Photovoltaic thin film solar cells based on kesterite Cu2ZnSn(SxSe1-x)4 compounds (CZTSSe) have reached >12% sunlight-to-electricity conversion efficiencies [1]. The performance of these devices has been limited by a low open circuit voltage (VOC), the origin of which has been the subject of intense debate. A leading explanation is that CZTSSe suffers from extreme band tailing due to structural disorder (intrinsic point defects) accounting for a significant part of the VOC loss [2].  The kesterite type structure (space group I-4) can be described by a stacking sequence of cation layers Cu/Sn - Cu/Zn - Sn/Cu - Cu/Zn - Cu/Sn perpendicular to the crystallographic z-direction. As the most likely origin of the band tailing the disorder in the Cu-Zn plane in the kesterite structure is discussed, but also recombination of charge carriers at deep defects attributes to band tailing. Such defects are resulting from deviations from the stoichiometric composition. The best performing kesterite-based thin film solar cells were obtained with a material quite different from the stoichiometric compound, especially with a Cu-poor/Zn-rich composition [1]. Thus besides Cu-Zn disorder, formed by CuZn and ZnCu antisites, various kind of intrinsic cationic point defects (vacancies, antisites, interstitials) occur in off-stoichiometric kesterite type semiconductors. The formation of such defects is driven thermodynamically by minimizing the Gibbs free energy of the crystal. We have developed a classification of intrinsic point defects in kesterites, named off-stoichiometry types [3].  Neutron scattering – in this case elastic coherent scattering (diffraction) was applied – allows a non-destructive analysis of the crystal structure of photovoltaic absorber materials like kesterites from the surface deep into the volume of the sample. Only with the use of neutrons a differentiation between the electronic similar elements copper and zinc in the crystal structure is possible. In a systematic study based on neutron powder diffraction, applying the average neutron scattering length analysis method [4], we were able to evaluate experimentally the off-stoichiometry type and intrinsic point defect concentrations as well as the Cu-Zn disorder in kesterite-type semiconductors.  In our structural investigations we have demonstrated, that kesterite type CZTS/Se as well as Cu2ZnGeSe4 can self-adapt to Cu-poor and Cu-rich compositions without any structural change except the cation distribution [5]. This ability to accept deviations from stoichiometry is correlated to the formation of intrinsic point defects. The Cu-Zn disorder, which is based on the formation of CuZn and ZnCu antisite defects in the Cu-Zn planes at z=1/4 and ¾, occurs always, in stoichiometric as well as off-stoichiometric kesterites [6, 7, 8]. We were able to show for the first time quantitatively that the Cu-Zn disorder in kesterites causes shifts in the energy band gap giving raise to band tailing [9], a possible performance limiting parameter for thin-film solar cell devices based on kesterite-type absorber layers.