Charge transfer and recombination kinetics at electrodes of molecular semiconductors investigated by intensity modulated photocurrent spectroscopy

Vapor-deposited thin films of phthalocyaninatozinc(II) (PcZn), hexadecafluo rophthalocyaninatozinc(H) (F16PcZn), and N,N'-dimethyl perylene tetracarbox ylic acid diimide (MePTCDI) were investigated by electrochemical impedance spectroscopy (EIS), photocurrent transient measurements in the millisecond- regime, and by intensity modulated photocurrent spectroscopy (RAPS). Interf acial states which act as traps and recombination sites (surface states) we re detected. Quantitative kinetic data could be obtained from IMPS for p-ty pe PcZn, where light-induced electron transfer to Fe(CN)(6)(3-) and p-benzo quinone was found to occur mainly from the LUMO to adsorbed reactant molecu les, whereas transfer from surface states plays a minor role. This was foun d to be opposite in the electron transfer from PcZn to oxygen which occurre d mainly via surface states. F16PcZn was found to behave as a compensated n -type semiconductor after storage in air. Surface states were detected whic h can be occupied by photogenerated electrons and led to their partial subs equent transfer to the electrolyte. Also found were near-surface states whi ch can be occupied by photogenerated holes but which do not lead to subsequ ent charge transfer to the electrolyte. At MePTCDI, another n-type material , adsorption of electroactive species from the electrolyte not only led to light-induced charge transfer to the adsorbed reactant but also to the reve rsible generation of additional surface traps. The results are rationalized by the rates of competing reactions, and implications for the use of such films in chemical sensors and organic photovoltaics are discussed.