Standard calculations performed by a team of researchers led by Masafumi Yamaguchi at the Toyota Technological Institute estimate that it may be possible for the photoconversion efficiency of single junction single-crystalline silicon solar cells to exceed 28%. In their article, published recently in the Journal of Materials Research, Yamaguchi and his colleagues discuss potential mechanisms that could help reach this record efficiency.

Research into new photovoltaics is motivated by a desire to meet the growing global energy demand with solar cells with higher efficiencies than now available. Silicon solar cells currently dominate the market, even though some other materials have achieved higher efficiencies, because silicon is an earth-abundant element and methods for creating silicon solar cells are well-established. Because of these advantages, “crystalline Si solar cells will continue to contribute as major solar cells in the future,” Yamaguchi says.

Despite years of development and production, many researchers are still investigating ways to improve silicon solar cell efficiency. In a solar cell, absorption of light creates charge carriers in the form of electrons and holes. Maintaining this separation until the carriers reach the electrodes of the solar cell is critical for generating electricity from the absorbed light. To make solar cells more efficient, the carriers have to be prevented from recombining. Recombination can occur inside the silicon at a grain boundary, dislocation, or crystal impurity or at the surface of the silicon. Scientists can calculate an external radiation efficiency (ERE) from a measurement of the external quantum efficiency to assess how much these parasitic recombination events hinder the overall efficiency for a given solar cell. “It’s clear that if we are able to reduce recombinations, we increase the ERE,” says Martin Hermle of Fraunhofer Institute for Solar Energy Systems (ISE). Hermle also conducts research into high efficiency silicon solar cells and is not affiliated with the research published by Yamaguchi and his colleagues.

Yamaguchi’s calculations demonstrated that silicon-based solar cells can become even more efficient by focusing on improving the ERE. Currently, most solar cells have EREs at or below 1% and the field is trying to figure out why this is the case and how to improve it. His calculations showed that by increasing the ERE to 20%, single-crystalline silicon solar cells could have an overall efficiency of 28.5%. Yamaguchi sees 20% ERE as an achievable target because it is similar to what has been demonstrated recently in gallium arsenide-based solar cells. An efficiency above 28% also requires a carrier lifetime greater than 30 ms inside the solar cell after processing. Achieving a similar ERE in single-crystalline silicon solar cells could still prove challenging for researchers.

To achieve this higher efficiency, Yamaguchi suggests improving the crystal quality of single-crystalline silicon solar cells by decreasing the number of defects and dislocations. Crystalline silicon solar cells often contain carbon and oxygen impurities from the fabrication process, which increase the non-radiative recombination. Dislocations are also present in multi-crystalline silicon solar cells; reducing them could make these types of solar cells more efficient as well.  

To achieve higher crystal quality in silicon solar cells, Hermle suggests that researchers could focus on improving processing methods. “Solar cells, especially silicon, are extremely sensitive to impurities introduced during processing,” Hermle says. Improving processing methods could occur in one of two ways: either steps can be chosen that do not impact the crystal quality or steps can be added to improve the quality of an imperfect material. For example, contaminants inside a silicon crystal could be driven to the surface and removed through etching before fabricating the structures needed to turn it into a photovoltaic device.

Beyond silicon solar cells, Yamaguchi says his approach could be generalized to other types of solar cells to guide progress in photovoltaic research. He hopes that the development of high-efficiency solar cells and photovoltaic devices will help other growing energy markets, such as electric cars.

Read the abstract in the Journal of Materials Research.