Cu2ZnSn(S,Se)4 solar cells from inks of heterogeneous Cu–Zn–Sn–S nanocrystals

• CZTS nanocrystals synthesized in solution were size-separated into two populations.
• The particle compositions and structures were studied via conventional methods.
• Sintered films and solar cells were fabricated and characterized.
• Device efficiencies reach 7.9% and 8.4% for two synthesis recipes, respectively.
• The effect of nanocrystal heterogeneity on sintering and efficiency was explored.

The solution-based synthesis of quaternary Cu2ZnSnS4 (CZTS) nanocrystals and further processing into a homogeneous photovoltaic absorber film prove challenging due to the stability of binary and ternary chalcogenide compounds comprised of Cu, Zn, and Sn cations. In this contribution, CZTS nanocrystals are synthesized according to a previously reported synthesis recipe yielding devices with reported efficiencies as high as 7.2%. The particles are separated by size into two populations exhibiting size-correlated composition variations. This observation highlights the challenges in synthesizing quaternary CZTS nanoparticles with interparticle composition uniformity. Films of particles from each population are sintered in a selenium atmosphere and found to exhibit a notable degree of phase segregation in the final film, while a mixture of particles from both populations converts into a relatively homogeneous film upon selenization. Devices fabricated from the two particle populations separately as well as the mixture of particles exhibit varying degrees of performance, with the mixed particle cells achieving total area efficiencies as high as 7.9%. A modified synthesis recipe producing CZTS particles with narrower composition variations is shown to produce devices with improved efficiencies up to 8.4%. Preliminary conclusions regarding the effect of nanocrystal heterogeneity on film sintering and device performance are presented.