Donor Derivative Incorporation: An Effective Strategy toward High Performance All‐Small‐Molecule Ternary Organic Solar Cells

Abstract Thick‐film all‐small‐molecule (ASM) organic solar cells (OSCs) are preferred for large‐scale fabrication with printing techniques due to the distinct advantages of monodispersion, easy purification, and negligible batch‐to‐batch variation. However, ASM OSCs are typically constrained by the morphology aspect to achieve high efficiency and maintain thick film simultaneously. Specifically, synchronously manipulating crystallinity, domain size, and phase segregation to a suitable level are extremely challenging. Herein, a derivative of benzodithiophene terthiophene rhodanine (BTR) (a successful small molecule donor for thick‐film OSCs), namely, BTR‐OH, is synthesized with similar chemical structure and absorption but less crystallinity relative to BTR, and is employed as a third component to construct BTR:BTR‐OH:PC71BM ternary devices. The power conversion efficiency (PCE) of 10.14% and fill factor (FF) of 74.2% are successfully obtained in ≈300 nm OSC, which outperforms BTR:PC71BM (9.05% and 69.6%) and BTR‐OH:PC71BM (8.00% and 65.3%) counterparts, and stands among the top values for thick‐film ASM OSCs. The performance enhancement results from the enhanced absorption, suppressed bimolecular/trap–assisted recombination, improved charge extraction, optimized domain size, and suitable crystallinity. These findings demonstrate that the donor derivative featuring similar chemical structure but different crystallinity provides a promising third component guideline for high‐performance ternary ASM OSCs.


General Experimental Details
All reactions were performed under nitrogen atmosphere and solvents were purified and dried from appropriate drying agents using standard techniques prior to use. Reagents available from commercial sources were used without further purification unless otherwise stated. Flash chromatography was performed by using Silicycle Silica Flash P60 (particle size 40-63 μm, 60 Å, 230-400 mesh) silica gel. Silica gel on TLC-PET foils from Fluka was used for TLC. Precursor 1 and 2 were prepared using literature methods. [1][2][3] All compounds were characterized by NMR spectroscopy on Bruker Avance III Ultrashield Plus instruments (600 MHz). The spectra were referenced on the internal standard TMS.
High-resolution mass spectrometry (HRMS) data was recorded using a Thermo Scientific-LTQ Velos Orbitrap MS. Elemental analyses were obtained commercially through Chemical & Analytical Services Pty Ltd. Note: Spectroscopy-grade CHCl3 was filtered through basic alumina prior to use in order to suppress solvent acidity and avoid undesired protonation reactions that may influence the spectral absorption of the molecular acceptors described in this study.

Synthesis of BTR-OH and Characterizations
Compound 1 (0.5 g, 0.3 mmol) was dissolved in a dry chloroform (10 mL) and few drops of triethylamine was added under nitrogen. Then, compound 2 (0.3 g, 1.3 mmol) was added and resulting solution was heated to reflux and stirred for 12 hours. The reaction mixture was cooled down to room temperature, poured into methanol. The precipitate was filtered and washed several times with methanol. After purified by silica gel column chromatography (eluent: CH2Cl2/MeOH = 98/2, v/v), a dark black solid BTR-OH (0.6 g, 94%) was obtained. 1 Figure S1. Cyclic voltammograms for SM donors BTR and BTR-OH.

Device Fabrication
The all-small-molecule organic solar cells were prepared on glass substrates with tin-doped indium oxide (ITO, 15 Ω/sq) patterned on the surface (device area: 0.08 cm 2 ). Substrates were prewashed with isopropanol to remove organic residues before immersing in an ultrasonic bath of soap for 15 min. Samples were rinsed in flowing deionized water for 5 min before being sonicated for 15 min each in successive baths of deionized water, acetone and isopropanol. Next, the samples were dried with pressurized nitrogen before being exposed to a UV-ozone plasma for 20 min. A thin layer of PEDOT:PSS (~30nm) (Clevios AL4083) was spin-coated onto the UV-treated substrates, the PEDOT-coated substrates were subsequently annealed on a hot plate at 150 °C for 20 min, and the substrates were then transferred into the glovebox for active layer deposition.
All solutions were prepared in the glovebox using the SM donors (BTR or BTR-OH) and the SM acceptor PC71BM; the SM donor BTR-OH was synthesized as mentioned above. The BTR was purchased from 1 Material Tech Inc., and the PC71BM was purchased from lumtech Inc. Optimized devices were obtained by dissolving BTR, BTR-OH and PC71BM in chloroform (CF) using a D/A ratio of 1:0:1, 0:1:1 and 0.8:0.2:1 (wt/wt), total concentration of 40mg/ml. Note: The as-prepared solutions were stirred for 3 hours at room temperature before being spin coat on the PEDOT:PSS substrates. The active layers were spin-coated at an optimized speed of 900~1200 rpm for time period of 45s, resulting in films of 250 to 300 nm in thickness. The active layers were then exposed to solvent vapor annealing (SVA) with Dichloromethane (DCM) vapors for 10-55s.
Then, a ~10nm-thin layer of Phen-NaDPO was coated on top as electron transport layer after SVA treatment. The samples were then dried at room temperature for 1 hour. Next, the samples were placed in a thermal evaporator for evaporation of a 90 nm-thick layer of Silver (Ag) evaporated at 1.5 Å s −1 ; pressure of less than 2x10 -6 Torr. Following electrode deposition, samples underwent J−V testing.
The current density-voltage (J-V) curves of devices were measured using a Keithley 2400 Source Meter in glove box under AM 1.5G (100 mW cm -2 ) using a Enlitech solar simulator. A 2×2 cm 2 monocrystalline silicon reference cell with KG5 filter (purchased from Enli Tech. Co., Ltd., Taiwan). The EQE spectra were measured using a Solar Cell Spectral Response Measurement System QE-R3011 (Enlitech Co., Ltd.). The light intensity at each wavelength was calibrated using a standard monocrystalline Si photovoltaic cell.  Samples for PL spectroscopy were spin-coated onto glass substrates subject to various conditions. Spectra were measured using a spectrofluorometer FluoroMax-4, HORIBA.

Transmission Electron Microscopy (TEM) Characterization
Films were spun-cast on PEDOT:PSS-coated glass substrates. The BTR-based and BTR-OH-based binary and ternary BHJ films were floated off the substrates in deionized water and collected on lacey carbon coated TEM grids (Electron Microscopy Sciences). TEM studies were performed a Thermo Fischer (former FEI) Titan Titan 80-300 TEM equipped with an electron monochromator and a Gatan Imaging Filter (GIF) Quantum 966.

Grazing Incidence Wide-angle X-ray Scattering
Silicon substrates for GIWAXS test were sonicated for 15 min each in successive baths of detergent, DI water, acetone and isopropanol. The substrates were then dried with pressurized nitrogen before being exposed to the UV−ozone plasma for 20 min. The BHJ layers were prepared following methods described in Section of Device Fabrication.
GIWAXS measurements were carried out at 5A beamline of the Pohang Light Source II (PLS-II) in South Korea. The GIWAX images were recorded at 0.13 incidence angle with X-rays of 11.57 keV (λ=1.0716Å) and MAR345 image plate detector.

SCLC Measurements
Hole-only devices were fabricated for the J-V measurements. The device structure was where J is the current density, L is the film thickness of the active layer, µ0 is the hole or electron mobility, εr is the relative dielectric constant of the transport medium, ε0 is the permittivity of free space (8.85 × 10 -12 F m -1 ), V (= Vappl -Vbi) is the internal voltage in the device, where Vappl is the applied voltage to the device and Vbi is the built-in voltage due to the relative work function difference of the two electrodes. 11. Solution NMR Spectra Figure S5. 1 H NMR spectrum of BTR-OH in CDCl3.