Application of plasma enhanced chemical vapor deposition silicon nitride as a double layer antireflection coating and passivation layer for polysilicon solar cells

Abstract

Reflection losses in passivated emitter solar cell polycrystalline silicon solar cells have been reduced by the application of a double layer antireflection coating of plasma enhanced chemical vapor deposition silicon nitride (PECVD SixNyHz). The layer was deposited in a single wafer parallel plate reactor powered by a 13.56 MHz rf power supply using SiH4 and NH3 as the reactive gases. The layers deposited had refractive indexes of 2.50 and 1.95 at HeNe wavelength and thicknesses of 42.5 and 64.5 nanometers (nm), respectively. The overall reflectance measured in the wavelength range between 350 and 1150 nm was 8.5%. The extinction coefficient of the high refractive index film showed a significant increase in absorption for short wavelengths. However, the improvement in current collection was higher than expected from the overall reflectance and absorption of the film. Short circuit current was increased by 49% and open circuit voltage increased by 3.3% when compared with uncoated cells. These results imply a passivation effect which increases the open circuit voltage beyond the value expected solely from the current collection addition.

Bulk and surface passivation of silicon solar cells accomplished by silicon nitride deposited on industrial scale by microwave PECVD

“…leading to the same conclusions as for the case of NH 3 . The increase of the refractive index in the cases mentioned above is generally accompanied by an increase of the absorption k. This enhanced absorption is probably due to an increased Si-Si coordination in the films.…”

“…This surprising result is explained by residual gas analysis (RGA) of the plasma. RGA shows that the depletion of N 2 in the MW plasma is about half of that for NH 3 . This implies that for a given molar flow of either N 2 or NH 3 , the concentration of NH x species in the plasma is the same.…”

“…For equivalent refractive indices at 630 nm, SiN x :H layers deposited with N 2 have higher absorption at shorter wavelengths than layers deposited using NH 3 . This effect is demonstrated in Figure 6 and implies that the absorption k <500 nm is not uniquely linked to the refractive index n 630 nm .…”

“…The increased refractive index with increased pressure can be explained with an adaptation of the models by Hanyaloglu and Kessels. In our model we assume the following dissociation chains for NH 3 , N 2 and SiH 4 :…”

“…By varying the ratio of Si/N in the SiN x :H layer, the refractive index at 630 nm can be varied over a wide range between 1Á9 and 2Á5 which is ideal for both bare and glass-encapsulated cells. [1][2][3] The second function of silicon nitride is surface passivation. The capabilities of SiN x :H, grown by plasma enhanced chemical vapor deposition (PECVD) to reduce the recombination of charge carriers at the Si surface, have been known for more than 20 years.…”