Electrochemically deposited p-n homojunction cuprous oxide solar cells

The electrical properties of both p- and n-type cuprous oxide (Cu₂O) films electrochemically deposited from two electrolyte solutions were examined by current-voltage measurements. The resistivity of p-type Cu₂O varied from 3.2×10⁵ to 2.0×10⁸ Ω cm, while that of n-type Cu₂O from 2.5×10⁷ to 8.0×10⁸ Ω cm, depending on deposition conditions such as solution pH, deposition potential and temperature. With optimized deposition conditions for minimum resistivity, p-n homojunction Cu₂O solar cells were fabricated by a two-step deposition process. The p-n homojunction Cu₂O solar cells showed a conversion efficiency of 0.1% under AM1 illumination. The low efficiency is attributed to the high resistivity of p- and n-type Cu₂O, which require doping to reduce.     

CdO/Cu2O solar cells by chemical deposition

CdO and Cu₂O thin films have been grown on glass substrates by chemical deposition method. Optical transmittances of the CdO and Cu₂O thin films have been measured as 60–70% and 3–8%, respectively in 400–900 nm range at room temperature. Bandgaps of the CdO and Cu₂O thin films were calculated as 2.3 and 2.1 eV respectively from the optical transmission curves. The X-ray diffraction spectra showed that films are polycrystalline. Their resistivity, as measured by Van der Pauw method yielded 10⁻²–10⁻³ Ω cm for CdO and approximately 10³ Ω cm for Cu₂O. CdO/Cu₂O solar cells were made by using CdO and Cu₂O thin films. Open circuit voltages and short circuit currents of these solar cells were measured by silver paste contacts and were found to be between 1–8 mV and 1–4 μA.     

Thin film deposition of Cu2O and application for solar cells

Deposition conditions of cuprous oxide (Cu₂O) thin films on glass substrates and nitrogen doping into Cu₂O were studied by using reactive radio-frequency magnetron sputtering method. The effects of defect passivation by crown-ether cyanide treatment, which simply involves immersion in KCN solutions containing 18-crown-6 followed by rinse, were also studied. By the crown-ether cyanide treatment, the luminescence intensity due to the near-band-edge emission of Cu₂O at around 680 nm was enhanced, and the hole density was increased from 10¹⁶ to 10¹⁷ cm⁻³. Finally, polycrystalline p-Cu₂O/n-ZnO heterojunctions were grown for use in solar cells. Two deposition sequences were studied, ZnO deposited on Cu₂O and Cu₂O deposited on ZnO. It was found that the crystallographic orientation and current–voltage characteristics of the heterojunction were significantly influenced by the deposition sequence, both being far superior for the heterojunction with structure Cu₂O on ZnO than for the inverse structure. We successfully obtained a photoresponse for the first time in the deposited thin film of Cu₂O/ZnO.     

A new route for fabricating CdO/c-Si heterojunction solar cells

CdO/c-Si solar cells have been made by depositing CdO thin films on p-type monocrystalline silicon substrate by means of the rapid thermal oxidation (RTO) technique using a halogen lamp at 350 °C/45 s in static air. Results on structural, optical, and electrical properties of grown CdO films are reported. The electrical and photovoltaic properties of CdO/Si solar cells are examined. Under AM1 illumination condition, the cell shows an open circuit voltage (V_OC) of 500 mV, a short circuit current density (J_SC) of 27.5 mA/cm², a fill factor (FF) of 60%, and a conversion efficiency (η) of 8.84% without using frontal grid contacts and/or post-deposition annealing. Furthermore, the stability of solar cells characteristics is tested.     

Improved n-Si/oxide junctions for environmentally safe solar energy conversion

The considerable interest in the practical use of solar energy has increased the importance of photovoltaic and photoelectrochemical systems. Metal oxide films like tin dioxide (SnO₂), titanium oxide (TiO₂) or indium tin oxide (ITO) are known to form stable photovoltaic junction with semiconductors of practical relevance like silicon (Si). Thin films of SnO₂ and TiO₂ were prepared easily and conveniently on the surface of silicon wafers by the spray pyrolysis technique. The prepared heterojunctions, i.e. the Si/oxide junction represent the main part of stable and efficient solar energy converter. In these systems, the solid/solid junction (n-Si/oxide) is separated from the site of the environmental interaction by the stable oxide film (SnO₂ or TiO₂) that protect the conventional semiconductor from photocorrosion.Besides its use in the fabrication of photovoltaic cells, the n-Si/oxide was used in the preparation of photoelectrochemical cells. The characteristics of the oxide film were subjected to a series of improvements either in the surface conductivity or the band gap energy, i.e. the position of the Fermi level of the semiconducting oxide by incorporating foreign atoms in the oxide film matrix during its preparation. The incorporation of Ru in the thin oxide film leads to the improvement of the solar conversion efficiency by improving the fill factor of the photovoltaic or photoelectrochemical cells. Such improvement enables the use of the prepared cells as clean energy converters. The fabricated solar cells have an average open-circuit potential of 0.44–0.62 V and a short circuit current of 28–30 mA/cm². A conversion efficiency up to 14% was achieved.     

Annealing effect on electrochromic properties of tungsten oxide nanowires

The effect of thermal annealing on the electrochromic properties of the tungsten oxide (WO_3−x) nanowires deposited on a transparent conducting substrate by vapor evaporation was investigated. The X-ray diffraction (XRD) indicated that the structures of the nanowries annealed below 500 °C had no significant change. The X-ray photoelectron spectroscopy (XPS) analysis suggested that the O/W ratio and the amount of W⁶⁺ ions in the annealed nanowire films could be increased as increasing annealing temperature. Increased annealing temperature could promote the coloration efficiency and contrast of the nanowire films; however, it could also affect the switching speed of the nanowire films.     

Deposition of transparent and conductive Al-doped ZnO thin films for photovoltaic solar cells

The effect of the substrate temperature on the optoelectronic properties of ZnO-based thin films prepared by rf magnetron sputtering has been studied. Three different targets (Zn/Al 98/2 at%, ZnO:Al 98/2 at% and ZnO:Al₂O₃ 98/2 wt%) have been investigated in order to compare resulting samples and try to reduce the substrate temperature down to room temperature. From the ZnO:Al₂O₃ target, transparent conductive zinc oxide has been obtained at 25°C with the average optical transmission in the 400–800 nm wavelength range, T = 80–90% and resistivity, = 3−5 × 10⁻³ Ωcm. In Al:Zn0 layers, the spatial distribution of the electrical properties across the substrate placed parallel to the target has been improved by depositing at high substrate temperatures, above 200°C. Besides, owing to diffusion processes of CuInSe₂ and CdS take place at 200°C, an AI:ZnO/CdS/CuInSe₂ polycrystalline solar cell made with the Al:ZnO deposited at 25°C as the transparent conductive oxide, has shown a more efficient photovoltaic response, η = 6.8%, than the one measured when the aluminium-doped zinc oxide has been prepared at 200°C, η = 1.8%.     

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.     

Low temperature silicon oxide and nitride for surface passivation of silicon solar cells

Surface passivation at low processing temperature becomes an important topic for crystalline and multicrystalline silicon solar cells. In this work, silicon oxide (250°C) and silicon nitride (300°C) have been developed by Photo-CVD and PECVD technique respectively. Effects of deposition parameters on the optoelectronic and structural properties of the films have been investigated. Interface-trap density (D_it) and fixed charge density (Qf) have been estimated by high frequency (1 MHz) capacitance-voltage measurement on Metal–Insulator–Silicon structure (CV-MIS). The effect of silicon oxide and silicon nitride on the performance of c-Si solar cells have been studied.     

Scanning tunneling microscopy of electrodeposited CuInSe2 nanoscale multilayers

We have investigated the electrochemical deposition of modulated thin films based on the Cu_xIn_2−xSe₂ system. CuInSe₂ is a leading alternative to silicon for use in thin film photovoltaic solar cells due to its optical absorption and electrical characteristics. Alternating layers of two different compositions based on the Cu_xIn_2−xSe₂ system were potentiostatically deposited. These nanometer-scale layers are used to form reduced-dimensionality structures such as superlattices that can be used in concentrator solar cells. We have used X-ray diffraction, energy-dispersive spectroscopy, and scanning tunneling microscopy to characterize our asdeposited thin films. The ability of the scanning tunneling microscope to resolve the individual nanoscale layers of our multilayered thin films is shown and is used to determine modulation wavelengths.     

Structure and optical properties of electrochromic copper oxide films prepared by reactive and conventional evaporation techniques

Copper oxide films (Cu_xO) are deposited by thermal evaporation techniques using copper oxide (CuO) or copper (Cu) as starting material. By varying the deposition parameters, two main types of Cu_xO film exhibiting different optical properties form. These are reddish gray and colorless films. The samples are characterised optically and morphologically. X-ray diffraction spectra reveal that evaporated Cu_xO films are amorphous. Fourier-transform infrared spectra of the samples were studied to evaluate chemical identification. The refractive index, the extinction coefficient and the thickness of the films are evluated from transmittance characteristics in the ultraviolet, visible and near-infrared regions. The refractive indices of the samples are between 2.9 and 3.1. The values determined for the optical constants are in aggreement with the results found in the literature. We report for the first time that Cu_xO films show reversible optical switching from the colored to bleached state. Optical transmittance measurements of the copper oxide film relative to indium tin oxide coated glass varied during coloring from spectral transmittance T_s = 85−40%     

High-efficiency μc-Si/c-Si heterojunction solar cells

P-type microcrystalline silicon (μc-Si (p)) on n-type crystalline silicon (c-Si(n)) heterojunction solar cells is investigated. Thin boron-doped μc-Si layers are deposited by plasma-enhanced chemical vapor deposition on CZ-Si and the V_oc of μc-Si/c-Si heterojunction solar cells is higher than that produced by a conventional thermal diffusion process. Under the appropriate conditions, the structure of thin μc-Si films on (1 0 0), (1 1 0), and (1 1 1) CZ-Si is ordered, so high V_oc of 0.579 V is achieved for 2×2 cm² μc-Si/multi-crystalline silicon (mc-Si) solar cells. The epitaxial-like growth is important in the fabrication of high-efficiency μc-Si/mc-Si heterojunction solar cells.     

Polycrystalline silicon solar cells on low cost foreign substrates

The use of polycrystalline silicon layers on low-cost substrates is a promising approach for the fabrication of low-cost solar cells. Using low-carbon steel and graphite as substrates, solar cell structures have been deposited by the thermal decomposition of silane and appropriate dopants.Steel was selected as a substrate on the sole basis of its low cost. However, steel and silicon are not compatible in their properties, and an interlayer of a diffusion barrier, such as borosilicate, must be used to minimize the diffusion of iron from the substrate into the deposit. The deposited silicon on borosilicate/steel substrates is polycrystalline with a grain size of 1–5 μm, depending on deposition conditions. P-n junction solar cells were found to have low open-circuit voltages and poor current-voltage characteristics, and Schottky-barrier solar cells were found to show negligible photovoltages.Graphite is more compatible with silicon in properties than steel, and silicon deposited on graphite substrates shows considerably better microstructures. A number of solar cells, 2·5×2·5 cm in area, have been fabricated from n⁺-silicon/p-silicon/p⁺-silicon/graphite structures. The best cell to date had a V_oc of 0·35 V and an AMO efficiency of 1·5% (no antireflection coating). This type of solar cell is very promising because of the simplicity in fabrication.     

Powder profile studies in electrodeposited cuprous oxide films

Thin films of copper oxide have been electrodeposited cathodically on copper substrates at room temperature. The deposited films have been characterized by X-ray diffraction and scanning electron microscopy. XRD showed the formation of crystalline cuprous oxide (Cu₂O). The XRD peaks are found to be shifted towards lower angle with narrowing of the profiles and the lattice parameter increases with annealing temperature. Scanning electron micrographs showed the formation of localized grain growth region which may be due to the non-uniform deposition of the films. The X-ray diffraction line broadening analysis of the as deposited as well as annealed films have been studied in order to evaluate the microstructural parameters which characterize the microstructural changes. The microstructural parameters like coherent domain size, RMS microstrain and dislocation density have been calculated using Warren-Averbach (Fourier) for multiple order, integral breadth (single and multiple line) and Williamson–Hall plot. The results of analysis obtained by different methods have been compared. The coherent domain size and RMS strain are not found to change appreciably with the increase of film thickness (4–13 μm). The optimum pH value of the electrolytic solution to deposit the films is found to lie in the range 9.2–9.3 where the strain variation is small compared to other pH values. The values of crystallite size and strains obtained by Warren–Averbach method and integral breadth method are comparable. However, Williamson–Hall plot overestimates the values of these two parameters. It is found that the crystallite size increases and RMS strain decreases with the increase of annealing temperature. The dislocation density is also found to decrease with annealing temperature.     

Mechanistic investigation on tuning the conductivity type of cuprous oxide (Cu2O) thin films via deposition potential

The conductivity type of cuprous oxide (Cu₂O) thin films is tuned by controlling the deposition potential of an electrochemical process in an acid cupric acetate solution containing sodium dodecyl sulfate. The morphology and chemical composition of the deposited Cu₂O films are studied by SEM, XRD and XPS. The change of the conductivity type of Cu₂O films is further studied through zero-bias photocurrent and Mott-Schottky measurements. The results indicate that the Cu₂O films behave as n-type semiconductors when the overpotentials are low (potentials higher than ?0.05 V) and p-type semiconductors when the overpotentials are high (potentials lower than ?0.10 V). The transformation of conductivity from n-type to p-type comes from the competition reactions between forming Cu₂O and forming metallic Cu from Cu²⁺. When the potential is lower than ?0.10 V, most of Cu²⁺ are consumed by the growth of metallic Cu at the film/solution interface, so that the Cu²⁺ provided to grow Cu₂O film are insufficient and copper vacancies form in the film, leading to the p-type conductivity.     

Photoelectrochemical characteristics of electrodeposited cuprous oxide with protective over layers for hydrogen evolution reactions

Cuprous oxide is one of the inexpensive options of highly efficient visible light-based photocathode for hydrogen generation in photoelectrochemical cells. Highly photoactive cuprous oxide (Cu₂O) films are obtained by cathodic electrodeposition using lactate stabilized copper sulphate precursor exhibiting a photo-current density of ~1 mA/cm² at ?0.1 V vs. RHE. Although Cu₂O is a decent choice for photoelectrochemical applications, including hydrogen evolution reaction (HER), it faces serious issues related to photodegradation and instability. To address this issue, a comparative study of two types of thin films, Al (2%)-doped ZnO (AZO) and NiO_x (usually, x > 2 at low T to x→1 at high T annealing) as photo-corrosion protective overlayers is made. The improved stability of the protected photoelectrodes is observed as noted from the photocurrent degradation of 3.5%, 0.16% and 0.03% in Cu₂O (bare), Cu₂O/AZO, and Cu₂O/NiO_x photocathodes, respectively. Furthermore, the electrochemical impedance spectroscopy reveals that electrode protected with NiO_x exhibit faster charge transfer kinetics and minimum photocurrent degradation as compare to the Cu₂O/AZO and Cu₂O(bare) photoelectrodes, proving its potential in HER kinetics.     

RF sputter deposition of the high-quality intrinsic and n-type ZnO window layers for Cu(In,Ga)Se2-based solar cell applications

Transparent ZnO films were prepared by rf magnetron sputtering, and their electrical, optical, and structural properties were investigated under various sputtering conditions. Aluminum-doped n-type(n-ZnO) and undoped intrinsic-ZnO (i-ZnO) layers were deposited on a glass substrate by incorporating different targets in the same reaction chamber. The n-ZnO films were strongly affected by argon ambient pressure and substrate temperature, and films deposited at 2 mTorr and 100°C showed superior properties in resistivity, transmission, and figure of merit (FOM). The sheet resistance of ZnO film was less dependent on film thickness when the substrate was heated during deposition. These positive effects of elevated substrate temperature are presumably attributed to the rearrangement of the sputtered atoms by the heat energy. Also, the films are electrically uniform through the 5 cm×5 cm substrate. The maximum deviation in sheet resistance is less than 10%. All of the films showed strong (0 0 2) diffraction peak near 2θ =34°. The undoped i-ZnO films deposited in the mixture of argon and oxygen gases showed high transmission properties in the visible range, independent of the Ar/O₂ ratio, while resistivity rose with increased oxygen partial pressure. The Cu(In,Ga)Se₂ solar cells, incorporating bi-layer ZnO films (n-ZnO/i-ZnO) as window layer, were finally fabricated. The fabricated solar cells showed 14.48% solar efficiency under AM 1.5 conditions (100 mW/cm²).     

The structural, transport and optical properties of screen printed CuxS thick films

The structural, transport and optical properties of screen printed CuxS thick films with possible application in photovoltaic and photothermal devices are reported. The X-ray diffraction studies show that the screen printed films are stable up to about 220°C in air and belong to the CuxS structure. Above this temperature it decomposes mainly to CuSO₄. The electrical conductivity depends on the sintering temperature and the amount of flux, Cu(NO₃)₂, used in the paste for screen printing. The differential scanning calorimetry studies reveal the phase changes occurring during heating and pertaining to the dependence of electrical conductivity on the sintering temperature. A configuration consisting of screen printed CuxS on chemically deposited and annealed (at 200°C) CdS thin film exhibited rectification.     

Improvement of a-Si solar cell properties by using SnO2:F TCO films coated with an ultra-thin TiO2 layer prepared by APCVD

TiO₂-overcoated SnO₂:F transparent conductive oxide films were prepared by atmospheric pressure chemical vapor deposition (APCVD) and an effect of TiO₂ layer thickness on a-Si solar cell properties was investigated. The optical properties and the structure of the TiO₂ films were evaluated by spectroscopic ellipsometry and X-ray difractometry. a-Si thin film solar cells were fabricated on the SnO₂:F films over-coated with TiO₂ films of various thicknesses (1.0, 1.5 and 2.0 nm) and I–V characteristics of these cells were measured under 1 sun (100 mW/cm² AM-1.5) illumination. It was found that the TiO₂ film deposited by APCVD has a refractive index of 2.4 at 550 nm and anatase crystal structure. The conversion efficiency of the a-Si solar cell fabricated on the 2.0 nm TiO₂-overcoated SnO₂:F film increased by 3%, which is mainly attributed to an increase in open circuit voltage (V_oc) of 30 mV.     

High-Ts amorphous top cells for increased top cell currents in micromorph tandem cells

In the present paper, the authors discuss the application of amorphous p–i–n solar cells containing i-layers which are deposited at high substrate temperatures as top cells in amorphous silicon/microcrystalline silicon tandem (“micromorph”) solar cells. Increasing the substrate temperature for the deposition of intrinsic a-Si : H results in a reduced optical gap. The optical absorption is enhanced and thereby the current generation. A high-current generation within a relatively thin amorphous top cell is very interesting in the context of micromorph tandem cells, where the amorphous top cell should contribute a current of at least 13 mA/cm² for a total cell current density of 26 mA/cm². A detailed study of the intrinsic material deposited by VHF-GD at 70 MHz at substrate temperatures between 220°C and 360°C is presented, including samples deposited from hydrogen-diluted silane plasmas. The stability of the films against light soaking is investigated employing the μ₀τ₀ parameter, which has been shown to be directly correlated to the cell performance. The paper discusses in detail the technological problems arising from the insertion of i-layers deposited at high substrate temperatures into solar cells. These problems are specially pronounced in the case of cells in the p–i–n (superstrate) structure. The authors demonstrate that an appropriate interface layer at the p/i-interface can largely reduce the detrimental effects of i-layer deposition at high temperatures. Finally, the application of such optimized high-temperature amorphous cells as top cells in micromorph tandem cells is discussed. Current densities of 13 mA/cm² in the top cell are available with a top cell i-layer thickness of only 250 nm.