Hole Diffusion Length Research Articles

Determination of hole diffusion length in n-GaN

The paper presents the derivation of a model for minority carriers collection based on the reciprocity theorem and its application for determination of hole diffusion length in n-GaN by means of photoluminescence. The estimated hole diffusion lengths at room temperature are 110 nm and 194 nm in the case of low and high excitation, respectively, which could be explained by saturation of non-radiative recombination centers in bulk GaN and at the surface with photogenerated carriers.

Defects in Hybrid Perovskites: The Secret of Efficient Charge Transport (Adv. Funct. Mater. 48/2021)

Charge Transport A combination of optical, electric, and thermal spectroscopy tools shed light on the ground-breaking interaction of defects with free charges in halide perovskite optoelectronic devices. For the first time, in article number 2104467, Artem Musiienko and co-workers demonstrate the effect of defects on diffusion length, lifetime, and mobility of holes and electrons separately. Advanced characterization of charge transport and defects can stimulate new device architectures and material modification pathways.

Surface Photo-Voltage (SPV) and Photo electrochemistry (PEC) on TiO2

Surface charge transfer processes at semiconductors have been of great importance as the quality of the material is vital in various applications such as electronics, photovoltaics, catalysis, electrocatalysis, photocatalysis, battery, supercapacitors to name but a few. Surface Photo-voltage (SPV) is a unique and simple technique based on the spectral dependence of the photovoltage as a function of photon energy applied as non-destructive and contactless method. It has been applied for the characterization of semiconductors such as band gap, surface potential, doping concentration, especially the minority carrier lifetime and diffusion length correlating to size, shape, thickness etc. Its simplicity including sample preparation, contacts, junction formation, etc. is an advantage over several other spectroscopic methods since it can offer information on submerged heterostructure layers.We will present characterization of several size and shapes of TiO2 (thin films, nanorods, etc.) towards its hole diffusion length using SPV (solid/solid) and compared with photoelectrochemical (PEC) technique (solid/liquid). We will point out the application of SPV and PEC on TiO2 coated LiCoO2, for possible insight on the stability and increased capacity in LIB (lithium-ion battery) from a thermodynamic and kinetic point of view. We will discuss the application of SPV technique in understanding the mechanism related to decreased surface oxygen activity and change in the activation energy of Li ion transfer.

Surface Photo-Voltage (SPV) and Photo electrochemistry (PEC) on TiO2

Hydrothermally grown rutile titanium dioxide (TiO2) nanorod array photoanode with various nanorod lengths were investigated for understanding the charge carrier separation including hole diffusion length using Photoelectrochemistry (PEC) and surface photo voltage (SPV) measurements for their importance in photoconversion as photoelectrode for solar water splitting. Various lengths of TiO2 nanorods with lengths 1.35, 1.85, 2.5 and 4.5 micrometers were prepared and used in the present investigation. Front and back illumination in SPV and PEC Mott-Schottky plots were investigated and found to show significant difference in charge carrier separation. The hole diffusion length calculations using PEC was up to 200 nm and using SPV was in the order of 600-700nm.

Photoelectrochemical Application and Charge Transport Dynamics of a Water-Stable Organic-Inorganic Halide (C6H4NH2CuCl2I) Film in Aqueous Solution.

A water-stable thin film composed of C6H4NH2CuCl2I was fabricated using spin-coating precursor solutions that dissolved equimolar amounts of C6H4NH2I and CuCl2 in N,N-dimethylformamide. Photoelectrochemical characteristics show that the C6H4NH2CuCl2I film demonstrated a stable photocurrent (∼1 μA/cm2) in an aqueous solution under white light (11.5 mW/cm2) even after 3000 s, while exhibiting a photon-to-current efficiency of 0.093% under AM1.5 (100 mW/cm2) illumination. However, these values were significantly lower than those of the CH3NH3PbX3 (X = I, Cl) film in solid devices. The electron diffusion length L(e-) (373 nm) and hole diffusion length L(h+) (177 nm) in the C6H4NH2CuCl2I photoelectrode were significantly lower than those of CH3NH3PbX3, limiting the photoelectrochemical and photocatalysis performances. Moreover, L(h+) was shorter than L(e-) in the C6H4NH2CuCl2I photoelectrode, resulting in the hole-collecting efficiency [ηc(h+)] being lower than the electron-collecting efficiency [ηc(e-)]. A CuO interlayer was introduced as a hole transport layer for the C6H4NH2CuCl2I photoelectrode, which improved L(h+) and ηc(h+).

Defects in Hybrid Perovskites: The Secret of Efficient Charge Transport

AbstractThe interaction of free carriers with defects and some critical defect properties are still unclear in methylammonium lead halide perovskites (MHPs). Here, a multi‐method approach is used to quantify and characterize defects in single crystal MAPbI3, giving a cross‐checked overview of their properties. Time of flight current waveform spectroscopy reveals the interaction of carriers with five shallow and deep defects. Photo‐Hall and thermoelectric effect spectroscopy assess the defect density, cross‐section, and relative (to the valence band) energy. The detailed reconstruction of free carrier relaxation through Monte Carlo simulation allows for quantifying the lifetime, mobility, and diffusion length of holes and electrons separately. Here, it is demonstrated that the dominant part of defects releases free carriers after trapping; this happens without non‐radiative recombination with consequent positive effects on the photoconversion and charge transport properties. On the other hand, shallow traps decrease drift mobility sensibly. The results are the key for the optimization of the charge transport properties and defects in MHP and contribute to the research aiming to improve perovskite stability. This study paves the way for doping and defect control, enhancing the scalability of perovskite devices with large diffusion lengths and lifetimes.

Photovoltaic behavior of centimeter-long lateral organic junctions

In this study, the photovoltaic behavior of centimeter-long lateral organic junctions, reaching 1.8 cm, is reported. The organic junctions are formed using organic semiconductor films with high mobilities of holes and electrons. The lateral diffusion lengths of photogenerated electrons and holes are 4.7 and 5.5 mm, respectively. The photovoltaic behavior in the centimeter-long lateral junctions is controlled by the trap-assisted recombination between the electrons and holes.

Preparation of multilayer periodic nanopatterned WO3-based photoanode by reverse nanoimprinting for water splitting

Photoelectrochemical (PEC) water splitting has been studied extensively as an environmentally friendly technology for hydrogen production using solar energy. WO3 is considered a promising semiconducting material for photoanodes due to its high electron mobility, good hole diffusion length, and chemical stability. Periodic nanostructures of WO3 have been investigated for enhancing the PEC performance of WO3-based photoanodes. In this study, facile fabrication of periodic nanostructures of WO3 was achieved using reverse nanoimprint lithography, and the multilayer stacking of nanopatterned WO3 film was also confirmed. The multilayer nanopatterned WO3 films were used as photoanodes for PEC water splitting. The performance of the fabricated photoanode in PEC was 2 times higher than that of planar WO3 film due to its higher light absorbance and lower charge transfer resistance.

High-resolution planar electron beam induced current in bulk diodes using high-energy electrons

Understanding the impact of high-energy electron radiation on device characteristics remains critical for the expanding use of semiconductor electronics in space-borne applications and other radiation harsh environments. Here, we report on in situ measurements of high-energy electron radiation effects on the hole diffusion length in low threading dislocation density homoepitaxial bulk n-GaN Schottky diodes using electron beam induced current (EBIC) in high-voltage scanning electron microscopy mode. Despite the large interaction volume in this system, quantitative EBIC imaging is possible due to the sustained collimation of the incident electron beam. This approach enables direct measurement of electron radiation effects without having to thin the specimen. Using a combination of experimental EBIC measurements and Monte Carlo simulations of electron trajectories, we determine a hole diffusion length of 264 ± 11 nm for n-GaN. Irradiation with 200 kV electron beam with an accumulated dose of 24 × 1016 electrons cm−2 led to an approximate 35% decrease in the minority carrier diffusion length.

Electron beam probing of non-equilibrium carrier dynamics in 18 MeV alpha particle- and 10 MeV proton-irradiated Si-doped β -Ga2O3 Schottky rectifiers

Minority hole diffusion length and lifetime were measured in independent experiments by electron beam-induced current and time-resolved cathodoluminescence in Si-doped β-Ga2O3 Schottky rectifiers irradiated with 18 MeV alpha particles and 10 MeV protons. Both diffusion length and lifetime exhibited a decrease with increasing temperature. The non-equilibrium minority hole mobility was calculated from the independently measured diffusion length and lifetime, indicating that the so-called hole self-trapping is most likely irrelevant in the 77–295 K temperature range.

On interface recombination, series resistance, and absorber diffusion length in BiI3 solar cells

Bismuth triiodide is a lead-free direct wide-bandgap solution-processable semiconductor that could be an alternative to lead-based perovskites in tandem or multijunction solar cells. However, the power conversion efficiency of single-junction BiI3 solar cells remains low. Here, we determine the main loss mechanisms of BiI3 solar cells in both n-i-p and p-i-n architectures. Overall, p-i-n devices have higher power conversion efficiency than that of n-i-p. It is found that n-i-p devices have higher (and significant) non-radiative recombination at the interface of the BiI3/transport layer, resulting in a lower open-circuit voltage than p-i-n devices. Moreover, the high series resistance (>70 Ω cm2) and a low average electron–hole diffusion length (∼60 nm) contributes to the low short-circuit current density (<5 mA/cm2) and fill factor (<40%) in all devices. In addition, interface recombination also reduces short-circuit current density. Finally, we demonstrate that lithium doping of BiI3 can increase the diffusion length of BiI3 to improve the performance of BiI3 solar cells. Solar cells with the configuration ITO/NiOx/Li:BiI3/PC61BM/bis-C60/LiF/Ag obtain a power conversion efficiency of 1.3% under AM 1.5 G illumination. The deep understanding of the main loss mechanisms of this work paves the way for future optimization of BiI3 solar cells.

How to Reduce Charge Recombination in Organic Solar Cells: There are Still Lessons to Learn from P3HT:PCBM

AbstractSuppressing charge recombination is key for organic solar cells to become commercial reality. However, there is still no conclusive picture of how recombination losses are influenced by the complex nanoscale morphology. Here, new insight is provided by revisiting the P3HT:PCBM blend, which is still one of the best performers regarding reduced recombination. By changing small details in the annealing procedure, two model morphologies are prepared that vary in phase separation, molecular order, and phase purity, as revealed by electron tomography and optical spectroscopy. Both systems behave very similarly with respect to charge generation and transport, but differ significantly in bimolecular recombination. Only the system containing P3HT aggregates of high crystalline quality and purity is found to achieve exceptionally low recombination rates. The high‐quality aggregates support charge delocalization, which assists the re‐dissociation of interfacial charge‐transfer states formed upon the encounter of free carriers. For devices with the optimized morphology, an exceptional long hole diffusion length is found, which allows them to work as Shockley‐type solar cells even in thick junctions of 300 nm. In contrast, the encounter rate and the size of the phase‐separated domains appear to be less important.

Enhanced Photoelectrochemical Performance of Hematite Photoanode by Decorating NiCoP Nanoparticles Through a Facile Spin Coating Method

Hematite (α-Fe2O3) is a potential photoanode material for photoelectrochemical (PEC) water splitting, but its short hole diffusion length and low water oxidation kinetics prevent its practical application. In general, the hole transport rate and the water oxidation kinetics could be improved by modifying the oxygen evolution cocatalysts (OECs) in the proper way. In this work, we designed and synthesized a bimetallic phosphides NiCoP nanoparticles (NPs) modified (Ti, Zr)-codoped hematite photoanode (TZ-H) by a facile spin coating method to achieve the uniform and close interface contact of semiconductor/OECs. Under the optimal conditions, the as-prepared photoanode exhibits high and stable PEC performance with a current density of 2.38 mA/cm2 at 1.23 V vs. RHE, which is one of the best performances of noble-metal-free hematite-based photoanodes for water oxidation. Moreover, the surface efficiency of charge separation is improved by NiCoP modification, which is increased from 59% to 91% at 1.23 V vs. RHE and reached nearly 99% at higher potential. This work not only proves that NiCoP NPs are outstanding OECs but also provides a new strategy for the design of noble-metal-free semiconductor/OECs photoanodes with high-quality interface contact.

How Does Bridging Core Modification Alter the Photovoltaic Characteristics of Triphenylamine-Based Hole Transport Materials? Theoretical Understanding and Prediction.

Perovskite solar cells have gained immense interest from researchers owing to their good photophysical properties, low-cost production, and high power conversion efficiencies. Hole transport materials (HTMs) play a dominant role in enhancing the power conversion efficiencies (PCEs) and long diffusion length of holes and electrons in perovskite solar cells. In hole transport materials, modification of π-linkers has proved to be an efficient approach for enhancing the overall PCE of perovskite solar cells. In this work, π-linker modification of a recently synthesized H-Bi molecule (R) is achieved with novel π-linkers. After structural modifications, ten novel HTMs (HB1-HB10) with a D-π-D backbone are obtained. The structure-property relationship, and optoelectronic and photovoltaic characteristics of these newly designed hole transport materials are examined comprehensively and compared with reference molecules. In addition, different geometric parameters are also examined with the assistance of density functional theory (DFT) and time-dependent DFT. All the designed molecules exhibit narrow HOMO-LUMO energy gaps (Eg =2.82-2.99 eV) compared with the R molecule (Eg =3.05 eV). The designed molecules express redshifting in their absorption spectra with low values of excitation energy, which in return offer high power conversion efficiencies. Further, density of states and molecular electrostatic potential analysis is performed to locate the different charge sites in the molecules. The reorganizational energies of holes and electrons are found to have good values, suggesting that these novel designed molecules are efficient hole transport materials for perovskite solar cells. In addition, the low binding energy values of the designed molecules (compared with R) offer high current charge density. Finally, complex study of HB9:PC61 BM is also undertaken to understand the charge transfer between the molecules of the complex. The results of all analyses advocate that these novel designed HTMs are promising candidates for the construction of future high-performance perovskite solar cells.

Controlling the carrier density in niobium oxynitride BaNbO2N via cation doping for efficient photoelectrochemical water splitting under visible light

Lower-valent cation doping enables appropriate reduction of undesirably high donor density in BaNbO2N, providing both a suitable electron conductivity and hole diffusion length with high photoelectrochemical performance for water splitting.

Boosting light harvesting and charge separation of WO3 via coupling with Cu2O/CuO towards highly efficient tandem photoanodes.

Photoanodes based on semiconductor WO3 have been attractive due to its good electron mobility, long hole-diffusion length, and suitable valence band potential for water oxidation. However, the semiconductor displays disadvantages including a relatively wide bandgap, poor charge separation and transfer, and quick electron–hole recombination at the interface with the electrolyte. Here we present a significantly improved photoanode with a tandem structure of ITO/WO3/Cu2O/CuO, which is prepared first by hydrothermally growing a layer of WO3 on the ITO surface, then by electrodepositing an additional layer of Cu2O, and finally by heat-treating in the air to form an exterior layer of CuO. Photocurrent measurements reveal that the prepared photoanode produces a maximum current density of 4.7 mA cm−2, which is, in comparison, about 1.4 and 5.5 times the measured values for ITO/WO3/Cu2O and ITO/WO3 ones, respectively. These enhancements are attributed to (1) harvested UV, visible, and NIR light of the solar spectrum, (2) accelerated charge separation at the heterojunction between WO3 and Cu2O/CuO, (3) better electrocatalytic activity of formed CuxO than pure Cu2O, (4) formation of a protective layer of CuO. This study thus may lead to a promising way to make high-performance and low-cost photoanodes for solar energy harvesting.

Vacancy defect engineering of BiVO4 photoanodes for photoelectrochemical water splitting.

Photoelectrochemical (PEC) water splitting has been regarded as a promising technology for sustainable hydrogen production. The development of efficient photoelectrode materials is the key to improve the solar-to-hydrogen (STH) conversion efficiency towards practical application. Bismuth vanadate (BiVO4) is one of the most promising photoanode materials with the advantages of visible light absorption, good chemical stability, nontoxic feature, and low cost. However, the PEC performance of BiVO4 photoanodes is limited by the relatively short hole diffusion length and poor electron transport properties. The recent rapid development of vacancy defect engineering has significantly improved the PEC performance of BiVO4. In this review article, the fundamental properties of BiVO4 are presented, followed by an overview of the methods for creating different kinds of vacancy defects in BiVO4 photoanodes. Then, the roles of vacancy defects in tuning the electronic structure, promoting charge separation, and increasing surface photoreaction kinetics of BiVO4 photoanodes are critically discussed. Finally, the major challenges and some encouraging perspectives for future research on vacancy defect engineering of BiVO4 photoanodes are presented, providing guidelines for the design of efficient BiVO4 photoanodes for solar fuel production.

Temperature dependence of diffusion length and mobility in mid-wavelength InAs/InAsSb superlattice infrared detectors

In the past decade, infrared detectors with InAs/InAsSb (Gallium-free) type-II strained layer superlattice absorbers became a technology of interest for many imaging applications. In this work, we study the dependence of minority carrier (hole) transport, absorption coefficient, and quantum efficiency (QE) of a 5.6 μm cutoff wavelength mid-wavelength infrared InAs/InAsSb detector on temperatures and applied bias. We found that the minority carrier lifetime is very long (τ ≈ 5.5 μs) and is temperature independent in the temperature range T = 50–150 K. The back-side illuminated QE without anti-reflection coating increases from ∼30% at T = 50 K to ∼60% at T = 180 K. The minority carrier (hole) diffusion length, Ldh, was found from QE and absorption coefficient. The hole diffusion length at T = 50 K is Ldh = 2.4 μm and increases monotonically to Ldh = 7.2 μm at T = 180 K. The hole mobility, calculated from diffusion length and minority carrier lifetime, is μh = 4.5 cm2/V s at T = 50 K and increases with temperature to reach μh = 7.2 cm2/V s at T = 150 K. In addition, we find that at lower temperatures where the diffusion length is shorter, the stronger QE dependence on applied bias is due to minority carrier collection from the depletion region, whose width increases with applied bias.

Photoactive Tungsten-Oxide Nanomaterials for Water-Splitting.

This review focuses on tungsten oxide (WO3) and its nanocomposites as photoactive nanomaterials for photoelectrochemical cell (PEC) applications since it possesses exceptional properties such as photostability, high electron mobility (~12 cm2 V−1 s−1) and a long hole-diffusion length (~150 nm). Although WO3 has demonstrated oxygen-evolution capability in PEC, further increase of its PEC efficiency is limited by high recombination rate of photogenerated electron/hole carriers and slow charge transfer at the liquid–solid interface. To further increase the PEC efficiency of the WO3 photocatalyst, designing WO3 nanocomposites via surface–interface engineering and doping would be a great strategy to enhance the PEC performance via improving charge separation. This review starts with the basic principle of water-splitting and physical chemistry properties of WO3, that extends to various strategies to produce binary/ternary nanocomposites for PEC, particulate photocatalysts, Z-schemes and tandem-cell applications. The effect of PEC crystalline structure and nanomorphologies on efficiency are included. For both binary and ternary WO3 nanocomposite systems, the PEC performance under different conditions—including synthesis approaches, various electrolytes, morphologies and applied bias—are summarized. At the end of the review, a conclusion and outlook section concluded the WO3 photocatalyst-based system with an overview of WO3 and their nanocomposites for photocatalytic applications and provided the readers with potential research directions.

Impact of electron injection on carrier transport and recombination in unintentionally doped GaN

The impact of electron injection on minority carrier (hole) diffusion length and lifetime at variable temperatures was studied using electron beam-induced current, continuous, and time-resolved cathodoluminescence techniques. The hole diffusion length increased from 306 nm to 347 nm with an electron injection charge density up to 117.5 nC/μm3, corresponding to the lifetime changing from 77 ps to 101 ps. Elongation of the diffusion length was attributed to the increase in the non-equilibrium carrier lifetime, which was determined using ultrafast time-resolved cathodoluminescence and related to non-equilibrium carrier trapping on gallium vacancy levels in the GaN forbidden gap.