High temperature oxidation pre-treatment of textured c-Si wafers passivated by a-Si:H
G. Limodio (TU Delft - Photovoltaic Materials and Devices)
G. D'Herouville (Student TU Delft)
L. Mazzarella (TU Delft - Photovoltaic Materials and Devices)
Y. Zhao (TU Delft - Photovoltaic Materials and Devices)
Guangtao Yang (TU Delft - Photovoltaic Materials and Devices)
O. Isabella (TU Delft - Photovoltaic Materials and Devices)
M. Zeman (TU Delft - Electrical Sustainable Energy)
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Abstract
This work shows an alternative surface cleaning method for c-Si wafers to replace the standard chemical procedures as RCA or HNO
3
which involve hazardous chemicals or unstable processes. The method consists in a high-temperature oxidation treatment (HTO) performed in a classical tube furnace that incorporates organic and metal particles present on the c-Si surfaces in the growing SiO
2
layer. The result is as a reliable pre-treatment method for obtaining less defective c-Si surfaces ready for solar cell fabrication after SiO
2
removal. To test the surface passivation quality obtained with our alternative cleaning method, we grow amorphous silicon (a-Si:H) layers by plasma enhanced chemical vapor deposition on both sides of the c-Si wafer and systematically compare the effective carrier lifetime (τ
eff
) and implied V
OC
(iV
oc
) to the wafer treated with the standard cleaning in our laboratory. We optimize HTO treatment time reaching τ
eff
of ∼6 ms and iV
oc
of 721 mV for the best sample. We ascribe the improved passivation quality using HTO to two concurrent factors. Firstly, the encapsulation of defects into SiO
2
layer that is then etched prior a-Si:H deposition and secondly, to modification of the pyramids’ morphology that facilitates the surface passivation. SEM pictures and reflection measurements support the latter hypothesis.