Enhanced Carrier Generation Research Articles

Photoelectrodes with Enhanced Carrier Generation and Collection Through Optical Simulation and Materials Engineering

AbstractThe efficiency of solar‐fuel conversion in photoelectrochemical (PEC) systems is hindered by significant losses of photons and photocarriers within the photoelectrodes. This study introduces an innovative ITO@In2S3 core‐shell nanowire structure designed to overcome these challenges through cutting‐edge materials engineering and sophisticated simulation techniques. Optical modeling and finite element simulations highlight the conflict between photocarrier generation and collection in planar In2S3 thin films and demonstrate how the core‐shell nanowire geometry can significantly enhance light absorption. The developed ITO@In2S3 nanowire photoanodes achieve a photocurrent of 11.3 mA cm−2 at 1.23 V versus RHE, which is nearly five times greater than that of planar ITO/In2S3 thin films. Characterization techniques, including UV–vis absorption and charge separation efficiency measurements, confirm the enhanced light absorption and improved carrier collection facilitated by broadening the optical absorption range and reducing carrier transport distances. This research not only deepens the understanding of the dynamics within thin‐film photoelectrodes but also paves the way for the development of more efficient technologies for solar fuel conversion.

Surface Rh-Boosted Photoelectrochemical Water Oxidation of α-Fe2O3 by Reduced Overpotential in the Rate-Determining Step.

The photoelectrochemical behavior of Rh cluster-deposited hematite (α-Fe2O3) photoanodes (α-Fe2O3@Rh) was investigated. The interactions between Rh clusters and α-Fe2O3 nanorods were elucidated both experimentally and computationally. A facile UV-assisted solution casting deposition method allowed the deposition of 2 nm Rh clusters on α-Fe2O3. The deposited Rh clusters effectively enhanced the photoelectrochemical performance of the α-Fe2O3 photoanode, and electrochemical impedance spectroscopy (EIS) and Mott-Schottky analysis were applied to understand the working mechanism for the α-Fe2O3@Rh photoanodes. The results revealed a distinctive carrier transport mechanism for α-Fe2O3@Rh and increased carrier density, while the absorbance spectra remained unchanged. Furthermore, density functional theory (DFT) calculations of the oxygen evolution reaction (OER) mechanism corresponded well with the experimental results, indicating a reduced overpotential of the rate-determining step. In addition, DFT calculation models based on the X-ray diffraction (XRD) measurements and X-ray photoelectron spectroscopy (XPS) results provided precise water-splitting mechanisms for the fabricated α-Fe2O3 and α-Fe2O3@Rh nanorods. Owing to enhanced carrier generation and hole transfer, the optimum α-Fe2O3@Rh3 sample showed 78% increased photocurrent density, reaching 1.12 mA/cm-2 at 1.23 VRHE compared to that of the pristine α-Fe2O3 nanorods electrode.

Hybrid Block Copolymer/Perovskite Heterointerfaces for Efficient Solar Cells.

Solution processable semiconductors like organics and emerging lead halide perovskites (LHPs) are ideal candidates for photovoltaics combining high performance and flexibility with reduced manufacturing cost. Moreover, the study of hybrid semiconductors would lead to advanced structures and deep understanding that will propel this field even further. Herein, a novel device architecture involving block copolymer/perovskite hybrid bulk heterointerfaces is investigated, such a modification could enhance light absorption, create an energy level cascade, and provides a thin hydrophobic layer, thus enabling enhanced carrier generation, promoting energy transfer and preventing moisture invasion, respectively. The resulting hybrid block copolymer/perovskite solar cell exhibits a champion efficiency of 24.07% for 0.0725cm2 -sized devices and 21.44% for 1cm2 -sized devices, respectively, together with enhanced stability, which is among the highest reports of organic/perovskite hybrid devices. More importantly, this approach has been effectively extended to other LHPs with different chemical compositions like MAPbI3 and CsPbI3 , which may shed light on the design of highly efficient block copolymer/perovskite hybrid materials and architectures that would overcome current limitations for realistic application exploration.

Beyond Light‐Trapping Benefits: The Effect of SiO2 Nanoparticles in Bifacial Semitransparent Ultrathin Cu(In,Ga)Se2 Solar Cells

Bifacial semitransparent ultrathin Cu(In,Ga)Se2 solar cells (BSTUT CIGSe SCs) enable efficient usage of light and reduced raw material. By inserting the SiO2 nanoparticles (NPs) at the CIGSe/back–contact interface, this work optimizes the performance of BSTUT SCs under front and especially rear illumination, which has not been studied much so far. For the SCs with NPs, the short‐circuit current density increases by 4.1–4.4 mA cm−2 for front and by 6.4–7.4 mA cm−2 for rear illumination. In addition, a significantly improved fill factor for rear illumination highlights a benefit of the NPs beyond light trapping. A jet‐like focusing behind the NPs is observed, which in this case leads to a higher field localization near the pn junction, joint with an enhanced carrier generation and separation. Furthermore, a thinner In2O3:Sn (ITO) back contact is noticed to be favorable for effective light trapping, whereas thicker ITO is preferred for higher open‐circuit voltage. Overall, inserting NPs in BSTUT SCs is an effective and practical strategy to achieve a higher cost‐to‐efficiency ratio in photovoltaic device production. In our case, a maximum of 12.2% under front and 9.2% under rear illumination is achieved leading to a calculated bifacial efficiency of 15% for the ultrathin device.

Detailed analyses on the role of nanobranches in photoelectrochemical performances of multiscale ZnO nanobrushes

In this study, brush-like hierarchical ZnO microrod/nanobranche photoanodes are fabricated via a facile two-step hydrothermal method. The application of the multiscale ZnO microrod backbone formed on a conductive glass substrate allows the growth of ZnO nanobrushes with controllable branch length, which enables detailed studies on the distinctive working mechanism of integrated ZnO microrod backbone and nanobranch structures. The growth and optimization of the uniformly formed secondary nanobranches significantly enhances PEC performance. In addition, the variations in photopotential and charge-carrier transfer are identified through open-circuit potential measurement and electrochemical impedance spectroscopy, to understand the electron–hole transfer kinetics of the ZnO nanobrushes. The results reveal that the distinctive performance of the ZnO nanobrushes originates from the synergetic effect of the ZnO microrod backbone/nanobranches, the subsequent effective charge separation, and enhanced carrier generation in the dominant space-charge region. In particular, the optimum photoelectrode is determined to be that containing the ZnO nanobrushes with nanobranches grown for 5 h, and the corresponding photocurrent density is measured to be 1.68 mA/cm2, which is 136.3% higher than that of the pristine ZnO microrod electrodes.

A hybrid system for plasmonic and surface polarization induced pyro-phototronic harvesting of light

Finding of surface polarization dependent pyroelectric phenomena that is independent of non-centrosymmetricity of the involved crystalline material has been an immense success for removing the bottlenecks in achieving pyroelectricity. With the realization of photo-induced surface polarization dependent pyroelectric effect, the pyro-phototronic harvesting of light has seen further ease. Here we present a conceptually novel work that combines photo-induced surface polarization generated pyro-phototronic effect in a completely organic crystalline system of plasma polymerized aniline-crystalline Rubrene (PPA-CRB) with plasmonic gold nanoparticles (Au NPs) in order to achieve enhanced light harvesting. In our work, we have effectively utilized the carriers generated from interband transition of Au NPs in the UV-A region, while enhanced carrier generation in the near infrared (NIR) region is obtained further owing to the tunability of the plasmonic absorption region of Au enabling the designed configuration for broadband light harvesting. The fabricated device exhibits fast response attributed to the surface polarization dependent pyro-phototronic effect at the interface of Au NPs and the organic semiconductor framework PPA-CRB. Plasma based single step and environmental friendly synthesis of PPA-CRB film adds further feasibility to the process. The fabricated device works well under zero biased condition, thus making this hybrid system eligible for self-powered broadband light harvesting applications.

Optical and electrical effects of nanobump structure combined with an undulated active layer on plasmonic organic solar cells

Abstract The effect of nanobump structure (NS) constructed with molybdenum oxide (MoO3) covering aerosol-derived Ag nanoparticles on organic solar cells is systematically explored by varying the MoO3 thickness. The amount of enhanced light absorption by the NS-induced scattering and plasmonic resonance effects increases with decreasing MoO3 thickness. Meanwhile, the oval-shape NS with fully enclosed Ag nanoparticles by MoO3 leads to significantly improved carrier extraction due to decreased ohmic loss, while the NS with partially covered Ag nanoparticles by thin MoO3 degrades device performance arising from increased recombination and leakage loss. Thus, the optimal MoO3 thickness occurs around 20 nm for 40 nm sized nanoparticles where the enhanced carrier generation and the improved carrier extraction by the NS effectively contribute to the improvement in photocurrent. Therefore, our analysis on carrier dynamics of plasmonic organic solar cells incorporating the NS would provide a clear guideline for optimizing device performance.

Nonlinear Optical Response in Graphene/WX2 (X = S, Se, and Te) van der Waals Heterostructures.

Light-frequency conversion based on two-dimensional (2D) materials is of great importance for modern nano- and integrated photonics. Herein, we report both the intrinsic (from the pure WX2 (X = S, Se, and Te)) and extrinsic (from the interface of graphene/WX2) second-order nonlinear coefficient tensor from graphene/WX2 van der Waals (vdW) heterostructures by first-principles calculations. The prominent peaks in the dispersion relation of the intrinsic second-order nonlinear coefficient in monolayer WX2 are due to the Van Hove singularity in the high-symmetry point or along the high-symmetry line with high joint density of states. The enhanced nonlinear optical response in the infrared band can be achieved in graphene/WS2 vdW heterostructures, resulting from the interlayer charge transfer between graphene and WS2. The value of the intrinsic second-order nonlinear coefficients of graphene/WSe2 vdW heterostructures is 1.5 times larger than that of pure monolayer WSe2 at the band gap energy of monolayer WSe2 because of the enhanced carrier generation after the heterostructure formation. Different from pure monolayer WX2, azimuthal angle-dependent second harmonic generation from graphene/WX2 vdW heterostructures exhibits extraordinary rotational symmetry at different photon energies, which can be used to deduce the extrinsic second-order nonlinear coefficient. These results pave the way for the nonlinear optical coefficient design based on 2D heterostructures for nonlinear nanophotonics and integrated devices.

Microscopic photoluminescence and photocurrent imaging spectroscopy of InAs nanostructures: Identification of photocarrier generation sites for intermediate-band solar cells

We performed microscopic imaging of photoluminescence (PL) and photocurrent (PC) on InAs nanostructures including disklike structures (nanodisks) and quantum dots (QDs). The correlation between PL and PC images indicates that the major fraction of upconverted carriers originates from nanodisks. By analyzing the excitation spectra, we find evidence that nanodisks and QDs need to be spatially separated to enhance PC generation via upconversion. The efficient simultaneous use of both QDs and nanodisks is an alternative approach to intermediate-band solar cells, where low-energy photons are upconverted in the QDs and high-energy photons are efficiently upconverted in the nanodisks, resulting in enhanced carrier generation yields. With spatially resolved upconverted PL, we show that PC generation in nanodisks is due to ejection of both electrons and holes.

Emitter controlled SiNx-free crystalline Si solar cells with a transparent conducting oxide film

A transparent conducting oxide (TCO) layer was applied in crystalline Si (c-Si) solar cells without use of the conventional SiNx-coating. A high quality indium-tin-oxide (ITO) layer was directly deposited on an emitter layer of a Si wafer. Three different types of emitters were formed by controlling the phosphorous diffusion conditions. A light-doped emitter forming a thinner emitter junction showed an improved photoconversion efficiency of 14.1% compared to the value of 13.2% for a heavy-doped emitter. This was induced by lower recombination within the narrower depletion region of the light-doped emitter.As for the electrical aspect, the ITO layer serves as an efficient electrical conductor and thus relieves the burden of high contact resistance of the light-doped emitter. From the aspect of light management, the intermediate refractive index of ITO is effective at reducing the light reflection, leading to an enhanced carrier generation in Si absorbers.

Field Effects in Plasmonic Photocatalyst by Precise SiO2 Thickness Control Using Atomic Layer Deposition

We report on TiO2 thin films with superior photocatalytic efficiency due to an increase in its exciton carrier generation induced by the plasmonic field of the underlying silver nanoparticles. TiO2 thin films are deposited on supported silver nanoparticles and are separated from each other by a fine-tunable thickness of SiO2 interlayer. The TiO2(15 nm)/SiO2/Ag nanoparticle architectures with systematic variation of SiO2 interlayer thickness of 2, 5, 10, and 20 nm show systematic increase in photocatalytic efficiency with decrease in the SiO2 thickness. The efficiency enhancement is shown to be caused by plasmonically enhanced carrier generation, which was confirmed through photocurrent measurements and Raman spectroscopy. With a 2 nm SiO2 interlayer that exhibited the best photocatalytic performance, a 3 times increase in photocurrent density, and a 200 times increase in Raman signal intensity of TiO2 is found. Atomic layer deposition was employed to achieve precise film thickness control of SiO2 and TiO2...

Fabrication of Subsurface Metallic Nanoparticles For Enhanced Carrier Generation in Silicon-based Photovoltaics

ABSTRACTDue to the low absorption coefficient of silicon in the bulk of the solar spectrum, the majority of silicon-based photovoltaic cells are at least 300 micrometers thick, limiting their economic feasibility. To achieve cost parity with conventional sources of energy, silicon-based photovoltaics have begun to move towards thinner substrates, on the order of a few micrometers; such thinner cells necessitate the use of light-trapping methods to increase the optical path length. Much research has begun investigating the use of trapped electromagnetic waves, or surface plasmons, to increase light scattering and interband carrier transition rates in the surrounding material. In one scheme, plasmonic modes can be supported through polarization of metallic nanoparticles. Past research has focused on the deposition of silver nanoparticles on the surface, and has shown that light absorbance can be increased markedly for certain bandwidths. Here, the relevant mechanism is increased lateral light scattering, which tends to guide the light into directions that are then totally internally reflected. When such metallic nanoparticles are polarized by incoming radiation, in addition to increased light scattering, the electric field in the vicinity of the nanoparticle is highly magnified. Such high fields can increase the carrier transition rates, and therefore the absorbance, in the surrounding silicon by orders of magnitude. The enhancement however decreases rapidly with the radial distance and is virtually diminished by 10 nm from the nanoparticle surface. Deposition on the surface of the solar cell, therefore, cannot exploit this effect, being isolated from the silicon by the passivating layer. It has been suggested that instead, such particles might be embedded into the silicon itself. To that end, we have developed a method to create subsurface silver nanospheres, using a combination of ion implantation, thermal deposition, and subsequent thermal annealing. By tailoring the implantation parameters, we can localize the layer of nanospheres and even create various bands at various depths. Through Rutherford backscattering and secondary ion mass spectroscopy characterization, we have found that the Ag has indeed annealed into the desired location. To ensure that the Ag has agglomerated to nanoparticles, we have confirmed this with TEM images and selected area diffraction patterns that indicate that such nanoparticles are indeed bulk phase silver and have diameters of 20-35 nanometers. This method can help realize more efficient surface plasmon-enhanced Si-based photovoltaics.

A Framework for Understanding Displacement Damage Mechanisms in Irradiated Silicon Devices

A framework is presented for understanding carrier generation and recombination mechanisms in irradiated silicon devices. Data obtained by many workers for irradiated bulk material and devices are analyzed and summarized, and key damage-factor dependences are identified. Measurements, simulations, and analyses support the conclusion that correlation of displacement damage effects with the rate of nonionizing energy loss (NIEL) for proton, neutron, pion, and heavy-ion bombardment is due to the dominant influence of defect subclusters. At low NIEL values (<5times10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> MeV-cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /g), isolated defects dominate the electrical properties. At relatively high NIEL (>2times10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> MeV-cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /g), subclusters dominate. Enhanced carrier generation and recombination produced by those small disordered regions is considered. Modeling results are presented for behavior observed in the transition region between domination by isolated defects and by clusters. The model presented in this paper simultaneously accommodates defect cluster models and NIEL scaling phenomena in the same framework

Photovoltaic Effect in Heterostructure Consisting of Poly(p-phenylene vinylene) Derivative and Polypyridine

The characteristics of a photovoltaic device based on a heterostructure consisting of poly(2-methoxy-5-dodecyloxy-p-phenylene vinylene) (MDOPPV) and polypyridine (PPy) have been studied. The heterostructure device shows a significant enhancement of the photovoltaic properties compared to the single-layer devices based on MDOPPV or PPy. For example, the device shows a significantly larger photocurrent than a device consisting only of MDOPPV. Moreover, notable quenching of the photoluminescence of the two polymers due to lamination has also been observed. These results indicate that a photoinduced charge transfer occurs between MDOPPV and PPy. However, the photoinduced charge transfer between these polymers cannot be explained by the bulk electronic energy structure of the polymers estimated from the optical absorption and the atmospheric photoemission spectra. A possible mechanism, which takes modulation of electronic energy structure at the interface due to a local lattice distortion into account, has been proposed. Another possible mechanism of the enhanced carrier generation based on the polarization at the MDOPPV/PPy interface is also mentioned.

Dark and photovoltaic properties of doped tetraphenylporphyrin sandwich cells. Part 2.—Properties of illuminated cells

The photovoltaic properties of electron-acceptor-doped thin-film sandwich cells of the structure Al|Al2O3|MTPP (meso-tetraphenylporphyrin)|Au, where M = Zn, H2, Cu, Ni or Co, fabricated using optimised doping procedures, and tested under high vacuum, are described. A wide variation in photovoltaic efficiency is observed with variation of the porphyrin central substituent, paralleling trends of device stability and affinity of the porphyrins to the dopant vapour. An exponential relationship is found between the value of peak quantum efficiency and the potential of the porphyrin first ring oxidation. The results indicate that the charge carrier photogeneration mechanism involves the formation of a weak ground-state donor-acceptor complex between the porphyrin and dopant molecules. Enhanced carrier generation is observed at wavelengths above 750 nm, possible resulting from the direct formation of triplet exciton states. Values of Soret band quantum yield of iodine-doped devices are 0.48, 0.083, 0.029, 0.026 and 0.018% for ZnTPP, H2TPP, CuTPP, NiTPP and CoTPP, respectively, and 0.22 and 0.04% for oxygen–water vapour-doped ZnTPP and H2TPP cells, respectively.

Field enhanced carrier generation in MOS-capacitors containing defects: C. Werner, A. Eder and H. Bernt Solid-St. Electron. 24, 275 (1981)

Field enhanced carrier generation in MOS-capacitors containing defects: C. Werner, A. Eder and H. Bernt Solid-St. Electron. 24, 275 (1981)

Field enhanced carrier generation in MOS-capacitors containing defects

Minority carrier generation lifetime was measured as a function of depletion width on wafers with different densities of defects. The results were compared with theoretical predictions for field enhanced carrier emission from coulomb centers. We found that capacitors with a very low lifetime due to stacking faults or metallic precipitations show a drastically enhanced field dependence of generation rate. Agreement with theory can be obtained if we assume a locally enhanced electric field at the defects which can be one order of magnitude higher than the average bulk field.

Field enhanced carrier generation in MOS-capacitors containing defects : C. Werner, A. Eder and H. Bernt. Solid-St. Electron.24, 275 (1981)

Field enhanced carrier generation in MOS-capacitors containing defects : C. Werner, A. Eder and H. Bernt. Solid-St. Electron.24, 275 (1981)