Oak Ridge National Laboratory’s Jun Xu a member of ORNL’s Chemical Sciences Division has boosted the light-to-power conversion efficiency of photovoltaic solar cells by nearly 80 percent with a new method to build photovoltaic solar cell.

Xu said he and his team, “designed the three-dimensional structure to provide an intrinsic electric field distribution that promotes efficient charge transport and high efficiency in converting energy from sunlight into electricity.”

This new technology development substantially overcomes the problem of poor transport of charges generated by solar photons. The charges of negative electrons and positive holes typically become trapped by defects in bulk materials and where the material interfaces – that degrades the performance.

Nanocone Photovoltaic Solar Cell Diagram. Click image for more info.

Xu explains their design, “To solve the entrapment problems that reduce solar cell efficiency, we created a nanocone-based solar cell, invented methods to synthesize these cells and demonstrated improved charge collection efficiency.”

The new nanostructure for solar cells consists of n-type nanocones surrounded by a p-type semiconductor. The n-type nanoncones are made of zinc oxide and serve as the junction framework and the electron conductor. The p-type matrix is made of polycrystalline cadmium telluride and serves as the primary photon absorber medium and hole conductor.

In the lab applying the structure to testing and comparing to a planer cell of the same materials Xu and the team colleagues were able to obtain a light-to-power conversion efficiency of 3.2 percent compared to 1.8 percent efficiency.

The team is answering the main challenges with key features of the solar material including its unique electric field distribution that achieves efficient charge transport; the synthesis of nanocones using inexpensive proprietary methods; and the minimization of defects and voids in semiconductors. The latter provides enhanced electric and optical properties for conversion of solar photons to electricity.

The newly designed efficient charge transport of the new solar cell can tolerate defective materials and reduce cost in fabricating next-generation solar cells.  How the technique will transfer to other popular materials could be the next item in the team’s list.

Xu said, “The important concept behind our invention is that the nanocone shape generates a high electric field in the vicinity of the tip junction, effectively separating, injecting and collecting minority carriers, resulting in a higher efficiency than that of a conventional planar cell made with the same materials.”

The design looks good, the team’s research that forms the foundation of this technology was accepted by this year’s Institute of Electrical and Electronics Engineers photovoltaic specialist conference and will be published in the IEEE Proceedings. The papers are titled “Efficient Charge Transport in Nanocone Tip-Film Solar Cells” and “Nanojunction solar cells based on polycrystalline CdTe films grown on ZnO nanocones.”  (Abstract Links Only)

As more skill and manufacturing applications make mass scale production possible this kind of research is going to gain ever more importance.  A 3.2% conversion factor of itself isn’t a great headline, but boosting from 1.8% by 80% is.

It may be that other materials won’t boost so much, or they boost even more.  If the technique can make it to printed cells and other and yet to be designed building methods, photovoltaic might get more consumer acceptance. Costs are going to have to fall; robust construction for wider range has to come, and net output needs to grow.  The market can’t wait for decade and longer paybacks.

But the technology gains are closing up the gap steadily and surely.


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