Al ₂ O ₃ /SiON stack layers for effective surface passivation and anti-reflection of high efficiency n-type c-Si solar cells

Excellent surface passivation and anti-reflection properties of double-stack layers is a prerequisite for high efficiency of n-type c-Si solar cells. The high positive fixed charge ( Q f ) density of N-rich hydrogenated amorphous silicon nitride (a-SiN x :H) films plays a poor role in boron emitter passivation. The more the refractive index ( n ) of a-SiN x :H is decreased, the more the positive Q f of a-SiN x :H is increased. Hydrogenated amorphous silicon oxynitride (SiON) films possess the properties of amorphous silicon oxide (a-SiO x ) and a-SiN x :H with variable n and less positive Q f compared with a-SiN x :H. In this study, we investigated the passivation and anti-reflection properties of Al 2 O 3 /SiON stacks. Initially, a SiON layer was deposited by plasma enhanced chemical vapor deposition with variable n and its chemical composition was analyzed by Fourier transform infrared spectroscopy. Then, the SiON layer was deposited as a capping layer on a 10 nm thick Al 2 O 3 layer, and the electrical and optical properties were analyzed. The SiON capping layer with n = 1.47 and a thickness of 70 nm resulted in an interface trap density of 4.74 = 10 10 cm −2 eV −1 and Q f of −2.59 = 10 12 cm −2 with a substantial improvement in lifetime of 1.52 ms after industrial firing. The incorporation of an Al 2 O 3 /SiON stack on the front side of the n-type solar cells results in an energy conversion efficiency of 18.34% compared to the one with Al 2 O 3 /a-SiN x :H showing 17.55% efficiency. The short circuit current density and open circuit voltage increase by up to 0.83 mA cm −2 and 12 mV, respectively, compared to the Al 2 O 3 /a-SiN x :H stack on the front side of the n-type solar cells due to the good anti-reflection and front side surface passivation.