Carrier Lifetime Research Articles

Enhanced surface passivation of GaAs nanostructures via an optimized SiO2 sol-gel shell growth

Gallium arsenide (GaAs) is an important compound semiconductor for optoelectronic devices. However, the fast nonradiative recombination velocity of GaAs requires surface passivation for the nano-optoelectronic and nanophotonic applications. Despite strides have been made in GaAs surface passivation, chemical treatments like sulfidation and nitridation suffer from reproducibility and stability issues, thus requiring protective layer encapsulation. Traditional vacuum based film passivation exposes samples to high temperature plasma, causing surface damage, and is expensive. Here, we demonstrate an effective passivation method via sol-gel growth of SiO2 shell on both dry etched GaAs nanopillar and bottom-up grown GaAs nanowires at room temperature with uniform coating and controlled thickness from sub-10 nm to around 100 nm, showing advantages in low cost as well as suitability for different types of nano morphology. X-ray photoelectron spectroscopy analysis confirms that the SiO2 sol-gel method with the addition of (NH4)2S can significantly remove the native oxides on the surface. Under the optimized SiO2 coating procedure, the emission intensity of dry etched GaAs nanopillar with a diameter of 0.5 μm is enhanced about 29.6 times and the decay lifetime of the minority carriers with a diameter of 1.8 μm increased from 43.6 to 477.5 ps. Thus, the easy fabrication, broad suitability, as well as effective passivation effect turn sol-gel growth of SiO2 into a cost-effective surface passivation approach for III–V nanostructures.

Thermoplasmonic TiN boosts photocatalysis in covalent-organic frameworks

Covalent-organic frameworks (COFs) hold promise for photocatalysis, but their efficiency is hindered by strong excitonic effects that limit charge separation and transfer. Here, we demonstrate a novel strategy to overcome this challenge by integrating thermoplasmonic TiN nanoparticles with a β-ketoenamine-linked COFs (TFP-BD). The enhanced local electromagnetic field generated by TiN promotes the separation of electron-hole pairs in TFP-BD. This enhancement is evidenced by a doubled carrier lifetime (∼0.99 ns) and photocurrent density (1.67 μA cm⁻²) in the 3% TiN@TFP-BD composite compared to pristine TFP-BD. Additionally, thermoplasmonic heating in 3% TiN@TFP-BD (69.2 °C) surpasses that of TFP-BD (62.15 °C), accelerating mass transport during the photocatalytic process. This synergistic effect leads to a remarkable hydrogen evolution rate (HER) of 37.59 mmol g⁻¹ h⁻¹, an 8.5-fold improvement over the pristine COFs. Our work highlights the potential of thermoplasmonic enhancement for solar fuel production and establishes a new approach to mitigate excitonic limitations in COF-based photocatalysts.

Transient-State Self-Bipolarized Organic Frameworks of Single Aromatic Units for Natural Sunlight-Driven Photosynthesis of H2O2.

Constructing π-conjugated polymer structures through covalent bonds dominates the design of organic framework photocatalysts, which significantly depends on the selection of multiple donor-acceptor building blocks to narrow the optical gap and increase the lifetimes of charge carriers. In this work, self-bipolarized organic frameworks of single aromatic units are demonstrated as novel broad-spectrum-responsive photocatalysts for H2O2 production. The preparation of such photocatalysts is only to fix the aromatic units (such as 1,3,5-triphenylbenzene) with alkane linkers in 3D space. Self-bipolarized aromatic units can drive the H2O2 production from H2O and O2 under natural sunlight, wide pH ranges (3.0-10.0) and natural water sources. Moreover, it can be extended to catalyze the oxidative coupling of amines. Experimental and theoretical investigation demonstrate that such a strategy obeys the mechanism of through-space π-conjugation, where the closely face-to-face overlapped aromatic rings permit the electron and energy transfer through the large-area delocalization of the electron cloud under visible light irradiation. This work introduces a novel design concept for the development of organic photocatalysts, which will break the restriction of conventional through-band π-conjugation structure and will open a new way in the synthesis of organic photocatalysts.

On-Substrate Fabrication of CsPbBr3 Single-Crystal Microstructures via Nanoparticle Self-Assembly-Assisted Low-Temperature Sintering.

The growth of all-inorganic perovskite single-crystal microstructures on substrates is a promising approach for constructing photonic and electronic microdevices. However, current preparation methods typically involve direct control of ions or atoms, which often depends on specific lattice-matched substrates for epitaxial growth and other stringent conditions that limit the mild preparation and flexibility of device integration. Herein, we present the on-substrate fabrication of CsPbBr3 single-crystal microstructures obtained via a nanoparticle self-assembly assisted low-temperature sintering (NSALS) method. Sintering guided by self-assembled atomically oriented superlattice embryos facilitated the formation of single-crystal microstructures under mild conditions without substrate dependence. The as-prepared on-substrate microstructures exhibited a consistent out-of-plane orientation with a carrier lifetime of up to 82.7 ns. Photodetectors fabricated by using these microstructures exhibited an excellent photoresponse of 9.15 A/W, and the dynamic optical response had a relative standard deviation as low as 0.1831%. The discrete photosensor microarray chip with 174000 pixels in a 100 mm2 area showed a response difference of less than 6%. This method of nanoscale particle-controlled single crystal growth on a substrate offers a perspective for mild-condition preparation and in situ repair of crystals of various types. This advancement can propel the flexible integration and widespread application of perovskite devices.

Effect of the column design and fabrication method on the reverse recovery characteristics of 1.2 kV SiC-superjunction-MOSFETs

We investigated the effects of the column length and fabrication method of the superjunction (SJ) structure on the reverse recovery characteristics of 1.2 kV silicon carbide (SiC) SJ metal–oxide–semiconductor field-effect transistors (MOSFETs). The voltage slope (dV/dt) and reverse recovery current slope (dir/dt) values during the reverse recovery of the SJ-MOSFETs with the SJ structure formed by Aluminum (Al) ion implantation were comparable to the values of MOSFETs without SJ structure at room temperature (RT), but they increased markedly with temperature. This is caused by the reduced p-column carrier concentration at RT during short recovery transient time due to the deep hole trap (DI center) introduced by ion implantation. At 175 °C, a softer reverse recovery waveform was obtained by shortening the SJ columns. Besides, Al ion implanted SJ-MOSFETs exhibited a weaker current dependence of the reverse recovery charge Qrr at 175 °C indicating a lower carrier injection level compared to the SJ-MOSFETs with the SJ structure formed by trench formation and epitaxial growth. This is caused by the short carrier lifetime of the SJ columns due to the Al ion implantation defects. The short and Al ion implanted SJ columns are effective for soft reverse recovery characteristics in SiC-SJ-MOSFETs.

Two Completely Non‐Fused Ring Acceptors Working in an Alloy‐Like Model for Efficient and Stable Organic Solar Cells

AbstractSimple chemical structure and simplified synthesis process of active layer materials are critical for advancing the practical application of organic solar cells. Herin, two completely non‐fused ring electron acceptors BTZT‐2Cl and BTZT‐4Cl are developed. BTZT‐4Cl exhibits an enhanced absorption band, increases electrostatic potential differences with D18, and improves crystallinity and molecular packing properties. Consequently, the binary device based on BTZT‐4Cl displays a markedly improved efficiency of 14.12%, compared to the BTZT‐2Cl‐based device, which only achieves a moderate efficiency of 11.25%. More importantly, an alloy‐like structure can be formed by incorporating a small amount of high miscibility and compatibility BTZT‐2Cl. The ternary blend exhibits more compact molecular packing, efficient exciton dissociation, and an extended charge carrier lifetime due to the formation of an alloy‐like structure. The ternary device achieves a decent efficiency of 15.41% with superior thermal stability and a high T80 lifetime over 1600 h after being aged at 65 °C. These results establish it as the most efficient among devices based on completely non‐fused ring acceptors with both high efficiency and voltage. This study demonstrates a simple material design strategy and high‐performance device optimization techniques, which are critical for advancing practical applications in the OSC field.

Single-layer GaInS3: Water-splitting photocatalyst with high solar conversion efficiency and long carrier lifetime from first-principles investigation

Two-dimensional (2D) materials with a high solar to hydrogen (STH) conversion are in high demand due to the escalating energy crisis and environmental degradation. The STH efficiency is strongly correlated with the charge carrier separation, which can be improved by the inside electric dipole. Herein, the water-splitting capabilities of single-layer (SL) GaInS3 are explored by employing first-principles calculations. Theoretical outcomes reveal that SL GaInS3 exhibits a large intrinsic electric dipole and has a thermally stable structure at 300 K. SL GaInS3 displays a direct bandgap of 1.83 eV, as well as suitable band edges and abundant visible absorption (105 cm−1) for solar-driven water-splitting. The electronic structure and carrier mobility calculations demonstrate the high separation efficiency of charge carriers, which is strongly affirmed by the quite long carrier lifetime of 8.26 ns. The overpotential analysis of charge carriers suggests that the oxygen generation of SL GaInS3 can be accomplished without a cocatalyst, while hydrogen generation can be completed with the aid of cocatalysts. The STH efficiency of SL GaInS3 reaches up to 16.8%, apparently surpassing the commercial standard (10%). In brief, the high STH and long carrier lifetime promise the possible application of SL GaInS3 for solar water splitting. What's more, this work provides straight evidence that 2D photocatalysts with inside dipoles exhibit a long carrier lifetime.

Enhanced photoelectrochemical water splitting via hydrogenated TiO2 nanotubes modified with Cu/CuO species

This study explores the collective influence of hydrogenation thermal treatment and the decoration of Cu/CuO onto TiO2 nanotubes (TiO2-NTs) concerning photoelectrochemical water splitting. To introduce point defects such as oxygen vacancies and Ti3+, electrochemically anodized Ti foils underwent hydrogenation in a 90 % Ar-10 % H2 atmosphere at 500 °C. Subsequently, Cu/CuO species were deposited on the hydrogenated TiO2-NTs through chemical decoration for varying stirring durations of 15, 30, 45, and 60 min. EPR analysis of the hydrogenated sample unveiled resonances at g-values distinct from the typical 1.997 signal associated with oxygen vacancies in TiO2-NTs, signifying the presence of Ti3+ and VO defects. Analysis via XPS identified Cu and CuO as the primary compositions of the decorated species on the hydrogenated TiO2-NTs. Additionally, SEM/EDS results showcased that longer stirring times, specifically 60 min., caused localized agglomeration of Cu/CuO species and TiO2-NTs breakage. The Cu/CuO-decorated hydrogenated TiO2-NTs exhibited enhanced absorption of visible light, leading to a band gap reduction to 2.09 eV after 45 min of Cu/CuO decoration. Notably, this particular sample demonstrated the highest photocurrent of 3.6 mA/cm2 and a photo-conversion efficiency of 2.48 % at 1.23 V vs. RHE. Overall, the presence of Cu/CuO species prolonged the lifetime of charge carriers, as evidenced by Nyquist plots and Mott-Schottky analysis. A schematic model is suggested based on the modified band gap of hydrogenated TiO2-NTs subsequent to Cu/CuO loading.

Space charge region recombination in highly efficient silicon solar cells

The recombination rate in the space charge region (SCR) of a silicon-based barrier structure with a long Shockley–Reed–Hall lifetime is calculated theoretically by taking into account the concentration gradient of excess electron-hole pairs in the base region. Effects of the SCR lifetime and applied voltage on the structure ideality factor have been analyzed. The ideality factor is significantly reduced by the concentration gradient of electron-hole pairs. This mechanism provides an increase of the effective lifetime compared to the case when it is insignificant, which is realized at sufficiently low pair concentrations. The theoretical results have been shown to be in agreement with experimental data. A method of finding the experimental recombination rate in SCR in highly efficient silicon solar cells (SCs) has been proposed and implemented. It has been shown that at the high excess carrier concentration exceeding 1015 cm–3 the contribution to the SCR recombination velocity from the initial region of SCR that became neutral is significant. From a comparison of theory with experiment, the SCR lifetime and the ratio of the hole to the electron capture cross sections are determined for a number of silicon SCs. The effect of SCR recombination on the key characteristics of highly efficient silicon SCs, such as photoconversion efficiency and open-circuit voltage, has been evaluated. It has been shown that they depend not only on the charge carrier lifetime in SCR, but also on the ratio of hole to electron capture cross sections σp /σn. When σp /σn < 1, this effect is significantly strengthened, while in the opposite case σp /σn > 1 it is weakened. It has been ascertained that in a number of highly efficient silicon SCs, the distribution of the inverse lifetime in SCR is described by the Gaussian one. The effect described in the paper is also significant for silicon diodes with a thin base, p-i-n structures, and for silicon transistors with p-n junctions. In Appendix 2, the need to take into account the lifetime of non-radiative excitonic Auger recombination with participation of deep impurities in silicon is analyzed in detail. It has been shown, in particular, that considering it enables to reconcile the theoretical and experimental dependences for the effective lifetime in the silicon bulk.

Impact of carbon injection in 4H-SiC on defect formation and minority carrier lifetime

Point defects in silicon carbide (SiC) can act as charge carrier traps and have a pronounced impact on material properties such as the mobility and carrier lifetime. Prominent among these traps is the carbon vacancy (VC) with a demonstrated detrimental effect on the minority carrier lifetime in 4H-SiC epitaxial layers. Hence, a variety of methods for VC removal have been proposed. Common to all is that they involve some sort of C-injection into the epi-layer that leads to annihilation of VC with C interstitials (Ci) via the reaction Ci＋ VC→0̸. However, many studies on injection of Ci for removal of VC do not take into account the potential effect of any additional defects that are formed as a result of excess C in the material, including electrically active defects that introduce energy levels in the SiC band gap. Herein, we study the formation and impact of carbon related defects that are introduced in 4H-SiC epi-layers by injection of excess carbon. C-injection is achieved by annealing 4H-SiC epi-layers covered by a graphitized photoresist known as a C-cap at 1250°C for different durations. The resulting appearance of defects in the samples is monitored using deep level transient spectroscopy (DLTS) and minority carrier transient spectroscopy (MCTS) measurements, which reveal the formation of both minority and majority carrier traps as a result of the C-injection. Intriguingly, the injected carbon is also found to interact with a perennially present impurity in 4H-SiC epi-layers and a potentially lifetime limiting defect, namely boron. Furthermore, we monitor the minority carrier lifetime as a function of C-injection time and depth from the surface using cross-sectional time-resolved photoluminescence (TRPL) measurements. Both the defect distributions and the minority carrier lifetime are found to depend strongly on the duration of the C-cap anneal, with a marked depth dependence being present for all samples studied. The moderate temperature C-cap annealing treatment is proposed as a method for enhancing the carrier lifetime in n-type 4H-SiC epi-layers for power device applications.

Auger-limited minority carrier lifetime in GeSn/SiGeSn quantum well

A minority carrier lifetime of 2.7 ns is measured at 77 K for a GeSn/SiGeSn single quantum well using time-resolved photoluminescence, and subsequent analyses indicate that the lifetime is Auger-limited. The 77 K lifetime is evaluated as a function of stepwise dose of 63 MeV proton irradiation up to a fluence of 1.5 × 1012 p+/cm2 with no discernable reduction in the observed lifetime, which implies that the lifetime damage factor slope to 63 MeV proton irradiation is not greater than 2 × 10−5 cm2/s. Steady-state photoluminescence is used to evaluate the total luminescence output as a function of excitation, yielding a 2/3 power law slope consistent with Auger-limited recombination. The observed Auger-limited behavior is consistent with reports of high p-type background carrier concentration in these alloys. The temperature dependence of the steady-state photoluminescence spectra also provides the Einstein single oscillator model parameters for the ground state transition energy and Urbach energy of the GeSn quantum well.

Discovery of the Zintl-phosphide BaCd2P2 as a long carrier lifetime and stable solar absorber

Discovery of the Zintl-phosphide BaCd2P2 as a long carrier lifetime and stable solar absorber

Light-activated surface passivation for more efficient silicon heterojunction solar cells: Origin, physics and stability

Silicon heterojunction (HJT) solar cells have world-leading efficiencies due to outstanding surface passivation. Yet, maintaining their performance during the lifetime of a photovoltaic module requires excellent quality and stability of the surface regions. It is well known that HJT solar cells can show an increase or reduction in performance under illumination, and this instability has been related to changes in the surface regions. This work investigates the stability of surface passivation in HJT solar cells by modelling the injection-dependent minority carrier lifetime of a range of symmetrically a-Si passivated silicon wafers. Fixed charges and defects at the interface are varied in the model to find the best fit to the injection-dependent lifetime before and after a high-intensity illumination treatment. The results indicate that the laser process induces an increase in field effect passivation at the surface, which is then reduced upon storage in the dark. The results show that lifetime spectroscopy is a useful tool to investigate the nature of a-Si passivation degradation.

Development of n-Type, Passivating Nanocrystalline Silicon Oxide Films via Plasma-Enhanced Chemical Vapor Deposition

Nanocrystalline silicon oxide (nc-SiOx:H) is a multipurpose material with varied applications in solar cells as a transparent front contact, intermediate reflector, back reflector layer, and even tunnel layer for passivating contacts, owing to the easy tailoring of its optical properties. In this work, we systematically investigate the influence of the gas mixture (SiH4, CO2, PH3, and H2), RF power, and process pressure on the optical, structural, and passivation properties of thin n-type nc-SiOx:H films prepared in an industrial, high-throughput, plasma-enhanced chemical vapor deposition (PECVD) reactor. We provide a detailed description of the n-type nc-SiOx:H material development using various structural and optical characterization techniques (scanning electron microscopy (SEM), energy dispersive X-ray (EDX), Raman spectroscopy, and spectroscopic ellipsometry) with a focus on the relationship between the material properties and the passivation they provide to n-type c-Si wafers characterized by their effective carrier lifetime (τeff). Furthermore, we also outline the parameters to be kept in mind while developing different n-type nc-SiOx:H layers for different solar cell applications. We report a tunable optical gap (1.8–2.3 eV) for our n-type nc-SiOx:H films as well as excellent passivation properties with a τeff of up to 4.1 ms (implied open-circuit voltage (iVoc)~715 mV) before annealing. Oxygen content plays an important role in determining the crystallinity and hence passivation quality of the deposited nanocrystalline silicon oxide films.

2-Iodoaniline-Modified High-Efficiency Carbon-Based CsPbIBr2 perovskite solar cells

The stability of carbon-based perovskite solar cells without a hole transport layer is better than that of hybrid perovskite solar cells, However, there will be serious interface recombination and carrier transport inhibition at the perovskite/carbon interface. Therefore, the interface between CsPbIBr2 perovskite and carbon electrodes was modified with 2-iodoaniline (2-IDA) to facilitate hole extraction in solar cell devices. –NH2 has a lone pair of electrons that can be coordinated with positively charged Lewis acid defects via coordination bonds. In addition, the introduction of iodide ions can compensate the halide vacancy on the grain boundary, inhibit the production of non-radiative recombination and passivate the defects in the film. After modification, the open-circuit voltage decays more slowly and the carrier lifetime is longer, resulting in a photoelectric conversion efficiency of 10.23% and Improve the moisture stability of the device.

Fluorinated Quasi 2D Perovskite Solar Cells with Improved Stability and Over 19% Efficiency

Abstract2D Ruddlesden–Popper perovskites (RPP) with excellent environmental and structural stability are emerging photovoltaic materials. Here, a benzylamine‐based spacer, namely 3,5‐difluorobenzylamine (DF‐BZA), is developed for stable and efficient quasi‐2D‐RP perovskite solar cells (PSCs). Compared to benzylamine (BZA)‐based quasi‐2D RPP, the DF‐BZA‐based perovskite film exhibited superior film quality with significantly enlarged grain size and improved charge carrier lifetime owing to the fluorine atoms in DF‐BZA. As a result, the optimized (DF‐BZA)2FA3Pb4I13 PSCs achieve a power conversion efficiency (PCE) of 19.24%, while BZA‐based PSCs ((BZA)2FA3Pb4I13) only achieve a PCE of 17.04%. This represents the champion PCE using FA as the A‐site cation in quasi‐2D RPPs with n = 4. Moreover, due to the effective insertion of fluorinated spacers into the inorganic layers, the moisture resistance stability and 85 °C thermal stability of (DF‐BZA)2FA3Pb4I13 are significantly improved. The improvement of photovoltaic performance and stability highlight the great potential of DF‐BZA‐based spacer for high‐performance quasi‐2D RP PSCs.

A Partial Metal Coordination Strategy toward High‐Performance and Cost‐Effective Electron Transporting Layer for Organic Solar Cells

Additives are frequently employed in the active layer of organic solar cells (OSCs) to fine‐tune morphology and improve overall device performance. The use of additive in electron transporting layer (ETL) has garnered less attention. In this study, poly(2‐vinyl pyridine) (P2VP) based cost‐effective ETLs are developed for OSCs. Addition of CoCl2 to the P2VP affords ETLs with enhanced conductivity and optimized work function. OSCs employing this ETL demonstrate prolonged carrier lifetime, suppressed charge recombination, and achieve higher power conversion efficiencies (PCEc) than the commonly used ETLs such as PFNBr and Phen‐NaDPO. The cost of the P2VP‐Co is merely one‐twentieth of that associated with Phen‐NaDPO or PFNBr. Mechanism studies and density functional theory (DFT) calculations reveal that the partial metal salt coordination of P2VP leaves free pyridine rings for dipole interactions with the Ag electrode, and provides higher electrostatic potentials and larger dipole moments compared to the polymer alone, thereby increasing conductivity. The polymer and metal salt additive strategy demonstrates its effectiveness in boosting device performance, offering valuable possibility for designing novel electronic materials and expanding the applications of widely available polymers.

Performance Enhancement of Three-Dimensional MAPbI3 Perovskite Solar Cells by Doping Perovskite Films with CsPbX3 Quantum Dots.

Perovskite thin films directly impact solar cell properties, making defect reduction crucial in perovskite solar cell research. In our study, we used perovskite quantum dots in the anti-solvent to act as nucleation centers in MAPbI3 thin films. These centers had lower nucleation barriers than homogeneous nucleation, improving perovskite crystallinity, reducing defects, and extending carrier lifetime. Fine-tuning the energy band also enhanced carrier transport. The most effective results were obtained using CsPb(Br0.5 I0.5)3 perovskite quantum dots. The resulting device, ITO/SnO2/MAPbI3 (300 nm)/spiro-OMeTAD (200 nm)/Ag (100 nm), achieved a 12.88% power conversion efficiency, a 16% increase from the standard element. The modified device maintained approximately 95% of its efficiency over 100 h in a 70% humidity environment.

Influence of Net Doping Concentration on Carrier Lifetime in 4H-SiC Substrates

Influence of Net Doping Concentration on Carrier Lifetime in 4H-SiC Substrates

Improved Carrier Separation and Recombination by Ferroelectric Polarization in the CuBiP2Se6/C2N Heterostructure: A Nonadiabatic Molecular Dynamics Study.

The rapid recombination of photogenerated carriers heavily restricts the photocatalytic efficiency. Here, we propose a new strategy to improve catalytic efficiency based on the ferroelectric van der Waals heterostructure (CuBiP2Se6/C2N). Combining density functional theory and the nonadiabatic molecular dynamics (NAMD) method, we have systematically analyzed the ground-state properties and carrier dynamics images in the CuBiP2Se6/C2N heterostructure. Our calculations showed that the ferroelectric polarization of CuBiP2Se6 provides the internal driving force for the photogenerated carriers separation. NAMD results demonstrate that the excited-state carrier transfer and recombination processes in the CuBiP2Se6/C2N are consistent with a type II mechanism. Meanwhile, constructing the ferroelectric heterostructure can effectively prolong the carrier lifetime, from ∼65.98 to ∼124.54 ps. Moreover, the high quantum efficiency and tunable band edge positions mean that the CuBiP2Se6/C2N heterostructure is an excellent potential candidate material for photocatalytic water splitting.