Figure 3 (a) J −V characteristics, (b) EQE
responses, (c) J ph−V eff,
(d) J sc−P light, (e) TPC,
and (f) TPV measurements of the optimized OSCs based on
PM6:Tz-H and PM6:Tz-Cl .
To further understand the charge recombination and extraction processes,
transient photocurrent (TPC) and transient photovoltage (TPV)
measurements were carried out. As presented in Figure 3e, the charge
sweep-out times under the short-circuit condition were roughly estimated
to be 0.97 and 0.84 μs, while the charge carrier lifetimes obtained from
the decay traces of TPV measurements (Figure 3f) were 1.59 and 1.83 μs
for Tz-H - and Tz-Cl -based devices, respectively. The
shorter charge extraction time
can be attributed to the higher electron mobility, and the relatively
longer lifetime of free carriers at open-circuit voltage may indicate
less recombination in the Tz-Cl -based device.
Film morphology
To evaluate the miscibility between the two NFREAs and PM6,
Flory-Huggins interaction parameters (χ ) were estimated by using
contact angle measurements. The contact angles of two liquids (deionized
water and ethylene glycol) on the neat PM6, Tz-H , andTz-Cl films were investigated (Figure
4a). The surface tensions
(γ ) of PM6, Tz-H , and Tz-Cl were calculated to
be 19.75, 26.02, and 23.90 mN m−1, respectively (Table
S4). According to the empirical equation\(\chi\ =\ K(\sqrt{\gamma_{D}}-\sqrt{\gamma_{A}})^{2}\), whereK is a constant, the χ PM6:Tz-H and χ PM6:Tz-Cl were 0.43 and 0.19K , respectively, indicating a better D/A miscibility for the
PM6:Tz-Cl blend film.