3,4-ethylenedioxy thiophene)–poly(styrenesulfonate) (G-PEDOT:PSS) hole transport layer (HTL) by introducing various concentrations of synthesized graphene (G) into poly(3,4-ethylene dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). The electrical conductivity of G-PEDOT:PSS was enhanced to 932781.17 S m−1 via the addition of 2 mg/mL of G to PEDOT:PSS. The O-IHSCs fabricated with the highly conductive G-PEDOT:PSS composite as HTL enhanced the power conversion efficiency (PCE) to 3.90%, a 70% increase compared to O-IHSCs fabricated with pristine PEDOT:PSS HTL. However, the accumulation of G at a higher concentration (2.5 mg/mL) degrades the performance of the solar cell, which generated further defects or film aggregation, interfering with the fast transport of free charge carriers toward their respective electrodes. The G-PEDOT:PSS composite contained various types of functionalization via interfacial reaction between the G and PEDOT:PSS based on Raman and X-ray photoelectron spectroscopy studies. These chemical functionalizations provide an additional mechanism of charge transport via bridges enhancing the carrier mobility and suppression of recombination of free charge carriers, resulting in significant improvement in photovoltaic performance of the O-IHSCs.

Photophysical Pathways in Highly Sensitive Cs2 AgBiBr6 Double-Perovskite Single-Crystal X-Ray Detectors.

The sensitive detection of X-rays embodies an important research area, being motivated by a common desire to minimize the radiation doses required for detection. Among metal halide perovskites, the double-perovskite Cs2 AgBiBr6 system has emerged as a promising candidate for the detection of X-rays, capable of high X-ray stability and sensitivity (105 μC Gy-1 cm-2 ). Herein, the important photophysical pathways in single-crystal Cs2 AgBiBr6 are detailed at both room (RT) and liquid-nitrogen (LN2 T) temperatures, with emphasis made toward understanding the carrier dynamics that influence X-ray sensitivity. This study draws upon several optical probes and an RT excitation model is developed which is far from optimal, being plagued by a large trap density and fast free-carrier recombination pathways. Substantially improved operating conditions are revealed at 77 K, with a long fundamental carrier lifetime (>1.5 µs) and a marked depopulation of parasitic recombination pathways. The temperature dependence of a single-crystal Cs2 AgBiBr6 X-ray detecting device is characterized and a strong and monotonic enhancement to the X-ray sensitivity upon cooling is demonstrated, moving from 316 μC Gy-1 cm-2 at RT to 988 μC Gy-1 cm-2 near LN2 T. It is concluded that even modest cooling-via a Peltier device-will facilitate a substantial enhancement in device performance, ultimately lowering the radiation doses required.

Hole Transport in Low-Donor-Content Organic Solar Cells

Organic solar cells with an electron donor diluted in a fullerene matrix have a reduced density of donor-fullerene contacts, resulting in decreased free-carrier recombination and increased open-circuit voltages. However, the low donor concentration prevents the formation of percolation pathways for holes. Notwithstanding, high (>75%) external quantum efficiencies can be reached, suggesting an effective hole-transport mechanism. Here, we perform a systematic study of the hole mobilities of 18 donors, diluted at ∼6 mol % in C60, with varying frontier energy level offsets and relaxation energies. We find that hole transport between isolated donor molecules occurs by long-range tunneling through several fullerene molecules, with the hole mobilities being correlated to the relaxation energy of the donor. The transport mechanism presented in this study is of general relevance to bulk heterojunction organic solar cells where mixed phases of fullerene containing a small fraction of a donor material or vice versa are present as well.