a)In chloroform solution; b)In neat film; c)Calculated from the formula:E gopt = 1240/λ onsetb);d)By cyclic voltammetry (CV) measurement.
To investigate the photovoltaic performance of BZO-4F and BZO-4Cl, we fabricate two devices with a conventional structure of ITO/ PEDOT: PSS/ Donor: Acceptor /PDINN[50]/ Ag. PBDB-T is selected as the polymer donor to blend with the acceptors because of the matched electrochemical energy levels and complementary optical absorption (Figures 1b and 1c). The device blend films adopt a bulk heterojunction (BHJ) structure with the optimized processing conditions such as donor/acceptor (D/A) concentration and weight ratio, additive ratio (chloronaphthalene, CN) and thermal annealing temperature/time (Table S1). As shown in the current density-voltage (J -V ) graphs in Figure 2a, the BZO-4F-based binary device shows a moderate PCE of 12.42% with the V OC of 0.73 V,J SC of 26.44 mA cm-2 and FF of 0.65. And the further charge generation, transport, recombination and morphology characterization are shown in Figures S8-S10. By comparison, the BZO-4Cl-based binary device delivers a comparable efficiency of 12.48% with an improved J SC of 26.86 mA cm-2 and a similar V OC, which agrees with the absorption and energy level analysis. The photovoltaic parameters are summarized in Table 2. To further improve the photovoltaic performance limited by the poor V OCand FF, a simple but efficient ternary strategy is considered. BZ4F-O-1, an alkoxyl-substituted conjugated pentacyclic acceptor reported by our group is chosen as the third component into the PBDB-T: BZO-4Cl blend[48]. BZ4F-O-1 has a strong light capture ability in 600-800 nm, which can compensate for the spectra gap between PBDB-T and BZO-4Cl (Figure 1b). Besides, the higher LUMO level of BZ4F-O-1 is favorable for the VOC. Furthermore, the introduction of BZ4F-O-1 might modulate the crystallinity and miscibility properties of the blend, thus improving the blend morphology.
First of all, the photovoltaic performance of PBDB-T: BZ4F-O-1 based binary device is studied. As shown in Figure 2a and Table 2, the PBDB-T: BZ4F-O-1 device shows a PCE of 10.48% with a V OCof 0.774V, J SC of 19.36 mA cm-2and FF of 0.70. The V OC and FF are much higher than that of the PBDB-T: BZO-4Cl binary devices. When BZ4F-O-1 is introduced to the PBDB-T: BZO-4Cl binary system, the PBDB-T: BZ4F-O-1: BZO-4Cl based ternary device achieved an optimal PCE of 15.51% with theV OC, J SC and FF simultaneously improved (Figure S7 and Table S2). The PCE-J SC statistics of the PBDB-T-based OSCs are shown in Figure 2d and Table S3. As far as we know, this is the best value for single-junction OSCs based on the PBDB-T donor and SMAs.