Current Conduction in Poly(3-Hexylthiophene) and in Poly(3-Hexylthiophene) doped [6,6]-Phenyl C61-Butyric Acid Methylester Composite Thin Film Devices

Transport properties of poly(3-hexylthiophene) (P3HT), and of its blend with [6,6]-phenyl C61-butyric acid methylester (PCBM), were studied by analysing temperature dependent current voltage characteristics of spin cast thin films sandwiched between aluminium electrodes in a metal insulator metal (MIM) configuration. It was found that in Al/P3HT/Al devices, the current is limited by space charge that accumulates near the hole injecting electrode due to the poor bulk transport properties of P3HT. At low temperatures and high applied electric fields the current density obeys a power law of the form J similar to V-m, characteristic of space charge limited current (SCLC) in the presence of exponentially distributed traps within the band gap. These traps are filled by charge that is injected by quantum mechanical tunnelling, which is adequately described by the Fowler-Nordheim (FN) theory. By calculating the majority charge carrier mobility in Al/P3HT/Al and Al/P3HT:PCBM/Al devices from the Ohmic, SCLC, and FN tunnelling fits at different temperatures, we have obtained that the charge carrier mobility in P3HT is two orders smaller than the electron mobility in the P3HT:PCBM blend at room temperature, but comparable at low temperatures. This information is important in determining the origin of open circuit voltage and short circuit current limit in solar cells that employ this blend as the active layer.