- Background and Motivation
Developing a new generation of low-cost and high performance photovoltaic solar cells requires both an strong light trapping/absorption and a high quality material to minimize carrier recombination. Radial p-i-n thin junction solar cells realized conformally on silicon nanowires (SiNW) are considered as a promising approach. A thin absorbent layer is designed to facilitate carrier separation, while the overall absorption of incident light is compensated and even enhanced by the significant light trapping effects among the SiNWs forest.
We have developped this strategy by combining the mature thin film technology and our newly-developed all-in-situ SiNWs fabrication techniques [link to nanowire growth paper]. The chosen Sn or Indium catalysts avoid the mid-bandgap recombination centers introduced otherwise by commonly used Au catalyst. By achieving effective morphology control and conformal thin film covering on the SiNWs structure, we are able to demonstrate SiNW-based solar cells with strongly enhanced light trapping effects, high short circuit current (Jsc=16.3 mA/cm2) and reasonable open circuit voltage (Voc=0.79 V) for an a-Si:H base absorber. This leads to a new record performance of the SiNWs solar cell based on randomly-oriented SiNWs arrays grown on low-cost substrate.
To optimize and engineer the light trapping by the SiNWs forest we use an in-situ spectroscopic ellipsometer module (MM-16) integrated with the PECVD system, to investigate the evolution of optical response of the growing SiNWs. This provides a useful and convenient diagnostic support to our all-in-situ fabrication strategy.
Last but not least, optical modeling in this disordered media is developed in order to understand/optimize light aborption and thus increase the short circuit current of the solar cells, while keeping a high carrier collection.