Photophysics of Lead-Halide Perovskite
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Abstract
The Sun provides Earth with a greenhouse gas free source of energy that exceeds the
annual energy consumption by orders of magnitude. Current solar cell technology limits
the percentage of harvested solar energy that can be used to generate electricity; this
is why new concepts and materials for photovoltaics are being explored. Lead-halide
perovskite solar cells offer the chance for low-cost high-performance devices. As a result of
the rapid technological development, perovskite solar cells have reached power conversion
efficiencies exceeding 22% at the laboratory stage, which is comparable to established
thin-film technologies. The success of lead-halide perovskites is based on the impressive
charge transport and remarkably low recombination rates for a solution-processable
semiconductor along with high optical absorption. Low transport and recombination
losses in combination with a thin light absorbing layer are essential for a high-performance
and low-cost solar cell. Despite their success in numerous optoelectronic devices, some of
the most fundamental material and physical properties of lead-halide perovskites are still
under intense debate.
The work included in this thesis is dedicated to the investigation of the photophysical
properties of lead-iodide perovskites. The photophysics is discussed for three
processes, here introduced in order of their occurrence, following photo-excitation of the
material: the formation of polarons on a sub-picosecond timescale, the slow cooling of
charge carriers on the order of hundred picoseconds and the radiative recombination in
the nanosecond time range. The first chapters introduce solar energy, the theoretical
background, and methodology followed by a summary of the experimental work performed
during this thesis work. Using time-resolved THz spectroscopy, the formation
of polarons was observed and quantified. Polarons in tetragonal CH3NH3PbI3 and
CH(NH2)2PbI3 are formed within 0.4 picoseconds independent of the temperature. The
formation of polarons resolves fundamental questions of the origin of the moderate
charge carrier mobility and low radiative recombination. Along with the formation of
polarons, charge carriers cool on two different timescales. The majority of the charge
carriers cools within a few picoseconds. A smaller fraction of charge carriers cools much
slower, requiring tens of picoseconds to reach the band minima, two orders of magnitude
slower than in GaAs. The slow cooling of charge carriers was investigated using transient
absorption spectroscopy. The recombination of charge carriers was investigated on two
different timescales using time-resolved photoluminescence spectroscopy. Concomitant to
the slow cooling of charge carriers, high-energy photon emission with a photon energy in
excess of up to 0.15 eV compared to the band edge emission was observed. The influence
of different processing techniques to the recombination of charge carriers was investigated
on a nanosecond timescale. As a result of the different processing conditions, the density
of trap states changes along with the morphology of the perovskite film.
These results provide new insights into the fundamental properties, such as polaron
formation and carrier cooling in lead-halide perovskites, and link the extrinsic
characteristics, for instance trap density, to photophysical processes like the radiative
recombination.