Novel push-pull thieno[2,3-b ]indole-based dyes for efficient dye-sensitized solar cells (DSSCs)

New metal-free sensitizers ( IK 3-6 ), based on the thieno[2,3-b ]indole ring system, bearing various aliphatic substituents at the nitrogen atom (electron-donating part), s everal thiophene units (π -bridge linker) and 2-cyanoacrylic acid (the electron-accepting and anchoring group) have been synthesized for application in dye-sensitized solar cells (DSSCs). The relationship between the IK dye structure and efficiency of the corresponding DSSC has been elucidated. Power conversion efficiency (PCE) up to 6.3% (short-circuit photocurrent density (J SC ) 19.0 mA cm -2 , open-circuit voltage (V OC ) 0.59 V, and fill factor (FF) 56.4%) were obtained for the DSSC, based on 2-cyano-3-{5-[8-(2-ethylhexyl)-8 H -thieno[2,3-b ]indol-2-yl]thiophen-2-yl}acrylic acid ( IK 3 ), which proved to be a highly synthetic available compound, under simulated AM 1.5 G irradiation (100 mW cm -2 ), thus indicating that thieno[2,3-b ]indole-based organic dyes are perspective candidates for DSSCs.


Introduction
Photovoltaic devices, based on crystalline, inorganic semiconductors, have reached impressive characteristics of their conversion efficiencies: over 28% in single-junction, and 37.9% in multi-junction device architectures under full illumination. 1However, this class of photovoltaics has disadvantages, concerning their high production and energy costs, resulting in a long financial payback time.The original paper by O'Regan and Grätzel, published in 1991, 2 revealed that a relatively high efficiency for the conversion of solar radiation to electricity can be achieved using dye-sensitized solar cells (DSSCs) based on organometallic dyes adsorbed on nanocrystalline semiconductor photoelectrodes (e.g., TiO2, SnO2, ZnO, NiO).Since then, great attention has been paid to development of organic dye sensitizers as the crucial components of DSSCs.
High values of power conversion efficiency (PCE) (up to 11.5%) have been reached for DSSCs, based on polypyridyl ruthenium complexes. 3However, their applications are limited owing to the high cost of this noble metal.In comparison with Ru-dyes, metal-free organic dyes have larger molar extinction coefficients (ε) and their facile synthesis and structural versatility enable tuning of the absorption wavelength and HOMO-LUMO energy levels.[6][7][8][9][10][11][12] In general, an organic dye molecule usually consists of an electron-donating part, a π-linker and an electron-accepting fragment, making the so-called "push-pull" structure.Push-pull dyes, which are based on electron-rich heterocyclic derivatives, such as carbazole, 13,14 indolo[3,2-b]carbazole, 15 and thieno [3,2-b]indole units, 16 with oligothiophene π-linkers and a 2-cyanoacetic acid fragment, as the acceptor moiety proved to exhibit high PCE values (up to 8.3%).In our previous publication, we reported the facile synthesis of push-pull dyes IK 1 and IK 2 with a new electron-donating thieno [2,3-b]indole system.The sensitization potential of these dyes in DSSCs was also established (Figure 1). 17Herein, we report on structural tuning of thieno [2,3-b]indole dyes by introducing a long alkyl-chain on the nitrogen atom of this heterocyclic system to suppress aggregation of dye molecules, 18,19 as well as by incorporation of thiophene units into the structure as π-linkers to enhance their light-harvesting ability.Effects of the structure of organic dyes on photovoltaic characteristics of the fabricated DSSCs have been elucidated.

Synthesis of the IK dyes
A new dye-sensitizer IK 3, bearing a 2-ehtylhexyl substituent at N-1 of the thieno[2,3-b]indole ring system, was obtained according to the same procedure, used for the synthesis of dyes IK 1 and IK 2, described in the recently published paper. 17Namely, 2-(thien-2-yl)-substituted thieno [2,3-b]indole 1a was formylated by using the Vilsmeier-Haack reaction to form the corresponding aldehyde 2a.This aldehyde was condensed with 2cyanoacetic acid to give the target dye IK 3, bearing a branched aliphatic chain at the nitrogen atom and only one thiophene unit, as a π-bridge linker for this push-pull system (Scheme 1).
The Vilsmeier-Haack formylation of compounds 1c and 1d was supposed to be the next step in the synthesis of the target dyes.Unfortunately, attempts to formylate the thieno [2,3-b]indoles in this way failed as rapid degradation of the starting materials 1c and 1d occured on treatment with the Vilsmeier complex.As such, an alternative synthetic strategy was elaborated to produce thieno [2,3-b]indole dyes with an expanded π-conjugated system.The approach exploited included the conversion of thieno [2,3-b]indoles 1b and 1e into the corresponding stannanes 1B and 1E, followed by the Stille cross-coupling reaction of intermediate 1B with 5-bromothiophene-2-carbaldehyde or intermediate 1E with 5'-bromo-[2,2'-bithiophene]-5-carbaldehyde to produce thienaldehydes 2b and 2d, respectively.In a similar manner, thieno [2,3-b]indole 1c, bearing 2,2'bithiophene moiety, was transformed into aldehyde 2c.Finally, new thieno [2,3-b]indole dyes IK 4-6 were synthesized through the Knoevenagel condensation of functional thienaldehydes 2b-d with 2-cyanoacetic acid in glacial acetic acid under reflux with piperidine, as the catalyst (Scheme 3).Optical and electrochemical measurements UV-vis absorption spectra of the dyes IK 3-6 in THF are shown in the Figure 2, and the spectral data are summarized in the Table 1.The 350-400 nm bands can be attributed to the localized π→π* electron transition.The main absorption bands in the visible region of ~400-600 nm with a high molar absorption are likely to originate from intramolecular charge transfer (ICT) transitions from the thieno[2,3-b]indole donating part of the molecules to electron-withdrawing fragment of 2-cyanoacrylic acid.The bathochromic shift caused by an increase in the number of thiophene units in the π-bridge can be observed due to π→π* electron transition, that changes from ~320 nm (IK 3) and 364 nm (IK 4) to 398 nm (IK 5 and IK 6).Also, this increase makes some bathochromic shift to ICT, but it can be observed only between one (IK 3) and two (IK 4) thiophene moieties.It is noteworthy that all dyes show a high visible molar extinction coefficient (from 42141 to 48773 M -1 cm -1 ), thus indicating their good light-harvesting capability.However, the value of molar extinction in the group of these dyes is enhanced slightly, when the number of thiophenes units in the π-bridge linker is increased from one to three., Rd is the absolute reflectance of the sample layer.The optical band gap energies (Egap opt ) were estimated from the extrapolation of the linear parts of corresponding curves to F(Rd) = 0.These data are summarized in the Table 1.The energy band gap can be easily tuned by increasing the number of tiophene units in the π-bridges, and the values of Egap opt lie in the range from 1.51 to 1.82 eV.These values indicate the prospects of using the studied compounds as light-harvesting materials.Electrochemical characteristics of the dyes IK 3-6 were measured by cyclic voltammetry (CV) in a THF/MeCN (v/v, 1:1) solution of 0.1 M LiClO4, as the supporting electrolyte at a scan rate of 100 mV/s, and the results obtained are listed in the Table 1.Thus, the HOMO energy levels of sensitizers IK 3-6, that correspond to the first oxidation potential of each dye, were determined from the CV data.The LUMO energy levels of these dyes were calculated from the optical energy band gaps, estimated from the diffuse reflectance spectra, and EHOMO values.The HOMO energy levels of all four dyes are more positive, than the I − /I3 − redox couple (−4.9 eV), making possible regeneration of oxidized dye molecules after injection of excited electrons into TiO2 electrode.On the other hand, the latter process is energetically permitted for each of these dyes because of their LUMO energy levels are more negative than the conduction edge of the TiO2 (−3.9 eV) (Figure 4).

Photovoltaic measurements
Photovoltaic properties of DSSCs.DSSCs were fabricated using the synthesized dyes IK 3-6, as photosensitizers, and their photovoltaic properties were measured under standard global AM 1.5 irradiation (100 mW cm 2 ).In addition, DSSCs based on recently described sensitizers IK 1 and 2, 17 that represent a structurally close series of thieno [2,3-b]indole dyes, were also tested under similar experimental conditions.It is important to note that only formal applicability of dyes IK 1 and 2 as sensitizers for DSSCs has been shown in our previous work. 17In particular, PCE values of the obtained solar cells were very low (PCE = 0.37 % for IK 1; PCE = 0.79% for IK 2).These low PCE values were attributed to a number of defects of TiO2 photoanodes, which were manufactured and used in that study. 17n contrast, the manufacturing technique of TiO2 electrodes has been significantly improved during current research, which allowed us to obtain photovoltaic devises based on the investigated dyes with better PCE characteristics.Furthermore, the photosensitize capabilities of all IK dyes have been compared with commercial ruthenium dye N3 cell performance.The photocurrent -voltage curves are depicted in the Figure 5, and the detailed parameters of short-circuit current density (JSC), open-circuit voltage (VOC), fill factor (FF) as well as total power conversion efficiency (PCE) are summarized in the Table 2.The formal relationship between the structure of the IK dyes and PCE of solar cells is visualized in the Figure 6.A significant increase of PCEs of cells based on dyes IK 1-3, having one thiophene unit (n = 1) in the πbridge linker, was observed with increasing length of aliphatic chain (Alk = Et, Bu and ETH) at the nitrogen atom of their thieno [2,3-b]indole donating part.The DSSC based on dye IK 3 showed the highest PCE of 6.3% with JSC of 19.0 mA cm -2 , VOC of 0.59 V and FF of 56.4%.Furthermore, the PCE value for DSSC with IK 3 was comparable with that of the cell based on the commercially available ruthenium dye N3, one of the well-known standard dyes.At the same time, the efficiency of cells based on IK 1, IK 4, and IK 5 (Alk = Et; n = 1, 2 and 3, respectively) proved first to be increasing, and then decreasing sharply, while more thiophene units are involved gradually into the structure of π-bridge linker.However, replacement of the ethyl group at the nitrogen atom of thieno[2,3-b]indole donating part (compound IK 5, PCE = 1.3%) with the butyl substituent (IK 6, PCE = 1.4%) does not influence the PCE values of the cells.Thus, DSSCs based on dyes IK 1, IK 5 and IK 6 were characterized by low PCE values linked with a poor photocurrent.The main reason for low JSC values is proposed to be that dyes IK 1, 5 and 6, having an insufficient size of aliphatic chain in their donating part and a high coplanar and π-conjugated structure, were capable of self-aggregation on the surface of TiO 2 photoanode, thus decreasing electron-injection yields.[25][26] Transients of photocurrent Figure 7 shows the transient of the photocurrent for DSSC with a photoanode sensitized by the dye IK 3. One can see that the potential shifted rapidly into the negative direction in case of illuminating photoanode with IK 3, which indicated a high rate of accumulation of injected electrons in the conduction band of nc-TiO2.The photocurrent density initially enhanced steeply under illumination with light output of 100mW cm −2 , and then the photocurrent stablized.The transient of the photocurrent has a good rectangular form, as one can see in the Figure 7.This indicated that there were no diffusion limitations for dye IK 3 regeneration and imposition of the dark electrochemical processes in case of the iodine-iodide mediator system.This result was confirmed by the absence of cathodic current at any potential of short circuit under dark conditions.

Conclusions
We have developed new organic dyes of the thieno [2,3-b]indole family, and used them successfully for sensitization of TiO2 films in the DSSCs.The optical and redox properties of these dyes were investigated.All of these dyes show a strong light-harvesting ability in the visible region, and the LUMO and HOMO levels (estimated from their electrochemical and optical data) were suitably located for electron-injection and regeneration processes in the solar cell.An aliphatic substituent, in respect of its length and branching, located in the electron-donating part of sensitizers, decisively influenced solar cell efficiency, since it inhibits aggregation of the molecules onto TiO2 surface by creating steric hindrances upon their adsorption.Indeed, the best result of solar energy-to-electricity conversion efficiency of 6.3% was achieved for the device based on the dye IK 3, having 2-Et-hexyl chain at the nitrogen atom of thieno [2,3-b]indole donor and only one thiophene unit in its π-bridge.In contrast, PCE values of the cells with dyes IK 5 (1.3%) and IK 6 (1.4%), containing 2,2':5',2''-terthiophene moiety, as the π-bridge linker, were significantly lower due to intermolecular π-stacking and aggregation processes in these dyes, proceeding on photoanode surface.8][29] On the other hand, dye-sensitizer IK 3 is a synthetic available compound due to its preparation route is rather short and does not require any transition metal-catalysts or pyrophoric reagents, and this dye appears to be a very promising for DSSC application.In this regards, results reported herein indicate that the thieno[2,3-b]indole ring system is very perspective as electron-donating block for the further engineering of new sensitizers for high-performance DSSCs.

Experimental Section
General. 1 H and 13 C NMR spectra were obtained on Bruker DRX-400 and AVANCE-500 spectrometers, with TMS as the internal standard.The 13 С NMR spectrum of dye IK 6 could not be determined due to a poor solubility of this dye in a range of deuterated solvents.Elemental analyses were carried out on a Eurovector EA 3000 automated analyzer.Melting points were determined on a Boetius apparatus and were not corrected.IR spectra of samples (solid powders) were recorded on a Spectrum One Fourier transform IR spectrometer (Perkin Elmer), equipped with a diffuse reflectance attachment (DRA).UV-visible spectra were recorded for a 2•10 -5 M THF solution with Shimadzu UV-2401PC spectrophotometer.Cyclic voltammetry of the IK dyes was carried out for their 1•10 -3 M THF/MeCN (1:1) solutions with Metrohm Autolab PGSTAT128N potentiostat using a standard three-electrode cell, which was equipped with a platinum working electrode, a Ag/AgNO3 (0.01 M) reference electrode, and a glassy carbon rod counter electrode.A three-electrode PECC-2 cell was used for photo-electrochemical measurements.The photoanode served as the working electrode and a platinum wire with the surface area of 5 cm 2 was used as the auxiliary electrode.In both electrochemical cells, a silver wire was used as the reference electrode.All potentials are given relative to this reference electrode.The voltammetric measurements were performed with an IPC Pro MF potentiostat under AM 1.5 global one sun of illumination (100 mW cm −2 ), provided by a solar simulator (Newport 96000).The illumination power at different distances was determined with a Nova apparatus (OPHIR-SPIRICON Inc.).The illuminated photoanode area was 0.125 cm 2 .The illumination was performed from the side of a TiO2 photoanode with the adsorbed dye.The time dependence of the photoanode and cathode potentials under the open-circuit conditions and the photocurrents at the short-circuit potential (transients) were measured under both illumination and in dark.TiO2 photoelectrode preparation.The preparation of photoelectrode was performed by adopting doctorblade technique on a conducting glassTCO22-15 (2.9 × 2.9 cm 2 pieces) coated with a conductive fluorinated tin oxide (specific surface resistivity about 15 Ω/sq, Solaronix), which was purified by aging in a sulfochromic mixture, followed by ultra-sonication in organic solvents (isopropanol and acetone) and distilled water, and then dried at 50 °C in air.Application of Ti-nanoxide D/SP paste (Solaronix), comprising a nanocrystalline titanium dioxide, was performed by the adopting doctor-blade procedure, using a stencil with a square hole of 2.0 × 2.0 cm (~90 μm depth).After application of the paste, raw photoanodes were dried at 50 °C in air, and then calcined in a muffle furnace at 450 °C for 1 h (heating rate 3 °C/min, in air) The thickness of thus obtained titanium dioxide film was about 15 μm.Sensitizing of titanium dioxide was performed by soaking photoanodes in 5•10 −4 M THF solutions of dyes IK 3-6 for 24 h.DSSCs properties test.To measure photocurrent-voltage (J-V) characteristics, the DSSC was illuminated by light with energy of 100 mW•cm -2 (AM 1.5) from a 150 W solar simulator (Newport, 96000).A computercontrolled IPC Pro MF potentiostat was employed to collect the J-V curves.
General procedure for the synthesis of indoline-2-ones (3b,d).The solution of 1-alkylisatin (3 mmol), corresponding acetylthiophene (3 mmol) and N,N-diethylamine (0.062 mL, 0.6 mmol) in EtOH (15 mL) was stirred at room temperature for 36 h.The resulting mixture was concentrated under reduced pressure and the residue was dissolved in dry CH2Cl2 (10 mL).SOCl2 (0.28 mL, 3.9 mmol) was then added to the solution and the mixture was stirred at room temperature for 2 h.After that it was concentrated under reduced pressure and the residue was recrystallized from EtOH affording indolin-2-ones 3b,d as dark-red needles.

Figure 1 .
Figure 1.Structures of push-pull dyes with the electron-rich heterocyclic donors.

Figure 5 .
Figure 5. Photocurrent -voltage curves of the DSSCs based on the IK dyes.

Figure 6 .
Figure 6.Relationship between the IK dye structure and PCE values of the related DSSCs.

Figure 7 .
Figure 7. Time dependence of photocurrent for DSSC with photoanode with dye IK 3 in the dark, and upon irradiation with light (100 mW/cm 2 ); scans recorded in acetonitrile solution in the presence of 0.5 M LiI + 0.05 M I 2 at the potential of short circuit current density.