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SUMMARY Organic photodetectors based on poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) blends with indium tin oxide (ITO) modified by phosphonic acid-based self-assembled monolayer (SAM) treatment in a short time are investigated. The mixed SAMs serve to tune the surface free energy and the work function of ITO by varying the blend ratio. The phosphonic acid-based SAM treatment results not only in lowering of the injection barrier at the ITO/organic layer interface but also in the lowering of the contact resistance between ITO and the organic layer, as shown by impedance spectroscopy in P3HT hole-only device with SAMs. P3HT:PCBM device with ITO modified by short treatment time of 1H,1H,2H,2H-perfluorooctane-phosphonic acid exhibits an incident-photon-to-current conversion efficiency of above 50% at -2 V, a high on/off ratio, and improved durability. [ABSTRACT FROM AUTHOR]