Figure 3 (a) J −V characteristics, (b) EQE
responses, (c) J ph−V eff,
(d) J sc−P 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 sc ∝P 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.