E loss = E g -qV oc = ∆E rad + (E g - E ct) + ∆E nor-rad = ∆E 1 + ∆E 2 + ∆E 3
of OC4-4Cl-Ph (12.29%) and OC4-4Cl-Th (11.00%). As far as we are aware, 16.56% is the best PCEs for NFREA based devices. Besides, the external quantum efficiency (EQE) measurement is further investigated to confirm the J sc values of OSCs. As depicted in Figure 3b, OC4-4Cl-Ph and OC4-4Cl-Thbased devices exhibit a good light to electricity response between 300 and 890 nm in wavelength; whereas the photocurrent response region ofOC4-4Cl-C8 -based devices is obviously red-shifted, being consistent with the UV-vis absorption results (vide supra ). Ulteriorly, the charge recombination behaviour was measured to investigates the reasons for the different photovoltaic parametrizations of OC4-4Cl-Ph , OC4-4Cl-Th andOC4-4Cl-C8- based OSCs. The charge dissociation probabilities (P diss) in the devices are assessed using the photocurrent density (J ph) vs effective voltage (V eff) curves. According to Figure 3c, theOC4-4Cl-C8 -based OSCs deliver higher exciton dissociation efficiency (99.25%) than OC4-4Cl-Ph(94.73%) and OC4-4Cl-Th(95.05%) based ones, indicating that D18:OC4-4Cl-C8 based devices have higher exciton dissociation and charge collection efficiency. Additionally, the light intensity (P light) dependent J sccharacteristics (JscP lightα) can be used to investigated the charge recombination process. As depicted in Figure 3d, the α values of OC4-4Cl-Ph , OC4-4Cl-Th andOC4-4Cl-C8 based devices are 0.968, 0.950 and 0.977, respectively. Obviously, the α value of OC4-4Cl-C8 is more approaching to 1, suggesting the relatively weak bimolecular recombination and more efficient exciton transport, thus achieving a higher FF value of the corresponding devices.[33-35]
Energy loss (E loss) analysis is performed to more fully understand the inpact of side chain engineering onV oc values. Furthermore, theE loss of these devices can be systematically studied by using highly sensitive EQE (sEQE) and electroluminescence (EL) measurements.[36-37] As depicted in Figure S5, the E loss can be calculated from this equation:E loss=E g-qV oc, where E g is defined based on the intersection of the absorption and photoluminescence spectra of the low-bandgap acceptor. The E g values based on theOC4-4Cl-Ph, OC4-4Cl-Th and OC4-4Cl-C8 devices are 1.50, 1.49, and 1.46 eV, respectively. Thus, theE loss values of OC4-4Cl-Ph ,OC4-4Cl-Th and OC4-4Cl-C8 based devices are calculated to be 0.55, 0.57 and 0.56 eV, respectively, which are equivalent to the high-performance OSCs based on FREAs such as BTP-4Cl (0.552 eV),[38] L8-BO (0.556 eV),[39] etc. Futhermore,E loss is generally contributed by charge generation (E g-E ct, ∆E 2) and charge recombination. The intersection of the sEQE and EL Gaussian fitting curves can be used to determineE ct which stands for the charge transfer state energy. As shown in Figure S6, hence, the charge generation energy loss (ΔE 2) values are calculated to be 0.06, 0.06 and 0.07 eV for OC4-4Cl-Ph , OC4-4Cl-Th andOC4-4Cl-C8 based devices, respectively. Further, the charge recombination energy loss can be divided into radiative energy loss (∆E rad, ∆E 1) and non-radiative energy loss (∆E nor-rad, ∆E 3). ΔE 3 can be calculated by the equation: ΔE 3= -kT ln(EQEEL), where k and T stand for the temperature and the Boltzman constant, respectively. The ∆E 3 values of OC4-4Cl-Ph ,OC4-4Cl-Th and OC4-4Cl-C8 -based devices are 0.18, 0.19 and 0.20 eV, respectively. Finally, by subtracting ∆E 2 and ∆E 3 fromE loss it is possible to calculate∆E 1 based on the OC4-4Cl-Ph, OC4-4Cl-Thand OC4-4Cl-C8 devices as 0.31, 0.32, and 0.29 eV, respectively. The higher E ct values and smaller energy loss can well explain the increased V oc ofOC4-4Cl-Ph based devices. By using the space charge limited current method, the hole (μ h) and electron (μ e) transport mobilities of OC4-4Cl-Ph, OC4-4Cl-Th and OC4-4Cl-C8 -based devices are calculated. Theμ h/μ e ofOC4-4Cl-Ph , OC4-4Cl-Th and OC4-4Cl-C8 -based devices are 1.6×10-4/1.3×10-4, 1.5×10-4/0.9×10-4 and 2.2×10-4/2.0×10-4cm2 V-1 s-1, respectively. The more balanced higher and higherμ h and μ e ofOC4-4Cl-C8 -based devices can be well explained the relatively high J sc and FF values.