Effects of conjugated polymer incorporation on the morphology and energy harvesting of solution-processed, phthalocyanine-based thin films

• Efficient energy transfer occurs between conjugated polymers and a phthalocyanine derivative.
• Absorption bandwidth is increased by a factor of more than 2.5 for in the ternary blend.
• A small fraction of polyfluorene improves crystallinity and energy transfer.

Here, we investigate the morphology, spectral absorption bandwidth and energy transfer in solution-processed, phthalocyanine-based thin films blended with conjugated polymer materials with complementary absorption bands. Unary, binary, and ternary solutions of the solution- processable phthalocyanine derivative 2,3,9,10,16,17,23,24-octakis(octyloxy)-29H,31H-phthalocyanine (Oct-Pc) and the conjugated polymers poly(9,9-dioctylfluorenyl-2,7-diyl) (PFO) and poly(3-hexylthiophene) (P3HT) were used to prepare sub-55-nm-thick unary-phase and blended thin films. Spectroscopic analysis shows that absorption band full-width-at-half-maximum (FWHM) values increase from between 60 nm and 160 nm for the individual materials to greater than 450 nm for the composite ternary-blend thin film due to the complementary bandgap energies and spectral absorption bands of the constituent materials. Additionally, photoluminescence and transient absorption measurements show very efficient transfer of excited-state energy in the wider band-gap materials (donors) to the narrower band-gap material (acceptor). Resonant energy or charge transfer occurs with efficiencies between 90% and 100% for the various blends. Atomic-force microscopy and grazing-incidence, wide-angle X-ray scattering data indicate that P3HT and Oct-Pc exhibit the poorest blending. This correlates with the lowest donor photoluminescence quenching efficiency due to the extended separation of the P3HT chains from Oct-Pc molecules. However, addition of a relatively small fraction of PFO disrupts Oct-Pc crystallinity and enables improved mixing and energy transfer between P3HT and Oct-Pc.