Diffusion Length Of Charge Carriers Research Articles

Performance Potential for Locally Contacted Perovskite Solar Cells

In perovskite solar cells (PSCs), a common characteristic of highly effective interface passivation materials is low conductivity. Gains in voltage are thus often disproportionately offset by resistive losses. Local contact approaches can minimize this trade‐off and have a proven track record in conventional silicon photovoltaics. Indeed, recent record efficiencies for centimeter‐scale PSCs exploit architectures where the passivation layer partially covers the perovskite‐transport layer interface. Herein, a three‐dimensional numerical device model is used to determine practical performance limits to local contact geometries and consider both the optimum contact dimensions and the trade‐offs involved in relaxing these dimensions for ease of fabrication. It is observed that the potential for substantial power conversion efficiency (PCE) increases with local contacts. In devices where power loss occurs solely through recombination at the contacted interface, PCE can be enhanced by up to 10% absolute compared to a full‐area contact. However, optimum PCEs depend on contacts on the order of nanometers. It is shown that more fabrication‐friendly micrometer‐scale contacts still boost PCE, but the absolute enhancement falls short due to the relatively low bulk perovskite charge carrier diffusion length. This may ultimately motivate methods of interface engineering that produce “effective” local contact geometries at nanometer scales, such as via self‐forming layers.

Trap states and carrier diffusion lengths in NiO/β-Ga2O3 heterojunctions

We report the electrical properties, deep trap spectra, and diffusion lengths of non-equilibrium carriers in Ni Schottky diodes and NiO/Ga2O3 heterojunctions (HJs) prepared on the same n−/n+ β-Ga2O3 epi structures. The heterojunctions decrease the reverse current of Ga2O3 high-power rectifiers. In HJs, in contrast to Schottky diodes, the capacitance and AC conductance show a prominent frequency and temperature dependence, suggesting the presence of two temperature activation processes with activation energies of 0.17 and 0.1 eV. The deep trap spectra of the Schottky diodes and HJs differ by the absence in the HJ of deep electron traps E2* with level near Ec − 0.7 eV considered to be an important center of non-radiative recombination. This correlates with the observed increase in the diffusion length of non-equilibrium charge carriers in the HJs to 370 nm compared to 240 nm in the Schottky diodes. The diffusion length of charge carriers in p-NiO was found to be quite short, 30 nm. Possible reasons for the observed differences and possible origin of the minority-trap-like feature commonly reported to be present in the deep level spectra of HJs and also observed in our experiments are discussed.

Performance analysis of ecofriendly Ge based perovskite solar cell using computational approach

The present work deals with the simulation of lead-free germanium (Ge) based perovskite solar cell having structure: FTO/SnO2/MAGeI3/Spiro-OMeTAD/Au using one dimensional SCAPS simulator. Initially, the optimisations of absorber thickness and absorber defect density are performed to obtain an efficient and stable device structure. This optimisation results into higher conversation efficiency i.e., 25.34 % (Voc = 1.18 V, Jsc = 24.79 mA/cm2 and FF = 86.54 %). Further, the relations between optimised thickness and absorber defects are also verified and it is found that due to lower diffusion length of charge carriers, the optimised thickness decreases with increase in absorber defect.

Trap States and Carrier Diffusion in Czochralski (100) Single Crystal β-Ga2O3

Deep trap spectra and carrier diffusion lengths were measured for unintentionally doped β-Ga2O3 bulk crystals with (100) orientation. The 20-mm diameter, 15-mm length boule was pulled by the Czochralski method from gallium oxide in (010) direction. It is found that the net density of shallow donors in (100) plates cleaved from the crystal was 2.6 × 1017 cm−3, with ionization energies of 0.05 eV measured from admittance spectra. Three deep electron traps with respective ionization energies of 0.6 eV (concentration 1.1 × 1014 cm−3), 0.8 eV (concentration 3.9 × 1016 cm−3) and 1.1 eV (concentration 8.9 × 1015 cm−3) were detected by Deep Level Transient Spectroscopy. The dominant 0.8 eV trap is associated with the E2 centers due to Fe acceptors, the two other traps are the well documented E1 and E3 centers. The major deep acceptors in the lower half of the bandgap have optical ionization threshold of 2.3 eV and concentration of 4 × 1015 cm−3 and are believed to be due to the split Ga vacancies acceptors. The diffusion length of non-equilibrium charge carriers was 90 nm. The electrical properties of these (100) oriented crystals grown by Czochralski are quite similar to those synthesized by the undoped Edge-defined Film-Fed Growth technique.

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.

Obviating Ligand Exchange Preserves the Intact Surface of HgTe Colloidal Quantum Dots and Enhances Performance of Short Wavelength Infrared Photodetectors.

A large volume, scalable synthesis procedure of HgTe quantum dots (QDs) capped initially with short-chain conductive ligands ensures ligand exchange-free and simple device fabrication. An effective n- or p-type self-doping of HgTe QDs is achieved by varying cation-anion ratio, as well as shifting the Fermi level position by introducing single- or double-cyclic thiol ligands, that is, 2-furanmethanethiol (FMT) or 2,5-dimercapto-3,4-thiadiasole (DMTD) in the synthesis. This allows for preserving the intact surface of the HgTe QDs, thus ensuring a one order of magnitude reduced surface trap density compared with HgTe subjected to solid-state ligand exchange. The charge carrier diffusion length can be extended from 50 to 90nm when the device active area consists of a bi-layer of cation-rich HgTe QDs capped with DMTD and FMT, respectively. As a result, the responsivity under 1340nm illumination is boosted to 1 AW-1 at zero bias and up to 40 AW-1 under -1V bias at room temperature. Due to high noise current density, the specific detectivity of these photodetectors reaches up to 1010 Jones at room temperature and under an inert atmosphere. Meanwhile, high photoconductive gain ensures a rise in the external quantum efficiency of up to 1000% under reverse bias.

CsPbBr3 Nanocrystals as Efficient Photocatalysts for Dehydrohalogenation: Toward Environmentally Friendly Trichloroethylene Synthesis.

The demand for a benign alternative to energy-intensive industrial chemical transformations is critical. Lead halide perovskites have emerged as promising candidates due to their unique optoelectronic properties, including high absorption coefficients in the visible region, tunable band gaps, and long charge carrier-diffusion lengths. In this study, we present a model reaction to showcase the photocatalytic utility of perovskite nanocrystals (NCs). Specifically, we demonstrate the synthesis of trichloroethylene (TCEt) from 1,1,2,2-tetrachloroethane (TCE) using CsPbBr3 NCs under white light illumination. The band-edge positions of the NCs and the redox potential of TCE enable efficient electron transfer for C-Cl bond activation. Furthermore, while ensuring operational stability, CsPbBr3 NCs undergo light-controlled modification, leading to the formation of mixed-halide perovskite (CsPbBrxCl3-x) NCs during the reaction. This procedure yields a mixed-halide perovskite that maintains stability while containing the desired halide content. Additionally, the reaction produces HBr as a byproduct, serving as a self-cleaning technique to eliminate excess Br- ions from the solution. Ultimately, we achieve nearly 100% conversion of CsPbBr3 to pure CsPbCl3 NCs, with a full width at half-maximum of approximately 11.2 nm. Our clean and efficient approach to synthesizing TCEt using perovskite NCs provides interesting insights into violet light-emitting diode (LED) fabrication and color patterning. This study highlights the promising potential of perovskite materials for sustainable chemical transformations and optoelectronic applications.

Impact of Intermolecular Interactions between Halogenated Volatile Solid Additives and the Nonfullerene Acceptor in Organic Solar Cells

AbstractThe halogenated volatile solid additives can delicately optimize the active layer morphology of organic solar cells, improving the devices' performance, stability, and reproducibility. However, what type of intermolecular interaction occurs between the solid additives and the active layer and whether the interaction truly impacts the donor or acceptor remains debatable. Herein, the focus is on halogenated volatile solid additives with conjugated benzene rings and their influence on the morphology of the active layer composed of PM6:Y6 as they evaporated. The absorbance spectra exhibit apparent red‐shift features in Y6 absorption regions, while the donor part is unaffected. The theoretical calculation results reveal that the additives can stay between two Y6 molecules and form halogen bonds, affecting the π–π aggregation properties of Y6. As a result, the crystalline features of the active layer are altered, leading to increased charge carrier mobilities, extended charge carrier diffusion lengths, reduced bimolecular charge recombination, and thus the device performance. Especially when 1,3,5‐tri bromobenzene is used, a champion power conversion efficiency of 17.9% is attained, among the best‐performed organic solar cells comprising PM6:Y6. The findings shed light on theoretical and experimental guidelines for designing and developing volatile solid additives for highly efficient nonfullerene organic solar cells.

Transport and trap states in proton irradiated ultra-thick κ-Ga2O3

Changes induced by irradiation with 1.1 MeV protons in the transport properties and deep trap spectra of thick (>80 μm) undoped κ-Ga2O3 layers grown on sapphire are reported. Prior to irradiation, the films had a donor concentration of ∼1015 cm−3, with the two dominant donors having ionization energies of 0.25 and 0.15 eV, respectively. The main electron traps were located at Ec−0.7 eV. Deep acceptor spectra measured by capacitance-voltage profiling under illumination showed optical ionization thresholds near 2, 2.8, and 3.4 eV. The diffusion length of nonequilibrium charge carriers for ɛ-Ga2O3 was 70 ± 5 nm prior to irradiation. After irradiation with 1.1 MeV protons to a fluence of 1014 cm−2, there was total depletion of mobile charge carriers in the top 4.5 μm of the film, close to the estimated proton range. The carrier removal rate was 10–20 cm−1, a factor of 5–10 lower than in β-Ga2O3, while the concentration of deep acceptors in the lower half of the bandgap and the diffusion length showed no significant change.

Electrical and Recombination Properties of Polar Orthorhombic κ-Ga2O3 Films Prepared by Halide Vapor Phase Epitaxy

In this study, the structural and electrical properties of orthorhombic κ-Ga2O3 films prepared using Halide Vapor Phase Epitaxy (HVPE) on AlN/Si and GaN/sapphire templates were studied. For κ-Ga2O3/AlN/Si structures, the formation of two-dimensional hole layers in the Ga2O3 was studied and, based on theoretical calculations, was explained by the impact of the difference in the spontaneous polarizations of κ-Ga2O3 and AlN. Structural studies indicated that in the thickest κ-Ga2O3/GaN/sapphire layer used, the formation of rotational nanodomains was suppressed. For thick (23 μm and 86 μm) κ-Ga2O3 films grown on GaN/sapphire, the good rectifying characteristics of Ni Schottky diodes were observed. In addition, deep trap spectra and electron beam-induced current measurements were performed for the first time in this polytype. These experiments show that the uppermost 2 µm layer of the grown films contains a high density of rather deep electron traps near Ec - 0.3 eV and Ec - 0.7 eV, whose presence results in the relatively high series resistance of the structures. The diffusion length of the excess charge carriers was measured for the first time in κ-Ga2O3. The film with the greatest thickness of 86 μm was irradiated with protons and the carrier removal rate was about 10 cm-1, which is considerably lower than that for β-Ga2O3.

Mesoporous CuFe2 O4 Photoanodes for Solar Water Oxidation: Impact of Surface Morphology on the Photoelectrochemical Properties.

Metal oxide-based photoelectrodes for solar water splitting often utilize nanostructures to increase the solid-liquid interface area. This reduces charge transport distances and increases the photocurrent for materials with short minority charge carrier diffusion lengths. While the merits of nanostructuring are well established, the effect of surface order on the photocurrent and carrier recombination has not yet received much attention in the literature. To evaluate the impact of pore ordering on the photoelectrochemical properties, mesoporous CuFe2 O4 (CFO) thin film photoanodes were prepared by dip-coating and soft-templating. Here, the pore order and geometry can be controlled by addition of copolymer surfactants poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (Pluronic® F-127), polyisobutylene-block-poly(ethylene oxide) (PIB-PEO) and poly(ethylene-co-butylene)-block-poly(ethylene oxide) (Kraton liquid™-PEO, KLE). The non-ordered CFO showed the highest photocurrent density of 0.2 mA/cm2 at 1.3 V vs. RHE for sulfite oxidation, but the least photocurrent density for water oxidation. Conversely, the ordered CFO presented the best photoelectrochemical water oxidation performance. These differences can be understood on the basis of the high surface area, which promotes hole transfer to sulfite (a fast hole acceptor), but retards oxidation of water (a slow hole acceptor) due to electron-hole recombination at the defective surface. This interpretation is confirmed by intensity-modulated photocurrent (IMPS) and vibrating Kelvin probe surface photovoltage spectroscopy (VKP-SPS). The lowest surface recombination rate was observed for the ordered KLE-based mesoporous CFO, which retains spherical pore shapes at the surface resulting in fewer surface defects. Overall, this work shows that the photoelectrochemical energy conversion efficiency of copper ferrite thin films is not just controlled by the surface area, but also by surface order.

Template-free formation of BiOCl double-shelled hollow microspheres with enhanced carbamazepine removal efficiency

Constructing three-dimensional (3D) hierarchical structure is regarded as an effective strategy to alleviate the inherent drawbacks of the BiOCl bulk structure and realize the optimization of photocatalytic performance. Herein, a unique BiOCl double-shelled hollow microsphere structure with adjustable shell thickness is fabricated through a template-free one-pot solvothermal method for the first time. A plausible formation mechanism is proposed by exploring the morphological changes during the growth process. The as-obtained hollow microspheres photocatalyst exhibits excellent carbamazepine (CBZ) degradation efficiency (97.5%) under simulated solar irradiation, the corresponding apparent rate constant is about 4 times and 3 times than that of conventional BiOCl solid microspheres. The experimental and characterization results show that the existence of nanosheet subunits and porous structures shorten the diffusion length of charge carriers, which is conducive to the separation and transfer of photogenerated. Meanwhile, the construction of double-shelled hollow structure achieves higher light utilization. This work demonstrates the superior performance of catalysts with hierarchical hollow structures for pollutant degradation and provides new ideas to further optimize the photocatalytic performance of BiOCl-based materials.

Review on Enhancement of Stability and Efficiency of Perovskite Solar Cell

For photovoltaic applications, organic-inorganic hybrid perovskite solar cells have a extensive array of characteristics, including elevated absorption coefficients, exceptional carrier mobility, long charge carrier diffusion lengths, low cost, and incredible development. As emerging solar cell with thin film technology, these solar cells have generated many concerns. The elevated efficiency along with the low cost of materials and process are the main benefit of this cell over commercial silicon or other organic and inorganic solar cells. The foundations behind the optoelectronic description of perovskite materials and important methods for creating highly efficient perovskite solar cells have been covered in this paper. The degradation mechanisms of unstable perovskite materials and the associated solar cells are discussed. There are two more ways to increase the stability of perovskite materials and perovskite solar cells: interface engineering between the hole transport layer and the perovskite active layer and interface alteration between the electron transport layer and the perovskite layer. The future development of PSC architecture engineering is finally given a perspective and outlook.

Non-steady-state photo-EMF in a periodically poled MgO : LiNbO-=SUB=-3-=/SUB=- crystal

The diffusion mode of the non-steady-state photo-EMF excitation is investigated in periodically poled MgO : LiNbO3 crystal at light wavelength λ=457 nm. The signal excitation is carried out along the polar axis of the crystal; the dependencies of the signal amplitude versus the phase modulation frequency, intensity and spatial frequency of the interference pattern are studied in this geometry. The analysis of these dependencies allows determining the photoelectric parameters of the material --- the type, value and relaxation time of the photoconductivity, as well as the diffusion length of charge carriers. Keywords: non-steady-state photo-EMF, periodic domain struc ture, lithium niobate.

Нестационарная фотоэдс в периодически поляризованном кристалле MgO : LiNbO-=SUB=-3-=/SUB=-

The diffusion mode of the non-steady-state photo-EMF excitation is investigated in periodically poled MgO:LiNbO₃ crystal at light wavelength λ=457 nm. The signal excitation is carried out along the polar axis of the crystal; the dependencies of the signal amplitude versus the phase modulation frequency, intensity and spatial frequency of the interference pattern are studied in this geometry. The analysis of these dependencies allows determining the photoelectric parameters of the material – the type, value and relaxation time of the photoconductivity, as well as the diffusion length of charge carriers.

Recent progress in the stabilization of low band-gap black-phase iodide perovskite solar cells.

All-inorganic and organic-inorganic hybrid perovskite solar cells (PSCs) have taken a quantum leap owing to their high performance and low-cost solution processability. Their efficiency has been dramatically increased up to ∼26%, matching the conventional inorganic photovoltaics like monocrystalline Si (26.1%), polycrystalline Si (21.6%), CdTe (22.1%), and CIGS (22.3%). Such outstanding performance has been achieved due to their excellent optoelectronic properties, such as a direct bandgap in the visible region, a very high absorption coefficient, a long charge-carrier diffusion length, and ambipolar carrier transport characteristics. FAPbI3 (FA = formamidinium) and CsPbI3 perovskites among the pool of perovskites are recommended for solar cell applications because they meet all the requirements for photovoltaic applications. However, the fundamental problem of these perovskites is that their photoactive black phase is highly unstable under ambient conditions due to small and large sizes of Cs+ and FA+ ions, respectively. The instability of the black phase of these perovskites hinders their applications in photovoltaic devices as a high-quality light absorber layer. Several approaches have been employed to prevent the formation of the photo-inactive yellow phase or to enhance the stability of the black phase of perovskites, such as dimensional and compositional engineering, the addition of external additives, and dimensional engineering. This perspective summarizes the various methods for stabilizing the black phase of CsPbI3 and FAPbI3 perovskites at room temperature as well as their application in photovoltaic devices.

Twist angle can expand charge carrier diffusion length in bilayer black phosphorus: ab initio quantum dynamics

Focusing on bilayer black phosphorus (BP) and performing nonadiabatic molecular dynamic (NA-MD) simulations, we demonstrate that appropriate twist angles can significantly improve the charge carrier lifetime and diffusion length.

Fullerene-Liquid-Crystal-Induced Micrometer-Scale Charge-Carrier Diffusion in Organic Bulk Heterojunction.

The short charge-carrier diffusion length (LD ) (100-300nm) in organic bulk heterojunction (BHJ) impedes the further improvement in power conversion efficiency (PCE) of organic solar cells (OSCs), especially for thick-film (>400nm) devices matching with industrial solution processing. Here a facile method is developed to efficiently increase LD and then improve PCEs of OSCs via introducing a fullerene liquid crystal, F1, into the active layer. F1 combines the inherent high electron mobility of fullerene and strong self-assembly capacity of liquid crystal, providing a fast channel for charge-carrier transportand reducing energetic disorder and trap density in BHJ film via enhancing crystallization. Typically, in PM6:Y6:F1 BHJ, the enhanced charge-carrier mobility (>10-2 cm-2 V-1 s-1 ) and prolonged charge-carrier lifetime (55.3 µs) are acquired to realize the record LD of 1.6 or 2.4µm for electron or hole, respectively, which are much higher than those of the PM6:Y6 binary sample and comparable to or even better than those values reported for some inorganic/hybrid materials, such as CuInx Ga(1- x ) Se2 (CIGS) and perovskite thin films. Benefitting from the micrometer-scale LD , the PM6:Y6:F1 ternary OSCs sustain a remarkable PCE of 15.23% with the active layer thickness approaching 500nm.

Recent Advances in the Functionalization of Perovskite Solar Cells/Photodetectors

AbstractMetal halide perovskite materials have excellent photoelectric properties, including high absorption coefficient, long charge carrier diffusion length, and low exciton binding energy, making them excellent candidates for photoconversion devices. In the past decade, great progress has been made in improving the performance of perovskite‐based solar cells and photodetectors. However, perovskite devices need to be endowed with more functionality to meet practical application requirements. In this regard, the three major trends in perovskite device development involving flexibility, transparency, and self‐healing have attracted wide interests. This review first summarizes the fundamental design principles of endowing perovskite devices with more functionality. Specifically, the functionalization is realized through elaborate substrate and electrode selection, perovskite layer modification, and device layout. This review presents the recent advances and specific requirements for functionalization of perovskite devices according to the three basic components: substrate, electrode, and perovskite layer. Finally, potential challenges and future research directions in this field are outlined. The aim of this review is not only to shed light on the basic design principles of functionalized perovskite devices, but also to serve as the roadmap for future exploration in perovskite devices.

Facile synthesis of Cs3Bi2Br9 perovskite nanoplates with low-polarity antisolvents for photodetection applications

Lead-free metal-halide perovskites have garnered tremendous attention in recent times for their excellent properties, such as high light absorption coefficient and high charge-carrier diffusion length. Herein, we report the synthesis and investigation of lead-free Cs3Bi2Br9 perovskite nanoplates for visible light photodetection applications. The Cs3Bi2Br9 nanoplates were synthesized by a facile reprecipitation method based on the selection of antisolvents. The synthesized nanoplates present phase-pure crystal quality and exhibit reduced trap states, resulting in a high on/off current ratio of ∼103 under illumination at a wavelength of 405 nm in ambient air. This study thus hints at new opportunities for stable and low-toxicity perovskite-based optoelectronic applications.