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

Abstract

Stationary photoconductivity has been measured for sandwich cells of poly͑phenylimino-1,4-phenylene-1,2ethenylene-2,5-dioctyloxy-1,4-phenylene-1,2-ethenylene-1,4-phenylene͒ between gold and aluminum electrodes. Films with thicknesses below and above 1 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.

Charge carrier generation and electron blocking at interlayers in polymer solar cells

“…This current might be largely affected by photogenerated charges, e.g., via photocurrent multiplication processes. 14,15 It is most likely due to this additional photocurrent contribution that the apparent built-in potential of the blend device is smaller than that of the strict bilayer.…”

Nanoparticle-assisted high photoconductive gain in composites of polymer and fullerene

“…While polymers present advantages of easy processing, flexibility with regard to additive concentration, and controllable morphology by solvent selection, high photoconductive gain under small bias has not yet been realized. Although photoconductive gain has been found in both pure polymer 17,18 and polymer/nanoparticle blend systems 19 , rather high voltage is needed to achieve high photoconductive gain which limits applications of these systems. In this manuscript, we show that by utilizing the advantages of both polymer and NPs (tunable optical and electrical properties), high photoconductive gain under small bias can be achieved with a simple polymer/NP blend film sandwiched between two metal electrodes.…”

<title>Photoconductivity and charge-carrier photogeneration in photorefractive polymers</title>

“…(3) the photoconductivity gain factor G is used to express that the number of carriers flowing through the external circuit in steady-state photoconductivity experiments, can be larger or smaller than the number of primarily photogenerated charge carriers 22,23 . Note, that an equation similar to Eq.…”