Abstract:
Tandem solar cells consist of two individual solar cells, stacked on top of each other. Using two absorbers with different band gap energies, they permit to reduce both, transmission and thermalization losses. Recently, especially silicon/perovskite tandem solar cells are widely investigated in the research community with power conversion efficiencies (PCE), currently close to 30 %, thus significantly above the respective single junction devices. The PCE crucially depends on the charge carrier transport across the interfaces, which is in a first approximation determined by the energy level alignment of the materials forming the interface and can be analysed by photoelectron spectroscopy (PES). In this thesis, a special variant of PES is applied: The energy of the incident photons is varied between 3 and 7 eV, while the photoelectron yield at one constant final state, thus a specific kinetic energy, is measured (constant final state yield spectroscopy, CFSYS). This method combines two main advantages: A high dynamic measurement range of up to seven orders of magnitude, which allows for the detection of very low densities of states in the valence band region, and a density of defect states in the band gap down to around 10^15 eV-1cm-3; furthermore, a high information depth of the photoelectrons of around 5 - 10nm is achieved, which enables the observation of buried interfaces and e.g. the direct determination of valence band offsets at heterojunctions. In this thesis, two interfaces between a photovoltaic absorber and a charge carrier selective contact layer are investigated. Firstly, based on the investigation of indium tungsten oxide (IWOx) as hole selective contact for n-doped crystalline silicon, the opto-electronic properties of IWOx upon high temperature annealing up to 700 ◦C are investigated. Thereafter, the focus of the work is on the investigation of the characteristically low density of states at the valence band maximum of methylammonium lead iodide and closely related perovskite compositions. By applying a model which describes both, a parabolic valence band maximum and an exponential band tail in one single equation, two valence band maxima can be distinguished in one angle-integrated CFSYS spectrum. Furthermore, the influence of different illumination (X-ray, UV) and environmental (air exposure, vacuum storage) conditions on the defect density of the perovskite is discussed. Based on the valence band modelling of the perovskite, the interface towards the fullerene C60 as electron transport layer is investigated in detail. Hereby, especially, the effect of a 1nm thin lithium fluoride interlayer between the perovskite and the C60 on the considered interface of perovskite/LiF/C60 is investigated. With the LiF interlayer an enhanced defect density is observed in the first monolayers of the C60. However, this is overcompensated by a reduced hole density at both sides of the interface, which decreases the non-radiative charge carrier recombination.