Abstract
While bulk heterojunction (BHJ) solar cells fabricated from high M-n PBDTTPD achieve power conversion efficiencies (PCE) as high as 7.3%, the short-circuit current density (J(SC)) 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 M-n PBDTTPD do not develop S-shaped J-V curves. However, S-shapes identical to those observed in high M-n PBDTTPD solar cells can be induced in low M-n devices through intentional contamination with the TPD monomer. Furthermore, when high M-n PBDTTPD is purified via size exclusion chromatography (SEC) to reduce the content of low molecular weight species, the J(SC) 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 J(SC) 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 M-n 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.