This work provides a detailed analysis of a high efficiency Cu₂ZnSnSe₄ device using a combination of advanced electron microscopy and spectroscopy techniques. In particular, a full picture of the different defects present at the interfaces of the device and in the bulk of the absorber is achieved through the combination of high resolution electron microscopy techniques with Raman, X-ray fluorescence and Auger spectroscopy measurements at the macro, micro and nano scales. The simultaneous investigation of the bulk and the interfaces allows assessing the impact of the defects found in each part of the device on its performance. Despite a good crystalline quality and homogeneous composition in the bulk, this work reports, for the first time, direct evidence of twinning defects in the bulk, of micro and nano-voids at the back interface and of grain-to-grain non-uniformities and dislocation defects at the front interface. These, together with other issues observed such as strong absorber thickness variations and a bilayer structure with small grains at the bottom, are shown to be the main factors limiting the performance of CZTSe devices. These results open the way to the identification of new solutions to further developing the kesterite technology and pushing it towards higher performances. Moreover, this study provides an example of how the advanced characterization of emergent multilayer-based devices can be employed to elucidate their main limitations.

This work presents the development of a novel chalcogenisation process for the fabrication of Cu₂ZnSn(S,Se)₄ (CZTSSe or kesterite) based solar cells that enables generating sharp graded anionic compositional profiles with high S content at the top and low S content at the bottom. This is achieved though the optimization of the annealing parameters including the study of several sulphur sources with different predicted reactivity (elemental S, thiourea, SnS and SeS₂). As a result, depending on the sulphur source employed, devices with superficially localized maximum sulphur content between 50-20 % within the charge depletion zone, and between 10-30 % towards the bulk material are obtained. This complex graded structure is confirmed and characterized by combining multi-wavelength depth-resolved Raman spectroscopy measurements together with in-depth Auger Electron Spectroscopy (AES) and X-Ray Fluorescence (XRF). In addition, the devices fabricated with such graded-band gap absorbers are shown to be fully functional with conversion efficiencies around 9 % and with improved V_OC deficit values that correlate with the presence of a gradient. These results represent one step forward towards anionic band gap grading in kesterite solar cells.

In this work, a device model for Cu₂ZnSnS₄ (CZTS) solar cell with certified world record efficiency is presented. A study of the most important loss mechanisms and its effect on solar cell performance was carried out. The trap-assisted tunneling and CdS/CZTS interface recombination are introduced as the most important loss mechanisms. Detailed comparison of the simulation results to the measured device parameters shows that our model is able to reproduce the experimental observations (quantum efficiency, efficiency, J_sc, FF, and V_oc) reported under normal operating conditions. Finally, a discussion about a further solar cell efficiency improvement is addressed.