Conductivity, charge carrier mobility and ageing of ZnPc/C60 solar cells

We have investigated conductivity and ageing of ZnPc/C60 solar cells as they are influenced by the charge carrier mobility and variation of the potential barrier height. The test structures with a reasonable energy conversion efficiency of ∼1.5% were investigated. The samples were aged for 1300 h upon illumination by the blue LED with peak emission at 475 nm. Upon ageing the devices showed a strong and fast degradation of the efficiency, short circuit current and of the fill factor within several hours followed by a much slower decrease of them. The reference samples kept in the dark at the room temperature did show only very small changes in their I–V curves. Carrier mobility dependencies on electric field strength at different temperatures were measured by the Charge Extraction by Linearly Increasing Voltage (CELIV) method. It was demonstrated that mobility values decrease during degradation as compared to the reference samples. Nevertheless only mobility changes cannot explain the observed drop of device current. The increase of the effective barrier height by about 0.1 eV from ∼0.55 eV up to ∼0.65 eV was observed in the aged samples. Meanwhile thermal activation energy values of the electrical conductivity grew from about 0.28 eV prior to degradation up to about 0.34 eV after ageing.