The Negative Effect of High‐Temperature Annealing on Charge‐Carrier Lifetimes in Microcrystalline PCBM

Abstract

The fullerene derivative, phenyl-C61-butyric acid methyl ester (PCBM) has been the subject of intense recent study as the electron-acceptor component in heterojunction blends for all-organic photovoltaic devices (OPVs) and as the electron-transport layer in organic field-effect transistors (FETs). In most of the studies referenced, the effect of high-temperature annealing has been investigated and has usually been found to have a positive influence on device characteristics, including the external quantum efficiency of OPVs and the electron mobility in FETs. This has been attributed to reorganization and/or aggregation within the initially amorphous or nanocrystalline spin-cast films resulting in the formation of larger well-organized domains. The occurrence of PCBM aggregation to form microcrystallites on annealing of spin-cast films has been well-documented by atomic force and electron beam microscopy studies of polymer blends with high PCBM content and pure PCBM films. In a previous paper we reported results on the mobility and decay kinetics of the mobile charge carriers formed on pulsed ionization of microcrystalline PCBM powder. An unexpected finding was that on returning to room temperature after a period of only a few minutes at approximately 120 °C, the lifetime of the mobile carriers was substantially shorter than found before heat treatment. The effect was even more pronounced on heating temporarily to about 170 °C. We concluded that a structural or chemical change of the material was occurring at elevated temperatures, which resulted in the irreversible formation of trapping sites for electrons and holes. Since the previous sample was not deaerated, it was thought that the effects observed could have been due to a thermally induced chemical change involving atmospheric gases such as oxygen or water vapor. It was therefore decided to carry out a further study on a sample which could be subjected to rigorous evacuation. The results, which are reported here, show that this negative influence of high-temperature annealing on the lifetime of charge carriers occurs even for this in vacuo sample. This could have important ramifications in view of the often routine thermal-annealing treatment applied to OPV and FET devices based on PCBM. In the present pulse-radiolysis time-resolved microwave conductivity (PR-TRMC) experiments, the microcrystalline powder sample of PCBM was contained in a cell that allowed the sample to be continuously evacuated in situ to a residual pressure of less than 10 Torr (1 Torr ≈ 133 Pa). All of the conductivity transients reported were obtained using singleshot 2 ns pulses of ionizing radiation (3 MeV electrons) with, at most, four pulses being used for signal averaging. Prior to presenting results on thermal annealing, we consider the effects of evacuation and radiation dose on the radiation-induced conductivity of the pristine sample at room temperature. In Figure 1, transients are shown for an air-equilibrated sample at room temperature and for the same sample after exposure to an accumulated radiation dose equivalent to about 150 pulses of 2 ns duration each. The end-of-pulse conductivity is seen to be unchanged and the decay to be only C O M M U N IC A TI O N S