Revealing the Sole Impact of Acceptor's Molecular Conformation to Energy Loss and Device Performance of Organic Solar Cells through Positional Isomers

Abstract Two new fused‐ring electron acceptor (FREA) isomers with nonlinear and linear molecular conformation, m‐BAIDIC and p‐BAIDIC, are designed and synthesized. Despite the similar light absorption range and energy levels, the two isomers exhibit distinct electron reorganization energies and molecular packing motifs, which are directly related to the molecular conformation. Compared with the nonlinear acceptor, the linear p‐BAIDIC shows more ordered molecular packing and higher crystallinity. Furthermore, p‐BAIDIC‐based devices exhibit reduced nonradiative energy loss and improved charge transport mobilities. It is beneficial to enhance the open‐circuit voltage (V OC) and short‐current current density (J SC) of the devices. Therefore, the linear FREA, p‐BAIDIC yields a relatively higher efficiency of 7.71% in the binary device with PM6, in comparison with the nonlinear m‐BAIDIC. When p‐BAIDIC is incorporated into the binary PM6/BO‐4Cl system to form a ternary system, synergistic enhancements in V OC, J SC, fill factor (FF), and ultimately a high efficiency of 17.6% are achieved.


Characterization
The 1 H NMR and 13 C NMR spectra were measured by Bruker AVANCE 400 or 700 MHz spectrometer. Mass spectra were performed by Bruker Daltonics Biflex III MALDI-TOF Analyzer in the MALDI mode. The ultraviolet-visible light (UV-vis) absorption spectra were measured using the JASCO-570 spectrophotometer (JASCO. Inc., Japan) in solution (chloroform) and the thin film (on a quartz substrate).

SCLC Measurements
Hole-only or electron-only devices were fabricated as follows: ITO/PEDOT:PSS/active layer/Au for holes and ITO/ZnO/active layer/Ca/Al for electrons. The mobility was extracted by fitting the J-V curves using space charge limited current (SCLC) method, [S3] which Here, J refers to the current density, μ is hole or electron mobility, ɛ r is relative dielectric constant of the transport medium, which is equal to 3, ɛ 0 is the permittivity of free space (8.85 × 10 -12 F m -1 ), V= V appl -V bi , where V appl is the applied voltage to the device, and V bi is the built-in voltage due to the difference in work function of the two electrodes (for hole-only diodes, V bi is 0.2 V; for electron-only diodes, V bi is 0 V).
E 0 is characteristic field, d is the thickness of the active layer and was measured by KLA-Tencor Alpha-Step D-600 Stylus Profiler.

EL Measurement
EL measurement was conducted by direct-current meter (PWS2326, Tectronix) to provide bias voltage for the test device, and the EL spectra were recorded by the fluorescence spectrometer (KYMERA-328I-B2, Andor technology LTD) with cooled silicon array and indium gallium arsenic detector, which was calibrated by standard light source (Ocean Optics).

EQE EL Measurement
The EQE EL was recorded with a built-in-house system comprising a standard S7 silicon photodiode (S1337-1010BR, Hamamatsu Electronics), Keithley 2400 source meter (for supplying voltages and recording injected currents), and Keithley 6482 picoammeter (for measuring the emitted light intensity).

Transient photovoltage (TPV) decay measurement
For TPV measurements, we use an LED connected to a Keithley 2400 as a background light source, while we use another LED as a pulsed light source. The pulsed light is controlled by an arbitrary wave generator (Tektronix, AFG3022C), and the final signal acquisition and processing is performed by an oscilloscope (Tektronix, MDO4104C).

The theory of V loss in OSCs
Under the open-circuit condition, the generation (G) rate and recombination (R) rate of charge carriers must be balanced (G = R). Assuming that the recombination of charge carriers takes place via CT states:

S8
(1) where k is the total recombination rate constant for the CT state decay, which is the sum of the radiative and the non-radiative recombination rate. [S4] (2) N CT is the density of populated CT states, which can be expressed as: [S5] ( ) where N CTC is the density of CT state complex.
The equation (3) could be reformulated, we then get the relation for the V OC : We identify the second term on the right side of Equation (4) to the overall voltage losses (ΔV OC ) Using equation (2) in equation (5), we derive ( ΔVr is the radiative voltage losses and ΔV nr is the non-radiative voltage losses. ( ) Now, could be divided in two parts: [S6] ΔV 1 and ΔV 2 S9 (8) Where, ΔV 1 is the inevitable part of the radiative recombination voltage loss, derived by the SQ, assuming that the absorption spectrum of the solar cell is a step-like function. ΔV 2 is the additional radiative voltage loss due to the fact that the real absorption spectrum of the solar cell is not a step-like function.

Transient Absorption Spectroscopy (TAS) Measurement
For femtosecond transient absorption spectroscopy, the fundamental output from Yb:KGW laser (1030 nm, 220 fs Gaussian fit, 100 kHz, Light Conversion Ltd) was separated to two light beam. One was introduced to NOPA (ORPHEUS-N, Light Conversion Ltd) to produce a certain wavelength for pump beam (here we use 720 nm), the other was focused onto a YAG plate to generate white light continuum as probe beam. The pump and probe overlapped on the sample at a small angle less than 10°. The transmitted probe light from sample was collected by a linear CCD array.
Then we obtained transient differential transmission signals by equation shown below: T T

T pump on T pump off T pump off
All the samples were measured in vacuum environments.

GIWAXS Measurements
GIWAXS measurement were accomplished with a Xeuss 2.0 WAXS/SAXS laboratory beamline using a Cu X-ray source (8.05 keV, 1.54 Å) and a Pilatus3R 300K detector. GIWAXS samples were prepared on silicon substrate by spin coating. S11 Scheme S1. Synthetic routes to m-BAIDIC and p-BAIDIC.       Representative TA spectra at indicated delay time.