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Abstract

This thesis examines the influence of surface functionalization by self-assembled monolayers (SAMs) on two different materials, (i) nickel oxide and (ii) gallium nitride. (i) Thin solution-processed films of nickel oxide (sNiO) were modified with dipolar self-assembled monolayers of 4-cyanophenyl-phosphonic acid (CYNOPPA) to improve the energy level alignment and the chemical compatibility to the donor in F4ZnPc:C60 organic photovoltaic devices. A detailed analysis of infrared and photoelectron spectroscopy showed the chemisorption of the molecules with a nominal layer thickness of around one monolayer. The spectroscopic results gave further insight into the chemical composition of the SAM. Density functional theory calculations were employed to reveal energetically allowed binding configurations of the phosphonate anchor group. CYNOPPA successfully increased the work function of the sNiO up to 5.1-5.2 eV, leading to a well-matched Fermi level of the hole-contact layer to the ionization potential of the donor. Unexpectedly, the improved alignment did not lead to a higher power conversion efficiency. Instead, a significant reduction of the fill factor was observed, which was assigned to the formation of a transport barrier due to a low conductive interface region. However, it was shown that the barrier can be reduced by doping the oxide hole-contact layer and thereby increase its charge carrier density. (ii) For the goal of GaN nanowire surface state passivation and control of the corresponding band bending, different GaN planes were functionalized with self-assembled monolayers. A step-by-step procedure of solvent cleaning, plasma treatment and HCl etching was developed to guarantee clean surfaces for SAM growth. X-ray photoelectron spectroscopy (XPS) measurements showed that oxygen as well as carbon contents could be reduced to a minimum with this procedure. SAM precursor with different anchoring groups were tested to find the best binding partner for the GaN surface. The SAM quality and passivation functionality was determined by Kelvin probe surface photovoltage and contact angle measurements. It was shown that amines and thiols suffer from low surface coverage on GaN(0001) under ambient conditions. Hence, an oxide template layer was grown for the attachment of phosphonic acids (PAs). Two phosphonic acids with different intrinsic dipole moments were applied to oxidized high quality GaN(0001) and GaN(1100) surfaces grown by molecular beam epitaxy. XPS and contact angle measurements revealed dense monolayer coverage of the PA-SAMs. Despite the oxide buffer layer, the band bending of the GaN was reduced by the chemisorption and molecular backbones of the PAs. Continuous-wavelength photoluminescence (cw-PL) measurements showed an increase of the total PL intensity, which suggests that a reduction of the band bending is essential for high luminescence quantum yields.