J. Hüpkes

The effect of front ZnO:Al surface texture and optical transparency on efficient light trapping in silicon thin-film solar cells

This study addresses the material properties of magnetron-sputtered aluminum-doped zinc oxide (ZnO:Al) films and their application as front contacts in silicon thin-film solar cells. Optimized films exhibit high conductivity and transparency, as well as a surface topography with adapted light-scattering properties to induce efficient light trapping in silicon thin-film solar cells. We investigated the influence on the ZnO:Al properties of the amount of alumina in the target as well as the substrate temperature during sputter deposition. The alumina content in the target influences the carrier concentration leading to different conductivity and free carrier absorption in the near infrared. Additionally, a distinct influence on the film growth of the ZnO:Al layer was found. The latter affects the surface topography which develops during wet-chemical etching in diluted hydrochloric acid. Depending on alumina content in the target and heater temperature, three different regimes of etching behavior have been i...

Modified Thornton model for magnetron sputtered zinc oxide: film structure and etching behaviour

ZnO:Al films were prepared on glass substrates with different sputter techniques from ceramic ZnO:Al2O3 target as well as metallic Zn:Al targets using a wide range of deposition parameters. Independent of the sputter technique, sputter pressure and substrate temperature were found to have a major influence on the electrical and structural properties of the ZnO:Al films. With an increasing deposition pressure, we observed a strong decrease in the carrier mobility and also an increase of the etching rate. The surface morphology obtained after etching of RF sputtered ZnO:Al systematically changes from crater-like to hill-like surface appearance with increasing pressure. The correlation of sputter parameters, film growth and structural properties is discussed in terms of a modified Thornton model.

Efforts to improve carrier mobility in radio frequency sputtered aluminum doped zinc oxide films

This study addresses the electrical and optical properties of radio frequency magnetron sputtered aluminum doped zinc oxide (ZnO:Al) films. The main focus was on the improvement in carrier mobility μ to achieve simultaneously high transparency for visible and particularly near-infrared light and low resistivity. The influence of Al concentration in the target, film thickness, sputter power, deposition pressure, and substrate temperature on material properties was investigated. The structural, compositional, electrical and optical properties were studied using x-ray diffraction, secondary ion mass spectrometry (SIMS), room temperature Hall effect measurements and spectral photometry, respectively. All ZnO:Al films were polycrystalline and preferentially oriented along [002]. The grain size along the direction of growth increased with higher Al doping and with increasing film thickness. The SIMS measurements revealed that the Al concentration in the film was nearly the same as in the target. Carrier concent...

Improved electrical transport in Al-doped zinc oxide by thermal treatment

A postdeposition thermal treatment has been applied to sputtered Al-doped zinc oxide films and shown to strongly decrease the resistivity of the films. While high temperature annealing usually leads to deterioration of electrical transport properties, a silicon capping layer successfully prevented the degradation of carrier concentration during the annealing step. The effect of annealing time and temperature has been studied in detail. A mobility increase from values of around 40 cm2/Vs up to 67 cm2/Vs, resulting in a resistivity of 1.4×10−4 Ω cm has been obtained for annealing at temperatures of 650 °C. The high mobility increase is most likely obtained by reduced grain boundary scattering. Changes in carrier concentration in the films caused by the thermal treatment are the result of two competing processes. For short annealing procedures we observed an increase in carrier concentration that we attribute to hydrogen diffusing into the zinc oxide film from a silicon nitride barrier layer between the zinc...

Organic solar cells on indium tin oxide and aluminum doped zinc oxide anodes

The authors compare organic solar cells using two different transparent conductive oxides as anode: indium tin oxide (ITO) and three kinds of aluminum doped zinc oxide (ZAO). These anodes with different work functions are used for small molecule photovoltaic devices based on an oligothiophene derivative as donor and fullerene C60 as acceptor molecule. It turns out that cells on ITO and ZAO have virtually identical properties. In particular, the authors demonstrate that the work function of the anode does not influence the Voc of the photovoltaic device due to the use of doped transport layers.

Optimization of the electrical properties of magnetron sputtered aluminum-doped zinc oxide films for opto-electronic applications

Abstract Magnetron sputtered ZnO:Al films are promising candidates as front electrode in a variety of opto-electronic devices. Here we report on efforts to obtain highly conductive and transparent ZnO:Al films using different deposition conditions for RF, DC and MF (mid frequency) sputtering. Investigations were made to see the effect of target doping concentration (TDC), film thickness, sputter pressure and deposition temperature. RF sputtering from ceramic targets yields low resistivities between 3 and 5×10−4 Ω cm for target doping concentrations between 4 and 0.5%. With decreasing TDC to 0.5% carrier mobilities up to 44 cm2/Vs were obtained, accompanied by the extension of the region of high transmission to the near infrared, due to a reduction in free carrier absorption and corresponding shift in plasma wavelength. DC and MF sputtering from metallic targets yielded similar low resistivities at deposition rates up to 200 nm/min. An analysis of mobility (μ) data of all films as function of the corresponding carrier densities (N) showed that the μ–N values obtained in this study are in the vicinity to limits suggested in the literature.

Temperature stability of ZnO:Al film properties for poly-Si thin-film devices

The crystallization of thin silicon films at temperatures between 425 and 600°C was investigated on glass substrates coated with Al-doped zinc oxide (ZnO:Al). Bare ZnO:Al layers degrade at the crystallization temperatures used. A silicon layer on top, however, efficiently prevents deterioration. The resistivity was even found to drop from 4.3×10−4Ωcm for the as deposited ZnO:Al to 2.2×10−4Ωcm in the case of aluminium induced crystallization and to 3.4×10−4Ωcm for solid phase crystallization. The temperature-stable conductivity of ZnO:Al films coated with Si opens up appealing options for the production of polycrystalline silicon thin-film solar cells with transparent front contacts.

Light trapping in periodically textured amorphous silicon thin film solar cells using realistic interface morphologies.

The influence of realistic interface morphologies on light trapping in amorphous silicon thin-film solar cells with periodic surface textures is studied. Realistic interface morphologies are obtained by a 3D surface coverage algorithm using the substrate morphology and layer thicknesses as input parameters. Finite difference time domain optical simulations are used to determine the absorption in the individual layers of the thin-film solar cell. The influence of realistic interface morphologies on light trapping is determined by using solar cells structures with the same front and back contact morphologies as a reference. Finally the optimal surface textures are derived.

Analyzing nanotextured transparent conductive oxides for efficient light trapping in silicon thin film solar cells

Nanotextured contact layers are used in silicon thin film solar cells for increasing the short circuit current and conversion efficiency. We developed an approach to analyze random nanotextured surfaces by atomic force microscopy and image segmentation. It was used to investigate sputtered and wet chemically etched aluminum doped zinc oxide films with various morphologies. The information extracted from the surfaces was correlated with optical simulations of periodically textured thin film solar cells. The results from the surface analysis and optical simulations were also compared with the experimental results obtained for amorphous silicon solar cells prepared on the nanotextured substrates.

Amorphous and Microcrystalline Silicon Based Solar Cells and Modules on Textured Zinc Oxide Coated Glass Substrates

This paper addresses scientific and technological efforts to develop highly efficient silicon thin film solar modules on glass substrates. We present a comprehensive study of μc-Si:H p-i-n single junction and a-Si:H/μc-Si:H stacked solar cells prepared by plasma-enhanced chemical vapour deposition (PECVD) at 13.56 MHz excitation frequency. In the first step cell development was performed in a small area PECVD reactor showing the relationship between deposition process and resulting solar cell performance. Subsequent up-scaling to a substrate area of 30×30 cm 2 confirmed the scalability to large area reactors. Moreover, we developed textured ZnO:Al films by sputtering and post deposition wet chemical etching as front contact TCO-material with excellent light scattering properties. A-Si:H/μc-Si:H tandem cells developed on this textured ZnO yielded stable efficiencies up to 11.2 % for a cell area of 1 cm 2 . First solar modules were prepared in our recently installed process technology, which includes PECVD, sputtering, texture etching and laser scribing on substrate sizes up to 30x30 cm 2 . Initial module efficiencies of 10.8 % and 10.1 % were achieved for aperture areas of 64 cm 2 and 676 cm 2 , respectively.