Efficient bulk photogeneration of charge carriers and photoconductivity gain in arylamino-PPV polymer sandwich cells

Stationary photoconductivity has been measured for sandwich cells of poly(phenylimino-1,4-phenylene-1,2-ethenylene-2,5-dioctyloxy-1,4-phenylene-1,2-ethenylene-1,4-phenylene) between gold and aluminum electrodes. Films with thicknesses below and above 1 mu m were examined in order to separate photocurrent contributions arising from processes at the polymer/electrode interfaces and the bulk. Spectra recorded under forward bias were almost identical in shape and size for irradiation through either the anode or the cathode indicating that the photocurrent is dominated by bulk photogeneration of charge carriers. Large charge carrier collection efficiencies exceeding those of typical PPV derivatives are derived from these experiments. The photogeneration is clearly enhanced within the tail of the polymer absorption and photoaction spectra recorded under forward bias resemble well the shape of solid-state photoexcitation spectra. We presume that both photoluminescence and photoconductivity in the studied arylamino-PPV compound are dominated by the excitation of states or sites rather deep in the density of states distribution. Under reverse bias the photoaction spectra depend strongly on illumination conditions. For illumination through the positively biased aluminum electrode the photocurrent spectra are almost symbatic with the polymer absorption while antibatic behavior is observed upon illumination through the gold electrode. The data can qualitatively be explained by the buildup of a negative space-charge region by immobile photogenerated electrons. Upon illumination through the positively biased aluminum electrode collection efficiencies of up to 2000% are observed. This is attributed to photocurrent multiplication arising from the accumulation of electrons near the interface, which promotes tunneling of holes into the film. [S0163-1829(99)02903-3].