The National Renewable Energy Laboratory (NREL) has produced highly bifacial perovskite cells with a front-side efficiency of 23%.
NREL’s bifacial perovskite solar cell research has resulted in 91-93 percent bifaciality, leading to the conclusion that bifacial perovskites could be a better way to get lower levelized electricity costs. It published results in Joule.
Perovskites are promising because of their low production costs, and their thinness allows them to be applied to most surfaces. Although challenges remain with the stability and longevity of perovskites, new efficiency records have been achieved every month in recent years.
Previous iterations of bifacial perovskites have struggled to keep pace with monofacial cells, which currently have a record 26% efficiency. Bifacial cells are also more expensive to manufacture than their unifacial counterparts. The NREL researchers concluded that for the technology to compete, the front-side efficiency of a bifacial cell should be close to the best-performing monofacial cell, and that the bifaciality rating should be close to 100% of the front-side efficiency.
A newly fabricated single-junction NREL bifacial cell was measured at 23% efficiency on the front side, and 91-93% efficiency level was achieved on the back side bifaciality. The research team said bifacial cells could produce 10 to 20 percent more power than unifacial cells.
Before building the record cell, the researchers used optical and electrical simulations to determine the ideal thickness. The front layer had to be thick enough to absorb most of the photons from a given part of the solar spectrum, but a perovskite layer that is too thick can block the photons.
Behind the cell, the NREL team had to determine the ideal thickness of the back electrode to minimize resistive loss. The team noted that the balance between resistive and optical losses must be considered. The transparent oxide back electrode must be thick enough to minimize resistive loss, but also thin enough to minimize parasitic absorption.
The research team found that the ideal thickness of the perovskite layer is about 850 nanometers, which is incredibly thin. For comparison, a human hair is about 70,000 nanometers thick.
Albedo, i.e. the reflectivity of the surface under the solar cell, affects the yield of the back side perovskites. The bifacial output power was measured to be 28.5 mW per square centimeter with an albedo of 0.3 and 30.1 mW per square centimeter with an albedo of 0.5.
This research was funded by the US Department of Energy Office of Solar Energy Technologies.