Impact of the oxygen content on the optoelectronic properties of the indium-tin-oxide based transparent electrodes for silicon heterojunction solar cells

Amorphous/crystalline silicon heterojunction (SHJ) solar cells technology is attracting tremendous attention in recent years due to its potential to achieve high power conversion efficiencies at low fabrication temperatures and using few process steps. However, the commercial mass production of this technology is still somehow restricted so far, which is mainly due to the high sensitivity of the SHJ solar cell parameters to the growth conditions. A significant distinctness between the SHJ configuration and the standard silicon wafer solar cell is the current collection scheme. Indeed, as the SHJ silicon wafer solar cell is limited by the low lateral conductivity of the thin-film-silicon layers used to form the contact, a transparent conductive oxide (TCO) is systematically employed to improve the carrier transport properties, whilst also acting as an antireflective coating (ARC) for the front side. From the variety of TCOs, indium tin oxide (ITO) is the most frequently used. In this work, we investigate the properties of ITO thin films grown by DC magnetron sputtering using different oxygen to total flow ratios [r(O2) = O2/(Ar+O2)] ranging from 0.01 (1%) to 0.08 (8 %). Hall effect measurements together with optical spectrometry were carried out and were found to be drastically affected by the oxygen content. Furthermore, time of flight-secondary ion mass spectrometry (TOF-SIMS) was used to determine the depth profiling of indium, oxygen, tin, silicon, phosphorous, and hydrogen throughout the ITO and silicon layers forming the solar cell. Finally, silicon heterojunction devices were fabricated and the associated photovoltaic performance were evaluated as a function of the r(O2) into the ITO electrodes. Lower oxygen flow ratio was found to yield the best performance which is attributed to lower parasitic resistive losses.