Imec has fabricated tandem
organic solar cells with peak conversion efficiencies of 5.15%. This was achieved
by stacking two different planar heterojunction devices, each with a high open-circuit
voltage (Voc).
The universal nature of the interconnection scheme makes it easy to incorporate
new promising materials in the tandem configuration. The screening of candidate
materials is ongoing at imec, with the focus on materials with absorption spectra
extended to higher wavelengths. The goal is to combine these new materials with
the current tandem set, creating a stack of 3 or more cells that will result
in an even broader coverage of the solar spectrum and in higher efficiencies.
Voltage-current density plots for tandem cells and single cells grown in the same run.
Organic solar cells are conventionally made from two materials: a donor and
an acceptor, which facilitates an efficient charge separation. For the acceptor,
the most commonly used molecule is one of the blue absorbing fullerenes. This
leaves the absorption spectrum of the donor material responsible to cover as
much as possible of the solar spectrum. But most organic semiconductors only
have a small optical bandwidth. Consequently, solar cells based on such materials
only catch a small part of the solar spectrum. This can be overcome with a properly
designed stacked or tandem configuration, in which several organic materials
each absorb a separate part of the spectrum.
In imec’s current configuration, two single cells are fabricated on top
of each other, connected in series to increase the total absorption. The choice
of materials is critical as the subcells should have complementary absorption
spectra. In this way, light that is not absorbed by one subcell can contribute
to the photocurrent in another subcell.
The donor materials in the subcells are phthalocyanine-based. We selected chloroaluminum
phthalocyanine (ClAlPc), subnaphthalocyanine (SubNc), and subphthalocyanine
(SubPc) as donor materials in combination with C60 as an acceptor. The materials
SubNc and ClAlPc have absorption spectra that are complementary with SubPc (580-820nm
for ClAlPc, 580-720nm for SubNc, and 470-620nm for SubPc). The energy levels
of all material pairs allow for a large Voc (0.85V, 0.82V, and 1.1V, respectively),
while the photocurrent is not limited by inefficient exciton dissociation. An
important property of the chosen phthalocyanine materials is their extremely
high extinction coefficient over a small spectral window. The small absorption
width opens possibilities to optimize the available energy after photon absorption,
as photo-generated excitons will thermally relax to the lowest excited state.
At the same time, the high absorption constant makes it possible to use very
thin layers (10-20nm) and still produce a considerable photocurrent (> 4mA/cm²).
Single cells were stacked in a tandem configuration with an all-evaporated,
novel interconnection scheme based on an electron-transport layer, metal nanoparticles
and a hole-transport layer. This configuration shows no potential drop, leading
to Voc values approaching 2V. At the same time, the fill-factor is not hampered
and efficiencies above 5% are obtained.
Tandem architectures like this, using carefully selected materials connected
in series, are a promising road to organic solar cells with higher efficiencies.
Imec is currently screening candidate materials with absorption spectra at higher
wavelengths. The goal is to combine these new materials in the current tandem,
creating a stack of 3 or more cells.