Annealing temperature dependence of the performance of bulk heterojunction polymer: Fullerene solar cells under short and open circuit conditions

Abstract The current density-voltage(J-V) characteristics and cole-cole impedance plots (-Im(Z) vs Re(Z)) of ITO/PEDOT:PSS/AnE-PVab:PCBM/Ca/Ag solar cells were measured under thermal annealing range 25–125 °C using AM 1.5 G (80 mW/cm2) solar simulator (white light). The fill factors (FF), the short-circuit current density (JSC) and the open-circuit voltage (VOC) initially increased up to 110 °C and then decayed considerably for higher annealing temperatures. Under short circuit conditions at 0 V DC, The Cole-Cole plots demonstrate an increase in the semicircle radius for devices annealed at 25 °C and 100 °C. Then, the semicircle radii decrease with increasing annealing temperatures from 105 °C and above. However, at VOC conditions, we observe that the semicircle radii increase with increasing the thermal annealing. To reproduce theoretically the observed cole-cole impedance plots at 0 V DC and VOC conditions for different annealing temperatures, we used an equivalent circuit in the framework of the transmission line model, incorporating the chemical capacitance (Cµ), the recombination resistance (Rrec), the transport resistance (Rt) and the contact electrical resistance (Rco). We determined the diffusion time ( τ d i f ) , the recombination time ( τ r e c ) , the diffusion length (Ln) at 0 V DC and VOC voltages. At VOC voltage, average mobility of global carriers for the device is around 4 10−3 cm2V−1s−1 , which is in good agreement with that derived using PCBM electron-only devices.