Evaporated MoOx as General Back-Side Hole Collector for Solar Cells.

Substoichiometric molybdenum oxide (MoO_x) has good potential as a hole-collecting layer in solar cells. In this paper, we report on the application of ultrathin evaporated MoO_x as a hole collector at the back side of two distinct photovoltaic technologies: polymeric and silicon heterojunction (SHJ). In the case of polymer solar cells, we test MoO_x as a hole transport layer in devices with inverted architecture. The higher transparency of the MoO_x film, compared to the commonly used poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), allows an enhanced back reflected light into the photoactive layer, thus boosting the photogeneration, as found from the illuminated J-V and external quantum efficiency (EQE) curves. The higher fill factor (FF) of the MoO_x-based device also suggests an improved charge collection efficiency compared to the cells with PEDOT:PSS. As for SHJ solar cells, we show that MoO_x offers the means for dopant-free hole collection with both p-type and n-type Si wafers. In the present comparison over planar test structures with Ag back reflecting electrodes, we observe an efficiency gain of approximately 1% absolute against a baseline with a conventional p-type amorphous silicon hole collector. The gain is linked to the increased V_OC, which is likely due to the reduced recombination at the Si wafer. [ABSTRACT FROM AUTHOR]