Carrier Lifetime Research Articles

Hydrogenation by catalytically generated atomic hydrogen for passivating contacts in crystalline silicon solar cells

Hydrogenation employing catalytically generated atomic hydrogen (Cat-H) is implemented on stacks of n-type polycrystalline silicon (poly-Si)/Si oxide (SiOx) to enhance passivation quality. The utilization of Cat-H demonstrates a substantial enhancement in surface passivation quality on crystalline Si (c-Si), resulting in an increase of the effective minority carrier lifetime (τeff) from 2.0 to 3.9 ms. The investigation reveals a dependency of τeff on the duration of Cat-H treatment: an initial rise followed by a plateau or slight decline. Secondary ion mass spectrometry (SIMS) measurements revealed the diffusion of H atoms into poly-Si and c-Si, which accumulate at the poly-Si/c-Si interface. This accumulation terminates dangling bonds in the poly-Si layer on the c-Si surface. Furthermore, the presence of an ultra-thin thermal Si oxide layer on the poly-Si layer formed during high-temperature annealing is crucial for protecting the film against etching induced by Cat-H.

Estimation of Influence on Carbon Vacancy Regarding 4H-SiC Substrate Grown by HTCVD Method

In order to increase productivity and reduce the cost of wafers, we have developed a high temperature chemical vapor deposition (HTCVD) method that can realize the high-speed growth of 4H-SiC crystals. Tokuda et al. reported an interesting study in which the carrier lifetime of a substrate grown by HTCVD (HTCVD substrate) was considerably shorter than that of the substrate grown by physical vapor transport (PVT); moreover, bipolar degradation was highly suppressed when the HTCVD substrate was applied to PiN diodes [1]. Herein, we demonstrate that the short carrier lifetime of the HTCVD substrate is mainly attributable to the carbon vacancy (VC) and that VC particularly diffuses from the HTCVD substrate to the epitaxial layer.

Ligand Controls Excited Charge Carrier Dynamics in Metal-Rich CdSe Quantum Dots: Computational Insights.

Small metal-rich semiconducting quantum dots (QDs) are promising for solid-state lighting and single-photon emission due to their highly tunable yet narrow emission line widths. Nonetheless, the anionic ligands commonly employed to passivate these QDs exert a substantial influence on the optoelectronic characteristics, primarily owing to strong electron-phonon interactions. In this work, we combine time-domain density functional theory and nonadiabatic molecular dynamics to investigate the excited charge carrier dynamics of Cd28Se17X22 QDs (X = HCOO-, OH-, Cl-, and SH-) at ambient conditions. These chemically distinct but regularly used molecular groups influence the dynamic surface-ligand interfacial interactions in Cd-rich QDs, drastically modifying their vibrational characteristics. The strong electron-phonon coupling leads to substantial transient variations at the band edge states. The strength of these interactions closely depends on the physicochemical characteristics of passivating ligands. Consequently, the ligands largely control the nonradiative recombination rates and emission characteristics in these QDs. Our simulations indicate that Cd28Se17(OH)22 has the fastest nonradiative recombination rate due to the strongest electron-phonon interactions. Conversely, QDs passivated with thiolate or chloride exhibit considerably longer carrier lifetimes and suppressed nonradiative processes. The ligand-controlled electron-phonon interactions further give rise to the broadest and narrowest intrinsic optical line widths for OH and Cl-passivated single QDs, respectively. Obtained computational insights lay the groundwork for designing appropriate passivating ligands on metal-rich QDs, making them suitable for a wide range of applications, from blue LEDs to quantum emitters.

Overcoming Microstructural Defects at the Buried Interface of Formamidinium-Based Perovskite Solar Cells.

Since the advent of formamidinium (FA)-based perovskite photovoltaics (PVs), significant performance enhancements have been achieved. However, a critical challenge persists: the propensity for void formation in the perovskite film at the buried perovskite-interlayer interface has a deleterious effect on device performance. With most emerging perovskite PVs adopting the p-i-n architecture, the specific challenge lies at the perovskite-hole transport layer (HTL) interface, with previous strategies to overcome this limitation being limited to specific perovskite-HTL combinations; thus, the lack of universal approaches represents a bottleneck. Here, we present a novel strategy that overcomes the formation of such voids (microstructural defects) through a film treatment with methylammonium chloride (MACl). Specifically, our work introduces MACl via a sequential deposition method, having a profound impact on the microstructural defect density at the critical buried interface. Our technique is independent of both the HTL and the perovskite film thickness, highlighting the universal nature of this approach. By employing device photoluminescence measurements and conductive atomic force microscopy, we reveal that when present, such voids impede charge extraction, thereby diminishing device short-circuit current. Through comprehensive steady-state and transient photoluminescence spectroscopy analysis, we demonstrate that by implementing our MACl treatment to remedy these voids, devices with reduced defect states, suppressed nonradiative recombination, and extended carrier lifetimes of up to 2.3 μs can be prepared. Furthermore, our novel treatment reduces the stringent constraints around antisolvent choice and dripping time, significantly extending the processing window for the perovskite absorber layer and offering significantly greater flexibility for device fabrication.

Exploring the Dynamics of Charge Transfer in Photocatalysis: Applications of Femtosecond Transient Absorption Spectroscopy.

Artificial photocatalytic energy conversion is a very interesting strategy to solve energy crises and environmental problems by directly collecting solar energy, but low photocatalytic conversion efficiency is a bottleneck that restricts the practical application of photocatalytic reactions. The key issue is that the photo-generated charge separation process spans a huge spatio-temporal scale from femtoseconds to seconds, and involves complex physical processes from microscopic atoms to macroscopic materials. Femtosecond transient absorption (fs-TA) spectroscopy is a powerful tool for studying electron transfer paths in photogenerated carrier dynamics of photocatalysts. By extracting the attenuation characteristics of the spectra, the quenching path and lifetimes of carriers can be simulated on femtosecond and picosecond time scales. This paper introduces the principle of transient absorption, typical dynamic processes and the application of femtosecond transient absorption spectroscopy in photocatalysis, and summarizes the bottlenecks faced by ultrafast spectroscopy in photocatalytic applications, as well as future research directions and solutions. This will provide inspiration for understanding the charge transfer mechanism of photocatalytic processes.

Toward Mass Production of Transition Metal Dichalcogenide Solar Cells: Scalable Growth of Photovoltaic-Grade Multilayer WSe2 by Tungsten Selenization.

Semiconducting transition metal dichalcogenides (TMDs) are promising for high-specific-power photovoltaics due to their desirable band gaps, high absorption coefficients, and ideally dangling-bond-free surfaces. Despite their potential, the majority of TMD solar cells to date are fabricated in a nonscalable fashion, with exfoliated materials, due to the lack of high-quality, large-area, multilayer TMDs. Here, we present the scalable, thickness-tunable synthesis of multilayer WSe2 films by selenizing prepatterned tungsten with either solid-source selenium at 900 °C or H2Se precursors at 650 °C. Both methods yield photovoltaic-grade, wafer-scale WSe2 films with a layered van der Waals structure and superior characteristics, including charge carrier lifetimes up to 144 ns, over 14× higher than those of any other large-area TMD films previously demonstrated. Simulations show that such carrier lifetimes correspond to ∼22% power conversion efficiency and ∼64 W g-1 specific power in a packaged solar cell, or ∼3 W g-1 in a fully packaged solar module. The results of this study could facilitate the mass production of high-efficiency multilayer WSe2 solar cells at low cost.

In-situ polymerization of dual monomers for constructing a highly elastic PAA-co-PAM network enables efficient and stable Carbon-based perovskite solar cells

During the preparation of carbon-based perovskite solar cells, the external stress applied to the carbon paste inevitably compresses the perovskite lattice, potentially damaging its structure and generating defects. This poses a significant threat to the efficiency and stability of devices. Herein, a dual monomer irregular in-situ polymerization method was developed to form a highly elastic network (acrylamide-co-acrylic acid, PAA-co-PAM) within perovskite films, serving as both a ligament to passivate defects and a buffer to improve film stability under stress. The results demonstrated that multiple active sites of PAA-co-PAM could simultaneously passivate positive and negative defects, which improved charge transport efficiency, reduced nonradiative recombination, and prolonged lifetime of carriers. Meanwhile, the cross-linking network in PAA-co-PAM exhibits high elasticity due to both internal hydrogen bonds between monomers and external hydrogen bonds between monomers and perovskite, resulting in films with exceptional structural stability and mechanical resilience. Notably, the corresponding device achieved the power conversion efficiency of 15.39%. Moreover, the unencapsulated photovoltaic device passivated by the elastic network exhibits better storage stability and mechanical stability compared to the control devices. This work provides a new perspective and insight for comprehensively enhancing the stability of carbon-based perovskite solar cells.

Anti-perovskites with long carrier lifetime for ultralow dose and stable X-ray detection

Anti-perovskites with long carrier lifetime for ultralow dose and stable X-ray detection

16.48% Efficient solution-processed CIGS solar cells with crystal growth and defects engineering enabled by Ag doping strategy

Solution-processed Cu(In,Ga)Se2 (CIGS) solar cells suffer from serious carrier recombination and power conversion efficiency (PCE) loss because of the poor film properties and easy formation of defects. Herein, we propose Ag&Se co-selenization strategy to enhance the crystallization and passivate harmful defects of the CIGS films. The formation of Ag-Se phase during the selenization process enables the formation of large grains and suppresses the deep level defects. It is found that Ag doping can enlarge the depletion region width, lower the Urbach energy and prolong the carrier lifetime. As a result, a champion solution-processed CIGS solar cell presents a high efficiency of 16.48% with the highly improved open-circuit voltage (VOC) of 662 mV and fill factor (FF) of 75.8%. This work provides an efficient strategy to prepare high quality solution-processed CIGS films for high-performance CIGS solar cells.

Charge Carrier Capture by Prominent Defect Centers in 4H-SiC

The knowledge of capture properties of electrically active defects is of primary importance as it helps to understand which deep states are effective in controlling the excess free carriers’ lifetime. Combining DLTS capture experiments with thermal emission measurements enables an overall thermodynamic description of deep states, thus making it possible to characterize recombination centers in semiconductor-based devices. In the present study, junction DLTS capture rate measurements were employed to extract the true capture cross-sections (inversely proportional to the carrier lifetime) and capture energy barriers for the main lifetime limiting defects in 4H-SiC (silicon carbide). A peculiar forward bias dependence of the capture parameters was observed for the shallow boron (B) hole trap. Capture rate measurements on the deep boron (D-center) trap also evidenced the presence of two capture mechanisms, thus allowing discrimination of D1 and D2 deep states within the D-center DLTS peak. The results were combined with activation energies and apparent capture cross-sections to obtain the free energy (ΔG) of electronic activation for the analysed deep states.

Device Modeling of 4H-SiC PIN Photodiodes with Shallow Implanted Al Emitters for VUV Sensor Applications

A numerical model is presented for the simulation of ultraviolet ion-implanted 4H-SiC photodiodes with shallow p- emitter doping profiles. An existing model for SiC pin photodiodes, taken from literature, is modified with a dedicated SiO2-SiC interface layer to account for degradation of carrier mobility and lifetime at the interface. Furthermore, aluminum compensation in 4H-SiC is included and its impact on the spectral response and carrier recombination is analyzed. The simulated spectral response in the wavelength range from 200 to 400 nm is compared to experimental data. While the existing model, taken from literature, fails to predict the performance of VUV photodiodes with a shallow p- emitter, the newly designed model successfully achieves high accuracy, even with a basic modeling approach featuring an abrupt material parameter transition.

Data-Driven Design of Electroactive Spacer Molecules to Tune Charge Carrier Dynamics in Layered Halide Perovskite Heterostructures.

Crafting rational heterojunctions with nanostructured materials is instrumental in fostering effective interfacial charge separation and transport for optoelectronics. Layered halide perovskites (LHPs) that form heterojunctions between organic spacer molecules and inorganic metal halide layers exhibit tunable photophysics owing to their customizable band alignment. However, controlling photogenerated carrier dynamics by strategically designing layered perovskite heterojunctions remains largely unexplored. We combine a data-driven approach with time-domain density functional theory (TD-DFT) and non-adiabatic molecular dynamics (NAMD) to screen and select electronically active spacer dications (A') that introduce a type-II heterojunction in the lead iodide-based Dion-Jacobson phase LHPs. The composition-structure-electronic property correlations reveal that the number of nitrogens in aromatic heterocycles is the key factor in designing electron-accepting spacers in these perovskites. The detailed atomistic simulations validate the design strategy further by modeling (A')PbI4 perovskites, which incorporate three different screened electroactive A' spacers. The computed excited charge carrier dynamics illustrate the phonon-mediated ultrafast interfacial electron transfer from the inorganic conduction band edge to the lower-lying unoccupied orbitals of spacers, exhibiting photoluminescence quenching in these (A')PbI4 perovskites. The spatially separated electrons and holes at the type-II heterojunction interface prolong the excited charge carrier lifetime, boosting the carrier transport and exciton dynamics. Our work illustrates a robust in silico approach for designing LHPs with exciting optoelectronic properties originating from their fine-tuned heterojunctions.

Strong absorption of silica over full solar spectrum boosted by interfacial junctions and light–heat–storage of Mg(OH)2–(CrOx–SiO2)

Strong absorption of near-infrared (NIR) light is essential for efficient solar energy applications. NIR absorption primarily depends on the surface plasmon resonance of incident light with free charge carriers (FCCs). We demonstrated that S-scheme interfacial junctions (IJs) can substantially enhance FCC density, light-harvesting, photocurrent intensity, long lifetime of the charge carriers and photothermal effects, paving a way for novel light-material design. Abundant S–scheme IJs are constructed in CrOx–SiO2 nanoparticles, which increase the FCCs outstandingly and enhance light absorption exceptionally over the entire solar spectrum. IJs’ construction enables silica to harvest solar energy strongly. Doped with multifunctional CrOx–SiO2 nanoparticles, the photothermal-dehydration conversion and the stored heat of Mg(OH)2 can be improved by 7.0- and 5.1-times upon 30-min irradiation, respectively. The reversibility of the photothermal charge–discharge cycles of Mg(OH)2 improves 19.3 times. The thermal-storage kinetics is substantially improved owing to the reduction of the dehydration activation energy (−15.2 %). Mg(OH)2–CrOx–SiO2 composite is an excellent one-step system of light–heat–storage.

Elucidating the Synergistic Promotional Mechanism of Water and Oxygen in the Aerobic C-C Homocoupling Reaction Catalyzed by Visible-Light-Derived Core-Shell Pd@A-CQDs Nanostructures.

Rational design and precise synthesis of biogenic noble-metal-based catalysts possessing distinctive structure and composition play a crucial role in the chemical industry, enabling sustainable construction of an inclusive range of chemical resources. In this study, we have effectively fabricated Pd@A-CQDs through a straightforward one-pot aqueous protocol assisted by visible light employing renewable biomass-derived amine-rich carbon quantum dots (A-CQDs). The remarkable visible light harnessing capability (bandgap, ca. 2.81 eV), high density (35.7 × 1018 cm-3), and long lifetime (25 ps) of photocharge carriers and amine-rich surface in A-CQDs make them ideal candidates as both reducing and stabilizing agents, thereby facilitating the in situ construction of metallic Pd(0) nanoparticles. Comprehensive physicochemical characterizations have provided compelling evidence for the spherical morphology of Pd@A-CQDs core-shell nanostructures, with ultrathin A-CQDs shells of ca. 1.9 nm and an average diameter of 14 ± 1 nm. The effectiveness of the synthesized Pd@A-CQDs catalysts was assessed in the ligand- and base-free homocoupling reaction of arylboronic acids in water at ambient temperature. The catalytic tests demonstrated the selective production of the homocoupled compound over protodeboronation products with excellent yield and high catalyst recyclability under ambient conditions. The protocol employed exhibited a high TOF (1.05 × 10-2 mol g-1 min-1) and a low E-factor, with a remarkably low palladium loading. XPS analysis confirmed the retention of the metallic nature of the palladium core within the catalysts during the reaction. The catalytic function of the palladium core in conjunction with the A-CQDs shell, along with the promotional effects provided by water and oxygen for the formation of nucleophilic tetravalent boron, was conclusively recognized by 11B NMR and O2-TPD measurements. The obtained experimental results deliver valuable insights into the probable reaction pathway for the homocoupling reaction catalyzed by the Pd@A-CQDs catalysts. Through a comprehensive and sustainable evaluation, the current methodology exhibits superior performance compared to previously documented techniques in relation to estimated circularity and adherence to good manufacturing practices (GMP).

The 2D porous NiSe/g-C3N4 nanosheets towards enhanced photocatalytic H2 evolution and degradation via synergism of co-catalyst and surface protonation

The 2D porous protonated NiSe/g-C3N4 nanosheets are synthesized via a chemical-calcination-etching-acidification-hydrothermal method. The obtained 2D NiSe/g-C3N4 nanosheets (NiSe/HCN-3) exhibit signally enhanced HER (∼1184.74 μmol g−1 h−1)/degradation (∼0.04645 min−1) than the monomer g-C3N4 (∼150/5-folds), including a decent stability (average HER ∼1152.71 μmol g−1 h−1). It mainly attributes to the NiSe/g-C3N4 interface owns appropriate potential gradient can promote the interface carrier driving to optimize efficiency, including increasing carrier transport, extending carrier lifetime, and reducing recombination, as well as the porous nanosheets and surface protonation obtain numerous active sites for decreasing overpotential and promoting electron solid/liquid interfacial diffusion. Additionally, the 2D porous nanosheets can improve the solar efficiency (multiple internal reflections) and shorten the carrier pathway.

Variation of carrier lifetime with optical excitation power density in micro-LEDs

GaN-based micro-LEDs are applied to visible light communication due to their high modulation bandwidth with reduced chip size. It requires a deep understanding of recombination processes and their impact on the bandwidth, which is mainly determined by the carrier lifetime. We employed confocal time-resolved photoluminescence (TRPL) to characterize the variation of carrier lifetime with optical excitation power density on micro-LEDs. We observed an initial increase followed by a sudden decrease within the power density range of 96.7 kW/cm2 to 546 kW/cm2 on a blue micro-LED with a chip size of 80 µm. We attribute this phenomenon to increased optical excitation power, gradually saturating the defect-dominated non-radiative recombination centers, with radiative recombination processes gradually taking over. We compared the power density at the inflection point for different regions on the sample and the samples with different sizes and sidewall structures. The power density for the lifetime inflection point at the center of the sample is smaller than that at the edge. We also find that the value is smaller for the sample with a chip size of 40 µm which prompts fewer total defects. The power density for sudden lifetime drop on samples with inclined sidewall structures is also smaller than those with vertical sidewall structures. Furthermore, we find that the excitation power density corresponding to the highest luminous efficiency is higher than that corresponding to the sudden drop at the beginning of the lifetime. This opens up possibilities for simultaneously achieving high modulation bandwidth and high efficiency between the two inflection points.

Insight into mechanism for remarkable photocatalytic hydrogen evolution of Cu/Pr dual atom co-modified TiO2.

The development of high-activity photocatalysts is crucial for the current large-scale development of photocatalytic hydrogen applications. Herein, we have developed a strategy to significantly enhance the hydrogen photocatalytic activity of Cu/Pr di-atom co-modified TiO2 architectures by selectively anchoring Cu single atoms on the oxygen vacancies of the TiO2 surface and replacing a trace of Ti atoms in the bulk with rare earth Pr atoms. Calculation results demonstrated that the synergistic effect between Cu single atoms and Pr atoms regulates the electronic structure of Cu/Pr-TiO2, thus promoting the separation of photogenerated carriers and their directional migration to Cu single atoms for the photocatalytic reaction. Furthermore, the d-band center of Cu/Pr-TiO2, which is located at -4.70 eV, optimizes the adsorption and desorption behavior of H*. Compared to TiO2, Pr-TiO2, and Cu/TiO2, Cu/Pr-TiO2 displays the best H* adsorption Gibbs free energy (-0.047 eV). Furthermore, experimental results confirmed that the photogenerated carrier lifetime of Cu/Pr-TiO2 is not only the longest (2.45 ns), but its hydrogen production rate (34.90 mmol g-1 h-1) also significantly surpasses those of Cu/TiO2 (13.39 mmol g-1 h-1) and Pr-TiO2 (0.89 mmol g-1 h-1). These findings open up a novel atomic perspective for the development of optimal hydrogen activity in dual-atom-modified TiO2 photocatalysts.

Magnetic doping engineering in perovskite microcrystals for boosted photocatalytic CO2 reduction

Conversion of carbon dioxide into value-added chemicals through sunlight is a promising approach to reducing our reliance on fossil fuels. Spin provides another freedom to manipulate the photocatalytic CO2 reduction (CO2RR) process and enhance the corresponding properties of optoelectronic applications. In this work, improved photocatalytic CO2RR efficiency was obtained by manipulating spin-polarized species in inorganic halide perovskite microcrystals (MCs) with the introduction of manganese cations (Mn2+) and an employment of a magnetic field. Mn2+-doped CsPbBr3 MCs were prepared through self-assembly process, where Mn2+ is incorporated into CsPbBr3 matrices successfully. Introducing Mn2+ into CsPbBr3 matrices not only induces structural change but also generates new emission band as well as magnetic properties. Thereby, Mn2+-doped CsPbBr3 MCs show significantly enhanced photocatalytic CO2RR properties in contrast with pristine samples (the CO yield is significantly elevated from 22.1 to 41.9 μmol/g in Mn2+-doped CsPbBr3 MCs), as they generate spin-polarized electrons with the introduction of Mn2+. Remarkably, the photocatalytic CO2RR of Mn2+-doped CsPbBr3 MCs is greatly improved under the employment of a magnetic field, achieving 5.0 times higher performance compared to pristine CsPbBr3 MCs when the magnetic field strength is 300 mT. This enhancement is experimentally demonstrated by using magnetic circular dichroism spectroscopy. The increased photocatalytic CO2RR efficiency of Mn2+-doped CsPbBr3 MCs stems from the cooperative doping of Mn2+ and the application of magnetic fields, which generates more spin-polarized photoexcited carriers, thereby leading to extended carrier lifetime and reduced recombination process. These findings demonstrate that introducing spin into optoelectronic semiconductors is a powerful approach to enhancing photocatalytic CO2RR efficiency.

Effect of doping level on the photoconduction and chemiresistive ethanol gas sensing of lanthanum-doped CuO films

Undoped and lanthanum (La)-doped copper oxide (CuO) thin films were prepared by the nebulizer spray pyrolysis technique. The X-ray diffraction analysis revealed that the effective doping of lanthanum does not alter the monoclinic crystalline structure of copper oxide thin films. Raman spectra confirmed the crystalline quality of undoped and La-incorporated CuO films. From, scanning electron micrograph images, the La-doped CuO films exhibit spherical particles. Energy dispersive X-ray analysis confirmed the existence of copper, lanthanum and oxygen elements. The doping of La-substituted CuO films showed a minimum bandgap when compared to undoped CuO film. The oxidation states of Cu 2p, La 3d and O 1s elements were observed. The electrical resistivity of the films was considerably decreased due to the doping of La ions in CuO matrix. The photoluminescence spectra show that the doping of La ions increases the defect states and shows increased intensity. The time resolved photoluminescence analysis revealed that an extended carrier lifetime was observed for the La-doped films. The La-doped CuO sensors achieved a superior gas sensing performance at room temperature and exhibited quick response and recovery times, suggesting that the sensor has the potential for the detection of harmful volatile gases in real-world applications. The photoconductive investigation revealed that the La-doped CuO films exhibited higher charge-transporting properties and increased light-harvesting ability. This study sheds light on designing gas sensors working under ambient conditions and photosensors for optoelectronic devices.

Crystalline silicon solar cell with an efficiency of 20.05 % remanufactured using 30 % silicon scraps recycled from a waste photovoltaic module

Several studies have aimed to develop methods for recycling and recovering valuable materials from waste photovoltaic (PV) modules to keep up with the increasingly stringent waste disposal guidelines. However, silicon (Si) recovered from disposed PV modules is rarely used as a raw material in the solar industry. In this study, 30 % Si recovered from waste PV modules was added to a feedstock to grow a 6inch single-crystalline Si ingot using the Czochralski method. The single-crystalline Si ingot was re-melted and manufactured to produce a higher quality single-crystalline Si ingot with a minority carrier lifetime of 274–1527 μs and purity of 7N7. Thereafter, a Si wafer was manufactured via cropping, squaring, and wafering. Finally, the Si wafer was used to fabricate a solar cell via a conventional process. Solar cell parameters, such as open circuit voltage, short circuit current density, fill factor, and efficiency, were analyzed using a solar simulator. The manufactured solar cell had an efficiency of 20.05 %, which is approximately 0.97 % lower than that of commercial wafer-based solar cells. Moreover, the factors influencing the solar cell efficiency were evaluated.