Recent progress in X-ray spectroscopies, intensified by the development of novel radiation sources such as X-ray free electron lasers, have opened the door to time-resolved measurements on the femtosecond time-scale. Such experimental studies may be able to investigate the molecular mechanisms of ultrafast chemical and photo-induced processes, such as those involved in solar conversion. However, in order to reach a fundamental understanding of the underlying processes, accompanying theoretical considerations are imperative. In this project we will apply the algebraic diagrammatic construction (ADC) approach for modelling X-ray absorption spectroscopy (XAS), X-ray emission spectroscopy (XES), and resonant inelastic X-ray scattering (RIXS) of a set of organic molecules used in photovoltaics. ADC is an ab initio excited-state method traditionally applied to valence-excited states, but the methodology has in recent years been adapted and successfully used to describe core-excitation processes. Multiple studies where ADC is utilized for the calculation of X-ray absorption spectra can be found in the literature, and very recent (as of yet unpublished) investigations have expanded the approach to also consider XES. In addition, a scheme to determine the electronic part of RIXS amplitudes has just been implemented in our group (JCTC 2017, 13, 5552). We plan to thoroughly test these implementations on a set of small molecules, ranging from ethenes to nucleic acids. In a second step, we will compute the corresponding X-ray spectra of several pristine molecules used as building blocks and active components of solar cells. These systems include the electron donors triphenylamine (TPA), N,N-diphenyl-2-tiophenamine (DPTA), and their derivatives. These molecules constitute building blocks for donor-pi-acceptor materials used in dye-sensitized solar cells (DSSC) and organic photovoltaics (OPVs). Furthermore, we will consider some larger molecular systems, such as the electron donor molecules PPT and DTDCTB, as well as monomers of poly(3-hexylthiophene) (P3HT). Here the photodegradation of P3HT (a process which strongly reduces the power conversion efficiency of OPVs) is particularly interesting and will be addressed by calculating XAS, XES, and RIXS spectra of the monomer, and then include a wide range of possible degradation final products involving oxygen. In the case of all electron donor molecules, the theoretical XAS results will be compared to experimental gas-phase spectra measured by the group of Carla Puglia (Uppsala University) at the synchrotrons MAX IV (Lund, SE) and Elettra (Trieste, IT).