THE PHOTOVOLTAIC RESPONSE IN POLY(P-PHENYLENE VINYLENE) THIN-FILM DEVICES

We report measurements of the photovoltaic effect in diode structures formed with thin films (100 nm) of the conjugated polymer poly(p-pheny lene vinylene), PPV, sandwiched between electrodes of indium/tin oxide , ITO, and either aluminium, magnesium or calcium. Under illumination incident through the ITO contact, large open-circuit voltages were mea sured, which saturated at approximately 1.2 V for Al and Mg devices, a nd approximately 1.7 V for Ca devices. Quantum efficiencies (short-cir cuit current/incident photon flux) of order 1% were measured at low in tensities (0.1 mW cm-2). The spectral response of the photocurrent dem onstrates that photon absorption near the electron-collecting electrod e optimizes the photocurrent, indicating that device performance is li mited by low electron mobilities in the bulk PPV. The photocurrent exh ibits a weak temperature dependence, with an activation energy that is a function of the electric field in the polymer. We have used these m easurements to estimate an exciton binding energy in PPV of approximat ely 0.4 eV.