Direct x-ray detection with conjugated polymer devices

The authors report the first direct detection of x-ray induced photocurrents in thick films (up to 20μm) of conjugated polymers. Schottky-based “sandwich” structures were fabricated from layers of either poly1-methoxy-4-(2-ethylhexyloxy)-phenylenevinylene (MEH-PPV) or poly(9,9-dioctylfluorene) (PFO) on indium tin oxide substrates using a top contact of aluminum. Good rectification was achieved from the Al-polymer contact, with a reverse bias leakage current density as low as 4nA∕cm2 at an electric field strength of 25kV∕cm. Irradiation with x-rays from a 50kV x-ray tube produced a linear increase in photocurrent over a dose rate range from 4to18mGy∕s. The observed x-ray sensitivities of 240nC∕mGy∕cm3 for MEH-PPV and 480nC∕mGy∕cm3 for PFO structures are comparable to that reported for Si devices. A response time of 150ms to pulsed x-ray irradiation was measured with no evidence of long-lived current transients. Conjugated polymers offer the advantage of easy coatability over large areas and on curved surfaces. Their low average atomic number provides tissue-equivalent dosimetric response, with many potential applications including medical x-ray and synchrotron photon detection.