Performance Evaluation of Hole-Selective and Passivating $\text{MoO}_{\mathrm{X}}$ Layers on Si: A Comparative Study of ALD and Evaporation

Abstract

Dopant-free heterojunction silicon solar cells are fabricated using sub-stoichiometric molybdenum trioxide $(\text{MoOx})$ as passivating and hole selective layer for n-type silicon. $\text{MoOx}$ is deposited by both, atomic layer deposition (ALD) and thermal evaporation (TE). No separate passivation layer is used in either case. While the ALD films give highly conformal deposition, their performance depends upon factors such as precursors used and process temperature. Our solar cells fabricated using $\text{ALD}-\text{MoOx}$ could reach nearly 7% efficiency as compared to solar cells using $\text{TE}-\text{MoOx}$, which reached 10.13%. This difference is mainly attributed to the recombination factor, which is four orders of magnitude higher for $\text{ALD}-\text{MoOx}$ than $\text{TE}-\text{MoOx}$. The ideality factor deviated from unity for ALD-MoOx based diodes. The average carrier lifetime obtained for TE-MoOx films crossed 100 microseconds, while for $\text{ALD}-\text{MoOx}$, it is about 55 microseconds. These observations indicate ALD-$\text{MoOx}-\text{Si}$ interface is influenced by detrimental parameters like high density of defect states or surface contaminants or both, which are incorporated during ALD of $\text{MoOx}$ as all the other processing steps are common for both routes. The undesirably high recombination factor may have resulted due to use of carbon-based precursor or use of ozone as oxygen precursor.