A group of researchers at the University of Michigan has developed an organic solar cell with a transparency of 40% and an efficiency of around 10%.
Conceived for applications in PV windows, the non-fullerene cell was fabricated with the polymer PCE-10 as a donor, and a near-infrared (NIR) absorbing, non-fullerene acceptor (NFA) known as BT-CIC, in an inverter design acceptor-donor-acceptor. “Non-fullerene acceptors cause very high efficiency but contain weak bonds that easily dissociate under high energy photons, especially the UV [ultraviolet] photons common in sunlight,” said researcher Yongxi Li, noting that these photons affect the device's durability.
In order to solve this issue, the research team needed to prevent UV photons hitting the cell and, for this purpose, they placed a zinc oxide (ZnO) layer on the front side of the device. “The ZnO layer (of around 30nm) was spin cast from a ZnO precursor solution onto the substrates and then thermally annealed at 150 degrees Celsius for 30 minutes in air,” the scientists explained.
This layer is able to conduct the solar-generated electrons to the electrode, thus mitigating the issue with UV photons but, at the same time, it affects the performance of the self-assembled monolayer used in the device, and a layer of a carbon-based material called IC-SAM had to be added as a buffer. Furthermore, a layer made of fullerene material known as C70 was applied at the electron and hole transporting layer interfaces. “To prevent chemical and morphological changes at organic/inorganic interfaces over time, buffer layers are inserted between the bulk-heterojuncton and charge transporting layers for improving the stability of the contact interface,” the scientists explained, noting that the cell was also encapsulated in a ‘glovebox' filled with ultra-high purity nitrogen.
The solar cell was tested under one-sun intensity with AM1.5 G irradiation for 1,900 hours at 55 degrees Celsius and lost only 6% of its efficiency, which shows, the U.S. group stated, that the device may still achieve an efficiency of 80% at the end of an estimated lifetime of 30 years.
The research team believes that the cell efficiency may be raised to 15% with a higher degree of transparency and that the device may be further improved by preparing the used materials as liquids. “Our results show that NFA-based solar cells have the potential to meet the market needs of high reliability, in addition to their high efficiency and potentially low manufacturing costs,” they concluded.
A completed description of the cell and the manufacturing method can be found in the paper Non-fullerene acceptor organic photovoltaics with intrinsic operational lifetimes over 30 years, published in nature communications.
The transparent organic solar cell was initially unveiled in 2017, when the research group said the materials used needed to adjust to the ultraviolet and near infrared wavelengths which are processed into electricity in the device. “Highly transparent solar cells represent the wave of the future for new solar applications,” said researcher Richard Lunt at the time. “We analyzed their potential and show that by harvesting only invisible light, these devices can provide a similar electricity-generation potential as rooftop solar, while providing additional functionality to enhance the efficiency of buildings, automobiles and mobile electronics.”
The group estimated that up to 7 billion square meters of glass surface in the U.S. could be covered by the material, with the potential of supplying 40% of energy demand in the U.S.
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