Markus Ermes

Influence of Interface Textures on Light Management in Thin-Film Silicon Solar Cells With Intermediate Reflector

High-efficiency thin-film silicon solar cells require advanced textures at the front contacts for light management. In this contribution, the influence of the texture of various transparent conductive oxides (TCO) on the effectiveness of an intermediate reflector layer (IRL) in a-Si:H/μc-Si:H tandem solar cells is investigated. The employed front side TCOs include several types of sputter-etched ZnO:Al, LPCVD ZnO:B and APCVD SnO2:F. The topographies after different stages of the deposition process of the tandem solar cell, at the front TCO, after deposition of the amorphous top cell and after the deposition of the microcrystalline bottom cell, were characterized by atomic force microscopy at precisely the same spot. The external quantum efficiency of the fabricated solar cells were measured and successfully reproduced by a finite-difference time-domain method applying the measured topographies at each interface of the solar cell. With these simulations, the impact of structure type and feature size on the effectiveness of the IRL is investigated. The highest IRL effectiveness in a tandem solar cell was found for double-textured ZnO:Al. In this contribution, we study the interplay between interface textures and parasitic losses. Our findings are relevant for the design of topography for optimized IRL performance.

Influence of film formation on light-trapping properties of randomly textured silicon thin-film solar cells

The influence of film formation on light-trapping properties of silicon thin-film solar cells prepared on randomly textured substrates was studied. Realistic interface morphologies were calculated with a three-dimensional (3D) surface coverage algorithm using the measured substrate morphology and nominal film thicknesses of the individual layers as input parameters. Calculated interface morphologies were used in finite-difference time-domain simulations to determine the quantum efficiency and absorption in the individual layers of the thin-film solar cells. The investigation shows that a realistic description of interface morphologies is required to accurately predict the light-trapping properties of randomly textured silicon thin-film solar cells.

Influence of texture modifications in silicon solar cells on absorption in the intrinsic layers

The influence of the front texture of an etched transparent conductive oxide with crater-like structures of various sizes on the absorption of a thin amorphous silicon (a-Si:H) layer is investigated by rigorous optical simulations as part of two simplified systems: A simplified single junction device, using a perfect metal as back contact and a top cell of an amorphous/microcrystalline silicon tandem device, using a microcrystalline silicon halfspace adjacent to the amorphous layer. The texture is modified by stretching either in height or laterally and the average absorption in the a-Si:H layer is investigated relative to the original structure. We investigate the average absorption for each wavelength as well as the total absorption, weighted with an AM1.5g spectrum. Furthermore, the local absorption distribution inside the a-Si:H layer is examined to improve the understanding of local texture features and their influence on absorption and cell performance. For both modifications, an optimal point can be found to improve the absorption in the amorphous layer by up to 15% and 6% for a simplified single junction and tandem top cell, respectively. In case of the top cell of the simplified tandem device, it is found that additionally, the transmission into the microcrystalline silicon can be improved. Also, the local absorption distribution shows that there is an optimal size of the surface craters for all modifications, while steeper crater rims in general lead to higher absorption.

Analysis of light propagation in thin-film solar cells by dual-probe scanning near-field optical microscopy

In this study, light propagation in textured hydrogenated microcrystalline silicon (μc-Si:H) thin-film solar cells is investigated on a sub-micron-scale by means of dual-probe scanning near-field optical microscopy (SNOM). Applying advanced modes of operation - exclusively available at dual probe SNOMs - light propagation is analyzed with subwavelength resolution. Measurements at μc-Si:H thin-film solar cells layer are presented visualizing the influence of local surface features on light propagation. Furthermore, the intensity decay of light guided inside the solar cell is mapped. The observed intensity decay agrees well with theory, verifying the validity of the method.

Reconstruction of SNOM near-field images from rigorous optical simulations by including topography artifacts

Scanning near-field optical microscopy (SNOM) is a powerful tool providing measurement of the near-field intensity of nano-structured surface layers. These measurements can be combined with rigorous solving of Maxwells equations to gain insight into light propagation inside the layer. However, there are often major differences between the simulated near-field intensity directly above the surface and SNOM measurements. The SNOM measurements are being performed in a way that sample and probe have a distance of about 20 nm at their closest point, therefore the finite size of the probe has a severe impact on the measurement, e.g. for textured surfaces. Any steep flank present in the topography leads to an increased distance between the aperture of the probe and the sample surface, since the shortest distance between sample and probe occurs at the side of the tip. This behavior modifies the measurement at all points where the geometry does not allow for the aperture to be placed 20 nm over the topography, since another part of the probe would get in contact with the surface. To account for these topography artifacts in our simulations, we developed an algorithm to calculate the height of the probe above each point of the surface. Taking this position into account for each point of the topography measurement, we are able to obtain an intensity distribution at the same positions as the SNOM measurement. This intensity distribution shows a much better agreement to experiment than assuming a constant distance of 20 nm from the surface. We illustrate this algorithm and its consequences for comparisons between SNOM measurements and simulation using the textured transparent front contact of a silicon-based thin-film solar cell as an example. In such devices, the absorber layer of the cell is typically thinner than the absorption length of the incident light, especially in the long wavelength region. Due to the texture, the effective light path can be prolonged, and near-field measurements allow for an insight into light intensity close to the interface as well as guided modes.

Investigation of the influence of the scanning probe on SNOM near-field images using rigorous simulations including the probe

To investigate light propagation and near-field effects above structured surfaces, scanning near-field optical microscopy is a powerful tool providing access to the near-field intensity. These measurements can be combined with rigorous solving of Maxwells equations to gain insight into light propagation inside the sample, which is not accessible via experiment. However, we find differences between the intensity distribution obtained via experiment and that observed in the simulation at a constant distance of 20 nm above the surface, which corresponds to the typical surface-to-probe distance in the experiment. A first explanation was given by topographic artefacts [Proc. SPIE 8789, 87890I (2013)]. To better understand the interaction between sample and probe in regard to light propagation, we include the probe in high-resolution simulations of different structures, with the position of the (finite-sized) probe resulting from its placement above each structure. While there is a visible difference in the overall light distribution of the system, caused by the probe, the relative intensity at the position of the probe is shown to be in very good agreement to the intensity in a system without the probe. This has been found for many probe positions along the surface of the structure. This result is applicable to many systems in different fields of research which use such measurements for obtaining information about near-field effects of samples. We show an application for thin-film photovoltaics, where light scattering textured surfaces are used to increase the path length of photons in the absorber layer to increase device performance.

Tuning Light Scattering Properties of Thin-Film Silicon Solar Cells by Means of Scalar Scattering Theory

A scalar scattering model is applied to rough thin-film silicon solar which allows to manipulate the surface texture with respect to its scattering properties. Generic textures are derives that outperforms the original texture.