W.R. Mateker, J.D. Douglas, C. Cabanetos, I.T. Sachs-Quintana, J.A. Bartelt, E.T. Hoke, A. El Labban, P.M. Beaujuge, J.M.J. Fréchet, M.D. McGehee
Energy and Environmental Science, 6(8), 2529-2537, (2013)
While bulk heterojunction (BHJ) solar cells fabricated from high Mn PBDTTPD achieve power conversion efficiencies (PCE) as high as 7.3%, the short-circuit current density (JSC)
of these devices can drop by 20% after seven days of storage in the
dark and under inert conditions. This degradation is characterized by
the appearance of S-shape features in the reverse bias region of
current–voltage (J–V) curves that increase in amplitude over time. Conversely, BHJ solar cells fabricated from low Mn PBDTTPD do not develop S-shaped J–V curves. However, S-shapes identical to those observed in high Mn PBDTTPD solar cells can be induced in low Mn devices through intentional contamination with the TPD monomer. Furthermore, when high Mn PBDTTPD is purified via size exclusion chromatography (SEC) to reduce the content of low molecular weight species, the JSC of polymer devices is significantly more stable over time. After 111 days of storage in the dark under inert conditions, the J–V curves do not develop S-shapes and the JSC
degrades by only 6%. The S-shape degradation feature, symptomatic of
low device lifetimes, appears to be linked to the presence of low
molecular weight contaminants, which may be trapped within samples of
high Mn polymer that have not been purified by SEC.
Although these impurities do not affect initial device PCE, they
significantly reduce device lifetime, and solar cell stability is
improved by increasing the purity of the polymer materials.