Silicon oxide treatment to promote crystallinity of p-type microcrystalline layers for silicon heterojunction solar cells

Many silicon heterojunction solar cells tend to suffer modest fill factors due to high series resistance compared to homojunction or high-temperature-passivated-contact-based solar cells. Loss analysis indicates that this limitation lies in the high contact resistance between the wafer and the electrode (through the intrinsic (i) amorphous silicon (a-Si:H) passivating layer and amorphous silicon doped layers), mostly originating from the p-type contact. We implement p-type microcrystalline doped layers in heterojunction cells and demonstrate with 2-side contacted devices that a low CO2 concentration silicon oxide (SiO) plasma treatment on the a-Si:H (i) passivating layer allows to reach high crystallinity with thin layers without impeding passivation. The influence of the treatment time and CO2 concentration on lifetime and nucleation of the microcrystallites is discussed. We then show the potential of such a SiO treatment by comparing, in 2-side contacted devices, amorphous (without SiO treatment) and microcrystalline p-layer, the latter showing efficiency up to 21.5% prior to optimization.