Efficiency improvement in nanorod amorphous silicon thin film with passivation layer and ultrathin metal electrode for photovoltaic application

In recent years, photovoltaic cells have attracted much attention and extensively been studied by many groups. The amorphous silicon (a-Si) thin film solar cells have the advantages of low cost, low material consumption and potential for the building-integrated applications although the conversion efficiency is usually below 10%. In this paper, we show an a-Si thin film solar cell with periodical nanorod structures for light trapping enhancement and an ultrathin silver film as transparent electrode with a lower resistance for performance improvement. In such a design, the conversion efficiency can be greatly improved. The periodicity and duty ratio of the nanorods were optimized to enhance the diffraction of the light within 500-900 nm into guided modes in the a-Si thin film and thus the total optical absorption can be enhanced. Furthermore, a SiO₂ layer was deposited onto the silicon nanorods, then the surface of SiO₂ layer was flattened by chemical-mechanical polishing, finally a 5-nm ultrathin metal film was deposited as a transparent electrode to replace the conventional transparent conductive oxide while having a lower sheet resistance of 9.6 Ω /□ and a transmittance ranging from 90% to 70% within the spectral range from 300 nm to 900 nm. Our design was analyzed by using the full-wave finite element method to calculate the optical absorption of the incident sunlight in the a-Si thin film. According to the simulation results, the light absorption can be relatively enhanced by 45% and the total conversion efficiency can be relatively improved by 41.2% compared to the conventional thin film a-Si solar cell without nanorod structures.