Low-Temperature High-Mobility Amorphous IZO for Silicon Heterojunction Solar Cells

Monica Morales-Masis,Christophe Ballif,Silvia Martin De Nicolas,Stefaan De Wolf,Jakub Holovsky

doi:10.1109/jphotov.2015.2450993

Abstract

Parasitic absorption in the transparent conductive oxide (TCO) front electrode is one of the limitations of silicon heterojunction (SHJ) solar cells efficiency. To avoid such absorption while retaining high conductivity, TCOs with high electron mobility are preferred over those with high carrier density. Here, we demonstrate improved SHJ solar cell efficiencies by applying high-mobility amorphous indium zinc oxide (a-IZO) as the front TCO. We sputtered a-IZO at low substrate temperature and low power density and investigated the optical and electrical properties, as well as subband tail formation-quantified by the Urbach energy (E U )-as a function of the sputtering oxygen partial pressure. We obtain an E U as low as 128 meV for films with the highest Hall mobility of 60 cm 2 /V ·s. When comparing the performance of a-IZO films with indium tin oxide (ITO) and hydrogenated indium oxide (IO:H), we find that IO:H (115 cm 2 /V · s) exhibits a similar E U of 130 meV, while ITO (25 cm 2 /V · s) presents a much larger E U of up to 270 meV. The high film quality, indicated by the low E U , the high mobility, and low free carrier absorption of the developed a-IZO electrodes, result in a significant current improvement, achieving conversion efficiencies over 21.5%, outperforming those with standard ITO.

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