An international team of researchers led by University College London (UCL) has developed a semi-transparent solar cell prototype that could be used for windows that harvest energy from both the sun and light coming from inside the building.
Described in a paper in Advanced Energy Materials, the technology could help turn buildings into power generators at all hours of the day.
“Rooftops are commonly fitted with solar panels, but the vast window areas of many modern buildings remain largely untapped as an energy resource,” said senior author Dr Mojtaba Abdi-Jalebi from the UCL Institute for Materials Discovery.
“In our study, we showed it is possible to keep the window transparent so it can let light through while maintaining the solar cells’ efficiency.”
Wrap-around solar?
He said the next step would be to engineer flexible solar cells for curved structures like windows on the Shard or in cars, as well as non-rigid surfaces such as clothes or backpacks.
He also wants to build bigger solar cells than the current prototypes.
“The longer-term vision is to make semi-transparent photovoltaics as easy to integrate as a window film,” Abdi-Jalebi said.
“As the technology matures, these devices could potentially be developed into flexible films that can be applied directly onto vehicle glass, sunroofs and other transparent surfaces to generate clean electricity without major structural changes.”
The solar windows let in 30% of sunlight where ordinary glass lets in 80% to 90%.
Lead author Siming Huang, a PhD student at UCL’s Institute for Materials Discovery, said: “Another advantage of this technology is that by obscuring some of the sunlight it acts in the same way as tinted windows, saving a portion of the energy required to keep the building cool.
“This is especially important in hotter areas of the world that use a high proportion of energy on air conditioning.”
Indoor wavelengths
The team used a light-absorbing material called perovskite, which can be adjusted to absorb indoor light’s specific wavelengths.
They used computer modelling to determine the best arrangement and thickness of perovskite layers in the cell for transparency and efficiency.
This led to a perovskite layer that was 185 nanometres thick, some 500 times thinner than a human hair. In typical solar cells, the perovskite layer is three or four times thicker.
They added a molecule – 3-trifluoromethyl-1H-1,2,4-triazole – that reduced defects in the perovskite called “traps”, which can cause electrons to get stuck before their energy can be harnessed.
This molecule also helped stabilise the perovskite crystal structure to prevent degradation over time.
Reducing gold’s glitter
The researchers also engineered a transparent electrode, the part that conducts electricity out of the cell.
In perovskite solar cells, the electrode is typically made of gold and blocks light. To make their electrode more transparent, the team sandwiched a thin layer of gold between two transparent layers of molybdenum oxide, which helped light pass through the gold by reducing reflection.
The team fabricated a 30cm x 30cm panel and found the solar cells could convert 22% of bright indoor light (1,000 lux) into electricity, as well as 14% of sunlight.
Under a standard accelerated durability test, the device retained 80% of its efficiency over 300 hours of continuous exposure to light.
The work was supported by the Henry Royce Institute for Advanced Materials. The team received funding from the UK’s Engineering and Physical Sciences Research Council (EPSRC) and Department for Energy Security and Net Zero, as well as from UCL, the British Council and London South Bank University.
