Microscopic Origins of Contact Deterioration During Annealing of Silicon Heterojunction Solar Cell Contacts

Over the last years, the dominating charge carrier transport barrier in silicon heterojunction (SHJ) solar cells could be boiled down to the contact of the indium–tin oxide (ITO) to the doped amorphous silicon (a-Si) layer. The formation of a parasitic oxide at this junction was hypothesized to act as a source for contact deterioration after annealing. However, no experimental proof could be obtained so far. In this contribution, we simultaneously investigate the contact resistivity and nanoscopic structure of the electron contact of a SHJ solar cell in the annealing temperature range between 140
$^\circ \text{C}$
and 240
$^\circ \text{C}$
. For this purpose, micro transfer length measurements, time-of-flight secondary ion mass spectrometry, and electron–energy loss spectroscopy as well as energy-dispersive X-ray spectroscopy in a (scanning) transmission electron microscope are applied. A minimum contact resistivity of around 120
$\text{m}\Omega {\text{cm}^{2}}$
is obtained at 160
$^\circ \text{C}$
. At higher temperatures, the contact resistivities increase rapidly. This contact degradation correlates with the thickening of a parasitic silicon oxide layer found at the ITO/a-Si junction and an increase in Si oxidation state within this interlayer. Additionally, Ag from the metallization diffuses into the Si, which may induce deep acceptor trap states. Furthermore, a TiO
${}_{\text{x}}$
layer with
${\text{x}}\sim {1}$
is proven between ITO and the AgPdTi metallization, which does not impair the current transport.