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29th European Photovoltaic Solar Energy Conference and Exhibition; 1933-1936, Light management concepts for the improvement of the absorption in silicon thin-film solar cells in n-i-p configuration mainly focus on the modification of the rear surface and are often expensive or limited to laboratory scale. In this work, we use electrochemical deposition to grow a ZnO nanorod antireflective layer on the front surface in n-i-p type silicon thin-film solar cells in different configurations (a-Si:H on glass, μc-Si:H on flexible metal substrate and a-Si:H/μc-Si:H tandem cells on glass). Our results show an enhanced quantum efficiency by implementation of the nanorod antireflective layer. As no deposition chambers and high temperatures are needed, the process is cost-effective and can be applied on industrial scale. Although the nanorods are grown directly on the terminated cell in aqueous solution, the electrical properties of the cell are not deteriorated by the deposition step.

29th European Photovoltaic Solar Energy Conference and Exhibition; 1667-1670, The joint project “Silicon-based thin-film solar cells on flexible metal substrates” (abbreviation: “SiSoFlex”) is funded as part of the Photovoltaics Innovation Alliance by the German Federal Ministry of Education and Research. The intended technical objectives of the project can be subdivided into various feasibility studies and into tests of new approaches towards the development of flexible substrates, contacts, cell designs and encapsulation films. Each of these approaches is ultimately linked by the overall objective of demonstrating an advanced cell design based on flexible substrates with a stable efficiency of more than 11%. The central issue is the interplay between the substrate, the back contact, the layer structure of the thin-film solar cells, the front contact and the encapsulation as well as the effects resulting from mutual interaction of the individual components on the cell efficiency. New alternative cell concepts and flexible contact layers are developed. By closely integrating simulation with the fabrication of solar cells, we are able to focus on the most promising cell design concepts.

27th European Photovoltaic Solar Energy Conference and Exhibition; 2714-2718, For optical and electrical modelling of microcrystalline silicon, the semiconductor material is usually described as effective medium without local differences in its optoelectronic behaviour and layer morphology. However, ratios between amorphous and crystalline phases of microcrystalline silicon can be affected by the deposition parameters and experimentally determined by Raman Spectroscopy. Furthermore, Conductive Atomic Force Microscopy and Transmission Electron Microscopy measurements give valuable information of microcrystalline layers about the growth process, the material composite and the detailed volume morphology. In consideration of published experimental results, two-dimensional simulation domains are investigated by modelling, treated microcrystalline silicon as a combined material of the two different phases of silicon. In particular, the influence of a-Si:H(i) buffer layers in μc-Si:H single junction solar cells as n-i-p and p-i-n configurations are presented.

27th European Photovoltaic Solar Energy Conference and Exhibition; 2497-2502, Light trapping due to rough interfaces is a common and industrially applied technique for amorphous silicon thin film solar cells. The induced scattering enhances the absorption and consequently the conversion efficiency of the device. There are several concepts to improve the light management beyond the standard transparent conductive oxide (TCO) structuring achieved by chemical wet etching. In this work, we investigate the application of novel light trapping solutions based on electrochemically deposited zinc oxide nanorod arrays, which goes beyond standard light management concepts for thin film silicon solar cells. We study the significant impact of array geometry on the solar cell efficiency and correlate our experimental results to multidimensional solar cell simulations. Our results demonstrate the need for an appropriate nanorod array design in order to optimize the electrical performance of the solar cell. In addition, geometry effects on light management and electrical properties for thin film silicon solar cells are studied by experiments and simulations of two-dimensional (2D) TCO gratings. The modeling of the structures aims to find the influence of the TCO structure height on the electrical cell performance.

26th European Photovoltaic Solar Energy Conference and Exhibition; 2667-2670, In this contribution three dimensional simulations of amorphous silicon thin film solar cells are presented. In order to simulate solar cells with rough interfaces, the surface topographies were measured via atomic microscopy and transferred to the commercial software Sentaurus TCAD (Synopsys). The model includes layer thicknesses and optoelectronic parameters like complex refractive index and defect structure. To calculate the space resolved optical generation rates of the virtual device, the optical solver Finite-Difference Time-Domain is used. A comparison of measured and simulated angular distribution functions illustrates a good validation of this model. However, a virtual device size of 1.5x1.5Am² is needed to reduce calculation errors due to the influence of the boundary conditions of lateral surfaces. For electrical modeling only divided parts from the initial virtual device are calculated in order to reduce the calculation time. A variation of subdomain volumes to identify the needed sizes and their results of illuminated current-voltage-characteristics are presented.

26th European Photovoltaic Solar Energy Conference and Exhibition; 2399-2402, In this contribution the growth conditions of microcrystalline and amorphous hydrogenated thin germanium layers on glass substrates and their potential as a bottom structure in silicon based multi junction solar cells is investigated. The absorption spectra of the thin germanium layers are several orders of magnitude higher than that of microcrystalline and amorphous silicon. Thus, the film thickness for the absorber layer can be significantly reduced compared to silicon thin films. By varying the deposition parameters like germane flow and total gas flow a transition from amorphous to microcrystalline germanium growth regime can be detected. The phase change can also be observed in dark conductivity measurements. Additionally to the growth experiments, first pin-solar cells are presented using also the n- and p-type germanium as the contact layers. Finally, the potential for thin germanium films used as bottom structures in tandem or triple junction solar cells is described.

25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain; 3032-3035, In this paper the activities of the photovoltaics group at the EWE research centre – NEXT ENERGY is described. The focus of the work lies on thin film silicon solar cells. A baseline process has been implemented allowing the deposition of state-of-the-art micromorph tandem solar cells. This is done with the goal to substantionally increase the efficiency of this kind of cells in the future and to build up the base for new concepts like triple junction solar cells. Detailed simulations were performed to get physical insight into the fundamental mechanisms in the solar cell and to extract further potential of efficiency enhancement. 3-dimensional optical simulations of structures with realistic topography will be presented. First experiments with ps-laser scribing have been performed for TCO and a-Si:H ablation At the end an outlook about the triple junction activities will be given.

25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain; 3133-3137, The Photovoltaic Division of NEXT ENERGY is focusing on thin film silicon solar cells. Beside the solar cell technology we deal with modeling and simulation of silicon based thin film solar cells to understand the physical mechanisms in such complex devices. Our goal is to create a model describing coupled optical and electrical behavior of a micromorph thin film silicon solar cell, which has a tandem structure consisting of amorphous and microcrystalline subcells. Sentaurus TCAD developed by Synopsys is used as simulation tool and allows computing the optical and electrical behavior in 1, 2 or 3 dimensions of a device. Three dimensional modeling and simulation is important for calculating the optical generation rate in solar cells with rough interfaces, which will be presented below. However, the main focus of this work is on electrical modeling based on material parameters and transport mechanisms.