Polymer
solar cells based on PDPP5T and PCBM as donor and acceptor materials,
respectively, were processed from aqueous nanoparticle dispersions. Careful monitoring and optimization
of the concentration of free and surface-bound surfactants in the
dispersion, by measuring the conductivity and ζ-potential, is
essential to avoid aggregation of nanoparticles at low concentration
and dewetting of the film at high concentration. The surfactant concentration
is crucial for creating reproducible processing conditions that aid
in further developing aqueous nanoparticle processed solar cells.
In addition, the effects of adding ethanol, of aging the dispersion,
and of replacing [60]PCBM with [70]PCBM to enhance light absorption
were studied. The highest power conversion efficiencies (PCEs) obtained
are 2.0% for [60]PCBM and 2.4% for [70]PCBM-based devices. These PCEs
are limited by bimolecular recombination of photogenerated charges.
Cryo-TEM reveals that the two components phase separate in the nanoparticles,
forming a PCBM-rich core and a PDPP5T-rich shell and causing a nonoptimal
film morphology.