Understanding the efficiency limiting factors for solar cell devices fabricated from compound semiconductors acting as absorber layer (chalcopyrites, kesterites) is one of the main tasks in current photovoltaic research. Most of the studies concerning electronic defects are done by spectroscopic methods mostly performed using thin films from different kinds of synthesis, without any real knowledge regarding the structural origin of these defects. The chalcopyrite compounds Cu(In,Ga)Se2 used as absorber layer in high efficient thin film solar cells generally show a non-stoichiometric composition. They are copper-poor (Cu/(In+Ga) < 1), whereby the chalcopyrite type crystal structure still persists but the occupation of the cations on the specific Wyckoff sites may change and various kinds of intrinsic point defects are formed within the material. The kind and concentration of these defects strongly correlate with the electronic and optical properties of the final device and the knowledge about them is crucial to tailor high efficient photovoltaic devices made of such compounds. Distinguishing between isoelectronic species like Cu+ and Ga3+ by conventional diffraction techniques, like laboratory X-ray powder diffraction, is almost impossible. Therefore, at first neutron powder diffraction with subsequent Rietveld refinement was applied to evaluate possible cation distribution models for copper – poor CuInSe2 and CuGaSe2 by the method of average neutron scattering length [1]. To decide if Cu+ and Ga3+ in Cu-poor CuGaSe2 are ordered or partially disordered distributed within the structure, anomalous X-ray diffraction experiments were performed complementary to the neutron diffraction experiments. The results show that the main existing defects in CuyIn1-ySe1/2+y are found to be copper vacancies and InCu anti-site defects [2]. In contrast to these findings off stoichiometric CuyGa1-ySe1/2+y and Cuy(In1-xGax)1-ySe1/2+y show a different behavior. As soon as gallium is introduced into the structure copper vacancies are still the predominant defects but a large concentration of interstitial defects of type BIII i (BIII = In, Ga) was observed as well. This was firstly seen in CuyGa1-ySe1/2+y [3] and the tendency continues in Cuy(In1-xGax)1-ySe1/2+y. Kesterites (Cu2ZnSn(S,Se)4; CZTS(Se)) have newly attracted attention as absorber material in thin film solar cells. The crystal structure of CZTS(Se) can be described according to the structural model of the natural mineral kesterite (s. g. I-4) [4]. The structure of CZTS(Se) seems to accept deviations from stoichiometry [5], particularly Zn excess and Cu deficiency. Occurrence of secondary phases is favoured in these compounds because of the small existence region of the single kesterite type phase, as can be derived from the analysis of the phase diagram of the Cu-Zn-Sn-S(Se) systems. Again, distinguishing between isoelectronic cations like Cu+ and Zn2+ by laboratory X-ray powder diffraction is almost impossible. Thus neutron diffraction was applied to study the cation distribution and disorder effects in CZTS(Se). Our results show, that CuZn and ZnCu are the main intrinsic point defects in stoichiometric CZTS(Se). The derived crystal structure and the cation disorder effects are in agreement with first-principles calculations [6].