Figure 3 (a) JV characteristics, (b) EQE responses, (c) J phV eff, (d) J scP light, (e) TPC, and (f) TPV measurements of the optimized OSCs based on J52:NoCA-17, J52:NoCA-18, and J52:NoCA-19.
To deep understand the variation in photovoltaic performance among the three OSC devices, charge generation and recombination were systematically studied. The photocurrent density (J ph) versus effective voltage (V eff) curves were measured to investigate the exciton dissociation and charge extraction mechanism. The exciton dissociation probability P (E , T ) can be reflected by the ratio of J ph to J sat, whereJ sat. is defined as the saturationJ ph at a sufficiently highV eff, indicating that all photogenerated excitons can be dissociated into free carriers and almost totally collected by the individual electrodes. As shown in Figure 3c, theP diss values were calculated as 95.1%, 96.0%, and 97.3% for the NoCA-17, NoCA-18, and NoCA-19-based devices, respectively. The higher P (E, T) suggests that the J52:NoCA-19 blend film exhibits more efficient exciton dissociation and charge collection, which is consistent with the higherJ sc value. In addition, the J -Vcurves were measured under various light intensities (P light) to investigate the charge recombination properties. The relationship between J sc andP light relationship can be described byJ scP lightα , where the bimolecular recombination can be completely suppressed when the 𝛼 value approaches unity. As shown in Figure 3d, the α values were calculated as 0.98, 0.96, and 0.99 for the NoCA-17, NoCA-18, and NoCA-19-based devices, respectively. The higher α for the NoCA-19 blend system indicates suppressed bimolecular recombination, which may contribute to the higher J sc and FF relative to the other two devices.
The charge extraction and recombination processes were further investigated by measuring the transient photocurrent (TPC) and transient photovoltage (TPV) decay kinetics of three devices. The charge sweep-out times at short-circuit condition were evaluated to be 0.58, 0.48, and 0.30 μs for NoCA-17, NoCA-18, and NoCA-19 based blends, respectively, indicating higher charge extraction efficiency and higher electron mobility (Figure 3e). The carrier lifetimes obtained from the decay traces of TPV measurements (Figure 3f) were 2.35, 3.30, and 8.89 μs for NoCA-17, NoCA-18, and NoCA-19 based devices, respectively. At open-circuit condition, the relatively longer lifetime of free carriers may imply less recombination. These above results are consistent to the improved J sc and FF in J52:NoCA-19 based device.