Semiconducting Polymer Thin Films Used in Organic Solar Cells: A Scanning Tunneling Microscopy Study

The widely used semiconducting polymers poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) (PH1000), poly(3-hexylthiophene-2,5-diyl) (P3HT), and Poly[[4,8-bis1(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b]dithiophene-2,6-diyl][3-fluoro2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7) for organic photo-voltaic (OPV) cells, are deposited on highly ordered pyrolytic graphite substrates and investigated by scanning tunneling microscopy (STM) under the liquid/solid interface technique. PEDOT:PSS film morphology shows an ellipsoidal shape of PEDOT surrounded by PSS with 4 nm size. STM images of P3HT reveal that films tend to form well-defined crystalline domains with an average interchain distance around 1.41 nm and chain length approximate to 41 nm. PTB7 chains are self-aggregated by using chlorobenzene as solvent; however, when using 1-phenyloctane with 3% of 1,8-diiodooctane, these chains show a worm-like pattern with a distance of approximate to 2 nm between backbone chains and a chain length of approximate to 90 nm. For P3HT and PTB7 based OPVs, local J-V plots are determined from photoconductive atomic force microscopy (pc-AFM) measurements and fitted with the modified Mott-Gurney equation, the reached parameters are correlated with those achieved from the macroscopic J-V graphs. Active films of OPVs cells based on PTB7 are also analyzed by using AFM phase imaging showing domains size of 80-120 nm. This current STM analysis can elucidate some new important insights on polymer film formation and morphology and therefore, their influence on the OPV performance.