A Rational Design of Isoindigo‐Based Conjugated Microporous n‐Type Semiconductors for High Electron Mobility and Conductivity

Abstract The development of n‐type organic semiconductors has evolved significantly slower in comparison to that of p‐type organic semiconductors mainly due to the lack of electron‐deficient building blocks with stability and processability. However, to realize a variety of organic optoelectronic devices, high‐performance n‐type polymer semiconductors are essential. Herein, conjugated microporous polymers (CMPs) comprising isoindigo acceptor units linked to benzene or pyrene donor units (BI and PI) showing n‐type semiconducting behavior are reported. In addition, considering the challenges of deposition of a continuous and homogeneous thin film of CMPs for accurate Hall measurements, a plasma‐assisted fabrication technique is developed to yield uniform thin films. The fully conjugated 2D networks in PI‐ and BI‐CMP films display high electron mobility of 6.6 and 3.5 cm2 V−1 s−1, respectively. The higher carrier concentration in PI results in high conductivity (5.3 mS cm−1). Both experimental and computational studies are adequately combined to investigate structure–property relations for this intriguing class of materials in the context of organic electronics.

The brick red reaction mixture was left to cool at room temperature and then filtrated by suction filtration.The resulting crude product was recrystallized by 1,2-dichlorobenzene to afford a dark brown solid (1.83 g, 98%), m.p. > 260 °C.

Instruments and Methods
Powder X-ray diffraction (PXRD): Powder X-ray diffraction measurements were performed on Rigaku Smart Lab II with Cu Kα (λ = 1.5405Å) radiation source operating at 40 kV and 40 mA.The patterns were recorded with a divergent slit of 1/16° over the 2θ range of 2-50° with step size = 0.02°.

Fourier transform infrared (FT-IR):
FT-IR spectra were taken on a Bruker Optics ALPHA-E spectrometer with a universal Zn-Se ATR (attenuated total reflection) accessory in the 600-4000 cm -1 region or using a Diamond ATR (Golden Gate) with 24 scan rate and 4 cm -1 resolution.
Solid-state 13 Carbon Cross-Polarization Magic Angle Spinning (CP MAS): 13 C CP MAS NMR spectra of the COFs were recorded on a Bruker Avance NEO 500MHz NMR spectrometer using a 4.0 mm MAS probe at ambient temperature and a magic angle spinning rate of 12.0 kHz.Spectra were acquired using a CP contact time of 2000us, a recycle delay of 2 sec and a total number of 42200 scans. 13C chemical shifts were externally referenced to the adamantane CH2 signal at 38.46 ppm.NMR data were processed using the software "TopSpin 4.1.4".

N2 adsorption
Porosity analyses were performed on the Anton Paar Autosorb iQ combined physisorption and chemisorption instrument.For each measurement, 20-30 mg of COF samples were used.The samples were activated at 80 °C for 16 hours before being subject to N2 gas adsorption in liquid N2 bath (77 K) to collect full isotherms.Surface areas were calculated using the multipoint Brunauer -Emmett -Teller (BET) model, and pore size distributions were found using the non-local density functional theory (NLDFT).P and P0 are the equilibrium at the saturation pressure of N2.

Scanning Electron Microscopy (SEM):
The FEI Nova NanoSEM 650 was employed for the SEM analysis of COFs samples.It combines an electron column with semi-in-lens detectors and an in-the-lens Schottky field emission gun to deliver ultrahigh-resolution with a wide range of probe currents (1 pA to more than 200 nA).With a voltage of 2.0 keV, the SEM images were recorded.The SEM samples were prepared by drop-casting (10 μL) of COFs dispersions in isopropyl alcohol on a silicon substrate and drying in air.
After complete evaporation of the solvent, the samples were coated with Pt (nano-sized film) using the JEOL JEC-300FC Auto Fine before SEM analysis.

Transmission Electron Microscopy (TEM):
The TEM images were recorded by FEI Tecnai TEM 20 kV.The TEM samples were prepared by the COFs powder dispersed in isopropyl alcohol solvent by 15-minute sonication, drop cast on carbon-coated copper grids TEM Window (TED PELLA, INC. 300 mesh), and allowed to dry overnight in desiccators.

X-ray photoelectron spectroscopy (XPS)
XPS measurements were performed using a Supra+ instrument (Kratos, Manchester, UK) equipped with an Al Kα excitation source and a monochromator.The charge neutralizer was on during the measurements.The take-off angle was 90°.XPS measurements and data processing were performed using ESCApe 1.5 software (Kratos).The powder samples were placed on a carbon tape attached to the silicon wafer.The area analysed was 300 by 700 microns.The measurements were performed at a pass energy of 20 eV.The base pressure in the main analysis chamber was 8•10 -8 mbar.The binding energy scale was corrected based on the C-C/C-H peak at 284.8 eV in the C 1s spectrum.

Cyclic Voltammetry (CV)
The electrochemical cyclic voltammetry experiment was carried out on a CHI 600 electrochemical workstation using a platinum disk, a platinum wire, and an Ag/Ag + electrode as working, auxiliary, and reference electrodes, respectively.The experiment was carried out in a deoxygenated anhydrous acetonitrile solution of tetra-n-butylammonium hexafluorophosphate (0.1 M) under an atmosphere of nitrogen at a scan rate of 50 mV s -1 .
The potential of the Ag/Ag + reference electrode was internally calibrated using the known energy level (-4.8) of the ferrocene/ferrocenium redox pair (Fc/Fc + ).The HOMO and LUMO energy levels were deduced from the oxidation onset (E ox onset) and the reduction onset (E red onset), respectively, using the following equations: EHOMO = -(4.8+Eox onset) and ELUMO= -(4.8+E red onset).

Ultraviolet photoelectron spectroscopy (UPS)
UPS analyses were performed on a Thermo Fisher Scientific Instruments UK, Sr.No.-KAS2020.Ultraviolet photoemission valance band spectrum (UPS) was performed on Thermo ESCALAB Xi+ with Helium light source (hv=21.2eV).Photoelectron spectrometer was under an ultrahigh vacuum of about 3×10 -9 Torr at 300 K.For UPS measurements, the samples were biased at -10 V for BI and -12 V for PI to observe the low-energy secondary electron cutoff.The samples were prepared by drop-casting BI and PI dispersions in isopropanol (10 μL) on a silicon substrate (freshly cleaned with pure isopropanol; dried) and dried under Ar in a desiccator.After complete evaporation of the solvent, the sample was directly used for UPS analysis.

Method for chemical exfoliation and thin preparation of CMPs (BI and PI)
The solution-based thin films, used in the Hall effect measurements, are developed by chemical exfoliation.50 mg of the CMP (BI and PI) powder is dispersed in 50 mL NMP (N-Methyl-2-Pyrrolidone) followed by sonicating the mixture for 8 hours using an ultraprobe-sonicator at a 10s/2s duty cycle in an ice bath.The solution is then centrifuged at 1500 rpm for 60 min to filter the unexfoliated particles and at 7500 rpm for 30 min to remove the insoluble impurities.After filtering, the CMPs are re-dispersed in 50 mL of IPA.

Hall effect measurement (HMS)
HMS was employed to characterize conductivity type, mobility, and carrier density in BI and PI thin films using Ecopia HMS-5000.The electrical contact was made by placing silver conductive paste on the four corners of thin films.The sample then was measured at a constant current in dark conditions at 25 ο C and a typical magnetic field of 0.55 T.

Surface Plasma Treatment
To employ plasma treatment on the surface of drop-casted thin films.Radio frequency (RF) glow discharge in the ambient gas of oxygen was used.By altering the surface of the substrate using plasma, extra charged electrons are lingering on the surface of the thin film, seeking lower energy states.The surface oxygen plasma activation is optimized in a RIE plasma reactor (SAMCO's RIE-200iP) with 90 W RF power at 50 sccm flow rate for 3 min.

Detailed information on Hall effect measurements and calculations
The substrate was chosen as a p-doped (100) silicon (n-Si) wafer, with 1-10 Ω.cm resistivity, and topped with a 300 nm wet thermal oxide.Before drop casting, 1×1 cm 2 square pieces of this wafer were first cleaned in acetone using ultrasonication and then rinsed with IPA, deionized water, and dried with N2 spray.20 µL of exfoliated polymer were drop-casted on the substrates for a total thickness of around 0.2 µm.The silver paste was applied at the corners of the films, which served as contacts.Then the measurements were carried out using the Ecopia HMS-5000 Hall effect measurement system by applying a constant current (1 mA) in dark conditions at 25 ο C and a typical magnetic field of 0.55 T.
The Hall-Effect measurement relies on the application of a magnetic field perpendicular to the direction of current flow in a conducting sample, resulting in a transverse voltage known as the Hall voltage.This voltage can be measured to extract information about the polymer material's electronic properties.Here's a summary of the process flow used to conduct the Hall-effect measurements: 1. Sample preparation: A 1×1 cm 2 square piece of the sample is prepared for measurements.The silver paste is applied at the corners of the sample, which served as current contacts, while a magnetic field is applied perpendicular to the current path.
2. Current and magnetic field setup: A constant current, I of 1 mA, is passed through the sample using the contacts.Simultaneously, a magnetic field, B of 0.55 T, is applied perpendicular to the current direction.The magnetic field is sufficiently strong to cause a measurable Hall voltage but not strong enough to affect the polymer's electronic properties.

Hall voltage measurement:
As the current flows through the sample under the influence of the magnetic field, a voltage perpendicular to both the current and the magnetic field, known as the Hall voltage (VH), is generated.The Hall voltage is measured using a voltmeter.

Hall coefficient calculation:
The Hall coefficient (RH) is related to carrier density (n) and mobility (μ).It can be calculated using the following formula: (1) Where t is the polymer film thickness in meters.Hall coefficient (RH) represents the ratio of the Hall voltage (VH) to the product of the magnetic field strength (B) and current (I).

Carrier density calculation:
The Hall coefficient (RH) allows the calculation of the carrier density (n) through the equation: (2) where q is the elementary charge.

Mobility calculation:
The carrier mobility (μ) can be calculated using the formula: where ρ is the resistivity.

Theoretical Framework
In order to shed some light from the theoretical perspective into the transport properties of both PI and BI compounds we have computed, as a proof of concept, the intrinsic carrier mobilities for the two monolayers within the deformation potential (DP) formalism. [2]to describe charge transport in nonpolar semiconductors.DP approach can be simplified into an effective mass approximation. [3,4]Given the structural symmetry of both systems, we consider the elastic modulus and effective mass purely isotropic within the 2D layered structures.Thus, the main DP parameters to be investigated are the elastic constant, the deformation constant and the effective mass. [5]The equation used to estimate the 2D sheet intrinsic carrier mobility is the following: where m * is the effective mass; τ is the relaxation time, T is the temperature, E 1 is the DP constant, which represents the strain-induced shift of the band edges, and C 2D is the elastic modulus, obtained from the lattice distortion by the strain.The main assumption is our case is the isotropy in the semiconductor 2D sheets, where electrons and phonons will behave in the same way in the two lattice directions.
Computational details.We have carried out a large battery of Density Functional Theory (DFT) based calculations to investigate the structural and electronic properties of both compounds, as well as all the DP parameters to get the intrinsic carrier mobilities.As a starting point, molecular fragments involved in the formation of the systems were studied within the framework of the DFT implemented within the Gaussian16 program. [6]For this purpose, we used as level of theory the hybrid generalized gradient approximation (GGA) long-range corrected hybrid functional CAM-B3LYP [7] together with the cc-pVDZ [8] basis set.All geometrical freedom degrees were allowed to vary independently.The calculated geometries of the molecular building blocks were confirmed as minima by frequency calculations.Periodic boundary conditions were used to perform simultaneous structure + cell optimizations of different stacked 3D layered system models based on their canonical 2D layer structures.The optimized molecular building-blocks were used to construct the different 2D networks with the QUANTUM EXPRESSO plane-wave DFT code [9] by using the GGA-PBE functional [10] to account for the exchange-correlation (XC) effects, and the Grimme DFT-D3 semi-empirical efficient vdW correction to include dispersion forces and energies in conventional DFT functionals [11] Ultra-soft pseudopotentials were used to model the ion-electron interaction within the atomic species. [12]Brillouin zones were sampled by means of optimal [2×2×1] and [2×2×8] Monkhorst-Pack grids [13] for the 2D layers and 3D crystals, respectively.One-electron wave-functions are expanded in a basis of plane-waves with a kinetic energy cutoff of 40 and 300 Ry for the kinetic energy and electronic density, respectively, which achieve sufficient accuracy to guarantee a full convergence in total energy and electronic density.In the full structure + cell optimizations for the different 3D crystal models the atomic relaxations were carried out within a conjugate gradient minimization scheme until the maximum force acting on any atom was below 0.02 eV Å -1 , including relaxation of interlayer distances.The crystal-bulk models were analyzed for their both eclipsed (AA) and staggered (AB) configurations, and some other intermediate ones.
Computation of the DP parameters.Within the DP theory we only consider the acousticphonon scattering mechanism, and the fundamental DP constants to be computed are the elastic constant (C2D), the deformation constant (E1) and the effective electron mass (me * ) for both PI and BI compounds.
, where E is the total energy of the unit cell, δ is the uniaxial strain applied to both lattice directions simultaneously, and Δ = S/S 0 describes the change in the surface area at a dilatation.C2D is then obtained by parabola fitting energy-strain curves.By dilating the lattice in the same way as for the calculation of C2D, E 1 can be calculated as dE/dδ, where E is the energy of CB edge for the δ strain.The effective mass, m * , is calculated using ℏ 2 [∂ 2 ε(k)/ ∂k 2 ] −1 , where ε(k) is the band energy, in this case, at  point to mimic the average electronic properties obtained from the experiments.For parabolic bands, the electron will move much like a free particle with m*, which results in constant effective masses.
After the simultaneous cell + structure relaxations, and after testing a bunch of different interlayer stacking configurations, the results of the simulations yield a structure for the PI compound with a unit cell of [2.28×2.28]nm 2 and an angle between lattice parameters of 104.1 o , with a preferential stacking configuration close to the eclipsed (AA) one and an interlayer distance of 5.11 Å, whilst the BI compound exhibits a perfectly hexagonal P6 symmetry with a unit cell of [3.55×3.55]nm 2 and an angle between lattice parameters of 60 o .In this case, the preferential stacking fashion is a perfectly eclipsed AA configuration with an interlayer distance of 4.98 Å. Regarding their electronic properties, in both cases, they exhibit a clear narrow-gap semiconducting character with band-gaps at X-and points of 1.06 and 1.57 eV for the PI and BI compounds, respectively, in good agreement with the experimental evidence.Based on the DFT calculations, the acoustic-phononlimited electron mobilities at room temperature (300 K) were obtained with the values E1, C2D, and me * .We have obtained values for C2D of 145.2 and 223.5 N m -1 for PI and BI, respectively; values for E1 of -5.7 and -6.5 eV for electrons for PI and BI, respectively; and, finally, values of me * of 1.42 and 3.95 me for PI and BI, respectively.All these values, according to (eq.1), provide intrinsic electron mobilities of 44.7 and 8.7 cm 2 V -1 s -1 .
Considering the use of only the acoustic phonon scattering mechanism approximation, together with the fact that the theoretical systems are highly idealized, the comparison between the experimental and theoretical mobilities is fairly good.

Figure S2 .
Figure S1.FT-IR spectra of BI and its constituent monomers.

Figure S11 .Figure S12 .
Figure S11.Pore-size distribution profile of a) BI and b) PI.

Figure S13 .
Figure S13.UPS spectrum of a) BI at -10 V bias Scan + 3 eV and b) PI at -10 V bias Scan + 2 eV.The work function (ϕ) was derived by subtracting the high-binding energy cut-off (16.4158 eV for BI 16.5412 eV for PI); BI(ϕ) = 21.22 eV -16.4158 eV= 4.805 eV, PI(ϕ) =21.22 eV-16.5412= 4.6788 eV from the radiation energy (21.22 eV; from non-voltage bias condition) Hence Ef for BI = -4.805eV vs vacuum and Ef for PI = -4.805eV vs vacuum.The low-energy tail of the UPS spectrum was used to determine the position of

Figure S14 .
Figure S14.Top and side pictorial views of the most stable DFT optimized structures for the a) PI and b) BI compounds, indicating in both cases the resulting optimized unit cell and interlayer distance.White, grey, blue and red spheres represent H, C, N and O atoms, respectively.

Figure S15 .
Figure S15.Synthesized CMP (BI and PI) drop-casted on 1×1 cm 2 pieces of thermal SiO2 and 3×3 cm 2 fused silica pieces without and with the plasma treatment.

Figure S16 .Figure S17 .
Figure S16.Comparison of UV/VIS absorption spectra of a) BI and b) PI in the solid state before and after oxygen plasma-treatment assisted thin film preparation.Tauc plot of c) BI and d) PI for bandgap calculations after oxygen plasma treatment-assisted thin film preparation.

Table S1 .
Electrochemical properties of BI and PI.

Table S2 .
Conductivity and mobility of BI and PI thin films relative to recent literature reports.