Synthesis and Photovoltaics of Novel 2,3,4,5-Tetrathienylthiophene-co-poly(3-hexylthiophene-2,5-diyl) Donor Polymer for Organic Solar Cell

This report focuses on the synthesis of novel 2,3,4,5-tetrathienylthiophene-co-poly(3-hexylthiophene-2,5-diyl) (TTT-co-P3HT) as a donor material for organic solar cells (OSCs). The properties of the synthesized TTT-co-P3HT were compared with those of poly(3-hexylthiophene-2,5-diyl (P3HT). The structure of TTT-co-P3HT was studied using nuclear magnetic resonance spectroscopy (NMR) and Fourier-transform infrared spectroscopy (FTIR). It was seen that TTT-co-P3HT possessed a broader electrochemical and optical band-gap as compared to P3HT. Cyclic voltammetry (CV) was used to determine lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy gaps of TTT-co-P3HT and P3HT were found to be 2.19 and 1.97 eV, respectively. Photoluminescence revealed that TTT-co-P3HT:PC71BM have insufficient electron/hole separation and charge transfer when compared to P3HT:PC71BM. All devices were fabricated outside a glovebox. Power conversion efficiency (PCE) of 1.15% was obtained for P3HT:PC71BM device and 0.14% was obtained for TTT-co-P3HT:PC71BM device. Further studies were done on fabricated OSCs during this work using electrochemical methods. The studies revealed that the presence of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) on the surface of indium tin oxide (ITO) causes a reduction in cyclic voltammogram oxidation/reduction peak current and increases the charge transfer resistance in comparison with a bare ITO. We also examined the ITO/PEDOT:PSS electrode coated with TTT-co-P3HT:PC71BM, TTT-co-P3HT:PC71BM/ZnO, P3HT:PC71BM and P3HT:PC71BM/ZnO. The study revealed that PEDOT:PSS does not completely block electrons from active layer to reach the ITO electrode.


Synthesis of TTT-co-P3HT
In a typical chemical polymerization, TTT (0.12 g, 0.28 mmol) and 3-hexylthiophene (1.87 g, 11.12 mmol, ≥99.0%, Merck (Pty) Ltd.) were added in 50 mL of chloroform (≥99.0%, Merck (Pty) Ltd.). The solution was stirred for 30 min and followed by addition of iron chloride (FeCl 3 , 1.8 g, 11.12 mmol, ≥99.99%, Merck (Pty) Ltd.). The mixture was refluxed at 60 • C for 24 h under nitrogen atmosphere, then polymerization was terminated by addition of MeOH. After, the precipitates were collected by filtration and were washed with Soxhlet extraction using acetone (99.3%, Merck (Pty) Ltd.) and MeOH as solvents for 24 h with each. Finally, the product was extracted with DCM for another 24 h. The DCM solvent was removed to obtain a dark brown solid (0.36 g).

Fabrication of OSCs
Devices fabrication and measurements were conducted in air. ITO (8 pixels, 20 Ω/square resistance, Ossila Ltd., Sheffield, UK) substrates were sonicated in 1% (by volume) hellmanex III (Merck (Pty) Ltd.) solution in hot water, followed by in acetone then 2-propylalcohol for 5 min with each. After every sonication step, the substrate was rinsed with water. PE-DOT:PSS (0.5-1.0 wt % in water, Merck (Pty) Ltd.) was sonicated for 3 h then filtered with 0.45 µm filter. It (30 µL PEDOT:PSS) was spun coated on ITO substrate for 30 s at Polymers 2021, 13, 2 4 of 14 4000 rpm. The film was baked at 150 • C for 5 min. Active layer was prepared by blending donor material polymer and PC 71 BM (mixture of isomers, 99%, Merck (Pty) Ltd.) with a total of 25 mg/mL in chlorobenzene (99.9%, Merck (Pty) Ltd.) as a solvent at 60 • C for overnight. The blend ratio of donor: acceptor used is 1:0.8. The solutions of 40 µL were spun at 2000 rpm for 30 s in air. The films were annealed for 5 min at 100 • C. ZnO solution (preparation method is given in supporting document) of 30 µL was spun coated at 4000 rpm for 30 s and was annealed at 100 • C for 30 min. Lastly, aluminum metal (Merck (Pty) Ltd.) was thermally evaporated at deposition pressure of about 10 −5 mbar to complete the device.

Results and Discussion
As shown in Schemes 1 and 2, synthesis of TTT and TTT-co-P3HT were performed by using Suzuki coupling and oxidation polymerization reaction methods, respectively. Results of size exclusion chromatography are shown in Table ?? and the chromatogram of TTT-co-P3HT is shown in Figure S1. The synthesized polymer, TT-co-P3HT, has a molecular weight Mn of 15,131 gmol −1 and a polydispersity index of 3.6. The chemical structure of TTT and TTT-co-P3HT were confirmed by NMR. The 1 H-NMR of TTT in Figure 1A reveals no presence of -OH protons from 2-thienylboronoic acid confirming successful synthesis. The spectrum was designed at a chemical shift range between 6.74 and 7.38 ppm as an insertion in Figure 1A. Number of protons obtained by integrating the signals corresponds to the number of protons from the chemical structure of TTT. Figure 1B shows the 13 C-NMR spectra of TTT and due to structural symmetry, only 10 signals were observed. For TTT-co-P3HT, only 1 H-NMR analysis was performed ( Figure 1C) and compared with the 1 H-NMR spectrum of P3HT ( Figure 1D). The 1 H-NMR spectrum of TTT-co-P3HT showed a new signal at 6.75 ppm which is absent on the 1 H-NMR spectrum of P3HT. This signal is due to the present of β-hydrogens from TTT. The signals from 1 H-NMR spectrum of TTT-co-P3HT are broader than those of P3HT. This is due to different P3HT chain lengths attached to TTT.

Results and Discussion
As shown in Scheme 1 and Scheme 2, synthesis of TTT and TTT-co-P3HT were performed by using Suzuki coupling and oxidation polymerization reaction methods, respectively. Results of size exclusion chromatography are shown in Table 1 and the chromatogram of TTT-co-P3HT is shown in Figure S1. The synthesized polymer, TT-co-P3HT, has a molecular weight Mn of 15,131 gmol −1 and a polydispersity index of 3.6. The chemical structure of TTT and TTT-co-P3HT were confirmed by NMR. The 1 H-NMR of TTT in Figure 1A reveals no presence of -OH protons from 2-thienylboronoic acid confirming successful synthesis. The spectrum was designed at a chemical shift range between 6.74 and 7.38 ppm as an insertion in Figure 1A. Number of protons obtained by integrating the signals corresponds to the number of protons from the chemical structure of TTT. Figure 1B shows the 13 C-NMR spectra of TTT and due to structural symmetry, only 10 signals were observed. For TTT-co-P3HT, only 1 H-NMR analysis was performed ( Figure 1C) and compared with the 1 H-NMR spectrum of P3HT ( Figure 1D). The 1 H-NMR spectrum of TTT-co-P3HT showed a new signal at 6.75 ppm which is absent on the 1 H-NMR spectrum of P3HT. This signal is due to the present of -hydrogens from TTT. The signals from 1 H-NMR spectrum of TTT-co-P3HT are broader than those of P3HT. This is due to different P3HT chain lengths attached to TTT.   FTIR spectra of TTT, TTT-co-P3HT and P3HT were recorded from 4000 cm −1 to 400 cm −1 . The spectra are shown in Figure 2. Unique vibrational bands can be observed at the wavenumber of 696 cm −1 , the range from 2934 to 2844 cm −1 and 3085 cm −1 on the spectrum of TTT. The vibrational band at 696 cm −1 is due to the aromatic C-H out of phase bending vibrations of thiophene [24][25][26], while the vibrational bands in the range from 2934 to 2844 cm −1 are due to the β-position C-H of the thiophene. The presence of the αposition C-H is confirmed by the vibrational band at the wavenumber of 3085 cm −1 [26,27]. In comparison with the spectra of P3HT and TTT-co-P3HT, the vibrational bands in the range from 2934 to 2844 cm −1 increases in intensity due to the presence of the hexyl group. The vibrational bands at 696 cm −1 and 3085 cm −1 disappears due to the occurrence of polymerization.

Optical and Electrochemical Characterization
UV-Vis spectra of TTT-co-P3HT and P3HT materials are presented in Figure 3A and were obtained in chlorobenzene solvent and as thin films. The data obtained from Figure  3A is given in Table 2. As thin films, TTT-co-P3HT and P3HT materials have maximum absorption at a longer wavelength as compared to the spectra obtained in chlorobenzene. This is an indication that intermolecular π-π stacking is stronger in thin films. In both chlorobenzene and thin films, TTT-co-P3HT showed maximum absorbance at shorter wavelengths. This shows that TTT disturbs the interchain delocalization of π-electrons of P3HT after functionalization [23]. The onset absorption wavelengths were used to determine optical band-gaps of TTT-co-P3HT and P3HT. The optical band gaps for TTTco-P3HT were determined to be 2.32 eV and 1.98 eV in chlorobenzene and thin film, respectively. As for P3HT, they were determined to be 2.21 eV in chlorobenzene and 1.91 eV in thin film. TTT-co-P3HT have broader band gaps indicating that lower number of photons are absorbed. Therefore, decreased numbers of electron/hole pairs are generated and TTT-co-P3HT OSCs is expected to have a decreased short circuit current-density (Jsc).  FTIR spectra of TTT, TTT-co-P3HT and P3HT were recorded from 4000 cm −1 to 400 cm −1 . The spectra are shown in Figure 2. Unique vibrational bands can be observed at the wavenumber of 696 696 cm −1 is due to the aromatic C-H out of phase bending vibrations of thiophene [24][25][26], while the vibrational bands in the range from 2934 to 2844 cm −1 are due to the -position C-H of the thiophene. The presence of the α-position C-H is confirmed by the vibrational band at the wavenumber of 3085 cm −1 [26,27]. In comparison with the spectra of P3HT and TTT-co-P3HT, the vibrational bands in the range from 2934 to 2844 cm −1 increases in intensity due to the presence of the hexyl group. The vibrational bands at 696 cm −1 and 3085 cm −1 disappears due to the occurrence of polymerization.

Optical and Electrochemical Characterization
UV-Vis spectra of TTT-co-P3HT and P3HT materials are presented in Figure 3A and were obtained in chlorobenzene solvent and as thin films. The data obtained from Figure 3A is given in Table 2. As thin films, TTT-co-P3HT and P3HT materials have maximum absorption at a longer wavelength as compared to the spectra obtained in chlorobenzene. This is an indication that intermolecular π-π stacking is stronger in thin films. In both chlorobenzene and thin films, TTT-co-P3HT showed maximum absorbance at shorter wavelengths. This shows that TTT disturbs the interchain delocalization of π-electrons of P3HT after functionalization [23]. The onset optical band gaps for TTT-co-P3HT were determined to be 2.32 eV and 1.98 eV in chlorobenzene and thin film, respectively. As for P3HT, they were determined to be 2.21 eV in chlorobenzene and 1.91 eV in thin film. TTT-co-P3HT have broader band gaps indicating that lower number of photons are absorbed. Therefore, decreased numbers of electron/hole pairs are generated and TTT-co-P3HT OSCs is expected to have a decreased short circuit current-density (Jsc).
CV was used to investigate electrochemical response of TTT-co-P3HT and P3HT donor polymers and the voltammograms are shown in Figure 3B. Table 2 displays the results of HOMO and LUMO energy levels obtained from Figure 3B. These energy levels offsets of the donor and acceptor materials are important factors in understanding the electron/hole pair separation dynamics. HOMO offset becomes of importance when the acceptor material absorbs light significantly [28,29]. In this work, we focus only on the LUMO offset. The LUMO offsets between donor and acceptor are determined from Figure 3C to be 0.96 eV for TTT-co-P3HT:PC71BM and 0.97 eV for P3HT:PC71BM. The values obtained for LUMO offsets are more than 3 times higher than the commonly known empirical threshold of 0.3 eV [30]. Gadisa et al. [31] studied the relationship between onset oxidation potentials of polythiophene derivatives and open circuit voltage (Voc). Their studies revealed that Voc decreases as onset oxidation potential increases. Gao et al. [32] reported donor and acceptor materials with LUMO offset less than threshold. They achieved low energy loss and high Voc. Therefore, TTT-co-P3HT:PC71BM and P3HT:PC71BM have high energy loss and this will have an impact on the inVoc.    CV was used to investigate electrochemical response of TTT-co-P3HT and P3HT donor polymers and the voltammograms are shown in Figure 3B. HOMO and LUMO energy levels obtained from Figure 3B. These energy levels offsets of the donor and acceptor materials are important factors in understanding the electron/hole pair separation dynamics. HOMO offset becomes of importance when the acceptor material absorbs light significantly [28,29]. In this work, we focus only on the LUMO offset. The LUMO offsets between donor and acceptor are determined from Figure 3C to be 0.96 eV for TTT-co-P3HT:PC 71 BM and 0.97 eV for P3HT:PC 71 BM. The values obtained for LUMO offsets are more than 3 times higher than the commonly known empirical threshold of 0.3 eV [30]. Gadisa et al. [31] studied the relationship between onset oxidation potentials of polythiophene derivatives and open circuit voltage (Voc). Their studies revealed that Voc decreases as onset oxidation potential increases. Gao et al. [32] reported donor and acceptor materials with LUMO offset less than threshold. They achieved low energy loss and high Voc. Therefore, TTT-co-P3HT:PC 71 BM and P3HT:PC 71 BM have high energy loss and this will have an impact on the inVoc.
Photoluminescence spectroscopy has been widely used to study electron/hole pair separation at the interface of donor and acceptor materials using the quenching effect [33][34][35][36]. Figure 4 depicts the photoluminescence results of (A) TTT-co-P3HT and TTT-co-P3HT:PC 71 BM and (B) P3HT and P3HT:PC 71 BM obtained using chlorobenzene as a solvent. Photoluminescence quenching was observed in both TTT-co-P3HT:PC71BM and P3HT:PC71BM blends. This quenching can be attributed to charge transfer and electron/hole pair separation efficiency which corroborates other results in literature [34]. To determine the quenching degree of TTT-co-P3HT:PC 71 BM and TTT-co-P3HT:PC 71 BM, the following Equation (1): was used to calculate photoluminescence quenching parameter q, where I donor is the intensity of donor material and I donor:acceptor is the intensity of donor:acceptor blend [33]. From Equation (1), for an outstanding degree of electron/hole pair separation and charge transfer without any recombination taking place, I donor:acceptor must be equal to zero resulting to q equal to 100%. John et al. [35] achieved q of 97.29% and 96.6% indicating excellent electron/hole separation and charge transfer for their blends. The quenching parameter q was found to be 36% in TTT-co-P3HT:PC 71 BM and 58% in P3HT:PC 71 BM. This reveals that some of the created electron/hole pair recombine in TTT-co-P3HT and P3HT [36]. When comparing two blends, the results show that electron/hole pair separation and charge transfer is sufficient in P3HT:PC 71 BM than in TTT-co-P3HT:PC 71 BM. Photoluminescence spectroscopy has been widely used to study electron/hole pair separation at the interface of donor and acceptor materials using the quenching effect [33][34][35][36]. Figure 4 depicts the photoluminescence results of (A) TTT-co-P3HT and TTT-co-P3HT:PC71BM and (B) P3HT and P3HT:PC71BM obtained using chlorobenzene as a solvent. Photoluminescence quenching was observed in both TTT-co-P3HT:PC71BM and P3HT:PC71BM blends. This quenching can be attributed to charge transfer and electron/hole pair separation efficiency which corroborates other results in literature [34]. To determine the quenching degree of TTT-co-P3HT:PC71BM and TTT-co-P3HT:PC71BM, the following Equation (1): was used to calculate photoluminescence quenching parameter q, where Idonor is the intensity of donor material and Idonor:acceptor is the intensity of donor:acceptor blend [33]. From Equation (1), for an outstanding degree of electron/hole pair separation and charge transfer without any recombination taking place, Idonor:acceptor must be equal to zero resulting to q equal to 100%. John et al. [35] achieved q of 97.29% and 96.6% indicating excellent electron/hole separation and charge transfer for their blends. The quenching parameter q was found to be 36% in TTT-co-P3HT:PC71BM and 58% in P3HT:PC71BM. This reveals that some of the created electron/hole pair recombine in TTT-co-P3HT and P3HT [36]. When comparing two blends, the results show that electron/hole pair separation and charge transfer is sufficient in P3HT:PC71BM than in TTT-co-P3HT:PC71BM.

Photovoltaic Properties
OSCs were fabricated during this study with conventional configuration as follows:

Photovoltaic Properties
OSCs were fabricated during this study with conventional configuration as follows:  Table 3. The organic bulk heterojunction solar cell containing TTT-co-P3HT produced smallest J SC (1.27 mA/cm 2 ), V OC (0.41 V), fill factor, FF (26.78%) and PCE (0.14%) in comparison with the device containing P3HT which produced the largest J SC (7.91 mA/cm 2 ), V OC (0.46 V), FF (31.64%) and PCE (1.15%). The improved performance of the device fabricated using P3HT was ascribed to sufficient electron/hole pair separation confirmed by photoluminescence and lower band-gap in comparison with TTT-co-P3HT. Low Jsc and Voc in TTT-co-P3HT based OSC is attributed to the disruption of the ordered lamellar stacking of P3HT by modification with TTT. This disruption results to a decrease in absorption and low hole mobility in TTT-co-P3HT [37,38]. P3HT and TTT-co-P3HT based OSCs were fabricated in air. Therefore, oxygen permeation does occur and will oxidize low work function aluminum electrode. Oxidized aluminum electrode will form a charge transport barrier. This induces S-shaped I-V curve and reduces the performance of the OSCs [39]. Additionally, penetrative oxygen in the active layer lead to different photo-oxidation reactions of an acceptor and donor materials [40,41]. Changes in the structures of an acceptor and donor materials will change their charge carrier mobilities, energy levels and photon absorption properties. The oxygen doping in the active layer will increase the concentration of holes, which results in an increase in trapping of electrons and a decrease in Voc and FF [42,43]. TTT-co-P3HT based OSCs were fabricated in air. Therefore, oxygen permeation does occur and will oxidize low work function aluminum electrode. Oxidized aluminum electrode will form a charge transport barrier. This induces S-shaped I-V curve and reduces the performance of the OSCs [39]. Additionally, penetrative oxygen in the active layer lead to different photo-oxidation reactions of an acceptor and donor materials [40,41]. Changes in the structures of an acceptor and donor materials will change their charge carrier mobilities, energy levels and photon absorption properties. The oxygen doping in the active layer will increase the concentration of holes, which results in an increase in trapping of electrons and a decrease in Voc and FF [42,43].

Characterization of OSCs with Electrochemical Methods
In order to further investigate the OSCs fabricated during this work, electrochemical techniques methods were used. ITO coated substrate was used as working electrode during electrochemical studies. The layers on the ITO substrates were prepared using OSCs fabrication conditions. Electrochemical techniques such as CV and EIS can be used to quantitatively study the electron-blocking ability of PEDOT:PSS interlayer. Parameters such as peak separation and peak current are very useful in evaluating electron-blocking properties because these parameters depend strongly on the amount of electroactive species from the electrolyte that are exposed to electroactive sites of the working electrode. Figure 6 depicts CV and Nyquist plots of ITO without/with PEDOT:PSS layer in 1 mM ferrocene prepared using 0.1 M TBAPF6 in acetonitrile. CV ( Figure 6A) obtained in the presence of PEDOT:PSS layer show a decrease in the oxidation/reduction peak currents and an increase in peak separations. The peak currents for a bare ITO is 0.27 mA for cathodic peak (Ipc) and for anodic peak (Ipa) is −0.18 mA, while for ITO/PEDOT:PSS are Ipc is 0.17 mA and Ipa is −0.07 mA. The extent to which the peak currents decreased after coating PEDOT:PSS onto the ITO substrate were estimated using Equation (2):

Characterization of OSCs with Electrochemical Methods
In order to further investigate the OSCs fabricated during this work, electrochemical techniques methods were used. ITO coated substrate was used as working electrode during electrochemical studies. The layers on the ITO substrates were prepared using OSCs fabrication conditions. Electrochemical techniques such as CV and EIS can be used to quantitatively study the electron-blocking ability of PEDOT:PSS interlayer. Parameters such as peak separation and peak current are very useful in evaluating electron-blocking properties because these parameters depend strongly on the amount of electroactive species from the electrolyte that are exposed to electroactive sites of the working electrode. Figure  6 depicts CV and Nyquist plots of ITO without/with PEDOT:PSS layer in 1 mM ferrocene prepared using 0.1 M TBAPF 6 in acetonitrile. CV ( Figure 6A) obtained in the presence of PEDOT:PSS layer show a decrease in the oxidation/reduction peak currents and an increase in peak separations. The peak currents for a bare ITO is 0.27 mA for cathodic peak (I pc ) and for anodic peak (I pa ) is −0.18 mA, while for ITO/PEDOT:PSS are I pc is 0.17 mA and I pa is −0.07 mA. The extent to which the peak currents decreased after coating PEDOT:PSS onto the ITO substrate were estimated using Equation (2): where I decreased is the amount of current decreased, I without is the peak current of bare ITO and I with is the peak current of ITO/PEDOT:PSS [44]. The values of I decreased was found to be 0.54 for cathodic peaks and 0.82 for anodic peaks. The peak separation for bare ITO was determined to be 0.42 V and for ITO/PEDOT:PSS was determined to be 1.17 V. The decrease in peak currents and an increase in peak separation for ITO/PEDOT:PSS suggest a decrease in the ITO electrode activity. These observations indicate that the PEDOT:PSS interlayer does not completely block electroactive species from ferrocene to reach ITO surface since the value of I decreased is not equal to 1. Therefore, PEDOT:PSS interlayer does not completely block electrons when is used as hole transport interlayer in OSCs and this might also be due to the presence of pinholes.
ferrocene to reach ITO surface since the value of Idecreased is not equal to 1. Therefore, PEDOT:PSS interlayer does not completely block electrons when is used as hole transport interlayer in OSCs and this might also be due to the presence of pinholes. EIS is an important instrument that can be used to research charge transfer processes at the interlayer [45][46][47] and evaluate the surface coverage at the electrode active area [44,48]. Figure 6B shows the Nyquist plots of bare ITO and ITO/PEDOT:PSS with the equivalent circuit as an inset. In the circuit, Rs is an Ohmic resistance, Rct is a resistance of charge transfer processes taking place at the interface and constant phase element (CPE) proposes a non-ideal behavior of the capacitor. CPE is well-defined by two adjustable values (CPE-T and CPE-P) and is mostly used as a capacitor-like element to compensate interfacial inhomogeneity (surface states or defects). In the case where CPE-P is equal to 1, then CPE and ideal capacitor are identical without defects [47,49,50]. The plots were fitted with zview software and the results of parameters obtained are recorded in Table 4. The value of Rct for bare ITO was found to be 655.10 Ω and for ITO/PEDOT:PSS was found to be 2935.00 Ω. These results reveal that PEDOT:PSS block the diffusion of electroactive species from ferrocene containing supporting electrolyte to the ITO surface. The electrode surface coverage (θ) can be estimated from Rct of bare ITO and ITO/PEDOT:PSS using Equation (3): where Rct bare ITO and Rct ITO/PEDOT:PSS are charge transfer resistances measured at bare ITO and ITO/PEDOT:PSS, respectively [48]. If the surface is completely covered, the value Rct ITO/PEDOT:PSS must be too big in such a way that θ will close to 1 [44]. From the Rct values attained by fitting the Nyquist plots, a value of θ = 0.78 was determined. Therefore, this indicate the presence of pinholes.   The results show that the presence of PEDOT:PSS cause a decrease in the peak currents. Cathodic and anodic peak currents are used to study the degree at which the peak current decreased using Equation (2). The results obtained are shown in Table 5. The extent at which the cathodic peak current is reduced because of PEDOT:PSS interlayer presence is 0.29 for TTT-co-P3HT:PC71BM, 0.25 for EIS is an important instrument that can be used to research charge transfer processes at the interlayer [45][46][47] and evaluate the surface coverage at the electrode active area [44,48]. Figure 6B shows the Nyquist plots of bare ITO and ITO/PEDOT:PSS with the equivalent circuit as an inset. In the circuit, R s is an Ohmic resistance, R ct is a resistance of charge transfer processes taking place at the interface and constant phase element (CPE) proposes a non-ideal behavior of the capacitor. CPE is well-defined by two adjustable values (CPE-T and CPE-P) and is mostly used as a capacitor-like element to compensate interfacial inhomogeneity (surface states or defects). In the case where CPE-P is equal to 1, then CPE and ideal capacitor are identical without defects [47,49,50]. The plots were fitted with zview software and the results of parameters obtained are recorded in Table 4. The value of R ct for bare ITO was found to be 655.10 Ω and for ITO/PEDOT:PSS was found to be 2935.00 Ω. These results reveal that PEDOT:PSS block the diffusion of electroactive species from ferrocene containing supporting electrolyte to the ITO surface. The electrode surface coverage (θ) can be estimated from R ct of bare ITO and ITO/PEDOT:PSS using Equation (3): where R ct bare ITO and R ct ITO/PEDOT:PSS are charge transfer resistances measured at bare ITO and ITO/PEDOT:PSS, respectively [48]. If the surface is completely covered, the value R ct ITO/PEDOT:PSS must be too big in such a way that θ will close to 1 [44]. From the R ct values attained by fitting the Nyquist plots, a value of θ = 0.78 was determined. Therefore, this indicate the presence of pinholes.  show that the presence of PEDOT:PSS cause a decrease in the peak currents. Cathodic and anodic peak currents are used to study the degree at which the peak current decreased using Equation (2). The results obtained are shown in Table 5. The extent at which the cathodic peak current is reduced because of PEDOT:PSS interlayer presence is 0.     is an anodic peak current obtained without PEDOT:PSS interlayer, b I pc (without) is cathodic peak current obtained without PEDOT:PSS interlayer, c I pa (with) is an anodic peak current obtained with PEDOT:PSS interlayer, d I pc (with) is cathodic peak current obtained with PEDOT:PSS interlayer, e I decreased (pa) is the extent at which anodic peak current decreased and f I decreased (pc) is an extent at which cathodic peak current decreased.

Conclusions
In summary, we have successfully synthesized TTT-co-P3HT using chemical oxidation polymerization for use as the donor material in OSCs. The properties of TTT-co-P3HT were compared with those of pristine P3HT. The optical band gaps of TTT-co-P3HT and P3HT in chlorobenzene were found to be 2.32 eV and 2.21 eV respectively. The LUMO offsets of active layers TTT-co-P3HT:PC 71 BM and P3HT:PC 71 BM were determined to be 0.96 eV and 0.97 eV, respectively. The TTT-co-P3HT:PC 71 BM active layer has insufficient electron/hole pair separation and charge transfer at the interface, which was confirmed by photoluminescence quenching studies. The OSCs device of P3HT:PC 71 BM exhibited a better performance with an efficiency of 1.15%, while TTT-co-P3HT:PC 71 BM exhibited an efficiency of 0.14%. Poor performance of TTT-co-P3HT in OSC is because of its low hole mobility and decreased photon absorbance due to the disturbance in ordered lamellar stacking of P3HT after functionalization. To further understand the deviation in TTT-co-P3HT:PC 71 BM and P3HT:PC 71 BM devices performance, we studied the layers of OSCs fabricated during this work using electrochemical methods. The study revealed that PEDOT:PSS interlayer does not completely block electrons from active layer to the ITO substrate. From cyclic voltammetry results, the diffusion of electrons to the ITO substrate was observed by a decrease in the current of the peaks. Therefore, this study gives an opportunity to further optimize OSCs using cheap and reliable electrochemical methods and also shows the importance of using electrochemical methods in study the interlayers behavior for OSCs use.

Conflicts of Interest:
The authors declare no known competing interest.