Opto-electrical characterization and modelling of defects in thin-film Cu(In,Ga)Se2 solar cells

In this thesis, we model and characterize defects in thin-film Cu(In,Ga)Se2 (CIGS) solar cells and study the passivation effect of two atomic layer deposited dielectric materials (AlOx and HfOx) at the CIGS/CdS (front) and the CdS/TCO (window) interfaces, along with the influence of post-deposition annealing. We use admittance spectroscopy (AS) on simulated and experimental CIGS solar cells to deeply characterize electronic defects in terms of characteristic frequency, activation energy and capture cross section. This method is also applied to study other loss mechanisms, such as parasitic resistances and interface barriers. The AS theory is completed with an innovative graphical representation called loss map, enabling to identify the signatures of these loss mechanisms. On top of this, optical experiments are conducted to evaluate relevant CIGS absorbers properties. This large panel of measurements allows to interpret the evolution of solar cells performance. In particular, we show that post-deposition annealing at 150°C has a positive impact on low performance solar cells, leading to a 10% Voc improvement and to an absolute increase of 1.2% efficiency. These cells are further enhanced when a 1nm AlOx passivation layer is introduced, resulting in a 30% Voc improvement and to an absolute increase of 4% efficiency compared to the reference. Opposite trends are observed for higher performance cells passivated with a 1nm layer of AlOx or HfOx. In these structures, we highlight the presence of a N1-type interface defect that is widely observed in the literature. In the case of front passivation, the N1 defect exhibits a higher response that is probably responsible for the reduced performance and is possibly explained by Fermi level pinning. For window passivation, the N1 defect density is reduced but this positive effect is neutralized by the barrier-induced roll-over that negatively affects the efficiency. Eventually, we also highlight the limitations of these characterization methods and discuss the need for further investigations using more advanced techniques.