Designing high‐performance nonfused ring electron acceptors via side‐chain engineering

The side‐chain has a significant influence on the optical properties and aggregation behaviors of the organic small molecule acceptors, which becomes an important strategy to optimize the photovoltaic performance of organic solar cells. In this work, we designed and synthesized three brand‐new nonfused ring electron acceptors (NFREAs) OC4‐4Cl‐Ph, OC4‐4Cl‐Th, and OC4‐4Cl‐C8 with hexylbenzene, hexylthiophene, and octyl side chains on the π‐bridge units. Compared with OC4‐4Cl‐Ph and OC4‐4Cl‐Th, OC4‐4Cl‐C8 with linear alkyl side chain has more red‐shift absorption, which is conducive to obtaining higher short‐circuit current density. Additionally, the OC4‐4Cl‐C8 film exhibits a longer exciton diffusion distance, and the D18:OC4‐4Cl‐C8 blend film displays faster hole transfer, weaker bimolecular recombination, and more efficient exciton transport. Furthermore, The D18:OC4‐4Cl‐C8 blend films may effectively form interpenetrating networks that resemble nanofibrils, which can facilitate exciton dissociation and charge transport. Finally, OC4‐4Cl‐C8‐based devices can be created a marvellously power conversion efficiency (PCE) of 16.56%, which is much higher than OC4‐4Cl‐Ph (12.29%)‐ and OC4‐4Cl‐Th‐based (11.00%) ones, being the highest PCE among the NFREA based binary devices. All in all, we have validated that side‐chain engineering is an efficient way to achieve high‐performance NFREAs.

Based on the aforementioned considerations, we focus on the design and synthesis of novel NFREAs using side chain engineering strategy.More specifically, NFREAs OC4-4Cl-Ph, OC4-4Cl-Th, and OC4-4Cl-C8 have similar molecular skeletons, but different side chains (hexylbenzene, hexylthiophene, and octyl).Side chains can effectively regulate the energy levels, absorption spectra, molecular packings, and blend film morphology of acceptors.As a result, OC4-4Cl-C8 displays a maximum exciton diffusion length, and the corresponding devices exhibit weaker bimolecular recombination, more effective exciton transport, as well as higher and better balanced mobility.Most encouragingly, devices based on OC4-4Cl-C8 demonstrated a champion PCE of 16.56% with a good short-circuit current (J sc ) (24.40 mA cm −2 ) and a suitable open-circuit voltage (V oc ) (0.90 V) with a high fill factor (FF) (75.10%) (Table 2).Our work shows that side-chain engineering is an efficient approach to enable high-performance NFREAs.
F I G U R E 1 Chemical structures of the polymer donor and small molecular acceptors.

TA B L E 2
Photovoltaic parameters of devices based on OC4-4Cl-Ph, OC4-4Cl-Th, and OC4-4Cl-C8.temperature, and additive was performed, for the corresponding optimization process was provided in the supporting information.The OC4-4Cl-C8-based devices can achieve a champion PCE of 16.56% with a good J sc (24.40 mA cm −2 ) and a suitable V oc (0.90 V) with a high FF (75.10%), which is much higher than that 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-Th-based devices exhibit a good light to electricity response between 300 and 890 nm in wavelength; whereas the photocurrent response region of OC4-4Cl-C8-based devices is obviously red-shifted, being consistent with the Ultraviolet-visible (UV-vis) absorption results (vide supra).Ulteriorly, the charge recombination behavior was measured to investigate the reasons for the different photovoltaic parametrizations of OC4-4Cl-Ph, OC4-4Cl-Th, and OC4-4Cl-C8-based OSCs.The charge dissociation probabilities (P diss ) in the devices are assessed using the photocurrent density (J ph ) versus effective voltage (V eff ) curves.According to Figure 3C, the OC4-4Cl-C8-based OSCs deliver higher exciton dissociation efficiency (99.25%) than OC4-4Cl-Ph (94.73%)-andOC4-4Cl-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 sc characteristics (J sc ∝ P light α ) can be used to investigate the charge recombination process.As depicted in Figure 3D, the α values of OC4-4Cl-Ph, OC4-4Cl-Th and OC4-4Cl-C8-based devices are 0.968, 0.950, and 0.977, respectively.38] Energy loss (E loss ) analysis is performed to more fully understand the impact of side chain engineering on V oc values.Furthermore, the E loss of these devices can be systematically studied by using highly sensitive EQE (sEQE) and electroluminescence (EL) 1117measurements. [39,40]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 photoluminescence and absorption spectra of the non-fullerene acceptors.The E g values based on the OC4-4Cl-Ph, OC4-4Cl-Th and OC4-4Cl-C8 devices are 1.50, 1.49, and 1.46 eV, respectively.Thus, the E loss values of OC4-4Cl-Ph, OC4-4Cl-Th-, and OC4-4Cl-C8based 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), [41] L8-BO (0.556 eV), [42] 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 determine charge transferstate (E ct ) which stands for the charge transfer state energy.As shown in Figure S6, the charge generation energy loss (ΔE 2 ) values for OC4-4Cl-Ph, OC4-4Cl-Th-and OC4-4Cl-C8-based devices are calculated to be 0.06, 0.06, and 0.07 eV, respectively.Furthermore, the charge recombination energy loss can be classified into two types: radiative energy loss (∆E rad , ∆E 1 ) and nonradiative energy loss (∆E nor-rad , ∆E 3 ).ΔE 3 can be calculated by the equation:

Exciton diffusion lengths
According to previous reports, a longer exciton diffusion distance is help to suppress charge recombination, which is beneficial for facilitating higher current density. [43]herefore, the exciton diffusion lengths (L D s) of OC4-4Cl-Ph, OC4-4Cl-Th and OC4-4Cl-C8 neat films are estimated by using transient absorption spectroscopy and the exciton-exciton annihilation (EEA) model.The groundstate-bleaching (GSB) signals (Figure S1) emerge after photoexcitation of OC4-4Cl-Ph, OC4-4Cl-Th, and OC4-4Cl-C8 films at 800 nm.The decay dynamics of the singlet excitons are shown in  S1), respectively.The large L D values of OC4-4Cl-C8 mean the longer exciton diffusion distances, which can effectively reduce the exciton recombination. [44]

Charge generation kinetics
The charge-generation process is studied using femtosecond transient absorption (fs-TA) spectroscopy of the blend films.The contour plots of the time-resolved absorption difference spectra of D18:OC4-4Cl-Ph, D18:OC4-4Cl-Th and D18:OC4-4Cl-C8 blend films pumped at 800 nm are shown in Figure 5.The GSB signal of polymer donor can reflect the hole transfer process. [45]Specifically, the kinetics of 602 nm is chosen to represent the D18 GSB dynamics.After excitation, strong GSB peaks around short wavelength region (550-660 nm) appears in OC4-4Cl-Ph-, OC4-4Cl-Th-, and OC4-4Cl-C8-based blend films, which represents the hole transport at the D/A interface.Biexponential function can be used to fit the hole transport kinetic process. [46]The hole transfer process consists of an ultrafast hole transfer process at the D/A contact, as defined by τ 1 , and a diffusion-mediated mechanism, that is, significantly influenced by domain size and aggregation, as defined by τ 2. [47] The τ 1 /τ 2 are estimated to be 2.46/23.95,2.81/22.22,and 1.73/18.95ps for the OC4-4Cl-Ph, OC4-4Cl-Th, and OC4-4Cl-C8 blend films, respectively.Therefore, the D18:OC4-4Cl-C8-based device is more conducive to the dissociation and diffusion of excitons, which is more advantageous to enhance the photovoltaic performance.

CONCLUSION
In summary, we have successfully designed and synthesized three novel NFREAs OC4-4Cl-Ph, OC4-4Cl-Th, and OC4-4Cl-C8 with hexylbenzene, hexylthiophene, and octyl side chains at the π-bridge units.According to our results, the LUMO energy levels decrease from OC4-4Cl-Ph to OC4-4Cl-Th and OC4-4Cl-C8.Notably, the OC4-4Cl-C8 exhibits a longer exciton diffusion distance, and the corresponding blend film (D18:OC4-4Cl-C8) displays faster hole transfer and diffusion-mediated processes, weaker bimolecular recombination, and more efficient exciton transport.Furthermore, the D18:OC4-4Cl-C8 blend films form favourable nano fibril-like interpenetrating networks, which could facilitate exciton dissociation and charge transport.Ultimately, the OC4-4Cl-C8 devices can achieve the highest PCE of 16.56% with a low E loss of 0.56 eV, which is much higher than OC4-4Cl-Ph (12.29%) to OC4-4Cl-Th (11.00%) based ones.To the extent that we know, 16.56% is the highest PCEs of NFREA-based devices up to now.Our work demonstrates that side-chain engineering is an efficient way to fabricate high-performance NFREAs.

C O N F L I C T O F I N T E R E S T S TAT E M E N T
The authors declare no conflict of interest.

D AT E AVA I L A B I L I T Y S TAT E M E N T
Supporting data are available from authors upon reasonable request.
This work financially supported by the National Natural Science Foundation of China (grant numbers: 52173174, 51933001, and 22109080), the Natural Science Foundation of Shandong Province (grant number: ZR2022YQ45), the Taishan Scholars Program (grant numbers: tstp20221121 and tsqnz20221134).A portion of this work is based on the data (GISAXS) obtained at BSRF-1W1A.The authors gratefully acknowledge the cooperation of the beamline scientists at BSRF-1W1A beamline.