Unveiling structure-performance relationships from multi-scales in non-fullerene organic photovoltaics

Unveiling the correlations among molecular structures, morphological characteristics, macroscopic properties and device performances is crucial for developing better photovoltaic materials and achieving higher efficiencies. To achieve this goal, a comprehensive study is performed based on four state-of-the-art non-fullerene acceptors (NFAs), which allows to systematically examine the above-mentioned correlations from different scales. It’s found that extending conjugation of NFA shows positive effects on charge separation promotion and non-radiative loss reduction, while asymmetric terminals can maximize benefits from both terminals. Another molecular optimization is from alkyl chain tuning. The shortened alkyl side chain results in strengthened terminal packing and decreased π-π distance, which contribute high carrier mobility and finally the high charge collection efficiency. With the most-acquired benefits from molecular structure and macroscopic factors, PM6:BTP-S9-based organic photovoltaics (OPVs) exhibit the optimal efficiency of 17.56% (certified: 17.4%) with a high fill factor of 78.44%, representing the best among asymmetric acceptor based OPVs. This work provides insight into the structure-performance relationships, and paves the way toward high-performance OPVs via molecular design.


Supplementary Tables
Supplementary Table 1 Photovoltaic parameters of OPVs based on PM6:BTP-S7 blend under various optimization conditions.

Electroluminescence
Measurement. An external current/voltage source was employed to provide an external electric field to the pristine and blended solar cells. The electroluminescence emissions were recorded with an Andor spectrometer.

FTPS-EQE Measurement. The FTPS-EQE was measured with a Vertex 70 from Bruker
Optics, which was equipped with a quartz tungsten halogen lamp, quartz beam-splitter and external detector option. A low-noise current amplifier (SR570) was used to amplify the photocurrent produced under illumination of the solar cells, with light modulated by the Fourier transform infrared spectroscope (FTIR). The output voltage of the current amplifier was fed back into the external detector port of the FTIR to use the FTIR software to collect the photocurrent spectra. EQEEL Measurement. The EQEEL was recorded with an in-house-built system comprising a Hamamatsu silicon photodiode 1010B, Keithley 2400 source meter (for supplying voltages and recording injected currents), and Keithley 485 picoammeter (for measuring the emitted light intensity). GIWAXS/GISAXS. GIWAXS/GISAXS measurements with Kα X-ray of Cu source (8.05 keV, 1.54 Å) and a Pilatus3R 300 K detector were conducted at a Xeuss 2.0 SAXS/WAXS laboratory beamline. Samples were prepared by spin coating identical chloroform blend solutions as those used in OPVs on Si substrates. The grazing incident angle were 0.2°.

S31
Time-Resolved Photoluminescence Spectroscopy (TRPL). The optically pumped lasing measurements were taken on a home-build far-field microfluorescence system (Olympus, IX73 inverted microscope). The crystal sample was immersed by diethylether and then dispersed onto a glass substrate. The excitation light (515 nm) was generated from the second harmonic of the fundamental output that was seeded by a mode-locked Ti:sapphire laser (Light Conversion Pharos, 1030 nm, < 300 fs, 1 MHz). The excitation light was filtered with a 515 nm band-pass filter and then diverged with a convex lens (f = 500 mm), finally focused down to a 140 μm diameter spot through an objective lens (Olympus MplanFLN, 20x, NA = 0.45). The emission light was collected by the same objective and focused into a spectrograph (Princeton Instruments, Acton SpectraPro, SP-2300i) with a 600 mm -1 grating and detected by a liquid-N2cooled CCD (PyLon 100B excelon). The instrument resolution (FWHM) was ~0.1 nm. All measurements were taken at room temperature with pulse picker = 1000. TRPL decay kinetics were collected using a TCSPC module (PicoHarp 300) and a SPAD detector (IDQ, id100) with an instrument response function ~100 ps. The two-photon pumped lasing performance was measured upon excitation at 1030 nm.

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 750 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: