Recombination Lifetime Of Charge Carriers Research Articles

Extending the Charge Carrier Recombination Lifetime by Octahedral Rotations in Ruddlesden-Popper Ba3Zr2S7 Perovskites.

Intentional distortions of [BX6] octahedra within perovskite structures have been recognized as a potent strategy for precise band gap adjustments and optimization of their photovoltaic properties, yet information regarding charge carrier dynamics linked to octahedral distortion under ambient conditions for chalcogenide perovskites remains limited. In this study, we utilize ab initio nonadiabatic molecular dynamics to explore the dynamics of photogenerated carriers in a representative two-dimensional Ba3Zr2S7 material in the Ruddlesden-Popper phase. The theoretical results highlight the influence of octahedral rotation on the materials' stability and carrier recombination lifetime of the system. Specifically, the octahedrally rotating P42/mnm phase exhibits a prolonged nonradiative carrier recombination lifetime attributed to the stabilized electron-phonon coupling. These findings offer valuable insights into the fundamental physical characteristics of imposed octahedral distortion and its potential for optimizing the optoelectronic performance of 2D Ruddlesden-Popper Ba-Zr-S chalcogenide materials.

Strain-Induced Modification of Photoluminescence in Quasi-2D Perovskite Thin Films

Quasi-2D perovskites are recognized as a promising material for lightweight photovoltaic applications. For these applications, it is critical to understand how mechanical stresses affect the perovskite’s optoelectronic properties. Here, we discuss how applied stress impacts the photoluminescence spectrum and lifetime of quasi-2D perovskite thin films and how these effects depend on interlayer cation choice (butylammonium or phenethylammonium) and film processing (hot-casting versus postdeposition annealing). We find that compressive stress induces an enhancement and spectral redshift (>10 meV per percent strain) in the emission from bulk-like grains in the film. Moreover, the charge carrier recombination lifetime is greatly enhanced (up to a factor of 14 with only 1% strain). We show that the phenethylammonium-based perovskites are more robust against strain-induced modifications compared to those containing the butylammonium cation. These results highlight how strongly strain can modify the optoelectronic properties of quasi-2D perovskite thin films and how cation choice and film processing can have important practical implications for developing lightweight photovoltaics.

Perovskite solar cells with embedded homojunction via nonuniform metal ion doping

A long photoluminescence decay lifetime has been regarded as a generic indication of long charge carrier recombination lifetime in semiconductors such as metal halide perovskites (MHPs), which have shown tremendous success in solar cells. Here, we report that MHP polycrystalline films with extrinsic metal ions have a very long charge recombination lifetime, but a much shorter photoluminescence decay lifetime, and this huge difference can be explained by a model of lateral homojunction within each individual grain. The lateral homojunction is formed due to the doping along grain boundaries by metal ions, and then verified by nanoscale potential mapping and transient photo-response mapping. The built-in electric field within each grain reduces the recombination of free charge carriers within the perovskite grain and along grain boundaries, while the free electrons and holes are collected to cathode and anode through the grain boundaries and grain interiors, respectively. Then, the efficiencies of MHP polycrystalline solar cells are increased.

Kinetic Monte Carlo Simulation of Perovskite Solar Cells to Probe Film Coverage and Thickness

Perovskite solar cells (PSCs) have received considerable attention in recent years due to their low processing cost and high‐power conversion efficiency. However, the mechanisms of PSCs are not fully understood. A model based on a probabilistic and statistical approach needs to be developed to simulate, optimize, and predict the photovoltaic (PV) performance of PSC. Herein, the 3D model based on the kinetic Monte Carlo (KMC) approach is developed to simulate 3D morphology of perovskite‐based solar cells and predict their PV performances and charge dynamics. The developed 3D model incorporates the temporal and physical behavior of perovskites, such as charge generation, transport, and recombination. The KMC simulation results show that pin holes‐free perovskite films with a homogenous 400 nm thick perovskite capping layer achieve the highest power conversion efficiency of 20.85%. However, the shortest apparent charge transport time (τt) and the longest apparent charge carrier recombination lifetime (τr) are found for the champion device. PV performance from the fabricated device is used to validate this simulation model. This model can provide a significant conceptual advance in identifying bottlenecks and guiding novel device designs to further improve the performance of perovskite PVs.

Inclusion of low cost activated carbon for improving hydrogen production performance of TiO2 nanoparticles under natural solar light irradiation

Photocatalytic studies are primarily focused on the low cost and sustainable materials with suitable bandgap and high surface area. The ultra-fast electron-hole pair recombination and limited light absorptions affect the efficiency of photocatalyst in an adverse manner, which can be unravelled by choosing an efficient combination of photocatalysts and suitable co-catalyst/support materials. The present work explores the combination of low-cost and high potential activated carbon and TiO2 as a nanocomposite, prepared through a one-pot hydrothermal process for hydrogen production under natural solar light irradiation. Among the synthesized photocatalysts, the one calcined at 400 °C for 2 h was found to be the best catalyst, which exhibited 3.5 times higher hydrogen production rate than the pristine TiO2 while tested with water containing 5 vol.% glycerol. Importantly, the optimized nanocomposite was also tested for hydrogen production from simulated seawater under same conditions and it showed a hydrogen production rate of 20,383 μmol g−1 h−1, which is 2.4 times higher than the glycerol water solution. The enhanced hydrogen production rate is due to the reduced bandgap of AC-TiO2 nanocomposite which offered more light absorption in the visible region compared to the pristine TiO2. The XRD, Raman spectroscopy, TEM, and PL analysis were also examined to investigate the crystallinity, purity, morphology, and charge carrier recombination life time of the synthesized catalysts.

Time-Resolved Photoluminescence in Heterostructures with InGaAs:Cr/GaAs Quantum Wells

The results of studies of the time-resolved photoluminescence in semiconductor heterostructures containing two noninteracting InGaAs quantum wells in a GaAs matrix are reported. One of the quantum wells was undoped, and the other was uniformly doped with chromium atoms (InGaAs:Cr). It was shown that the introduction of Cr had a profound effect on the recombination lifetime of charge carriers in quantum wells. The change in the photoluminescence intensity after excitation cannot be described by a monoexponential decay function, which is attributed to a change in the built-in electric field of the surface barrier in the quantum wells because of screening by photoexcited charge carriers.

Overcoming the Limitations of Transient Photovoltage Measurements for Studying Recombination in Organic Solar Cells

Transient photovoltage (TPV) measurements are frequently used to study recombination processes in thin‐film solar cells by probing the decay of a small optically induced voltage perturbation to infer the charge carrier dynamics of devices at open circuit. However, the validity of this method to probe organic semiconductors has recently come into doubt due to large discrepancies in the reported carrier lifetime values for the same systems and the reporting of unrealistic reaction order values. Herein, the validity of TPV to extract reliable charge carrier lifetimes in thin‐film solar cells is explored through the use of time‐dependent drift‐diffusion simulations and measurements. It is found that in low‐mobility materials, TPV serves primarily as a probe of charge carrier redistribution in the bulk rather than bulk recombination dynamics and that the extracted time constant is highly mobility dependent. To address this shortcoming, transient photocharge, a new technique to measure the charge carrier density during photovoltage decay, is introduced and applied to study the recombination dynamics in a series of (fullerene and nonfullerene) organic solar cell systems. It is shown that using this technique the charge carrier recombination lifetime in the active layer is more accurately determined.

Effect of Pelargonidin on carrier recombination lifetime of CTS quantum dots

In the current work, we have prepared stacks of Copper tin sulphide (CTS)/Pelargonidin core/shell quantum dots on soda lime substrates by SILAR technique. Copper tin sulphide quantum dots were of tetragonal phase matching well with JCPDS 089-4714. The absorption coefficient of the core/shell quantum dots increased while the photoluminescence intensity decreased with increasing shell thickness. Decrease of charge carrier confinement energy due to leakage of wavefunction into the shell is the cause of fall in PL intensity. Three transitions were found in all the samples corresponding to 1Se-1Sh, 1Pe-1Ph and 1De-1Dh transitions using the non-interacting particle model. Charge carrier recombination lifetime of P state was found to be in the order of 100 ns, with lifetime increasing with shell thickness, probably due to the P state being the most occupied at room temperature and decrease of surface trap sites with increasing shell thickness. Owing to the high lifetime of CTS/Pelargonidin core/shell quantum dots, these can be used effectively in solar cells where recombination time is a deciding factor for efficiency and also in bioimaging of living cells and tissues.

Elucidating the Origins of Subgap Tail States and Open‐Circuit Voltage in Methylammonium Lead Triiodide Perovskite Solar Cells

AbstractRecombination via subgap trap states is considered a limiting factor in the development of organometal halide perovskite solar cells. Here, the impact of active layer crystallinity on the accumulated charge and open‐circuit voltage (Voc) in solar cells based on methylammonium lead triiodide (CH3NH3PbI3, MAPI) is demonstrated. It is shown that MAPI crystallinity can be systematically tailored by modulating the stoichiometry of the precursor mix, where small quantities of excess methylammonium iodide (MAI) improve crystallinity, increasing device Voc by ≈200 mV. Using in situ differential charging and transient photovoltage measurements, charge density and charge carrier recombination lifetime are determined under operational conditions. Increased Voc is correlated to improved active layer crystallinity and a reduction in the density of trap states in MAPI. Photoluminescence spectroscopy shows that an increase in trap state density correlates with faster carrier trapping and more nonradiative recombination pathways. Fundamental insights into the origin of Voc in perovskite photovoltaics are provided and it is demonstrated why highly crystalline perovskite films are paramount for high‐performance devices.

Solar light triggered catalytic performance of graphene-CuO nanocomposite for waste water treatment

Abstract In this work, the effect of a new nanocomposite material on the photodegradation of methylene blue is demonstrated. The nanocomposite is synthesized by the combination of cupric oxide (CuO) with graphene nanoplatelets (GNPs) and characterized by XRD, FE-SEM, FTIR, Raman, and UV–vis spectroscopy. Interestingly, the inclusion of GNPs with CuO enhances the photocatalytic properties of nanocomposite without tweaking inherent properties such as the conductive nature of GNPs. Within 80 min of exposure to the solar light, a 99.44% photodegradation of methylene blue is achieved, which is much better than using CuO alone, for which the degradation is only 75%. The much-improved photocatalytic performance of the nanocomposite is a consequence of low bandgap energy of GCuO as well as the conducting nature of GNPs, that causes fast photo excitation of electrons and lengthens the recombination lifetime of charge carriers respectively.

A Simple Approach Low-Temperature Solution Process for Preparation of Bismuth-Doped ZnO Nanorods and Its Application in Hybrid Solar Cells

A simple low-temperature solution processed bismuth-doped ZnO nanorods (NRs) and poly(3-hexylthiophene) (P3HT) were used as electron acceptor and donor, respectively, in a hybrid inorganic–organic photovoltaic system. Controlling Bi precursor concentration via solution processing (hydrothermal method) plays an important role in altering the morphology, structure, and intrinsic defects of ZnO NRs. Interstitial doping of Bi–Bi2O3 into ZnO (BiZO) NRs results in simultaneous improvement of the open circuit voltage and short circuit current density primarily due to prolonged charge carrier recombination lifetime, increased donor–acceptor interfacial areas with efficient exciton dissociation, and charge carrier mobility. As a result, the power conversion efficiency of the 2 wt % BiZO NRs-P3HT device was significantly enhanced by 55% compared with that of the pristine device. Overall, our study highlighted the immense potential of BiZO NRs as an excellent electron acceptor for fabrication of hybrid optoelectroni...

Interplay of nanoscale domain purity and size on charge transport and recombination dynamics in polymer solar cells

Charge transport and bimolecular recombination dynamics were correlated with nanomorphology in polymer solar cells. The morphology of poly(diketopyrrolopyrrole-terthiophene) (PDPP3T) and phenyl-C61-butyric acid methyl ester (PC60BM) blend films was modified using different solvent additives namely 1-chloronaphthalene (CN), 1,8-diiodooctane (DIO) and 1,8-octanedithiol (ODT) and their role on steady state and transient optoelectronic properties was investigated. The energy filtered transmission electron microscopy (EFTEM) images showed that additives (e.g. CN and DIO) improved the domain purity which leads to significantly higher short circuit current densities (Jsc). However when the cells were processed with the ODT additive, the fill factor (FF) and open circuit voltage (Voc) decreased dramatically. Films processed with the ODT additive showed a smaller domain size but were more connected compared to films processed using CN and DIO additives. Transient photocurrent analysis indicates faster charge collection in the case of CN and DIO processed solar cells and the slowest charge collection in ODT processed solar cells. Interestingly devices processed with the ODT additive also showed the longest charge carrier recombination lifetime and lowest bimolecular recombination coefficient. This is attributed to the smaller donor domains that are connected with each other to provide a more interconnected and efficient charge transport matrix but longer pathways in ODT films. Such a matrix helped the charge to escape from the donor-acceptor interfaces and thus reduces the bimolecular recombination, while the longer pathway increases the charge collection time. Further insight is provided into the selection of processing conditions to achieve an ideal active layer morphology consisting of domains with higher polymer purity and optimal size that lead to higher Jsc and FF.

Separation of local bulk and surface recombination in crystalline silicon from luminescence reabsorption

AbstractSpontaneous photoemission of crystalline silicon provides information on excess charge carrier density and thereby on electronic properties such as charge carrier recombination lifetime and series resistance. This paper is dedicated to separating bulk recombination from surface recombination in silicon solar cells and wafers by exploiting reabsorption of spontaneously emitted photons. The approach is based on a comparison between luminescence images acquired with different optical short pass filters and a comprehensive mathematical model. An algorithm to separate both front and back surface recombination velocities and minority carrier diffusion length from photoluminescence (PL) images on silicon wafers is introduced. This algorithm can likewise be used to simultaneously determine back surface recombination velocity and minority carrier diffusion length in the base of a standard crystalline silicon solar cell from electroluminescence (EL) images. The proposed method is successfully tested experimentally. Copyright © 2009 John Wiley & Sons, Ltd.

Relation between physical structure and electrical properties of diamond-like carbon thin films

Diamond-like carbon (DLC) belongs to very interesting materials used for a number of practical applications. Its properties strongly depend on the method of deposition and on the deposition parameters. In particular, the electrical properties of DLC films obtained by RF PCVD discharge depend substantially on the preparation conditions. Influence of deposition conditions on the physical structure and chemical content is discussed. Both the drift mobility of charge carriers and the electrical conductivity of DLC films strongly depend on deposition conditions. The electrical conductivity results are explained in terms of hopping mechanism. At the temperatures below 260 K, the hopping among states in a narrow band of states dominates; at the higher temperatures, the band-to-band transitions are suggested to occur. A model of the forbidden gap explaining the observed electrical properties of DLC films is proposed. Experimental results concerning the electrical conduction of DLC/Si heterostructures are also presented and discussed. The difference in the properties of DLC/n-Si and DLC/p-Si structures may suggest that the electron transport via localised states dominate in the investigated DLC films. Recombination lifetime of charge carriers in DLC films is measured using both dielectric spectroscopy and detection of decay of photocurrents. The lifetime is found to be of the order of 0.3–0.4 ms, which is quite long in comparison with classical doped silicon semiconductors.

Recombination lifetime of charge carriers in DLC thin films

Decay of photocurrent in diamond-like carbon (DLC) thin films and broadband dielectric measurements of rectifying n-Si/DLC junctions were used to find the recombination lifetime of charge carriers in DLC films. The recombination lifetime found from the two methods was between 10−4 and 10−3 s.

The effect of impurities on the silicon detector's radiation hardness

Charged carriers recombination and generation lifetimes as well as DLTS measurements of neutron irradiated silicon detectors have been performed. Radiation-induced changes of the coefficient related to the charge carrier recombination lifetime have been observed to be independent of doping level, type conductivity and original silicon manufacturers. Suggestions about how to avoid the conductivity inversion in n-type silicon detectors at high irradiation levels are also given.

Time-resolved microwave conductivity. Part 1.—TiO2photoreactivity and size quantization

Charge-carrier recombination dynamics after laser excitation are investigated by time-resolved microwave conductivity (TRMC) measurements of quantum-sized (Q-) TiO_2, Fe^(III)-doped Q-TiO_2, ZnO and CdS, and several commercial bulk-sized TiO2 samples. After pulsed laser excitation of charge carriers, holes that escape recombination react with sorbed trans-decalin within ns while the measured conductivity signal is due to conduction-band electrons remaining in the semiconductor lattice. The charge-carrier recombination lifetime and the interfacial electron-transfer rate constant that are derived from the TRMC measurements correlate with the CW photo-oxidation quantum efficiency obtained for aqueous chloroform in the presence of TiO_2. The quantum efficiencies are 0. 4 % for Q-TiO_2, 1. 6 % for Degussa P25, and 2. 0 % for Fe^(III)-doped Q-TiO_2. The lower quantum efficiencies for Q-TiO_2 are consistent with the relative interfacial electron-transfer rates observed by TRMC for Q-TiO_2 and Degussa P25. The increased quantum efficiencies of Fe^(III)-doped Q-TiO_2 and the observed TRMC decays are consistent with a mechanism involving fast trapping of valence-band holes as Fe^(IV) and inhibition of charge-order recombination.