Synthesis of nanostructured Al-doped zinc oxide films on Si for solar cells applications

Al-doped ZnO thin films have been prepared by a novel successive chemical solution deposition technique. The variation in morphological, structural, electrical, and optical properties of nanostructured films with doping concentration is investigated in details. It was demonstrated that rapid photothermal processing (RPP) improves the quality of nanostructured ZnO films according to the enhancement of resonant Raman scattering efficiency, and the suppression of the visible luminescence with the increase of RPP temperature. It was found from the I-V characteristics of ZnO/Si heterojunction that the average short-circuit current density is about 8 mA/cm². For 1%Al-doped ZnO/SiO₂/Si structure, the short-circuit current density is about 28 mA/cm². The improvement shown in the characteristics may be assigned partially to the reduction of the defect density in the nanostructured Al-doped ZnO films after RPP. The correlations between the composition, microstructure of the films and the properties of the solar cell structures are discussed. The successive chemically deposited Al-doped ZnO thin film offers wider applications of low-cost solar cells in heterojunction structures.     

Investigation of lateral parameter variations of Al-doped zinc oxide films prepared on glass substrates by rf magnetron sputtering

Sputter-deposited Al-doped zinc oxide (ZnO : Al) is an interesting transparent conductive oxide (TCO) material for application in electronic devices and thin-film solar cells. A phenomenon in the planar magnetron sputtering of the ZnO : Al films that is not well investigated as yet are the laterally non-uniform film properties resulting from the laterally inhomogeneous erosion of the target material, whereby the lateral distribution of the film properties depends strongly on the sputtering parameters. In this work, the lateral distributions of the electrical, optical, and surface structure properties of the ZnO : Al films prepared by the rf magnetron sputtering on glass substrates are investigated across a distance of 64 mm using the four-point probe technique, optical transmission and reflection measurements, X-ray diffraction, and scanning electron microscopy. We find that the lateral variations of the parameters of the ZnO : Al films prepared by the rf magnetron sputtering can be reduced to acceptable levels by optimising the deposition parameters. Hence, it seems that the sputter-deposited ZnO : Al is a promising TCO material for large-area thin-film solar cells.     

A new catalyst of AlCu@ZnO for hydrogen evolution reaction

Thin films of undoped ZnO, Al-doped ZnO, Cu-doped ZnO, and AlCu@ZnO deposited on indium tin oxide were performed by the sol-gel spin coating method. The prepared ZnO thin films were investigated for their structural and electrical properties after annealing at 500 °C for 1 h. ZnO thin films were characterized by electrochemical impedance spectroscopy, linear sweep voltammetry, scanning electron microscopy, Fourier transform infrared spectroscopy and Mott Schottky. According to the results obtained from the Nyquist diagrams of the ZnO thin films, the resistance value was found to decrease with binary doping and the resistance value was found to be lowest in AlCu@ZnO doped thin film containing 0.01 M Al and 0.1 M Cu. As ZnO thin films go to cathodic potentials, it is seen that the cathodic current value of ZnO with undoped is the lowest. It has been found that only Al and Cu doping showed less cathodic current than double doping.     

Impact of Zn1-xMgxO:Al transparent electrode for buffer-less Cu(In,Ga)Se2 solar cells

Buffer layers such as CdS and ZnS are used in high efficiency Cu(In,Ga)Se₂ (CIGS) thin film solar cells. Eliminating buffer layer is attractive to realize low-cost thin film solar cells by reducing fabrication process. However, the elimination of the buffer layers leads to shunting due to the interface recombination between transparent conductive oxide (TCO) and CIGS layers. To reduce the interface recombination, the control of conduction band offset (CBO) is effective. In this study, we fabricated Zn_1−xMg_xO:Al (ZMO:Al) as the TCO for the CBO control. ZMO:Al was prepared by co-sputtering of ZnO:Al₂O₃ (ZnO:Al) and MgO:Al₂O₃ targets. ZMO:Al shows high transmittance in visible region and the band gap energy widen with the addition of Mg to ZnO:Al. Buffer-less CIGS solar cells with an Al/NiCr/TCO/CIGS/Mo/soda-lime glass structure using ZMO:Al and ZnO:Al were fabricated. For comparison, ZnO/CdS buffered cell was also fabricated. Current density-voltage characteristics of the devices showed the cell with ZMO:Al film achieved higher efficiency compared to the buffer-less cell with ZnO:Al. This result suggested that the control of CBO is important to reduce interface recombination between TCO layer and CIGS absorber.     

Improvement of electrical and optical properties of ZnO thin films prepared by MOCVD using UV light irradiation and in situ H2 post-treatment

We have investigated the effect of ultraviolet (UV) light irradiation during the deposition of nominally undoped ZnO thin films using metalorganic chemical vapor deposition (MOCVD) and/or in situ hydrogen post-treatment. Due to the desorption of oxygen and incorporation of hydrogen as a shallow donor at the surface, the ZnO film prepared by the photo-MOCVD and in situ hydrogen post-treatment shows the highest film quality as a transparent conductive electrode for thin-film solar cells.     

Antireflective coating fabricated by chemical deposition of ZnO for spherical Si solar cells

ZnO thin films as an antireflective (AR) coating have been successfully fabricated on spherical Si solar cells by chemical deposition, which enables uniform film formation. ZnO films were prepared chemically by immersing the cell in an aqueous solution of zinc nitrate and dimethylamineborane maintained at 80 °C. The current–voltage measurements of the solar cells confirmed the increase in short circuit current induced by the AR effect. The open circuit voltage and fill factor were improved by surface passivation. As a result, the conversion efficiency of cells without an AR coating (9.45%) increased to 11.8%, which represents a 25% (relative) increase. The results indicate that the chemical deposition of ZnO is effective for the AR coating of spherical Si solar cells.     

Development of n-μc-SiOx:H as cost effective back reflector and its application to thin film amorphous silicon solar cells

Development of doped silicon oxide based microcrystalline material as a potential candidate for cost-effective and reliable back reflector layer (BRL) for single junction solar cells is discussed in this article. Phosphorus doped μc-SiO_x:H layers with a refractive index ∼2 and with suitable electrical properties were fabricated by radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) technique, using the conventional capacitively coupled reactors. Optoelectronic properties of these layers were controlled by varying the oxygen content within the film. The performance of these layers as BRL have been investigated by incorporating them in a single junction amorphous silicon solar cell and compared with the conventional ZnO:Al based reflector layer. Single junction thin film a-Si solar cells with efficiency ∼9.12% have been successfully demonstrated by using doped SiO:H based material as a back reflector. It is found that the oxide based back reflector shows analogous performance to that of conventional ZnO:Al BRL layer. The main advantage with this technology is that, it can avoid the ex-situ deposition of ZnO:Al, by using doped μc-SiO:H based material grown in the same reactor and with the same process gases as used for thin-film silicon solar cells.     

Solid-phase crystallization of amorphous silicon on ZnO:Al for thin-film solar cells

The suitability of ZnO:Al thin films for polycrystalline silicon (poly-Si) thin-film solar cell fabrication was investigated. The electrical and optical properties of 700 -nm-thick ZnO:Al films on glass were analyzed after typical annealing steps occurring during poly-Si film preparation. If the ZnO:Al layer is covered by a 30 nm thin silicon film, the initial sheet resistance of ZnO:Al drops from 4.2 to 2.2 Ω after 22 h annealing at 600 °C and only slightly increases for a 200 s heat treatment at 900 °C. A thin-film solar cell concept consisting of poly-Si films on ZnO:Al coated glass is introduced. First solar cell results will be presented using absorber layers either prepared by solid-phase crystallization (SPC) or by direct deposition at 600 °C.     

Novel device structure for Cu(In,Ga)Se2 thin film solar cells using transparent conducting oxide back and front contacts

Cu(In_1−xGa_x)Se₂ (CIGS)-based thin film solar cells fabricated using transparent conducting oxide (TCO) front and back contacts were investigated. The cell performance of substrate-type CIGS devices using TCO back contacts was almost the same as that of conventional CIGS solar cells with metallic Mo back contacts when the CIGS deposition temperatures were below 500 °C for SnO₂:F and 520 °C for ITO. CIGS thin film solar cells fabricated with ITO back contacts had an efficiency of 15.2% without anti-reflection coatings. However, the cell performance deteriorated at deposition temperatures above 520 °C. This is attributed to the increased resistivity of the TCO’s due to the removal of fluorine from SnO₂ or undesirable formation of a Ga₂O₃ thin layer at the CIGS/ITO interface. The formation of Ga₂O₃ was eliminated by inserting an intermediate layer such as Mo between ITO and CIGS. Furthermore, bifacial CIGS thin film solar cells were demonstrated as being one of the applications of semi-transparent CIGS devices. The cell performance of bifacial devices was improved by controlling the thickness of the CIGS absorber layer. Superstrate-type CIGS thin film solar cells with an efficiency of 12.8% were fabricated using a ZnO:Al front contact. Key techniques include the use of a graded band gap Cu(In,Ga)₃Se₅ phase absorber layer and a ZnO buffer layer along with the inclusion of Na₂S during CIGS deposition.     

Electrical and optical properties of ZnO thin film as a function of deposition parameters

Al-doped zinc oxide (AZO) and undoped zinc oxide (ZO) films have been prepared by rf magnetron sputtering. Films with low resistivities were achieved by using an Al-doped ZnO target and films with higher resistivities can be obtained by introducing oxygen during deposition. An AZO thin film which was fabricated with an rf power of 180 W, a sputtering pressure of 10 mTorr and thickness of 5000 Å showed the lowest resistivity of 1.4×10⁻⁴ Ω cm and transmittance of 95% in the visible range, and ZO film made by reactive sputtering with the above 10% oxygen content had the highest resistivity of 6×10¹⁴ Ω cm.     

ZnO interface layer and CO2 plasma treatment for improving efficiency of micromorph silicon solar cells

We have developed zinc oxide (ZnO) film and CO₂ plasma treatment for the use as an intermediate layer between top and bottom cell in order to improve performance of micromorph silicon solar cells. The CO₂ plasma treatment was performed by very high frequency plasma-enhanced chemical vapor deposition (VHF PECVD) technique, and the ZnO interface layer was deposited by DC-magnetron sputtering method. Effects of both techniques on the cell performance were comparatively investigated. We found that the ZnO interface layer and CO₂ plasma treatment were effective in enhancing V_oc, J_sc as well as FF of the cells as the same. The micromorph solar cells using an optimized ZnO interface layer and the CO₂ plasma treatment indicated initial conversion efficiency of 11.4% and 11.2%, respectively. Experimental results indicated that the CO₂ plasma treatment technique is more suitable for using in cell fabrication process than the ZnO interface layer since it is simpler and has no negative impact of possible shunts.     

Electrophoretic deposition of ZnO film and its compression for a plastic based flexible dye-sensitized solar cell

A room temperature fabrication method is developed for the preparation of a ZnO porous film on a plastic substrate, involving an electrophoresis deposition (EPD) process, followed by the compression of the film as the post-treatment. The thus prepared ZnO film is used for the photoanode of a dye-sensitized solar cell (DSSC). Besides, an indoline dye is employed as the sensitizer (referred to as D149) for the ZnO semiconductor. Performances of such DSSCs are studied at various compression pressures used for their ZnO films. Electrochemical impedance spectroscopy (EIS) is employed to quantify the charge transport resistance at the ZnO/dye/electrolyte interface (R_ct2) and the electron lifetime (τ_e) in the ZnO film. As for the thickness effect, ZnO film with a thickness of about 22 μm renders the best efficiency for the ZnO based DSSC. In addition, UV-O₃ is applied in two ways; in one way only the compressed ZnO film is treated in one step, and in the second way both the substrate and the compressed ZnO film are treated separately in two steps. The adherence of the ZnO film is shown by a photograph. Scanning electron microscopy is used to characterize the morphologies of the ZnO films.     

Effects of intrinsic ZnO buffer layer based on P3HT/PCBM organic solar cells with Al-doped ZnO electrode

Organic solar cell devices were fabricated using poly(3-hexylthiophene) (P3HT) and 6,6-phenyl C61-butyric acid methyl ester (PCBM), which play the role of an electron donor and acceptor, respectively. The transparent electrode of organic solar cells, indium tin oxide (ITO), was replaced by Al-doped ZnO (AZO). ZnO has been studied extensively in recent years on account of its high optical transmittance, electrical conduction and low material cost. This paper reports organic solar cells based on Al-doped ZnO as an alternative to ITO. Organic solar cells with intrinsic ZnO inserted between the P3HT/PCBM layer and AZO were also fabricated. The intrinsic ZnO layer prevented the shunt path in the device. The performance of the cells with a layer of intrinsic ZnO was superior to that without the intrinsic ZnO layer.     

复合绒面透明导电薄膜研究

在常规非晶硅电池绒面SnO2衬底上，采用Zn：Al重量比为5％的金属靶直流反应磁控溅射沉积ZnO，构成复合绒面SnO2／ZnO透明导电膜。控制适当ZnO厚度，既能保持SnO2绒面效果，又可阻挡H离子对SnO2的还原作用，可作为微晶硅电池的前电极。文中对ZnO沉积条件以及复合膜的形貌、电光性能进行了讨论。     

Study on diffusion barrier layer of silicon-based thin-film solar cells on polyimide substrate

ZnO and Ni films were used as the diffusion barrier layer between Al and n-type μc-Si:H for the hydrogenated amorphous silicon (a-Si:H) solar cells on polyimide (PI) substrate. The electrical, optical and uniformity properties of ZnO or Ni film influence strongly the performance and uniformity of solar cells. The uniformity of the solar cells with ZnO diffusion barrier layer degraded with the increasing thickness of ZnO film. The uniformity of solar cells with Ni diffusion barrier layer was more than 90%, which was generally better than those with ZnO film. A power-to-weight ratio of 200 W/kg was obtained for a-Si:H thin-film solar cell on PI substrate with a size of 14.8 cm².     

Solar cells prepared on multicrystalline silicon subjected to new gettering and passivation treatments

New combined gettering and passivating procedures for solar cells prepared from multicrystalline silicon (mc-Si) have been considered. Passivation has been performed by (i) diamond-like carbon films deposition onto front or rear side of the wafers with following annealing, or (ii) hydrogen plasma treatments. Gettering region has been formed by deposition of Al film on specially prepared Si with developed surface. The advantages of such a gettering process in comparison with traditional gettering with Al are demonstrated. The improving influence of the treatments on diffusion length in mc-Si and efficiency of prepared solar cells have been found out. Physical mechanisms responsible for the observed effects of gettering and passivation are discussed.     

The role of Al and Mg in the hydrogen storage of electrospun ZnO nanofibers

Pure and doped ZnO nanofibers with Al and Mg were successfully synthesized via an electrospinning method using a sol–gel containing Polyvinylpyrrolidone as a spinning aid and a zinc nitrate precursor. Calcination of the doped and undoped electrospun nanofibers was conducted at 500 °C in air, and the resultant structures were analyzed by X-ray diffraction (XRD) and Raman spectroscopy. The diameter of the doped nanofibers decreased with increasing viscosity and conductivity, as measured by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Energy dispersive spectroscopy (EDS) showed that Mg and Al are present in ZnO nanofibers. The pressure composition isotherm (PCI) demonstrated that the capacity of hydrogen storage in pure zinc oxide nanofibers is a factor of two greater than that of zinc oxide nanoparticles. However, Al-doped ZnO nanofibers have the highest capacity of hydrogen storage (2.81 wt%) at room temperature.     

Simple and fast fabrication of a-Si:H/c-Si hetero-junction solar cells by dual-chamber hot wire chemical vapor deposition

One of the fabrication issues in hetero-junction crystalline Si solar cells is the overhead time between the deposition steps of the top and bottom surfaces, because flipping of the progressing wafer is necessary to process the both sides of the wafer. To reduce the overall processing time by reducing the overhead time, we propose a dual-chamber deposition system, where thin films on the top and bottom surfaces of the Si wafer are simultaneously deposited. We have evaluated the proposed deposition system by demonstrating fabricated hetero-junction crystalline Si solar cells, which were compared with solar cells fabricated by a conventional plasma-enhanced chemical deposition system. We have obtained the power conversion efficiency of 15.5% from solar cells fabricated by our dual-chamber system; and additional analyses confirmed that the proposed dual-chamber system is, in principle, competitive with conventional systems in terms of the fabricated solar cell performance. This novel concept for the fabrication of a hetero-junction crystalline Si solar cell is expected to lay an important foundation in the future thin film crystalline Si based photovoltaic industry.     

New materials and deposition techniques for highly efficient silicon thin film solar cells

This paper reviews recent efforts to provide the scientific and technological basis for cost-effective and highly efficient thin film solar modules based on amorphous (a-Si:H) and microcrystalline (μc-Si:H) silicon. Textured ZnO:Al films prepared by sputtering and wet chemical etching were applied to design optimised light-trapping schemes. Necessary prerequisite was the detailed knowledge of the relationship between film growth, structural properties and surface morphology obtained after etching. High rate deposition using plasma enhanced chemical vapour deposition at 13.56 MHz plasma excitation frequency was developed for μc-Si:H solar cells yielding efficiencies of 8.1% and 7.5% at deposition rates of 5 and 9 Å/s, respectively. These μc-Si:H solar cells were successfully up-scaled to a substrate area of 30×30 cm² and applied in a-Si:H/μc-Si:H tandem cells showing initial test cell efficiencies up to 11.9%.     

Fabrication of flexible CdTe solar modules with monolithic cell interconnection

CdTe solar cells and modules have been manufactured on polyimide (PI) substrates. Aluminum doped zinc oxide (ZnO:Al) was used as a transparent conductive oxide (TCO) front contact, while a thin high resistive transparent layer of intrinsic zinc oxide (i-ZnO) was used between the front contact and the CdS layer. The CdS and CdTe layers were evaporated onto the ZnO:Al/i-ZnO coated PI films in a high vacuum evaporation system followed by a CdCl₂ activation treatment and a Cu–Au electrical back contact deposition. In some cases prior to the cell deposition, the PI film was coated with MgF₂ on the light facing side and the effects on the optical and electrical properties of TCO and solar cells were investigated. The limitations on current density of solar cells due to optical losses in the PI substrate were estimated and compared to the experimentally achieved values. Flexible CdTe solar cells of highest efficiencies of 12.4% and 12.7% were achieved with and without anti-reflection MgF₂ coating, respectively.Laser scribing was used for patterning of layers and monolithically interconnected flexible solar modules exhibiting 8.0% total area efficiency on 31.9 cm² were developed by interconnection of 11 solar cells in series.