Kimihiko Saito

Relationship between the cell thickness and the optimum period of textured back reflectors in thin-film microcrystalline silicon solar cells

Periodically textured back reflectors with hexagonal dimple arrays are applied to thin-film microcrystalline silicon (μc-Si:H) solar cells. When the textures have a moderate aspect ratio, the optimum period for obtaining a high short circuit current density (JSC ) is found to be equal to or slightly larger than the cell thickness. If the cell thickness exceeds the texture period, the cell surface tends to be flattened and texture-induced defects are generated, which constrain the improvement in JSC . Based on these findings, we have fabricated optimized μc-Si:H cells achieving a high efficiency exceeding 10% and a JSC of 30 mA/cm2.

Enhanced photocurrent and conversion efficiency in thin-film microcrystalline silicon solar cells using periodically textured back reflectors with hexagonal dimple arrays

Periodically textured back reflectors with hexagonal dimple arrays are applied to thin-film microcrystalline silicon (μc-Si:H) solar cells for enhancing their photon absorption and photovoltaic performance. In a systematic survey of 1 -μm-thick μc-Si:H cells, the best performance is obtained with a period of 1.5 μm and an aspect ratio of 0.20–0.25 with a high current density exceeding 26 mA/cm2 and a marked efficiency of 10.1%. These results demonstrate the high potential of periodic textures or surfacegratings for improving the conversion efficiency of thin-film silicon solar cells.

Investigation of Textured Back Reflectors With Periodic Honeycomb Patterns in Thin-Film Silicon Solar Cells for Improved Photovoltaic Performance

Periodically textured back reflectors with a honeycomb dimple pattern are investigated for improving the infrared response and conversion efficiency in substrate-type microcrystalline (μc-Si:H) silicon solar cells. For 1-μm-thick μc-Si:H cells, the best current density is obtained at the period of 1.4 µm, while a substantial enhancement in the external quantum efficiency is achievable in a wide range of period of 1–2 µm. In addition, photocurrent of the cells is improved with increasing the aspect ratio of the honeycomb textures. As a result of a high open-circuit voltage and fill factor, as well as the improved current densities, high conversion efficiencies of 9.4% and 9.9% are realized in the 1-µm-thick and 2-µm-thick μc-Si:H cells, respectively.

Periodic structures boost performance of thin-film solar cells

Photovoltaic power generation, a key renewable energy resource, commonly uses wafer-based crystalline silicon solar cells. An alternative is thin-film silicon solar cells (TFSSCs), which would particularly benefit systems with generation capacity of more than one gigawatt because of the abundance and non-toxicity of their source materials. In TFSSCs, trapping the incident light within thin silicon films is crucial to improve the photon absorption and conversion efficiency. To scatter the incident light and elongate the optical path length inside the cell, TFSSCs use textured substrates, which normally have randomized textures with sizes ranging from sub-microns to several microns. Potentially, a TFSSC’s optical path length can be enhanced by a factor of 4n2, where n is the refractive index of thin-film silicon.1 In recent years, developers have sought a more sophisticated platform for light trapping, studying periodically textured substrates or surface gratings.2, 3 For example, if we apply an optimized periodic texture to amorphous silicon (a-Si:H) solar cells, it improves the conversion efficiency as well as the shortcircuit current density (JSC) at the same level as state-of-theart random textures.3 However, we have yet to demonstrate the full potential of using periodic textures in microcrystalline silicon ( c-Si:H) solar cells, which need more efficient light confinement because of their very small absorption coefficient in the IR region. Excessively steep textures, such as binary surface-relief gratings or pyramidal textures with V-shaped valleys, can induce defects in c-Si:H films and impair photovoltaic performance,4 so it is important to find textures that are suitable for high-quality film growth as well as good light confinement. Figure 1. Scanning electron microscope images of four periodic honeycomb textures for thin microcrystalline solar cells with periods of (a) 1 m, (b) 1.5 m, (c) 2 m, and (d) 3 m.