Magazine Feature

Tandem Solar: Two Layers Are Better Than One


Perovskite/ CIGS semiconductor pairing promises to boost photovoltaic efficiency

Researchers engaged in a project called Capitano (funded by the German Ministry for Economic Affairs and Energy) are combining thin-film solar modules based on perovskite semiconductors and CIGS.

combination, they think, is the key to building remarkably efficient
tandem solar cells with all the advantages of thin-film technology and
an efficiency factor that could top the 30-percent mark. The Karlsruhe
Institute of Technology (KIT), the Schwäbisch Hall-based enterprise NICE
Solar Energy, and the Centre for Solar Energy and Hydrogen Research
Baden-Württemberg (ZSW) have joined forces in this project with ZSW
acting as coordinator.

solar modules consist of two different types of modules in a layered
array that puts the solar spectrum to much better use than any single
solar cell. This combination is far more efficient. Multi-junction solar
cells' efficiency could extend beyond 30 percent, in theory. The
ceiling for single-layer silicon solar cells, for example, is 29

variants of tandem modules are now available. The perovskite solar cell
in the CIGS/ perovskite version converts the light in the visible part
of the solar spectrum into electricity. The underlying CIGS solar cell
absorbs the light in the near-infrared spectrum that penetrates the
perovskite solar cell. One of the reasons why these tandem solar cells
look to be so promising is that they could be realised as thin-film
technologies on substrates of several square meters. This would not only
boost efficiency; it would also cut costs.

on this consortium's excellent skill-set, Michael Powalla, ZSW board
member, head of the Photovoltaics division at ZSW and professor at KIT,
says, “Given the wide spectrum of skills at work in this project ranging
from fundamental science to mass manufacturing, I expect great advances
to be made in the further development of this promising technology.”
Dr. Ulrich W. Paetzold, head of the junior research group at KIT, adds,
"We are developing the next generation of highly efficient thin-film
tandem cells with an efficiency potential above 30 percent. Promising
applications include highly efficient solar modules for
building-integrated photovoltaic solutions, for example.”

The Capitano project

in July 2019, the Capitano project is to run for three years with the
German Ministry for Economic Affairs and Energy providing around €5.2
million in funding. This project aims to develop cells with higher yet
stable efficiency factors, and then combine these cells to make
efficient tandem solar modules. The industry partner NICE Solar Energy
will assess the possibility and cost of manufacturing these modules on
an industrial scale.

achieve this project's goals, the ZSW is developing CIGS modules with
an adapted bandgap and optimised surface, and investigating
semitransparent perovskite solar cells and highly efficient and
transparent modules. Eager to test industrial processes such as slot-die
coating for the perovskite layer, researchers are focusing on producing
optimised intermediate layers and transparent contact layers that have
been adapted accordingly. The results will flow into efforts to make
tandem solar cells and modules that are interconnected in monolithic
array. These scientists also want to assess the manufacturing process's
ecological impact.

is doing its part for this project by developing new materials,
processes, and prototypes for manufacturing semi-transparent perovskite
solar cells and highly transparent modules with an adapted bandgap a
high efficiency factor. The institute's researchers are particularly
interested in exploring scalable manufacturing processes such as
slot-die coating and gas-phase vacuum deposition. Its scientists are
developing a light management concept to improved light yield amid the
complex architecture of the tandem solar cells. They have also been
tasked to calculate yields.

Solar Energy GmbH is providing small CIGS solar modules from its CIGS
innovation line to underpin the other two partners' efforts to make
tandem solar modules. The enterprise will then assess these tandem
modules' suitability for manufacturing at industrial-scale, 300
megawatts capacity. A cost comparison with single-junction CIGS solar
modules is also on the company's agenda.

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