Electrochemical and optical studies of new symmetrical and unsymmetrical imines with thiazole and thiophene moieties

Electrochimica Acta 332 (2020) 135476

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Electrochemical and optical studies of new symmetrical and unsymmetrical imines with thiazole and thiophene moieties Krzysztof Artur Bogdanowicz a, **, Beata Jewloszewicz a, Karolina Dysz a, Wojciech Przybyl a, Agnieszka Dylong a, Wojciech Mech b, Krzysztof P. Korona b,
ska b, Agnieszka Iwan a, * Magdalena Skompska c, Andrzej Kaim c, Maria Kamin a b c

Military Institute of Engineer Technology, Obornicka 136 Str, 50-961, Wroclaw, Poland Faculty of Physics, University of Warsaw, ul. Pasteura 5, 02-093, Warsaw, Poland Faculty of Chemistry, University of Warsaw, Pasteura 1, 02-093, Warsaw, Poland

a r t i c l e i n f o

a b s t r a c t

Article history: Received 18 September 2019 Received in revised form 27 November 2019 Accepted 8 December 2019 Available online xxx

In this study new symmetrical and unsymmetrical imines based on 4-[4-(4-fluorophenyl)1,3-thiazol-2or 2,2’:50 , 2’’-terthiophene-5yl]aniline and 2,20 ,50 ,2”-terthiophene-5,5”-dicarboxaldehyde carboxaldehyde, respectively, were synthesized by melt condensation with the yield of 86% and 40%. The 5% weight loss of the imines was found in temperature range from 294 to 390  C in nitrogen, depending on the symmetry of aldehyde used in synthesis. The band gap of symmetrical imine (PV-BLJSC13) is lower by about ~0.33 eV than that of unsymmetrical PV-BLJ-SC14. For both imines the electron density of HOMO level is mainly located on thiophene moieties and is extended to imine bonds in thiazole rings and imine moieties. The theoretical calculations of their HOMO-LUMO levels by DFT methods were also provided. The UVeVis absorption spectra and cyclic voltammograms of imines both in the form of thin films cast on the electrode as well as dissolved in 1,2-dichloromethane were studied to determine the electrochemical and optical band gap energies as well as correlate the positions of HOMO and LUMO levels obtained by DFT and electrochemical methods. The resistance of imines was approximately 113.8 U and 102.4 U for PV-BLJ-SC13 and PV-BLJ-SC14, respectively as determined by IR thermal images and have a tendency to decrease by about 17e24 U when mixed with PTB7 and PC71BM. Both imines showed very bright luminescence. Moreover, optical absorption, photoluminescence and IR thermal images were measured for imine:PTB7 or imine:PTB7:PC71BM composite. Finally, simple organic solar cells based on (PTB7:imine):PC71BM in weight ratio (4:1):8 and (8:1):13 w/w were constructed and characterized. The highest power conversion efficiency, which was equal to 0.42% was determined for the ITO/PEDOT:PSS/PTB7:PV-BLJ-SC13:PC71BM (4:1):8/Al device. In addition, the power conversion efficiency was strongly dependent on the weight ratio of imine in active layer. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Imines Azomethines Electrochemistry Thermographic camera Photoluminescence Organic solar cells

1. Introduction In the 21st century new organic compounds such as small molecules, polymers and dendrimers are synthesized as materials for optoelectronic devices such as solar cells and light emitting diodes [1e5]. Syntheses of many organic materials proposed in the literature often require multiple step procedures or/and application

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (K.A. Bogdanowicz), iwan@witi. wroc.pl (A. Iwan). https://doi.org/10.1016/j.electacta.2019.135476 0013-4686/© 2019 Elsevier Ltd. All rights reserved.

of inorganic catalysts and toxic solvents [6e11]. Due to increased environmental concerns, the rules of Green Chemistry should be more than ever taken into account during synthesis of new compounds [12]. In this context, a group of azomethine compounds seems very interesting. Our previous review study in the field of photovoltaics revealed only selected information about azomethines and some other data are also only fragmentally given in a few papers [13e18]. Azomethines are more promising than polyazomethines [19e26] for application in organic photovoltaics taking into consideration their solubility and purity. The values of power conversion efficiency (PCE) for imines investigated as components of organic solar cells are found in the range of 0.1e1.21% [13e17]. Petrus et al. received the highest value of PCE ¼ 1.21% for

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device with the architecture ITO/MoO3/TPA-N]CHeTheCH]NTPA:PC71BM/LiF/Al, where TPA is triphenylamine and Th stands for thiophene [13]. On the other hand, Canli et al. proposed imine (5(10-undecyloxy)-2-[[[4-hexylphenyl]imino]methyl]phenol) with liquid crystalline properties as an acceptor in the active layer with poly(3- hexylthiophene) P3HT as a donor material [14]. The reported PCE value was 0.27% after annealing at the temperature of phase transition (PCE before annealing was 0.16%). Moussalem et al. received the PCE ¼ 1.18% for bilayer cell containing imine with benzodifuran moieties and thiophene rings as the donor material of a thickness 20e25 nm and C60 as acceptor material [15]. Additionally, unsymmetrical imine with porphyrin moieties were tested in dye-sensitized solar cells (DSSC) and showed the value of PCE ¼ 1.75% [17]. Recently, the power efficiency of 2.2% has been obtained for the cell of ITO/MoOx/imine:PC71BM (1:5)/Mg/Al architecture containing imines with triphenylamine, thiophene and thiazolo-moieties [18]. This study is an attempt to develop new symmetrical and unsymmetrical imines in one step melt condensation reaction, ecologically friendly, without any inorganic catalyst. The imines with thiophene and thiazole moieties were then investigated as additional components in several active layers (imine:PTB7, imine:PC71BM and imine:PTB7:PC71BM) for organic solar cell. As follows from the above, the optical and electrochemical properties of the new active layers were modulated by changing the symmetry of investigated imines, namely the presence of one or two imine bonds and thiazole moieties. The special attention was focused on the problem of how the above relationships affect the power conversion efficiency of constructed simple devices based on imine:PTB7 or imine:PC71BM, while defects on the formed photoactive surface were also taken into account, which were studied using IR thermal images. It should be noted, that is the first time IR thermal images have been used to analyze surface defects in devices based on imine:PTB7:PC71BM. The synthesized imines exhibited: (i) good thermal stability, (ii) deep HOMO level (about 6  5.6 eV), (iii) very good photoluminescence, (iv) absorption band in the range 350e500 nm. The studies of the new imines with thiophene and thiazole moieties are focused on several aspects: (i) influence of the symmetry in the imine on UVeVis absorption and photoluminescence properties, (ii) influence of the symmetry in the imine on HOMO-LUMO levels, (iii) influence of the symmetry in the imine on thermal stability, (iv) influence of the weight ratio of imine in the active layer based on PTB7:PC71BM on photovoltaic properties of BHJ organic solar cells, (v) influence of active layer composition on the electrical properties in binary and ternary organic layers. The investigations were performed by means of numerous physicochemical techniques: UVevis and photoluminescence, cyclic voltammetry, IR thermal imaging, conductivity, and currentvoltage measurements. The HOMO and LUMO levels of the imines were also determined by DFT calculations. In accordance with the best of our knowledge the optical and electrical properties of the symmetrical and unsymmetrical imines with thiophene and thiazole moieties presented in this work were not investigated so far. Moreover, for the first time the IR thermal images were used to analyze defects in devices based on imine:PTB7:PC71BM.

2. Experimental 2.1. Materials and synthesis The compounds PV-BLJ-SC13and PV-BLJ-SC14 were synthesized using the precursor materials 4-[4-(4-fluorophenyl)1,3-thiazol-2yl]aniline, and 2,20 ,50 ,2”-terthiophene-5,5”-dicarboxaldehyde and 2,2’:50 ,2’’-terthiophene-5-carboxaldehyde as shown in Fig. 1 and in Supporting Information [35,36]. Our study showed that PV-BLJSC13 and PV-BLJ-SC14 imines cannot be obtained by condensation in solution. For this reason, we proposed a simple and ecologically friendly synthesis in melt. A single step synthesis was carried out by the condensation reaction of aldehyde (dialdehyde) and amine in the presence of p-toluenosulfonic acid (p-TosOH) at 180  C for 34 h. The progress and completion of the reaction was monitored by a thin layer chromatography. Then, dimethylacetamide (DMA) was added to dissolve the reaction mixture and the product was filtered, washed with ethanol and acetone and recrystallized from acetone/ hexane. Finally, the samples were dried for 24 h at 60  C. Further details on synthesis are presented in Supporting Information. 2.2. Methods The samples were characterized with 1H NMR, using DMSO‑d6 as a solvent by means of a Jeol ECZ-400 S spectrometer (1H 400 MHz) with delay time 5 s. Measurements were carried out at room temperature using 10e15% (w/v) sample solutions. The FTIR spectra with 2 cm1 resolution were obtained by means of Nicolet-Nexus Fourier spectrometer, applying 32 scans and KBr pellet technique for middle-IR (4000e400 cm1). DSC measurements were done using TA Q20 DSC calorimeter. The temperature was calibrated on the onsets of melting points of ice and indium. The sample was hermetically sealed in aluminium pans of 30 ml. Measurements were done with heating rate of 5  C/ min in a nitrogen atmosphere. Thermogravimetric (TGA) measurements were performed by means of TA Q50 TGA using Pt holders. Behavior of the imines was examined under nitrogen atmosphere. The gas flow supplied in the vicinity of the studied sample was 20 ml min1. Each TGA test included heating in a temperature range of 40e800  C with a rate of 5  C/min. The theoretical calculations were performed using GAUSSIAN 16 [27] software package. The structures of the investigated compounds were optimized at the hybrid density functional DFT/B3LYP level [28e30] with the 6-31G(d) basis set, to a stationary point on the Born-Oppenheimer potential energy surface proved by the absence of imaginary frequencies. Several starting geometries were applied for each compound, and for further study the structure with the lowest enthalpy was considered. The choice of the relative inexpensive DFT method with the comparatively small 6-31G(d) basis set was conditioned by the size of the investigated molecules, and the fact that for closed-shell organic compounds containing C, H, N, and O elements improvement with increasing basis-set size is modest for the heats of formation [31]. The frontier HOMO and LUMO were calculated on fully optimized structures by employing both the DFT/B3LYP 6-31G(d) and PBE/6-311G(d,p) [32] approaches. The last procedure turned out to be successful in accurate reflecting the experimental HOMO and LUMO results for large molecules, especially for fullerenes and their derivatives [33]. UVevis reflection measurements were carried out with the use of spectrophotometer Shimadzu 3600 equipped with an integration sphere for diffuse reflectance measurements of powders. BaSO4 was used as the reference. Cyclic voltammograms were performed in a standard three

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Fig. 1. Scheme of condensation reaction to form PV-BLJ-SC13 and PV-BLJ-SC14 imines.

electrode electrochemical cell, with a Pt disc of the surface area of 0.03 cm2 as the working electrode, a Pt wire as the counter electrode and Ag/Agþ (0.1 M) in AN as the reference electrode. The potential of Ag/Agþ electrode was calibrated vs. Ag/AgCl/Cl-aq (3 M) using ferrocene/ferrocenium (Fc/Fcþ) redox couple in acetonitrile. Then, the obtained value, 0.311 V vs. Ag/AgCl was referred to the standard hydrogen electrode (SHE) assuming that the difference between the potential of Ag/AgCl/Cl(3 M) and SHE is 0.194 V. Thus, the potential of Ag/Agþ (0.1 M, AN) electrode is 0.505 V vs. SHE. All solutions before experiments were deoxygenated by bubbling with a stream of Ar for 10 min. The Pt electrode before each use was polished with a polycrystalline diamond suspension (3 mm) (MetaDiTM Supreme, Buehler) on polishing cloth. The measurements were performed for: a) imines deposited on the Pt electrode and b) imines dissolved in DCM. In the first approach, a small amount of the imine was dissolved in DCM or THF and then, a drop of the imine solution was cast on Pt electrode. After solvent evaporation, the Pt electrode covered with a thin layer of the imine was immersed in AN solution containing 0.1 M TBAPF6 supporting electrolyte and cycled at the scan rate of 100 mV s1. In the second approach, the cyclic voltammograms were obtained on the polished Pt electrode immersed in DCM containing imine and 0.1 M TBAPF6 supporting electrolyte. Photoluminescence (PL) spectra were performed in the range of 300e900 nm using the Hamamatsu C7557 CCD detector. Timeresolved PL (TRPL) measurements were performed using frequency-tripled (to wavelength of 300 nm) output pulses (tP ¼ 130 fs) from a mode-locked Ti:sapphire laser. The temporal distribution of the PL was analyzed by the Hamamatsu streak camera. For PL spectroscopy, the samples were drop cast on copper substrates from the solutions used for organic solar cell construction. The working solar cells were characterized by IeV measurements and photo-electric spectroscopy. The current densityvoltage curves of bulk heterojunction (BHJ) polymer solar cells were measured using Ossila 8 pixel test board under dark and under AM1.5 solar illumination. The illumination source was a Newport VeraSol-2 LED Class AAA Solar Simulator of 1000 W/m2 power output. The photo-electric spectroscopy was made with use of tungsten lamp and 0.5-m monochromator in the range from 300 nm to 1400 nm. The photo-excited current was measured with a Keithley logarithmic pico-amperometer. The excitation intensity

value that was necessary for external quantum efficiency (EQE) calculation was controlled by calibrated power meter PM320. The excitation density was of the order of 1 mW/cm2. Thermal behaviour was observed using thermographic camera (VIGOcam v50, VIGO System S.A, Poland), while applying bias voltage between 0 and 10 V and using a multichannel potentiostatgalvanostat (PGStat Autolab M101, Metrohm, Nederland) connected to computer. In the experiment the voltage was applied in range from 0 V to 10 V with 0.5 V step increment; each voltage value was maintained for 3 min. The current response was recorded during all these 3-min-periods and in the last 10 s the IR image was collected. The work of both camera and power source was controlled via computer software. For the experiment the active layers from solution in dichloroethane with total concentration 15 mg/mL were prepared by spin-coating method at 5000 rpm for 20 s. This experimental procedures were described in details in our previous work [34]. Samples for thermal behaviour study were prepared in a form of sandwich like structure between two ITO-coated glass giving the total area of approximately 2.35 cm2 for PTB7 and 1.5 cm2 for other samples. IR-Vis-UV optical absorption spectra were recorded at room temperature using Carry 5000 spectrophotometer in the range 300e800 nm. The thickness of layers was measured with profilometer, so the absorption coefficients could be calculated. Thin layer of organic compounds for absorption measurements were spin coated (900 rpm, 60s) or drop cast (PV-BLJ-SC13 and PV-BLJSC14) from the solutions described above on ozone-UV activated pure glass substrates. 2.3. Organic solar cells construction Solar cells were fabricated on an indium tin oxide (ITO)-coated glass substrates with the structure ITO/PEDOT:PSS/(donor material):PC71BM/Al in Ar atmosphere. The acceptor material was fullerene C70 ester with phenyl and butyric acid methyl (PC71BM). As reference donor material we used poly(4,8-bis[(2-ethylhexyl) oxy]benzo[1,2-b:4,5-b0 ]dithiophene-2,6-diyl-alt-3-fluoro-2-[(2ethylhexyl)carbonyl]thieno [3,4-b]thiophene-4,6-diyl) (PTB7). The investigated imines PV-BLJ-SC13 and PV-BLJ-SC14 were blended with PTB7 and also were used as donor materials. The substrates had 8 separated ITO pixels giving the single cell area of 4 mm2 per each pixel. The most important ingredients: PTB7 (M215), PC71BM,

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PEDOT:PSS (Heraeus Clevios AI 4083) and ITO 8 pixel glass substrates were purchased from Ossila. The ITO-coated glass substrates were first cleaned with deionised water and dried by compressed nitrogen. Next ITO surface was activated by 5 min treatment in Ossila UV Ozone Cleaner. On such prepared ITO substrates, a PEDOT:PSS layer was deposited by spin coating (5000 rpm) from PEDOT:PSS aqueous solution which was filtered through 0.45 mm PES syringe filter. The obtained thin PEDOT:PSS film was annealed on a hot plate at 150  C for 15 min and the substrates were transferred to Ar glove box. A chlorobenzene solutions of PV-BLJ-SC13 and PV-BLJ-SC14 imines, PTB7, PC71BM and mixture of PTB7:PC71BM were prepared in Ar glove box and stirred for 24 h on the hotplate at 50  C. Next, the solutions of PTB7, PC71BM and mixture of PTB7:PC71BM were filtered through 0.45 mm PTFE syringe filter. In order to prepare the imine:PTB7:PC71BM mixture, the filtered PTB7:PC71BM solution was mixed with 15 mg/ml imines solutions to target concentration and stirred for next 3 h at temperature of 50  C. A total concentration of imine:PTB7:PC71BM was 25 mg/ml. Two solutions with different imine:PTB7:PC71BM ratios: (1:4):8 and (1:8):13 as well as the reference solution without imine (0:2):3 were studied. The prepared solutions were spin-coated (900 rpm, 60s) on ITO glass substrates with previously deposited PEDOT:PSS layer. Then, an aluminium electrode was deposited by thermal evaporation in a vacuum of about 3  105 mbar.

3.1. Cyclic voltammetry The onset potentials of oxidation and reduction of both imines red (Eox onset E onset , respectively), at which the initial injection of holes to HOMO level or electrons to LUMO level occurs, were determined from the onsets of anodic and cathodic currents in the cyclic voltammograms recorded at the scan rate of 100 mV s1. These values were referred to the potential of standard hydrogen electrode (SHE), and then converted into the absolute scale (in eV), assuming that 0 V vs NHE corresponds to 4.43 eV [37]. Thus, the ELUMO and EHOMO levels were determined from the equations:

  ELUMO ¼  Ered onset þ 4:43 eV
EHOMO ¼  Eox onset þ 4:43 eV

3. Results and discussion

Before electrochemical studies of the imines, the Pt electrode was cycled in deaerated acetonitrile containing 0.1 M TBAPF6 supporting electrolyte to determine the width of “electrochemical window”. It was especially important to determine the onset potential of reduction of water traces present in the solvent to avoid overlapping with reduction of the monomers. As displayed in Fig. S5, the negative potential limit is about 2.1 V vs Ag/Agþ. The analysis of electrochemical results for both imines studied will be described in details below. In addition, HOMO-LUMO levels for investigated imines were theoretically calculated by two DFT methods. The experimental and theoretical values are listed in Tables 1 and 2, respectively. For geometries of optimized structures see Supporting Information, while the HOMO and LUMO molecular orbitals of both imines are depicted further in the text.

The molecular structure and purity of the PV-BLJ-SC13 and PVBLJ-SC14 imines were ascertained by using 1H NMR, FTIR and UVeVis spectroscopy (see Figs. S1eS2). The 1H NMR spectra of both imines showed characteristic singlet peak due to HC]N- proton and multiple peaks due to aromatic protons. The proton attributed to the imine bond appeared at 8.56 ppm for PV-BLJ-SC13 and 8.82 ppm for PV-BLJ-SC14 (see Supporting Information). Moreover, the strong bands at 1605 cm1 and 1610 cm1 in the FTIR spectra for PV-BLJ-SC13 and PV-BLJ-SC14, respectively corresponding to C]N stretching (n(C]N)) were found (see Supporting Information). The UVeVis absorption spectrum of PV-BLJ-SC14 in chloroform revealed three bands as shown in Fig. S3. The imine PV-BLJSC14 exhibited two sharp peaks with absorption maxima at 246 nm (corresponding to the p-p* transition) and 353 nm, and one broad peak with maximum at 444 nm (corresponding to intramolecular charge transfer between thiophene and fluorophenyl1,3-thiazol-2-yl]aniline). The thermal properties of the imines were evaluated by thermogravimetric analysis (TGA), differential thermogravimetry (DTG) and differential scanning calorimetry (DSC) (see Fig. S4). The TGA curve for symmetrical imine PV-BLJ-SC13 indicates that the compound is thermally stable up to about 390  C. The similar behavior was observed for unsymmetrical imine PV-BLJ-SC14 up to about 294  C. Decomposition of both compounds occurred in two steps. In the first one, in the range 390 e 430  C for PV-BLJ-SC13 and 294 e 410  C for PV-BLJ-SC14, the observed weight loss was of about 35% and 55%, respectively. After the second step of decomposition at 800  C, the char yield percentage in the symmetrical imine PV-BLJ-SC13 was a little higher (49%) than that in unsymmetrical PV-BLJ-SC14 (approximately 31%). The temperature of the maximum decomposition point, evidenced by the DTG curves, was observed at 400 and 390  C for PV-BLJ-SC13 and PV-BLJ-SC14, respectively. These relatively high decomposition temperatures are important from the point of view of application of both imines in organic solar cells.

3.1.1. HOMO-LUMO of PV-BLJ-SC13 The electrochemical measurements were performed on Pt covered with a thin layer of PV-BLJ-SC13, in acetonitrile containing 0.1 M TBAPF6 as the supporting electrolyte. The imine was deposited on Pt by drop cast from THF solution. At first, the cyclic voltammograms were recorded in the limited potential range (from 0 V towards negative and then positive values), to select the most suitable polarization range and detect precisely the oxidation and reduction onsets. Then, the potential range was extended to 1.7 V and 1.5 V. As presented in Fig. 2a, the reduction of the imine starts at about 1.2 V, while the irreversible reduction peak is located at 1.43 V. An interesting behavior is observed in the positive potential range. Namely, a characteristic loop (the current in the reverse scan is higher than that in the forward one), typical for nucleation and growth process, appears in the cyclic voltammogram. Since the PV-BLJ-SC13 molecule contains three thiophene units, the oxidation current may be ascribed to thiophene oxidation. Then, the compound immobilized on the electrode probably undergo some transformation (for example coupling or crosslinking) which was manifested by the loop in the CV. Moreover, in effect of this irreversible oxidation the color of the PV-BLJ-SC13 immobilized on the electrode changed from orange to blue-violet. The similar experiments were carried out on the bare Pt electrode immersed in the DCM containing supporting electrolyte (0.1 M TBAPF6) and PV-BLJ-SC13 in the form of suspension. As presented in Fig. 2b, there is no loop in the cyclic voltammogram in the range of positive potentials, and no reduction peak is observed in the first scan. However, the latter one gradually develops in the subsequent cycles. It suggests that the imine molecules (probably oxidized in the positive potential range) are adsorbed at the negatively charged electrode and then reduced at about 1.3 V, as it was observed for PV-BLJ-SC13 immobilized on the electrode. However, the concentration of the imine at the electrode was

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Table 1 Energies of HOMO and LUMO levels and the band gap energy (Eg) determined from electrochemical data for the imines PV-BLJ-SC13 and PV-BLJ-SC14 grafted on the Pt electrode and dissolved in dichloromethane. Imine

þ Eox onset =Vvs. Ag/Ag

þ Ered onset /V vs. Ag/Ag

E ox onset /V vs. NHE

Ered onset /V vs. NHE

Eg,ec/eV

HOMO vs. vacuum/eV

LUMO vs. vacuum/eV

Eg

1.17 1.16

1.20 1.02

1.68 1.66

0.69 0.515

2.37 2.18

6.11 6.09

3.80 3.91

2.10 2.27

0.85 0.78

1.85 1.60

1.35 1.285

1.34 1.60

2.70 2.38

5.79 5.71

3.09 3.33

2.20 2.50

opt/eV

PV-BLJ-SC13 on Pt electrode in the solution PV-BLJ-SC14 on Pt electrode in the solution

Table 2 The HOMO and LUMO energies of two investigated imines calculated by two DFT methods: PBE/6-311G(d,p) and B3LYP/6-31G(d). Method

Eg [eV]

HOMO [eV]

LUMO [eV]

1.57 2.62

4.98 5.28

3.41 2.59

1.79 2.88

4.92 5.25

3.12 2.37

PV-BLJ-SC13 PBE/6-311G(d,p) B3LYP/6-31G(d)

positive potentials resulted in the formation of voltammetric loop and appearance of a reduction peak in the backward scan, at about 0.6 V (peak A0 in Fig. 4a). The consecutive cycling leads to formation of several new peaks in the voltammograms. A gradual increase of their intensity upon cycling (Fig. 4b) suggests the growth of a conducting film on the Pt electrode. The oxidation current at E > 0.9 V may be ascribed to oxidation of a terminal thiophene

PV-BLJ-SC14 PBE/6-311G(d,p) B3LYP/6-31G(d)

probably too small to undergo the coupling/crosslinking after oxidation in the positive potential range. The cathodic peak observed in both cases may be ascribed to reduction of imine to radical anion, according to the Scheme 1: The onset potentials for oxidation and reduction were used to determine the HOMO and LUMO levels of PV-BLJ-SC13 and band gap energy, Eg,ec (listed in Table 1). The band gap energy was also determined from UVevis spectrum of PV-BLJ-SC-13 solution in NMP (1-methyl-2-pyrrolidinone), since the solubility of this imine in acetonitrile (AN), 1,2dichloromethane (DCM) and chloroform is rather poor. The optical band gap (Eg, opt) of PV-BLJ-SC13 determined from the edge of the absorption band is about 2.27 eV (Fig. S6a), which is quite close to the value obtained from the cyclic voltammograms of the compound in the solution (2.18 eV). For comparison, the UVeVis measurements were also done for PV-BLJ-SC13 in the solid state. The diffuse reflectance spectra of the imine cast on the ITO substrate and PV-BLJ-SC13 powder are presented in Fig. S6b and Fig. S6c, respectively. The spatial distribution of the HOMO and LUMO frontier orbitals for PV-BLJ-SC13 imine is presented in Fig. 3. The surfaces depicted in the figure correspond to positive (orange and green) and negative (violet and yellow) contour values outlining current isosurface. Our theoretical study shown that in the case of PV-BLJ-SC13 the electron density of HOMO is mainly located on thiophene moieties (see Fig. 3 - orange and violet colors), and is extended to imine bond (as result of condensation reaction of aldehyde and amine) as well as to C]N bond in thiazole rings and phenylene rings. On the other hand, LUMO (Fig. 3 - green and yellow) is located only on thiophene moieties. 3.1.2. HOMO-LUMO of PV-BLJ- SC14 As in the previous case, the PV-BLJ-SC14 imine was drop cast on Pt and studied in acetonitrile (AN) containing 0.1 M TBAPF6. In the first cyclic voltammogram of freshly prepared film, the reduction starts at 1.85 V, while the oxidation onset potential is about 0.9 V (Fig. 4a). These two potentials were taken to determine the position of HOMO and LUMO levels (see Table 1) and the band gap energy (2.75 eV). Polarization of Pt/PV-BLJ-SC14 electrode to higher

Fig. 2. Cyclic voltammograms of: PV-BLJ-SC13 drop cast on Pt electrode, in 0.1 M TBAPF6 in AN (a) and bare Pt electrode immersed in the solution of PV-BLJ-SC13 in 0.1 M TBAPF6 in DCM (numbers 1, 3 and 5 denote the numbers of CV cycles) (b).

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Scheme 1. The cathodic peak observed in both cases may be ascribed to reduction of imine to radical anion, according to the reaction.

group in the molecule of PV-BLJ-SC14 to radical cation. Then, two radical cations undergo coupling and form dimer after abstraction of two protons. The dimers containing 6 thiophene units undergo reversible oxidation/reduction which is manifested by the presence of symmetrical redox peaks (A and A’ in Fig. 4b). The electrode cycling resulted also in the changes in the range of negative potentials. Namely, a new reduction peak (B0 ) is formed at 0.55 V and a shoulder (C0 ) appears at the potentials more negative than 1.3 V. This suggests that the radical anions may be created both on the thiophene and imine moieties. In order to determine reversibility of these processes, the cyclic voltammograms were recorded in various potential ranges. As visible in Fig. 5, when the potential scan was reversed at 0.4 V, only one redox couple (peaks A and A0 ) was formed. The increase of the potential range to 1.0 V, resulted in appearance of the reduction peak B’ at 0.55 V and its anodic counterpart (B) was formed at 0.5 V. Finally, when the potential range was extended to 1.9 V, the moieties reduced below 1.3 V are then oxidized in the reverse scan in the potential range from 0.5 V to 0.93 V (red curve in Fig. 5). This suggests that the PV-BLJ-SC14 dimers may be reversibly doped both in p- and n-doping range. Next, we investigated the electrochemical behavior of PV-BLJSC14 on the bare Pt electrode in 1,2-dichloromethane (DCM) solution (see Fig. 6). In the first voltammetric scan the onset of reduction current is about 1.6 V, while oxidation starts at about 0.78 V (Fig. 6a) and these two values were taken to determine the positions of LUMO and HOMO levels and the band gap energy (2.38 eV). In the consecutive cyclic voltammograms two additional reduction peaks, B0 and C’, develop at 0.55 V and 1.3 V, respectively. Thus, the shape of the voltammogram becomes similar to that recorded on the electrode covered with the film of PV-BLJ-SC14 (after

dimerization). Although the current intensities of the oxidation and reduction peaks are much lower due to low concentration of imine in the solution, one can conclude that the PV-BLJ-SC14 molecules are oxidized on the Pt electrode and undergo coupling via terminal thiophene units. It was also important to note that the electrode cycling resulted in a gradual change of the solution color from orange to red which means that the dimers are well soluble in dichloromethane. The formation of the compound of longer conjugation length (six thiophene units instead of three) results in the formation of a new absorption peak at the wavelength of 540 nm in UVevis spectrum of the solution after electrochemical studies and diminution of the peak corresponding to the monomer (at 400 nm) (see Fig. S7d). The results presented above indicate that the PV-BLJ-SC14 is very promising compound because the thiophene terminal groups after oxidation may be coupled with another thiophene e bearing compounds for example the fullerenes functionalized with thiophene groups. The co-polymerization of these two type of compounds may lead to the formation of polymers of donor-acceptor properties which may be used as photoactive materials in organic solar cells. Model of HOMO and LUMO orbitals for PV-BLJ-SC14 is presented in Fig. 7. The electron density of HOMO level of unsymmetrical imine PVBLJ-SC14 is, as in the case of PV-BLJ-SC13 (symmetrical imine), located mainly on the thiophene moieties (in Fig. 7 - orange and violet colors) and extended to imine bond, thiazole ring and phenylene ring. As in the case of PV-BLJ-SC13, the LUMO (in Fig. 7green and yellow) is located only on thiophene moieties. The HOMO-LUMO gap of PV-BLJ-SC14 was calculated to be 1.79 eV and 2.88 eV by PBE/6-311G(d,p) and B3LYP/6-32G(d) method, respectively (see Table 2). Comparison of the results obtained by electrochemical, optical and DFT methods are presented in Fig. S8. When analyzing the theoretical results, two aspects should be discussed. The first concerns the compatibility of theoretical results with each other, i.e. the results obtained with the PBE and B3LYP methods. It is known that the functionalities used in both methods differ from each other in the percentage of amount of HartreeeFock exact-exchange, which in consequence clearly leads to differences in theoretically designated Eg Ref. [38]. It should be noted, however, that the differences in Eg values obtained by both methods are for

Fig. 3. Models of HOMO (orange and violet) and LUMO (green and yellow) orbitals for optimized PV-BLJ-SC13 molecule. Orange and green lobes correspond to positive, whereas violet and yellow lobes to negative isosurface values. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

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PV-BLJ-SC13 and PV-BLJ-SC14 imines by 1.05 and 1.09 eV, respectively, therefore they are similar to the difference in HOMO-LUMO gaps (0.98 eV) determined by similar methods for the thiophene hexamer [39]. The second aspect is the compatibility of theoretical results obtained by both methods with the experimental results. Both popular methods of theoretical determination of frontier orbitals energy, i.e. B3LYP and PBE, used in this work do not accurately reproduce Eg using the experimental methods we used, which, as can be seen in Tables 1 and 2, also differ from each other. It is however shown that the accuracy of prediction strongly depends on the studied system and the functional applied [40]. For example, PBE functional has been postulated to yield reliable results for fullerene systems while the B3LYP functional performs rather poorly [41]. When it comes to functionals used in the present research, the pure or hybrid functionals such as PBE and B3LYP significantly overestimate HOMO energies and underestimate

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LUMO energies, respectively, resulting in much smaller HOMO e LUMO gaps [38]. Thus, comparing the results for frontier orbitals obtained experimentally and theoretically in this paper it is not clear which theoretical method best simulates the experimental results. The advantage of the calculations made is that they reflect the mutual ratio of the determined orbital energies, consistent with the results of the experiment, i.e. both the B3LYP and PBE methods indicate that, for example, Eg for PV-BLJ-SC14 is greater than for PVBLJ-SC13.

3.2. Thermal imaging In the current work, for the first time thermographic camera was used to detect the location of defects in the created simple devices and to follow electrical behavior of two imines and their mixtures with PTB7 or PC71BM compounds in weight ratio 1:1. For

Fig. 5. Cyclic voltammograms on Pt electrode covered with the PV-BLJ-SC14 layer (grafted from THF solution) in different potential ranges, in the solution of 0.1 M TBAPF6 in AN.

Fig. 4. The first (a) and consecutive (b) cyclic voltammograms obtained on Pt electrode covered with the imine PV-BLJ-SC14 layer (grafted from THF solution), in 0.1 TBAPF6 in AN. Dashed line corresponds to CV obtained in the same solution on the bare Pt electrode.

Fig. 6. Evolution of the cyclic voltammograms on Pt electrode in a fresh solution of PVBLJ-SC14 þ 0.1 M TBAPF6 in DCM (the numbers at the lines correspond to the cycle numbers).

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Fig. 7. Models of HOMO (orange and violet) and LUMO (green and yellow) orbitals for optimized PV-BLJ-SC14 molecule. Orange and green lobes correspond to positive, whereas violet and yellow lobes to negative isosurface values. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

thermal imaging, the samples were prepared using spin-coating technique to form the active layers over glass support covered with indium-tin oxide (ITO) conductive layer. The architecture of the studied devices is denoted as ITO/active layer/Ag/ITO, whereas active layer PTB7, PC71BM, PV-BLJ-SC13, PV-BLJ-SC14, PV-BLJSC13:PTB7, PV-BLJ-SC13:PC71BM, PV-BLJ-SC14:PTB7 and PV-BLJSC14:PC71BM, and as silver source an Ag conductive paste were used. In the studied architecture all layers were morphologically uniform. Fig. 8 presents constructed devices containing imine and mixtures with PTB7 and PC71BM. Generally, all devices for one and two component active layers demonstrated very similar thermal topographic responses to the applied voltage: as it can be seen on the exemplary images presented in Fig. 8. It is interesting to note that the increase of the voltage leads to appearance of two main heating centers located in proximity to the external metallic contacts (crocodile clamps). This kind of behavior was observed by us in our previous works [34,42]. The heating centers located close to crocodile clamps can be explained by the concentration of the current flow close to the metallic connections and due to imperfect interface between metal and ITO. The presence of overheating spots influence partially the thermal response of the device, hence for further analysis we selected circular areas (see Fig. 8), least affected by these spots, to collect thermal data. During the experiment the maximum thermal response ranged between 40  C and 130  C (see Fig. 9). The highest observed temperature was recorded for PTB7, whereas the thermal response of PC71BM was slightly higher, up to 53  C, than those of imines and their mixtures with PTB7 or PC71BM. The imine showed the lowest thermal response, 43  C and 40  C for PV-BLJ-SC13 and PV-BLJ-SC14, respectively. The addition of PC71BM and PTB7 to the imine increase the thermal response compared to pure imine, giving temperatures: 48  C and 43  C for PV-BLJ-SC13: PC71BM and PV-BLJ-SC14: PC71BM, respectively (Fig. 9a); 52  C and 53  C for PVBLJ-SC13:PTB7 and PV-BLJ-SC14:PTB7, respectively (Fig. 9b). Observed behaviour could be related to distortion original packing

of structure in a layer observed by PV-BLJ-SC13 and PV-BLJ-SC14. The addition of PTB7 and PC71BM could cause partial phase separation increasing the internal resistance since presence of better conductor does not improve the current passage. Regarding the electrical conductivity all samples acted as electrical conductors with displaying linear increment of current upon increasing applied potential. The resistance of 1 cm2 ranged from 20.6 U to 113.8 U. The lowest resistance was registered for PTB7 (Fig. 9c), what aligns very well with above mentioned thermal behavior: when the current flow is high, due to low sample resistance, the sample temperature increase tends to be also high. A general tendency was observed: the resistance of imines (113.8 U and 102.4 U for PV-BLJ-SC13 and PV-BLJ-SC14, respectively) have a tendency to decrease its value when mixed with PTB7 and PC71BM of about 17e24 U, except PV-BLJ-SC14:PC71BM (Fig. 9d). On the other hand, PV-BLJ-SC14:PC71BM mixture showed negligible difference, 0.6 U, compared to device containing only PV-BLJ-SC14 layer. Summary of electrical parameters of imines and their mixture with PTB7 or PC71BM are presented in Table S1.

3.3. Optical absorption spectroscopy The absorption spectra of different materials used in construction of the solar cells are plotted in Figs. 10 and 11. Moreover, the absorption spectra of the polymer:fullerene blends and polymer:imine:fullerene blends were measured. The weight ratios of imine to PTB7 in the active layer were 1:4 and 1:8. The thickness of measured layers is shown in Table S2. Pure PTB7 donor material of 1.6 eV bang gap absorbs light in 1.6e2.5 eV spectral range showing two maxima at 1.7eV and 2.0 eV with absorption coefficient of about 1.1  105 cm-1 PC71BM acceptor material have absorption in wide spectral range starting from band gap energy at about 2.0 eV up to UV, with absorption coefficient of half of PTB7 value. The absorption spectrum of PTB7:PC71BM blend (in 2:3 ratio) shows strong absorption in the whole visible light

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Fig. 8. IR images obtained for devices architecture: ITO/PV-BLJ-SC13/Ag/ITO (left side), ITO/PV-BLJ-SC13:PTB7/Ag/ITO (middle side) and ITO/PV-BLJ-SC13:PC71BM/Ag/ITO (right side) at 2.5 V, 4.0 V, 6.0 V, 8.0 V and 9.5 V.

range with characteristic maxima from the component materials. Absorption range of the PTB7:PC71BM active layer is well matched to the solar radiation spectrum. Spin cast deposited imines films were very porous due to low solubility of PV-BLJ-SC13 and PV-BLJ-SC14 in chlorobenzene. Extinction caused by light scattering on the porous structure was stronger than absorption in imines. Therefore, the absorption spectra of drop cast layers of PV-BLJ-SC13 and PV-BLJ-SC14 were measured. However, the transmission signal registered for the drop cast samples was reduced due to relatively high thickness of films. Intensity of PV-BLJ-SC14 spectrum was reduced by factor 3 due to high thickness of drop cast layer. The absorption of the PV-BLJ-SC13 and PV-BLJ-SC14 imines was very broad and we attributed it to the p- p* transition in the imine and thiophene groups. For the PV-BLJSC13 absorption started below 2 eV and covered the whole range up to 4 eV. In the case of PV-BLJ-SC14 the absorption started at 2.3 eV. High absorption at low photon energy range suggested that PV-BLJ-SC13 imine has lower band gap than PV-BLJ-SC14 imine.

This is confirmed by electrochemical data and the results of DFT calculations presented in section 3.1. The spectral absorption range of both imines PV-BLJ-SC13 and PV-BLJ-SC14 is complementary to that of PTB7 absorption, which can lead to the increase of generated photocurrent in PTB7:imine:PC71BM organic solar cell by widening the absorption range. In the case of imines and PTB7:PC71BM mixtures the absorption spectra show no visible signal related to imines. Even direct comparison of pure imines and mixtures spectra (see Fig. 11) does not reveal influence of imines on absorption spectrum of the mixtures. Probably, it can be explained by low solubility of PV-BLJ-SC13 and PV-BLJ-SC14 in chlorobenzene resulting in low concentration of imines in PTB7:PC71BM blends. 3.4. Luminescence spectroscopy Illumination of the studied materials with high-energy photons (4.2 eV) results in a transfer of electrons to high excited states. Then,

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Fig. 9. The correlation of temperature versus applied potential for constructed devices containing PV-BLJ-SC13 or PV-BLJ-SC14 and their mixtures with PTB7 (a) or PC71BM (b) together with the correlation of current versus applied potential for constructed devices containing PV-BLJ-SC13 or PV-BLJ-SC14 and their mixtures with PTB7 (c) or PC71BM (d).

the electrons relax to lower excited states and emit light from these states. Thus, the photoluminescence (PL) energy is lower than the excitation energy (Stockes shift). The emission energy tells us what energy is available (also for photovoltaic effect) after relaxation. The dynamics of photoluminescence processes was measured with time resolved (TRPL) spectroscopy. Particles absorb light by optically allowed transitions that are usually from ground state to singlet excited states. During relaxation some of the excited

electrons are transferred to long living states that are usually triplet, p states. Electrons from these states have lifetime of the order of nanosecond. This time is long enough for these electrons to diffuse to the acceptor material accelerating the photoluminescence decay, what was observed for example in 2252ThDMB:PCBM mixture [24] and 25Th-cardo polyazomethine:PCBM mixture [43]. The time-resolved PL (TRPL) spectra of PV-BLJ-SC13 and PV-BLJ-

Fig. 10. The optical absorption spectra of: (a) the pure materials, (b) the imines and PTB7:PC71BM mixtures with different compositions.

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Fig. 11. The optical absorption spectra of the layers containing: (a) imine PV-BLJ-SC13 and (b) imine PV-BLJ-SC14.

SC14 samples are plotted in Fig. 12 as intensity contour maps. The contour lines (isolines) in these figures determine the e-fold difference in intensity (they are equally spaced in logarithmic intensity scale). Therefore, the distance along time axis between two contour lines shows an estimation of the luminescence decay time, t. It can be noticed that the recombination dynamics changed slightly with time, which means that decay is not strictly exponential. We may, nevertheless, estimate the average decay time. In the case of PV-BLJ-SC13 and PV-BLJ-SC14 the average t was about 0.7 ns and 0.4 ns, respectively. It can be observed that isolines in Fig. 12 are slightly modulated, which points to the existence of a

few peaks. However, the shape of this modulation does not change with time, which means that all peaks have the same evolution in time. This observation strongly suggests that we observed one peak related to electron transition and its vibrational replicas. TRPL data from Fig. 12 are easier to analyze along single axis: time or energy. Cross-sections of the TRPL data along energy axis give time-averaged photoluminescence spectra. The PL spectra of the investigated compounds were plotted in Fig. 13a. One can observe that PV-BLJ-SC13 and PV-BLJ-SC14 imines had very bright luminescence. Comparing to them, PCBM and PTB7 had more than order of magnitude weaker emission. Fullerenes emitted at about 1.7 eV and PTB7 at about 1.6 eV. The imines had luminescence bands at about 2 eV that consisted of a few peaks. The difference between absorption and emission maxima was about 0.5 eV, due to vibrational shift and exciton formation energy. Energy positions of the peaks were at 1.87 eV, 2.01 eV, and 2.14 eV for PV-BLJ-SC13, and at 1.92 eV, 2.06 eV, and 2.19 eV for PV-BLJ-SC14. They can be interpreted as electron transitions and their vibrational bands [44]. Vibration energy was hn ¼ 0.13e0.14 eV. Zero vibration peaks were at 1.88 and 1.92 eV in PV-BLJ-SC13 and PV-BLJ-SC14, respectively. Cross-sections of the TRPL data along time axis give energyaveraged photoluminescence transients. The plots of PL transients for the investigated imines and their blends with PC71BM are plotted in Fig. 13b. It can be observed that decays of PV-BLJ-SC14 were clearly faster than decays of PV-BLJ-SC13. However, decays of pure imines and their blends with PCBM were nearly parallel, what means that PCBM did not influence electron recombination in imines. In the case of imine:PC71BM blends one can observe that PL intensities were lower than in the pure imines. It is due to escape of carriers to PC71BM. In the case of PV-BLJ-SC13 and PV-BLJ-SC14 the escape takes place when carriers are in highly excited singlet states. After relaxation, the rest of carriers stay in the polymer, so the PL decays are identical as those in the pure materials. 3.5. Photovoltaic properties

Fig. 12. TRPL spectra of PV-BLJ-SC13 and PV-BLJ-SC14 imines.

The results of current-voltage measurements for constructed solar cells are plotted in Fig. 14 and the electrical properties are presented in Table S3. The J-V characteristics show that the highest efficiency has standard PTB7:PC71BM (2:3) cell without the imine addition. The electrical parameters of this standard cells were as follows: a short circuit current JSC of 5 mA/cm2, open circuit voltage VOC of 0.76 V, and a fill factor FF of 49%, giving a power conversion efficiency PCE of 1.87%. The examples of EQE spectra of solar cells with different imines are presented in Fig. S9.

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Fig. 13. (a) Photoluminescence spectra of imines, PTB7 and fullerenes PC61BM and PC71BM. (b) Photoluminescence transients of the investigated imines and their blends with PC71BM.

Fig. 14. J-V characteristics of organic PTB7:PC71BM solar cells with and without imines. (The right figure is magnification of the left one).

For imine PV-BLJ-SC13 cell with higher concentration of imine ((4:1):8) we obtained efficiency of 0.42%, where main decrease relative to the reference sample was in current density and fill factor. Also decrease of open circuit voltage was observed which can suggest energy level mismatch of imine PV-BLJ-SC13 and PTB7:PC71BM materials. Closer research on IeV curves indicates the increase of serial resistance for the cell after PV-BLJ-SC13 imine addition. For lower concentration of PV-BLJ-SC13 imine ((8:1):13) the efficiency drastically decreased to 0.0003% due to high serial resistance. Solar cells with imine PV-BLJ-SC14 had almost the same efficiency 0.02% and 0.06% for respectively high and low concentrations of imine. Low efficiency was caused by low current density and minimal fill factor (S-shape IeV curve). This observation suggests a huge decrease in carriers transport in bulk heterojunction. Such effect can be caused by disorder in PTB7:PC71BM crystal structure caused by PV-BLJ-SC14 imine addition, low carrier mobility in PV-BLJ-SC14 or energy level mismatch between PV-BLJSC14 and PTB7:PC71BM. 4. Conclusions In summary, two new imines with thiazole and thiophene moieties were synthesized by condensation reaction to create symmetrical and unsymmetrical compound with good thermal stability and opto-electrical properties. Our engineering concept to develop a solar cell containing symmetrical and unsymmetrical imines was verified by experimental and theoretical study. The

main conclusions are as follows: 1. It is possible obtain soluble imines with thiophene and thiazole moieties by ecological melt condensation reaction, however their solubility depends on symmetry of investigated imine. Better solubility in chloroform was found for unsymmetrical one (PV-BLJ-SC14). 2. Both imines were thermally stable up to 300e400  C, depending on their symmetry. Better stability was found for symmetrical PV-BLJ-SC13. 3. Energy band gaps for both investigated imines obtained from optical measurements and electrochemical analyses differ in comparison with theoretical values calculated using quantum chemistry methods by ca. 0.35e0.53 eV and 0.47e0.96 eV for Eg.opt and Eg,ec, respectively. The difference between theoretical value of HOMO-LUMO energy band gap and the band gap determined from experiments is a common finding, and may be explained by the fact that the measured electronic parameters are not solely driven by the energy levels of the materials. Clearly, the chemical environment with corresponding interactions and ultimate organization of the molecule at the site of measurement are other important key parameters to consider when experimental data have to be assessed. It is interesting that for both imines 3LYP functional employing the 6-31G(d) basis set turn out to reflect better the experimental data (especially obtained with FT-IR method) when compared with PBE/6-311G(d,p) approach.

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4. The molecules of unsymmetrical imine PV-BLJ-SC14 may undergo dimerization upon electrochemical oxidation. 5. The PV-BLJ-SC14 dimers are electroactive and may be reversible doped both in p- and n-type doping range. 6. The PV-BLJ-SC13 and PV-BLJ-SC14 imines have very bright luminescence what coincides with low charge transfer. Both investigated imines have very broad absorption in visible spectrum of light, but we couldn’t observe imine influence on absorption spectra related to a mixture of imines and PTB7:PC71BM blend, most probably due to insufficient solubility of imines. 7. The addition of PC71BM and PTB7 to the imine increased the thermal response compared to pure imine. Linear increment of current upon increasing applied potential was found for all investigated devices by IR thermal images. The resistance of both imines is higher than those of imine:PTB7 and imine:PC71BM mixtures by about 20%. 8. In our opinion both proposed imines could be interesting materials for organic opto-electronics and photovoltaics. However, in order to create more efficient devices one should provide better solubility of imines in the standard solvents, for example by incorporation of alkyl substituents in the imine molecules. The problem of their insolubility in standard solvents must by overcame at least through alternative architecture with evaporated layer.

Author contributions section Krzysztof Artur Bogdanowicz: Investigation, Beata Jewloszewicz: Investigation, Karolina Dysz: Investigation, Wojciech Przybyl: Investigation, Agnieszka Dylong: Investigation, Wojciech Mech: Investigation, Krzysztof P. Korona: Investigation, Reviewing and Editing, Magdalena Skompska: Investigation, Writing- Reviewing and Editing, Andrzej Kaim: Investigation,
ska: Reviewing and Writing- Reviewing and Editing, Maria Kamin Editing, Agnieszka Iwan: Writing- Original draft preparation, Supervision, Writing- Reviewing and Editing, Conceptualization. Acknowledgments Authors are grateful for financial support from Polish National Centre of Research and Development (TECHMATSTRATEG1/ 347431/14/NCBR/2018). The theoretical results presented in this work were obtained with the computational resources of the Interdisciplinary Center for Mathematical and Computational Modeling at Warsaw University (Grant G15-11). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.electacta.2019.135476. References [1] K.D.G.I. Jayawardena, L.J. Rozanski, C.A. Mills, M.J. Beliatis, N.A. Nismy, S.R.P. Silva, Nanoscale 5 (2013) 8411e8427. [2] S. Cao, B. Li, R. Zhu, H. Pang, Chem. Eng. J. 355 (2019) 602e623. [3] I.Y. Kanal, S.G. Owens, J.S. Bechtel, G.R. Hutchison, J. Phys. Chem. Lett. 4 (2013) 1613e1623. [4] K. Lee, J.Y. Kim, S.H. Park, S.H. Kim, S. Cho, A.J. Heeger, Adv. Mater. 19 (2007) 2445e2449. [5] C. Sekine, Y. Tsubata, T. Yamada, M. Kitano, S. Doi, Sci. Technol. Adv. Mater. 15

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