Optically rough and physically flat TCO substrate formed by coating ZnO thin film on pyramid-patterned glass substrate

Abstract

An optically rough and physically flat (OR-PF) TCO substrate was developed by spin-coating Al-doped ZnO thin film on glass substrate patterned with random pyramidal morphology to enhance the light absorption in superstrate-type thin-film silicon solar cells without deteriorating the growth of photoactive layers. This substrate possesses flat surface with a root mean square roughness of 10.9 nm, high visible light transmittance of over 86%, a sheet resistance of 29.5 Ω/sq, and moderate light-scattering ability. A surface morphology of random pyramidal morphology was formed on the glass substrate by replicating the pyramidal surface texture of a ZnO thin film with the room-temperature nanoimprint technique. Employed as the front electrode of hydrogenated amorphous Si single-junction solar cells, this OR-PF substrate enabled a Voc of 0.91 eV, an FF of 0.71, a Jsc gain of approximately 8%, and an efficiency gain of approximately 20% with respect to the AZO-coated flat glass substrate. The Jsc gain is attributed to the enhancement in the light absorption of a-Si:H solar cells triggered by the diffraction behaviour of the interfacial pyramidal morphology, while the achievement of large Voc and FF is ascribed to the improvement in the growth quality of the photoactive layers. Based on the calculation results of the bidirectional scattering distribution function, this substrate empowers comparable intensity of the diffraction peaks for 600–700 nm wavelength light but scatters less light away from the specular direction with respect to the conventional pyramid-textured ZnO substrate. This OR-PF substrate would enable efficient photon management and the growth of high-quality photoactive materials simultaneously for thin-film silicon solar cells.