In Photovoltaics (PV), the direct conversion of light into electricity, thin film technologies constitute an emerging alternative because of short energy payback time and minimum use of higt purity materials, addressing the urgent need for cost-competitive renewable energy technologies [1]. Compound semiconductors, like chalcopyrite type Cu(In,Ga)(Se,S)2 (CIGSe) are the most advanced and most efficient absorber materials. CIGSe-based solar cells are very well positioned in the field of PV technologies with present record efficiencies for small cells of 22.3% (production size modules 16.5%) [1].One reason for their success is related to the high flexibility of the chalcopyrite crystal structure, accepting strong deviations from stoichiometry forming native point defects, such as vacancies, interstitials and antisites [2]. But CIGSe relies on the scarce elements In and Ga, which may severely limit the mass deployment of this PV technology.In the recent years, research has been focused on developing high efficient thin film devices based on earth abundant elements, like the kesterite-type semiconductors Cu2SnZn(S,Se)4 (CZTSSe) [3]. These materials have high absorption coefficients that allow the use as active absorber layers in thin film solar cells. The best performance of kesterite-based thin film solar cells with conversion efficiency of 12.6% [4] were obtained with a material quite different from the stoichiometric compound, especially with a Cu-poor/Zn-rich composition. We have demonstrated, that kesterite type CZTSe can self adapt to Cu-poor and Cu-rich compositions without any structural change except the cation distribution [5]. Thus kesterite type CZTSe shows, like chalcopyrite type Cu(In,Ga)Se2, a structural flexibility. Again, the ability to accept deviations from stoichiometry is correlated to the formation of intrinsic point defects. Taking into account the nescessity to keep the charge balance, certain cation substitution processes lead to the formation of non-stoichiometric CZTS/Se. These off-stoichio-metry types are corrlated to certain point defects. In order to determine the cation distribution in the crystal structure and thus the point defects of the compound semiconductor, the method of the average neutron scattering length analysis has been adopted [7]. In a systematic study we have shown, that the evaluated off-stoichiometry type and the concentration of intrinsic point defects  correlate with the variation in chemical composition in CZTS/Se. Thus we revealed the possibility to deduce occuring point defects from the chemical composition of the kesterite phase. Moreover we have shown the correlation between Cu-Zn disorder and energy band gap variations.