Open-Circuit Voltage Characteristics in Polyfluorene Based Photovoltaic Devices

Abstract

AbstractIn this work the origin of the open-circuit voltage is studied in bi-layer devices based on polyfluorenes. The devices were fabricated using a lamination technique, which avoids the problems that usually arise when making well-defined bi-layers from soluble polymers. Often the open-circuit voltage in devices is attributed to the difference in the workfunctions of the electrodes. Whilst a cathode dependence of the open-circuit voltage was observed here, an additional constant voltage of ∼1V was also present in the devices investigated. We conclude that this additional open-circuit voltage is largely a result of a photo-induced dipole at the donoracceptor interface. This dipole is formed by charges that are separated as a result of exciton dissociation when the device is illuminated. Devices where the order of the polymer layers was reversed showed very different open-circuit voltages which confirms that the open-circuit voltage is heavily dependent on the structure of the active layer in the device. The effects of device thickness and excitation intensity on open-circuit voltage are also discussed.