We have realized highly efficient photodetectors based on composites of the semiconducting polymer poly͓2-methoxy-5-͑2Ј-ethylhexyloxy͒-1,4-phenylenevinylene͔ and PbSe nanocrystal quantum dots. The external quantum efficiency in these devices is greater than 1 for electric fields E ϳ 7 ϫ 10 5 V / cm. The observed photocurrent gain could be attributed to the carrier multiplication in PbSe nanocrystal quantum dots via multiple exciton generation, and the efficient charge conduction through the host polymer material. This photocurrent gain is observed only when the PbSe nanocrystal band gap is at least three times smaller than the optical energy gap of the active polymer material.

Poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene-vinylene] (MEH-PPV) adopts a preferential orientation on indium tin oxide. Although the basic building block of this polymer provides a negligible overall point-group symmetry, the polymer MEH-PPV packs with suffi cient order to exhibit band structure. The polymer is fragile with bond cleavage evident following both argon-ion impact and ultraviolet radiation, but annealing leads to the restoration of much of the bond order.

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Polymer optoelectronics and microelectronics have been recognized as next generation technologies. One of the widely investigated materials for photodiode, LED and solar cell applications is the insoluble conjugated polymer poly(p-phenylene vinylene) or PPV. In this paper we present experimental results of a blended polymer-organic compound photodiode. This diode is based on a soluble derivative of PPV, poly(2-methoxy-5- (2,9-ethyl-hexyloxy)-1,4- phenylene vinylene) or MEH-PPV, and the organic material ethyl viologen dibromide or EVD. In making the photodiodes, solutions of MEH-PPV and EVD were spin-coated on indium tin oxide coated glass substrates. The thicknesses of these polymer-organic thin films were approximately 190 nm. An aluminum cathode was deposited by thermal evaporation. These devices were illuminated under monochromatic light in UV and visible range wavelengths. These thin polymer-organic blend photodiodes have shown an eight-fold increase in responsivity and quantum efficiency compared to pure MEH-PPV photodiode devices. The increase in photoconductivity of blended MEH-PPV:EVD photodiodes may be due to charge transfer by EVD dication. The results from this work clearly demonstrate the application of the reported approach for the realization of polymer photodiodes with increased photoconductivity characteristics.

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