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

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 degrees C using AM 1.5 G (80 mW/cm(2)) solar simulator (white light). The fill factors (FF), the short-circuit current density (JSC) and the open-circuit voltage (V-OC) initially increased up to 110 degrees 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 degrees C and 100 degrees C. Then, the semicircle radii decrease with increasing annealing temperatures from 105 degrees C and above. However, at V-OC conditions, we observe that the semicircle radii increase with increasing the thermal annealing. To reproduce theoretically the observed colecole impedance plots at 0 V DC and V-OC conditions for different annealing temperatures, we used an equivalent circuit in the framework of the transmission line model, incorporating the chemical capacitance (C-mu), the recombination resistance (R-rec), the transport resistance (R-t) and the contact electrical resistance (Rco). We determined the diffusion time (tau(dif)), the recombination time (tau(rec)), the diffusion length (L-n) at 0 V DC and V-OC voltages. At VOC voltage, average mobility of global carriers for the device is around 4 10 3 cm(2)V(-1)s(-1), which is in good agreement with that derived using PCBM electron-only devices.