Factors controlling hole injection in single conjugated polymer molecules.

New insights on the molecular level details of the recently reported light-assisted injection of positive charge into single conjugated polymer chains are reported. Extensive new fluorescence-voltage single molecule spectroscopy (FV-SMS) measurements were performed on single chains of the archetypical conjugated polymer MEH-PPV embedded in a capacitor device to complement previous studies of the influence of the bias scan rate and optical excitation intensity. The use of a vacuum microscope allowed for the precise control of the device atmosphere, demonstrating the influence of triplet states in the MEH-PPV on the FV-SMS modulation. For identical device conditions, little variation was observed in the rate and yield of charging from molecule to molecule. Through the use of thicker supporting matrices and insulating polymer "blocking layers", it was determined that good electrical contact between the hole transport layers and the single molecules was necessary for charge injection. The results demonstrate the complexity of charge transfer processes at the interface of organic semiconductors and highlight the ability of single molecule methods to advance the understanding of such processes at the nanoscale.