Solution processable organic p–n junction bilayer vertical photodiodes

Organic p–n bilayer photodiodes were produced by solution casting poly(3-hexylthiophene) (P3HT) from chlorobenzene and phenyl-C61-butyric acid methyl ester (PCBM):poly(4-chlorostyrene) (PClS) blends from the nearly orthogonal solvent dichloromethane onto flexible indium tin oxide (ITO)/polyester as a substrate. This is the first demonstration of PCBM–inert polymer blends for such a device. The electron mobility of a 90% PCBM–10% PClS blend was 3.5 × 10−3 cm2/V s in a field-effect transistor. The diodes showed a rectification ratio of 2.0 × 103 at ±2.0 V with a forward bias current density as high as 340 μA/cm2 at 2.0 V in the dark. Irradiation with various light sources (0.013–291 mW/cm2) under ambient atmosphere generated a linear increase in photocurrent. Photodiodes with thinner active layers showed larger photocurrent and relative photoresponse, probably because of lower series resistance and lower recombination probability. The reverse bias response was less dependent on device area than the forward bias response. Photocurrents from multiple devices in parallel were additive as expected. The results demonstrate a simple fabrication route to light detectors compatible with solution processes and flexible substrates.

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► A poly(chlorostyrene)–PCBM blend had sufficient electron mobility to serve as the “n” component in a photodiode.
► Dichloromethane was used as an orthogonal solvent to deposit this n-layer on P3HT, giving diode rectification ratio 1000.
► We compared forward and reverse bias performances; reverse bias mode was more sensitive and independent of device area.
► Photoresponse was greater for thinner films, and was nearly linear over a large intensity range.
► Multiple photodiodes could be connected in parallel, giving the expect additive signals.