Overcoming efficiency limitations in sustainable Cu2ZnSn(S,Se)4 based solar cells: new approaches for old problemsThin film photovoltaic (PV) Cu2ZnSn(S,Se)4 (CZTSSe, kesterite) based technology needs to overcome in the near future the current efficiency limitations to move towards large scale industrialization. Currently, the record conversion efficiency of kesterites (12.6%) [1] barely exceeds half of the values reported for Cu(In,Ga)Se2 and CdTe (higher than 22%) [2]. The main challenge for CZTSSe solar cells, currently under intensive research, is the large Voc deficit, which is remarkably higher in comparison to well stablish solar cell technologies. In this work, we will review the most relevant aspects of CZTSSe as absorber material for photovoltaic applications, describing the main conversion efficiency limitations, including: Cu/Zn disorder, formation and characteristics of deep defects, nature and passivation of grain boundaries and surfaces, secondary phases formation and distribution, band-gap and compositional fluctuations, and origin of tail states [3]. In view of these restraints, advanced technological solutions will be presented, as a pathway towards a cost-efficient and sustainable thin film PV technology based on kesterites.We will introduce the positive effect of a Ge nanolayer on the morphological and electrical properties of kesterites that has been recently discovered by IREC (see Figure 1) [4-6]. Other technological solutions will be discussed in detail covering different aspects such as the accurate control of the group IV elements composition, the management of the Cu/Zn disorder, the fine-tuning of grain boundaries properties and the incorporation of advanced concepts like front-back band-gap grading control. Finally, the perspectives to catch up with the high efficiencies of other thin film PV technologies will be presented together with the requisites to reach this challenging objective.[1] W. Wang, M. T. Winkler, O. Gunawan, T. Gokmen, T. K. Todorov, Y. Zhu, B. B. Mitzi. Advanced Energy Materials 4 (2014) 1301465. [2] M. A. Green, K. Emery, Y. Hishikawa, W. Warta, E. D. Dunlop. Progress Photovolt.: Res. Appl. 23 (2015) 805-812.[3] T. J. Huang, X. Yin, G. Qi, H. Gong. Phys. Status Solidi RRL 8 (2014) 1-28.[4] S. Giraldo, M. Neuschitzer, T. Thersleff, S. López-Marino, Y. Sánchez, H. Xie, M. Colina, M. Placidi, P. Pistor, V. Izquierdo-Roca, K. Leifer, A. Pérez-Rodríguez, E. Saucedo. Adv. Energy Mater. 5 (2015) 1501070.[5] S. Giraldo, M. Neuschitzer, M. Placidi, P. Pistor, A. Perez-Rodrıguez, E. Saucedo. IEEE J. Photovolt. 6 (2016) 754-759.[6] S. Giraldo, T. Thersleff, G. Larramona, M. Neuschitzer, P. Pistor, K. Leifer, A. Pérez-Rodríguez, C. Moisan, G. Dennler, E. Saucedo. Progress. Photovolt.: Res. Appl. (2016) DOI: 10.1002/pip.2797.