Ricerc@Sapienza

The achievable current in a silicon based heterojunction solar cell is a limit in its conversion efficiency. This drawback depends on the optical band-gap of the sun-lighted amorphous layers, that reduces the light spectrum reaching crystalline silicon absorber. The search for new materials to overcome this hurdle is recently focusing its attention on hydrogenated amorphous silicon oxide (a-SiOx:H), which offers effective crystalline silicon surface passivation and optical band-gap larger than the commonly used hydrogenated amorphous silicon (a-Si:H). Despite a large literature on a-Si:H deposition and treatments, a-SiOx:H film obtained by plasma-enhanced chemical vapor deposition (PECVD) is relatively new in heterojunction cell application, therefore material properties should be investigated and confirmed. To obtain a-SiOx:H film, one common technique descends from dissociation of silane in hydrogen dilution by PECVD, as for a-Si:H film, introducing CO2 as source of oxygen. This addition heavily modifies the film growth, composition and hydrogen inclusion, since the new discharge conditions influence the passivation properties and also increase the amorphous layer thermal stability [1]. Recently we studied the effect of various thermal annealing and UV light soaking on a-SiOx:H layers finding out that combining together both treatments and optimizing certain process parameters allows quality increase and stability of c-Si passivation [2] [3]. In this work is presented the comparison between two heterojunction solar cells (illustrated and described in fig.1) which differ one from another only in the front side passivation layer: a-Si:H in one case, a-SiOx:H in the other case.. We monitored lifetime and implied Voc during fabrication steps and evaluated the final I-V characteristics and quantum efficiency. Furthermore, we validated the metastability proprieties of a-SiOx:H film, previously studied on single layer [2], when adopted to fabricate a complete cell structure. In particular, we established that combination of UV light exposure and thermal treatment provides a relevant performance improvement in cells having a-SiOx:H buffer, especially when compared with a-Si:H cells. In conclusion cells with a-SiOx:H show an increase of all electrical parameters: indeed Jsc and Voc were respectively 1mA/cm2 and 20mV higher in cell with a-SiOx:H than in cell with a-Si:H. The difference in voltage was also observed before cells finalization with contacts formation, by measuring the implied Voc, which was 751mV for the structure passivated both side by a-SiOx:H (Fig.2).These results will be useful for the further optimization of a-SiOx:H/c-Si heterojunction cells fabrication process.