Optically-rough and physically-flat TCO substrates for superstrate-type thin-film solar cells: Sol-gel Zn1−xMgxO coating on reaction-ion etched glass substrates

Abstract

Novel optically-rough and physically-flat transparent conductive oxides (TCO) substrates were developed by spin-coating Al doped Zn1−xMgxO (AZMO) transparent conductive thin film on roughened glass substrates treated by a reactive-ion etching (RIE) process, for enhancing the light-absorption in superstrate-type solar cells without deteriorating growth of photoactive layers. These substrates possess features of high transmittance, low surface roughness and strong light-scattering behavior. A surface morphology of randomly distributed irregular cones was formed on the Corning 7059 glass substrate through the overlapping of several lens-shaped cavities etched by the carbon tetrafluoride-oxygen plasma during the RIE process. This rough interface morphology not only has light-scattering ability, but also shows an antireflection effect at the interface between the AZMO and RIE etched glass substrate. One prototype with 40 cycles AZMO coating on a RIE etched substrate showed a root-mean-square surface roughness of 4.6nm, a sheet resistance of 75.1Ω/sq, and a haze ratio in transmission of 12.2% at the wavelength of 700nm. These TCO substrates were applied to the front electrode of superstrate-type hydrogenated amorphous Si single junction solar cells. The solar cells exhibited a current gain of 11% and an efficiency gain of 14% as well as similar open-circuit voltage and fill factor with respect to the ones deposited on the AZMO coated flat glass substrate. These substrates would enable an efficient light management and the growth of high quality photoactive materials simultaneously for superstrate-type thin-film solar cells.