Investigation of non-aqueous electrodeposited CdS/Cd1−xZnxTe heterojunction solar cells

Polycrystalline Cd_1−xZn_xTe solar cells with efficiency of 8.3% were grown by cathodic electrodeposition on glass/ITO/CdS substrates using non-aqueous ethylene glycol bath. The deposit is characterised versus the process conditions by XRD and found to possess a preferred (1 1 1) orientation on Sb doping in the electroplating bath. The surface morphology of the deposit is studied using atomic force microscope. The average RMS roughness for the ternary film was higher than that for the binary CdTe. Optical properties of the films were carried out to study the band gap and calculation of molar concentration ‘x’. The effects of Sb doping in CdS/Cd_1−xZn_xTe heterojunctions have been studied. The short circuit current density (c) was found to improve and series resistance (R_s) reduced drastically upon Sb doping. This improvement in J_sc is attributed to an increase in quantum efficiency. The evaluation of solar cell parameters was also carried out using the current–voltage characteristics in dark and illumination. The best results were obtained when 2×10⁻³ M ZnCl₂ along with antimony were present in the deposition bath. Under AM 1.5 conditions the open circuit voltage, short circuit current density, and fill factor of our best cell were V_oc=600 mV, J_sc=26.66 mA/cm², FF=0.42 and efficiency, η=8.3%. The carrier concentration and built-in potential of Cd_1−xZn_xTe calculated from Mott–Schottky plot was 2.72×10¹⁷ cm⁻³ and 1.02 eV.     

Calculation of band offsets at the CdS/SnS heterojunction

SnS is a promising material for heterojunction solar cells, but the energy band alignment is not known for SnS-based heterojunctions. In this study, the energy band offset at the CdS/SnS heterojunction is calculated using the first principle, density-functional, pseudopotential method. A procedure analogous to that used in the core-level photoemission spectroscopy is adopted to calculate the band offset. The 4d core-level difference between Cd and Sn was estimated from the energy calculation of a superstructure consisting of zincblende CdS and rock-salt or zincblende SnS. The calculated valence-band offset is 0.1 eV when the rock-salt SnS is assumed and 0.84 eV when the zincblende SnS is assumed.     

Initial results of CdS/CuInTe2 heterojunction formed by flash evaporation

The CuInTe₂ thin films is one of the most attractive semiconductors for solar cells applications, since its direct band gap energy (Eg≈1 eV) is suitable as an absorber in photovoltaic conversion. In this letter the CuInTe₂ thin films are prepared by flash evaporation technique. X-ray diffraction measurements on the as-deposited CuInTe₂ film showed that these films consist mainly of the chalcopyrite phase. The junction formation in the n-CdS/p-CuInTe₂ cell has been investigated using current–voltage (I–V) and capacitance-voltage (C–V) measurements.     

Determination of photo-active region in organic thin film solar cells with an organic heterojunction

The photo-active region in the solar cells consisting of Cu-phthalocyanine (CuPc) and perylene-derivative (PV) layers was determined by using exciton blocking layers (EBLs) inserted in these layers. The photocurrent density was low when the EBL was placed near the CuPc/PV interface. With the increase of the distance between the EBL and the CuPc/PV interface, the photocurrent increased. However, when the distance reached a certain value, it leveled off owing to the limited diffusion length of excitons. From the analysis of the relationship between the position of EBL and the photocurrent density, the photo-active regions in the CuPc and PV layers were estimated to be 8 and 12 nm thick from the interface, respectively.     

Chemical stability of sputtered Mo/Sb2Te3 and Ni/Sb2Te3 layers in view of stable back contacts for CdTe/CdS thin film solar cells

Sb–Te phases sputtered from an Sb₂Te₃/Sb/Te target in a substrate temperature range from 293 to 523 K are characterised by X-ray diffraction (XRD) and Hall measurements. A simple thermodynamic model is introduced for estimating the chemical stability of the Ni/Sb₂Te₃ and the Mo/Sb₂Te₃ interface. These data and the results of kinetic test reactions for sputtered Ni/Sb₂Te₃, Ni/Sb–Te, Mo/Sb₂Te₃ and Mo/Sb–Te layers are compared using XRD measurements. Metal/Sb₂Te₃ thin film double-layer systems are used as a model for an innovative back contact for CdTe/CdS thin film solar cells offering an improved long-term stability.     

Photocarrier generation in organic thin-film solar cells with an organic heterojunction

Photocurrent–voltage characteristics for organic solar cells with a heterojunction formed between copper phthalocyanine and a perylene derivative (or C₆₀) were studied. The photocurrent was observed under both reverse and forward biases. From the analysis of the photocurrent action spectra, the origin of the reverse photocurrent was attributed to the excitons formed in both the organic layers, whereas that of the forward photocurrent was attributed to the excitons formed in the perylene derivative (or C₆₀) layer. The photocurrent density under reverse bias increased at higher temperatures, suggesting that the charge recombination possibility was lowered at higher temperatures. On the basis of the time responses of the photocurrents observed after pulsed photoirradiation, the charge separation and transport processes are discussed.     

CF4/O2 dry etching of textured crystalline silicon surface in a-Si:H/c-Si heterojunction for photovoltaic applications

We investigated a dry cleaning procedure of the crystalline substrate, both mono- and multi crystalline silicon, to leave an uncontaminated surface using an etching process involving CF₄/O₂ mixture. A detailed investigation was performed to find compatibility and optimisation of amorphous layer depositions both on flat and textured silicon by changing the plasma process parameters. We found evidence that plasma etching acts by removing the native oxide and the damages of textured silicon and by leaving an active layer on silicon surface suitable for the emitter deposition. SEM analysis confirmed that it is possible to find plasma process conditions where no appreciable damages and change in surface morphology are induced. By using this process we achieved on amorphous crystalline heterostructure a photovoltaic conversion efficiency of 13% on 51 cm² and 14.5% on 1.26 cm² active area. We also investigated compatibility of the process with industrial production of large area devices.     

Amorphous silicon/p-type crystalline silicon heterojunction solar cells with a microcrystalline silicon buffer layer

Undoped hydrogenated amorphous silicon (a-Si:H)/p-type crystalline silicon (c-Si) structures with and without a microcrystalline silicon (μc-Si) buffer layer have been investigated as a potential low-cost heterojunction (HJ) solar cell. Unlike the conventional HJ silicon solar cell with a highly doped window layer, the undoped a-Si:H emitter was photovoltaically active, and a thicker emitter layer was proven to be advantageous for more light absorption, as long as the carriers generated in the layer are effectively collected at the junction. In addition, without using heavy doping and transparent front contacts, the solar cell exhibited a fill factor comparable to the conventional HJ silicon solar cell. The optimized configuration consisted of an undoped a-Si:H emitter layer (700 Å), providing an excellent light absorption and defect passivation, and a thin μc-Si buffer layer (200 Å), providing an improved carrier collection by lowering barrier height at the interface, resulting in a maximum conversion efficiency of 10% without an anti-reflective coating.     

Epitaxial n-Si/p-CuInS2 heterojunction devices

We present and characterize the first epitaxial Si/CuInS₂ heterojunction devices. By means of molecular beam epitaxy (MBE), slightly copper-rich CuInS₂ epilayers are deposited on sulphur-terminated Si-(1 1 1) surfaces of n-type wafers. Both the quality of the substrate and the deposited epilayer are controlled in situ using low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES) and various other methods including transmission electron microscopy (TEM) are applied in the ex situ structural characterization. The heterojunction diodes are completed by the deposition of an indium-tin oxide (ITO) layer and of metallic contacts. Their electronic and structural properties are discussed.     

Effect of sodium addition on Cu-deficient CuIn1−xGaxS2 thin film solar cells

Chalcopyrites are important contenders among solar-cell materials due to direct band gap and very high-absorption coefficients. Copper-indium-gallium disulfide (CIGS2) is a chalcopyrite material with a near-optimum band gap of 1.5 eV for terrestrial as well as space applications. At FSEC PV Materials Laboratory, record efficiency of 11.99% has been achieved on a 2.7 μm CIGS2 thin film prepared by sulfurization. There are reports of influence of sodium on copper-indium-gallium selenide (CIGS) as well as copper-indium disulfide (CIS2) solar cells. However, this is the first of its kind approach to study the effect of sodium on CIGS2 solar cells and resulting in encouraging efficiencies. Copper-deficient CIGS2 thin films were prepared with and without the addition of sodium fluoride (NaF). Effects of addition of NaF on the microstructure and device electrical properties are presented in this work.     

Fabrication and characterisation of Si micropillar PV structures

Abstract

Arrays of vertical silicon micropillar radial junction solar cells have been fabricated by diffusion of direct application spin on dopant and from the vapour phase through proximity rapid thermal diffusion. The micropillars were fabricated by optical lithography and deep reactive ion etching. The micropillar arrays show superior antireflective properties over the measured spectrum and good correlation to finite difference time domain modelling of identical geometry arrays. Junctions formed by a conventional spin on doping process of phosphorus containing dopant solution produced Suns-V_oc values in the region of 0·3 V. This value is likely due to difficulties encountered in achieving an even distribution of dopant over the entire surface of the arrays. An alternative method utilising spin on dopant but employing an intermediate vapour phase diffusion step produced promising results with Suns-V_oc values reaching 0·5 V following a post-diffusion drive-in step.     

Fabricating CdS/BiVO4 and BiVO4/CdS heterostructured film photoelectrodes for photoelectrochemical applications

The photoelectrochemical (PEC) properties of heterostructured CdS/BiVO₄ and BiVO₄/CdS film electrodes on conducting glass for hydrogen production under visible light were investigated. These two types heterostructured film electrodes were prepared using spin coating method and ultrasonic spray pyrolysis method. The structural analyses of the prepared films were determined by using XRD, SEM, EDX and UV–vis. Photoelectrochemical measurements were carried out in a convenient three electrodes cell with 0.5 M Na₂SO₃ aqueous solution. In order to investigate band gap influence of electrode PEC property, a series ITO/Cd_1−xZn_xS/BiVO₄ and ITO/BiVO₄/Cd_1−xZn_xS (x = 0 ∼ 1) film electrodes were also synthesized. After PEC test, a maximum photocurrent density from ITO/CdS/BiVO₄ film electrode was confirmed. The maximum photocurrent density, 3 times and 113 times as that of single CdS film electrode and single BiVO₄ film electrode, respectively. Incident photon to current conversion (IPCE) of as prepared film electrodes were measured and the value were 65% (ITO/CdS/BiVO₄), 22% (single CdS film) and 10% (ITO/BiVO₄/CdS) at 480 nm with 0.3 V external bias. Comparison with ITO/BiVO₄/CdS electrode and single Cd_1−xZn_xS electrodes, the heterostructured ITO/CdS/BiVO₄ electrode can effectively suppress photogenerated electron-hole recombination and enhance light harvesting. Therefore, the ITO/CdS/BiVO₄ electrode gave the maximum photocurrent density and IPCE value.     

Thin film silicon solar cells for space applications: Study of proton irradiation and thermal annealing effects on the characteristics of solar cells and individual layers

The paper reports on the effects of a proton irradiation campaign on a series of thin-film silicon solar cells (single- and double-junction). The effect of subsequent thermal annealing on solar cells degraded by proton irradiation is investigated. A low-temperature annealing behaviour can be observed (at temperatures around 100 to 160°C) for microcrystalline silicon solar cells. To further explore this effect, a second proton irradiation campaign has been carried out, but this time on microcrystalline silicon layers. The effect of proton irradiation and subsequent thermal annealing on the optical and electronic properties of microcrystalline silicon is, thus, thoroughly investigated.     

Enhanced photocatalytic hydrogen evolution using a novel in situ heterojunction yttrium-doped Bi4NbO8Cl@Nb2O5

The novel in situ Z-scheme heterostructure materials Y-doped Bi₄NbO₈Cl@Nb₂O₅ (Bi_4-xY_xNbO₈Cl, x = 0, 1, 1.33, 2, 2.67, 3) have been synthesized successfully via a solid-state method. The as-prepared samples were characterized by XRD, Raman spectrum, SEM, EDS, element mapping, HRTEM, XPS and UV–vis spectrum to explore the structures, morphologies and optical properties. Photocatalytic activities were evaluated for hydrogen generation using the Pt as the co-catalysts. HRTEM results indicated the Pt particles were deposited on the surface of the Bi₄NbO₈Cl. Photocatalytic activities were evaluated by hydrogen generation. While photocatalytic results showed that BiY₃NbO₈Cl composites exhibited the best performance of hydrogen production under the full-range irradiation (λ > 300 nm) while the Y-doped Bi₄NbO₈Cl@Nb₂O₅ with Y:Bi molar ratio 1:1 obtained the highest efficiency with ultraviolet light eliminated. The H₂ production was 1.35 mmol and 0.9 mmol in 8 h, respectively. Furthermore, a direct Z-scheme mechanism with enhanced hydrogen evolution competent for accelerating the separation of photogenerated carries has been presented and proved by electrochemical impedance spectroscopy (EIS). Finally, considering the conclusions of the electron spin-resonance spectroscopy (EPR), ·OH radicals served as an active species played an important role in the hydrogen production. Mechanisms about the action of the ·OH radicals were also proposed.     

Effect of hydrogen dilution on intrinsic a-Si:H layer between emitter and Si wafer in silicon heterojunction solar cell

In silicon heterojunction solar cells, a thin intrinsic amorphous-silicon (a-Si:H) buffer layer between a doped emitter and a c-Si wafer is essential to minimize carrier recombination. This study examines the effect of H₂ dilution on the properties of the intrinsic a-Si:H layers deposited on Si wafers by plasma-enhanced chemical vapor deposition. A H₂/SiH₄ ratio of 24 led to improvements in the quality of intrinsic a-Si:H films and in the performance of passivation compared to a-Si:H film without H₂ dilution. A high H₂-dilution ratio, however, degraded the passivation of the a-Si:H film. The Si heterojunction solar cells with an optimal intrinsic a-Si:H layer showed an efficiency of 12.3%.     

The n-CdIn2Se4/p-CdTe heterojunction solar cells

The n-CdIn₂Se₄/p-CdTe heterojunction solar cells have been fabricated by deposition of n-CdIn₂Se₄ thin films using spray pyrolysis on to p-CdTe. Current density-voltage and capacitance-voltage measurements were performed to determine the electrical properties of the structures. The capacitance-voltage behavior indicates an abrupt interface. The junction quality factor (n), series and shunt resistance (R_s and R_sh), fill factor (FF) and efficiency (η) for the cell have been estimated. The device exhibit maximum fill factor (FF), power conversion efficiency (η) of about 0.55% and 0.67%.     

Nanostructured La0.6Sr0.4Co0.8Fe0.2O3/Y0.08Zr0.92O1.96/La0.6Sr0.4Co0.8Fe0.2O3 (LSCF/YSZ/LSCF) symmetric thin film solid oxide fuel cells

Functional all-oxide thin film micro-solid oxide fuel cells (μSOFCs) that are free of platinum (Pt) are discussed in this report. The μSOFCs, with widths of 160 μm, consist of thin film La_0.6Sr_0.4Co_0.8Fe_0.2O₃ (LSCF) as both the anode and cathode and Y_0.08Zr_0.92O_1.96 (YSZ) as the electrolyte. Open circuit voltage and peak power density at 545 °C are 0.18 V and 210 μW cm⁻², respectively. The LSCF anodes show good lattice and microstructure stability and do not form reaction products with YSZ. The all-oxide μSOFCs endure long-term stability testing at 500 °C for over 100 h, as manifested by stable membrane morphology and crack-free microstructure.     

Properties of amorphous silicon solar cells fabricated from SiH2Cl2

Chlorinated intrinsic amorphous silicon films [a-Si:H(Cl)] and solar cell i-layers were fabricated using electron cyclotron resonance-assisted chemical vapor deposition (ECR-CVD) and SiH₂Cl₂ source gas. n–i–p solar cells deposited on ZnO–coated SnO₂ substrates had poor photovoltaic performances despite the good electronic properties measured on the a-Si:H(Cl) films. Improved open–circuit voltage (V_oc) of 0.84 V and fill factor (FF) of 54% were observed in n–i–p solar cells by providing an n/i buffer layer and by using Ga-doped ZnO coated glass substrates. However, the FF improvement was still rather poor, which is thought to originate from high interface recombination in the ECR deposited solar cells. The V_oc and the FF showed much stable feature against light soaking.     

Cooperative effect between BaTiO3 and CaFe2O4 in a cocatalyst-free heterojunction composite for improved photochemical H2 generation

A series of novel BaTiO₃/CaFe₂O₄ heterojunction composites with different weight ratios of CaFe₂O₄ vs BaTiO₃ was successfully fabricated by sonication-calcination method using the pre-prepared BaTiO₃ and CaFe₂O₄ powders synthesized in hydrothermal and sol-gel methods, respectively. The composites were well characterized using XRD, UV–vis DRS, SEM, TEM, EDS and XPS to substantiate that BaTiO₃ and CaFe₂O₄ coexist in the heterojunction composite. The highest photocatalytic hydrogen generation rate was obtained for BaTiO₃/CaFe₂O₄ (40 wt%) compared to either of its individual counterparts and this improvement indicated the existence of a cooperative effect between BaTiO₃ and CaFe₂O₄ in the heterojunction. Based on UV-vis-DRS, photoluminescence and time-resolved fluorescence lifetime measurements, the cooperative effect between BaTiO₃ and CaFe₂O₄ originated from the improved photoresponse in the visible light region and efficient separation of the photogenerated electron–hole pairs augmenting their availability for the photocatalytic reaction. A plausible photocatalytic mechanism was also deduced using electrochemical impedance spectroscopy measurements, describing the migration direction of the separated charge carriers. Moreover, the best composite BaTiO₃/CaFe₂O₄ (40 wt%) exhibited fairly stable photoactivity for H₂ production using the sacrificial agent (Na₂S and Na₂SO₃) without the assistance of any noble metals as cocatalysts.