Proc. SPIE. 9890, Optical Micro- and Nanometrology VI
KEYWORDS: Solar cells, Crystals, Silicon, Optical microscopy, Optical microscopy, Near field scanning optical microscopy, Near field scanning optical microscopy, Thin film solar cells, Near field, Light wave propagation, Near field optics, Absorption
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.
In thin optoelectronic devices, like organic light emitting diodes (OLED) or thin-film solar cells (TFSC), light propagation, which is initiated by a local point source, is of particular importance. In OLEDs, light is generated in the layer by the luminescence of single molecules, whereas in TFSCs, light is coupled into the devices by scattering at small surface features. In both applications, light propagation within the active layers has a significant impact on the optical device performance. Scanning near-field optical microscopy (SNOM) using aperture probes is a powerful tool to investigate this propagation with a high spatial resolution. Dual-probe SNOM allows simulating the local light generation by an illumination probe as well as the detection of the light propagated through the layer. In our work, we focus on the light propagation in thin silicon films as used in thin-film silicon solar cells. We investigate the light-in-coupling from an illuminating probe via rigorous solution of Maxwell's equations using a Finite-Difference Time-Domain approach, especially to gain insight into the light distribution inside a thin layer, which is not accessible in the experiment. The structures investigated include at and structured surfaces with varying illumination positions and wavelengths. From the performed simulations, we define a "spatial sensitivity" which is characteristic for the local structure and illumination position. This quantity can help to identify structures which are beneficial as well as detrimental to absorption inside the investigated layer. We find a strong dependence of the spatial sensitivity on the surface structure as well as both the absorption coefficient and the probe position. Furthermore, we investigate inhomogeneity in local light propagation resulting from different surface structures and illumination positions.