Monolithic Tandem Perovskite/Silicon Solar Cells Set New World Record in Solar Power Efficiency

Achievement date: 
2017
Outcome/accomplishment: 

Researchers at the NSF-funded Engineering Research Center (ERC) for Quantum Energy and Sustainable Solar Technologies (QESST) at Arizona State University (ASU), in collaboration with Stanford University (SU), set a new world record of 23.6 percent efficiency for a novel tandem perovskite/silicon monolithic solar cell. The National Renewable Energy Laboratory certified the new solar cell efficiency record, which exceeds the previous record by 2.4 percent absolute.

Impact/benefits: 

Once they exceed 30 percent efficiency, perovskite/silicon tandem modules have the potential to dramatically level the cost of solar electricity at a module cost below one hundred dollars per square meter. The record-breaking tandem solar cell from QESST brings this possibility and widespread commercialization closer to reality as the perovskite cell withstood industry-standard reliability tests that have caused failure in previous, less robust cells.

Explanation/Background: 

Module efficiency remains a primary driver in the photovoltaics market, but the silicon cells that comprise 90 percent of the modules in the market are approaching their theoretical single-junction efficiency limit. Tandem solar cells that use silicon as a bottom cell are an attractive route to further increase efficiency with potentially minimal cost increase, but excellent top-cell candidates have not existed historically.

QESST researchers have been tuning silicon solar cells for maximum performance in the infrared portion of the spectrum. The team paired silicon cells with perovskite top cells fabricated by Stanford researchers to enable their record-breaking tandem cell. This pairing consists of inexpensive ingredients and crystals that can be grown easily at low temperatures.

Silicon absorbs reddish light photons well, while low-grade perovskites easily absorb those that are blue and green. When paired, the perovskite/silicon tandem cells are optimized to efficiently capture different parts of the solar spectrum. The perovskite top cell maintained its efficiency during 1000 hours of constant illumination while exposed to ambient atmosphere, and during 1000 hours of damp-heat testing after encapsulation. Results were published in Nature Energy, publicly announced at a meeting of the Materials Research Society (MRS), and noted in the “Solar cell efficiency tables (version 49).”

Innovations that brought the tandem to its record-setting performance include a new perovskite composition utilizing cesium and formamidinium that enhances stability to heat; a thin metal oxide layer at the front of the perovskite cell that renders the absorber tolerant of transparent conductive oxide sputtering; a silicon cell with a polished front surface compatible with perovskite spin coating and a textured rear surface that enhances light scattering; and an excellent reflector at the rear of the silicon cell that employs a novel nanoparticle coating. Additional challenges for pursuing perovskite adoption include addressing improvements to quality, durability, and safety.