Photo-generated Current Density Research Articles

Upright Pyramid Surface Textures for Light Trapping and MoOx Layer in Ultrathin Crystalline Silicon Solar Cells

In this work, ray tracing is used to investigate the optical characteristics of various surface structures in ultrathin crystalline silicon (c-Si) for solar cells. Ultrathin c-Si with a thickness of 20 μm is used as the substrate. The light trapping includes front upright pyramids with a molybdenum oxides (MoOx) anti-reflection (AR) layer. Planar ultrathin c-Si (without a MoOx AR layer and upright pyramids) is used as a reference. The wafer ray tracer was developed by a photovoltaic (PV) lighthouse to model the MoOx AR layer to reduce the front surface reflectance and impacts of the AR layer on ultrathin Si solar cells. The optical properties are calculated on the AM1.5 global solar energy spectrum across the 200–1200 nm wavelength region. From the absorbance profile, the photogenerated current density (Jph) in the substrate is also calculated with various surface structures. The front upright pyramids with the MoOx layer result in the largest absorbance enhancement due to the enhanced light scattering by the pyramids and MoOx AR layer. The Jph of 37.41 mA/cm2 is improved when compared to the planar ultrathin c-Si reference. This study is significant as it illustrates the potential of ultrathin c-Si as a promising PV module technology in the future.

Fabrication and optimization of perovskite-based photoanodes for solar-driven CO2 photoelectroreduction to formate

The photoelectrochemical reduction of CO2 to value-added products represents a promising strategy for mitigating CO2 emissions. However, further research efforts need to be undertaken to enable the technology for scale-up, including the design and fabrication of efficient photoelectrodes capable of yielding substantial photogenerated current densities. In this work, a photoanode combining a commercial calcium titanate perovskite (CaTiO3) and BiVO4 layers coated onto a transparent FTO substrate by automated spray pyrolysis is proposed. Different photoanode configurations are tested, with the most favourable results achieved when BiVO4 is positioned as the top layer with back illumination. The optimization of the catalytic loading is also assessed, finding an optimal at 1 mg cm−2 of CaTiO3 and 3 mg cm−2 of BiVO4, resulting in an impressive current density of –71 mA cm−2 at –1.8 V vs. Ag/AgCl. This optimal photoanode is then integrated into an electrolyzer for continuous visible light-driven CO2 reduction in the gas phase to formate, obtaining a concentration of 63.8 g L−1, with a Faradaic Efficiency of 79.1 %, and solar-to-fuel conversion efficiency of 7.6 %. These results represent a significant advancement in the development of photoanodes, offering promise for the future scalability of photoelectrochemical CO2 reduction processes.

Mechanism for Improving Kesterite Solar Cells Performance via Filed Passivation Effect Induced by V‐Doped MoSe2 Interface Layer at Back Interface

One of the key issues impeding the enhancement of power conversion efficiency (PCE) of Cu2ZnSn(S,Se)4 (CZTSSe) solar cells is the severe carrier recombination at CZTSSe/MoSe2 back interface, primarily arising from the reverse electric field formed between CZTSSe and n‐type MoSe2 produced after selenization. To inhibit recombination at back interface, herein, the MoSe2 layer is converted from n‐type to p‐type by V doping in site through reaction of V‐alloyed Mo (Mo:V) back electrode with Se during selenization, and CZTSSe solar cells with p+‐type V‐doped MoSe2 (MoSe2:V) interface layer are fabricated. It is found that the PCE of the device rises from 8.34% to 9.63% as back contact changes from soda lime glass (SLG)/Mo/n‐MoSe2 to SLG/Mo:V/p+‐MoSe2:V. The quantitative analysis demonstrates that the increased PCE predominantly originated from the decreased reverse saturated current density (J0), followed by the decreased series resistance (RS), and lastly by the increased photogenerated current density (JL). The influence mechanism of the SLG/Mo:V/MoSe2:V back contact on device performance is suggested by studying the properties of Mo:V and MoSe2:V films and CZTSSe/MoSe2:V heterojunction. This work emphasizes the vital significance of the back surface passivation field induced by p+‐MoSe2:V/p‐CZTSSe heterojunction, which is enlightening for optimizing the back contact in kesterite photovoltaics.

ZnO/Au/GaN heterojunction-based self-powered photoelectrochemical Sensor for alpha-fetoprotein detection

In recent years, the development of miniature and portable sensors has been a major focus of research. PEC self-powered sensors have emerged as a potential solution to the power supply issue, eliminating the need for external power supplies and operating without bias voltage. This study developed a ZnO/Au/GaN sensor for highly sensitive detection of alpha-fetoprotein (AFP). The sensor uses GaN substrates with nanogold films to provide an auxiliary bias voltage, promoting high photogenerated current density. Using ZnO/Au/GaN as a photoanode resulted in significantly higher photocurrent generated by the sensor compared to Au/GaN or ZnO/ITO alone. To enable selective detection of AFP, antibody modification of the ZnO nanorod arrays was employed. The linear range of the sensor response to AFP was determined to be 0.080–5.0 ng/mL, with an impressively low detection limit of 0.027 ng/mL (S/N = 3). These results demonstrate the potential of this self-powered sensor for detecting AFP content in human serum samples. Overall, this study presents a novel approach for developing highly sensitive and selective self-powered sensors for biomarker detection, which could facilitate early detection and clinical diagnosis of various types of cancer.

Boosting the photocathode performances of protected Cu2O inverse opals using photonic-crystal heterostructures

Cuprous oxide (Cu2O) is a highly efficient p-type semiconductor photocatalytic material with the capability for visible light absorption. In this work, three-dimensional Cu2O inverse opals with different diameters were prepared using the electrochemical deposition method. The photocathode performances were enhanced by adjusting the wavelength of photonic band gap, being co-modified by Au and Ag nanoparticles, applying the protective layers, and fabricating a photonic-crystal heterostructure with another p-type semiconductor. The slow-light effect leads a 23.9 % increase in photocurrent density. The co-modification of Au and Ag nanoparticles achieves a 77 % average increase in photocurrent density compared with pure Cu2O inverse opals. Conformal protective layers, consisting of ZnO and Al2O3 and deposited using atomic layer deposition, prevent the photocorrosion of the Cu2O photocathode while preserving its photocatalytic activity. After one hour of illumination, the photo-generated current density of protected Cu2O inverse opals is nearly three times higher than that of their uncoated counterparts. By fabricating Ag-doped ZnO with p-type characteristics and combining it with Cu2O into a heterostructure, the saturated photocurrent density of the Ag-doped ZnO/Cu2O heterostructure reaches −3.78 mA/cm2, which is almost 3.2 times higher than that of pristine Cu2O inverse opal at −0.12 V vs RHE.

Bifunctional Co-TiO2 co-promoted bulk and interfacial charge transfer of BiVO4 photoanodes for solar water oxidation

The primary causes of photogenerated carrier recombination in BiVO4 photoanodes are the formation of bulk polarons and sluggish water oxidation kinetics. To address these issues effectively, we designed a bifunctional Co-TiO2 to modify the BiVO4 photoanodes. UV–vis DRS and Mott-Schottky results indicate that Co adjusts the band gap of TiO2, and there forms a type-II heterojunction between BiVO4 and Co-TiO2. The experimental tests and theoretical calculations for OER as well as EIS results demonstrate that Co sites in Co-TiO2 provide more reaction sites for the interfacial oxidation reaction, reducing the overpotential. Under the dual action of Co-TiO2, the co-promotion for the migration ability of photogenerated carriers in the bulk and on the surface of BiVO4 photoanode has been realized. This conclusion is supported by analysis of photogenerated charge transfer behavior through transient photocurrent and IMPS. As a result, the photogenerated current density of BiVO4/Co7 %-TiO2 photoanode reaches 4.38 mA cm−2 at 1.23 V vs. RHE under AM 1.5 G illumination, which is four times that of pure BiVO4 photoanodes.

Enhancement of power conversion efficiency of Ag-substituted Cu2ZnSn(S,Se)4 solar cells via tuning Cu2+/(Cu++Cu2+) percentage in precursor solution

Although trace silver (Ag) substitution for Cu in Cu2ZnSn(S, Se)4 (CZTSSe) offers benefits in improving open-circuit voltage (VOC) of CZTSSe solar cells, its further development is still hindered by the fairly low short-circuit current density (JSC) owing to the increase of bandgap (Eg) of the absorber layer. Herein, a strategy is demonstrated to mitigate the poor JSC arising from the Ag substitution by tuning the Cu2+/(Cu++Cu2+) percentage in the (Cu, Ag)2ZnSnS4 (CAZTS) precursor solution. Depth characterization indicates that the appropriate Cu2+/(Cu++Cu2+) percentage in the CAZTS precursor solution increases the photogenerated charges separation and carrier transport processes for Ag-substituted CZTSSe (CAZTSSe) solar cells. Benefiting from this strategy, the power conversion efficiency (PCE) of 9.57% is achieved when the Cu2+/(Cu++Cu2+) is 50 at.%. The main factor for the improvement of PCE is due to the increase in photogenerated current density (JL) and decrease in shunt resistance (Rsh). It is demonstrated that the increase in JL results from a widening in the depletion layer width (Wd) of the p-n junction and an increase in carrier lifetime, induced by optimization of the Cu2+/(Cu++Cu2+) percentage. While the decrease in Rsh comes from the change of reaction pathways due to the change in Cu2+/(Cu++Cu2+) percentage.

In situ depositing a metal–organic framework film on TiO2 nanoarray toward stable photogenerated cathodic protection for nickel-coated magnesium alloy

The photoelectrochemical performance of single TiO2 nanotubes (NTs) under visible light irradiation is unacceptable for application as a photogenerated cathodic protection photoanode owing to the wide band gap and narrow light absorption range. To develop a stable photoanode for photogenerated cathodic protection, this paper proposes preparing a new class of metal–organic framework (MOF, i.e., UIO-66-NH2) modified TiO2 NTs composite by a solvothermal method. An ortho-octahedral structure of the synthesized MOF powder was identified by different characterizations, including high-resolution transmission electron microscopy (HRTEM). X-ray photoelectron spectroscopy (XPS) and HRTEM show that the MOF and TiO2 form a type-II heterojunction, significantly enhancing the photoelectrochemical performance, confirmed by the electrochemical measurements. Compared with the single-component TiO2 photoanode, the MOF/TiO2 composite exhibits significantly higher photogenerated current density by a factor of 3.3, remarkably lower charge transfer resistance, and a negatively shifted bias threshold. The MOF/TiO2 composite coupled with nickel- phosphorus-plated magnesium alloy (Mg/Ni) is shown to impart the Mg/Ni with more negative and stable polarization potential. This finding indicates that the photoanode can provide stable photogenerated cathodic protection for Mg/Ni electrode. These findings offer insight into preparing highly efficient and durable photocatalytic materials and composite photoanodes for photogenerated cathodic protection.

Glutathione-sensitized SnS2 nanoflake/CdS nanorod heterojunction for enhancing cathodic protection of 304 stainless steel with remarkable photoelectric conversion performance

An effective way to enhance photochemical performance can be achieved by optimizing the microstructure of composite semiconductor materials. Here, a two-step hydrothermal method was used to synthesize SnS2/CdS composite photoanodes for photocathode protection applications, and glutathione was introduced as a structure-directing agent to enhance the separation efficiency of photogenerated electron-hole pair of SnS2/CdS. In the presence of glutathione, SnS2 became a smaller and thinner nanoflake and hexagonal CdS extended into a rod shape, the combination of them resulted in larger light collection area and higher charge transfer rate. The electrochemical test results demonstrated that the open circuit potential of 304 SS coupled with SnS2/CdS/GSH photoanode was negatively shifted to about −0.5 V under light, which not only far exceeded the self-corrosion potential of 304 SS (-0.18 V), but also improved from −0.42 V of SnS2/CdS. Further, the photogenerated current density of SnS2/CdS/GSH was nearly three times that of SnS2/CdS, EIS and polarization curve results also indicated that glutathione enhances the PEC performance of SnS2/CdS. Obviously, the type-II heterojunction formed by glutathione-sensitized SnS2/CdS provides more electrons for 304 SS, which is beneficial to achieve better photocathodic protection.

Simultaneous S-scheme promoted Ag@AgVO3/g-C3N4/CeVO4 heterojunction with enhanced charge separation and photo redox ability towards solar photocatalysis

Photocatalytic removal of toxic contaminants is one of the emerging techniques for water remediation, but it suffers from low redox ability, charge recombination and poor light harvesting efficiency. The present work reports a simultaneous S-scheme promoted by CeVO4/g-C3N4/Ag@AgVO3. The formation of the S-scheme mechanism enhanced the generation of photogenerated carriers and also improved the redox ability of the electrons and holes in the reduction and oxidation photocatalysts. The ternary demonstrated remarkable photo switching properties along with efficient charge separation which was achieved through dual interfacial interaction within the ternary (Ag@AgVO3/g-C3N4 and CeVO4/g-C3N4). The heterojunction formation was verified through the shift in binding energy spectra in the X-ray Photoelectron spectroscopy (XPS), and high-resolution transmission electron microscopy analysis (HR-TEM). The ternary demonstrated reduced PL intensity, width of space charge region and an upsurge in photogenerated current density in the order of 93 μA/cm2 (∼6X higher than all the pristine). This resulted in efficient removal of methyl orange, methylene blue and endocrine disruptive bisphenol-A with a removal rate of 0.02 min−1, 0.03 min−1 and 0.0087 min−1 and an apparent quantum yield of 4.6 × 10−9 (Methylene Orange), 6.89 × 10−9 (Methylene Blue) and 2 × 10−9 (Bisphenol A/H2O2).

Regulating divalent metal species in aluminum-based layered double hydroxides to selectively promote photocatalytic CO production from CO2

Photocatalysis is a potential technology to reduce CO2 greenhouse gases. Layered double hydroxides (LDHs) are promising photocatalysts due to good catalytic activity, low synthesis cost, and large-scale preparation. However, their CO selectivity in photocatalytic CO2 reduction is still unsatisfactory. Herein, we have systematically studied the effects of different divalent metal species (e.g., Mg2+, Ni2+, and Zn2+) on the CO selectivity in aluminum-based LDHs. The results indicate that ZnAl-LDH can negatively shift the conduction band to obtain a large driving power for CO2 reduction to CO, which much improves the CO selectivity. In addition, the ZnAl-LDH also reduces the charge transfer resistance leading to a higher photo-generated current density, thus achieving a high CO2 reduction performance. Specifically, ZnAl-LDH presents a better CO selectivity of 72.8 % and a higher CO yield of 1.58 μmol·g−1·h−1, than MgAl-LDH (66.2 %, 0.96 μmol·g−1·h−1) and NiAl-LDH (59.8 %, 1.01 μmol·g−1·h−1). This study provides a feasible strategy to improve the selectivity of CO photocatalytic reduction of CO2 by regulating divalent metal species in aluminum-based layered double hydroxides.

Efficiency enhancement of Cu2ZnSn(S, Se)4 solar cells by addition a CuSe intermediate layer between Cu2ZnSn(S, Se)4 and Mo electrode

A CuSe intermediate layer (IL) is prepared between CZTSSe and Mo electrode to decay the carrier recombination on the rear surface of the CZTSSe absorber. The power conversion efficiency (PCE) can be increased from 7.52% to 10.09% by optimizing the thickness of CuSe IL. The increased PCE comes from improvement in filling factor (FF), short-circuit current density (JSC), and open-circuit voltage (VOC), and their contribution percent is calculated to be 63.08%, 24.83%, and 12.09%, respectively. It is demonstrated that boosted FF is mainly due to decreased reverse saturation current density (J0), raised JSC owing to higher photogenerated current density (JL), and enhanced VOC caused by decreased J0 and higher JL. The contribution percent of (ideal factor (A), J0), JL, Rs, and shunt resistance (Rsh) to increased PCE is calculated to be 60.84%, 27.41%, 10.33%, and 1.42%, respectively. By experimental characterization and SCAPS-1D simulation, it is suggested that decreased J0 results from the formation of passivation field and high electron potential barrier at the rear surface of CZTSSe due to the addition of suitable thickness CuSe IL, higher JL from the increase in width of the depletion region of CZTSSe/CdS, lower Rs from decrease in thickness of Mo(S, Se)2, and bigger Rsh from improved crystal quality of CZTSSe absorber.

Mg2Si is the new black: Introducing a black silicide with >95% average absorption at 200–1800 nm wavelengths

Textured silicon surface structures, in particular black silicon (b-Si), open up possibilities for Si-based solar cells and photodetectors to be extremely thin and highly sensitive owing to perfect light-trapping and antireflection properties. However, near-infrared (NIR) performance of bare b-Si is limited by Si band gap of 1.12 eV. This work reports a simple method to increase NIR absorption of b-Si by in vacuo silicidation with magnesium. Obtained Mg2Si/b-Si heterostructure has a complex geometry where b-Si nanocones are covered by Mg2Si shells and crowned with flake-like Mg2Si hexagons. After growth optimization, Mg2Si cover atop b-Si resulted in 5-fold lower total reflectivity (3.7%) and optical absorption to be no lower than 88% over 200–1800 nm spectral range. More importantly, Mg2Si/b-Si heterostructure is more adjusted to match AM-1.5 solar spectrum with theoretically higher photogenerated current density. The maximal advantage is demonstrated in the NIR region compared to bare b-Si in full accordance with one‘s expectations about NIR-sensitive narrow band gap (0.75 eV) semiconductor with high absorption coefficient, which is Mg2Si. Results of optical simulation confirmed the superiority of Mg2Si/b-Si NIR performance. Therefore, this new wide-band optical absorber called black silicide proved rather competitive alongside state-of-the-art approaches to extend b-Si spectral blackness.

Improvement of power conversion efficiency of Cu2ZnSn(S,Se)4 solar cells by Al doped CdS

In this work, we substituted Al doped CdS(CdS:Al) for CdS buffer layer of Cu2ZnSn(S,Se)4 (CZTSSe) solar cell to increase built-in field and decrease interface recombination, and studied influence of this substitution on power conversion efficiency (PCE) of CZTSSe solar cell. It is found that the PCE can be increased from 6.57% to 8.99% by substituting CdS:Al with Al doping concentration of 3.3 at% for CdS. The increased PCE is mainly attributed to increase in photogenerated current density (JL) and decrease in reverse saturation current density (J0). Mechanisms of increase in JL and decrease in J0 are suggested by analysis of influence of change of bandgap, transmission, electron density and lattice constants of CdS induced by Al doping on light intensity passing through CdS, built-in field and lattice mismatch of CZTSSe/CdS heterojunction. This strategy provides a rational design for optimizing the heterojunction of CZTSSe solar cells to achieve high efficiency.

Improvement of Photovoltaic Performance of Cu2ZnSn(S,Se)4 Solar Cells by Modification of Back Electrode Interface with Amorphous Boron Nitride

AbstractAn amorphous boron nitride (aBN) layer is inserted between Cu2ZnSn(S,Se)4 (CZTSSe) films and Mo back electrode of CZTSSe solar cell by magnetron sputtering to mitigate the carrier recombination at back interface by tuning energy band alignment and suppressing formation of secondary phases and Mo(S,Se)2. The effect of the aBN layer on power conversion efficiency (PCE) of CZTSSe solar cell is investigated by numerical simulation and experiment. It is found that PCE is greatly improved via the insertion of suitable thick aBN layer, which is attributed to increased open‐circuit voltage (VOC) and fill factor (FF). By quantitative analysis, it is deduced that the increased VOC originates mainly from decrease in reverse saturation current density (J0), followed by increase in shunt resistance (RSh), and the increased FF is mainly due to the decreased J0, followed by increased RSh and decreased photogenerated current density (JL). By optimizing aBN thickness through tuning sputtering time, the PCE increases from 8.68% of CZTSSe solar cell without the aBN layer to 9.51% of the CZTSSe solar cell with aBN layer. The influence mechanism of the aBN layer on the PCE is suggested by analyzing quantitatively the effect of the aBN layer on JL and electrical parameters.

High-efficiency black silicon tunnel oxide passivating contact solar cells through modifying the nano-texture on micron-pyramid surface

Optical loss is a significant factor restricting the conversion efficiency of conventional bifacial tunnel oxide passivating contact (TOPCon) solar cells. Black silicon structure is commonly used to enhance the photogenerated current density ( J ph ) of crystalline solar cells due to its excellent light-trapping capability. However, the photogenerated current gain is cancelled by the increased emitter recombination current originated from the black silicon structure with a high enhanced surface area ratio. In this work, we used a buffered oxide etching solution to modify the surface morphology of nanopore/micron-pyramid composite (NPP) structure silicon. Further, we studied the effects of NPP structures with different enhanced surface area ratio on front-side reflection, boron atom doping, emitter passivation, and cell performance. By identifying the appropriate surface modification processing, we fabricated the large-scale (158.75 mm × 158.75 mm) bifacial TOPCon solar cells using industrial equipment and processes with an average short-circuit current density of 41.12 mA/cm 2 and average conversion efficiency of 23.08%. Through adequately widening nanostructure size and depositing high-quality Al 2 O 3 /SiN x stacked passivation films on NPP structure surface, we achieved lower carrier recombination while maintaining high J ph . • Conversion efficiency of 23.2% achieved by b-Si TOPCon solar cells. • Surface modification of b-Si improves emitter passivation using buffer etching. • Medium specific area of b-Si achieve a balance of light-trapping and electrical loss.

Thin films of p-SnS and n-Sn2S3 for solar cells produced by thermal processing of chemically deposited SnS

Post-deposition heat treatments of chemically deposited SnS-orthorhombic thin films at 450 °C in a controlled ambient greatly influence their structural, morphological, compositional, optical and electrical properties. Heating of the film with a tin chloride layer (90 nm in thickness) produced by thermal evaporation led to an increased crystalline grain diameter (23 nm) in the film, with characteristics suitable for solar cell application: a bandgap (Eg) of 1.2 eV (indirect), p-type electrical conductivity of 10–3 Ω–1 cm–1, hole concentration of 1015 cm–3 and hole mobility of 2.4 cm2/(V s). However, heating the SnS film in a controlled S-vapour ambient converted it to Sn2S3 with a crystalline grain diameter of 24 nm, Eg of 1.2 eV (direct), n-type electrical conductivity of 3.4 × 10–3 Ω–1 cm–1, and an electron concentration of 1016 cm–3. Optical absorption spectra of these materials suggest a photo-generated current density (JL) of 32 mA/cm2 under air mass 1.5 Global (1000 W/m2) solar radiation for either material for a thickness of 600 nm, which is attractive for solar cells.

Outstanding stability and photoelectrochemical catalytic performance of (Fe, Ni) co-doped Co3O4 photoelectrodes for solar hydrogen production

In this work, pure and (Fe, Ni) co-doped Co3O4 nanostructured photoelectrodes of different doping levels and thicknesses were manufactured at constant substrate temperature (450 °C) using the spray pyrolysis technique. In addition to the chemical compositions; the structural, optical, electrical, and photoelectrochemical (PEC) properties were investigated through the use of various analysis techniques. By increasing the codpoing ratio to 6%, the low energy band gap is decreased from 1.43 to 1.3 eV and the high energy bandgap is increased from 2.63 to 2.87 eV, in addition to the reduction in particle size from 30.2 to 12.0 nm. The high energy gap vanishes by increasing the codoped film's spread volume to 60 ml. X-ray photoelectron spectroscopy of 6%(Fe, Ni)-60ml Co3O4 confirms the existence of Ni2+,3+ and Fe2+,3+. Among the studied photoelectrodes, the 6%(Fe, Ni)-60ml Co3O4 photoelectrode displays a photocatalytic hydrogen output rate of 150 mmol/h.cm2 @-1V in 0.3M Na2SO4 electrolyte. The photocurrent density of 6%(Fe, Ni)-60ml photoelectrode reached up to 13.6 mA/cm2@-1V with an IPCE (incident photon to current conversion efficiency) of ∼42%@405 nm and STH (solar to hydrogen conversion efficiency) of ∼11.37%, which are the highest values yet for Co3O4-based photocatalysts. The value of ABPE(applied bias photon-to-current efficiency) is 0.34%@(-0.28V and 636 nm). Interestingly, this photoelectrode shows a photogenerated current density of −0.14 mAcm−2 at 0 V and a PEC current onset over 0.266V. The thermodynamic parameters, corrosion parameters, PEC surface areas, Tafel slopes, and impedance spectroscopies are also being studied to confirm and classify the PEC H2 production mechanism. The 6%(Fe, Ni)-60ml Co3O4 photoelectrode stability/reusability shows only a 6.6% reduction in PEC performance after ten successive runs at -1V with a corrosion rate of 1.2 nm/year. This work offered a new codoping strategy for the design of a highly active Co3O4 based photocatalyst for the generation of solar light-driven hydrogen.

Intermediate-band-assisted near-field thermophotovoltaic devices with InAs, GaSb, and Si based absorbers

A new scheme of near-field thermophotovoltaic devices is proposed by introducing the intermediate-band absorber. The two-step excitation via the intermediate band provides a large photogenerated current density and maintains a high voltage output for the thermal-electric conversion. Energy dissipation processes in devices are analyzed by using the detailed balance model. Results show that the powers and efficiencies of thermophotovoltaic devices with intermediate bands in appropriate positions are clearly larger than those of the conventional counterparts. Based on the optical properties observed in experiments, three types of absorber materials are compared, which indicates that InAs with low energy bandgap is more suitable as a high-temperature material for photon absorptions than GaSb and Si. Comparing the performance of our model with experiments, we show that the InAs based thermophotovoltaic device allows the enhancement of efficiency over a range of gap sizes. The proposed model may open a new field in the application of thermophotovoltaic devices.

An in-depth study of photocathodic protection of SS304 steel by electrodeposited layers of ZnO nanoparticles

Thin layers of ZnO nanoparticles were deposited potentiostaticly on the surface of stainless steel 304 (SS304) at various potentials (−0.7 V, −1.0 V, −1.3 V, −1.6 V) to study the effect of photo-generated cathodic protection on the substrate. Electrochemical methods of open circuit potential (OCP), potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS) were employed to investigate the electrochemical behaviors of ZnO-covered SS304 system (ZnO/SS304) in 3.5 wt% NaCl solution under the conditions with and without UV illumination. It is found that the ZnO nanoparticles deposited at −1.0 V was in the shape of hexagonal nanorods, which caused a negative shift to lowest OCP upon UV illumination. Polarization curves were also performed on ZnO/ITO glass system to reveal the photoelectrochemical behavior of ZnO nanoparticles only. Possible anodic and cathodic reactions on the ZnO/SS304 and ZnO/ITO systems at the absence and presence of light illumination have been analysed and discussed in detail. It is found that traditional Tafel extrapolation of the polarization curves cannot be used to get the free corrosion current density of ZnO/SS304 system under light illumination due to the superposition of photo-generated current density. The charge transfer resistance obtained from EIS also includes the influence from the photo-generated reactions on the ZnO/SS304 system. Long-term immersion of the ZnO/SS304 in NaCl solution indicates that the ZnO nanorods underwent morphological transitions to nanotubes at 5 days and finally to needles and thin plates at 10 days. Correspondingly, the photocathodic protection of the ZnO layer decreases with the extension of immersion time. The transformation in morphology is postulated to be caused by the attack of Cl− ions on the surface of ZnO nanoparticles.