Stephan Lehnen

Nanoscale Observation of Waveguide Modes Enhancing the Efficiency of Solar Cells

Nanophotonic light management concepts are on the way to advance photovoltaic technologies and accelerate their economical breakthrough. Most of these concepts make use of the coupling of incident sunlight to waveguide modes via nanophotonic structures such as photonic crystals, nanowires, or plasmonic gratings. Experimentally, light coupling to these modes was so far exclusively investigated with indirect and macroscopic methods, and thus, the nanoscale physics of light coupling and propagation of waveguide modes remain vague. In this contribution, we present a nanoscopic observation of light coupling to waveguide modes in a nanophotonic thin-film silicon solar cell. Making use of the subwavelength resolution of the scanning near-field optical microscopy, we resolve the electric field intensities of a propagating waveguide mode at the surface of a state-of-the-art nanophotonic thin-film solar cell. We identify the resonance condition for light coupling to this individual waveguide mode and associate it to a pronounced resonance in the external quantum efficiency that is found to increase significantly the power conversion efficiency of the device. We show that a maximum of the incident light couples to the investigated waveguide mode if the period of the electric field intensity of the waveguide mode matches the periodicity of the nanophotonic two-dimensional grating. Our novel experimental approach establishes experimental access to the local analysis of light coupling to waveguide modes in a number of optoelectronic devices concerned with nanophotonic light-trapping as well as nanophotonic light emission.

High-resolution photocurrent mapping of thin-film solar cells using scanning near-field optical microscopy

The efficiency of thin-film solar cells strongly depends on the plasmonic structures, cloaking, and especially the microscopic and nanoscopic material inhomogeneity and surface topography of the absorber. However, the understanding of the latter requires optoelectronic characterization on a nanoscale. In this study, by applying an aperture-type scanning near-field optical microscope (SNOM) in illumination mode, direct photocurrent measurements with sub-100 nm resolution were performed on randomly textured hydrogenated microcrystalline silicon (μc-Si:H) thin-film solar cell, flat μc-Si:H thin-film solar cell and flat hydrogenated amorphous silicon (a-Si:H) thin-film solar cell in order to investigate the influence of material inhomogeneity and surface topography on the local photocurrent generation. While in case of the randomly textured μc-Si:H solar cell, contrary behaviors of the photocurrent response between short and long wavelengths were identified, the same correlation between the photocurrent signal and the surface topography was observed for the two flat solar cells at all wavelengths. The measurement results can be explained by a combination of two dominant effects, (i) local light coupling into the sample and (ii) light propagation inside the sample. By this study, on the one hand the importance of surface texturing as a concept to increase the efficiency is demonstrated. On the other hand, the influence of the interaction between the SNOM probe and the surface on the photocurrent measurements has been investigated.

Nanoscale Investigation of Polarization-Dependent Light Coupling to Individual Waveguide Modes in Nanophotonic Thin-Film Solar Cells

Nanophotonic light management concepts are essential building blocks of advance photovoltaic technologies. These concepts make use of light coupling to waveguide modes supported by the photoactive absorber material of the solar cell. In this contribution, we will explain on the basis of our recently published results the development of scanning near field optical microscopy as a new method that enables the direct nanoscale observation of light coupling to an individual waveguide mode in a nanophotonic thin-film silicon solar cell. Beyond this, we present a new detailed study based on this new method on the polarization dependence of the light coupling to an individual waveguide mode.

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.

Electro-optical characterization of solar cells with scanning near-field optical microscopy

We present three different working modes of scanning near-field optical microscopy which provide complementary information about the electro-optical properties of solar cells. Those working modes allows to study the local light scattering and trapping in textured solar cells, the local light propagation inside the cell and the impact of lateral inhomogeneity on a nanoscopic scale. Due to the presence of evanescent light modes, the presented techniques give access to important physical effects which is only possible in the optical near-field.

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.

Nanophotonic Light Management in Thin-Film Solar Cells

Periodic or random nanostructures in thin-film solar cells at the front, rear and as interlayer enhance solar cell efficiencies. The underlying effect of dielectric or plasmonic light scattering will be discussed combining experiments and theory.