Optically-rough and physically-flat TCO substrates for superstrate-type thin-film solar cells: sol-gel Zn1−xMgxO coating on nanoimprint patterned glass substrates

•New optically-rough and physically-flat TCO substrates were developed in simple way.•It showed low surface roughness and strong diffraction behavior.•It enabled a-Si solar cells with large open-circuit voltage and good spectral response.

Novel optically-rough and physically-flat transparent conductive oxides (TCO) substrates were developed for thin-film solar cells with superstrate configuration. These substrates consist of a widegap Al doped Zn1−xMgxO (AZMO) transparent conductive thin film prepared by sol-gel process and glass substrates with periodic hole array pattern formed by a nanoimprinting technique. These substrates showed low optical absorption, low surface roughness, strong light-diffraction behavior, and moderate sheet resistance, simultaneously. The number of AZMO coating layers and the dimension of the hole feature on the pattern influenced the surface roughness and diffraction behavior of these substrates significantly. A substrate with 40 AZMO coating layers on a hole pattern (diameter=2 µm, pitch=2.2 µm, and depth=1 µm) showed a root-mean-square surface roughness of 5.5 nm, a sheet resistance of 34.3 Ω/sq, and a haze ratio of 11.2% at the wavelength of 700 nm. These TCO substrates enabled hydrogenated amorphous Si single junction solar cells with large open-circuit voltage (Voc=0.92 V) and good spectral response through acting as the front electrode. These substrates would enable an efficient light management and the growth of high quality photoactive materials, simultaneously.

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