Space-Charge-Limited Bipolar Currents at High Fields in Polymer/C₆₀ Diodes: A Simple Model Description

Abstract

We investigate the dark transport properties of organic photovoltaic devices formed by a heterojunction of a semiconducting co-polymer and the fullerene (C60) [. The copolymers are composed of thiophene, phenylene and fluorene units, where the thiophene content was kept constant while the fluorene/phenylene ratio was varied. Measurements show that for low voltage, the current versus voltage (j x V) characteristics presents an exponential increasing, typical of a diode behavior. On the other hand, the j x V curves are space-charge limited at high voltages. This latter response follows the theoretical predictions proposed by Koehler et al in Ref. [ (see also [), which explains the dark current in polymer/fullerene photodiodes at high V's. To describe the full j x V characteristics, we have extended the analysis in Ref. [ assuming a space-charge dependent inner series resistance. From such model we are able to nicely fit the experimental curves, to estimate the effective mobility in the whole device and to obtain the diode reverse saturation current j0. Finally, the results indicate that the copolymer with the highest effective total mobility is the PFT [(9,9dihexyl-9H-fluorene-2,7-diyl)-1,2-ethenediyl-2,5-thiophene-1,2ethenediy.