Sketch of the tandem
cell. Credit: H. Cords/HZB
(January 8, 2016) Tandem solar cells based on silicon and
perovskites have raised high hopes for future high efficiency solar
modules. A team led by perovskite solar
cell pioneer Henry Snaith at the University of Oxford has now shown, with
contributions by Bernd Rech and Lars Korte of the Helmholtz-Zentrum Berlin,
that an ultimate efficiency of 30% should be attainable with such tandem cells.
They discovered a structurally stable perovskite composition with its band gap
tuned to an optimum value of 1.75 eV. The results have been published in
"Science".
Tandem solar cells based on silicon and perovskites have
raised high hopes for future high efficiency solar modules (see also results
here). A tandem solar cell works by absorbing the high energy photons (visible
light) in a top cell which generates a high voltage, and the lower energy
photons (Infra red) in a rear cell, which generates a lower voltage. This
increases the theoretical maximum efficiency by about 50% in comparison to a
standalone silicon cell.
To maximise efficiency, the amount of light absorbed in top
cell has to precisely match the light absorbed in the rear cell. However, the band
gap of ~1.6eV of the standard perovskite material is too small to fully exploit
the efficiency potential of this technology.
A team led by perovskite solar cell pioneer Prof. Henry
Snaith FRS at the University of Oxford, in collaboration with silicon solar
cell experts Prof. Bernd Rech and Dr. Lars Korte of the Helmholtz-Zentrum
Berlin, have shown that an ultimate efficiency of 30% should be attainable with
such tandem cells.
They conceived together a tandem cell, in a configuration
where the perovskite and the silicon
cell are mechanically stacked and contacted separately. The HZB team
contributed the silicon cell. The Oxford group did vary systematically the
chemical composition of the perovskite layer, and with a precise cocktail of
ions discovered a structurally stable perovsksite with its band gap tuned to an optimum value
of 1.75 electron volts, maintaining a high electronic quality of the layer. At
the same time, they increased the chemical and thermal stability of the
material significantly.