Silica nanosphere textured back reflectors for increased absorption in thin film amorphous silicon solar cells

Abstract

Absorption of long-wavelength photons in thin-film hydrogenated amorphous silicon (a-Si:H) solar cells is inherently low due to an absorption layer thickness of 250300nm and long absorption lengths for photons of wavelengths above 700nm. When used in a thin-film solar cell, back reflectors that exhibit diffuse reflection increase the probability of absorption by lengthening the path-lengths of photons. Ordered monolayers of silica nanospheres coated with a reflective surface were investigated for use as a diffuse back reflector. A method for fabricating a monolayer of silica nanospheres was developed using a custom built dip-coating apparatus. Repeatable monolayers of 500nm nanospheres were fabricated on glass and stainless steel substrates. The monolayers were covered with a 200nm layer of silver (Ag) and a 200nm layer of aluminum-doped zinc oxide (ZnO:Al) to be used as a back-reflector in a-Si:H solar cells . The back-reflector substrates were measured to have a much higher percentage of diffuse reflectance when compared to flat silver back-reflector substrates. Increases in long-wavelength absorption were observed by normalized external quantum efficiency (NEQE) measurements of the a-Si:H solar cells. Short circuit current density increases were seen in both I-V measurements under AM1.5 lighting as well as in NEQE measurements.