TCO work function related transport losses at the a-Si:H/TCO-contact in SHJ solar cells

•Fermi level pinning is relevant for the a-Si:H/TCO contact.•The pinning factor for the contact with (i)-a-Si:H layers was determined to be 0.2–0.3.•Thus a minor difference in WF will result in only small change in FF.•For work function engineering to have an effect the WF should be considerably higher or lower than 4.7 eV of ITO.•WOx (high WF TCO) lowers Schottky barrier at the p-contact.

Silicon based heterojunction (SHJ) solar cells show high efficiency enabled through excellent passivation by amorphous silicon (a-Si:H), the use of light trapping schemes and transparent conductive layers. Many different research groups and companies have achieved high open-circuit voltages above 730 mV, but fill factors (FFs) at the level of conventional silicon solar cells are rare. One reason for a lower FF may be the (p)a-Si:H/TCO-contact, where a Schottky diode results. This can lead to an increased transport barrier adversely affecting the hole collection and thus the FF. The role of TCOs with work functions (WF) higher (tungsten oxide (WOx)) and lower (aluminium doped zinc oxide, AZO) than the standard ITO (indium tin oxide) will be examined experimentally in this paper. The aim of this paper is the proof of concept that WF engineering can improve the contact. Schottky theory with TCO WF values taken from the literature was the main assumption for the simulations and in the case of WOx at the hole contact, it could be shown that adjusting the TCO WF to (p)-a-Si:H decreases the Schottky barrier height.