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At least 397 records · Page 22
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Enhancement of Photoconductivity by Carrier Screening Effect in n-GaSb/InAs/p-GaSb Heterostructure with Single Deep Quantum Well

n-GaSb/n-InAs/p-GaSb heterostructure with a single InAs QW was grown for the first time by MOVPE. Photocurrent spectra were obtained at reverse bias in the range from 0 to 0.8 V. It was shown that the photocurrent increases nonlinearly. The maximum of differential photoconductivity is archived at low applied voltage up to 0.2 V. This effect was explained by electrostatic screening of electrons localized in QW.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Photoanodization of n-Si in the Presence of Hydrogen Peroxide: Voltage Dependence

The fundamental aspects of the electrochemical etching of lightly doped n-Si under backside illumination conditions are studied in a solution with a low HF concentration and a high concentration of hydrogen peroxide. The data obtained are compared with those for a control electrolyte containing no H{sub 2}O{sub 2}. The morphology of the self-organized macropores, their growth rate, porosity, effective valence, and the amount of dissolved silicon are examined in relation to the applied voltage. The anodization kinetics at low and high bias voltages is analyzed. It is found that, under the same illumination, the initial photocurrent in the peroxide electrolyte is approximately twice lower than in the aqueous electrolyte, which makes it possible to state that the quantum efficiency of the photocurrent is lower. As, however, the etching duration is made longer, the current in the peroxide electrolyte strongly increases to become higher than that in the control electrolyte based on H{sub 2}O. It is found that, in the presence of H{sub 2}O{sub 2}, the depthwise growth rate of the macropores increases by more than a factor of 2, and the porosity decreases. The vertical macropore channels have a diameter smaller than that for macropores formed in the aqueous electrolyte and their walls are poorly passivated, which causes branching and the formation of secondary mesopores, the number of which grows with increasing voltage. The effective valence of silicon dissolution in the presence of H{sub 2}O{sub 2} decreases to less than 2. The results are interpreted in terms of the Gerischer and Kolasinski models.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

Photoconductivity and Non-Exponential Relaxation at Insulating LaAlO3/SrTiO3 Interfaces

LaAlO{sub 3} is grown on SrO terminated (100) SrTiO{sub 3}, and (110) SrTiO{sub 3}, producing insulating heterointerfaces without light. Photocurrent spectroscopy at low temperatures reveals a broad distribution of interface states between 2 eV and 2.7 eV at both interfaces, with a higher density in the (110) case concomitant with relatively shallow traps. The photocurrent relaxation can be well fitted by a stretched exponential form, confirming energetically distributed electron traps. Photo-carrier lifetimes are larger than a few hundred seconds for optical excitation approaching the SrTiO{sub 3} band-gap energy, providing the opportunity to study transient light-induced properties at low temperatures.

99 GENERAL AND MISCELLANEOUS↗
DOE OSTI2017

Enhanced Performance of a Self-Powered Organic/Inorganic Photodetector by Pyro-Phototronic and Piezo-Phototronic Effects

Self-powered photodetectors (PDs) have long been realized by utilizing photovoltaic effect and their performances can be effectively enhanced by introducing the piezo-phototronic effect. Recently, a novel pyro-phototronic effect is invented as an alternative approach for performance enhancement of self-powered PDs. Here, a self-powered organic/inorganic PD is demonstrated and the influences of externally applied strain on the pyro-phototronic and the photovoltaic effects are thoroughly investigated. Under 325 nm 2.30 mW cm -2 UV illumination and at a -0.45% compressive strain, the PD's photocurrent is dramatically enhanced from ≈14.5 to ≈103 nA by combining the pyro-phototronic and piezo-phototronic effects together, showing a significant improvement of over 600%. Theoretical simulations have been carried out via the finite element method to propose the underlying working mechanism. Moreover, the pyro-phototronic effect can be introduced by applying a -0.45% compressive strain to greatly enhance the PD's response to 442 nm illumination, including photocurrent, rise time, and fall time. This work provides in-depth understandings about the pyro-phototronic and the piezo-phototronic effects on the performances of self-powered PD to light sources with different wavelengths and indicates huge potential of these two effects in optoelectronic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Catalyst-Free and Morphology-Controlled Growth of 2D Perovskite Nanowires for Polarized Light Detection

Ruddleson–Popper (RP) perovskites have emerged as a class of material inheriting the superior optoelectronic properties of two materials: perovskites and 2D materials. The large exciton binding energy and natural quantum well structure not only make these materials ideal platforms to study light–matter interactions but also render them suitable for fabrication of various functional optoelectronic devices. Nanoscale structuring and morphology control have led to semiconductors with enhanced functionalities. Nanowires of semiconducting materials are extensively used for important applications like lasing and sensing. However, catalyst and template-free scalable growth of nanowires of 2D perovskites has remained elusive. In this paper, a facile approach for morphology-controlled growth of nanowires of 2D perovskite, (BA) 2 PbI 4 , is demonstrated. Additionally, it is shown that the photoluminescence (PL) from the nanowires is highly polarized with a polarization ratio as large as ≈0.73, which is one of the largest reported for perovskites. Finally, it is further shown that the photocurrent from the hybrid nanowire/graphene device is also sensitive to the polarization of the incident light with the photocurrent anisotropy ratio of ≈3.62 (much larger than the previously reported value of 2.68 for perovskites), thus demonstrating the potential of these nanowires as highly efficient photodetectors for polarized light.

2D perovskites↗
DOE OSTI2025

Efficient Broadband Terahertz Generation by Above‐Band‐Gap Excitation of the Pyroelectric ZnSnN 2

Terahertz (THz) radiation is a powerful probe of low-energy excitations in all phases of matter. However, it remains a challenge to find materials that efficiently generate THz radiation in a broad range of frequencies following optical excitation. Here, we investigate a pyroelectric material, ZnSnN 2 , and find that its above-band-gap excitation results in the efficient formation of an ultrafast photocurrent generating THz radiation. The resulting THz electric field spans a frequency range from below 1 THz to above 30 THz. The results suggest that the photocurrent is primarily driven by an ultrafast pyroelectric effect where the photo-excited carriers screen the spontaneous electric polarization of ZnSnN 2 . Strong structural disorder reduces the photocarrier lifetime significantly and, thus, enables broadband operation. ZnSnN2 shows a similar THz-emitter performance as the best spintronic THz emitters regarding bandwidth and amplitude. The study unveils the large potential of pyroelectric materials as efficient and broadband THz emitters with built-in bias fields.

36 MATERIALS SCIENCE↗
DOE OSTI2024

In Situ Exploration of Dipole Field Effects on Weak Hysteresis in 3D/2D Perovskites

This research delves into the effects of 2D layers on the functionality of 3D perovskite using lock-in amplifier-based in situ surface photovoltage (SPV) and its phase spectroscopy, with an emphasis on elucidating the connection between the tuning of dipole moments and the photocurrent hysteresis. Conventionally, the SPV of a perovskite/hole transport layer is observed to diminish as positive bias escalates. However, this trend is reversed in the case of 3D perovskite samples, where an augmentation in SPV is noted under positive bias. Notably, 3D/2D perovskite structures initially show a decrease, then an increase in SPV as bias intensifies, a phenomenon more pronounced with larger dipole moments in 2D. However, there is no linear relationship between the dipole moment and the hysteresis factor. Furthermore, using in situ light-chopping-frequency-modulated SPV and Kelvin Probe Force Microscopy, it is revealed that the dipole fields of 2D layers can hinder ion migration. This leads to efficient hole transfer and minimal photocurrent hysteresis in 3D/2D perovskites, providing strong evidence for the underlying cause of hysteresis. Additionally, these findings suggest intricate interplays among the external electric field, interface dipole moments, and surface photovoltaics, offering significant insights into perovskite optoelectronics.

3D/2D perovskite↗
DOE OSTI2018

Unassisted Water Splitting Using a GaSb x P (1- x ) Photoanode

Here in this work, unbiased water splitting with 2% solar-to-hydrogen efficiency under AM 1.5 G illumination using new materials based on GaSb 0.03 P 0.97 alloy is reported. Freestanding GaSb x P 1-x is grown using halide vapor phase epitaxy. The native conductivity type of the alloy is modified by silicon doping, resulting in an open-circuit potential (OCP) of 750 mV, photocurrents of 7 mA cm -2 at 10 sun illumination, and corrosion resistance in an aqueous acidic environment. Alloying GaP with Sb at 3 at% improves the absorption of high-energy photons above 2.68 eV compared to pure GaP material. Electrochemical Impedance Spectroscopy and illuminated OCP measurements show that the conduction band of GaSb x P 1-x is at -0.55 V versus RHE irrespective of the Sb concentration, while photocurrent spectroscopy indicates that only radiation with photon energies greater than 2.68 eV generate mobile and extractable charges, thus suggesting that the higher-laying conduction bands in the Γ 1 valley of the alloys are responsible for exciton generation.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Higher Mobility and Carrier Lifetimes in Solution-Processable Small-Molecule Ternary Solar Cells with 11% Efficiency

Solution–processed small molecule (SM) solar cells have the prospect to outperform their polymer–fullerene counterparts. Considering that both SM donors/acceptors absorb in visible spectral range, higher expected photocurrents should in principle translate into higher power conversion efficiencies (PCEs). However, limited bulk–heterojunction (BHJ) charge carrier mobility (<10 –4 cm 2 V –1 s –1 ) and carrier lifetimes (<1 µs) often impose active layer thickness constraints on BHJ devices (≈100 nm), limiting external quantum efficiencies (EQEs) and photocurrent, and making large–scale processing techniques particularly challenging. In this report, it is shown that ternary BHJs composed of the SM donor DR3TBDTT (DR3), the SM acceptor ICC6 and the fullerene acceptor PC71BM can be used to achieve SM–based ternary BHJ solar cells with active layer thicknesses >200 nm and PCEs nearing 11%. The examinations show that these remarkable figures are the result of i) significantly improved electron mobility (8.2 × 10 –4 cm 2 V –1 s –1 ), ii) longer carrier lifetimes (2.4 µs), and iii) reduced geminate recombination within BHJ active layers to which PC 71 BM has been added as ternary component. As a result, optically thick (up to ≈500 nm) devices are shown to maintain PCEs >8%, and optimized DR3:ICC6:PC 71 BM solar cells demonstrate long–term shelf stability (dark) for >1000 h, in 55% humidity air environment.

14 SOLAR ENERGY↗
DOE OSTI2020

Retarded Charge-Carrier Recombination in Photoelectrochemical Cells from Plasmon-Induced Resonance Energy Transfer

N-type metal oxides such as hematite (α-Fe 2 O 3 ) and bismuth vanadate (BiVO 4 ) are promising candidate materials for efficient photoelectrochemical water splitting; however, their short minority carrier diffusion length and restricted carrier lifetime result in undesired rapid charge recombination. In this paper, a 2D arranged globular Au nanosphere (NS) monolayer array with a highly ordered hexagonal hole pattern (hereafter, Au array) is introduced onto the surface of photoanodes comprised of metal oxide films via a facile drying and transfer-printing process. Through plasmon-induced resonance energy transfer, the Au array provides a strong electromagnetic field in the near-surface area of the metal oxide film. The near-field coupling interaction and amplification of the electromagnetic field suppress the charge recombination with long-lived photogenerated holes and simultaneously enhance the light harvesting and charge transfer efficiencies. Consequently, an over 3.3-fold higher photocurrent density at 1.23 V versus reversible hydrogen electrode (RHE) is achieved for the Au array/α-Fe 2 O 3 . Furthermore, the high versatility of this transfer printing of Au arrays is demonstrated by introducing it on the molybdenum-doped BiVO 4 film, resulting in 1.5-fold higher photocurrent density at 1.23 V versus RHE. The tailored metal film design can provide a potential strategy for the versatile application in various light-mediated energy conversion and optoelectronic devices.

2D pattern array↗
DOE OSTI2024

Identifying and Amplifying the Spontaneously Formed Photo‐Charge Contribution in Opaque and Semitransparent Organic Photovoltaics

Abstract Spontaneous photo‐charge (SP) generation within the non‐fullerene (NFA) bulk helps avoid exciton (EX) related loss as compared to the traditional heterojunction route in organic solar cells (OSCs). Unfortunately, the promising SP utilization attracts little attention at this time due to the lack of knowledge on its specific contribution to the photovoltaic performance and rational optimization in high‐efficiency devices. To fill the gap, the NFAs’ SP characteristics are related with their photocurrent loss and power conversion efficiency (PCE) in state‐of‐the‐art materials. Though higher SP population is good for efficient EX utilization, it simultaneously acts as an additional photo‐charge recombination channel. The former can be further enhanced by increasing the NFAs’ crystallinity with preferred lamellar growth, and the latter can be suppressed via electric doping. With the aid of the two‐step optimization strategy, the total photocurrent loss is reduced from 14.4% to 4.8%, accompanying with PCE increment from 16.9% to 18.5%. The SP utilization strategy is finally applied in the semitransparent OSCs (ST‐OSCs), due to its less heterojunction area sensitive photo‐charge generation characteristic. A light utilization efficiency (LUE) is then increased from 3.21% to 3.77%. The findings highlight the pursue of efficient SP utilization as a new design philosophy in future OSCs’ progresses.

Chemistry↗
DOE OSTI2023

Correlation of Broad Absorption Band with Small Singlet‐Triplet Energy Gap in Organic Photovoltaics

Abstract Organic photovoltaics (OPV) are one of the most effective ways to harvest renewable solar energy, with the power conversion efficiency (PCE) of the devices soaring above 19 % when processed with halogenated solvents. The superior photocurrent of OPV over other emerging photovoltaics offers more opportunities to further improve the efficiency. Tailoring the absorption band of photoactive materials is an effective way to further enhance OPV photocurrent. However, the field has mostly been focusing on improving the near‐infrared region photo‐response, with the absorption shoulders in short‐wavelength region (SWR) usually being neglected. Herein, by developing a series of non‐fullerene acceptors (NFAs) with varied side‐group conjugations, we observe an enhanced SWR absorption band with increased side‐group conjugation length. The underpinning factors of how molecular structures and geometries improve SWR absorption are clearly elucidated through theoretical modelling and crystallography. Moreover, a clear relationship between the enhanced SWR absorption and reduced singlet‐triplet energy gap is established, both of which are favorable for the OPV performance and can be tailored by rational structure design of NFAs. Finally, the rationally designed NFA, BO‐TTBr, affords a decent PCE of 18.5 % when processed with a non‐halogenated green solvent.

Chemistry↗
DOE OSTI2016

Polymer‐Based Ruthenium(II) Polypyridyl Chromophores on TiO 2 for Solar Energy Conversion

Abstract A polychromophoric light‐harvesting assembly featuring a polystyrene (PS) backbone with ionic carboxylate‐functionalized Ru II polypyridyl complexes as pendant groups (PS‐Ru‐A) was synthesized and successfully anchored onto mesoporous structured TiO 2 films (TiO 2 //PS‐Ru‐A). Studies of the resulting TiO 2 //PS‐Ru‐A films carried out by transmission electron microscopy (TEM), scanning electron microscopy (SEM), and atomic force microscopy (AFM) confirmed that the ionic carboxylated Ru II complexes from PS‐Ru‐A led to the surface immobilization on the TiO 2 film. Monochromatic light photocurrent spectroscopy (IPCE) and white light (AM1.5G) current–voltage studies of dye‐sensitized solar cells using the TiO 2 //PS‐Ru‐A photoanode give rise to modest photocurrent and white light efficiency (24 % peak IPCE and 0.33 % PCE, respectively). The photostability of surface‐bound TiO 2 //PS‐Ru‐A films was tested and compared to a monomeric Ru II complex (TiO 2 //Ru‐A), showing an enhancement of ∼14 % in the photostability of PS‐Ru‐A. Transient absorption measurements reveal that electron injection from surface‐bound pendants occurs on the picosecond time scale, similar to TiO 2 //Ru‐A, while time‐resolved emission measurements reveal delayed electron injection occurring in TiO 2 //PS‐Ru‐A on the nanosecond time scale, underscoring energy transport from unbound to surface‐bound complexes. Additionally, charge recombination is delayed in PS‐Ru‐A, pointing towards intra‐assembly hole transport to complexes away from the surface. Molecular dynamics simulations of PS‐Ru‐A in fluid solution indicate that a majority of the pendant Ru II complexes lie within 10–20 Å of each other, facilitating efficient energy‐ and charge transport among the pendant complexes.

Leem, Gyu↗
DOE OSTI2019

SnO 2 Transparent Printing Pastes from Powders for Photon Conversion in SnO 2 -Based Dye-Sensitized Solar Cells

Tin oxide (SnO 2 ) is the most enticing alternative to titanium oxide (TiO 2 ) with the aim of identifying a more positive conduction band material for dye-sensitized solar cells (DSCs). We put forward a protocol based on grinding, sonication, and centrifuge to generate transparent SnO 2 pastes to minimize light reflectance losses from the metal oxide. Under optimized conditions, a highly transparent film with substantially enhanced light penetration depth through active layer SnO 2 is realized for efficient light harvesting from two different commercially available powders (18 and 35 nm nanoparticle sizes). A ruthenium sensitizer ( B11 ) and two organic sensitizers ( NL3 and MK2 ) are shown to achieve higher or comparable photocurrent densities with SnO 2 relative to standard TiO 2 -based DSCs. SnO 2 -based DSCs show minimum recombination losses, comparable charge collection efficiencies, and minimal photovoltage losses relative to TiO 2 DSCs. Thus, the option of a transparent metal oxide, which can facilitate high photocurrents (>16 mA cm -2 observed) and lower recombination rates than TiO 2 is an attractive material for DSC applications.

14 SOLAR ENERGY↗
DOE OSTI2017

Atomically Altered Hematite for Highly Efficient Perovskite Tandem Water‐Splitting Devices

Abstract Photoelectrochemical (PEC) cells are attractive for storing solar energy in chemical bonds through cleaving of water into oxygen and hydrogen. Although hematite (α‐Fe 2 O 3 ) is a promising photoanode material owing to its chemical stability, suitable band gap, low cost, and environmental friendliness, its performance is limited by short carrier lifetimes, poor conductivity, and sluggish kinetics leading to low (solar‐to‐hydrogen) STH efficiency. Herein, we combine solution‐based hydrothermal growth and a post‐growth surface exposure through atomic layer deposition (ALD) to show a dramatic enhancement of the efficiency for water photolysis. These modified photoanodes show a high photocurrent of 3.12 mA cm −2 at 1.23 V versus RHE, (>5 times higher than Fe 2 O 3 ) and a plateau photocurrent of 4.5 mA cm −2 at 1.5 V versus RHE. We demonstrate that these photoanodes in tandem with a CH 3 NH 3 PbI 3 perovskite solar cell achieves overall unassisted water splitting with an STH conversion efficiency of 3.4 %, constituting a new benchmark for hematite‐based tandem systems.

Gurudayal↗
DOE OSTI2024

Evidence of hot carrier extraction in metal halide perovskite solar cells

The presence of hot carriers is presented in the operational properties of an (FA,Cs)Pb(I, Br, Cl) 3 solar cell at ambient temperatures and under practical solar concentration. Albeit, in a device architecture that is not suitably designed as a functional hot carrier solar cell. At 100 K, clear evidence of hot carriers is observed in both the high energy tail of the photoluminescence spectra and from the appearance of a nonequilibrium photocurrent at higher fluence in light J–V measurements. At room temperature, however, the presence of hot carriers in the emission at elevated laser fluence is shown to compete with a gradual red shift in the PL peak energy as photoinduced halide segregation begins to occur at higher lattice temperature. The effects of thermionic emission of hot carriers and the presence of a nonequilibrium carrier distribution are also shown to be distinct from simple lattice heating. In conclusion, this results in large unsaturated photocurrents at high powers as the Fermi distribution exceeds that of the heterointerface controlling carrier transport and rectification.

14 SOLAR ENERGY↗
DOE OSTI2018

Enhancing the photoresponse and photocatalytic properties of TiO 2 by controllably tuning defects across {101} facets

Introducing defects into semiconductors with well-controlled exposed facets offers an effective route for the development of photocatalytic materials with greatly improved properties. Here, we report a facile ethylene glycol reduction procedure to make anatase titanium dioxide (TiO 2 ) with different concentrations of exposed {001} and {101} facets, leading to different surficial defects. TiO 2 with increased concentrations of {101} facets shows a 5-fold improvement in photocurrent generation as well as improved photocatalytic activity towards water splitting under visible light irradiation. Thus, the improved activity is ascribed to the oxygen vacancies as well as the variable surface chemical states, which collectively induce a slower recombination rate of photo-induced electron-hole pairs. This work also highlights a feasible strategy to obtain the defective TiO 2 and explore the synergistic effect of surface defects and different concentrations of exposed {001} and {101} facets for photocurrent and photocatalytic properties under visible light irradiation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗