Interface engineering in chalcopyrite thin film solar devices

Successful interface engineering requires compositional and electronic material characterization as a prerequisite for understanding and intentionally generating interfaces in photovoltaic devices. The paper gives an overview with several examples, all referring to Cu(In,Ga)(S,Se)₂ (“CIGSSe”)-based solar cells, with an emphasis on characterization using highly specialized methods, such as elastic recoil detection analysis, X-ray emission spectroscopy and photoelectron spectroscopy using synchrotron and ultraviolet light for excitation, inverse photoemission spectroscopy and Kelvin probe force microscopy. First, the determination of the depth profile of the band gap energy E_g in the absorber layer is demonstrated. The modification of E_g towards both interfaces is discussed in terms of beneficial electronic effects. Next, the interface between absorber and buffer layers with alternative and promising non-toxic materials is considered. Between CIGSSe and a ZnSe buffer deposited by the metalorganic chemical vapor deposition (MOCVD) method a buried ZnS interface was found. For a Zn(O,OH) buffer processed with an ion layer gas reaction (ILGAR) the correlation of surface composition, valence band maximum and efficiency of the resulting solar cell is shown. In addition, another approach is considered where a ZnMgO window layer is sputtered directly on the absorber omitting any buffer layer. The determination of the potential distribution at the ZnMgO/CIGSSe interface supports the understanding of this new and simpler way to get good cell performances even without any buffer. Finally, monolithically integrated solar modules without encapsulation were investigated before and after accelerated aging tests and changes at the interconnects were identified.     

Stable cycling behavior of the light invariant AlGaAs/Si/metal hydride solar cell

The extended stability (eight month) of an unusual solar cell which provides a nearly constant energetic output in illuminated or dark conditions is demonstrated. Bipolar multiple bandgap semiconductors are selected to provide a high photopotential and efficient conversion of insolation. A redox couple is chosen with an electrochemical potential closely matched to the photopotential, and provides efficient high-capacity electrochemical storage. The single cell combines bipolar AlGaAs (E_g=1.6 eV) and Si (E_g=1.0 eV) and metal hydride/NiOOH storage, and generates a light variation insensitive potential of 1.2–1.3 V at an overall solar conversion efficiency of 18.2%. The cell generates power under illumination and in the dark, and in the dark stored energy is spontaneously (without external switching) released.     

ZnO–InN nanostructures with tailored photocatalytic properties for overall water-splitting

ZnO-based electrodes for one-step photocatalytic water splitting are designed by incorporating InN. The electronic and optical properties of (ZnO)_1−x(InN)_x alloys and ZnO with InN-like cluster formations ZnO:(InN)_x are analyzed by means of first-principles approaches. We calculate the energy gaps E_g, the band-edge energies relative to the vacuum level, and the optical absorption, employing the GW₀ method to describe single-particle excitations and the Bethe–Salpeter equation to model the two-particle exciton interactions. For ZnO and InN, the valence-band maximum (VBM) is E_VBM ≈ −7.3 and −5.7 eV, and the energy gap is E_g ≈ 3.3 and 0.7 eV, respectively. Incorporating InN into ZnO, the random (ZnO)_1−x(InN)_x alloys up-shifts the VBM and down-shifts the conduction-band minimum (CBM). In addition, the presence of InN-like clusters enhances this effect and significantly narrows the band gap. For instance, the VBM and the energy gap for 12.5% InN are E_VBM ≈ −6.5 and −6.1 eV, and E_g ≈ 2.2 and 1.9 eV for the alloy and the cluster structure, respectively. This impact on the electronic structure favors thus visible light absorption. With proper nanoclusters, the band edges straddle the redox potential levels of H⁺/H₂ and O₂/H₂O, suggesting that ZnO–InN nanostructures can enhance the photocatalytic activity for overall solar-driven water splitting.     

Well-defined core/shell pDA@Ni-MOF heterostructures with photostable polydopamine as electron-transfer-template for efficient photoelectrochemical H2 evolution

Here, novel core/shell polydopamine@Ni-MOF (pDA@Ni-MOF) heterogeneous nanostructures are synthesized via a simple one-pot nucleation-growth technique. This rational core/shell design method provide a uniform Ni-MOF shell thickness (shell: ～ 10 nm) as well as homogeneous wrapping of pDA templates with quite narrow size distributions. The obtained band properties of bare pDA (E_CB = ?0.35 eV and E_VB = 2.95 eV vs normal hydrogen electrode (NHE)) and bare Ni-MOF (E_CB = ?0.49 eV and E_VB = 2.85 eV vs NHE) clearly revealed charge separation is occurred on pDA by absorbing light due to π-π1 transition, and photogenerated electrons on conduction band (CB) of pDA was migrated to CB of Ni-MOF. Specifically, the photoelectrochemical (PEC) water performance of pDA@Ni-MOF photoanodes with highest current density is recorded as 8.61 mA/cm² at 0.77 V vs. RHE under visible LED irradiation, which is significantly higher than bare pDA (0.008 V vs. RHE) and bare Ni-MOF (0.011 V vs. RHE) at the same conditions. Note that, the higher photon absorption properties of pDA in core together with high interaction valence bond between two semiconductors could generate electron rich state giving rise to faster electron transfer kinetics as next generation of MOF based hybrid materials with regular morphologies.     

Optical and photoconductive properties of SnS thin films prepared by electron beam evaporation

Thin films of SnS have been prepared by electron beam evaporation. The films represent Herzbergite orthorhombic structure, established by their XRD patterns. The band gap energy and type of optical transitions were determined from transmission spectra and an optical band gap of E_g(tr)=1.23 eV for indirect transitions and E_g(tr)=1.38 eV for direct transitions were estimated. Using the dependence of photoconductivity from wavelength, a band gap of E_g(ph)=1.2 eV was determined as well. A thermal band gap of E_g(T)=1.29 eV was evaluated from the temperature dependence of the dark resistivity, and admixture level with activation energies (0.25 and 0.36 eV) were found. Roughness of the surface of SnS thin films was evaluated using atomic force microscopy.     

Efficiency variation of the intermediate band solar cell due to the overlap between absorption coefficients

This paper studies the efficiency variation of the intermediate band solar cell (IBSC) due to the variation of amount of the overlap in the energy range of the different transitions in three band model, using detailed balance model. The effect of overlap on the performance of IBSC has been investigated between 0 and 4 eV range, under 46,000 sun concentration, for E_G=1.95 and 1.12 eV using equal and nonequal absorption coefficients. If there is no overlap (0 eV), maximum efficiency of 63.2% for E_G=1.95 and 54.8% for E_G=1.12 eV are obtained which are independent of the value of absorption coefficients. Under maximum overlap condition (4 eV), in the case of equal absorption coefficients, the efficiency of IBSC reduces to 43.2% for E_G=1.95 eV and 35% for E_G=1.12 eV; the optimum intermediate band (IB) level also shifts towards the half of the band gap. The impact of overlap can be minimized when absorption coefficients increase with energy. The requirement of large base width which is the reciprocal of the smallest absorption coefficient is the bottleneck of the nonequal absorption coefficients even the absorption coefficient and the base width product reduces from 5 to 1 for maximum overlap condition. A further reduction in the efficiency due to the nonradiative recombination is also obtained for the larger base width.     

Growth of Zn1−xMgxO films with single wurtzite structure by MOCVD process and their application to Cu(InGa)(SSe)2 solar cells

The effect of the growth temperature and Mg/(Mg+Zn) molar flow rate ratio of metal organic sources on the crystalline structure of Zn_1−xMg_xO (ZMO) films is investigated in thin films prepared by metal organic chemical vapor deposition (MOCVD) process on fused silica in order to obtain the wide-bandgap ZMO films with single wurtzite structure, which is very important to achieve high-efficiency chalcopyrite solar cells. Based on the measurements and analysis of the fabricated samples, the ZMO films with the controllable bandgap from 3.3 to 3.72 eV can exhibit a single wurtzite phase depending on the growth temperature and Mg content. Furthermore, the resistivity of ZMO films is comparable to that of ZnO film. It is a good indication that ZMO film is superior to CdS or ZnO films as buffer and window layers mainly due to its controllable bandgap energy and safety. As a result, the solar cells with ZMO buffer were fabricated without any surface treatment of Cu(InGa)(SSe)₂ (CIGSSe) absorber or antireflection coating, and the efficiency of 10.24% was obtained.     

High photocatalytic performance for hydrogen production under visible light on the hetero-junction Pani-ZnO nanoparticles

The present work is devoted to the preparation of the hetero-junction of Polyaniline-Zinc oxide nanoparticles (Pani-ZnO_Nps) and its photo-electrochemistry to assess its photocatalytic properties for the water reduction into hydrogen. The semiconducting characterization of the Pani-ZnO_Nps synthetized by in situ chemical oxidative polymerization was studied for the hydrogen evolution reaction (HER) upon visible light illumination. The forbidden bands E_g (= 1.64 eV, Pani) and (3.20 eV, ZnO_NPS) were extracted from the UV–Visible diffuse reflectance data. The Electrochemical Impedance Spectroscopy (EIS) showed the predominance of the intrinsic material with a bulk impedance of 71 kΩ cm². The semi conductivity was demonstrated by the capacitance measurements with flat band potentials (E_fb = - 0.7 and - 0.3 V_SCE) and carriers concentrations (N_A = 1.77 × 10¹⁹ and N_D = 4.80 × 10²⁰ cm^?3) respectively for Pani and ZnO_NPS. The energetic diagram of the hetero-junction Pani-ZnO_Nps predicts electrons injection from Pani to ZnO_NPS in KOH electrolyte. An improvement of 78% for the evolved hydrogen was obtained, compared to Pani alone; a liberation rate of 61.16 μmol g^?1 min^?1 and a quantum yield of 1.15% were obtained. More interestingly, the photoactivity was fully restored after three consecutive cycles with a zero-deactivation effect, indicating clearly the reusability of the catalyst over several cycles.     

New developments in Cu(In,Ga)(S, Se)2 thin film modules formed by rapid thermal processing of stacked elemental layers

A second-generation process for high-efficiency large-area Cu(In,Ga)(Se,S)₂ thin film (CIGSSe) solar modules has been developed applying controlled sodium doping and rapid thermal processing for absorber formation. The pilot line delivers aperture area efficiencies of 12.2%±0.5% (average) for 30×30 cm² modules and a certified champion efficiency of 13.1% for an unencapsulated 60×90 cm² demonstrator module. The stability of the frameless pilot line modules with a low-cost package against humidity is confirmed externally by passing the damp heat test sequence according to IEC 61646. Substitution of CBD-CdS by CBD-Zn(S, OH) buffer layers yields efficiencies up to 12% on 30×30 cm² circuits. First CIGSSe-cells on flexible substrates were also developed applying 30 μm thin titanium foils, 8×8 cm² in size. Average cell efficiencies on a substrate up to 12.4% were achieved.     

Solar syngas production from CO2 and H2O in a two-step thermochemical cycle via Zn/ZnO redox reactions: Thermodynamic cycle analysis

Solar syngas production from CO₂ and H₂O is considered in a two-step thermochemical cycle via Zn/ZnO redox reactions, encompassing: 1) the ZnO thermolysis to Zn and O₂ using concentrated solar radiation as the source of process heat, and 2) Zn reacting with mixtures of H₂O and CO₂ yielding high-quality syngas (mainly H₂ and CO) and ZnO; the ZnO is recycled to the first, solar step, resulting in net reaction βCO₂ + (1 − β)H₂O → βCO + (1 − β)H₂. Syngas is further processed to liquid hydrocarbon fuels via Fischer-Tropsch or other catalytic processes. Second-law thermodynamic analysis is applied to determine the cycle efficiencies attainable with and without heat recuperation for varying molar fractions of CO₂:H₂O and solar reactor temperatures in the range 1900-2300 K. Considered is the energy penalty of using Ar dilution in the solar step below 2235 K for shifting the equilibrium to favor Zn production.     

Vanadium based zinc spinel oxides: Potential materials as photoanode for water oxidation and optoelectronic devices

In the recent past, layered zinc-based vanadium spinel oxides (ZnVOs) have shown an intriguing way to accomplish the challenges of energy conversion, storage, and utilization issues. Here, through first-principles calculations, a comprehensive study has been carried out to investigate the AV₂M (where A = Zn, Zn₂, Zn₃, Zn_4, and M = O₄, O₆, O₇, O₈, O₉ respectively) electronic, photocatalytic, and optical properties. Formation energies with a negative sign express that the final compounds from the pure elements are possible and cohesive energies revealed that compounds are energetically stable. Spin-polarized calculations are also taken into account for better approximation of the electronic properties (band structure and density of states). All layered structures show indirect bandgap for spin-up calculations in range 0.3 eV–2.4 eV, while spin-down calculations show mix trends in range 2.3 eV–3.50 eV. An appropriate band edge with large enough kinetic over-potentials of the oxygen evolution reaction (ΔE_V ≥ 1.244 eV) makes them potential candidates as photoanode for water splitting. ZnV₂O₄ is more suitable for OER as it has small kinetic overpotential as compared to the oxidation potential of water. Interestingly, all ZnVOs display a dramatically large coefficient (~10⁵ cm⁻¹) for optical absorption. Photogenerated electrons and holes on the layered zinc-based vanadium spinel oxide surfaces could make these spinel oxides promising materials for photocatalytic water splitting and solar energy conversion.     

Control of conduction band offset in wide-gap Cu(In,Ga)Se2 solar cells

The effects of conduction band offset of window/Cu(In,Ga)Se₂ (CIGS) layers in wide-gap CIGS based solar cells are investigated. In order to control the conduction band offset, a Zn_1−xMg_xO film was utilized as the window layer. We fabricated CIGS solar cells consisting of an ITO/Zn_1−xMg_xO/CdS/CIGS/Mo/glass structure with various CIGS band gaps (E_g≈0.97–1.43 eV). The solar cells with CIGS band gaps wider than 1.15 eV showed higher open circuit voltages and fill factors than those of conventional ZnO/CdS/CIGS solar cells. The improvement is attributed to the reduction of the CdS/CIGS interface recombination, and it is also supported by the theoretical analysis using device simulation.     

Graphitic carbon wrapped Co on Zn,Co, N-codoped 3D tremella-like carbon with abundant nanocages for high-performance oxygen reduction

To develop green and renewable energy, subtle design and synthesis of non-precious metal catalysts with low-price, high-efficiency, and robust stability are of paramount significance for oxygen reduction reaction (ORR). In this study, we synthesized graphitic carbon wrapped Co on Zn, Co, N-codoped 3D tremella-like carbon with abundant nanocages (G-Co/Zn, Co, N-TCs) by one-step coordination pyrolysis in the presence of azacytosine. The pyrolysis temperature and Zn/Co feeding ratio were carefully investigated. Integrating the unique structure and synergistic effect of the bimetals, the resulting G-Co/Zn, Co, N-TCs exhibited remarkable ORR activity (E_onset = 0.99 V; E_1/2 = 0.88 V vs. RHE), and excellent stability (only 13 mV negative shift of E_1/2 after scanning 2000 cycles), outperforming the control groups and commercial Pt/C under alkaline condition. This research presents some precious viewpoints for fabricating advanced N- and transition metal-dual doped carbon electrocatalysts in energy conversion and storage correlated devices.     

ZnO nanorods-PANI heterojunction dielectric,electrochemical properties,and photodegradation study of organic pollutant under solar light

In the last years, the development of photocatalysts has gained a growing interest in the photoelectrochemical conversion. Here, we report a hetero-junction organic/inorganic (Polyaniline (PANI)) on n type Silicon substrate prepared by electrophoretic deposition (EPD). ZnO nanorods (NRs) are grown by chemical bath deposition (CBD) on seed layer of ZnO prepared by dip coating onto the PANI layer. The relief and morphological properties are investigated by both atomic force microscope (AFM) and scanning electron microscopy (SEM). The forbidden bands (E_g) of 1.12 and 3.26 eV are obtained for PANI and ZnO_NRS respectively. The optical and dielectric constants are determined by the diffuse reflectance where the extinction coefficient (k), the refractive index (n), optical conductivity, dissipation factor and relaxation time are extracted. The semi conductivity is highlighted by the capacitance measurement; a flat band potential (E_fb = 0.541 V) for ZnO_NRs-PANI is deduced from the Mott-Schottky plot. The contributions of the bulk and grain boundaries of the electrode are evidenced from the electrochemical impedance spectroscopy (EIS). The as-fabricated samples show improved photoactivity compared to PANI and ZnO_NRs. Indeed, the hetero-junction exhibits a high stability and recyclability for the photodegradation of Orange II (OII) after six cycling runs. The current research offers a new understanding of designing a high performance of the photocatalyst under solar light based on PANI for the wastewater treatment.     

Enhancement of Bi-based niobate pyrochlores conductivity with Ru-doping. Structural,optical, and electrical properties

New Ru-doped bismuth-based pyrochlores were synthesized. The effects of Ru-doping in Bi_1.5Mg_0.375Cu_0.375Nb_1.5-xRu_xO_7-δ on the formation of the crystal structure, stability, optical, and electrical properties of the compositions were studied. For compositions up to x = 0.1, only a phase of the pyrochlore type was detected by X-ray diffraction method (XRD) and energy dispersive X-ray analysis (EDX). Small impurities of Bi₂Ru₂O₇, RuO₂ and CuO were found for samples with x ≥ 0.25. The lattice parameters of pyrochlores decrease upon doping with Ru. X-ray diffraction modeling revealed the structural stability of the doped compositions and the distribution of Ru⁴⁺ cations in the B sites of the A₂B₂O₆O' pyrochlore structure. A decrease in the band gap energy E_g was observed upon doping with Ru: 2.40 (x = 0) – 2.10 eV (x = 0.5) for the direct transition and 1.52 (x = 0) – 0.16 eV (x = 0.5) for indirect transition as a result of optical spectra analysis and DFT-HSE03 calculation. The chemical compatibility of the obtained pyrochlores with the manganite perovskite La_0.7Sr_0.3MnO₃ was studied. The pyrochlores can be considered as a component of cathode composite material for using in intermediate temperature solid oxide fuel cells (IT-SOFC).     

Interstitial carbon doped of setaria viridis-like Znln2S4 hollow tubes for efficient the performance of photocatalytic hydrogen production

In the photocatalytic water splitting hydrogen production system, the key to efficient use of solar energy is to choose a suitable photocatalyst. As an important ternary sulfide, ZnIn₂S₄ (ZIS) has attracted wide attention because of its narrow band gap (E_g = 2.3–2.8 eV) and wide light absorption range. However, further modification was still needed. Therefore, in this work, the unique C/ZIS hollow tubes with nano-flakes were prepared by a simple solvothermal method. To a large extent, this unique structure increased the utilization of light and active sites. Moreover, the dissolution of PAN during the solvothermal process caused the carbon element to be uniformly doped into the hollow tube framework. After a series of characterization results, C/ZIS-3.0 has the best hydrogen release rate (1241.94 μmol g⁻¹ h⁻¹) and good cyclability under visible light irradiation (λ ≥ 420 nm). And its unique morphology and possible catalytic mechanism were further discussed.     

Evaluation of a hybrid photovoltaic-wind system with hydrogen storage performance using exergy analysis

This article presents and discusses the results of an exergy analysis conducted during the operation of a test-bed hybrid wind/solar generator with hydrogen support, designed and constructed at the Industrial Engineering School of the University of Extremadura, Badajoz (Spain). An exergy analysis is made of the different components of the system, calculating their exergy efficiencies and exergy losses, and proposing future improvements to increase the efficiency of the use of the surplus energy produced by the wind/solar generator. The results show the electrolyzer to have an acceptable efficiency (η_ex = 68.75%), but the photovoltaic modules a low exergy efficiency (η_ex = 8.39%) as also is the case, though to a lesser extent, for the fuel cell (η_ex = 35.9%).     

Synthesis and characterization of 2,1,3-benzothiadiazole-thieno[3,2-b]thiophene-based charge transferred-type polymers for photovoltaic application

New low-band-gap copolymers, including thieno[3,2-b]thiophene and 2,1,3-benzothiadiazole, were synthesized as photovoltaic materials. Thiophene was introduced to provide extended π-conjugation length and charge transfer properties. A band gap (E_g^op=1.62 eV, E_g^ec=1.51 eV) of this polymer was investigated through UV-vis spectroscopy and cyclic voltammetry. A bulk heterojunction structure of glass/indium tin oxide (ITO)/PEDOT:PSS/polymer-PCBM(1:3)/LiF/Al was fabricated for investigating photovoltaic properties. PC₇₁BM was used as an acceptor material, due to its increased absorption in the visible region, in comparison with PC₆₁BM. In this polymer, incident photon-to-current conversion efficiency (IPCE) was as high as 50%. Moreover, maximum power conversion efficiency (PCE) of up to 1.72% was achieved under AM 1.5 G conditions. It demonstrated relatively high V_OC (0.67 V) and J_SC (6.86 mA/cm²), while a low fill factor value (0.37) was obtained.     

Laser raman measurements of temperature and species concentration in swirling lifted hydrogen jet diffusion flames

Simultaneous spatially and temporally resolved point measurements of temperature, mixture fraction, major species (H₂, H₂O, O₂, N₂), and minor species (OH) concentrations are performed in unswirled (S_g = 0), low swirl (S_g = 0.12), and high swirl (S_g = 0.5) lifted turbulent hydrogen jet diffusion flames into still air. Ultraviolet (UV) Raman scattering and laser-induced predissociative fluorescence (LIPF) techniques are combined to make the multi-parameter measurements using a single KrF excimer laser. Experimental results are compared to the fast chemistry (equilibrium) limit, to the mixing without reaction limit, and to simulations of steady stretched laminar opposed-flow flames. It is found that in the lifted region where the swirling effects are strong, the measured chemical compositions are inconsistent with those calculated from stretched laminar diffusion flames or stretched partially premixed flames. Sub-equilibrium values of temperature, sub-flamelet values of H₂O, and super-flamelet values of OH are found in an intermittent annular turbulent brush of the swirled flame but not in the unswirled flame. Farther downstream of the nozzle exit (x/D ≥ 50), swirl has little effect on the finite-rate chemistry.