High sensitivity organic photodiodes with low dark currents and increased lifetimes

Organic semiconductors hold the promise for large-area, low-cost image sensors and monolithically integrated photonic microsystems. This requires the availability of photodiodes offering at the same time high quantum efficiency, low noise and long lifetimes. Although published structures of organic photodiodes offer high external quantum efficiencies (EQE) of up to 76% [F. Padinger, R.S. Rittberber, N.S. Sariciftci, Effects of postproduction treatment on plastic solar cells, Advanced Functional Materials 13 (2003) 1, P. Schilinsky, C. Waldauf, C.J. Brabec, Recombination loss analysis in polythiophene based bulk heterojunction photodetectors, Applied Physics Letters 81 (20) (2002) 3885], [1] and [2] they normally suffer from short lifetimes of only a few hundred hours as well as large dark currents. In our work the lifetime of a poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) heterojunction photodiode structure was increased to several thousand hours by omitting the widely used poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) anode layer. In addition, a simple model of optical interference and absorption effects was used to find the optimum thickness that combines high quantum efficiency with low dark current. As a result, we report on organic photodiodes with state-of-the-art EQE of 70% at 0 V bias, an on/off current ratio of 106 at −1 V and 40 mW/cm2 illumination, dark current densities below 10 nA/cm2 at −1 V, and a lifetime of at least 3000 h.