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

Cao, Zhao; Taubner, Thomas (Thesis advisor); Rau, Uwe (Thesis advisor)

Jülich : Forschungszentrum Jülich GmbH, Zentralbibliothek, Verlag (2021)
Book, Dissertation / PhD Thesis

In: Schriften des Forschungszentrums Jülich. Reihe Energie & Umwelt = Energy & environment 536
Page(s)/Article-Nr.: 1 Online-Ressource : Illustrationen, Diagramme

Dissertation, RWTH Aachen University, 2021

Abstract

A solar cell is used to directly convert the sunlight into electrical energy. The key indicator for the performance of a solar cell, hence for its competitiveness compared with other forms of renewable energies is the conversion efficiency. The conversion efficiency of a solar cell strongly depends on its local optoelectronic properties, such as local light coupling efficiency or local material inhomogeneity, on the microscopic or even nanoscopic length scales. Therefore, an accurate understanding and assessment of the specific effects of these factors on the photogenerated current would provide valuable information for the improvement of the solar cell performance. This work presents local photocurrent measurements of various thin-film silicon solar cells with subwavelength spatial resolution by using an aperture-type scanning near-field optical microscope (a-SNOM) as the illumination source. The measurement method allows for direct access to the local optoelectronic properties. With the support of finite-difference time-domain (FDTD) simulations, their individual contributions to the photocurrent generation are analyzed. Starting with the flat microcrystalline silicon (μc-Si:H) thin-film solar cell, the SNOM photocurrent measurements are conducted with five different wavelengths ranging from 473nm to 780nm. The measurement results show a distinct correlation between the local photocurrent signal and the local topography, which is observed for all wavelengths. Corresponding FDTD simulations with an idealized topography reveal that the origin of this correlation is the topography-dependent local light coupling efficiency, which can be ~ 30% higher at local topographic minimums than that at local topographic maximums. In addition, the effect of the light polarization on the local light coupling is investigated with a periodically textured amorphous silicon (a-Si:H) thin-film solar cell. Both, the SNOM photocurrent measurements and FDTD simulations show that, at topographic structures with high rotational asymmetry, the difference of the light coupling efficiencies can be as large as 30% between two perpendicular polarizations. Next, SNOM photocurrent measurement results of the flat polycrystalline silicon (poly-Si) thin-film solar cell fabricated by liquid phase crystallization (LPC) are presented. Due to the absence of significant topographic structures, the measured photocurrent is mainly the result of the local material inhomogeneity: a grain boundary in the presented case. In order to derive information of characteristic electrical parameters, i.e. the minority carrier diffusion length $L_\mathrm{D}$ and the grain boundary recombination velocity $S_{\mathrm{gb}}$, electrical simulations are performed and fitted to the photocurrent profiles extracted from the recorded photocurrent maps. The determined values, $L_\mathrm{D} = 4.6$ μm and $S_{\mathrm{gb}} = 5.2\,\times\,10^{5}$ cm/s, are in good agreement with the figures found in other references. The last investigated sample is a solar cell with more complex structure where both, topographic structures and local material inhomogeneity would have a considerable impact on the measured photocurrent: a randomly textured μc-Si:H thin-film solar cell. The corresponding SNOM photocurrent measurement results exhibit reversed correlations of the photocurrent signal to the local topography between short and long wavelengths: while this correlation remains the same for the short wavelengths as in the case of flat μc-Si:H thin-film solar cell, higher photocurrent signal is measured at local topographic maximum for long wavelengths. FDTD simulations with the real topography acquired from the atomic force microscopy predict however, still the same correlation for all wavelengths. This leads to the conclusion that, the reversed trend observed at long wavelengths is probably caused by the electrical defects below the surface, which are commonly formed at sharp micro-valleys, i.e. at local topographic minimums. As a consequence, the photocurrent response is strongly reduced at local topographic minimums, which results in the observed reversal of the correlation between the photocurent signal and local topography at long wavelengths. This work clearly demonstrates the power of the SNOM photocurrent measurements for the nanoscale optoelectronic characterization of solar cells. In particular, the combination with supporting FDTD simulations facilitates the specific analysis of the individual effects of the local topography variations and the local material iv inhomogeneity on the photocurrent generation. Consequently, the presented techniques will allow for more specific, hence more efficient approaches when exploring concepts for the improvement of the solar cell efficiency.