Investigation Of High Efficiency Nc-Si: H/C-Si Heterojunction Solar Cells

Silicon heterojunction（SHJ） solar cells consisting of thin hydrogenated amorphous silicon（a-Si: H） layers deposited on crystalline silicon（c-Si） wafers have attracted a lot of attention for their high performance and low production cost. In order to improve the efficiency of SHJ solar cells, one effective method is to use the window materials with wide bandgap and high conductivity. As compared to a-Si: H thin films, nc-Si: H thin films with lower Fermi energy, significantly wider optical bandgap and higher conductivity is an excellent candidate for the emitter layer.In this paper, SHJ solar cells were fabricated by RF-PECVD at low temperature and low RF power in con?guration of Al/ ITO/ nc-Si: H（p） /a-Si: H（i）/c-Si（n） /c-Si（n+）/Al. The influence of the properties of nc-Si: H（p） and a-Si: H thin films, wet chemical passivation and post-deposition hydrogen plasma treatment（HPT）on the performance of solar cells was systemically investigated by various test methods. The main conclusions are drawn as followings:（1） Hydrogen dilution ratio, B₂H₆-to-Si H4 ratio and film thickness have strong influence on the structural, optical and electronic properties of p type nc-Si: H films. The action mechanisms of these factors were deeply analyzed. It was found that increasing the hydrogen dilution ratio or decreasing the B2H6 doping concentration could lead to a continuous increase of crystalline fraction. In addition, with the increase of Xc the CH decreases while RIR increases, and meanwhile the bonding mode of H changes from Si H bonds to clustered Si H bonds in GB regions. The 20 nm thick p type nc-Si: H films with high quality were obtained, and by employing the films as window layers, the fill factor of SHJ solar cells was significantly improved.（2） With the decrease of silane concentration, substantial changes of crystalline volume fraction, hydrogen concentration, structure factor, optical bandgap and photosensitivity of films take place in transition zone. The passivation quality of intrinsic silicon thin ?lms is decided by the hydrogen content and bonding structure of films. The films close to transition zone show good compactness and photosensitivity, high hydrogen concentration and low state density and contain abundant Si H bonds. Thereby, the films can provide excellent passivation for c-Si surfaces. But the passivation quality deteriorates seriously when the films are too thin. By optimizing the film thickness of the passivation layers, we obtained a nc-Si: H/c-Si SHJ solar cell with an efficiency of 17.4%.（3） The influence of wet-chemical passivation and post-deposition hydrogen treatments（HPT） on the interface quality of SHJ solar cells is investigated. A minority carrier lifetime of 981 us was achieved on c-Si by employing a new wet-chemical passivation method. And it is shown that the interface quality can be efficiently improved by the HPT treatment due to the film quality and hydrogen content were increased. However, the prolonged HPT treatment time would result in a rapidly drop of the interface quality due to the prolonged etching effect of hydrogen. The optimum HPT treatment time was proven to be 60 s in this work. A nc-Si: H/c-Si SHJ solar cell with an efficiency of 18.8% was achieved.（4） By employing the optimized results, the 148cm2 nc-Si: H/c-Si SHJ solar cells with Voc=713m V,Jsc=35.6m A·cm-2,FF=0.780,η=19.8% was achieved.