Louis Grenet

Sodium-doped Mo back contacts for Cu(In,Ga)Se 2 solar cells on Ti foils: Growth, morphology, and sodium diffusion

Sodium addition is necessary to reach high efficiencies with Cu(In,Ga)Se2 (CIGS) solar cells on metallic substrates. This can be achieved using DC-sputtered multilayer Mo back contacts including a sodium-doped layer (Mo:Na). In this study, 450 nm-thick Mo:Na layers were sputter-deposited on Ti foils using a working pressure ranging from 3.4 mTorr to 30 mTorr, and capped with 100 nm-thick, dense, pure Mo layers. The deposition pressure of the Mo:Na layers strongly affected their electrical resistance, optical reflectance, morphology, and crystal quality. The top Mo layer allowed controlling the electrical and optical properties of the back contact, but its own morphology was affected by the structure of the underlying Mo:Na layer. The Na concentration in the CIGS layer varied as a function of the deposition pressure of the Mo:Na layer, inducing changes in the absorber grain size and Ga distribution, as well as in the carrier density. Absolute conversion efficiency improvements over 3% were obtained using the sodium-doped back contacts on Ti foils, leading to a performance which was equivalent to the soda-lime glass reference.

Comparing strategies for improving efficiencies in vacuum processed Cu2ZnSnSe4 solar cells

In this study, we detail a Cu2ZnSnSe4 based solar cell fabrication process based on the selenization of metallic precursor stacks with elemental Se. 9.4% efficient devices without antireflection coating have been obtained. First, reproducibility issues of the process are carefully shown and discussed. It is demonstrated that device performances are strongly impacted by the precise control of the precursor composition. Then, starting from this robust process, a review of existing strategies to improve kesterite efficiencies is conducted. A significant increase in efficiency (+1.4% absolute efficiency and +50 mV VOC) is obtained with absorber surface treatment and post-annealing, while no effect of Ge incorporation in the precursor stack is observed. This contradictory result to most of the recent publications raises the question of the universality of this strategy to improve kesterite solar cell performance. Finding a universal activation step to boost kesterite efficiencies and bring it to the market remains a crucial need for the community.In this study, we detail a Cu2ZnSnSe4 based solar cell fabrication process based on the selenization of metallic precursor stacks with elemental Se. 9.4% efficient devices without antireflection coating have been obtained. First, reproducibility issues of the process are carefully shown and discussed. It is demonstrated that device performances are strongly impacted by the precise control of the precursor composition. Then, starting from this robust process, a review of existing strategies to improve kesterite efficiencies is conducted. A significant increase in efficiency (+1.4% absolute efficiency and +50 mV VOC) is obtained with absorber surface treatment and post-annealing, while no effect of Ge incorporation in the precursor stack is observed. This contradictory result to most of the recent publications raises the question of the universality of this strategy to improve kesterite solar cell performance. Finding a universal activation step to boost kesterite efficiencies and bring it to the market rem...