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 J −V measurements.
Table 2 Photovoltaic parameters of OSCs based on PM6:NFREAs
under AM1.5G illumination, 100 mW cm-2.