Efficient indium-tin-oxide (ITO) free top-absorbing organic photodetector with highly transparent polymer top electrode

We report on an efficient solution-processable, top-absorbing organic photodetector with a polymer top electrode. The layer sequence is inverted, starting with the cathode as bottom electrode instead of an indium-tin-oxide (ITO) anode used in state-of-the-art organic detectors. The device comprises a bulk heterojunction of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) as photoactive layer and a double layer of different formulations of poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS). The lower PEDOT:PSS layer reduces the thermally generated dark current and the upper layer acts as anode. To increase the conductivity of the highly transparent (>90% at 500 nm wave length) PEDOT:PSS electrode ethylene glycol is added. The influence of the anode sheet resistance R_sheet on important photodiode characteristics is investigated. For R_sheet = 100 Ω/sq high external quantum efficiency (>70% at 500 nm) and high cut-off frequencies (>400 kHz) can be reached. The performance of these inverted organic photodetectors is shown to be equal to state-of-the-art non-inverted detectors containing an ITO anode.