Hole Diffusion Length Research Articles

Layered double hydroxide modified bismuth vanadate as an efficient photoanode for enhancing photoelectrochemical water splitting.

Photoelectrochemical (PEC) water splitting has attracted significant interest as a promising approach for producing clean and sustainable hydrogen fuel. An efficient photoanode is critical for enhancing PEC water splitting. Bismuth vanadate (BiVO4) is a widely recognized photoanode for PEC applications due to its visible light absorption, suitable valence band position for water oxidation, and outstanding potential for modifications. Nevertheless, sluggish water oxidation rates, severe charge recombination, limited hole diffusion length, and inadequate electron transport properties restrict the PEC performance of BiVO4. To surmount these constraints, incorporating layered double hydroxides (LDHs) onto BiVO4 photoanodes has emerged as a promising approach for enhancing the performance. Herein, the latest advancements in employing LDHs to decorate BiVO4 photoanodes for enhancing PEC water splitting have been thoroughly studied and outlined. Initially, the fundamental principles of PEC water splitting and the roles of LDHs are summarized. Secondly, it covers the development of different composite structures, including BiVO4 combined with bimetallic and trimetallic LDHs, as well as other BiVO4-based composites such as BiVO4/metal oxide, metal sulfide, and various charge transport layers integrated with LDHs. Additionally, LDH composites incorporating materials like graphene, carbon dots, quantum dots, single-atom catalysts, and techniques for surface engineering and LDH exfoliation with BiVO4 are discussed. The research analyzes the design principles of these composites, with a specific focus on how LDHs enhance the performance of BiVO4 by increasing the efficiency and stability through synergistic effects. Finally, challenges and perspectives in future research toward developing efficient and stable BiVO4/LDHs photoelectrodes for PEC water splitting are described.

(Invited) Morphology-Engineered Hematite for Addressing Short Hole-Diffusion Lengths

This study addresses challenges in photoelectrochemical (PEC) water oxidation on hematite photoanodes, with a focus on overcoming the limited hole diffusion length (~4 nm) and poor electrical properties. To achieve this, an efficient overlayer is integrated onto the branched structure, generating a highly porous architecture by leveraging additional reaction interfaces exploiting the Kirkendall effect. This phenomenon occurs during high-temperature annealing, particularly at the interface of the overlayer and Ti-FeOOH (hematite precursor), utilizing distinct diffusion rates of metal atoms. By introducing additional interfaces on the hematite precursor surface, porosity and pore size are effectively controlled, resulting in a highly nanoporous structure. Through morphological engineering, the overlayer facilitates heteroatom diffusion (doping) into the hematite lattice, thereby promoting facile charge transport. The overlayer material selection is guided by density functional theory calculations. The resulting optimized photoanode, combined with an efficient cocatalyst (NiFe(OH)x), exhibits a maximum photocurrent density of 5.1 mA cm-2 at 1.23 V vs. RHE, marking a 3.2-fold increase compared to the reference hematite photoanode. This enhancement is attributed to the highly nanoporous structure and optimal doping. Thus, this study represents a significant advancement in enhancing the PEC performance of hematite-based photoanodes for future applications.

Surface and Defect Modulation via Flame Technique for Efficient Photoelectrochemical Water-Splitting

The conversion of solar energy into chemical fuel via photoelectrochemical (PEC) water splitting shows great promise for solving the energy crisis, achieving carbon neutrality, and a sustainable future. Iron (Fe)-based materials have attracted tremendous attention from various light absorber materials due to their earth abundance, high light absorption, and robust PEC stability under harsh conditions[1]. However, diverse shortcomings, such as the short hole diffusion length, severe bulk/interface recombination, and sluggish water oxidation kinetics, limit the photocurrent generation for achieving high solar-to-hydrogen efficiency[2,3]. In particular, the high-temperature annealing requirement necessitates suitable synthesis and activation strategies to awaken their PEC activity.In this talk, we present a novel high-temperature flame technique to activate two Fe-based model photoanodes, i.e., zinc ferrite (ZnFe2O4, ZFO) and hematite (Fe2O3). The flame-activated ZFO (just a few tens of seconds) exhibits an enhanced charge collection ability and enriched active sites compared to the non-activated ZFO. These improvements are attributed to the high-temperature induced grain growth, enhanced grain connection, and rich defect formation caused by the fuel-rich flame environment[4]. On the other hand, a unique textured and mesoporous hematite (tm-hematite) photoanode can be synthesized via the same flame technique combining solution chemistry. The resultant tm-hematite exhibited enlarged surface area, enhanced charge transport, and transfer properties. We show that the unique mesoporous morphology, dominated (110) plane, and rich Ti dopant concentration on grain surface are responsible for the enhanced PEC activity. Notably, the tm-hematite showed greatly enhanced photocurrent density (3.1 mA/cm2 at 1.23 V vs the reversible hydrogen electrode, RHE) compared to the conventional nanorod hematite (~1.4 mA/cm2 at 1.23 VRHE) under 1 sun illumination. The flame-activated ferrite photoanodes demonstrated excellent PEC stability for 50 hours, even without additional coating/treatments. Our flame technique shows extraordinary potential for synthesizing high-efficiency ferrite-based photoelectrodes and puts a step forward to achieving the goal of sustainable energy and environment future. Keywords: Photoelectrochemical water splitting, Ferrites, Flame treatment, defect control, mesoporous structure, charge collection, photocurrent density, stability References [1] R. Tan, A. Sivanantham, B. J. Rani, Y. J. Jeong, I. S. Cho, Coord. Chem. Rev. 2023, 494, 215362.[2] I. S. Cho, M. Logar, C. H. Lee, L. Cai, F. B. Prinz, X. Zheng, Nano Lett. 2014, 14, 24.[3] I. S. Cho, H. S. Han, M. Logar, J. Park, X. Zheng, Adv. Energy Mater. 2016, 6, 1501840.[4] R. Tan, Y. J. Jeong, Q. Li, M. Kang, I. S. Cho, J. Adv. Ceram. 2023, 12(3): 612-624. Figure 1

Fe2O3 Nanoflakes - WS2 Nanosheets Heterojunctions for Multi-Fold Enhancement in Photoelectrochemical Solar Energy Conversion.

Fe2O3-based photoanodes show great potential in photoelectrochemical water splitting due to their excellent stability, moderate band gap, and abundance. However, a short hole diffusion length limits its photocurrent density. Here, a multi-fold enhancement in photocurrent density from Fe2O3 nanoflakes - WS2 nanosheets heterojunction is reported. The heterojunction exhibits a synergistic photocurrent density of 0.52mA cm-2 at 1.3V (versus RHE) under AM 1.5G simulated sunlight, which is 2.23 times higher than pristine Fe2O3 nanoflakes. The Mott-Schottky and Nyquist plots indicate a higher charge density with lower charge transfer resistance at the semiconductor-electrolyte junction. The density functional theory (DFT) -based first-principles calculations are performed by designing a heterostructure between Fe2O3(110) and WS2(001) similar to the experimentally found arrangement of crystal planes. Free energy analysis and relative band extrema positions, obtained from DFT calculations and valence band spectroscopy, indicate the formation of type II heterojunction with partial oxygen terminated surface of Fe2O3. The type-II band alignment with a charge transfer of 4.8 × 10-4 e per interfacial WS2 to Fe2O3, helps in easy separation of photogenerated charges. The work establishes both an experimental design and a theoretical framework of highly crystalline nanoflakes-nanosheet heterojunctions for efficient photoelectrochemical solar energy conversion.

Charge Carrier Collection Losses in Lead‐Halide Perovskite Solar Cells

AbstractThe collection of photogenerated charges in halide perovskite solar cells depends on the thickness of the absorber layer, with larger thicknesses leading to a reduced collection efficiency. This observation has traditionally been associated with insufficiently high electron and hole diffusion lengths in the absorber layers. However, it is shown that in the presence of low‐mobility contact layers, charge collection can be thickness‐dependent, even if the absorber layer has infinite mobility. Here, analytical equations are derived for the thickness dependence of charge collection losses in situations where recombination is bulk or interface‐limited and show how to relate these equations to voltage‐dependent photoluminescence data. The analytical equations are compared to experimental data and numerical simulations and it is observed that experimental data on triple‐cation perovskite devices with different thicknesses approximately follows the case, where bulk recombination dominates.

Hole diffusion length and mobility of a long wavelength infrared InAs/InAsSb type-II superlattice nBn design

We demonstrated high-performance 8.9 μm cutoff wavelength nBn InAs/InAsSb type-II strained-layer superlattice (T2SL). These detectors exhibit a long minority carrier (hole) lifetime of 1.2 μs at 80 K, high quantum efficiency of 40% for back-side illuminated devices without antireflection coating, and low dark current density of 4.6 × 10−6 A/cm2 at 80 K. We measured absorption, minority carrier (hole) lifetime, quantum efficiency, and spectral response as a function of the temperature and applied bias. We investigated the temperature dependence of the hole diffusion length and mobility and found that their values increase with temperature from 1.3 μm and 6.5 cm2/Vs at 30 K to 6.5 μm and 36 cm2/Vs at T = 90 K. We compared the measured diffusion length and mobility of holes in long-wavelength infrared (LWIR) T2SL with these parameters of a high-performance mid-wavelength infrared (MWIR) T2SL. Unexpectedly, hole mobility in LWIR T2SL was found to be higher than in MWIR that is contrary to the theoretical predictions.

Time-resolved cathodoluminescence measurement of the effects of α -particle-related damage on minority hole lifetime in free-standing n-GaN

Time-resolved cathodoluminescence using 30 keV ultrafast electron pulses has been used to perform direct measurements of the minority hole lifetime τh as a function of 3.7 MeV α-particle fluence in high-quality free-standing n-type GaN substrates. The lifetime damage factor K calculated from these measurements was found to monotonically decrease from 6.9 × 10−2 to 6.4 × 10−4 cm2 s−1 ion−1 with increasing α-fluence from 108 to 1012 cm−2, implying a reduction in trap cross section and/or an aggregation of α-induced traps. The small, ∼200–300 nm, hole diffusion length estimated from the minority hole lifetime for the highest α-fluence necessitates the deployment of α-voltaic device strategies and architectures that emphasize depletion and drift over diffusion for effective charge collection and optimal power conversion efficiency.

Electron Diffusion Length Effect on Direction of Irradiance in Transparent FAPbBr3 Perovskite Solar Cells.

Transparent photovoltaics for building integration represent a promising approach for renewable energy deployment. These devices require transparent electrodes to manage transmittance and to ensure proper cell operation. In this study, transparent FAPbBr3-based perovskite solar cells optimized via a passivation treatment were demonstrated with average visible transmittance values above 60% and light utilization efficiencies up to 5.0%. Experiments under varying ultraviolet (UV) irradiance intensities from both front and rear directions revealed performance differences correlated with diffusion-limited transport and open-circuit voltage changes. Combining the UV-radiated experiments and drift-diffusion simulations, an asymmetry between the diffusion lengths of electrons and holes in the perovskite is revealed, with estimated values resulting in less than 50 nm and more than 99 nm, respectively. Our methods not only identify electron-hole diffusion length differences but also introduce a general protocol for characterizing solar cells with transparent electrodes.

Synthesis, characterization and photoelectrochemical performance of Bi2Fe4O9 thin films

Mullite-type Bi2Fe4O9 has been little explored in thin film form as a photoanode for photoelectrochemical water splitting. In this study, Bi2Fe4O9 thin films have been prepared using the sol-gel technique from a simple precursor solution based on the corresponding metal salts, acetic acid, and polyvinyl alcohol. The films were deposited by dip-coating onto fluorine-doped tin oxide substrates and dried at 350 °C, repeating the dipping-drying cycle six times, and finally sintered at 600 °C. The films were characterized by GIXRD, revealing the formation of the material in its orthorhombic phase. Raman spectroscopy showed the Ag and B1g vibrational modes, validating the formation of the bismuth iron oxide. UV–Vis transmittance measurements revealed that the material exhibits two optical transitions: a direct band gap of 2.86 eV and an indirect band gap of 1.98 eV. FESEM micrographs and AFM images showed a uniform nanostructured surface morphology. The photoelectrochemical properties of the Bi2Fe4O9 films were studied using cyclic voltammetry and chronoamperometry with front side illumination, demonstrating the stability of the material in aqueous media and the generation of photocurrent in the presence of H2O2. Furthermore, results from intensity-modulated photocurrent spectroscopy (IMPS) revealed that the photocurrent is limited by both bulk and surface recombination and a short hole diffusion length.

Improving the efficiency and stability of betavoltaic batteries based on understanding efficiency fluctuations and gaps with theoretical limits

Wide-bandgap semiconductors are regarded as preferred materials for preparing semiconductor conversion devices in betavoltaic batteries due to their high theoretical conversion efficiency (ηc). However, there are a few comprehensive analytical studies on why the experimental values of ηc are generally much lower than the theoretical limit of ηc (ηc-limit) and how to improve ηc and its stability. In this work, combined with the energy deposition distributions of Ti3H2, 63Ni, and 147Pm2O3 radioactive sources in SiC obtained from Monte Carlo simulations, a multi-physical mechanism, multi-parameter coupling numerical model was established. This model can comprehensively analyze the output characteristics of betavoltaic batteries under the influence of actual device structural and material parameter changes. Our results show that changes in structural and material parameters cause significant variations in the collection efficiency (Q) of the radiation-generated electron–hole pair (RG-EHP). Considering structural parameters are easy to control, instabilities in actual SiC material parameters, which include electron diffusion length (Ln), hole diffusion length (Lp), and surface recombination velocity (S), are the main reason that ηc fluctuates significantly and is generally far lower than ηc-limit. Due to differences in the distribution of RG-EHP produced by different radioactive sources in SiC, the dominant parameters causing ηc fluctuations differ. By analyzing differences in recombination loss mechanisms under different radioactive sources, the device structures were designed in a targeted manner to make ηc closer to ηc-limit. Meanwhile, when the SiC material quality fluctuates, the stability of ηc increases by 58.5%, 35.3%, and 48.2% under Ti3H2, 63Ni, and 147Pm2O3, respectively.

Carrier diffusion in long wavelength InGaN quantum well LEDs after injection through V-defects

The efficiency of operation of GaN-based light emitting diodes (LEDs) to a large degree relies on realization of a uniform hole distribution between multiple quantum wells (QWs) of the active region. Since the direct thermionic transport between the QWs is inefficient, the hole injection through semipolar 101¯1 QWs that form on the facets of V-defects has been suggested as an alternative approach. However, for an efficient LED operation, the carrier distribution should be uniform not only vertically, between the QWs but also laterally, within individual QWs. In this work, the lateral carrier distribution in long wavelength InGaN/GaN QW LEDs is studied by the scanning near-field optical microscopy. The measurements have shown that emission is concentrated around the V-defect injectors. At high currents, the diffusion length of holes in polar QWs was found to be ∼0.6–1 μm and the hole diffusion coefficient ∼0.6 cm2/s. The obtained data should aid design of the V-defect injectors for a laterally uniform carrier distribution in the active region QWs.

BiVO4 Photoanode with NiV2O6 Back Contact Interfacial Layer for Improved Hole-Diffusion Length and Photoelectrochemical Water Oxidation Activity.

The short hole diffusion length (HDL) and high interfacial recombination are among the main drawbacks of semiconductor-based solar energy systems. Surface passivation and introducing an interfacial layer are recognized for enhancing HDL and charge carrier separation. Herein, we introduced a facile recipe to design a pinholes-free BiVO4 photoanode with a NiV2O6 back contact interfacial (BCI) layer, marking a significant advancement in the HDL and photoelectrochemical activity. The fabricated BiVO4 photoanode with NiV2O6 BCI layer exhibits a 2-fold increase in the HDL compared to pristine BiVO4. Despite this improvement, we found that the front surface recombination still hinders the water oxidation process, as revealed by photoelectrochemical (PEC) studies employing Na2SO3 electron donors and by intensity-modulated photocurrent spectroscopy measurements. To address this limitation, the surface of the NiV2O6/BiVO4 photoanode was passivated with a cobalt phosphate electrocatalyst, resulting in a dramatic enhancement in the PEC performance. The optimized photoanode achieved a stable photocurrent density of 4.8 mA cm-2 at 1.23 VRHE, which is 12-fold higher than that of the pristine BiVO4 photoanode. Density Functional Theory (DFT) simulations revealed an abrupt electrostatic potential transition at the NiV2O6/BiVO4 interface with BiVO4 being more negative than NiV2O6. A strong built-in electric field is thus generated at the interface and drifts photogenerated electrons toward the NiV2O6 BCI layer and photogenerated holes toward the BiVO4 top layer. As a result, the back-surface recombination is minimized, and ultimately, the HDL is extended in agreement with the experimental findings.

Combined effects of annealing treatment and compounding with FeOOH cocatalyst on efficiency and stability of BiVO4/Ag3PO4 photoanode

Bismuth vanadate (BiVO4) is a promising metal oxide photoabsorber for photoanode applications but is limited by its relatively short hole diffusion length resulting in severe charge complexation and limited performance. Herein, Ag3PO4 was compounded on BiVO4 through photodeposition of Ag followed by solution oxidation. The annealing of the composite BiVO4/Ag3PO4 heterojunction at 400 °C precipitated some Ag from Ag3PO4 on BiVO4/Ag3PO4 to form a new ternary photoanode BiVO4/Ag3PO4/Ag. Compared to the unannealed sample, the light trapping and crystal quality enhanced, resulting in increased photocurrent density of the annealed BiVO4/Ag3PO4 photoanode (4.46 mA cm−2) by 59.9 % when compared to the unannealed photoanode (2.79 mA cm−2). To further optimize the photoanode, co-catalyst iron hydroxyl oxide (FeOOH) was deposited to yield BiVO4/Ag3PO4/Ag/FeOOH photoanode with maximum photocurrent density of 4.71 mA cm−2 at 1.23 V (vs. reversible hydrogen electrode, VRHE), equivalent to about 3.7-fold the value of BiVO4 photoanode. The photoanode maintained 90.8 % of its initial current density after 3 h stability tests. Besides, the photoelectrocatalytic degradation of Rhodamine B pollutant by BiVO4/Ag3PO4/Ag and BiVO4/Ag3PO4/Ag/FeOOH photoanode showed performance enhancement when compared to BiVO4/Ag3PO4. Overall, the proposed strategy looks very promising for the rational fabrication of high-efficiency heterojunction photoanodes.

Ideal Structures of Isolated BiVO4 Nanoparticles on WO3 Nanocorals to Enhance Photoelectrochemical Hydrogen Evolution

Metal oxide nanoparticles (NPs) of n-type semiconductors have recently garnered significant attention in the photoelectrochemical (PEC) field. In this study, isolated BiVO4 NPs on anodized WO3 nanocorals are investigated with respect to the formation mechanism of the NPs and the correlation between particle size and PEC efficiency. The sizes and distances of the BiVO4 NPs on the WO3 nanocorals are controlled via spin-coating and heat-treatment. The optimized BiVO4 NPs, with ideal sizes of 10 nm and distances of 100 nm between them, exhibit satisfactory electron and hole diffusion lengths, suppressing electron–hole recombination and promoting the exposure of WO3 to ultraviolet light. Furthermore, high-quality BiVO4, without the formation of by-products due to the optimized concentrations of Bi and V in the precursor solution, results in a 2.3-fold higher photocurrent density and H2 production, and a 6-fold higher incident photon-to-current conversion efficiency (IPCE) compared to those of pristine WO3 nanocorals.

Decoupled Crystallization and Particle Growth of BiVO4 via Rapid Thermal Process for Enhanced Charge Separation

AbstractBiVO4 is one of the most promising candidates for photoelectrochemical water oxidation. However, the poor crystallinity and short hole diffusion length limit its charge separation. One bottleneck stems from the contradiction between high crystallinity and small particles via conventional furnace heating processes. This paper describes the design and fabrication of BiVO4 photoanodes via rapid thermal process (RTP), rather than furnace heating, to decouple the constraints between nucleation, crystallization, and growth processes of BiVO4. The higher heating ramp rate of RTP compared with furnace heating promotes the fast diffusion of reactant molecules, which elevates the nucleation rate above the particle growth rate of BiVO4, leading to small particles with high crystallinity. Moreover, the ultra‐high heating temperature makes it possible to crystallize the small BiVO4 particle within a short time. Thus, a high crystallinity can be obtained for the RTP‐treated BiVO4 while maintaining small particle size, achieving a charge separation efficiency of up to 82%, 30% higher than that of furnace‐treated BiVO4 photoanode.

Hematite (α-Fe2O3) with Oxygen Defects: The Effect of Heating Rate for Photocatalytic Performance.

Hematite (α-Fe2O3) emerges as an enticing material for visible-light-driven photocatalysis owing to its remarkable stability, low toxicity, and abundance. However, its inherent shortcomings, such as a short hole diffusion length and high recombination rate, hinder its practical application. Recently, oxygen vacancies (Vo) within hematite have been demonstrated to modulate its photocatalytic attributes. The effects of Vo can be broadly categorized into two opposing aspects: (1) acting as electron donors, enhancing carrier conductivity, and improving photocatalytic performance and (2) acting as surface carrier traps, accelerating excited carrier recombination, and deteriorating performance. Critically, the generation rate, distribution, role, and behavior of Vo significantly differ for synthesis methods due to differences in formation mechanisms and oxygen diffusion. This complexity hampers simplified discussions of Vo, necessitating careful investigation and nuanced discussion tailored to the specific method and conditions employed. Among various approaches, hydrothermal synthesis offers a simple and cost-effective route. Here, we demonstrate a hydrothermal synthesis method for Vo introduction to hematite using a carbon source, where variations in the heating rate have not been previously explored in terms of their influence on Vo generation. The analyses revealed that the concentration of Vo was maximized at a heating rate of 16 °C/min, indicative of a high density of surface defects. With regard to photocatalytic performance, elevated heating rates (16 °C/min) fostered the formation of Vo primarily on the hematite surface. The photocatalytic activity was 7.1 times greater than that of the sample prepared at a low heating rate (2 °C/min). These findings highlight the crucial role of surface defects, as opposed to bulk defects, in promoting hematite photocatalysis. Furthermore, the facile control over Vo concentration achievable via manipulating the heating rate underscores the promising potential of this approach for optimizing hematite photocatalysts.

Nanostructured porous WO3 for photoelectrochemical splitting of seawater

Seawater, due to its salinity, can serve as an electrolyte in photoelectrochemical (PEC) systems, which have the potential to enable solar-driven production of hydrogen and oxidizing species. In this study several micrometer thick and highly porous nanostructured WO3 films were formed and their performance in photoanodic oxidation of chloride and sulfate ions, the most abundant anionic species in seawater, was investigated. Layers were characterized using scanning electron microscopy, X-ray diffraction, diffuse reflectance spectroscopy, and BET surface area measurements. The study demonstrates that efficient PEC performance is achieved through optimal light absorption by sufficiently thick WO3 layers (3–5 μm), a highly porous structure that facilitates electrolyte permeation and ionic transport and provides a large electrochemically active interfacial area, and a nanosheet morphology where the sheet thickness is shorter than the hole diffusion length in WO3. The Faradaic efficiency of ClO- + ClO2- generation in neutral chloride-based electrolytes was found to be close to 100 %, but decreased significantly with photoelectrolysis time, presumably, due to the formation of higher chlorine oxo compounds. In addition, photocurrent stability and dissolution of WO3 photoanodes in acidic sulfate and neutral chloride media was compared. Though dissolution of WO3 in 0.5 M NaCl was found to be up to 3 times higher, photocurrent decay was more significant in 0.5 M H2SO4. The phenomenon was attributed to effective hole scavenging by surface-adsorbed Cl- ions, which prevents passivation of the photoelectrode surface due to formation of W(VI) peroxospecies.

Temperature‐Dependent Interplay between Structural and Charge Carrier Dynamics in CsMAFA‐Based Perovskites

AbstractState‐of‐the‐art triple cation, mixed halide perovskites are extensively studied in perovskite solar cells, showing very promising performance and stability. However, an in‐depth fundamental understanding of how the phase behavior in Cs0.05FA0.85MA0.10Pb(I0.97Br0.03)3 (CsMAFA) affects the optoelectronic properties is still lacking. The refined unit cell parameters a and c in combination with the thermal expansion coefficients derived from X‐ray diffraction patterns reveal that CsMAFA undergoes an α–β phase transition at ≈280 K and another transition to the γ‐phase at ≈180 K. From the analyses of the electrodeless microwave photoconductivity measurements it is shown that shallow traps only in the γ‐phase negatively affect the charge carrier dynamics. Most importantly, CsMAFA exhibits the lowest amount of microstrain in the β‐phase at around 240 K, corresponding to the lowest amount of trap density, which translates into the longest charge carrier diffusion length for electrons and holes. Below 200 K a considerable increase in deep trap states is found most likely related to the temperature‐induced compressive microstrain leading to a huge imbalance in charge carrier diffusion lengths between electrons and holes. This work provides valuable insight into how temperature‐dependent changes in structure affect the charge carrier dynamics in FA‐rich perovskites.

Hole diffusion effect on the minority trap detection and non-ideal behavior of NiO/β-Ga2O3 heterojunction

A NiO/β-Ga2O3 heterojunction was fabricated by sputtering a highly p-doped NiO layer onto β-Ga2O3. This heterojunction showed a low leakage current and a high turn-on voltage (Von) compared to a Ni/β-Ga2O3 Schottky barrier diode. The extracted Von from the NiO/β-Ga2O3 heterojunction's forward current–voltage characteristics was ∼1.64 V, which was lower than the extracted built-in potential voltage (Vbi) obtained from the capacitance–voltage curve. To explain this difference, deep level transient spectroscopy and Laplace-deep level transient spectroscopy were employed to study majority and minority traps in β-Ga2O3 films. A minority trap was detected near the surface of β-Ga2O3 under a reverse bias of −1 V but was not observed at −4 V, indicating its dependence on hole injection density. Using Silvaco TCAD, the hole diffusion length from P+-NiO to β-Ga2O3 was determined to be 0.15 μm in equilibrium, which is increased with increasing forward voltage. This finding explained why the trap level was not detected at a large reverse bias. Moreover, hole diffusion from NiO into β-Ga2O3 significantly affected the β-Ga2O3 surface band bending and impacted transport mechanisms. It was noted that the energy difference between the conduction band minimum (CBM) of β-Ga2O3 and the valence band maximum (VBM) of NiO was reduced to 1.60 eV, which closely matched the extracted Von value. This supported the dominance of direct band-to-band tunneling of electrons from the CBM of β-Ga2O3 to the VBM of NiO under forward bias voltage.

Impact of Solid-State Charge Injection on Spectral Photoresponse of NiO/Ga2O3 p–n Heterojunction

Forward bias hole injection from 10-nm-thick p-type nickel oxide layers into 10-μm-thick n-type gallium oxide in a vertical NiO/Ga2O3 p–n heterojunction leads to enhancement of photoresponse of more than a factor of 2 when measured from this junction. While it takes only 600 s to obtain such a pronounced increase in photoresponse, it persists for hours, indicating the feasibility of photovoltaic device performance control. The effect is ascribed to a charge injection-induced increase in minority carrier (hole) diffusion length (resulting in improved collection of photogenerated non-equilibrium carriers) in n-type β-Ga2O3 epitaxial layers due to trapping of injected charge (holes) on deep meta-stable levels in the material and the subsequent blocking of non-equilibrium carrier recombination through these levels. Suppressed recombination leads to increased non-equilibrium carrier lifetime, in turn determining a longer diffusion length and being the root-cause of the effect of charge injection.