We review in this article recent developments within the framework of ab initio many-body perturbation theory aiming at providing an accurate description of the electronic and excitonic properties of pi-conjugated organic systems currently used in organic photovoltaic cells. In particular, techniques such as the GW and Bethe-Salpeter formalisms are being benchmarked for acenes, fullerenes, porphyrins, phthalocyanines, and other molecules of interest for solar energy applications. It is shown that not only the electronic properties, but also the electron-phonon coupling matrix elements, and the charge-transfer excitations in donor/acceptor complexes, are accurately described. The present calculations on molecules containing up to a hundred atoms are based on a recently developed Gaussian auxiliary basis implementation of the GW and Bethe-Salpeter formalism, including full dynamics with contour-deformation techniques, as implemented in the Fiesta code.