Correlating Electronic Structure and Device Physics with Mixing Region Morphology in High‐Efficiency Organic Solar Cells

Abstract The donor/acceptor interaction in non‐fullerene organic photovoltaics leads to the mixing domain that dictates the morphology and electronic structure of the blended thin film. Initiative effort is paid to understand how these domain properties affect the device performances on high‐efficiency PM6:Y6 blends. Different fullerenes acceptors are used to manipulate the feature of mixing domain. It is seen that a tight packing in the mixing region is critical, which could effectively enhance the hole transfer and lead to the enlarged and narrow electron density of state (DOS). As a result, short‐circuit current (J SC) and fill factor (FF) are improved. The distribution of DOS and energy levels strongly influences open‐circuit voltage (V OC). The raised filling state of electron Fermi level is seen to be key in determining device V OC. Energy disorder is found to be a key factor to energy loss, which is highly correlated with the intermolecular distance in the mixing region. A 17.53% efficiency is obtained for optimized ternary devices, which is the highest value for similar systems. The current results indicate that a delicate optimization of the mixing domain property is an effective route to improve the V OC, J SC, and FF simultaneously, which provides new guidelines for morphology control toward high‐performance organic solar cells.


Modification of Chemical Fibers and Polymer Materials, College of Materials Science
and Engineering, Donghua University, Shanghai 201620, P. R. China

Experimental Section and device characterization
General Methods The morphologies of the nanostructures were characterized by transmission electron microscopy (TEM, JEM-1400, JEOL, Japan). The GIWAXS characterization of the thin films was performed at the Advanced Light Source (Lawrence Berkeley National Laboratory) on beamline 7.3.3. The incidence angle was 0.16°, and the beam energy was 10 keV. Samples were prepared under device conditions on the Si substrates. R-SoXS was performed at beamline 11.0.1.2 (ALS, LBNL) with a bean energy of 285.0 eV. Samples were prepared under device conditions on the Si/PEDOT:PSS substrates, then placed in water and transferred to a silicon nitride window.

Chemicals and Reagents
All reagents and chemicals were purchased from commercial sources (Aldrich or Acros) and used without further purification. PM6 and Y6 were purchased from Solarmer company. C 61 DMI was syntehsized by Prof.
Device Fabrication Organic solar cell devices with ITO/PEDOT:PSS/active layer/PFN-Br/Ag structures were fabricated. Patterned ITO glass substrates were sequential cleaned by ultrasonicating in acetone, detergent, deionized water and isopropyl alcohol for 15 min each and then dried under dry oven. The precleaned substrates were treated in an ultraviolet-ozone chamber for 15 min, then a ~40 nm thick PEDOT:PSS (Clevious PVP AI 4083 H. C. Stark, Germany) thin film was deposited onto the ITO surface by spin-coating and baked at 150 °C for 15 min. The active layer solution of PM6:Y6 (1:1.2, wt%, 6.5 mg mL -1 for PM6) in CF (with 0.5% CN solvent additive) and PM6:Y6:FAs (1:1.2:0.2, wt%, 6.5 mg mL -1 for PM6) in CF (with 0.5% CN solvent additive) were stirred at 25 °C for 2h, and then spin-coated on top of the PEDOT:PSS layer (2300 rpm,40s). The optimum thickness of the active layer is 140 nm. The prepared films were treated with thermal annealing at different temperature for 5 min. After cooling to room temperature, a ~5 nm thick of PFN-Br was spin-coated on the top of active layer. Then, those samples were brought into to an evaporate chamber and a 150 nm thick silver layer was thermally evaporated on the PFN-Br layer at a base pressure of 1 × 10 -6 mbar. The evaporation thickness was controlled by SQC-310C deposition controller (INFICON, Germany). Twelve devices were fabricated on one substrate and the active area of each device was 0.05 cm 2 defined by a shadow mask.
Device Characterization. Device performance was measured by using a 510 Air Mass 1.5 Global (AM 1.5 G) solar simulator (SS-F5-3A, Enlitech) with an irradiation intensity of 100 mW cm -2 , which was demarcated by a calibrated silicon solar cell (SRC2020, Enlitech). The J-V characteristics were measured along the forward scan direction from -0.5 to 1 V, with a scan step of 50 mV and a dwell time is 10 ms using a Keithley 2400 Source Measure Unit. EQE spectra were measured by using a solar-cell spectral-response measurement system (QE-R3011, Enlitech).
Ultraviolet photoelectron spectroscopy measurement. Ultraviolet Photoelectron Spectroscopy analysis was conducted using an AXIS Ultra DLD spectrometer (Kratos Analytical Inc., Mancheser, UK) with a He discharge UV lamp with He I radiation (incident photo energy, 21.22 eV), a hemispherical analyzer operating in the fixed analyzer transmission mode and the standard aperture (analysis area: 0.3 mm * 0.7 mm).

SCLC Mobility Measurements.
The electron-only devices were fabricated with ITO/ZnO/PM6:Y6/ZnO/Ag structures and hole-only devices were fabricated with ITO/PEDOT:PSS/PM6:Y6/MoOx/Al structures. The thickness of the active layer is 140 nm. The space charge limited current (SCLC) mobility was calculated according to the Mott-Gurney square law J = 9ε r ε 0 μV 2 /8L 3 , where J is the current density, ε r is the relative dielectric constant of the transport medium component, ε0 is the vacuum permittivity, μ is the electron or hole mobility, V is the effective voltage, and L is the thickness of active layer.
Electroluminescence measurement. Electroluminescence spectrum measurement was conducted by direct-current meter (PWS2326, Tectronix) to provide bias voltage for the test device, and the electroluminescence emissions were recorded by the fluorescence spectrometer (KYMERA-328I-B2, Andor technology LTD). EQE EL measurement. The EQE EL was recorded with an in-house-built system comprising a standard silicon photodiode 1010B, Keithley 2400 source meter (for supplying voltages and recording injected currents), and Keithley 6482 picoammeter (for measuring the emitted light intensity).

Highly sensitive EQE (s-EQE)
The halogen light source (LSH-75, Newport) passed through the monochromator (CS260-RG-3-MC-A, Newport) to form monochromatic light, which was focused on the device to generate electrical signals. Signals were finally collected by the front-end current amplifier (SR570, Stanford) and phase-locked amplifier (Newport). A corrected silicon solar cell (S1337-1010BR) was used as a standard detector.
Transient absorption spectroscopy (TA) 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 550 and 750 nm, 30 fs pulse duration), 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.

Impedance Spectroscopy (IS) measurement
a, The equivalent circuit for impedance spectroscopy fitting. b, Chemical capacitance of the devices with binary and ternary BHJ layer extracted from impedance spectra. The method of obtaining DOS through IS is as reported in the literature. [1,2,3,4] Impedance measurements were carried out by illumination with a 1.5G illumination source (1000 W m -2 ) using a Solar Simulator. The illumination intensity was tuned by filters. Impedance spectra were measured for different light intensities by applying a small voltage perturbation (10 mV rms) at frequencies from 8 MHz to 4 Hz, for different bias voltages. To measure in open circuit voltage conditions a bias voltage equals to V oc at each light intensity was applied. The original data measured at different voltages are shown in Figure S29. These measurements were performed with LCR meeter equipped with a frequency analyzer module, always at room temperature. The equivalent circuit has been broadly used in bulk heterojunction organic photovoltaic devices. [1,2,3,4] R internal are internal (bulk) resistance. R rec and CPE are the recombination resistance and the chemical capacitance, respectively. R rec physically models the recombination paths through the device. CPE relates with the accumulation of photogenerated electrons and holes in the quasi-Fermi levels for electrons (E Fn ) and holes (E Fp ). The chemical capacitance (CPE) were extracted from the low-frequency region. [5,6] It has been reported that the chemical capacitance follows the shape of the electron DOS (g n ) as: [1,2,3,4] = 2 ( ) Then the DOS results are fitted by exponential fitting to extract the DOS information.

Transient photovoltage (TPV) and photocurrent (TPC) measurement.
The lifetime of carriers can be measured by the transient photovoltage measurement. The background illumination was provided by a normal LED light source, and pulsed light was provided by arbitrary wave generator. The photovoltage traces were registered by the oscilloscope. The photocurrente traces were registered with the resistance of 50 Ω, switching open-circuit mode to short-circuit mode. The integrated TPC signal provides a measure of the total charge generated by the laser pulse (∆Q). Empirically, the differential capacitance values are found to follow the exponential dependence on the open-circuit voltage given by = ∆ ∆ = 0 ( ) + , and so the charge-carrier density as a function of V OC is given by treating the device as a parallel-plate capacitor and integrating with respect to voltage, as , where A is the active layer area, and d is the active layer thickness.   Figure S1. UV-vis absorption spectroscopy of pure films.