A promising, but also challenging approach for hydrogen production from renewable energy is
the so-called direct solar water splitting. In this integrated approach, a functionalised solar cell
is immersed in water forming a photoelectrochemical cell [1]. To generate both sufficient poten-
tial and current to split water into hydrogen and oxygen, the solar cell must comprise ideally two
semiconductors of different bandgaps. For high-efficiency absorbers in operation, the low stabil-
ity of the top-most semiconductor layer in contact with an electrolyte and a catalyst is currently
the biggest bottleneck with respect to achieving longer lifetime and optimised performance of the
device [2]. Therefore, to design the solid-liquid interface in a way that prevents corrosion pro-
cesses while absorbing enough light, is required fundamental understanding at an atomistic level.
Here, we use reflection anisotropy spectroscopy (RAS), a highly surface sensitive technique, to
investigate in situ the surface functionalisation procedure. During the first step, a GaAs protective
layer is chemically etched until the second layer, an AlInP window layer, is reached. RAS allows a
real-time monitoring of the etching process and the determination of the etching time with a pre-
cision in the order of a second [3]. The occurrence of Fabry-Perot oscillations on RAS transient
measurements during etching indicates a well-ordered process and provides information about
GaAs etching depth and etching rate. The second step of the surface functionalisation process
consists in a photoelectrochemical modification of the AlInP surface oxide in an aqueous RhCl₃
solution and under external illumination. Since this process currently lacks understanding and
reproducibility needs to be improved, a simpler system than AlInP-RhCl₃ , namely InP(100) in
contact with HCl, was investigated under applied potentials with in situ RAS. The InP-HCl inter-
face has demonstrated a reversible build up of an anisotropy in the cathodic region, originating
from an interface restructuring with the formation of a well-ordered, thin InCl_x surface film [4]. The
in situ monitoring of electrochemical surface passivation together with the Fabry-Perot oscillations
suggest that structural changes of the AlInP surface upon photoelectrochemical functionalisation
can be precisely controlled with RAS.