Charge Transfer and Recombination Kinetics at Electrodes of Molecular Semiconductors Investigated by Intensity Modulated Photocurrent Spectroscopy

Vapor-deposited thin films of phthalocyaninatozinc(II) (PcZn), hexadecafluorophthalocyaninatozinc(II) (F₁₆PcZn), and N,N‘-dimethyl perylene tetracarboxylic acid diimide (MePTCDI) were investigated by electrochemical impedance spectroscopy (EIS), photocurrent transient measurements in the millisecond-regime, and by intensity modulated photocurrent spectroscopy (IMPS). Interfacial states which act as traps and recombination sites (surface states) were detected. Quantitative kinetic data could be obtained from IMPS for p-type PcZn, where light-induced electron transfer to Fe(CN)₆^3- and p-benzoquinone was found to occur mainly from the LUMO to adsorbed reactant molecules, whereas transfer from surface states plays a minor role. This was found to be opposite in the electron transfer from PcZn to oxygen which occurred mainly via surface states. F₁₆PcZn was found to behave as a compensated n-type semiconductor after storage in air. Surface states were detected which can be occupied by photogenerated electrons and led to their partial subsequent transfer to the electrolyte. Also found were near-surface states which can be occupied by photogenerated holes but which do not lead to subsequent 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 reversible 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.