Figure 3. a) Hole mobility; b) Electron mobility; c)J SC versus light intensity; d)V OC versus light intensity of the optimized
binary and ternary OSCs.
The dependences of J SC andV OC on light intensity
(P light) of the binary and ternary devices are
shown in Figure 3c, 3d and Table S6 to evaluate the charge
recombination. The relationship between J SC andP light can be described byJ SC ∝P lightα. All the α values of
the devices are approaching 1, indicating negligible bimolecular
recombination in the devices. Meanwhile, V OC ∝nkT/e ln(P light) is used to define the
correlation between V OC and Plight, wherek is the Boltzmann constant, T is the Kelvin temperature,
and e is the elemental charge. Generally, if the slope is close
to kT/e , the dominant recombination mechanism is bimolecular
recombination. If not, the mechanism is a complex recombination process.
The slopes of the V OC -
ln(P light) curves of the PBDB-T: BZ4F-O-1,
PBDB-T: BZO-4Cl and PBDB-T: BZ4F-O-1: BZO-4Cl devices are
1.48kT/q , 1.30kT/q and 1.26kT/q , respectively. The
above results demonstrate that the ternary device possesses a more
suppressed charge recombination.
Active layer morphology has great effects on the exciton dissociation,
charge transport and recombination process, thus the photovoltaic
performance of the resulting OSCs. Differential scanning calorimetry
(DSC) is employed to understand the miscibility of the donor and
acceptor materials (Figure S11). In the heating process, BZ4F-O-1 and
BZO-4Cl exhibit their endothermic peaks at 240.4 oC
and 215.9 oC, respectively. The BZ4F-O-1: BZO-4Cl (1:
1, w/w) mixture shows a medium value of melting temperature at 223.4oC, while the melting peaks of neat BZ4F-O-1 and
BZO-4Cl are disappeared, suggesting good miscibility between BZ4F-O-1
and BZO-4Cl[51].
Atomic force microscopy (AFM) is employed to investigate the surface
morphology of the devices based on PBDB-T: BZ4F-O-1, PBDB-T: BZO-4Cl and
PBDB-T: BZ4F-O-1: BZO-4Cl blends. The AFM images are shown in Figure 4.
The phase images present obvious fiber-like interpenetrating morphology.
According to the height images, all the blend films have uniform
surfaces with the root-mean-square (RMS) roughness of 1.510, 0.967 and
0.972 nm. PBDB-T: BZO-4Cl film possesses the lowest RMS value. When the
BZ4F-O-1 is introduced to the PBDB-T: BZO-4Cl binary system, the RMS
value of the ternary film is slightly increased. It is evident that the
crystallinity and miscibility of the blend might be finely improved,
favoring the charge dissociation and transport. In addition,
transmission electron microscopy (TEM) patterns (Figure 4) also prove
the uniform morphology of the binary and ternary blend films.