Investigation of light propagation in thin-film silicon solar cells by dual-probe scanning near-field optical microscopy
Jülich / Forschungszentrum Jülich, Zentralbibliothek (2015) [Book, Dissertation / PhD Thesis]
Page(s): 120 S. : Ill., graph. Darst.
In this work, the light propagation in microcrystalline silicon thin film solar cells is investigated. For this purpose, a dual-probe scanning near-field optical microscope (SNOM) was developed and set up from scratch. The microscope is equipped with two separated probes for local illumination and detection on a subwavelength scale. Applying newly developed modes of measurement, exclusively available at dual probe SNOMs, the microscope allows for measuring the propagation of light in thin layers with high precision. Within the framework of this thesis, the different physical challenges of dual probe scanning near-field optical microscopy are outlined and the technological solutions are described. The reliability of the setup was thoroughly tested at measurements of light propagation in flat and textured microcrystalline silicon (µc-Si:H) thin film solar cells in nip-configuration. The measured raw data are analyzed by multiple methods. It is observed, that the lateral intensity decay of light is strongly influenced by local surface features. Therefore, an advanced dual-probe scan mode is introduced which compensates for the non-constant coupling efficiencies, caused by local surface features. In dual-probe operation, only a small share of the photons emitted by the illumination probe finally reaches the detection probe. Hence, the different loss mechanisms, which are accounted for the strong attenuation of the propagating light, are theoretically investigated by means of a ray-tracing approach. Ray-tracing allows to examine the loss mechanisms separately. The simulation reveals, that for a wavelength of 750nm the intensity decay within the first 5 µm is dominated by the radial distribution of light inside the layer. At longer distance, absorption is the major mechanism for a decrease in light intensity. The intensity decay due to a transmittance at the front interface strongly attenuates the propagation of light with small angles of incidence. Although ray-tracing neglects the wave nature of light, the approach is capable to reproduce the lateral intensity decay of light propagating in a 1 µm thick µc-Si:H layer. The ray-tracing approach is supplemented with finite-difference time-domain (FDTD) simulations which provide the field distribution on a sub-wavelength scale. By including the illumination probe to the simulated layer stack, an improved compliance of simulation and experiment is achieved. Based on the FDTD simulation, the light distribution inside and above the layer stack is investigated and compared to a half-space model which represents a system without light-trapping properties. It is demonstrated, that at least for undamaged, perfect probes, a direct light transfer between illumination and detection probe, bypassing the absorber layer, is negligible. Furthermore, the FDTD simulations provide the angular distribution of the light emitted by a SNOM probe, placed at subwavelength distance above the surface. Thereby, the simulations complement the experimentally determined angular resolved far-field emission characteristic of a probe in air. It is shown that a large share of 64% of the light intensity emitted by a SNOM probe, placed at subwavelength distance above a µc-Si:H absorber layer, is coupled into angles which exceeds the angle of total reflection. Finally, the intensity decay of the propagating light, determined by FDTD simulations, revealed good accordance with the measured data. A modulation of the intensity decay, which originates from interference induced by multiple reflections, is observed in the measurement as well as in FDTD simulations. The good compliance between simulation and experiment indicates that macroscopic absorption coefficients are suitable for the description of light propagation even on microscopic scales.
von Plessen, Gero