High-rate Deposition, Interface Control And Solar Cell Applications Of Microcrystalline Silicon Films

Hydrogenated microcrystalline silicon (?c-Si:H) films have attracted intense interests in photovoltaic applications, as they potentially offer higher efficiency and higher stability compared with amorphous silicon (a-Si). The optical absorption of?c-Si:H film is very low due to its indirect energy band structure. For application of this material in solar cells, a film thickness of 1-3?m is required for sufficient absorption and utilization of sun light. Microcrystalline silicon films are generally prepared with plasma enhanced chemical vapor deposition using very high hydrogen diluted silane as source gas, which makes the deposition rate very lower than that of a-Si:H.To apply?c-Si:H film in mass production of solar cells, a high deposition rate is required while maintaining reasonable material quality.Microcrystalline silicon are generally prepared on foreign substrate at low substrate temperature. Under this condition,?c-Si experiences an incubation phase to nucleate, leaving an amorphous transition layer, an incubation layer, at film/substrate interface. The interface between the?c-Si:H film and underlying layer (substrate or other layer) involved the transportation and the recombination of carriers has important influence on device performance. The presence of the amorphous incubation layer complicates this interface, thereby, it is significant to control the incubation layer and the film/substrate interface.This study is mainly devoted to the deposition of?c-Si film at high rate, the control on film/substrate interface structure, and the application in heterojunction and nanowire solar cells.A technique combining very high frequency inductively coupled plasma with gas-jet was developed to deposit?c-Si films, and a high deposition rate of over 20 nm/s was obtained at a low substrate temperature. The high generation rate and the rapid transportation of film growth precursors contribute to this high deposition rate. In jet-ICPCVD, abundant hydrogen atoms are generated, which induces a high crystallinity in film through a hydrogen-induced chemical annealing. Furthermore, the low dark conductivity and high photosensitivity were obtained due to the suppression over oxygen doping and activation by low temperature process in jet-ICPCVD. P-type doping was realized in this system, and a high conductivity of 1.24 S/cm was obtained in the film with a thickness of less than 20 nm.We researched the control over the interface structure of?c-Si/substrate to eliminate the amorphous incubation layer. Firstly, we realized the deposition of?c-Si film free of amorphous interface layer and homogeneous in the growth direction at a high rate and a low substrate temperature in the jet-ICPCVD system. The results reveal that high-density plasma, spatial plasma potential, and a high pressure are of substantive benefit to the generation of abundant energetic hydrogen atoms, which can reach and anneal the incubation layer through hydrogen chemical annealing, resulting in the crystallization of the amorphous incubation layer during the growth process. Secondly, it was found from the study on PECVD process that the nucleation of?c-Si is closely correlated with the thickness of the H-treated a-Si underlying layer. On a H-treated ultrathin a-Si:H layer, very thin P-doped?c-Si:H films (-20 nm) with high conductivity (>1 S/cm) were obtained owing to rapid nucleation. H2 plasma treatment results in an increased compressive stress. Consequently, abundant strained Si-Si bonds and SiHn complexes are generated, promoting the rapid nucleation of?c-Si. Furthermore,?c-Si:H films were applied to silicon heterojunction (SHJ) solar cells with (n)?c-Si:H/(i)a-Si:H/(p)c-Si structure. It was demonstrated that the application of?c-Si:H as emitter can increase the short circuit current compared with an a-Si one. Coaxial nanowire array of ZnO:Al/(n)a-Si:H/(p)c-Si structure was prepared successfully, and will be applied in silicon nanowire solar cells.The significant results from our research on high-rate deposition, interface control, and solar cell application of?c-Si films, will contribute to the improvement of?c-Si:H solar cell performance.