Interplay between Side Chain Density and Polymer Alignment: Two Competing Strategies for Enhancing the Thermoelectric Performance of P3HT Analogues

A series of polythiophenes with varying side chain density was synthesized, and their electrical and thermoelectric properties were investigated. Aligned and non-aligned thin films of the polymers were characterized in the neutral and chemically doped states. Optical and diffraction measurements revealed an overall lower order in the thin films with lower side chain density, also confirmed using polarized optical experiments on aligned thin films. However, upon doping the non-aligned films, a sixfold increase in electrical conductivity was observed for the polythiophene with the lowest side chain density compared to poly(3-hexylthiophene) (P3HT). We found that the improvement in conductivity was not due to a larger charge carrier density but an increase in charge carrier mobility after doping with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ). On the other hand, doped aligned films did not show the same trend; lower side chain density instead led to a lower conductivity and Seebeck coefficient compared to those for P3HT. This was attributed to the poorer alignment of the polymer thin films with lower side chain density. The study demonstrates that optimizing side chain density is a synthetically simple and effective way to improve electrical conductivity in polythiophene films relevant to thermoelectric applications.

Synthesis of poly(3-hexylthiophene) (90% RR) (P3HT) 2,5-Dibromo-3-hexyl-thiophene (534 mg, 1.64 mmol) dissolved in dry THF (3.8 mL) in an oven dried microwave vial under nitrogen.It is paramount to ensure all glassware is dry to provide the highest yield.Added isopropyl magnesium chloride lithium chloride complex (1.3 M in THF, 1.2 mL, 0.98 eq) to the solution dropwise and the reaction mixture was stirred for 2 hr at 70 °C (dark yellow solution).This was then added to a suspension of Ni(dppp)Cl2 (9 mg, 0.02 mmol, 1 mol%) in dry THF (3 mL), in a separate oven dried microwave vial and the polymerisation was stirred at 70 °C overnight.After cooling to room temperature, the reaction was terminated by addition of 1 mL of HCl (10 % (v/v)) followed again by stirring for 20 minutes.

Nuclear Magnetic Spectroscopy (NMR)
1 H NMR spectroscopy was carried out on a Bruker AV400 or Bruker AVIII400 spectrometer.
Samples were dissolved in 0.4 -0.6 mL.High temperature NMR spectra were obtained via heating the samples in the NMR spectrometer at 363 K (90 °C).Due to relaxation times commonly seen in conjugated polymer NMR, we note that the integration between the aromatic and aliphatic protons to not match in the P3HT spectra.Therefore, we have corrected the integration of 1.11 to 1.00 for P3HT and similarly for the other polymers in the series.If we then consider the statistical copolymer to have 1-x hexylthiophene units and x unsubstituted thiophene units, then we would expect 1+x aromatic protons and 2-2x aliphatic protons in the 2.8 ppm region (methylene group next to thiophene).With R defined as the ratio of aromatic protons to aliphatic protons (2.8 ppm region), we get R = (1+x)/(2-2x).Solving for x, we get x = (2R-1)/(2R+1).Thus, we calculate the thiophene content (x) from the ratio (R) of aromatic protons to aliphatic protons (2.8 ppm region) in the table below.
HPLC grade chloroform was purchased from Acros Organics and polystyrene standards were purchased from Agilent.GPC Samples were prepared via dissolving the polymers in chlorobenzene at 1 mg mL -1 at 80 °C for 1 hr, then cooling to room temperature and filtering through a 0.45 μm PTFE filter.100 μL was injected into the system and run at 1 mL min -1 in chlorobenzene through the oven at 80 °C.Analysis was carried out on Shimadzu's LabSolutions software.a Measured from the integration under the GPC trace vs polystyrene standards.b Đ = MW/Mn.c Calculated using DP = (fTMT + f3HTM3HT)/Mn where fT and f3HT represent the fraction of thiophene and 3-hexylthiophene monomer calculated from NMR integrations and MT and M3HT represent the molecular weight of the monomers.

Thermal characteristics
TGA was carried out on a TA instruments Q500 using a platinum pan at 10 °C min -1 between 25 -800 °C under nitrogen then 800 -1000 °C under air.Between 0.5 -3 mg of polymer powder was used for TGA.DSC was carried out on TA Instruments DSC25 running at 10 °C min -1 under a nitrogen atmosphere.Between 1 -5 mg of polymer powder was used for DSC.Table S3.

Thin film fabrication
Non-aligned polymer thin films were fabricated by spin coating thin films onto glass sides from 10 mg mL -1 polymer solutions in ODCB dissolved at 80 °C.Solutions were spun at 2000 rpm for 90 seconds then 8000 rpm for 30 seconds from 80 °C solutions in air on a Laurell Technologies Corporation Model WS-650MZ-23NPPB spin coater.Glass slides were cleaned by sonicating in soapy water, DI water, acetone, and IPA for 15 minutes.Prior to spin coating substrates were plasma cleaned using a Harrick Plasma PDC-32G plasma cleaner for 30 minutes.Note that decreasing the side chains density decreases the polymer solubility in nonpolar and apolar solvents.One should consider low concentration (<20 mg mL -1 ) to ensure complete solubility of the polymer before casting.
The aligned polymer films were prepared by doctor-blading a hot solution in ODCB (10 mg mL -1 ) at 160 °C on cleaned glass slides covered with a sacrificial polymer film of water-soluble NaPSS (10 mg mL -1 aq).The orientation of the films by high-temperature rubbing followed the protocol described in previous publications. 3 4Rubbing is performed by using a homemade set-up.It consists of a rotating cylinder covered with a polyester cloth and a translating hot plate.

UV-Vis spectroscopy
Solution and thin film UV-Vis spectroscopy of the non-aligned films neutral films was carried out on a Shimadzu UV3600 UV-Vis-NIR spectrometer.Temperature dependent solution UV-Vis spectroscopy was carried out in quartz cuvettes from solutions of polymers in chlorobenzene (CB) at 0.01 mg mL -1 at 20 to 100 °C using a VICI DBS PCB 1500 Plus Peltier Cryobath with cuvette holder as the heater/cooler.All spectra were baselined to chlorobenzene at 20 °C.

Electrochemistry and Spectroelectrochemical Experiments
CV was carried out using a PalmSens EmStat3 potentiostat with degassed acetonitrile with 0.1 M tetrabutylammonium hexafluorophosphate as the supporting electrolyte measured at 50 mV s -1 scan rate.The reference, counter and working electrode were Ag/Ag + , platinum and glassy carbon, respectively.Thin films of each polymer were drop cast from 1 mg mL -1 chloroform solutions onto glassy carbon electrode.Ferrocene was used as the standard, measured using 0.01 M ferrocene in degassed acetonitrile with 0.1 M tetrabutylammonium hexafluorophosphate as supporting electrolyte at a scan rate of 50, 100 and 200 mV s -1 .
Diameter of the working electrode is 3 mm.For the spectroelectrochemistry experiments, thin films of P3HT and T24 were spun onto 2 x 2 cm ITO coated glass at 2000 rpm for 60 seconds then 8000 rpm for 30 seconds from 10 mg mL -1 ODCB solutions at 80 °C.The polymer films were then swabbed down to 1 x 2 cm using IPA and cotton buds, then the slides were scored and split in half to produce 1 x 1 cm polymer films on 1 x 2 cm slides.The exposed ITO on the substrate was clipped using crocodile clips and submerged into degassed acetonitrile with 0.1 M tetrabutylammonium hexafluorophosphate as the supporting electrolytes in a quartz cuvette.Platinum wire, Ag/Ag + and the ITO coated glass slide were used as the counter, reference and working electrode respectively.ITO slides were cleaned using, soapy water, DI water, acetone and IPA via sonicating for 15 minutes.The whole cuvette was placed into the Shimadzu UV3600 UV-Vis-NIR spectrometer and potentials were applied using PalmSens EmStat3 potentiostat.Spectra were recorded after 1 minute of applying each new potential.

Work Function Measurements
Photo

X-ray diffraction
Grazing incidence x-ray diffraction measurements in the out-of-plane direction were carried out on PANalytical X'Pert Pro diffractometer using Cu Kα X-rays, setup in grazing incidence configuration.Thin films were drop cast onto boron doped single side polished silicon substrates of <100> orientation, purchased form PI-KEM, from 20 mg mL -1 polymer solutions in ODCB at 80 °C.Films were covered with glass petri dish and allowed to dry overnight in air.
Due to the low power of the diffraction measurement, thick films were required to record significant signals for analysis.Sufficient thickness was not achieved by spin-coating, so the polymers were drop casted instead.Therefore, we note here that the morphology-property relationships drawn for non-aligned films rely on the assumption that the order observed for drop-casted films is representative for the spin-coated films.The films were doped via immersion in 2 mg mL -1 F4TCNQ solutions in degassed acetonitrile overnight in air on a shaker plate.The films were then washed with acetonitrile and dried under vacuum.We would like to note here that using the Scherrer equation limits the analysis as beak broadening is due to paracrystalline disorder, not finite grain size which is assumed by the Scherrer equation.
These results are shown in lieu of a more complete linewidth analysis.a Calculated using Braggs law using Cu Kα (1.5406 Å) as the wavelength.b FWHM was obtained from fitting a gaussian peak to the (100) peak using Origin Pro's fitting tool.c The coherence lengths were calculated using the Scherrer equation.

Roughness measurements
AFM was carried out in air using a Bruker Dimension Icon system.ScanAsyst Air tips were used to image the samples in PeakForce Quantitative Nanomechanical Property Mapping (QNM) mode.Roughness was calculated using Nanoscope Analysis software.

Organic Field Effect Transistor (OFET) Measurements
The back-gate and gate dielectrics were chosen to be highly n-doped silicon and thermally grown SiO2 (300 nm), respectively.After cleaning Si/SiO2 substrates with oxygen plasma at 300W for 10 min, the substrates were then immersed in 3 wt% PTS/toluene solution for 15 hours at 90 °C.The excess PTS on Si/SiO2 substrates were cleaned by sonication with toluene, followed by toluene, acetone and isopropanol rinse.The polymers solutions were preheated at 80°C for 1-2 hours before film deposition.The polymers (10 mg mL -1 ) in ODCB were spin-coated on PTS-functionalized Si wafer.The Cr/Au electrodes (5/250 nm) were thermally evaporated under high vacuum (10 -6 mbar) as the source and drain electrodes (W/L = 1000/20).The prepared OFETs were placed in a nitrogen glove box prior to testing.The mobility was calculated from the gradient of the transfer curve after any 'kinks' in the curve.S6.Saturation mobility of P3HT, Tref, T19, T24 thin films.

Polymer
Saturation Mobility (x10 -4 cm 2 V -1 s -1 ) P3HT  HighSqP doping was carried out by depositing a 1 mg mL -1 solution of F4TCNQ in MeCN, leaving on the films for 60 seconds, then removed via spinning at 8000 rpm for 30 seconds.
This was followed by another 1 mg mL -1 solution of F4TCNQ in ODCB leaving on the film for 120 seconds, then removed by spinning at 8000 rpm for 30 seconds.LowSqP doping was carried out via depositing a 0.1 mg mL -1 solution of F4TCNQ in MeCN, leaving on for 10 secs, then spinning off at 8000 rpm for 30 secs.Film thickness was measured using a Bruker DekTak XT.The higher performing batch of P3HT used to confirm our findings (named here P3HT99) was synthesised previously. 7The RR% was measured to be above 99 % measured by high temperature NMR.The Mn, Mw and Đ are 49 kDa, 75 kDa and 1.5 respectively.
Conductivity measurements were measured on a Karl Suss probe station under a nitrogen atmosphere (<20 ppm O2) using an Agilent 4155B sourcemeter following the standard van der Pauw method.Four I-V measurements of each sample were performed, corresponding to sourcing current between each pair of adjacent contacts, while measuring the voltage at the other pair of contacts.These data were checked for current reversal and reciprocity consistency to <5% variance, following NIST recommendations; the van der Pauw equation was then used to determine the sheet conductivity.UV-Vis measurements were performed on a Shimadzu UV-3600i UV-Vis-NIR spectrometer.
IR spectroscopy was carried out on a Bruker Vertex 70v FT-IR spectrometer under vacuum using a DLaTGS detector.The data drops below 0 absorbance as the beam splitter is only rated to 6000 cm -1 in the spectrometer.Thin film interference present in the data were filtered out in MATLAB using a stopband filter.To extract the F4TCNQ anion concentration we used the same fitting parameters as our previous work.Table S8.a Extracted from the fitting results.b Estimated using the relation σ = μen where σ is the measured conductivity (S cm -1 ), e is the electric charge (1.602 x 10 -19 C), n is the F4TCNQ concentration (cm -1 ) and μ is the doped mobility (cm 2 V -1 s -1 ).

UV-Vis-NIR Spectroscopy, Electron Diffraction and Thermoelectric Characteristics of Doped Aligned Thin Films
The doping was performed following the incremental concentration doping (ICD) procedure introduced in a previous publication.8 9, 10 Samples were immersed for 10 s in the dopant solution of increasing concentration.Such short doping times are sufficient to reach saturation of the film doping level as shown earlier for P3HT.No rinsing step was conducted as it results in de-doping of the films.Doping was performed in a jacomex glovebox under inert atmosphere (< 1 ppm O2 and < 1 ppm H2O).
All devices were fabricated on glass substrates cleaned by ultrasonication in acetone, ethanol, hellmanex and deionized water (x3 times).The cleaned substrates were dried under nitrogen and exposed to plasma prior to film deposition.Gold electrical contacts (40 nm thick) in a fourpoints probe geometry (1 mm spacing between electrodes, 5 mm length) were deposited by evaporation at an average rate of 4-6 Å s -1 through a shadow mask.The geometry of deposited gold electrodes was used to measure the charge transport and thermopower on a same sample in both parallel and perpendicular directions to the rubbing.Oriented polymer films were floated on water and carefully recovered on the device with pre-deposited gold electrodes.They were subsequently doped using the ICD protocol.11 Four-point probe measurements of electrical conductivity were performed using a Keithley 2634B and a Lab Assistant Semiprobe station in a Jacomex glovebox under N2 atmosphere.The resistivity ρ was derived from the sheet resistance R such that ρ = 1.81.R.t where t is the film thickness (the geometrical correction factor was determined following the method in reference 3).The film thickness was extracted from the UV-Vis absorbance spectra using the calibration curves described below.The average conductivity value for a given rubbing temperature was taken as the average of two devices.
Thermopower measurements were conducted on the same devices.The thermopower was measured using a differential temperature method.A variable temperature gradient ∆T was established and the corresponding thermovoltage ∆V was measured.The Seebeck coefficient was extracted from the slope of ∆V versus ∆T.Calibration of the Seebeck coefficient measurement was performed using a constantan wire.
The orientation of the aligned polymer films was probed by UV-Vis-NIR absorption (350 -2500 nm) using a Varian Cary5000 spectrometer with polarized incident light (spectral resolution of 1 nm).Table S9.To get a thickness for the aligned films a calibration curve of measured thickness vs absorption of the non-aligned films was created using AFM and UV-Vis spectroscopy.Thin films were fabricated as previously however solutions of 5, 10, 15 and 20 mg mL -1 were used to ensure the calibration curve fit in the region between 0 -100 nm.A small section (~1 x 2 mm) of polymer thin films were measured using UV-Vis spectroscopy using a Shimadzu UV3600 UV-Vis-NIR spectrometer.Then a small section of the film was removed adjacent to the spot measured using a wooden toothpick to ensure no scratching of the glass.Thickness measurements were taken from the removed section to the section where the absorption of the polymer film was measured, using the glass as the base line.

Figure S2. 1 H 2 FigureFigure S4. 1 H
Figure S2.1  H NMR of P3HT in d2-TCE (top).Zoomed in of the triplet at 2.87 ppm and broad peak at 2.65 ppm are from the alpha protons on the alkyl chain.The integrated ratio between these peaks indicates the head to tail content/regioregularity.2

Figure S7 .
Figure S7.GPC traces of P3HT, Tref, T19, T24.The peak for P3HT is bimodal indicating two different regimes however the molecule weight was estimated via integration over both peaks.

Figure S12 .
Figure S12.Cyclic voltammograms of P3HT, Tref, T19, T24 at 50, 100 and 200 mV s -1(top left, top right and bottom left respectively).Plot of current max vs scan rate across the polymer series (bottom right).The dotted line of each colour represents the linear extrapolation from 0 to 50 mV s -1 .
Photo-Electron Spectroscopy in Air (PESA) has been performed on an AC-2 Model from Riken Instruments.UV photons are emitted from a deuterium lamp, then monochromated by a grating spectrometer and finally focused on the sample film.The photoelectrons emitted by the sample are detected by an open counter.When the sample's surface is bombarded with a slowly increasing amount of UV energy, photoelectrons start to emit at a certain energy level which corresponds to the photoelectron work function.Thin films were fabricated using 10 mg ml -1 ODCB solutions, doctor bladed onto ITO coated glass substrates.

Figure S15 .
Figure S15.Photoemission spectra of P3HT thin films with increasing energy (eV) along the x-axis against standardised photoelectron yield (Yield^n, where n is 0.33) along the y-axis.

Figure S16 .
Figure S16.Photoemission spectra of Tref thin films with increasing energy (eV) along the x-axis against standardised photoelectron yield (Yield^n, where n is 0.33) along the y-axis.

Figure S17 .
Figure S17.Photoemission spectra of T19 thin films with increasing energy (eV) along the x-axis against standardised photoelectron yield (Yield^n, where n is 0.33) along the y-axis.

Figure S18 .
Figure S18.Photoemission spectra of T24 thin films with increasing energy (eV) along the x-axis against standardised photoelectron yield (Yield^n, where n is 0.33) along the y-axis.

Figure S19 .
Figure S19.GIXRD diffraction patterns in the out-of-plane direction of P3HT, Tref, T19, T24 drop cast films from 20 mg mL -1 ODCB solutions at 80 °C in the pristine and doped state.The scale bar represents counts.

Figure S25 .Figure S26 .
Figure S25.Transfer characteristics at a drain voltage of -60 V (left) and Output curve (right) of a T24 OFET.The saturation mobility was extracted from the gradient of the grey line on the (Id) 1/2 vs Vg plot.

13.
Conductivity, UV-Vis Absorbance and FTIR Spectroscopy of Doped Non-Aligned Thin FilmsGlass substrates patterned with van der Pauw contacts (1 mm contacts at each corner) was chosen to permit conductivity, UV-Vis spectroscopy, and FTIR measurements on a single sample.Substrates were first prepatterned with Cr/Au electrodes (5/25 nm) by thermal evaporation through a shadow mask, then cleaned by sequential sonication in Decon 90, DI water, acetone, and isopropanol, dried under nitrogen flow, and exposed to oxygen plasma for 10 minutes.After transferring to a nitrogen glovebox (MBraun Labmaster 130, <1 ppm H20, O2), polymer thin films were spun cast at 1500 rpm for 60 secs (Specialty Coating Systems G3P) from 10 mg mL -1 ODCB solutions at 80 °C, then annealed at 180 °C for 20 mins under N2.Thin films were doped with two different concentrations under nitrogen atmosphere.

Figure S27 .
Figure S27.Thin film UV-Vis absorbance spectra of the polymer series doped under high (left) and low (right) doping conditions.

Figure S28 .
Figure S28.Fits of the high (top row) and low (bottom row) doped UV-Vis spectra using the same fitting parameters according to our previous work.1

Figure S29 .
Figure S29.FTIR spectrum of low doped polymer films.

Figure S38 .
Figure S38.Thin film UV-Vis spectra of the polymer series of increasing thickness obtained by increasing the solution concentration in ODCB used for the calibration curves.

Figure S39 .Figure S40 .
Figure S39.Calibration curves of thin films across the polymer series.

Table S1 .
Table summarising the values calculated to obtain the thiophene content.

Polymer Mw (kg mol -1 ) a Mn (kg mol -1 ) a Đ b Degree of Polymerisation c
Table showing the thermal properties estimated from TGA and DSC spectra of P3HT, Tref, T19, T24.
a Obtained from thin films on ITO coated glass substrates.

Neutral Doped d(100) (Å) a FWHM b Coherence Lengths (Å) d(100) (Å) a FWHM b Coherence
Table showing the d-spacings, FWHM and coherence lengths of the neutral and doped nonaligned drop cast films.

Table S7 .
Table showing the conductivity results of the non-aligned polymer thin films.
cAverage over four measurements.
Table showing the d-spacings of the (100) and (020) of doped aligned films at 1 mg ml -1 .