Development of High Efficiency Four-Terminal Perovskite-Silicon Tandems
Date
2017
Authors
Duong, The
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Abstract
This thesis is concerned with the development of high
efficiency four-terminal perovskite-silicon tandem solar cells
with the potential to reduce the cost of solar energy. The work
focuses on perovskite top cells and can be divided into three
main parts: developing low parasitic absorption and efficient
semi-transparent perovskite cells, doping perovskite materials
with rubidium, and optimizing perovskite material’s bandgap
with quadruple-cation and mixed-halide. A further section
investigates the light stability of optimized bandgap perovskite
cells. In a four-terminal mechanically stacked tandem, the
perovskite top cell requires two transparent contacts at both the
front and rear sides. Through detailed optical and electrical
power loss analysis of the tandem efficiency due to non-ideal
properties of the two transparent contacts, optimal contact
parameters in term of sheet resistance and transparency are
identified. Indium doped tin oxide by sputtering is used for both
two transparent contacts and their deposition parameters are
optimized separately. The semi-transparent perovskite cell using
MAPbI3 has an efficiency of more than 12% with less than 12%
parasitic absorption and up to 80% transparency in the long
wavelength region. Using a textured foil as anti-reflection
coating, an outstanding average transparency of 84% in the long
wavelength is obtained. The low parasitic absorption allows an
opaque version of the semi-transparent perovskite cell to operate
efficiently in a filterless spectrum splitting perovskite-silicon
tandem configuration. To further enhance the performance of
perovskite cells, it is essential to improve the quality of
perovskite films. This can be achieved with mixed-perovskite
FAPbI3/MAPbBr3. However, mixed-perovskite films normally contain
small a small amount of a non-perovskite phase, which is
detrimental for the cell performance. Rb-doping is found to
eliminate the formation of the non-perovskite phase and enhance
the crystallinity of the films. Rb-doping is studied under
different excess PbI2 concentrations and the optimal condition is
found to be 5% Rb-doping and 15% excess PbI2 concentration. The
addition of more than 10% Rb results in the formation of an
unwanted Rb-rich phase due to the significant lattice mismatch
between Rb and FA/MA cations. An efficiency of 18.8% is achieved
for the champion cell as compared to 16% with control cells.
Importantly, Rb-doping improves the light, moisture and thermal
stability of perovskite cells. The optimal bandgap of the
perovskite top cell in perovskite-silicon tandems is between 1.7
eV and 1.8 eV. A quadruple-cation Rb/Cs/FA/MA mixed-halide I/Br
perovskite composition is explored to obtain high quality
perovskite films with a bandgap of 1.73 eV. The ratio between
Cs/FA/MA cations is critical to the morphology, crystal
orientation and electronic properties of perovskite films.
Furthermore, 5% Rb-doping enhances the crystallinity and
suppresses defect migration in the films. Semi-transparent cells
with efficiencies up to 16% and negligible hysteresis are
achieved using this material. With excellent transparency and
optimal bandgap of the semi-transparent perovskite cell, a record
four-terminal mechanically stacked perovskite-silicon tandem
efficiency of 26.4% is achieved.
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solar cells, perovskite, silicon, tandem, energy, material, engineering
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Thesis (PhD)
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