Wide gap microcrystalline silicon carbide emitter for amorphous silicon oxide passivated heterojunction solar cells

Abstract

Wide gap n-type microcrystalline silicon carbide [µc-SiC:H(n)] is highly suitable as window layer material for silicon heterojunction (SHJ) solar cells due to its high optical transparency combined with high electrical conductivity. However, the hot wire chemical vapor deposition (HWCVD) of highly crystalline µc-SiC:H(n) requires a high hydrogen radical density in the gas phase that gives rise to strong deterioration of the intrinsic amorphous silicon oxide [a-SiOx:H(i)] surface passivation. Introducing an n-type microcrystalline silicon oxide [µc-SiOx:H(n)] protection layer between the µc-SiC:H(n) and the a-SiOx:H(i) prevents the deterioration of the passivation by providing an etch resistance and by blocking the diffusion of hydrogen radicals. We fabricated solar cells with µc-SiC:H(n)/µc-SiOx:H(n)/a-SiOx:H(i) stack for the front side and varied the µc-SiOx:H(n) material properties by changing the microstructure of the µc-SiOx:H(n) to evaluate the potential of such stack implemented in SHJ solar cells and to identify the limiting parameters of the protection layer in the device. With this approach we achieved a maximum open circuit voltage of 677 mV and a maximum energy conversion efficiency of 18.9% for a planar solar cell.

Transparent silicon carbide/tunnel SiO2 passivation for c‐Si solar cell front side: Enabling Jsc > 42 mA/cm2 and iVoc of 742 mV

“…For a wafer thickness of 165 μm, the estimated practical limit for iV oc is 748 mV according to Yoshikawa et al 23 To achieve such values close to the practical limit, Taguchi et al 22 claim that an ultraclean surface is needed, which was not assured for the sample preparation of this work as the HWCVD growth of μc-SiC: the great potential of this layer. To further increase the transparency of the μc-SiC:H(n), a possible way is to increase the filament temperature during the HWCVD growth of μc-SiC:H(n) as it was reported before in previous studies., 6,7 However, we also reported in Köhler et al 7 26,28 and which also follow the idea of simple fabrication process. On high-quality c-Si wafer, with τ bulk = 12 ms, we calculated 26.6% for η, 42.24 mA/cm 2 for J sc , 738 mV for V oc , and 85.2% for FF.…”

“…As compared with the parasitic absorption loss of 5 nm a‐Si:H(i), which is approximately 1.5 mA/cm 2 , 30 nm μc‐SiC:H(n) absorbs 10 times less incoming sunlight (0.15 mA/cm 2 ), which shows the great potential of this layer. To further increase the transparency of the μc‐SiC:H(n), a possible way is to increase the filament temperature during the HWCVD growth of μc‐SiC:H(n) as it was reported before in previous studies., However, we also reported in Köhler et al that higher filament temperatures decrease the μc‐SiC:H(n)/SiO 2 passivation quality strongly. How to overcome this trade‐off between transparency and passivation is currently under investigation.…”

“…N‐type microcrystalline silicon carbide (μc‐SiC:H(n)) grown by hot wire chemical vapor deposition (HWCVD) is known for its large optical bandgap – of 2.4 to 3.4 eV. This high optical transparency makes it a promising window layer material, which was developed for silicon thin‐film solar cells as well as for silicon heterojunction solar cells, .…”

“…This high optical transparency makes it a promising window layer material, which was developed for silicon thin‐film solar cells as well as for silicon heterojunction solar cells, . However, the deposition conditions for growing highly transparent μc‐SiC:H(n) on top of a‐Si:H(i) lead to strong deterioration of the underlying amorphous silicon layers, because of etch‐off of the films, , . This trade‐off between high‐quality passivation and attractive transparency has limited the beneficial implementation of μc‐SiC:H(n) in silicon solar cells for a long time.…”

Phosphorous Catalytic‐Doping of Silicon Alloys for the Use in Silicon Heterojunction Solar Cells

“…[5] However, there is still optical loss from the parasitic absorption in the solar spectrum range due to the use of doped a-Si:H layers at the front side [1,6] as well as loss from the lower conductive a-Si:H layers. Silicon alloy thin-films, such as microcrystalline silicon (μc-Si:H), nanocrystalline silicon oxide (nc-SiO x :H), and microcrystalline silicon carbide (μc-SiC:H), [7][8][9] are very promising alternatives to be used in a SHJ solar cell to reduce the contact resistance and parasitic absorption due to their higher conductivity and lower absorption coefficient in short wavelength range.…”