Thermal-assisted band to band tunneling at the electron donor/acceptor energy harvesting assembly

Understanding interfacial electronic properties at the electron donor/acceptor energy harvesting assembly in photovoltaic devices is important in that it can provide insights into revealing charge carrier recombination mechanism at the interface. From temperature dependent contact resistance measurements at the aluminum-doped zinc oxide (AZO)/poly-hexylthiophene (P3HT)/aluminum-doped zinc oxide (AZO) structure in which AZO (electron acceptor) acts as highly conductive source and drain electrodes, charge transfer mechanism at the electron donor/acceptor interface is elucidated. In the mechanism, charge transfer process is initiated by band to band tunneling (BTBT) of hole carriers from AZO to P3HT through localized electronic states populated near the highest occupied molecular orbital (HOMO) level of the P3HT close to the AZO favored by energetic alignment between the conduction band edge of AZO and the HOMO of P3HT. Following BTBT, charge transfer process is completed by thermal activated hopping in the P3HT (electron donor) close to the AZO. It is found that thermally activated transport in the P3HT close to the AZO dominates the magnitude of the contact resistance, providing insights into the origin of carrier recombination mechanism at the donor/acceptor interface in photovoltaic devices.