Improved performance of polymer photodetectors using indium–tin-oxide modified by phosphonic acid-based self-assembled monolayer treatment

• ITO modified by phosphonic acid-based self-assembled monolayer (SAM).
• 1H,1H,2H,2H-Perfluorooctanephosphonic acid (FOPA) is used as SAM.
• Lowering of the injection barrier at the ITO/organic layer interface.
• Lowering of the contact resistance between ITO and the organic layer.
• Polymer photodetector with FOPA exhibits high efficiency and high on/off ratio.

For organic photodetectors (OPDs), in order to realize high incident-photon-to-current conversion efficiency (IPCE) and high on/off ratio, the characteristics of hole-only devices and polymer photodetectors with indium–tin-oxide (ITO) modified by phosphonic acid-based self-assembled monolayer treatment in a short time are investigated. 1H,1H,2H,2H-Perfluorooctanephosphonic acid (FOPA) 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. An OPD based on a blend of a donor, poly(9,9-dioctylfluorene-co-bithiophene) (F8T2), and an acceptor, fullerene derivative [6-6]phenyl-C61-butyric acid methyl ester (PCBM) with FOPA treatment exhibits blue-light sensitivity and IPCE of approximately 75% at −4 V. For the F8T2:PCBM device with FOPA, the IPCE is improved, and the dark current hardly increases. F8T2:PCBM device with FOPA treatment exhibits high on/off ratio, wide detector dynamic range and cut-off frequency of more than 10 MHz under reverse voltage.

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