Window layer with p doped silicon oxide for high Voc thin-film silicon n-i-p solar cells

Abstract

We investigate the influence of the oxygen content in boron-doped nanocrystalline silicon oxide films (p-nc-SiOx) and introduce this material as window layer in n-i-p solar cells. The dependence of both, optical and electrical properties on the oxygen content is consistent with a bi-phase model which describes the p-nc-SiOx material as a mixture of an oxygen-rich (O-rich) phase and a silicon-rich (Si-rich) phase. We observe that increasing the oxygen content enhances the optical gap E04 while deteriorating the activation energy and the planar conductivity. These trends are ascribed to a higher volume fraction of the O-rich phase. Incorporated into n-i-p a-Si:H cells, p-nc-SiOx layers with moderate oxygen content yield open circuit voltage (Voc) up to 945 mV, which corresponds to a relative gain of 11% compared to an oxygen-free p-layer. As a similar gain is obtained on planar and on textured substrates, we attribute the increase in Voc to the higher work function of the p-nc-SiOx layer made possible by its wider band gap. These results are attained without changing the dilution ratio of the 250 nm thick intrinsic layer. We also observe an enhancement of 0.6 mA cm−2 in short circuit current density in the short wavelengths due to the higher transparency of the p-nc-SiOx layer. Finally, an initial efficiency of 9.9% for a single junction 250 nm a-Si:H n-i-p solar cell on plastic foil is achieved with the optimization of the p layer thickness, the doping ratio of the front transparent conductive oxide, and the optical properties of the back reflector.