Structure-Property Relationship of New Organic Sensitizers Based on Multicarbazole Derivatives for Dye-Sensitized Solar Cells

A new multicarbazole based organic dye (C2A1, C2S1A1) with a twisted structure was designed and synthesized, and the corresponding dye (C1A1) without the twisted structure was synthesized for comparison. They were successfully applied in dyesensitized solar cells (DSSCs). The results showed that the nonplanar structure of C2A1 and C2S1A1 can efficiently retard the dye aggregation and charge recombination. The organic dye (C2S1A1) with thiophene units also exhibited a higher molar extinction coefficient and red-shifted absorption, which leads to an improved light harvesting efficiency. The C2S1A1-sensitized solar cell produced a solar-to-electricity conversion efficiency of 5.1%, high open circuit voltage (Voc) of 0.69V, and short-circuit photocurrent density of 10.83mA cm under AM 1.5 irradiation (100mWcm) conditions.


Introduction
Since the first successful fabrication of sandwich type solar cells by O'Regan and Grätzel in 1991 [1], the dye-sensitized solar cells (DSSCs) have received significant attention in both the academic and industrial fields, owing to their efficiency, high adaptability, economic feasibility, and relatively less environmental issues compared with the traditional Si-based solar cells.A DSSC consists of three main components: a photoanode, an electrolyte, and a sensitizer.Among these components, the sensitizer plays the important role of capturing the photons and generating the electrons, which are injected into the conduction band of the semiconductor (e.g., TiO 2 ).Significant research efforts have been made to develop efficient sensitizers to enhance the efficiency of DSSCs.Among dyes used as sensitizers, the sensitization of nanocrystalline TiO 2 solar cells with Ru-complex photosensitizers (e.g., N3 and N719) has been intensively studied.As a result, power conversion efficiencies (PCEs) higher than 11% under AM 1.5 irradiation have been achieved [2][3][4].However, metalfree organic sensitizers have shown PCEs between 6% and 10% [5][6][7][8][9] under the same conditions.Nevertheless, organic dyes possess many advantages, such as high molar extinction coefficients (), ease of customized molecular design for the desired photophysical and photochemical properties, cost effectiveness without the need for transition metals, and in somecases being environmentally friendly.However, one drawback of organic dyes is that the electron lifetimes (  ) of the DSSCs with organic dyes were shorter than with a Ru dye.This is due to the charge recombination between the injected electrons in the TiO 2 electrode and I 3 − ion in the liquid electrolyte and the aggregation of the dyes on TiO 2 (- stacking).Usually, charge recombination can be decreased by introducing alkyl side chains into the dye molecule backbones [10,11], and dye aggregation can be restrained via molecular design that changes the molecular structure from planar to nonplanar or twisted [12][13][14].Hence, careful design of dyes containing a twisted structure is a preferred strategy for the development of high performance DSSCs [15][16][17].The attachment of a carbazole unit to the conjugated polymer backbone can efficiently depress -stacking of the polymers in the solid state [18][19][20][21], and such a unit has been introduced to the dye molecules used in the DSSCs.

International Journal of Photoenergy
In this study, a new multicarbazole based organic dye (C2A1 and C2S1A1) with a twisted structure was designed and synthesized, and the corresponding dye (C1A1) without the twisted structure was synthesized for comparison.The results showed that nonplanar molecular structures prevented charge recombination and dye aggregation.Furthermore, the organic dye (C2S1A1) with thiophene units exhibited a higher  value and red-shifted absorption band because of the improved electron extraction paths from the extension of -conjugation.All the aforementioned factors contributed to an improved light harvesting ability.To verify the strategy, the photovoltaic performances of the DSSCs containing the dyes were compared using their current-voltage (I-V) curves, monochromatic photon-to-current efficiencies, and impedance spectroscopy (EIS) analysis, which were used to study the interfacial electron transfer process, light harvesting efficiency for photons of particular wavelengths, and estimate   , respectively.

Instrumental Analysis.
Structural analysis was performed using the 1 H NMR spectra recorded on a Bruker Avance NMR 400 spectrometer in CDCl 3 and DMSO- 6 .UV/Vis spectra were recorded using a CARY5000 UV/Vis/NIR spectrophotometer.The redox properties were examined by cyclic voltammetry (CV, model: IviumStat).The electrolyte solution of 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF 6 ) was prepared in freshly dried dimethylformamide (CHCl 3 ) solution.The Ag/AgCl and platinum wire (0.5 mm in diameter) electrodes were used as the reference and counter electrodes, respectively.

Synthesis.
All the starting materials and solvents were commercially available and were purchased from Aldrich, TCI, and Alfa Aesar.They were used without further purification.The synthetic procedure of C1A1, C2A2, and C2S1A1 is illustrated in Scheme 1.

Synthesis of 2-Cyano
56 mmol), 2cyanoacetic acid (0.15 g, 1.71 mmol), and a catalytic amount of piperidine in CH 3 CN (15 mL) were mixed and heated under reflux for 8 h.After the solution was cooled to room temperature, the mixture was poured into ice water.The precipitate was filtered, washed with distilled water, and dried under vacuum.The product was obtained as a dark yellow powder.Yield: (0.4 g, 44.4%). 1 (2)

Fabrication and Characterization of DSSCs.
The TiO 2 paste was coated on a precleaned glass substrate containing fluorine doped tin oxide (FTO, TEC8, Pilkington, 8 Ωcm −2 , thickness: 2.3 mm) using the doctor-blade coating method and sintered at 500 ∘ C for 1 h.The other TiO 2 paste was recoated over the sintered layer using TiO 2 particles (approximately 400 nm) as the scattering layer, and the glass substrate was sintered again at 500 ∘ C for 1 h.The prepared TiO 2 film was dipped in an aqueous solution of 0.04 M titanium tetrachloride (TiCl 4 ) at 70 ∘ C for 30 min.For dye adsorption, the annealed TiO 2 electrodes were immersed in the dye solution (0.3 mM of dye in ethanol) at room temperature for 24 h.The dye-adsorbed TiO 2 electrode and platinum counter electrode were assembled using a 60 m thick Surlyn (Dupont, 1702)  as the bonding agent.The liquid electrolyte was introduced through a prepunctured hole on the counter electrode.The electrolyte comprised 3-propyl-1-methyl-imidazolium iodide (PMII, 1 M), lithium iodide (LiI, 0.2 M), iodide (I 2 , 0.05 M), and tert-butylpyridine (TBP, 0.5 M) in CH 3 CN/valeronitrile (85 : 15).The active areas of the dye-adsorbed TiO 2 films were estimated using a digital microscope camera with image analysis software (Moticam 1000).
The photovoltaic I-V characteristics of the prepared DSSCs were measured under 1 sunlight intensity (100 mW cm −2 , AM 1.5), which was verified using a standard Si-solar cell (Keithley 2400, ORIEL, Newport, PV Measurement Inc.).The monochromatic incident photon-to-current efficiencies (IPCEs) were plotted as a function of the wavelength of light by using an IPCE measurement system (PEC-S20, Peccell Technologies, Inc.).

Electronic Absorption Properties of Organic Dyes.
The UV/Vis absorption spectra of the organic dyes in CHCl 3 are shown in Figure 1 and the corresponding data are summarized in Table 1.The absorption band at 390-450 nm can be attributed to the intramolecular charge transfer (ICT) between the donor and acceptor.The absorption maxima of the charge-transfer band in CHCl 3 are at 392, 417, and 442 nm for C1A1, C2A1, and C2S1A1, respectively.Compared to the C1A1 dye, the C2A1 and C2S1A1 dyes exhibited red-shifted absorption at 25 nm and 50 nm, respectively.This shows that the added carbazole units are beneficial to extend the light absorption and to increase the electron donating ability in comparison to a single carbazole unit (C1A1).The  values of the organic dyes are larger than that of the N719 dye, which indicates that these dyes have good light harvesting ability.
The electrochemical behavior of the organic dyes was measured by CV, as shown in Figure 2. The detailed data are listed in Table 1.The highest occupied molecular orbital (HOMO) levels of these dyes were 0.84 V, 0.68 V, and 0.5 V for C1A1, C2A1, and C2S1A1 versus normal hydrogen electrode (NHE), respectively.The obtained values are more positive than the I 3 − /I − redox potential value (0.4 V versus NHE).This indicates that the oxidized dyes formed after the electron injection into the conduction band of TiO 2 could accept electrons from the electrolyte thermodynamically.They could also accept electrons from the LUMO levels that are more negative than the TiO 2 conduction band.This indicates that the electrons from the excited LUMO level can be easily injected onto the photoelectrode and that the oxidized dyes may be regenerated using the I 3 − /I − redox couple.

Photovoltaic Measurements.
The photovoltaic performances of the DSSCs based on organic dyes were compared using the variation of flow current with the bias voltage, IPCE, impedance, and electron lifetime analysis.Figure 3(a) shows the I-V curves of the DSSCs with the different organic dyes, as summarized in Table 2.
Under the standard global AM 1.5 solar irradiation, the cells based on C2A1S1 and C2A1 dyes containing two carbazole units exhibited higher efficiency compared to those based on the C1A1 dye.The short-circuit current density ( sc ), open circuit voltage ( oc ), and overall yield () of the three dyes are in the order of C2S1A1 > C2A1 > C1A1.This is due to the improved light absorption ability by the added carbazole units and the existence of twisted structures, which resulted in an increased current density and inhibited dye aggregation and charge recombination [22].The higher efficiency of the C2S1A1 dye can be explained by the increased electron donating ability and  values due to the introduction of thiophene.The thiophene unit in the C2S1A1 dye may have caused strong - interactions that could be attributed to the light harvesting efficiency.
In order to rationalize these observations, the spectra of the monochromatic IPCE of the DSSCs based on the organic dyes are shown in Figure 3(b).The carbazole-based sensitizers efficiently converted visible light to photocurrent across the higher energy region over the wavelength range of 350-550 nm.A maximum IPCE of 74% was realized at 480 nm for the C2S1A1 dye, while the C2A1 and C1A1 dyes exhibited a maximum IPCE of 69% and 63% at 440 nm, respectively.This is probably due to the fact that the C2S1A1 dye has a much broader absorption spectrum whose contributions are expected to enhance the photogenerated current values.
In addition, EIS was employed to study the electron recombination in the DSSCs.The EIS measurement is shown in Figure 4, and the data is listed in

Conclusions
In this study, a new multicarbazole based organic dye (C2A1 and C2S1A1) with a twisted structure was designed and synthesized, and the corresponding dye (C1A1) without twisted structure was synthesized for comparison.The addition of carbazole units to the organic dyes is an effective method to adjust and control the photochemical and electrochemical properties of the dyes, which determine the charge recombination and overall energy conversion efficiency.The C2S1A1 dye exhibited the highest PCE of 5.1% with a  oc of 0.69 V and short-circuit photocurrent density of 10.83 mA cm −2 .The increased electron donating ability of the C2S1A1 molecule provided higher  values and a much broader absorption spectrum.

Figure 2 :
Figure 2: The cyclic voltammetric curves of the dyes in chloroform containing 0.1 M TBAPF 6 as supporting electrolyte at a scan rate of 50 mV s −1 .

Figure 3 :
Figure 3: (a) Current density-voltage characteristics of dye-sensitized solar cells containing organic dyes under illumination using simulated solar light (AM 1.5, 100 mW cm −2 ).(b) Incident photon-to-current efficiency curves for dye-sensitized solar cells containing organic dyes.

Figure 4 :
Figure 4: (a) Measured dye-sensitized solar cell impedance spectrum at forward bias condition under illumination.(b) Bode-phase plots for the dye-sensitized solar cells.

Table 1 :
Electrochemical parameters of organic dyes.

Table 2 :
Photovoltaic performance of dye-sensitized solar cells a .

Table 3 .
[23] s and  rec represent the series resistance and charge-transfer resistance at the dye/TiO 2 /electrolyte interface, respectively, and  CE represents the resistance at the counter electrode.The values of  s and  CE (the first semicircle in the Nyquist plot) were almost the same for the three dyes because of the same electrode material and same electrolyte used.The  rec was determined by the middle semicircle in the Nyquist plot.From the EIS measurements, the   , which expresses the electron recombination between the electrolyte and TiO 2 , was calculated following a literature procedure[23].The  rec for the dyes C1A1, C2A1, and C2S1A1 was 36.65,19.71, and 13.84 Ω, respectively.Under illumination, the smaller  rec values indicated fast charge generation and transport.The calculated   of C1A1, C2A1, and C2S1A1 was 2.36, 3.52, and 4.7 ms, respectively.Among the dyes, the C2S1A1-based cell had a longer   , which led to a lower rate of charge recombination and thus improved  oc .Therefore, the C2S1A1 dye provided a much faster electron transport and prolonged   .The improved values of  sc and  oc of the DSSCs with the C2S1A1 dye can be mainly attributed to the improved light harvesting efficiency.

Table 3 :
Performances of mercurochrome and organic dye based dye-sensitized solar cells and electron transport properties of their photoanodes as determined by impedance analysis.Cell areas are 0.24 cm 2 . 1 is fluorine doped tin oxide interface resistance.b  2 is due to resistance at interface between counter electrode and electrolyte.c  3 possibly originated from backward charge transfer from TiO 2 to electrolyte and electron conduction in porous TiO 2 film.d  is lifetime of an electron in dye-sensitized solar cells. a