The Energy Research Centre of the Netherlands (ECN) says along with Solliance it has achieved 30.2% ‘equivalent’ conversion efficiency for a bifacial tandem solar device. It measures this level as an equivalent because energy output of monofacial and bifacial devices is dependent on several factors as location and orientation of the PV system.
“The accepted practice to determine the equivalent efficiency for bifacial designs is measuring the solar cell from the front in standard test condition (STC) and adding another 20% of this illumination from the rear. By adopting this method, the bifacial equivalent tandem efficiency of our device is 30.2%, thus surpassing the limit of monofacial silicon solar cells,” said ECN.
To achieve this efficiency level of 30.2%, they placed a newly developed perovskite solar cell on top of an industrial bifacial crystalline silicon solar cell. This was then integrated into a novel device architecture harvesting light from both sides, resulting into a bifacial tandem solar cell. In order to make the bifacial tandem cell respond to the variable nature of the sunlight, the tandem device created by ECN and Solliance used a 4-terminal configuration, which means it has separate circuits for the top and bottom cells that allow for ‘dynamic finetuning and optimization of the energy yield’.
Gianluca Coletti, Program Manager Tandem Technology at ECN, said the technology roadmap is to work on the scalability, integration and reliability of this technology, increase the equivalent efficiency to 35% in the next 3 to 5 years and get the technology ready for mass production. ECN says Choshu Industry Co., Forschungszentrum Jülich, imec, and Eindhoven University of Technology contributed to this work.
In January 2019, China’s LONGi produced a first bifacial monocrystalline silicon PERC solar cell exceeding 24% on commercial wafer size (see LONGi 24.06% Efficiency PERC Cell World Record). Earlier this year, Solliance and Hanergy’s MiaSole Hi-Tech reported 21.5% power conversion efficiency using a perovskite solar cell on top and flexible CIGS on bottom (see 21.5% Efficiency For Flexible Tandem Cell).