Figure 1. (a) Chemical structures of PM6, PYF-T-o, PJ1-γ and PC71BM; (b) energy level diagram; (c) UV‐Vis absorption spectra of pure PM6, pure acceptor and acceptor:SA (1000:5 weight ratio) films; (d) Schematic diagram showing the difference between blend‐casting and sequential processing; (e) Current density‐voltage (J‐V) curves.
Figure 1a depicts the molecular structures of poly(benzodithiophene-4,8-dione) (PM6), PYF-T-o , PJ1-γ, and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM).Figure 2b illustrates the energy levels of the active layer materials,[26, 47-48] where PJ1-γ exhibits higher LUMO (-3.72 eV) and HOMO levels (-5.61 eV) than PYF-T-o . Therefore, the PJ1-γ based solar cell has a greater potential to realize a high V oc upon similar energy losses. Figure 1cpresents the UV-vis absorption spectra of the pure films of the donor and acceptors as well as those of the polymer acceptor:SA films. The main absorption of PM6 ranges from 500 nm to 650 nm, while the acceptors exhibit strong absorption in the range of 750 nm to 850 nm. The absorption profiles of the donor and acceptor complement each other, allowing for the absorption of a broader range of photons. Furthermore, the introduction of the tiny amount of PC71BM into the active layer does not alter the optical density (in the resolution relevant for OSC devices), which suggests that the role of PC71BM in promoting device performance should mainly be its contribution in morphology and electrical properties. The UV-Vis absorption spectra of the blend films are shown in the Supporting Information (SI, Figure S1 ), which shows a similar donor-to-acceptor ratio between the different films. This is advantageous in minimizing the influence of the donor-to-acceptor ratio on device performance when comparing different device engineering techniques.
Conventional devices with a structure of ITO/PEDOT:PSS/active layer/PDNIT-F3N/Ag were fabricated, where the active layer was prepared using either the BC or the SqP method (see Figure 1d ). To investigate the effect of solvents on device performance, the BC device was fabricated from either chloroform or toluene. More detailed device preparation procedure is available in the SI . The device configurations using the BC method were denoted as (BC)PM6:PYF-T-o , (BC)PM6:PYF-T-o +SA (with the addition of SA), (BC)PM6:PJ1 , and (BC)PM6:PJ1 +SA. The corresponding SqP devices were denoted by (SqP)PM6/PYF-T-o , (SqP)PM6/PYF-T-o +SA, (SqP)PM6/PJ1 , and (SqP)PM6/PJ1 +SA. Figure 1e shows the J-V curves of the devices obtained from the measurements, and the detailed photovoltaic parameters are summarized in Table 1,Table S2 and Table S3 . When toluene (Tol) was used as the processing solvent, the maximum power conversion efficiencies (PCEs) of the (BC)PM6:PJ1 (Tol) and (BC)PM6:PJ1 +SA(Tol) devices are 15.8% and 16.2%, respectively. The best performance for BC devices can only be achieved using chloroform (CF) as the processing solvent: The (BC)PM6:PJ1 and (BC)PM6:PJ1 +SA show maximum PCEs of 16.5% and 16.7%, respectively. As we mentioned above, we also prepared PYF-T-o and PJ1 devices using the SqP method where the processing solvent for both the donor and acceptor materials was toluene. First, in the binary device, the SqP method improved the efficiency of PJ1-γ- based device by 1.6%, resulting in a PCE of 17.4% for the binary device of (SqP)PM6/PJ1-γ, with aV OC of 0.947 V, a J SC of 25.18 mA cm-2, and a FF of 0.731. Next, the addition of the small amount of solid additive successfully boosted the PCE of the toluene-processed SqP device, (SqP)PM6/PJ1 +SA, to 18.0%, which is among the highest PCEs for all-polymer solar cells processed from non-halogen solvent. The improvement by the SA was also observed in the PYF-T-o based all-PSC devices.
Table 1. Summary of photovoltaic parameters for PM6, PYF-T-o , PJ1-γ based all-PCSs processed from different methods, measured under AM 1.5 G illumination at 100 mW cm−2.