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 20 mu m) of conjugated polymers. Schottky-based sandwich structures were fabricated from layers of either poly[1-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 4 nA/cm(2) at an electric field strength of 25 kV/cm. Irradiation with x-rays from a 50 kV x-ray tube produced a linear increase in photocurrent over a dose rate range from 4 to 18 mGy/s. The observed x-ray sensitivities of 240 nC/mGy/cm(3) for MEH-PPV and 480 nC/mGy/cm(3) for PFO structures are comparable to that reported for Si devices. A response time of < 150 ms 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. (C) 2007 American Institute of Physics.