Femtosecond to nanosecond excited state dynamics of vapor deposited copper phthalocyanine thin films

Abstract

Vapor deposited thin films of copper phthalocyanine (CuPc) were investigated using transient absorption spectroscopy. Exciton-exciton annihilation dominated the kinetics at high exciton densities. When annihilation was minimized, the observed lifetime was measured to be 8.6 AE 0.6 ns, which is over an order of magnitude longer than previous reports. In comparison with metal free phthalocyanine (H 2 Pc), the data show evidence that the presence of copper induces an ultrafast relaxation process taking place on the ca. 500 fs timescale.By comparison to recent time-resolved photoemission studies, this is assigned as ultrafast intersystem crossing. As the intersystem crossing occurs ca. 10 4 times faster than lifetime decay, it is likely that triplets are the dominant excitons in vapor deposited CuPc films. The exciton lifetime of CuPc thin films is ca. 35 times longer than H 2 Pc thin films, while the diffusion lengths reported in the literature are typically quite similar for the two materials. These findings suggest that despite appearing to be similar materials at first glance, CuPc and H 2 Pc may transport energy in dramatically different ways. This has important implications on the design and mechanistic understanding of devices where phthalocyanines are used as an excitonic material.

Short and long-range electron transfer compete to determine free-charge yield in organic semiconductors

“…The extensive red-shifting and broadening typical of aggregated phthalocyanines is absent in these films, suggesting that we have indeed prepared samples that resemble a solid solution -it does not resemble the typical aggregate spectra of pure phthalocyanine films. [18][19][20][21] The symmetric broadening and slight red shift of phthalocyanine spectrum in the PCBM host may be due to a combination of effects. First, we note that this phenomenon is universal for all our sensitizers, even those (Sq1 and Sq2) that we will show below not to exhibit strong PL quenching or free-charge generation.…”

Efficient charge generation from triplet excitons in metal-organic heterojunctions

“…Comparison of the intersystem crossing time of ≈500 fs in CuPc [5,17,18] with the 9 ns average excitation transfer time within the CuPc domain strongly suggests that the vast majority of donor states contributing to the long-term charge component must have relaxed to the triplet manifold. Based on ultrafast measurements on few-monolayer systems and energy considerations, injection from these states is generally considered extremely disfavored [17,18].…”

Ring currents modulate optoelectronic properties of aromatic chromophores at 25 T

“…Upon nanoparticle formation using flash nanoprecipitation (39,40), the CuPc spectrum starkly blue-shifts by 70 nm and broadens, giving rise to a new excitonic absorption band due to the strong intermolecular interactions concomitant with molecular aggregation, characteristic of H-aggregates. This absorption spectrum differs from the absorption spectra of many recorded CuPc thin films and single crystals because of the lack of crystallinity in the nanoparticle samples (41). The absorption spectra for H 2 Pc in toluene and in nanoparticle form can be found in SI Appendix, Figs.…”