Record-Setting Photovoltaic Efficiency with Four-Junction Device

Mark Keevers with the four-junction solar mini-module built at the University of New South Wales. (Image courtesy of Rob Largent/UNSW.)

New records are continuously being set and broken as we inch closer to the theoretical limits of solar cell efficiency. The latest is an efficiency record of 34.5 percent for sunlight-to-electricity conversion in solar cells using unfocused sunlight—a 44-percent gain over the previous record.

Novel Solar Cell Arrangement

This efficiency is achieved using a 28 cm² four-junction mini-module embedded in a prism. Maximum energy is extracted from sunlight by dividing the rays into four bands. A hybrid four-junction receiver further extracts electricity from each incoming beam. A silicon cell on one face of the glass prism is combined with a triple-junction solar cell on the other face of the prism.

A 2-D diagram of how the prism spectrum splitter mini-module works. (Image courtesy of Rob Largent/UNSW.)

The triple-junction cell consists of a combination of three layers. These are indium-gallium-phosphide, indium-gallium-arsenide, and germanium. Energy is extracted by each junction at its most efficient wavelength as sunlight passes through each layer. The unused parts of the beam pass through each subsequent layer. A portion of the infrared band of incoming sunlight which is not processed by the triple-junction cell is bounced onto the silicon cell. This results in the extraction of nearly all the energy from each beam of sunlight hitting the device.

Prospective Applications for Light-Splitting Cells

This light-splitting method for solar cells is ideal for solar towers, but not yet for homes and offices. This is due to the increased amount of effort required to manufacture multi-junction solar cells, as well as their increased cost. Crystalline silicon, single-junction cells remain the more feasible option for these applications, at least for now.

The efficiency record of 34.5 percent was set by a team of engineers at the University of New South Wales in Sydney, Australia. This record is nearly 44 percent better than the previous record, set by the American company, Alta Devices, which had reached 24 percent efficiency. However, Alta Devices’ device had a larger surface area of 800 cm².

Scaling up the record-setting mini-module to 800 cm² is entirely possible, though the engineers on the UNSW team which developed the new mini-module admit that there would be some marginal loss from interconnection. This type of four-junction device is estimated to have an efficiency of 53 percent. This is two-thirds of the theoretical limit for solar cell efficiency for multiple junction solar cells.

Over USD $8 billion has been generated through Australia’s solar cell research and UNSW’s passivated emitter rear cells (PERC cells) alone are forecast to save $750 million USD in domestic electricity generation over the next ten years due to efficiency gains alone.

For further reading on the development of solar cells, check out this article on a photovoltaic manufacturing company founded by MIT graduates which emphasizes aesthetics and performance.