Improving the long-term stability of PBDTTPD polymer solar cells through material purification aimed at removing organic impurities

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.