Bifacial perovskite solar cells achieve 93% backside efficiency

A bifacial perovskite solar cell, which allows sunlight to reach both sides of the device, could produce higher energy yields at lower overall costs, according to scientists at the US Department of Energy’s National Renewable Energy Laboratory (NREL). The dual nature of a bifacial solar cell enables the capture of direct sunlight on the front and the capture of reflected sunlight on the back, allowing this type of device to outperform its monofacial counterparts.

Kai Zhu, lead author of the research paper published in the journal Joule, said this perovskite can operate effectively on either side. Past bifacial solar cell research has yielded devices considered inadequate in comparison to monofacial cells, which have a current record of 26% efficiency. Ideally, a bifacial cell should have a front-side efficiency close to the best-performing monofacial cell and a similar back-side efficiency.

The researchers were able to make a solar cell where the efficiency under illumination of both sides is similar. The lab-measured efficiency of the front illumination reached above 23%. From the back illumination, the efficiency was about 91–93% of the front. Before constructing the cell, researchers relied on optical and electrical simulations to determine the necessary thickness. The perovskite layer on the front of the cell had to be sufficiently thick to absorb most of the photons from a certain part of the solar spectrum, but a perovskite layer that is too thick can block the photons. On the back of the cell, the NREL team had to determine the thickness of the rear electrode to reduce resistive loss.

According to Zhu, simulations guided the design of the bifacial cell, and without that assistance the researchers would have had to experimentally produce cell after cell to determine the ideal thickness; for a perovskite layer, this is around 850 nanometres. By comparison, a human hair is approximately 70,000 nanometres. To determine the efficiency gained through bifacial illumination, the researchers placed the cell between two solar simulators. Direct light was aimed at the front side, while the back side received reflected light. The efficiency of the cell climbed as the ratio of reflected light to the front illumination increased.

While a bifacial perovskite solar module would cost more to manufacture than a monofacial module, over time they could end up being better financial investments because they generate 10–20% more power.

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