Optical characterization methods were applied to a series of microcrystalline silicon thin films and solar cells deposited by the very high frequency glow discharge technique. Bulk and surface light scattering effects were analyzed. A detailed theory for evaluation of the optical absorption coefficient α from transmittance, reflectance and absorptance (with the help of constant photocurrent method) measurements in a broad spectral region is presented for the case of surface and bulk light scattering. The spectral dependence of α is interpreted in terms of defect density, disorder, crystalline/amorphous fraction and material morphology. The enhanced light absorption in microcrystalline silicon films and solar cells is mainly due to a longer optical path as the result of an efficient diffuse light scattering at the textured film surface. This light scattering effect is a key characteristic of efficient thin-film-silicon solar cells.

We investigate the efficiency potential of organic-inorganic halide perovskite/crystalline silicon tandem solar cells, a new class of photovoltaic devices targeting long-term cost reductions by ultrahigh conversion efficiencies. Methyl ammonium lead triiodide perovskite solar cells are particularly interesting as the top cell in Si-based tandem devices due to their suitable band gap, high photovoltage, and low sub-bandgap absorption. We derive optical models for a perovskite/Si tandem cell with Lambertian light trapping in the perovskite top cell, as well as for a top cell in the single pass limit. We find that unlike for other thin-film device architectures, light trapping is not required for the triiodide perovskite/Si tandem to reach matched top and bottom cell currents. While a Lambertian top cell could be employed in a four-terminal tandem, a top cell in the single pass limit enables a current-matched monolithic device with realistic top cell thicknesses. We calculate a limiting efficiency of 35.67% for an ideal (no parasitic absorption, ideal contacts) monolithic tandem, assuming a top cell open-circuit voltage of 1100 mV.

We report on the experimental realization of amorphous/microcrystalline silicon tandem solar cells (Micromorph) based on our three-dimensional design. An enhancement is reached in the short-circuit current by 40%, with an excellent open-circuit voltage of 1.41V and a fill factor of 72%. We have used nanoholes or microholes dry etched into the ZnO front contact layer. Monte Carlo optical modeling shows that stable efficiency of amorphous silicon p-i-n solar cells in over 12% range is possible. For the Micromorph cells, efficiency over 15% with the thickness of amorphous Si below 200 nm and of microcrystalline Si around 500 nm is possible.