Post-annealing to recover the reduced open-circuit voltage caused by solvent annealing in organic solar cells†
Abstract
The tremendous loss of 246 mV in open-circuit voltage (Voc) 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 Voc, 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 Voc to 225 mV on average. Through analysis of photocurrent density (Jph) versus the effective voltage (Veff) and short-circuit current (Jsc) 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 (J0) value reduced to as low as 6.85 × 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.