NiO as a P-Type TCO for Inorganic Thin-Film Photovoltaics

A p-type TCO for wider applicability in inorganic absorbers is urgently needed to enable bifacial and tandem thin-film architectures. NiO is a p-type transparent conductive oxide (TCO) that is successfully used in low-temperature processed organic and perovskite photovoltaics. Thin-film absorbers such as chalcogenides often employ temperatures that are higher than those used in organic or perovskite devices. In this paper we study the feasibility of using sputtered NiO as a hole transport layer in chalcogenide PV. NiO thin films deposited using RF magnetron sputtering were studied at varying substrate-temperature, gas pressure and composition. The antiferromagnetic property of NiO prevented the determination of carrier properties using Hall effect, therefore a hot-point-probe employing the Seebeck effect was used to determine the carrier type. This was shown to change from p-type to n-type as the resisitivity decreases. NiO films with resistivity less than 0.3
$\Omega$
-cm were n-type, whereas those with resistivities larger than 1
$\Omega$
-cm are p-type. Even though n-type in transport behavior, the workfunction of the highly conductive films remained close to the valence band-position suggesting that the n-type behavior is likely due electron transport in a acceptor-like defect-band. The conductivity degraded when the films were subjected to post-deposition temperatures
$> 200C$
. The Haacke figure-of-merit was calculated for the as-deposited films with a median value of
$1.1\times 10^{-9}\Omega^{-1}$
which suggests
$NiO$
is a promising
$p+layer$
as long as it is maintained at lower post-deposition temperatures.