Intermolecular Interactions Determine Exciton Lifetimes in Neat Films and Solid State Solutions of Metal-Free Phthalocyanine

Thin films of vapor-deposited metal-free phthalocyanine (H₂Pc) were studied using ultrafast transient absorption spectroscopy in the visible region. Following photoexcitation, an excited state absorption feature located near 532 nm was observed which served as a probe of the excited state. For exciton densities larger than 5 × 10¹⁸ excitons/cm³ the time-dependent measurements of the excited state absorption included the presence of nonexponential decay kinetics attributed to exciton-exciton annihilation. At exciton densities less than 5 × 10¹⁸ excitons/cm³ annihilation was negligible, and the decay kinetics appeared single exponential within the signal-to-noise. The fitted time constant, 239 ± 24 ps, was attributed to the lifetime decay of the singlet excitons. When the H₂Pc was diluted into a wide energy gap host via vapor deposition, the observed lifetime was significantly reduced, reaching 87 ± 9 ps for a concentration of 25% H₂Pc. The decreased exciton lifetime with dilution was remarkable since it has been commonly reported that excited state lifetimes decrease as the chromophore concentration is increased. The reduced lifetime was correlated to the loss of α-phase ordering as indicated in the UV/vis spectra of the films. Within the context of photovoltaic applications this highlights the importance of both molecular level ordering and chromophore concentration when trying to engineer fundamental material properties such as exciton diffusion length.