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 SCP lightα. All the α values of the devices are approaching 1, indicating negligible bimolecular recombination in the devices. Meanwhile, V OCnkT/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.