Electronic properties of titania (and AZO) and its interface to organic acceptor materials

The focus of this work is on the investigation of the electronic structure at interfaces of inverted organic solar cells and energetic states in the band gap of the anatase (101) surface. Inverted organic solar cells are a promising alternative to conventional inorganic solar cells, regarding the potentially low production costs and its variety in possible applications. Inverted organic solar cells sometimes exhibit S-shaped I-V characteristics instead of diode-like I-V characteristics, which results in a decrease of the efﬁciency of the solar cells. The electronic alignment at the interface between the different materials within the solar cell inﬂuences strongly the functionality and efﬁciency of the solarcell. The contact formation between the materials depends on the materials itself, but also on the speciﬁc surface, e.g. adsorbates covering it. This work compares I-V characteristics of different solar cell device stacks with the respective energy diagrams. The investigation of the interface is mainly done by photoelectron spectroscopy, which is a powerful method to determine the electronic alignment at the interface between different materials. The shape of the I-V characteristics of inverted organic solar cells oftendepends on the used metal oxide (which acts as electron transporting layer) within the solar cell device stack. Typical electron transport layers are TiOx and aluminum doped zinc oxide (AZO). Inverted organic solar cells with PC61BM:P3HT as absorber material and AZO as electron transport layer show diode-like I-V characteristics, whereas solar cells with TiOx as electron transport layer show S-shaped I-V characteristics. Using a bilayer consisting of TiOx:AZO or AZO:TiOx as electron transport layer, only solar cells where the TiOx forms the interface to PC 61 BM show S-shaped I-V characteristics. In model experiments, C60 replaces PC61BM and the interface of C60 to TiOx and AZO is determined by means of photoelectron spectroscopy. The interface energy diagram of the TiOx/C60 interface displays a barrier for electron extraction, whereas the AZO/C60 interface does not. Interface experiments of C 60 with in situ (adsorbate free) and ex situ(adsorbate contaminated) cleaved anatase single crystals show the crucial inﬂuence of adsorbates on the formation of the electronic interface between the metal oxide and the organic absorber. Additional experiments show that the S-shaped I-V characteristics with TiOx as electron transport layer transform into diode-like characteristics upon illumination with UV light. In situ UV illumination of the TiOx/C 60 interface with UV light induces changes of the band alignment, which result in a decrease of the electron extraction barrier at the TiOx/C60 interface. Furthermore the reduction of the barrier is most probably caused by UV induced desorption of oxygen from the TiOx surface. The second part focuses on fundamental investigations of the anatase (101) surface. For this purpose, anatase single crystals are cleaved along the (101) surface plane by pliers and a clean (101) surface, without any further preparation methods such as sputtering and annealing, is obtained. Analyzing and comparing differently prepared titania samples by normal and resonant photoemission reveals the existence of shallow band gap states at the crystalline anatase surface. The origin of those shallow gap states is assigned to step edges of monoatomic height at the anatase (101) surface and respectively to the intersections of (101) surface planes of nanocrystalline anatase. In water adsorption experiments, the adsorption kinetics of water onto the anatase (101) surface are investigated in more detail.