Transparent WOx window layers for silicon based heterojunction solar cells
Francesca Menchini1, Luca Serenelli1, Luca Martini1,2, Andrea Albano1, Ferdinando Alessio1, Pietro Mangiapane1, Enrico Salza1, Glauco Stracci1, Giampiero De Cesare2, Domenico Caputo2, Mario Tucci1
1ENEA, Casaccia Research Center, Rome, Italy
/2DIET University of Rom , Rome, Italy

Despite the very high efficiency reached with heterojunction solar cells based on crystalline/amorphous silicon (26.7% by Kaneka), some aspects still leave room for improvements to increase the cells efficiency and reduce the manufacture cost. The possibility to replace the doped amorphous silicon (a-Si:H) with some transition metal oxide (TMO) layer has been recently investigated. TMOs possess high transparency and high band gap energy which, together with a high work function, could ensure good hole-extraction capabilities.
In this work we show heterostructure solar cells containing WOx layers grown by different methods. We have substituted the conventional passivating intrinsic a-Si:H layer by a more transparent amorphous hydrogenated silicon oxide film (a-SiOx:H). This layer, together with WOx, gives rise to a highly transparent front window for a silicon absorber substrate, which can be manufactured without any toxic gases during the growth process.
We find out that the devices based on evaporated WOx have better performances than those based on sputtered WOx films, even if the overall cell efficiency is still lower than the literature reported values. Sputtered WOx films induce plasma damage over the a-SiOx:H layer, that reflects in a deterioration of the silicon surface passivation. Moreover work function values lower than the expectation in the WOx layer still require further optimisation to achieve the desired cell efficiency.