Four organic solar cell (OSC) devices with the bilayer heterojunction architecture were investigated, where carbon nanotubes (CNTs) were doped within the acceptor layer. The power conversion efficiency (PCE) of the CNT-incorporated device with a concentration of 0.004 wt% is approximately
20% point higher than that of the reference one. As the concentration of CNTs became higher, the PCE of the devices deteriorated; this could be caused by the percolative connection of CNTs within the layer. The voltage dependence on the effective lifetime of the charge carriers, determined
by Cole–Cole curves of the impedance analysis, was different for the reference and CNT-incorporating devices—the lifetime of the CNT-incorporated ones was shorter, possibly owing to the high local electric field near the CNTs. Controlling the concentration of CNTs below the critical
concentration of percolation is a key factor in achieving high photovoltaic performance.
Effect of polar molecules on the transport and localization of charge carriers in molecular materials
