Solution processed perovskite solar cells

Today’s carbon-based economy will not be sustainable in the future.
Not only will the known reserves of fossil fuels, like oil, natural
gas or coal, be significantly reduced within the next 100 years,
but the continued burning of fossil fuels also emits greenhouse
gases, which have led to a global increase in temperature, called
global warming. To preserve the environment for future generations
and to prepare for the time when we will inevitably run out of
fossil fuel, we have to change the way we produce our primary
energy and focus research and investments on renewable energy
sources. While energy from wind and water is already harvested with
very high efficiencies, the utilization of solar energy still
offers big room for improvements. Although conventional crystalline
silicon cells achieve efficiencies around 25 %, their production is
very energy intensive and relies on advanced production
technologies, which makes them still rather expensive. To make
photovoltaics a major part of our energy landscape, an easily
prepared type of solar cell consisting of cheap and abundant
materials is required. Novel organometal halide perovskite-type
materials fulfill these requirements and have proven to be serious
competitors for conventional photovoltaics. After only four years
of research they already achieve power conversion efficiencies
above 20 %. This thesis introduces a fast and easy way to prepare
planar heterojunction solar cells based on methylammonium lead
iodide (MAPbI3). The photoactive layer is deposited in a 2-step
deposition approach, where a thin film of the lead precursor is
converted into the final perovskite simply by immersing it into a
solution of the other component. The resulting films consist of
individual crystals sizes a few 100 nm and covering the whole
substrate without significant gaps or holes. Solar cells prepared
by this method achieve power conversion efficiencies of 15 %.
Furthermore, by adjusting the temperature of the immersion bath,
the orientation of the perovskite crystals can be controlled. The
orientation, together with the resulting change in efficiency and
resistance, gives interesting insights into the anisotropic charge
transport properties of this class of materials. Additionally, the
conventionally used hole blocking layer, titanium dioxide, was
replaced by one made of fullerene molecules. The efficiencies
achieved by solar cells employing this kind of electron selective
contact reached almost 10 %, although the reproducibility was
initially very low. This was attributed to a partial dissolution of
the fullerene film during the subsequent preparation steps. To
increase the stability of the layer, it was photo-polymerized using
UV radiation. This not only reduces the solubility and therefore
increases the fraction of solar cells achieving high efficiencies;
it also changed the energy levels close to the bandgap. The bandgap
energy of organic lead halide perovskite materials is strongly
dependent on the composition. By exchanging some or all of the
iodide in MAPbI3 with bromide, the difference between valence and
conduction band can be changed from 1.5 eV (pure iodide) to 2.25 eV
(pure bromide). This substitution can be performed gradually, so
that phase pure materials with properties in between the two
extremes are obtained. The pure bromide MAPbBr3 perovskite,
however, does not perform efficiently in a planar heterojunction
solar cell. Its close relative based on formamidinium FAPbBr3 has
also been investigated for its suitability as active solar cell
material. Although it is structurally very similar to MAPbBr3, with
equivalent light absorption and emission properties, a 10 fold
higher efficiency was observed for the FA-based compound. This
striking difference is mainly attributed to an increased
photoluminescence lifetime, resulting in an increased diffusion
length of the free charge carriers. Apart from their application as
light absorbing materials in solar cells, perovskites have also
been investigated for their application as light emitters.
Depending on the perovskite used, it was possible to demonstrate
red light emission (MAPbI3) or green emission (MAPbBr3).