| Čoahkkáigeassu: | <p>With increasing global surface temperatures due to greenhouse gas emissions from
human activity, the global society is in urgent need of pivoting the energy system
towards low or zero-emission energy sources. Photovoltaic solar energy has the
potential to deliver substantial emission reductions and is expected to make up a major
portion of the energy mix in the remainder of the 21st century. Over the last decade,
halide perovskite materials have shown great promise in bolstering and diversifying
photovoltaic solar technologies, as well as the potential for efficient and colour-pure
light-emitting diodes.</p>
<p>Leaps of progress have been made on halide perovskite-based single-junction
and tandem solar cell technologies, but triple-junction solar cells, which theoretically
could provide an even higher power conversion efficiency, are still in the infancy. For
mainly perovskite-based triple-junctions to reach the highest realistic power conversion
efficiencies, top cell bandgaps in the range 1.95 to 2.05 eV are required. Herein, we
review the achievements and progress that have been made on the topic of perovskite
triple-junction solar cells and the important wide-bandgap (2 eV) top cell. We then go
on to characterize our FAPb(Br<sub>0.7</sub>I<sub>0.3</sub>)<sub>3</sub>-based 2.0 eV single-junction solar cells. We identify the main reasons for V<sub>OC</sub>-loss in these cells and look into the effect of halide
segregation on the performance. Finding that nonradiative recombination in the bulk
is the main cause of V<sub>OC</sub>-loss and that halide segregation may be significantly more
detrimental to the J<sub>SC</sub> than the V<sub>OC</sub>. Further, we investigate halide segregation and
degradation trends in isolated films of this 2 eV material under the influence of different
atmospheres. We observe untypical wavelength-shifts in the photoluminescence
spectrum which may be linked to iodide-depletion under light exposure.</p>
<p>Finally, we improve the performance of our green perovskite LEDs by introducing
a novel composite p-contact comprising the organic hole-transporting polymer TFB
deposited by solution processing and Al<sub>2</sub>O<sub>3</sub> deposited by atomic layer deposition.
Due to the different chemical reactivity of the trimethyl aluminium precursor of the
ALD process towards the oxide surface groups and the TFB polymer, the Al<sub>2</sub>O<sub>3</sub>
has the dual function of being able to insulate any bare patches of the underlying
transparent conducting oxide electrode, which were not adequately covered during the
TFB processing, and intergrow in the porosities of the TFB layer and cause a swelling,
further blocking any potential nonradiative recombination sites.</p>
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