Tk. Daubler et al., Efficient bulk photogeneration of charge carriers and photoconductivity gain in arylamino-PPV polymer sandwich cells, PHYS REV B, 59(3), 1999, pp. 1964-1972
Stationary photoconductivity has been measured for sandwich cells of poly(p
henylimino-1,4-phenylene-1,2-ethenylene-2,5-dioctyloxy-1,4-phenylene-1,2-et
henylene-1,4-phenylene) between gold and aluminum electrodes. Films with th
icknesses below and above 1 mu m were examined in order to separate photocu
rrent contributions arising from processes at the polymer/electrode interfa
ces 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 ch
arge carriers. Large charge carrier collection efficiencies exceeding those
of typical PPV derivatives are derived from these experiments. The photoge
neration 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 d
istribution. Under reverse bias the photoaction spectra depend strongly on
illumination conditions. For illumination through the positively biased alu
minum electrode the photocurrent spectra are almost symbatic with the polym
er absorption while antibatic behavior is observed upon illumination throug
h the gold electrode. The data can qualitatively be explained by the buildu
p of a negative space-charge region by immobile photogenerated electrons. U
pon illumination through the positively biased aluminum electrode collectio
n efficiencies of up to 2000% are observed. This is attributed to photocurr
ent multiplication arising from the accumulation of electrons near the inte
rface, which promotes tunneling of holes into the film. [S0163-1829(99)0290
3-3].