Solution-processable inverted organic photodetectors using oxygen plasma treatment Daniela Baierl a, * , Bernhard Fabel a , Paolo Gabos b , Lucio Pancheri c , Paolo Lugli a , Giuseppe Scarpa a a Institute for Nanoelectronics, Technische Universität München, Theresienstraße 90, 80333 München, Germany b DISI, University of Trento, Via Sommarive 14, 38123 Trento, Italy c FBK-irst, Via Sommarive 18, 38123 Trento, Italy article info Article history: Received 22 March 2010 Received in revised form 21 April 2010 Accepted 24 April 2010 Available online 8 May 2010 Keywords: Inverted organic photodetector Oxygen plasma Hydrophilic PCBM:P3HT Solution-processable Space charge limited current abstract In this work, we present a solution-processed inverted organic photodetector with a bulk heterojunction of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) as photoactive layer and poly(3,4-ethylenedioxythiophene):poly(styrenesul- fonate) (PEDOT:PSS) as hole-conductor. By the exposure of the hydrophobic photoactive layer to a low-pressure oxygen plasma the challenge of spin-coating the water-based PED- OT:PSS solution on top of it is overcome. Impacts of the oxygen plasma on the device regarding electrical and material properties were investigated. It is shown that the plasma treatment does not damage the photoactive layer but increases its wettability due to for- mation of polar oxygen compounds on the surface. These oxides influence the carrier trans- port due to space charge limited current, leading to a decreased photocurrent for external bias smaller than 1 V, whereas for larger reverse bias the device performance is not neg- atively influenced. A comparison with non-inverted reference photodiodes is also reported. We found that the main limitation of the inverted devices, a decreased external quantum efficiency, is not due to the impact of plasma treatment but to the use of a semi-transpar- ent gold anode with a transmission of about 55% @ 500 nm instead of ITO (indium-tin- oxide) used in the reference diodes. Taking this reduced transmission into account, the inverted photodiodes show device performance comparable to the non-inverted devices. Ó 2010 Elsevier B.V. All rights reserved. 1. Introduction Solution-processable organic photovoltaics are a prom- ising alternative to inorganic semiconductor devices due to their low-cost and large-scale fabrication capability. Bulk- heterojunction photodetectors, based on a blend of regio- regular poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM), begin to match the device performance of their inorganic counterparts, e.g. showing high quantum efficiencies [1]. Furthermore, conjugated polymers have the advantage of chemical tunability of the bandgap, e.g. enabling the fabrication of infrared sensitive photodiodes with high detectivities of > 10 13 cm Hz 1 2 =W as reported by Gong et al. [2]. The handling as liquids allows an easy combination of organic polymers with inorganic materials [3] and technol- ogy, in particular with CMOS circuits. Thus, a new kind of hybrid CMOS-imagers with fill factors up to 100% [4] and sensitivity beyond the visible range of the spectrum would be feasible. Usually organic detectors and solar cells are bottom- absorbing devices in which the light enters the devices through the bottom electrode, which typically consists of a transparent ITO-anode on a glass substrate. The deposi- tion of organics on top of opaque substrates like silicon 1566-1199/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2010.04.023 * Corresponding author. Tel.: +49 8928925320; fax: +49 8928925337. E-mail address: baierl@nano.ei.tum.de (D. Baierl). Organic Electronics 11 (2010) 1199–1206 Contents lists available at ScienceDirect Organic Electronics journal homepage: www.elsevier.com/locate/orgel