Scientists propose novel bilayer structure for crystalline silicon solar cells

Researchers from the Shanghai Advanced Research Institute (SARI) of the Chinese Academy of Sciences have proposed a novel "bilayer" structure composing of different transition metal oxide (TMO) thin films in crystalline silicon (c-Si) solar cells in order to improve the cells' efficiency.  

The researchers combined NiOx and MoOx films into a bilayer structure that extracts "hole carriers" from c-Si more efficiently than single-layer films can. The results were published in Cell Reports Physical Science. 

"Hole carriers" carry a positive charge. Together with electrons, which have opposite polarity, they were created after c-Si absorbs sun light. By extracting positive holes and negative electrons from c-Si to external circuits, the sun light is converted to usable electricity – this is what a c-Si solar cell does. "Extracting" carriers from c-Si is critical and they can be realized by carrier-selective contacts (CSCs) such as TMO films.  

CSCs play an important role in improving the power conversion efficiency (PCE) of c-Si cells. Existing single-layer, thin TMO films such as MoO_x or NiO_x cannot effectively extract the desired carriers-mainly holes, thus leading to c-Si solar cells with mediocre efficiency. 

In a NiO_x/MoO_x bilayer structure, however, MoOx can induce band bending at the interface, which is favorable for hole carrier extraction. Moreover, NiO_x helps to block undesired electron carriers. This is confirmed by both band alignment simulation and minority carrier lifetime measurements. 

Taking advantages of these features, the researchers reported a remarkable PCE of 21.31% in c-Si solar cells employing NiO_x/MoO_x bilayers. 

Moreover, forming an additional ultra-thin SiO_x layer on the silicon surface can further suppress loss pathways such as recombination, etc. 

As a consequence, using an NiOx/SiOx/MoOx structure can further boost the device's PCE to 21.60%. This is the highest reported efficiency of any c-Si solar cell employing MoO_x-based hole-selective contacts instead of a costly a-Si:H passivation layer, according to the researchers. 

This study highlights a promising and robust approach to employing bilayers as efficient structures for extracting hole carriers. It serves as an inspiring guide for tackling challenges in the field of passivating contact c-Si solar cells. 

This work was supported by the National Natural Science Foundation of China, the Natural Science Foundation of Shanghai, and the Shanxi Science and Technology Department, among others.