Post-annealing to recover the reduced open-circuit voltage caused by solvent annealing in organic solar cells

The tremendous loss of 246 mV in open-circuit voltage (V-oc) upon solvent annealing in p-DTS(FBTTh2)(2):PC71BM solar cells that has been observed could be recovered by post-annealing. Following solvent annealing, subsequent thermal annealing of the active layer could not achieve full recovery of V-oc, which is attributed to minor variation in morphology, crystalline behavior and absorption edge. However, after completing fabrication of the entire device, subsequent post-annealing succeeded in recovery of V-oc to 225 mV on average. Through analysis of photocurrent density (J(ph)) versus the effective voltage (V-eff) and short-circuit current (J(sc)) versus light intensity, it is shown that the charge collection ability increased and bimolecular recombination decreased in the device after post-annealing. Based on dark J-V characteristics and fitting curves, the reverse saturation current (J(0)) value reduced to as low as 6.85 x 10(-11) A cm(-2), demonstrating that the method of post-annealing has an advantage over thermal annealing, as it leads to better interfacial contact between active layer and back electrode.