Preparation and characterization of p-type hydrogenated amorphous silicon oxide film and its application to solar cell

Abstract

Thin film wide band gap p-type hydrogenated amorphous silicon (a-Si) oxide (p-a-SiOx:H) materials were prepared at 175°C substrate temperature in a radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) and applied to the window layer of a-Si solar cell. We used nitrous oxide (N2O), hydrogen (H2), silane (SiH4), and diborane (B2H6) as source gases. Optical band gap of the 1% diborane doped films is in the range of 1.71eV to 2.0eV for films with increased oxygen content. Dark conductivity of these films is in the range of 8.7×10−5S/cm to 5.1×10−7S/cm. The fall in conductivity, that is nearly two orders of magnitude, for about 0.3eV increase in the optical gap can be understood with the help of Arrhenius relation of conductivity and activation energy, and may not be significantly dependant on defects associated to oxygen incorporation. Defect density, estimated from spectroscopic ellipsometry data, is found to decrease for samples with higher oxygen content and wider optical gap. Few of these p-type samples were used to fabricate p-i-n type solar cells. Measured photo voltaic parameters of one of the cells are as follows, open circuit voltage (Voc)=800mV, short circuit current density (Jsc)=16.3mA/cm2, fill-factor (FF)=72%, and photovoltaic conversion efficiency (η)=9.4%, which may be due to improved band gap matching between p-a-SiOx:H and intrinsic layer. Jsc, FF and Voc of the cell can further be improved at optimized cell structure and with intrinsic layer having a lower number of defects.