C₆₀Fullerene Nanocolumns–Polythiophene Heterojunctions for Inverted Organic Photovoltaic Cells

Inverted organic photovoltaic cells have been fabricated based on vertical C(60) nanocolumns filled with spin-coated poly[3-(4-carboxybutyl)thiophene-2,5-diyl] (P3CBT). These C(60) nanocolumns were prepared via glancing angle deposition (GLAD), an efficient synthetic approach that controls the morphology of the resulting film, including intercolumn spacing, nanostructure shapes, and overall film thickness, among others. Intercolumn spacing was tuned to better match the expected P3CBT exciton diffusion length while simultaneously increasing heterointerface area. Due to observed in situ dissolution of the C(60) nanocolumns in solvents typically used to spin-coat polythiophene-based polymers (i.e., chloroform and chlorobenzene), the carboxylic acid-substituted polythiophene, P3CBT, was used as it is soluble in dimethyl sulfoxide (DMSO), a solvent that did not affect the structure of the GLAD-produced C(60) nanostructures. Preservation of the C(60) nanocolumnar structure in the presence of DMSO, with and without P3CBT, was verified by absorbance spectroscopy and SEM imaging. Incorporating these nanostructured C(60)/P3CBT films into photovoltaic devices on indium tin oxide (ITO) showed that the engineered nanomorphology yielded a 5-fold increase in short-circuit current and a power conversion efficiency (PCE) increase from (0.2 ± 0.03)% to (0.8 ± 0.2)% when compared to a planar device. When compared to a standard bulk heterojunction (BHJ) device based upon the same materials, the C(60)-GLAD device outperformed fully solution-processed bulk heterojunctions, which were observed to have PCEs of (0.49 ± 0.03)%.