Reduced Surface Recombination Research Articles

Carbon-Protected BiVO4—Cu2O Thin Film Tandem Cell for Solar Water Splitting Applications

Carbon-protected BiVO4 photoanode and Cu2O photocathode tandem photoelectrochemical (PEC) system has been explored to reduce surface recombination and enhance the stability of the photoelectrodes. In addition to the carbon layer, the electrodeposited FeOOH nanolayer and drop-casted MoS2 co-catalyst layer on the photoanode and photocathode, respectively improve the reaction kinetics. The optimized photoanode (Mo-BiVO4/C/FeOOH) and photocathode (Cu2O/C/MoS2) produces current densities of ~1.22 mA cm−2 at 1.23 V vs. RHE and ~−1.48 mA cm−2 at 0 V vs. RHE, respectively. The obtained photocurrent is higher than bare photoelectrodes without a carbon layer. Finally, a tandem cell has been constructed, and an unassisted current density of ~0.107 mA cm−2 is obtained for a carbon-protected BiVO4–Cu2O tandem PEC cell at zero bias. The improved stability and enhanced photocurrent of the carbon protective layer are attributed to its better charge transfer resistance and minimized surface defects. Carbon protective layer can be a viable option to improve the stability of photoelectrodes in aqueous media.

Passivation of perovskite layer surface states with pyridine in flexible and printed perovskite solar cells

Perovskite solar cells (PSCs), prepared by using solution-processed printing techniques, gained much attention over the past few years and a considerable progress has been achieved in improving the power conversion efficiencies of these devices. Nevertheless, there are still some advancements that can be implemented, especially in terms of passivation of surface defects in the perovskite photoactive layer. Passivation can afford considerable reduction in surface recombination of charge carriers in the photoactive layer and help to obtain devices with better performance. In this work, poly(3-hexylthiophene-2,5-diyl)-based inks with small amount of pyridine as an additive are used to deposit the hole transport layer and simultaneously passivate the surface defects of the perovskite layer in flexible and printed PSCs. The devices are fabricated on flexible conductive plastic substrates using a slot-die coating method. It is found that 2.5 wt.% pyridine-containing inks for preparing hole transport layer have a positive effect on the performance of resulting PSCs. On average, around 13% improvement in the power conversion efficiency is observed for the devices with passivation as opposed to the reference devices without passivation. The effect of pyridine passivation on the structural and electronic properties of the perovskite layer on a flexible substrate is studied using experimental and analytical techniques, whereas the computer simulation methods are employed to rule out the possible mechanisms for the performance improvements in the devices with passivation. The approach presented here can be useful for developing simplified protocols for printing of flexible PSCs with the passivated perovskite layer and improved device efficiency.

A Transparent Electrode Based on Solution-Processed ZnO for Organic Optoelectronic Devices

Achieving high-efficiency indium tin oxide (ITO)-free organic optoelectronic devices requires the development of high-conductivity and high-transparency materials for being used as the front electrode. Herein, sol-gel-grown zinc oxide (ZnO) films with high conductivity (460 S cm−1) and low optical absorption losses in both visible and near-infrared (NIR) spectral regions are realized utilizing the persistent photoinduced doping effect. The origin of the increased conductivity after photo-doping is ascribed to selective trapping of photogenerated holes by oxygen vacancies at the surface of the ZnO film. Then, the conductivity of the sol-gel-grown ZnO is further increased by stacking the ZnO using a newly developed sequential deposition strategy. Finally, the stacked ZnO is used as the cathode to construct ITO-free organic solar cells, photodetectors, and light emitting diodes: The devices based on ZnO outperform those based on ITO, owing to the reduced surface recombination losses at the cathode/active layer interface, and the reduced parasitic absorption losses in the electrodes of the ZnO based devices.

Charge Dynamics at Surface-Modified, Nanostructured Hematite Photoelectrodes for Solar Water Splitting

The balance of the charge transfer and recombination kinetics of photoelectrodes governs the device efficiency for solar water splitting. Hematite (α-Fe2O3) is a photoanode typically used because of advantages such as its abundance, low cost, multiple convenient deposition methods, and an attractive bandgap energy; however, poor electrical properties prevent high solar energy to hydrogen conversion efficiencies. In this work, we evaluate and compare several strategies to address this issue, using a nanorod array morphology and incorporation of overlayers of one or more materials that favor the charge carrier transfer kinetics and reduce surface recombination. We use intensity-modulated photocurrent spectroscopy (IMPS) to evaluate these systems, and demonstrate that the presence of TiO2 and MoO x overlayers successfully suppresses surface recombination through passivation of hematite interfacial recombination sites. However, the hole transfer process at the overlayers occurs at more positive potentials due to the location of the new surface states at the overlayer—electrolyte interface. We show that the deposition of the CoPi oxygen evolution reaction co-catalyst partially addresses this disadvantage. The best efficiencies were obtained for the CoPi-TiO2/α−Fe2O3 and CoPi-MoO x /TiO2/α−Fe2O3 photoelectrodes, with internal quantum efficiencies of 0.42−0.44 under 455 nm irradiation.

Effective passivation of TiO2/Si by interlayer SiOx controlled by scanning zone annealing for perovskite/Si tandem solar cell

A 2-terminal tandem solar cell that utilized organometal halide perovskite as the top cell and crystalline silicon (Si) as the bottom cell has attracted much attention in recent years for its high theoretical energy conversion efficiency. Among many different approaches, high energy conversion efficiency with relatively simple device architecture can be achieved by direct deposition of electron transport material of a perovskite top cell on a Si bottom cell. To maximize the amount of extractable current from this type of solar cell, precise control of surface recombination at the interface between the electron transport layer and Si is a crucial factor. In this study, we proposed the utilization of Scanning Zone Annealing (SZA), a unique heating treatment developed by our research group, to reduce surface recombination by selective treatment of a TiO2/Si interface. This method was proven effective to increase the minority carrier lifetime of a sputtered-TiO2/Si interface from 6.3 μs to 355 μs for p-type float-zone Si, from 7.9 μs to 310 μs for n-type float-zone Si, and from 9.3 μs to 271 μs for p-type Czochralski Si. Furthermore, the origin behind the reduction in surface recombination at a TiO2/Si interface by SZA was proposed based on direct observation of the interface. Here, we found that the thickness of a natively formed SiOx interlayer is crucial to the reduction of the surface recombination at a TiO2/Si interface and is controllable at nanometer scale by adjusting the amount of heat transferred during SZA treatment.

Comment on "Enhanced Charge Selectivity via Anodic-C60 Layer Reduces Nonradiative Losses in Organic Solar Cells".

Understanding interface-related phenomena is important for improving the performance of thin-film solar cells. In ACS Appl. Mater. Interfaces2021, 13, 12603–12609, Pranav et al. report that incorporating a thin C60 interlayer at the MoO3 anode results in reduced surface recombination of electrons, which is ascribed to a decreased electron accumulation near the anode on account of an increased built-in voltage. Here, we offer an alternative explanation: the introduction of a C60 interlayer renders the MoO3 contact Ohmic. The reduced anode barrier simultaneously increases the built-in voltage, minimizes nonradiative voltage losses upon the extraction of majority carriers (holes), and suppresses minority-carrier (electron) surface recombination, the latter being the result of hole accumulation and associated band bending near the Ohmic hole contact. We therefore argue that Ohmic contact formation suppresses both majority- and minority-carrier surface recombination losses, whereas the built-in voltage per se does not play a major role in this respect.

Engineering Surface Architectures for Improved Durability in III–V Photocathodes

GaInP2 has shown promise as the wide bandgap top junction in tandem absorber photoelectrochemical (PEC) water splitting devices. Among previously reported dual-junction PEC devices with a GaInP2 top cell, those with the highest performance incorporate an AlInP2 window layer (WL) to reduce surface recombination and a thin GaInP2 capping layer (CL) to protect the WL from corrosion in electrolytes. However, the stability of these III-V systems is limited, and durability continues to be a major challenge broadly in the field of PEC water splitting. This work provides a systematic investigation into the durability of GaInP2 systems, examining the impacts of the window layer and capping layer among single junction pn-GaInP2 photocathodes coated with an MoS2 catalytic and protective layer. The photocathode with both a CL and WL demonstrates the highest PEC performance and longest lifetime, producing a significant current for >125 h. In situ optical imaging and post-test characterization illustrate the progression of macroscopic degradation and chemical state. The surface architecture combining an MoS2 catalyst, CL, and WL can be translated to dual-junction PEC devices with GaInP2 or other III-V top junctions to enable more efficient and stable PEC systems.

Simulation of a Novel Junction-less Solar Cell Structure Using PC-1D Software

The continues demand for Photovoltaic device improvement has led us to search for new sources of photovoltaic action. For that, this paper proves the existence of other sources for photovoltaic action in addition to the main source which is the built-in electrostatic fild generated through controlled composition gradientThese new sources are uncovered through mathematical examination of a specifi material system followed by the construction of four-layer p-type semi-conducting materials (i.e., without pn-junction formation). First three layers of this specifi material system are fabricated with AlxGα1-xAs with diffrent Aluminum fractions, while the fourth layer is fabricated with Ge with a diffsed back surface fild (BSF) layer to reduce surface recombination velocities (SRV).The complete structure is analyzed using the PC-1D simulation softare, and it shows that the junctionless structure possesses the photovoltaic action with low performance.

Recombination Dynamics in PbS Nanocrystal Quantum Dot Solar Cells Studied through Drift–Diffusion Simulations

The significant performance increase in nanocrystal (NC)-based solar cells over the last decade is very encouraging. However, many of these gains have been achieved by trial-and-error optimization, and a systematic understanding of what limits the device performance is lacking. In parallel, experimental and computational techniques provide increasing insights into the electronic properties of individual NCs and their assemblies in thin films. Here, we utilize these insights to parameterize drift–diffusion simulations of PbS NC solar cells, which enable us to track the distribution of charge carriers in the device and quantify recombination dynamics, which limit the device performance. We simulate both Schottky- and heterojunction-type devices and, through temperature-dependent measurements in the light and dark, experimentally validate the appropriateness of the parameterization. The results reveal that Schottky-type devices are limited by surface recombination between the PbS and aluminum contact, while heterojunction devices are currently limited by NC dopants and electronic defects in the PbS layer. The simulations highlight a number of opportunities for further performance enhancement, including the reduction of dopants in the nanocrystal active layer, the control over doping and electronic structure in electron- and hole-blocking layers (e.g., ZnO), and the optimization of the interfaces to improve the band alignment and reduce surface recombination. For example, reduction in the percentage of p-type NCs from the current 1–0.01% in the heterojunction device can lead to a 25% percent increase in the power conversion efficiency.

Reducing Surface Recombination Velocity of Methylammonium-Free Mixed-Cation Mixed-Halide Perovskites via Surface Passivation

Reducing Surface Recombination Velocity of Methylammonium-Free Mixed-Cation Mixed-Halide Perovskites via Surface Passivation

Perovskite Solar Cells with Front Surface Gradient

AbstractThe recombination flux in a solar cell is determined by not only recombination centers, but also the spatial distribution of minority carriers. For halide perovskite solar cells (PSCs), although there has been a tremendous amount of work focusing on defect passivation, the issue of carrier distribution is not as well studied as for other types of solar cells. Here in this work, with the incorporation of perovskite quantum dots, the concept of the front surface gradient in PSCs using a solution process is successfully realized. Evidenced by multiple characterization techniques, the minority carriers are pushed away from the defect‐rich surface by the gradient of valence band maximum, which effectively reduces surface recombination without compromising photocurrent. As a result, the normal structured hybrid PSCs and MAPbI3 cells exhibit open‐circuit voltages exceeding 93% and 90% of their respective Shockley–Queisser limits, and the power conversion efficiencies reach 22.36% and 20.53%, respectively.

Hydrothermal Synthesis of CaMn2O4·xH2O Nanorods as Co‐Catalysts on GaN Nanowire Photoanode

Co‐catalysts play a crucial role in photoelectrochemical (PEC) water splitting reactions by improving slow kinetics and reducing surface recombination, thereby enhancing PEC performance. However, achieving a well‐defined interface between low‐dimensional semiconductors and co‐catalysts is challenging due to difficulties in depositing a uniform co‐catalyst layer and attaining mixed and disordered states. CaMn2O4·xH2O has been extensively studied for the purpose of decreasing the overpotential in the oxygen evolution reaction. In this study, CaMn2O4·xH2O was synthesized via a hydrothermal method, and loaded onto a GaN nanowire (NW) photoanode. Optimized CaMn2O4·xH2O layers were decorated on GaN NWs to create CaMn2O4@GaN, which delivered a maximum photocurrent of 0.4 mA/cm2 at 0 V vs. the Ag/AgCl reference electrode under an AM 1.5 G solar simulator, 1.6‐fold higher than that (0.25 mA/cm2) of bare GaN NWs. Thus, highly efficient and stable PEC water splitting was achieved on the GaN NW photoanode.

Dielectric screening in perovskite photovoltaics

The performance of perovskite photovoltaics is fundamentally impeded by the presence of undesirable defects that contribute to non-radiative losses within the devices. Although mitigating these losses has been extensively reported by numerous passivation strategies, a detailed understanding of loss origins within the devices remains elusive. Here, we demonstrate that the defect capturing probability estimated by the capture cross-section is decreased by varying the dielectric response, producing the dielectric screening effect in the perovskite. The resulting perovskites also show reduced surface recombination and a weaker electron-phonon coupling. All of these boost the power conversion efficiency to 22.3% for an inverted perovskite photovoltaic device with a high open-circuit voltage of 1.25 V and a low voltage deficit of 0.37 V (a bandgap ~1.62 eV). Our results provide not only an in-depth understanding of the carrier capture processes in perovskites, but also a promising pathway for realizing highly efficient devices via dielectric regulation.

Highly Efficient 1D/3D Ferroelectric Perovskite Solar Cell

AbstractWith the capability to manipulate the built‐in field in solar cells, ferroelectricity is found to be a promising attribute for harvesting solar energy in solar cell devices by influencing associated device parameters. Researchers have devoted themselves to the exploration of ferroelectric materials that simultaneously possess strong light absorption and good electric transport properties for a long time. Here, it is presented a novel and facile approach of combining state‐of‐art light absorption and electric transport properties with ferroelectricity by the incorporation of room temperature 1D ferroelectric perovskite with 3D organic–inorganic hybrid perovskite (OIHP). The 1D/3D mixed OIHP films are found to exhibit evident ferroelectric properties. It is notable that the poling of the 1D/3D mixed ferroelectric OIHP solar cell can increase the average Voc can be increased from 1.13 to 1.16 V, the average PCE from 20.7% to 21.5%. A maximum power conversion efficiency of 22.7%, along with an enhanced fill factor of over 80% and open‐circuit voltage of 1.19 V, can be achieved in the champion device. The enhancement is by virtue of reduced surface recombination by ferroelectricity‐induced modification of the built‐in field. The maximum power point tracking measurement substantiates the retention of ferroelectric‐polarization during the continued operation.

Charge collection in h-BN neutron detectors at elevated temperatures

Many of the neutron detector applications are in the environments with harsh conditions such as high temperatures. We report here the measurements of charge collection parameters of hexagonal boron nitride (h-BN) detectors at elevated temperatures, including the electron mobility-lifetime product (μτ) and surface recombination field (s/μ). It was found that μτ is increased, while s/μ is decreased as temperature increases. The temperature dependence of the surface recombination field (s/μ) revealed that electrons trapped in the surface states tend to thermally activate to the bulk region at higher temperatures with an activation energy of about 0.22 eV, leading to a reduction in the density of the charged surface states at elevated temperatures. Consequently, the charge collection efficiency is enhanced at elevated temperatures due to a reduced surface recombination field and increased electron mobility-lifetime product. The results suggested that h-BN neutron detectors are favorable for high temperature operation.

Promoting Photoelectrochemical Water Oxidation on Ti-Doped Fe2O3 Nanowires Photoanode by O2 Plasma Treatment

Surface electron traps on semiconductor photoanodes mediate surface recombination and deteriorate the photoelectrochemical (PEC) water oxidation performance of the photoanode. Developing convenient methods to reduce surface electron traps is therefore essential for high efficiency PEC water oxidation on semiconductor photoanodes, particularly for nanostructured photoanodes with large surface area. Herein, we employ a O2 plasma treatment to boost the PEC water oxidation performance of Ti-doped Fe2O3 (Ti-Fe2O3) nanowires photoanodes, aiming to reduce surface oxygen vacancies, the dominant electron traps on Ti-Fe2O3 surface. X-ray diffraction (XRD), scanning electron microscopy and spectroscopic analyses show that the oxygen plasma treatment changes the structural, morphological and optical properties negligibly, but it does reduce the content of surface oxygen vacancies, as estimated from O1s X-ray photoelectron spectroscopy spectra. An optimal O2 plasma treatment (200 W, 70 s) increases the photocurrent density of the Ti-Fe2O3 nanowire photoanode to 2.14 mA·cm−2 (1.23 V vs. RHE) under air mass 1.5G simulated solar light, which is 1.95 times higher than the pristine Ti-Fe2O3 nanowire photoanode. The surface hole transfer efficiency is also improved by 1.66 times due to the reduced surface recombination. The work suggests that O2 plasma treatment is a convenient but effective method to boost the PEC water oxidation performance of Ti-Fe2O3 photoanode and might be applicable to other semiconducting oxide photoanodes for high efficiency PEC water splitting.

Advanced architecture ofCdHgTe photodiode for detection of weak infrared radiation

The dark currents of CdHgTe photodiodes based on the advanced architecture, which isused inthreshold FPAfordetection ofweak infrared radiation,have been analyzed. Theopposite conductivity regions areformedof in wide-gap layers, thatreducesthe contribution of theSRHgeneration-recombination currents of to the total dark current.By using various compositions layers with transition sublayers reducing surface recombination at the interfaces, one canre-duce the surface mechanismcontributionto the total dark current of the photodiode. Finally, due to the correct choice of the absorption layer composition and wide-gap layer parameters, dark current in the space charge region ismuch lower than аdiffusion currentcausing by Auger mechanismin the absorption region for a given cutoff wavelength.

Efficient light extraction in subwavelength GaAs/AlGaAs nanopillars for nanoscale light-emitting devices.

This work reports on high extraction efficiency in subwavelength GaAs/AlGaAs semiconductor nanopillars. We achieve up to 37-fold enhancement of the photoluminescence (PL) intensity from sub-micrometer (sub-µm) pillars without requiring back reflectors, high-Q dielectric cavities, nor large 2D arrays or plasmonic effects. This is a result of a large extraction efficiency for nanopillars <500 nm width, estimated in the range of 33-57%, which is much larger than the typical low efficiency (∼2%) of micrometer pillars limited by total internal reflection. Time-resolved PL measurements allow us to estimate the nonradiative surface recombination of fabricated pillars. We conclusively show that vertical-emitting nanopillar-based LEDs, in the best case scenario of both reduced surface recombination and efficient light out-coupling, have the potential to achieve notable large external quantum efficiency (∼45%), whereas the efficiency of large µm-pillar planar LEDs, without further methods, saturates at ∼2%. These results offer a versatile method of light management in nanostructures with prospects to improve the performance of optoelectronic devices including nanoscale LEDs, nanolasers, single photon sources, photodetectors, and solar cells.

Genetic Algorithm Optimization of Core-Shell Nanowire Betavoltaic Generators

Numerical optimization has been used to determine the optimum junction design for core–shell nanowires used in betavoltaic generators. A genetic algorithm has been used to calculate the relative thickness, height, and doping of each segment within silicon, gallium arsenide, and gallium phosphide nanowires. Using the simulated spectra and energy deposition of nickel-63, nickel citrate, tritium, and tritiated butyl, devices with power output and overall efficiency up to 8 µW.cm−2 and 12%, respectively, have been predicted. Compared to previously investigated axial nanowires, the core–shell structures simulated here have realized drastic improvements by reducing surface recombination for longer nanowires. In addition, core–shell nanowires are shown to be capable of nearly matching the ideal performance predicted for this device structure. A new approach for calculating the practical upper limit of betavoltaic performance is presented and additional methods for improvement are discussed.

Fermi Level Engineering of Passivation and Electron Transport Materials for p‐Type CuBi2O4 Employing a High‐Throughput Methodology

AbstractMetal oxide semiconductors are promising for solar photochemistry if the issues of excessive charge carrier recombination and material degradation can be resolved, which are both influenced by surface quality and interface chemistry. Coating the semiconductor with an overlayer to passivate surface states is a common remedial strategy but is less desirable than application of a functional coating that can improve carrier extraction and reduce recombination while mitigating corrosion. In this work, a data‐driven materials science approach utilizing high‐throughput methodologies, including inkjet printing and scanning droplet electrochemical cell measurements, is used to create and evaluate multi‐element coating libraries to discover new classes of candidate passivation and electron‐selective contact materials for p‐type CuBi2O4. The optimized overlayer (Cu1.5TiOz) improves the onset potential by 110 mV, the photocurrent by 2.8×, and the absorbed photon‐to‐current efficiency by 15.5% compared to non‐coated photoelectrodes. It is shown that these enhancements are related to reduced surface recombination through passivation of surface defect states as well as improved carrier extraction efficiency through Fermi level engineering. This work presents a generalizable, high‐throughput method to design and optimize passivation materials for a variety of semiconductors, providing a powerful platform for development of high‐performance photoelectrodes for incorporation into solar‐fuel generation systems.