Tandem solar cells
The previous section showed that a single junction solar cell makes poor use
of the solar spectrum for two reasons; first, photons with energies
less than the bandgap do not contribute at all. Second, each photon with
higher energy contributes one electron to the current, however, all the
energy exceeding the bandgap is lost. For a better use of the photon energies
we could try to use different semiconductor materials and guide each photon
into an absorber where the bandgap matches the photon energy.
The simplest structure of this kind is the so called tandem where two
absorbers are stacked. For the combination of a high and a low bandgap material
the illumination should first strike the absorber with the high bandgap because
there light with high energy will be absorbed with a high output voltage.
Furthermore, this material will be transparent for low energy
light wich can be passed on to the second absorber with the lower bandgap.
Technologically the fabrication of tandem devices is quite demanding because the
extraction of the currents is not trivial. Basically two designs exist, the
monolithically integrated tandem and the mechanically stacked tandem. The
basic designs are shown in the figure below.
Figure 1: Schematic design of monolithic (left) and stacked (right) tandem devices.
Irradiation enters from the top into a high bandgap absorber for high energy photons
("blue light"). Light with lower energy is passed through to a low bandgap absorber (red).
The monolithic approach (left) requires a transparent tunneling junction. The stacked
devices (right) consist of two isolated cells, here a thin film design for the
combination of a superstrate and a substrate solar cell.
Stacking of indvidual solar cells into a tandem structure seems plausible and
straightforward. One would usually contact the top and the bottom cells separately
which is the reason why these tandems are also called four terminal devices. Due to
the separate connections of top and bottom cells they do not require current matching
which makes the combination of bandgaps quite flexible. An estimation of the maximum
efficiencies in dependence on the two involved bandgaps is shown below. The calculation
is similar to the case of the single cell of the previous section; it was assumed
that the top cell absorbs all photons with energies higher than the bandgap and
is ideally transparent for sub-bandgap light. Thus, the illumination of the bottom
cell is through the top cell. Contributions due to radiative recombination in
the top cell were omitted for simplicity.
Figure 2: Efficiency map of a four terminal tandem device. Efficiencies of more than
30 % are expected for a broad range of bandgaps.
A problem of stacked cells is the requirement of transparent contacts. Tandems
based on thin film technology were first manufactured by combining
an a-Si top module and a CuInSe2 bottom device .
Other top absorber materials still require the development of transparent
back contacts. This was shown for the case of CdTe solar cells  whereas
transparent CuGaSe2 superstrate solar cells require still some research
(some of the authors contributions toward this goal are presented on this website).
In the area of crystalline devices stacking is taken a big step ahead; usually it
means putting a monolithic (In,Ga)P/GaAs tandem on top of a GaSb booster cell
or even another monolithic (Al,Ga)(As,Sb) tandem. The resulting triple or quadrupel
cells show efficiencies well above 30%.
In monolithically integrated tandems the two cells are series connected. For
a given spectral distribution the absorber thicknesses must be adopted
to yield equal currents. Spectral changes during the day or throughout
the year (see the chapter about illumination) may give rise to spectral
mismatch. In this case the subcell with less current acts as load on the
second cell and the overall efficiency is lowered.
Figure 3: Efficiency map of a two terminal tandem device. The range for high
efficiencies is narrower because the condition of current matching must be met.
High efficiency tandem cells are fabricated from crystalline materials where the
required flexibility in band gap energies is best met by the III-V
semiconductors. Lattice matched (In,Ga)P and GaAs yield the highest
reported efficiencies. These materials can be grown epitaxially on
Ge single crystal substrates. In a typical design of manufacturer
Germanium is not only used as substrate for the
epitaxy but also serves as IR sensitive bottom cell in a
triple junction device. Single crystal substrates and epitaxial
processes make these cells quite expensive, mostly they are used
for space applications.
Monolithic integration also established in thin film
technolgy. Amorphous and microcrystalline silicon with their respective
bandgaps of 1.8 and 1.1 eV form an ideal pair. Such cells are often called
micromorph tandems which first introduced by the PV-lab of the University of Neuchtel
(since 2009 part of the EPFL).
The technology was first commercialized by the
Purely amorphous tandem cells are fabricated from
a-Si and a-SiGe. The bandgap in the Si-Ge system can be varied in a wide range
and allows for the series connection of absorbers with three different bandgaps.
Xunlight uses this concept for
their triple junction cells on flexible substrates.
Note: As in the previous section, more accurate assumptios made in the literature will
yield different values. Based on purely thermodynamic limitations, DeVos finds a
limiting efficiency of 42.3% for a four terminal device with band gaps of 1.0
and 1.9 eV . Assuming material parameters of typical devices, Nell finds
an optimum efficiency of 39.1% for a four terminal device with bandgaps of 1.16
and 1.91 eV .
 K. W. Mitchell, D. Willett, C. Eberspacher, J. Ermer, J.
Proc. 21st IEEE PVSC 1990 p. 1481-6
 A. N. Tiwari, G. Krypunov, F. Kurdesau, D. L. Bätzner, A. Romeo,
H. Zogg, Progress in Photovoltaics, 12 (2004), p33-38
 A. DeVos, J. Phys. D 13, p839 (1980)
M. E. Nell, A. M. Barnett, IEEE Trans. Electron Dev. 34(2) (1987)