Photosensitivity of MEH-PPV Sandwich Devices and Its Implication to Polymer Electronic Structure

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Abstract
Thin film devices made with Poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene), MEH-PPV, are known to be efficient light emitting diodes. The same devices, under reverse bias, exhibit excellent photosensitivity. Large photovoltages were observed in devices with selected metals as the cathode and anode electrodes. The open-circuit voltage, Voc, varies with the work functions of the electrodes; in most cases, Voc approaches the work function difference between the two electrodes at high excitation levels. The photosensitivity increases significantly under reverse bias. At −10 V, the sensitivity in Ca/MEH-PPV devices reaches 45 mA/Watt at 20 mW/cm2 (quantum yield of ∼ 13% electrons/photon). The action spectra of the photovoltaic signal at zero bias, and the photocurrent at 2V forward bias follow the absorption spectrum at the band edge. Thus, mobile carriers can be generated either by charge injection at the contacts or by interband photoexsitation, as observed in inorganic semiconductors.