Stabilization of PCBM domains in bulk heterojunctions using polystyrene-tethered fullerene

• Efficiencies and lifetimes of P3HT:PCBM bulk heterojunction devices increased with the addition of polymer-tethered fullerenes (PTFs).
• PCBM domain sizes within bulk heterojunctions were reduced by up to 400% after annealing.
• Photoluminescence measurements demonstrate that PTFs do not interfere with charge transfer between donor and acceptor domains.
• Polystyrene-tethered fullerenes have the potential to be a universal additive for systems that use fullerene based acceptors.

The domain size of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) in the bulk heterojunction (BHJ) of organic photovoltaic (OPV) devices is of great importance to the performance and lifetime of a device. This study focuses on the use of polystyrene-tethered fullerenes (PTFs) as an additive to reduce the initial domain size of PCBM as well as to mitigate the growth of these domains over time that typically results in decreased device efficiency. Films were prepared from a 1:1 mixture of P3HT:PCBM on silicon wafers to approximate a BHJ system. PTFs were added in concentrations ranging from 0 to 20%. The samples were then annealed at 150 °C for varying times ranging from 5 to 60 min and examined by SEM to determine the average PCBM domain sizes for each sample. As expected, the control sample containing no additive showed significant growth in the PCBM domains as the sample was annealed with a final average domain size of 290 μm2. However, addition of as little as 5% PTFs produced average PCBM domain sizes after 60 min of annealing of 28 μm2. Addition of 20% PTF resulted in an average PCBM domain size of 2 μm2. Photoluminescence spectroscopy (PL) studies were performed on the films to study their electrical properties and based on these encouraging results; devices containing these architectures were prepared. OPVs with PTFs showed an increased lifetime, and in some cases, an increase in efficiency over devices containing no PTF.

Top Row: Films containing P3HT:PCBM and varying percentages of PTFs annealed for 30 mins @ 150°C. Bottom Row: IV curves for devices containing P3HT:PCBM (1:0.7) and varying percentages of PTFs annealed for 30 mins @ 150°C and a plot of the average PCE over the annealing period.Figure optionsDownload as PowerPoint slide