• Energy Research

Abstract A simple method is developed to determine the light trapping properties of arbitrarily textured solar cells with high accuracy. The method allows for determining the quantum efficiency and short circuit current density of thin film solar cells prepared on randomly nanotextured surfaces. The light trapping of the randomly textured solar cell is described by the area weighted superposition of periodically textured solar cells. The necessary input parameters for the calculations are determined by analyzing the randomly textured surfaces of the solar cells using atomic force microscopy and image processing. The analysis of the atomic force microscope images and the calculation of the quantum efficiency and short circuit current can be determined from current maps, without complex and time-consuming calculations. The calculated solar cell parameters exhibit excellent agreement with experimentally measured quantum efficiencies and short circuit current densities for amorphous and microcrystalline silicon thin film solar cells prepared on randomly textured substrates. Finally, the work contributes to a comparison of random and periodic light trapping structures.

Abstract A simple method is developed to determine the light trapping properties of arbitrarily textured solar cells with high accuracy. The method allows for determining the quantum efficiency and short circuit current density of thin film solar cells prepared on randomly nanotextured surfaces. The light trapping of the randomly textured solar cell is described by the area weighted superposition of periodically textured solar cells. The necessary input parameters for the calculations are determined by analyzing the randomly textured surfaces of the solar cells using atomic force microscopy and image processing. The analysis of the atomic force microscope images and the calculation of the quantum efficiency and short circuit current can be determined from current maps, without complex and time-consuming calculations. The calculated solar cell parameters exhibit excellent agreement with experimentally measured quantum efficiencies and short circuit current densities for amorphous and microcrystalline silicon thin film solar cells prepared on randomly textured substrates. Finally, the work contributes to a comparison of random and periodic light trapping structures.

Abstract 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.

Abstract 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.

Abstract Multijunction solar cells, proven technological route for achievements of highest PV conversion efficiencies, require accurate tuning of the sub-cell absorption to ensure that every cell in the stack delivers the same current density. Even though currents of the sub-cells can be precisely matched for a fixed illumination spectrum, current mismatch cannot be avoided in real terrestrial applications due to variations of the irradiance spectrum. The issue becomes critical when a solar cell has to cover wider range of applications such as a mixture of direct sun / shadow / artificial light – the case for various distributed PV-powered electronics. Furthermore the current matching constrains choice of materials and designs of sub-cells for a tandem device. A straightforward basic configuration with decoupling the sub-cell's currents is a 3-terminal configuration with one additional contact sheared by the top and bottom cell. This concept requires voltage-matching between the top and bottom cell when these cells are integrated in modules. A realistic concept for the voltage-matched 3-terminal cell reported recently includes a wide gap top cell combined with a tandem bottom cell made of 2 sub-cells with lower bandgaps. The concept is a hybrid between 2 and 3-terminal configurations with voltage matching and relaxed current matching constrains. Established thin film silicon solar cell technology provides interesting option to realize the hybrid 3-terminal cell with amorphous Si top cell (VOC ≈ 0.9 V) and two microcrystalline Si cells (VOC ≈ 0.5 V). In this work we present proof of concept of the voltage matched 3-terminal tandem cell prepared with highly transparent and conductive IOH intermediate contact. The efficiency of 10.4% has been achieved made up of independently operating 7.9% efficient top cell and 2.5% efficient bottom tandem cell. The paper summarizes the development and discusses optical losses identified in the 3-T devices.

Abstract Multijunction solar cells, proven technological route for achievements of highest PV conversion efficiencies, require accurate tuning of the sub-cell absorption to ensure that every cell in the stack delivers the same current density. Even though currents of the sub-cells can be precisely matched for a fixed illumination spectrum, current mismatch cannot be avoided in real terrestrial applications due to variations of the irradiance spectrum. The issue becomes critical when a solar cell has to cover wider range of applications such as a mixture of direct sun / shadow / artificial light – the case for various distributed PV-powered electronics. Furthermore the current matching constrains choice of materials and designs of sub-cells for a tandem device. A straightforward basic configuration with decoupling the sub-cell's currents is a 3-terminal configuration with one additional contact sheared by the top and bottom cell. This concept requires voltage-matching between the top and bottom cell when these cells are integrated in modules. A realistic concept for the voltage-matched 3-terminal cell reported recently includes a wide gap top cell combined with a tandem bottom cell made of 2 sub-cells with lower bandgaps. The concept is a hybrid between 2 and 3-terminal configurations with voltage matching and relaxed current matching constrains. Established thin film silicon solar cell technology provides interesting option to realize the hybrid 3-terminal cell with amorphous Si top cell (VOC ≈ 0.9 V) and two microcrystalline Si cells (VOC ≈ 0.5 V). In this work we present proof of concept of the voltage matched 3-terminal tandem cell prepared with highly transparent and conductive IOH intermediate contact. The efficiency of 10.4% has been achieved made up of independently operating 7.9% efficient top cell and 2.5% efficient bottom tandem cell. The paper summarizes the development and discusses optical losses identified in the 3-T devices.

AbstractSurface morphology of ZnO layers with as-deposited thickness between 550 to 1050nm was varied over wide range by changing etching time. The surface morphology of these ZnO layers was measured by atomic force microscopy (AFM) and statistically analyzed in terms of root mean square roughness and the diameter and depth of the craters. The texture-etched ZnO covered with a Ag and ZnO layers were applied into n-i-p μc-Si:H solar cells as back reflectors. The effects of the back reflector morphologies on solar cell performances were investigated. The link between the morphology, the scattering properties of ZnO layers, and the solar cell performance is discussed.

AbstractSurface morphology of ZnO layers with as-deposited thickness between 550 to 1050nm was varied over wide range by changing etching time. The surface morphology of these ZnO layers was measured by atomic force microscopy (AFM) and statistically analyzed in terms of root mean square roughness and the diameter and depth of the craters. The texture-etched ZnO covered with a Ag and ZnO layers were applied into n-i-p μc-Si:H solar cells as back reflectors. The effects of the back reflector morphologies on solar cell performances were investigated. The link between the morphology, the scattering properties of ZnO layers, and the solar cell performance is discussed.

AbstractWe investigate the scattering behavior of nano‐textured ZnO–Air and ZnO–Silicon interfaces for the application in thin film silicon solar cells. Contrary to the common approach, the numerical solution of the Maxwell's equations, we introduce a ray tracing approach based on geometric optics and the measured interface topography. The validity of this model is discussed by means of scanning near‐field optical microscopy (SNOM) measurements and numerical solutions of the Maxwell's equations. We show, that the ray tracing model can qualitatively describe the formation of micro lenses, which are the dominant feature of the local scattering properties of the investigated interfaces. A quantitative analysis for the ZnO–Silicon interface at λ = 488 and 780 nm shows that the ray tracing model corresponds well to the numerical solution of the Maxwell's equations, especially within the first 1.5 µm distance from the interface. Direct correlations between the locally scattered intensity and the interface topographies are found. Copyright © 2011 John Wiley & Sons, Ltd.