Florian Ruske

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...

Microstructure and photovoltaic performance of polycrystalline silicon thin films on temperature-stable ZnO:Al layers

Polycrystalline silicon (poly-Si) thin films have been prepared by electron-beam evaporation and thermal annealing for the development of thin-film solar cells on glass coated with ZnO:Al as a transparent, conductive layer. The poly-Si microstructure and photovoltaic performance were investigated as functions of the deposition temperature by Raman spectroscopy, scanning and transmission electron microscopies including defect analysis, x-ray diffraction, external quantum efficiency, and open circuit measurements. It is found that two temperature regimes can be distinguished: Poly-Si films fabricated by deposition at low temperatures (Tdep 400 °C) directly in crystalline phase reveal columnar, up to 300 nm big crystals with a strong ⟨110⟩ orientation and much better solar cell parameters. It ...

Hard x-ray photoelectron spectroscopy study of the buried Si/ZnO thin-film solar cell interface: Direct evidence for the formation of Si–O at the expense of Zn-O bonds

The chemical structure of the interface between silicon thin films and the transparent conductive oxide ZnO:Al has been investigated by hard x-ray photoelectron spectroscopy. By varying the excitation energy between 2010 and 8040 eV, we were able to probe the Si/ZnO interface buried below 12 nm Si. This allowed for the identification of changes induced by solid phase crystallization (SPC). Based on in-situ SPC annealing experiments, we find clear indications that the formation of Si–O bonds takes place at the expense of Zn–O bonds. Hence, the ZnO:Al acts as the oxygen source for the interfacial Si oxidation.

Analysis of Urbach-like absorption tails in thermally treated ZnO:Al thin films

The sub bandgap absorption of as deposited and thermally treated ZnO:Al has been investigated and described applying Urbach tail theory. In the as deposited state Urbach energies show a decreasing trend for higher deposition temperatures. Annealing leads to much less band tailing, and resulting Urbach energies are the same as for undoped ZnO. It has been concluded that structural disorder is the main contribution to the bandtails in the as deposited state. In summary, films with excellent electrical properties due to very high carrier mobilities and excellent optical properties due to the strong reduction in sub bandgap absorption were obtained.

Rigorous optical simulation of light management in crystalline silicon thin film solar cells with rough interface textures

We apply a hybrid finite element / transfer matrix solver to calculate generation rate spectra of thin film silicon solar cells with textured interfaces. Our focus lies on interfaces with statistical rough textures. Due to limited computational domain size the treatment of such textures requires a Monte Carlo sampling of texture representations to obtain a statistical average of integral target quantities. This contribution discusses our choice of synthetic rough interface generation, the Monte Carlo sampling and the need for an incorporation of the cells substrate into optical simulation when illumination of the cell happens through the substrate. We present results of the numerical characterization and generation rates for a single junction cell layout.

Optical characterization of high mobility polycrystalline ZnO:Al films

Optical methods are powerful and non-destructive means to characterize highly doped transparent conducting oxide thin films. In order to describe the optical properties of high-mobility ZnO films we present a dielectric function composed of different analytic expressions to describe the different contributions to the dielectric function of the films. This allows for the correct description of measured optical spectra and reduces the complex functions to a set of fitting parameters. In a second step we compare the obtained parameters to theoretical models. The basic theories are nicely reproduced and the basic link between optical and electrical properties can be understood. The findings can help on the route to a complete presiction of optical properties from the basic material properties or vice versa.

Challenges and opportunities of electron beam evaporation in the preparation of poly-Si thin film solar cells

Electron-beam (e-beam) evaporation provides both exciting opportunities and challenges for the preparation of poly-crystalline silicon (poly-Si) thin film solar cells. A conversion efficiency of 6.7% was recently achieved for solid phase crystallized poly-Si mini-modules on planar SiN-coated glass deposited at a deposition rate of 600 nm/min, demonstrating the excellent electronic quality of e-beam evaporated silicon. Even at significantly increased background pressures of 5×10−6 mbar, the photovoltaic performance of the mini-modules was considerably high, showing a decline in open circuit voltage of 17 mV per cell. The implementation of light trapping structures into the device led to an efficiency increase of 1.1%, yielding module efficiencies of 7.8%. By systematically studying the implementation of ZnO:Al as a front contact layer into the poly-Si solar cell device structure, we unraveled novel features that prove the supreme suitability of ZnO:Al for poly-Si thin film solar cells. Not only can etched ZnO:Al be utilized as a front side texture, but its electrical properties can also improve during the crystallization process of the Si layer, showing a record charge carrier mobility of 67 cm2/Vs after thermal annealing. In addition, ZnO:Al drastically modifies the crystallization kinetics of the Si on ZnO:Al, enabling us to control the crystallization process by adjusting the deposition temperature. The nucleation process of Si on ZnO:Al was found to be influenced by a variation of the deposition temperature of the amorphous Si in a critical temperature regime of 200 °C to 300 °C. The nucleation rate decreased significantly with decreasing deposition temperature, while the activation energy for nucleation increased from 2.9 eV at a deposition temperature of 300 °C to 5.1 eV at 200 °C, resulting in poly-Si which comprised grains with features sizes of several µm.

High mobility annealing of Transparent Conductive Oxides

To improve electrical properties a high temperature annealing treatment was applied to several transparent conductive oxides (TCO), namely tin doped indium oxide (ITO), Ga- or Al- doped ZnO (ZnO:Al/Ga), ion beam assisted deposited (IBAD) ZnO:Ga and Ga doped zinc magnesium oxide (ZnMgO:Ga). All these films were grown by magnetron sputtering. During the annealing process all TCO films were capped with 50 nm of amorphous silicon in order to protect the films from environmental impact. Increase in mobility up to 72 cm2/Vs and low resistivity of 1.6 × 10−4 Ωcm was achieved for ZnO:Al after annealing at 650°C for 24 h. Independent of the deposition conditions and doping or alloying material almost all ZnO based films show a consistent improvement in mobility. Also for ITO films a decrease in resistivity with partially improved mobility was found after annealing. However, not all ITO films show consistent improvement, but carrier density above 1021 cm−3 while ZnO films show no clear trend for carrier density but a remarkable increase in mobility. Thus we propose the healing of defects and the activation of donors to be most significant effects for ZnO and ITO films, respectively.

ZnO:Al with tuned properties for photovoltaic applications: thin layers and high mobility material

TCO films are crucial components of almost all thin-film solar cells and a-Si:H/c-Si heterojunction solar cells. As they are used as front contacts, the requirements for electrical conductivity and optical trnamission are generally very high. Further restrictions are imposed onto the deposition process by the cell manufacturing process, in which e.g. the maximum substrate temperature can be limited. In this paper the optimization of ZnO:Al layer deposited by magnetron sputtering to different solar cells is discussed. For a-Si:H/c-Si heterojunction solar cells the advantages and limitations of different variations of magnetron sputtering of ZnO:Al are discussed and compared to standard ITO deposition. For a-Si:H/μc-Si:H the usage of post-deposition treatments to improve the optical and electrical performance is briefly discussed.

Light trapping in polycrystalline silicon thin-film solar cells based on liquid phase crystallization on textured substrates

Liquid phase crystallization (LPC) is a promising technique to fabricate high-quality polycrystalline silicon absorber layers on cheap glass substrates. Recently, we achieved open-circuit voltages above 580mV using a silicon heterojunction and a newly developed single-sided contact system. However, the still moderate efficiency of 5.7% can be attributed to short circuit current densities not exceeding 16mA/cm2, caused by optical losses and absorber recombination. An approach to tackle the first problem is presented in this work. In general, a proper light management concept for thin film devices involve specially designed anti reflective coatings (ARC) and textured surfaces on both sides of the device, to enhance the optical path of the cell. As the crystals achieved using LPC are up to cm in length, commonly used wet chemical treatments using in wafer-based PV are applicable here, too. However providing a texture to the substrate:silicon interface is more complex as the texture as well as all layers deposited before the crystallization process must withstand the high temperatures present during LPC. However it is possible to deposit adequate inter-layers that enable LPC on randomly textured substrates as shown in this work.