Investigations of New Phenothiazine-Based Compounds for Dye-Sensitized Solar Cells with Theoretical Insight

New D-π-D-π-A low-molecular-weight compounds, based on a phenothiazine scaffold linked via an acetylene unit with various donor moiety and cyanoacrylic acid anchoring groups, respectively, were successfully synthesized. The prepared phenothiazine dyes were entirely characterized using nuclear magnetic resonance (NMR) spectroscopy and elemental analysis. The compounds were designed to study the relationship between end-capping donor groups’ structure on their optoelectronic and thermal properties as well as the dye-sensitized solar cells’ performance. The effect of π-conjugation enlargement by incorporation of different heterocyclic substituents possessing various electron–donor affinities was systematically experimentally and theoretically examined. Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations were implemented to determine the electronic properties of the novel molecules.


Introduction
Currently, dye-sensitized solar cells (DSSCs) based on metal-free organic dyes have drawn greater and greater attention owing to their advantages, such as a relatively low cost, compliance with environmental requirements, and easy device preparation. Opposite to the complexes with heavy atoms utilized in DSSCs, which show high or even comparable power conversion efficiency (PCE) of 13% [1][2][3][4][5], the use of metal-free sensitizers in DSSCs makes it possible to obtain ever-growing PCE up to 14% [6][7][8][9][10]. The synthesis of organic dyes such as carbazole, thiophene, pyrene, phenothiazine to act as donors (D), and quinoline and pyridine as acceptor (A) units, produce in combination D-A systems which have been evolved for the utilization of DSSCs, organic light-emitting diodes (OLEDs), sensors and bio-imaging [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23]. Phenothiazine (PTZ), as a heterocyclic compound, encloses electron-abundant sulfur and nitrogen atoms, and plays the role of an strong electron-donating (D) moiety in D-A systems. The large π-conjugated structure, high molar absorption coefficients (ε), high thermal and chemical stability, and also the non-planar conformation preventing aggregation of phenothiazine are important requirements making PTZ and its derivatives desirable candidates for photovoltaic applications. The many ways to modify phenothiazine structure by addition of different types of in the air. Such photoanodes with adsorbed dye's molecules were employed to assembly a sandwich-structure solar cells (FTO/TiO 2 +dye/EL-HSE/Pt/FTO) by fixing it with the counter electrode (Pt/FTO). The electrolyte consist of iodide/triiodide redox couple was injected between the electrodes. The photovoltaic performance of fabricated devices was registered under standard 100 mW/cm 2 (AM 1.5 G) irradiation.
Materials 2020, 13, x FOR PEER REVIEW 4 of 17 cells (FTO/TiO2+dye/EL-HSE/Pt/FTO) by fixing it with the counter electrode (Pt/FTO). The electrolyte consist of iodide/triiodide redox couple was injected between the electrodes. The photovoltaic performance of fabricated devices was registered under standard 100 mW/cm 2 (AM 1.5 G) irradiation.
Materials 2020, 13, x FOR PEER REVIEW 4 of 17 cells (FTO/TiO2+dye/EL-HSE/Pt/FTO) by fixing it with the counter electrode (Pt/FTO). The electrolyte consist of iodide/triiodide redox couple was injected between the electrodes. The photovoltaic performance of fabricated devices was registered under standard 100 mW/cm 2 (AM 1.5 G) irradiation.

Synthesis and Characterization
The novel phenothiazine derivatives (2a-2c) of D-π-D-π-A type with various donor end-capped 2,2′-bithienyl, dibenzothiophenyl, and 9,9'-dibutylfluorenyl moieties linked by ethylene with phenothiazine were synthesized by multi-step reactions. First, the 7-bromo-10-octyl-10Hphenothiazine-3-carbaldehyde (I) obtained in multistep reactions according to well-known synthetic procedures was used as a starting material in the synthesis of compound 1a (Scheme 1) [23,31,32]. Compound I was converted to 2-[(trimethylsilyl)ethynyl]-10-octyl-10H-phenothiazin-3carbaldehyde (II) via [Pd]-catalyzed cross-coupling reaction using TMSA (79% yield). Subsequently, prior deprotection of II using tetrabutylammonium fluoride (TBAF) followed by in-situ [Pd]catalyzed reaction with 2-bromodibenzothiophene gave compound 1a a 40% yield. The compounds were fully characterized by the NMR spectroscopic technique (Supporting Information, Figures S1-S12). The purity of prepared molecules was confirmed by mass spectrometry and elemental analysis (Supporting Information, Synthesis and characterization). In the 1 H NMR Materials 2020, 13, 2292 5 of 17 spectra for all compounds 2a-2c, the characteristic signal for a proton from the methylene group appeared as a singlet at 8.00 ppm. Apparently, in the proton NMR spectra of compounds 2a-2c, the signal of a carbaldehyde proton (with a chemical shift at 10 ppm), which is characteristic for aldehyde derivatives 1a-1c, disappeared. Additionally, comparing the proton NMR spectra of aldehydes 1b and 1c with the target compounds 2b and 2c, the signals from the proton at the C4 phenothiazine ring were moved to higher chemical shifts, at about 0.15 ppm. In the 13 C NMR spectra of 2a-2c dyes, the lack of peaks typical for carbons from a carbaldehyde group at about 190 ppm, and the appearance of a signal at about 165 ppm, which is the standard signal for the carbon of carboxyl groups (-COOH), was observed. Further, in the carbon NMR spectra of 2a-2c, peaks for methylene carbon (-CH=C-) at about 100 ppm were seen. The compounds were fully characterized by the NMR spectroscopic technique (Supporting Information, Figures S1-S12). The purity of prepared molecules was confirmed by mass spectrometry and elemental analysis (Supporting Information, Synthesis and characterization). In the 1 H NMR spectra for all compounds 2a-2c, the characteristic signal for a proton from the methylene group appeared as a singlet at 8.00 ppm. Apparently, in the proton NMR spectra of compounds 2a-2c, the signal of a carbaldehyde proton (with a chemical shift at 10 ppm), which is characteristic for aldehyde derivatives 1a-1c, disappeared. Additionally, comparing the proton NMR spectra of aldehydes 1b and 1c with the target compounds 2b and 2c, the signals from the proton at the C4 phenothiazine ring were moved to higher chemical shifts, at about 0.15 ppm. In the 13 C NMR spectra of 2a-2c dyes, the lack of peaks typical for carbons from a carbaldehyde group at about 190 ppm, and the appearance of a signal at about 165 ppm, which is the standard signal for the carbon of carboxyl groups (-COOH), was observed. Further, in the carbon NMR spectra of 2a-2c, peaks for methylene carbon (-CH=C-) at about 100 ppm were seen.
The target phenothiazine derivatives 2a-2c were obtained as crystalline compounds from reaction with melting temperature (Tm), identified by DSC during the first heating scan, in the range of 56-68 C (Experimental section in Supporting Information and Figure S13). The observed broadmelting endotherm proves a low degree of crystallinity of these molecules, probably due to defects, which cause partial disruption of the crystalline order, decreasing the crystallinity [35]. Defects such as crystal imperfections, which are defects in the regular geometrical arrangement of the atoms in a crystalline solid, may result from the crystallization process. On the other hand, impurities can also affect the broad peak obtained during the melting process. In the case of presented compounds, the purity was confirmed by elemental analysis. The differences in calculated and found C, N and H content are in the acceptable range, not exceeding 0.3% (Experimental Section in Supporting Information). However, it should be kept in mind that even a trace amount of impurities can impact the DSC curve trace. When the isotropic liquid was cooled down and heated again, the glass transition (Tg) phenomena was seen, with Tg ranging from 43 to 64 C ( Figure S13). Thus, the synthesized compounds are molecular glasses which can form glassy phases above room temperature and uniform amorphous thin films can be prepared. Considering the DSC thermograms of 2a-2c, it can be concluded that they form stable amorphous molecular materials because on further heating above their Tg no peaks due to crystallization and melting were seen ( Figure S13). It was found that introduction to phenothiazine-derivative dibenzothiophenyl unit (2a) increased both Tm and Tg compared to other substituents, that is, 2,2′-bithienyl (2b) and 9,9′-dibutylfluorenyl (2c). Summarizing, the synthesized phenothiazine derivatives were characterized by a low degree of The target phenothiazine derivatives 2a-2c were obtained as crystalline compounds from reaction with melting temperature (T m ), identified by DSC during the first heating scan, in the range of 56-68 • C (Experimental section in Supporting Information and Figure S13). The observed broad-melting endotherm proves a low degree of crystallinity of these molecules, probably due to defects, which cause partial disruption of the crystalline order, decreasing the crystallinity [35]. Defects such as crystal imperfections, which are defects in the regular geometrical arrangement of the atoms in a crystalline solid, may result from the crystallization process. On the other hand, impurities can also affect the broad peak obtained during the melting process. In the case of presented compounds, the purity was confirmed by elemental analysis. The differences in calculated and found C, N and H content are in the acceptable range, not exceeding 0.3% (Experimental Section in Supporting Information). However, it should be kept in mind that even a trace amount of impurities can impact the DSC curve trace. When the isotropic liquid was cooled down and heated again, the glass transition (T g ) phenomena was seen, with T g ranging from 43 to 64 • C ( Figure S13). Thus, the synthesized compounds are molecular glasses which can form glassy phases above room temperature and uniform amorphous thin films can be prepared. Considering the DSC thermograms of 2a-2c, it can be concluded that they form stable amorphous molecular materials because on further heating above their T g no peaks due to crystallization and melting were seen ( Figure S13). It was found that introduction to phenothiazine-derivative dibenzothiophenyl unit (2a) increased both T m and T g compared to other substituents, that is, 2,2 -bithienyl (2b) and 9,9 -dibutylfluorenyl (2c). Summarizing, the synthesized phenothiazine derivatives were characterized by a low degree of crystallinity, confirmed by broad-melting endotherm with low enthalpy and showed a significant tendency for the creation of amorphous material.

Electrochemical Properties
In the case of compounds tested as dyes in DSSCs, the electrochemical properties are crucial considering the electron injection and dye regeneration process, which take place in the device during the conversion of light on electricity. Two techniques, cyclic voltammetry (CV) and differential pulse voltammetry (DPV), were used for characterizing the electrochemical behaviour of 2a-2c. Results of Materials 2020, 13, 2292 6 of 17 these investigations are presented in Table 1. The highest occupied molecular orbitals and the lowest unoccupied molecular orbitals (LUMO) of compounds were calculated using the electrochemical oxidation and reduction onset potentials, respectively. Compounds 2a-2c were characterized by a non-reversible reduction process. For phenothiazine derivatives, the onset of the reduction peak was registered at the potential of −1.34 V vs. Fc/Fc + for 2b to −1.22 V vs. Fc/Fc + for 2c ( Figure 1). Reduction potentials correspond to the reduction of cyanoacrylic acid [36]. However, taking into account the structure, not only the acid unit but also the PTZ moiety takes part [17]. voltammetry (DPV), were used for characterizing the electrochemical behaviour of 2a-2c. Results of these investigations are presented in Table 1. The highest occupied molecular orbitals and the lowest unoccupied molecular orbitals (LUMO) of compounds were calculated using the electrochemical oxidation and reduction onset potentials, respectively.  Compounds 2a-2c were characterized by a non-reversible reduction process. For phenothiazine derivatives, the onset of the reduction peak was registered at the potential of −1.34 V vs. Fc/Fc + for 2b to −1.22 V vs. Fc/Fc + for 2c ( Figure 1). Reduction potentials correspond to the reduction of cyanoacrylic acid [36]. However, taking into account the structure, not only the acid unit but also the PTZ moiety takes part [17].

Code Eox onset (V) Ered onset (V) HOMO (eV) LUMO (eV) Eg CV (eV) Eg
The oxidation process is important for compounds tested as dyes in DSSCs. The compounds should exhibit similar potential to the potential of the pair I3 − /I − being 0.42 V, enabling the regeneration of dyes by I − in the electrolyte [37]. Compound 2a was characterized by a non-reversible oxidation process. After three scans, no product of electrode surface was found ( Figure S14). Based on the peak-to-peak separation (cathodic/anodic waves) (Table S1) oxidation processes for 2b and 2c were quasi-reversible. Oxidation to potentials higher than first oxidation potentials cause an irreversible process. The oxidation process accords to the oxidation of the PTZ unit [38]. The lowest oxidation potential of 0.40 V was observed for 2a and the highest, 0.47 V, for 2c. These values characterized the electron-donating nature of the PTZ part. Differences in oxidation potentials can be caused by the donating effect of linked units. The highest influence on phenothiazine dyes has bithiophene in 2b, but this effect is notably smaller than in the case of the 3,5-bis(trifluoromethyl)phenyl and p-methoxyphenyl moieties previously reported [17].
On the other hand, for efficient electron injection, the energy state level (LUMO) should be above that of the conductivity band of TiO2 (4.0 eV) [39]. Considering the electrochemical data, it can be  [37]. Compound 2a was characterized by a non-reversible oxidation process. After three scans, no product of electrode surface was found ( Figure S14). Based on the peak-to-peak separation (cathodic/anodic waves) (Table S1) oxidation processes for 2b and 2c were quasi-reversible. Oxidation to potentials higher than first oxidation potentials cause an irreversible process. The oxidation process accords to the oxidation of the PTZ unit [38]. The lowest oxidation potential of 0.40 V was observed for 2a and the highest, 0.47 V, for 2c. These values characterized the electron-donating nature of the PTZ part. Differences in oxidation potentials can be caused by the donating effect of linked units. The highest influence on phenothiazine dyes has bithiophene in 2b, but this effect is notably smaller than in the case of the 3,5-bis(trifluoromethyl)phenyl and p-methoxyphenyl moieties previously reported [17].
On the other hand, for efficient electron injection, the energy state level (LUMO) should be above that of the conductivity band of TiO 2 (4.0 eV) [39]. Considering the electrochemical data, it can be concluded that the synthesized new phenothiazine derivatives show the existence of a driving force for electron injection into the TiO 2 conductivity band and regeneration of dyes in the electrolyte based on an iodine redox pair.
Energy gaps were determined using two methods, that is, electrochemical measurements and UV-Vis electronic spectra registered in dichloromethane. It was found that the presence of 9,9 -dibutylfluorene substituent in 2c reduced the value of E g CV to 1.69 eV in comparison with molecules containing dibenzothiophene (2a) and 2,2 -bithiophene (2b) units.

Structure Optimization and Frontier Molecular Orbitals
The structures of the target compounds were optimized by using the method based on density functional theory (DFT) and also with the time-dependent extension (TD-DFT). DFT calculations were performed in the program Gaussian 09, with the B3LYP [40] exchange-correlation functional with the 6-31G** basis set, whereas in the case of TD-DFT, the CAM-B3LYP [41] and wB97XD [42] exchange-correlation functionals with the 6-31G** basis set were applied [43]. All calculations were performed in a chloroform solution in the polarizable continuous model (PCM) [44]. The optimized structures of molecules 2a-2c, with the contours and energies of HOMO-1, HOMO, LUMO and LUMO+1, are presented in Table 2. concluded that the synthesized new phenothiazine derivatives show the existence of a driving force for electron injection into the TiO2 conductivity band and regeneration of dyes in the electrolyte based on an iodine redox pair. Energy gaps were determined using two methods, that is, electrochemical measurements and UV-Vis electronic spectra registered in dichloromethane. It was found that the presence of 9,9′dibutylfluorene substituent in 2c reduced the value of Eg CV to 1.69 eV in comparison with molecules containing dibenzothiophene (2a) and 2,2′-bithiophene (2b) units.

Structure Optimization and Frontier Molecular Orbitals
The structures of the target compounds were optimized by using the method based on density functional theory (DFT) and also with the time-dependent extension (TD-DFT). DFT calculations were performed in the program Gaussian 09, with the B3LYP [40] exchange-correlation functional with the 6-31G** basis set, whereas in the case of TD-DFT, the CAM-B3LYP [41] and wB97XD [42] exchange-correlation functionals with the 6-31G** basis set were applied [43]. All calculations were performed in a chloroform solution in the polarizable continuous model (PCM) [44]. The optimized structures of molecules 2a-2c, with the contours and energies of HOMO-1, HOMO, LUMO and LUMO+1, are presented in Table 2. The highest value of energy gap ΔE = 2.79 eV was achieved by 2a, followed by 2c (ΔE = 2.76 eV) and 2b (ΔE = 2.63 eV). The percentage contribution of particular parts of the molecules 2a-2c in the creations of selected orbitals is presented in Figure 2. concluded that the synthesized new phenothiazine derivatives show the existence of a driving force for electron injection into the TiO2 conductivity band and regeneration of dyes in the electrolyte based on an iodine redox pair. Energy gaps were determined using two methods, that is, electrochemical measurements and UV-Vis electronic spectra registered in dichloromethane. It was found that the presence of 9,9′dibutylfluorene substituent in 2c reduced the value of Eg CV to 1.69 eV in comparison with molecules containing dibenzothiophene (2a) and 2,2′-bithiophene (2b) units.

Structure Optimization and Frontier Molecular Orbitals
The structures of the target compounds were optimized by using the method based on density functional theory (DFT) and also with the time-dependent extension (TD-DFT). DFT calculations were performed in the program Gaussian 09, with the B3LYP [40] exchange-correlation functional with the 6-31G** basis set, whereas in the case of TD-DFT, the CAM-B3LYP [41] and wB97XD [42] exchange-correlation functionals with the 6-31G** basis set were applied [43]. All calculations were performed in a chloroform solution in the polarizable continuous model (PCM) [44]. The optimized structures of molecules 2a-2c, with the contours and energies of HOMO-1, HOMO, LUMO and LUMO+1, are presented in Table 2. The highest value of energy gap ΔE = 2.79 eV was achieved by 2a, followed by 2c (ΔE = 2.76 eV) and 2b (ΔE = 2.63 eV). The percentage contribution of particular parts of the molecules 2a-2c in the creations of selected orbitals is presented in Figure 2. concluded that the synthesized new phenothiazine derivatives show the existence of a driving force for electron injection into the TiO2 conductivity band and regeneration of dyes in the electrolyte based on an iodine redox pair. Energy gaps were determined using two methods, that is, electrochemical measurements and UV-Vis electronic spectra registered in dichloromethane. It was found that the presence of 9,9′dibutylfluorene substituent in 2c reduced the value of Eg CV to 1.69 eV in comparison with molecules containing dibenzothiophene (2a) and 2,2′-bithiophene (2b) units.

Structure Optimization and Frontier Molecular Orbitals
The structures of the target compounds were optimized by using the method based on density functional theory (DFT) and also with the time-dependent extension (TD-DFT). DFT calculations were performed in the program Gaussian 09, with the B3LYP [40] exchange-correlation functional with the 6-31G** basis set, whereas in the case of TD-DFT, the CAM-B3LYP [41] and wB97XD [42] exchange-correlation functionals with the 6-31G** basis set were applied [43]. All calculations were performed in a chloroform solution in the polarizable continuous model (PCM) [44]. The optimized structures of molecules 2a-2c, with the contours and energies of HOMO-1, HOMO, LUMO and LUMO+1, are presented in Table 2. The highest value of energy gap ΔE = 2.79 eV was achieved by 2a, followed by 2c (ΔE = 2.76 eV) and 2b (ΔE = 2.63 eV). The percentage contribution of particular parts of the molecules 2a-2c in the creations of selected orbitals is presented in Figure 2. concluded that the synthesized new phenothiazine derivatives show the existence of a driving force for electron injection into the TiO2 conductivity band and regeneration of dyes in the electrolyte based on an iodine redox pair. Energy gaps were determined using two methods, that is, electrochemical measurements and UV-Vis electronic spectra registered in dichloromethane. It was found that the presence of 9,9′dibutylfluorene substituent in 2c reduced the value of Eg CV to 1.69 eV in comparison with molecules containing dibenzothiophene (2a) and 2,2′-bithiophene (2b) units.

Structure Optimization and Frontier Molecular Orbitals
The structures of the target compounds were optimized by using the method based on density functional theory (DFT) and also with the time-dependent extension (TD-DFT). DFT calculations were performed in the program Gaussian 09, with the B3LYP [40] exchange-correlation functional with the 6-31G** basis set, whereas in the case of TD-DFT, the CAM-B3LYP [41] and wB97XD [42] exchange-correlation functionals with the 6-31G** basis set were applied [43]. All calculations were performed in a chloroform solution in the polarizable continuous model (PCM) [44]. The optimized structures of molecules 2a-2c, with the contours and energies of HOMO-1, HOMO, LUMO and LUMO+1, are presented in Table 2. The highest value of energy gap ΔE = 2.79 eV was achieved by 2a, followed by 2c (ΔE = 2.76 eV) and 2b (ΔE = 2.63 eV). The percentage contribution of particular parts of the molecules 2a-2c in the creations of selected orbitals is presented in Figure 2. concluded that the synthesized new phenothiazine derivatives show the existence of a driving force for electron injection into the TiO2 conductivity band and regeneration of dyes in the electrolyte based on an iodine redox pair. Energy gaps were determined using two methods, that is, electrochemical measurements and UV-Vis electronic spectra registered in dichloromethane. It was found that the presence of 9,9′dibutylfluorene substituent in 2c reduced the value of Eg CV to 1.69 eV in comparison with molecules containing dibenzothiophene (2a) and 2,2′-bithiophene (2b) units.

Structure Optimization and Frontier Molecular Orbitals
The structures of the target compounds were optimized by using the method based on density functional theory (DFT) and also with the time-dependent extension (TD-DFT). DFT calculations were performed in the program Gaussian 09, with the B3LYP [40] exchange-correlation functional with the 6-31G** basis set, whereas in the case of TD-DFT, the CAM-B3LYP [41] and wB97XD [42] exchange-correlation functionals with the 6-31G** basis set were applied [43]. All calculations were performed in a chloroform solution in the polarizable continuous model (PCM) [44]. The optimized structures of molecules 2a-2c, with the contours and energies of HOMO-1, HOMO, LUMO and LUMO+1, are presented in Table 2. The highest value of energy gap ΔE = 2.79 eV was achieved by 2a, followed by 2c (ΔE = 2.76 eV) and 2b (ΔE = 2.63 eV). The percentage contribution of particular parts of the molecules 2a-2c in the creations of selected orbitals is presented in Figure 2. concluded that the synthesized new phenothiazine derivatives show the existence of a driving force for electron injection into the TiO2 conductivity band and regeneration of dyes in the electrolyte based on an iodine redox pair. Energy gaps were determined using two methods, that is, electrochemical measurements and UV-Vis electronic spectra registered in dichloromethane. It was found that the presence of 9,9′dibutylfluorene substituent in 2c reduced the value of Eg CV to 1.69 eV in comparison with molecules containing dibenzothiophene (2a) and 2,2′-bithiophene (2b) units.

Structure Optimization and Frontier Molecular Orbitals
The structures of the target compounds were optimized by using the method based on density functional theory (DFT) and also with the time-dependent extension (TD-DFT). DFT calculations were performed in the program Gaussian 09, with the B3LYP [40] exchange-correlation functional with the 6-31G** basis set, whereas in the case of TD-DFT, the CAM-B3LYP [41] and wB97XD [42] exchange-correlation functionals with the 6-31G** basis set were applied [43]. All calculations were performed in a chloroform solution in the polarizable continuous model (PCM) [44]. The optimized structures of molecules 2a-2c, with the contours and energies of HOMO-1, HOMO, LUMO and LUMO+1, are presented in Table 2.  2.63 eV). The percentage contribution of particular parts of the molecules 2a-2c in the creations of selected orbitals is presented in Figure 2. concluded that the synthesized new phenothiazine derivatives show the existence of a driving force for electron injection into the TiO2 conductivity band and regeneration of dyes in the electrolyte based on an iodine redox pair. Energy gaps were determined using two methods, that is, electrochemical measurements and UV-Vis electronic spectra registered in dichloromethane. It was found that the presence of 9,9′dibutylfluorene substituent in 2c reduced the value of Eg CV to 1.69 eV in comparison with molecules containing dibenzothiophene (2a) and 2,2′-bithiophene (2b) units.

Structure Optimization and Frontier Molecular Orbitals
The structures of the target compounds were optimized by using the method based on density functional theory (DFT) and also with the time-dependent extension (TD-DFT). DFT calculations were performed in the program Gaussian 09, with the B3LYP [40] exchange-correlation functional with the 6-31G** basis set, whereas in the case of TD-DFT, the CAM-B3LYP [41] and wB97XD [42] exchange-correlation functionals with the 6-31G** basis set were applied [43]. All calculations were performed in a chloroform solution in the polarizable continuous model (PCM) [44]. The optimized structures of molecules 2a-2c, with the contours and energies of HOMO-1, HOMO, LUMO and LUMO+1, are presented in Table 2.  2.63 eV). The percentage contribution of particular parts of the molecules 2a-2c in the creations of selected orbitals is presented in Figure 2. concluded that the synthesized new phenothiazine derivatives show the existence of a driving force for electron injection into the TiO2 conductivity band and regeneration of dyes in the electrolyte based on an iodine redox pair. Energy gaps were determined using two methods, that is, electrochemical measurements and UV-Vis electronic spectra registered in dichloromethane. It was found that the presence of 9,9′dibutylfluorene substituent in 2c reduced the value of Eg CV to 1.69 eV in comparison with molecules containing dibenzothiophene (2a) and 2,2′-bithiophene (2b) units.

Structure Optimization and Frontier Molecular Orbitals
The structures of the target compounds were optimized by using the method based on density functional theory (DFT) and also with the time-dependent extension (TD-DFT). DFT calculations were performed in the program Gaussian 09, with the B3LYP [40] exchange-correlation functional with the 6-31G** basis set, whereas in the case of TD-DFT, the CAM-B3LYP [41] and wB97XD [42] exchange-correlation functionals with the 6-31G** basis set were applied [43]. All calculations were performed in a chloroform solution in the polarizable continuous model (PCM) [44]. The optimized structures of molecules 2a-2c, with the contours and energies of HOMO-1, HOMO, LUMO and LUMO+1, are presented in Table 2.  2.63 eV). The percentage contribution of particular parts of the molecules 2a-2c in the creations of selected orbitals is presented in Figure 2. concluded that the synthesized new phenothiazine derivatives show the existence of a driving force for electron injection into the TiO2 conductivity band and regeneration of dyes in the electrolyte based on an iodine redox pair. Energy gaps were determined using two methods, that is, electrochemical measurements and UV-Vis electronic spectra registered in dichloromethane. It was found that the presence of 9,9′dibutylfluorene substituent in 2c reduced the value of Eg CV to 1.69 eV in comparison with molecules containing dibenzothiophene (2a) and 2,2′-bithiophene (2b) units.

Structure Optimization and Frontier Molecular Orbitals
The structures of the target compounds were optimized by using the method based on density functional theory (DFT) and also with the time-dependent extension (TD-DFT). DFT calculations were performed in the program Gaussian 09, with the B3LYP [40] exchange-correlation functional with the 6-31G** basis set, whereas in the case of TD-DFT, the CAM-B3LYP [41] and wB97XD [42] exchange-correlation functionals with the 6-31G** basis set were applied [43]. All calculations were performed in a chloroform solution in the polarizable continuous model (PCM) [44]. The optimized structures of molecules 2a-2c, with the contours and energies of HOMO-1, HOMO, LUMO and LUMO+1, are presented in Table 2. The highest value of energy gap ΔE = 2.79 eV was achieved by 2a, followed by 2c (ΔE = 2.76 eV) and 2b (ΔE = 2.63 eV). The percentage contribution of particular parts of the molecules 2a-2c in the creations of selected orbitals is presented in Figure 2. concluded that the synthesized new phenothiazine derivatives show the existence of a driving force for electron injection into the TiO2 conductivity band and regeneration of dyes in the electrolyte based on an iodine redox pair. Energy gaps were determined using two methods, that is, electrochemical measurements and UV-Vis electronic spectra registered in dichloromethane. It was found that the presence of 9,9′dibutylfluorene substituent in 2c reduced the value of Eg CV to 1.69 eV in comparison with molecules containing dibenzothiophene (2a) and 2,2′-bithiophene (2b) units.

Structure Optimization and Frontier Molecular Orbitals
The structures of the target compounds were optimized by using the method based on density functional theory (DFT) and also with the time-dependent extension (TD-DFT). DFT calculations were performed in the program Gaussian 09, with the B3LYP [40] exchange-correlation functional with the 6-31G** basis set, whereas in the case of TD-DFT, the CAM-B3LYP [41] and wB97XD [42] exchange-correlation functionals with the 6-31G** basis set were applied [43]. All calculations were performed in a chloroform solution in the polarizable continuous model (PCM) [44]. The optimized structures of molecules 2a-2c, with the contours and energies of HOMO-1, HOMO, LUMO and LUMO+1, are presented in Table 2. The highest value of energy gap ΔE = 2.79 eV was achieved by 2a, followed by 2c (ΔE = 2.76 eV) and 2b (ΔE = 2.63 eV). The percentage contribution of particular parts of the molecules 2a-2c in the creations of selected orbitals is presented in Figure 2. concluded that the synthesized new phenothiazine derivatives show the existence of a driving force for electron injection into the TiO2 conductivity band and regeneration of dyes in the electrolyte based on an iodine redox pair. Energy gaps were determined using two methods, that is, electrochemical measurements and UV-Vis electronic spectra registered in dichloromethane. It was found that the presence of 9,9′dibutylfluorene substituent in 2c reduced the value of Eg CV to 1.69 eV in comparison with molecules containing dibenzothiophene (2a) and 2,2′-bithiophene (2b) units.

Structure Optimization and Frontier Molecular Orbitals
The structures of the target compounds were optimized by using the method based on density functional theory (DFT) and also with the time-dependent extension (TD-DFT). DFT calculations were performed in the program Gaussian 09, with the B3LYP [40] exchange-correlation functional with the 6-31G** basis set, whereas in the case of TD-DFT, the CAM-B3LYP [41] and wB97XD [42] exchange-correlation functionals with the 6-31G** basis set were applied [43]. All calculations were performed in a chloroform solution in the polarizable continuous model (PCM) [44]. The optimized structures of molecules 2a-2c, with the contours and energies of HOMO-1, HOMO, LUMO and LUMO+1, are presented in Table 2. The highest value of energy gap ΔE = 2.79 eV was achieved by 2a, followed by 2c (ΔE = 2.76 eV) and 2b (ΔE = 2.63 eV). The percentage contribution of particular parts of the molecules 2a-2c in the creations of selected orbitals is presented in Figure 2. concluded that the synthesized new phenothiazine derivatives show the existence of a driving force for electron injection into the TiO2 conductivity band and regeneration of dyes in the electrolyte based on an iodine redox pair. Energy gaps were determined using two methods, that is, electrochemical measurements and UV-Vis electronic spectra registered in dichloromethane. It was found that the presence of 9,9′dibutylfluorene substituent in 2c reduced the value of Eg CV to 1.69 eV in comparison with molecules containing dibenzothiophene (2a) and 2,2′-bithiophene (2b) units.

Structure Optimization and Frontier Molecular Orbitals
The structures of the target compounds were optimized by using the method based on density functional theory (DFT) and also with the time-dependent extension (TD-DFT). DFT calculations were performed in the program Gaussian 09, with the B3LYP [40] exchange-correlation functional with the 6-31G** basis set, whereas in the case of TD-DFT, the CAM-B3LYP [41] and wB97XD [42] exchange-correlation functionals with the 6-31G** basis set were applied [43]. All calculations were performed in a chloroform solution in the polarizable continuous model (PCM) [44]. The optimized structures of molecules 2a-2c, with the contours and energies of HOMO-1, HOMO, LUMO and LUMO+1, are presented in Table 2.  2.63 eV). The percentage contribution of particular parts of the molecules 2a-2c in the creations of selected orbitals is presented in Figure 2. Due to the various electron-donating characters of the substituents, i.e., 2,2′-bithienyl, dibenzothiophenyl, and 9,9′-dibutylfluorenyl, the contributions to the creation of HOMO differ from each other; the highest (42%) was observed for 2,2′-bithienyl (2b) and the lowest (19%) for dibenzothiophenyl (2a) group. The lead structure, 10-octyl-10H-phenothiazine, has a dominant contribution to the creation of the highest molecular orbital (44-68%), whereas the difference of acetylene linker and anchoring group (-CHC(CN)COOH) between the studied compounds is not significant. Moreover, a deep analysis of the optimized structures of 2a-2c revealed that substituents are coplanar in relation to the wing of the butterfly structure of phenothiazine.
Considering the fact that the prepared phenothiazine derivatives will be tested as a sensitizer in DSSCs, the theoretical calculations of systems-type dye/TiO2 (based on chemisorption model) were realized at the B3LYP/6-31G(d,p) level of theory, with the LANL2DZ basis set for Ti atoms with implemented titanium clusters (TiO2)9 [17]. The optimized structure of systems 2a-2c/(TiO2)9 are presented in Table 3. The same trend of the localization of the highest occupied molecular orbital for studied systems 2a-2c/(TiO2)9, as for the dyes 2a-2c, was followed. LUMOs are located only on the (TiO2)9 cluster (Figure 3). Calculated energy gaps for 2a/(TiO2)9 (1.95 eV) are the highest, whereas the lowest was achieved for 2b/(TiO2)9 (1.80 eV). The optimized geometries did not demonstrate any noteworthy differences between the optimized dyes and their corresponding dye-TiO2 systems.  Due to the various electron-donating characters of the substituents, i.e., 2,2 -bithienyl, dibenzothiophenyl, and 9,9 -dibutylfluorenyl, the contributions to the creation of HOMO differ from each other; the highest (42%) was observed for 2,2 -bithienyl (2b) and the lowest (19%) for dibenzothiophenyl (2a) group. The lead structure, 10-octyl-10H-phenothiazine, has a dominant contribution to the creation of the highest molecular orbital (44-68%), whereas the difference of acetylene linker and anchoring group (-CHC(CN)COOH) between the studied compounds is not significant. Moreover, a deep analysis of the optimized structures of 2a-2c revealed that substituents are coplanar in relation to the wing of the butterfly structure of phenothiazine.
Considering the fact that the prepared phenothiazine derivatives will be tested as a sensitizer in DSSCs, the theoretical calculations of systems-type dye/TiO 2 (based on chemisorption model) were realized at the B3LYP/6-31G(d,p) level of theory, with the LANL2DZ basis set for Ti atoms with implemented titanium clusters (TiO 2 ) 9 [17]. The optimized structure of systems 2a-2c/(TiO 2 ) 9 are presented in Table 3. The same trend of the localization of the highest occupied molecular orbital for studied systems 2a-2c/(TiO 2 ) 9 , as for the dyes 2a-2c, was followed. LUMOs are located only on the (TiO 2 ) 9 cluster (Figure 3). Calculated energy gaps for 2a/(TiO 2 ) 9 (1.95 eV) are the highest, whereas the lowest was achieved for 2b/(TiO 2 ) 9 (1.80 eV). The optimized geometries did not demonstrate any noteworthy differences between the optimized dyes and their corresponding dye-TiO 2 systems.

Experimental and Theoretical Optical Properties
The UV-Vis spectra of precursors 1a-1c and target dyes 2a-2c in chloroform solution are presented in Figure 4, and the optical data are summarized in Table 4. The precursors 1a-1c display several absorption peaks in the UV region, ascribed to π-π* transition and the other peaks at 395 nm caused by ICT from the donor to acceptor. The dyes 2a-2c show absorption peaks of π-π* transition at similar wavelengths as precursors 1a-1c, whereas the ICT bands are red-shifted around 50 nm.  realized at the B3LYP/6-31G(d,p) level of theory, with the LANL2DZ basis set for Ti atoms with implemented titanium clusters (TiO2)9 [17]. The optimized structure of systems 2a-2c/(TiO2)9 are presented in Table 3. The same trend of the localization of the highest occupied molecular orbital for studied systems 2a-2c/(TiO2)9, as for the dyes 2a-2c, was followed. LUMOs are located only on the (TiO2)9 cluster (Figure 3). Calculated energy gaps for 2a/(TiO2)9 (1.95 eV) are the highest, whereas the lowest was achieved for 2b/(TiO2)9 (1.80 eV). The optimized geometries did not demonstrate any noteworthy differences between the optimized dyes and their corresponding dye-TiO2 systems. realized at the B3LYP/6-31G(d,p) level of theory, with the LANL2DZ basis set for Ti atoms with implemented titanium clusters (TiO2)9 [17]. The optimized structure of systems 2a-2c/(TiO2)9 are presented in Table 3. The same trend of the localization of the highest occupied molecular orbital for studied systems 2a-2c/(TiO2)9, as for the dyes 2a-2c, was followed. LUMOs are located only on the (TiO2)9 cluster (Figure 3). Calculated energy gaps for 2a/(TiO2)9 (1.95 eV) are the highest, whereas the lowest was achieved for 2b/(TiO2)9 (1.80 eV). The optimized geometries did not demonstrate any noteworthy differences between the optimized dyes and their corresponding dye-TiO2 systems.

Experimental and Theoretical Optical Properties
The UV-Vis spectra of precursors 1a-1c and target dyes 2a-2c in chloroform solution are presented in Figure 4, and the optical data are summarized in Table 4. The precursors 1a-1c display several absorption peaks in the UV region, ascribed to π-π* transition and the other peaks at 395 nm caused by ICT from the donor to acceptor. The dyes 2a-2c show absorption peaks of π-π* transition at similar wavelengths as precursors 1a-1c, whereas the ICT bands are red-shifted around 50 nm. With regards to the donor terminal unit, the ICT transition peak of 2a-2c is both hyper-and bathochromic shifted with an increase of donor ability of the substituents in the sequence 1c→1a→1b.

Experimental and Theoretical Optical Properties
The UV-Vis spectra of precursors 1a-1c and target dyes 2a-2c in chloroform solution are presented in Figure 4, and the optical data are summarized in Table 4. The precursors 1a-1c display several absorption peaks in the UV region, ascribed to π-π* transition and the other peaks at 395 nm caused by ICT from the donor to acceptor. The dyes 2a-2c show absorption peaks of π-π* transition at similar wavelengths as precursors 1a-1c, whereas the ICT bands are red-shifted around 50 nm. With regards to the donor terminal unit, the ICT transition peak of 2a-2c is both hyper-and bathochromic shifted with an increase of donor ability of the substituents in the sequence 1c→1a→1b.

Experimental and Theoretical Optical Properties
The UV-Vis spectra of precursors 1a-1c and target dyes 2a-2c in chloroform solution are presented in Figure 4, and the optical data are summarized in Table 4. The precursors 1a-1c display several absorption peaks in the UV region, ascribed to π-π* transition and the other peaks at 395 nm caused by ICT from the donor to acceptor. The dyes 2a-2c show absorption peaks of π-π* transition at similar wavelengths as precursors 1a-1c, whereas the ICT bands are red-shifted around 50 nm. With regards to the donor terminal unit, the ICT transition peak of 2a-2c is both hyper-and bathochromic shifted with an increase of donor ability of the substituents in the sequence 1c→1a→1b.

Experimental and Theoretical Optical Properties
The UV-Vis spectra of precursors 1a-1c and target dyes 2a-2c in chloroform solution are presented in Figure 4, and the optical data are summarized in Table 4. The precursors 1a-1c display several absorption peaks in the UV region, ascribed to π-π* transition and the other peaks at 395 nm caused by ICT from the donor to acceptor. The dyes 2a-2c show absorption peaks of π-π* transition at similar wavelengths as precursors 1a-1c, whereas the ICT bands are red-shifted around 50 nm. With regards to the donor terminal unit, the ICT transition peak of 2a-2c is both hyper-and bathochromic shifted with an increase of donor ability of the substituents in the sequence 1c→1a→1b.

Experimental and Theoretical Optical Properties
The UV-Vis spectra of precursors 1a-1c and target dyes 2a-2c in chloroform solution are presented in Figure 4, and the optical data are summarized in Table 4. The precursors 1a-1c display several absorption peaks in the UV region, ascribed to π-π* transition and the other peaks at 395 nm caused by ICT from the donor to acceptor. The dyes 2a-2c show absorption peaks of π-π* transition at similar wavelengths as precursors 1a-1c, whereas the ICT bands are red-shifted around 50 nm. With regards to the donor terminal unit, the ICT transition peak of 2a-2c is both hyper-and bathochromic shifted with an increase of donor ability of the substituents in the sequence 1c→1a→1b. In general, the replacement of fluorene with thiophene or dibenzothiophene as terminal substituents in the presented compounds facilitates effective ICT and extends the region of light absorption.  The theoretical investigations of prediction of the absorption spectra of 2a-2c using CAM-B3LYP and wB97XD have been utilized, and the calculated UV-Vis spectra are presented in Figure 5. The computed and the experimental absorption maxima are in reasonable agreement, differing by about 60 nm. It can be seen in Figure 5 that the stimulated absorption spectra using both methods CAM-B3LYP and wB97XD follow the same trend, being red-shifted in the direction 2a→2b→2c, differing from the experimental data although the differences in the calculated absorption maxima are precisely small. TD-DFT calculations outcomes appear at least in two excitation wavelengths in the range of 250 to 450 nm with high oscillator strength (f) from 0.76 to 1.51. In particular, compound 2b displays the strongest oscillator strength (f = 1.51) among all the investigated dyes, as well as the  The theoretical investigations of prediction of the absorption spectra of 2a-2c using CAM-B3LYP and wB97XD have been utilized, and the calculated UV-Vis spectra are presented in Figure 5. The computed and the experimental absorption maxima are in reasonable agreement, differing by about 60 nm.  The theoretical investigations of prediction of the absorption spectra of 2a-2c using CAM-B3LYP and wB97XD have been utilized, and the calculated UV-Vis spectra are presented in Figure 5. The computed and the experimental absorption maxima are in reasonable agreement, differing by about 60 nm. It can be seen in Figure 5 that the stimulated absorption spectra using both methods CAM-B3LYP and wB97XD follow the same trend, being red-shifted in the direction 2a→2b→2c, differing from the experimental data although the differences in the calculated absorption maxima are precisely small. TD-DFT calculations outcomes appear at least in two excitation wavelengths in the range of 250 to 450 nm with high oscillator strength (f) from 0.76 to 1.51. In particular, compound 2b displays the strongest oscillator strength (f = 1.51) among all the investigated dyes, as well as the It can be seen in Figure 5 that the stimulated absorption spectra using both methods CAM-B3LYP and wB97XD follow the same trend, being red-shifted in the direction 2a→2b→2c, differing from the experimental data although the differences in the calculated absorption maxima are precisely small. TD-DFT calculations outcomes appear at least in two excitation wavelengths in the range of 250 to 450 nm with high oscillator strength (f) from 0.76 to 1.51. In particular, compound 2b displays the strongest oscillator strength (f = 1.51) among all the investigated dyes, as well as the smallest band gap of 2.63 eV. In addition, there is a slight difference in the value of the computed band gap among the studied dyes 2a-2c and the optical band gap (Tables 2 and 4), about 0.4 eV. For all dyes 2a-2c, the theoretical calculations demonstrate that the reasonably strong excitation at 356-384 nm is mainly dominated by a HOMO-LUMO transition, and in some part described by a HOMO-1-LUMO transition (Table 5).
Before focusing on the photovoltaic (PV) performance of prepared cells containing phenothiazine derivatives 2a-2c as sensitizers, it is important to discuss the light-harvesting efficiency (LHE) of 2a-2c and electronic absorption properties with surface morphology of TiO 2 with anchored dyes. LHE, being an important parameter for the high efficiency of a DSSC, was calculated from the oscillator strength of the dye molecule. The theoretical UV-Vis absorption spectra for 2a-2c obtained using CAM-B3LYP and wB97XD ( Figure 5) allowed calculating of LHE. The obtained data are presented in Table 5.
As can be seen from Table 5, higher oscillator strength may enhance the LHE. The synthesized phenothiazine derivatives demonstrate high LHE, which is required for an efficient photocurrent response. The differences in the calculated LHE values of the presented compounds are insignificant.
Considering the registered UV-Vis spectra of TiO 2 with phenothiazine derivatives it can be noticed that all such systems exhibited higher light absorption intensity than semiconductor layer with N719 ( Figure 6). Thus, it can be expected that devices based on 2a-2c may provide better PV efficiency than a cell with N719.
The surface morphology of photoanode, which impacts on cell efficiency, was investigated using atomic force microscopy (AFM). The AFM micrographs of the prepared TiO 2 with adsorbed 2a-2c are presented in Figure 7. Materials 2020, 13, x FOR PEER REVIEW 12 of 17 The surface morphology of photoanode, which impacts on cell efficiency, was investigated using atomic force microscopy (AFM). The AFM micrographs of the prepared TiO2 with adsorbed 2a-2c are presented in Figure 7.  The surface morphology of photoanode, which impacts on cell efficiency, was investigated using atomic force microscopy (AFM). The AFM micrographs of the prepared TiO2 with adsorbed 2a-2c are presented in Figure 7.  The quality of photoanodes was determined by measuring root-mean-square roughness (RMS). All photoanodes with phenothiazine derivatives were quite planar, as indicated by the low RMS values, which were 18, 17 and 18 nm for 2a, 2b and 2c, respectively. A significant smoothing of the photoanode surface compared to TiO 2 with adsorbed N719 (RMS = 52 nm) or TiO 2 without anchored dye (RMS = 86 nm) was observed. This may indicate that the TiO 2 pores were well filled by the compounds 2a-2c comparing to N719. The lower roughness of photoelectrodes could impact on both the stability of the device and the improvement of PV performance.
In the next step of investigations, the complete solar cells were fabricated. At the beginning, the incident photon-to-current efficiency (IPCE) of DSSCs was registered. Figure 8 shows the IPCE spectra of constructed DSSCs.
Materials 2020, 13, x FOR PEER REVIEW 13 of 17 The quality of photoanodes was determined by measuring root-mean-square roughness (RMS). All photoanodes with phenothiazine derivatives were quite planar, as indicated by the low RMS values, which were 18, 17 and 18 nm for 2a, 2b and 2c, respectively. A significant smoothing of the photoanode surface compared to TiO2 with adsorbed N719 (RMS = 52 nm) or TiO2 without anchored dye (RMS = 86 nm) was observed. This may indicate that the TiO2 pores were well filled by the compounds 2a-2c comparing to N719. The lower roughness of photoelectrodes could impact on both the stability of the device and the improvement of PV performance.
In the next step of investigations, the complete solar cells were fabricated. At the beginning, the incident photon-to-current efficiency (IPCE) of DSSCs was registered. Figure 8 shows the IPCE spectra of constructed DSSCs.
(a) (b) (c) (d) As can be seen from Figure 8 the highest photocurrent response about 60% exhibited the device with phenothiazine containing dibenzothiophenyl unit 2a compare to the cell with 2b (18%) and 2c (27%). Furthermore, all DSSCs based on phenothiazine derivatives showed higher IPCE values than a reference solar cell containing N719 (15%). In the case of a device with compound 2b, the hypochromic shift of the maximum IPCE curve was observed in relation to the other molecules ( Figure S15).
Finally, the photovoltaic parameters such as open-circuit voltage (VOC), short circuit photocurrent density (JSC), fill factor (FF) and the power conversion efficiency (PCE) were calculated, based on registered the current density-voltage (J-V) characteristics of prepared solar cells (Figure 9). The PV parameters are collected in Table 6. As can be seen from Figure 8 the highest photocurrent response about 60% exhibited the device with phenothiazine containing dibenzothiophenyl unit 2a compare to the cell with 2b (18%) and 2c (27%). Furthermore, all DSSCs based on phenothiazine derivatives showed higher IPCE values than a reference solar cell containing N719 (15%). In the case of a device with compound 2b, the hypochromic shift of the maximum IPCE curve was observed in relation to the other molecules ( Figure S15).
Finally, the photovoltaic parameters such as open-circuit voltage (V OC ), short circuit photocurrent density (J SC ), fill factor (FF) and the power conversion efficiency (PCE) were calculated, based on registered the current density-voltage (J-V) characteristics of prepared solar cells (Figure 9). The PV parameters are collected in Table 6. Materials 2020, 13, x FOR PEER REVIEW 14 of 17 Figure 9. Photocurrent density-voltage (J-V) curves of the prepared devices with dyes 2a-2c. As can be seen in Table 6 and Figure 9, all of the tested devices containing phenothiazine derivatives demonstrated better PV performance than the reference solar cell with commercial N719. The DSSC with a compound bearing a dibenzothiophenyl donor moiety 2a exhibited the best conversion efficiency of 6.22% (about 57% of the PV performance of the cell sensitized with N719). The best conversion efficiency of the cell sensitized with 2a results from the improved electron injection and less dark current, as indicated by the fact it had the highest JSC (17.96 mA/cm 2 ) and a high VOC (700 mV). The device fabricated using phenothiazines substituted with 2,2′-bithienyl (2b) and 9,9′-dibutylfluorene (2c) showed a lower PCE value than a cell with 2a, probably due to less efficient electron injection into the TiO2 conduction band indicated by the lower JSC. The obtained results correlate very well with IPCE response ( Figure S15). The device based on 2a demonstrated a higher maximum IPCE value than the others, 60%. The IPCE of devices based on 2a and 2c displayed a broader band between 350 and 650 nm than those with 2b. Moreover, DSSC with 2b showed a significantly low intense band, with a maximum IPCE value of 18% compared to 2a and 2c (27%). The better IPCE response can be interpreted in terms of a higher Jsc value, which correlates well with the obtained short circuit photocurrent density. It can be concluded that the most promising, as sensitized for DSSC, is 2a, and optimization of the photoanode preparation by changing a solvent or semiconductor layer modification may further enhance the PV parameters.

Conclusions
In summary, the novel dyes 2a-2c of D-π-D-π-A based on phenothiazine (PTZ) with diverse donor (D) terminal groups connected by ethylene spacer with PTZ, and cyanoacrylic acid as an anchoring group (A) were successfully synthesized and fully characterized. The impact of endcapped groups in dyes 2a-2c on thermal, electrochemical and photophysical properties has been thoroughly studied and widely discussed with DFT/TD-DFT theoretical calculations. It was established that the electron-donating property of the end-capped groups has a moderate influence on the optical absorption properties and HOMO/LUMO energy levels, but a considerable impact on the morphology of the resulting active layers and photovoltaic behaviour of dyes 2a-2c. The DSSC device based on 2a demonstrated an overall PCE of 6.22% (Jsc = 17.96 mA/cm 2 , Voc = 700 mV, and FF  As can be seen in Table 6 and Figure 9, all of the tested devices containing phenothiazine derivatives demonstrated better PV performance than the reference solar cell with commercial N719. The DSSC with a compound bearing a dibenzothiophenyl donor moiety 2a exhibited the best conversion efficiency of 6.22% (about 57% of the PV performance of the cell sensitized with N719). The best conversion efficiency of the cell sensitized with 2a results from the improved electron injection and less dark current, as indicated by the fact it had the highest J SC (17.96 mA/cm 2 ) and a high V OC (700 mV). The device fabricated using phenothiazines substituted with 2,2 -bithienyl (2b) and 9,9 -dibutylfluorene (2c) showed a lower PCE value than a cell with 2a, probably due to less efficient electron injection into the TiO 2 conduction band indicated by the lower J SC . The obtained results correlate very well with IPCE response ( Figure S15). The device based on 2a demonstrated a higher maximum IPCE value than the others, 60%. The IPCE of devices based on 2a and 2c displayed a broader band between 350 and 650 nm than those with 2b. Moreover, DSSC with 2b showed a significantly low intense band, with a maximum IPCE value of 18% compared to 2a and 2c (27%). The better IPCE response can be interpreted in terms of a higher J sc value, which correlates well with the obtained short circuit photocurrent density. It can be concluded that the most promising, as sensitized for DSSC, is 2a, and optimization of the photoanode preparation by changing a solvent or semiconductor layer modification may further enhance the PV parameters.

Conclusions
In summary, the novel dyes 2a-2c of D-π-D-π-A based on phenothiazine (PTZ) with diverse donor (D) terminal groups connected by ethylene spacer with PTZ, and cyanoacrylic acid as an anchoring group (A) were successfully synthesized and fully characterized. The impact of end-capped groups in dyes 2a-2c on thermal, electrochemical and photophysical properties has been thoroughly studied and widely discussed with DFT/TD-DFT theoretical calculations. It was established that the electron-donating property of the end-capped groups has a moderate influence on the optical absorption properties and HOMO/LUMO energy levels, but a considerable impact on the morphology of the resulting active layers and photovoltaic behaviour of dyes 2a-2c. The DSSC device based on 2a demonstrated an overall PCE of 6.22% (J sc = 17.96 mA/cm 2 , V oc = 700 mV, and FF = 0.48) which was considerably higher than that of 2b (PCE = 4.22%, J sc = 11.87 mA/cm 2 , V oc = 631 mV, FF = 0.54) and 2c (PCE = 4.80%, J sc = 12.80 mA/cm 2 , V oc = 703 mV, FF = 0.56). Moreover, compared to the reference N719-based dye (PCE = 3.56%), the investigated dyes 2a-2c showed appreciably improved photovoltaic performance. These results point out that the terminal donor group in novel dyes 2a-2c plays an important role in the enhancement of the optoelectronic properties and efficiency of DSSC devices.