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

Benjamin W Caplins,Russell J Holmes,Tyler K Mullenbach,David A Blank

doi:10.1039/c6cp00958a

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 ± 0.6 ns, which is over an order of magnitude longer than previous reports. In comparison with metal free phthalocyanine (H2Pc), 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 H2Pc 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 H2Pc 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.

Highlights

Copper phthalocyanine has been one of the most widely studied materials in organic electronics for several decades. For photovoltaic applications it was used in the first modern two-layer structure by Tang in 1986.1 Given its historical significance and the continual interest in CuPc as a low cost active material in organic electronic devices it is somewhat surprising that the electronic excited states of the material have not been thoroughly characterized with time-resolved techniques
We have monitored the excited states of a vapor deposited CuPc thin film using transient absorption spectroscopy
Evolution of the transient spectrum on the sub-picosecond timescale is assigned to ultrafast intersystem crossing to the triplet state

Summary

Introduction

Copper phthalocyanine has been one of the most widely studied materials in organic electronics for several decades. For photovoltaic applications it was used in the first modern two-layer structure by Tang in 1986.1 Given its historical significance and the continual interest in CuPc as a low cost active material in organic electronic devices it is somewhat surprising that the electronic excited states of the material have not been thoroughly characterized with time-resolved techniques. Two of the simplest and most obvious questions are: what is the character of the electronic excited states in these films? These two pieces of information are critical to developing a mechanistic understanding of the transport of energy in materials.[2,3,4]. Experiments on H2Pc are presented to help understand the role of the copper atom in the excited state dynamics (Fig. 1)

Experimental

Sample preparation

Results and discussion

Conclusions