Abstract :
[en] Hybrid organic-inorganic perovskites (HOIPs) are the trending materials when discussing
solar cells. Their power conversion efficiency went from 3.8% to 25.5% in twelve years,
making them extremely promising, especially when combined with silicon in a tandem
configuration. This improvement has been achieved by finding the best candidates for
charge extraction and by interface engineering, compositional engineering and surface
passivation. However, the surface of the HOIPs is still not well understood, and the
role of grains boundaries for example is still highly debated. Determining the intrinsic
surface properties of HOIPs is therefore crucial to find the best passivation strategies or
fabrication designs to limit the surface and interfacial losses, and further improve solar
cell efficiencies. Currently, solution-based processes are the most used techniques for
fabrication, even though their upscalability towards commercialization is highly unlikely,
and the use of solvents, sometimes toxic, considerably alters the perovskite surface, which
makes the interpretation of their characterization challenging and sometimes misleading.
The aim of this thesis is to clarify the intrinsic surface properties of HOIPs, and mainly
CH3NH3PbI3 (or MAPbI3), by using surface-sensitive techniques such as scanning tunneling
microscopy and spectroscopy (STM and STS) and Kelvin probe force microscopy
(KPFM). To that end, HOIP thin films are mainly fabricated by thermal co-evaporation
to achieve pristine surfaces, and inert-gas transfer is used to avoid contamination before
their characterization.
The lateral variations of the local density of states of MAPbI3 and mixed halide HOIPs
are investigated. The grain-to-grain and facet variations are linked to different density of
surface states that pin the Fermi level at the surface, and different workfunctions (WF),
which are both attributed for MAPbI3 to different surface terminations, and for the mixed
HOIPs to an additional degradation of the perovskites.
The effect of varying the methylammonium iodide (MAI) content, via the partial pressure,
in co-evaporated MAPbI3 is studied and the excess of MAI proves to be detrimental,
as it introduces low-dimensional perovskites and stacked perovskite sheets that considerably
reduce its intrinsic stability. Therefore near-stoichiometric conditions are preferred
and yield films more stable to light and heat and without photostriction.
Nevertheless this intrinsic stability is still not optimal, and the continuous variations
of the WF measured by KPFM upon prolonged illumination is investigated. Combined
with X-ray photoelectron spectroscopy (XPS), the photo-induced degradation of MAPbI3,
and evaporation of I2 are revealed as the causes of these variations. Besides, by combining
KPFM and photoluminescence (PL) techniques for different thicknesses and substrates,
energy band diagrams can be drawn and unveil a bending of the bands in the bulk.
Lastly, the surface sensitivity of HOIPs is investigated when they are intentionally put
in contact with extrinsic factors such as oxygen and solvents, and the surface properties
are shown to be considerably altered. In addition, passivation strategies are used to
demonstrate how surfaces can be improved.
