Poole-Frenkel emission and defect density in a-Si:H/nc-Si:H multilayer films for “all silicon” third generation photovoltaics

Abstract

A layered structure of nanocrystalline Silicon (nc-Si) and Si compound dielectric (SiO2, Si3N4, SiC) is becoming popular for the third generation photovoltaics because of its tunable optical bandgap. However, this structure makes poor current conduction, as the barrier potential between Si and Si dielectric is very high. An excellent current conduction can be obtained by replacing the Si dielectric with amorphous Si (a-Si:H), as the barrier potential between a-Si and nc-Si is very low compared to nc-Si/Si dielectric structure. This paper presents the electrical properties of an a-Si:H/nc-Si:H multilayer film fabricated using hot-wire chemical vapour deposition. Hydrogen (H2) dilution in the nc-Si:H layers was considered as a variable parameter in different multilayer films. Formation of a nc-Si:H layer sandwiched between two a-Si:H layers was confirmed by Raman spectroscopy and X-ray diffraction. The current-voltage measurements performed in low temperature reveal different vertical charge transport mechanisms, such as Poole-Frenkel (PF) emission, phonon assisted tunnelling and direct tunnelling. Below room temperature, the PF emission rate increases the current with increase in H2 dilution in the multilayer films. The capacitance-voltage measurement performed near the room temperature explains the increased defects in the multilayer films with an increase in H2 dilution.