Figure 2 Normalized dilute solution (a) and thin film (b) optical absorption spectra of Tz-H and Tz-Cl .
The UV-vis absorption spectra of the two NFREAs in dilute chloroform solution and thin film were shown in Figures 2a and 2b, and the corresponding optical parameters were summarized in Table 1. Due to their similar chemical structures, both Tz-H and Tz-Clexhibited almost identical solution absorption spectra with the maximum peaks located at 717 and 718 nm, respectively. In solid state,Tz-H and Tz-Cl possessed a strong absorption in the region of 600−850 nm, showing a red-shift of ~ 50 nm relative to that in solution state (724 nm). Specially in the S0→S1 band, the absorption intensity of 0–0 vibronic transition was relatively lower than that of 0–1 transition, indicating the formation of H-aggregates in the two NFREA films.[42] The optical bandgaps of Tz-Hand Tz-Cl were 1.41 and 1.43 eV, respectively, according to the equation: E gopt = 1240/λ onset. According to the cyclic voltammetry (CV) measurements, the HOMO and the lowest unoccupied molecular orbital (LUMO) energy levels of Tz-H and Tz-Clwere estimated to be −5.62/−3.94 and −5.65/−3.97 eV, respectively (Figure S1 and Table 1). Compared with Tz-H ,Tz-Cl possessed slightly lower-lying HOMO/LUMO energy levels, which should be attributed to the introduction of electron-withdrawing chlorine atom.
Photovoltaic properties
To investigate the photovoltaic performance of the two NFREAs, OSC devices with an conventional device structure of ITO/PEDOT:PSS/active layer/PDIN/Ag based on PM6:Tz-H and PM6:Tz-Cl blends were fabricated. The detailed optimization process of Tz-H - andTz-Cl -based photovoltaic devices was shown in Tables S2 and S3, including the weight ratio of donor and acceptor as well as the content of additive. The optimized device based on PM6:Tz-H showed an inferior PCE of 6.41%, with an open-circuit voltage (V oc) of 0.838 V, a short-circuit current density (J sc) of 15.74 mA cm−2, and a fill factor (FF) of 48.57% (Figure 3a and Table 2). Impressively, Tz-Cl -based device exhibited a much higher PCE of 11.10%, with simultaneously improvedV oc (0.841 V), J sc (20.71 mA cm−2), and FF (63.72%). To further confirm the accuracy of the J sc values, the external quantum efficiency (EQE) spectra of the devices were shown in Figure 3b. The integrated J sc values obtained from the EQE responses were 15.61 and 20.27 mA cm−2 forTz-H - and Tz-Cl -based devices, respectively, which were well consistent with the JV measurements.
Table 2 Photovoltaic parameters of OSCs based on PM6:NFREAs under AM1.5G illumination, 100 mW cm-2.