Belgian researchers IMEC have taken another step towards mechanically-stacked triple junction solar cells, which are expected to eventually become the most efficient available.
In a triple junction cell, each junction is tuned to absorb a different part of the solar spectrum, with the upper layers designed to be transparent to wavelengths absorbed in the lower layers.
This broad coverage of the spectrum is why high efficiencies are obtained.
However, with monolithic (epitaxially-grown) construction current flows vertically through the stack, with the lowest current cell limiting the output of the whole stack.
See also: IMEC reveals solar cells with 18.4% conversion efficiency
Mechanical stacking allows electrical isolation between layers and separate connections to be made to each cell.
"We have succeeded in transferring this GaAs top cell onto a Ge bottom cell, creating a mechanical stack," said research lab IMEC. "In that stack, the Ge bottom cell is separately contacted. It has a potential efficiency of 3-3.5%, which is higher than Ge bottom cells in state-of-the-art monolithically stacked InGaP/(In)GaAs/Ge cells."
This is the first promising demonstrator of IMEC stacking technology, which it predicts will produce cells with efficiencies above 40%.
At the top of the stack is a one-side contacted GaAs top cell that is 4µm thick and that is transparent for infrared light.
Its efficiency is 23.4%, which is close to the efficiency of standard GaAs cells.
Team manager Giovanni Flamand expects to show IMEC's first stacked triple-junction cell at the beginning of next year.
The lab expects mechanically stacked InGaP/GaAs/Ge triple-junction cells to have conversion efficiencies 1-2% higher than those obtained today with monolithic triple-junction solar cells which is over 40% with concentrated illumination.
"Mechanical stacks are more complex to handle and interconnect. But they definitely offer a way to increase the conversion efficiency and energy yield of high-efficiency solar cells," said IMEC photovoltaics program director Dr Jef Poortmans. "And they also enable an efficient way to try and use new combinations of materials."