Abstract
1,3,4-Oxadiazole based photosensitizers with biphenyl, naphthalene, anthracene, and triphenylamine as the electron-donating moiety were synthesized for solar cell applications. In these photosensitizers, cyano groups were introduced as the electron acceptor and the anchor group because of their high electron-withdrawing ability and strong bonding to the semiconductor. Oxadiazole isomers were used as the π-conjugation system, which bridges the donor-acceptor systems. The electrochemical and optical properties of the sensitizers were investigated both in their native form and upon incorporation into dye sensitized solar cells. The results of UV-visible absorption spectroscopy, electrochemical impedance spectroscopic measurements, and photocurrent voltage characteristics indicate that 1,3,4-oxadiazole pi-spacer with the anthracene moiety has the highest efficiency of 2.58%. Density functional theory was employed to optimize the structures of the sensitizers and the TiO2 cluster.
1. Introduction
The wide spread application of the photovoltaic (PV) cells is limited mainly due to their high cost to energy output ratio. PV cells are generally classified as inorganic based cells and organic based ones. Inorganic PV cells (Si or quantum dots based solar cells) cost has dropped a lot in the last decades but is still noncompetitive compared to conventional electricity production sources. The motive to achieve high cost/efficiency ratios derived the emergence of the thin film dye sensitized solar cell. It was developed by O’Regan and Grätzel in 1991. The efficiency of this PV cell, which became known as dye sensitized solar cell (DSSC), was reported as 7.1 to 7.9% [1]. DSSCs have got attention due to an ideal tradeoff between efficiency and cost performance [2–5].
The dye, which is the major component of the DSSC, absorbs sunlight and produces excitons [6, 7]. It is chemically bonded to the porous surface of a semiconductor. Currently, DSSCs based on ruthenium(II) polypyridyl complexes have the best conversion efficiencies (PCE) of 11% under standard (Global Air Mass 1.5) illumination [8–10]. But it is still low for commercial applications. The low absorption of light in the red/near infrared region (NIR) is the major drawback in the ruthenium based sensitizers that limits the further improvement in efficiency. To overcome these problems, metal-free organic sensitizers have been developed [11]. The basic structural unit of organic dye is donor-π spacer-acceptor. Substituents acting as the donor and the acceptor are separated by a π-conjugated spacer. The studies suggest that the donor groups to form efficient sensitizers should be selected from the electron rich aryl amines family including phenylamine, aminocoumarin, indoline, (difluorenyl)triphenylamine, and biphenyl. The π-conjugated connector must be selected from compounds containing thiophene and oxadiazole units due to their outstanding charge transfer characteristics. Acrylic acid group is considered the best acceptor moiety [12]. Tian et al. synthesized methylthiophene based photosensitizers for high-performance DSSCs. They introduced vinyl unit as pi-bridge and found that the DSSC assembled with 2-cyano-3-(5-(4-(diphenylamino)phenyl)-4-methylthiophenyl-2-yl) acrylic acid showed an efficiency of 8.27% under simulated AM 1.5 G solar irradiation (100 mW cm2) [13]. Mao et al. synthesized a series of new metal-free organic dyes with either a boron dipyrromethene (BODIPY) phenylene or thiophene as a pi-conjugated bridge. The structure-property relationship of these dyes shows that the introduction of a methoxy group as the donor and a BODIPY thiophene unit as the pi-conjugated bridge are favorable to improve the efficiency of DSSCs [14]. Diketopyrrolopyrrole (DPP) sensitizers were synthesized by Qu et al. The sensitizer containing diketopyrrolopyrrole moiety as the pi-bridge exhibited good stability and better photovoltaic performance of up to 4.41% power conversion efficiency [15].
DFT/TD-DFT is an effective tool to investigate the ground and excited state properties of photosensitizer complexes as compared to other high level quantum approaches because the computed orbitals are suitable for the typical MO-theoretical analyses and interpretations [16]. Many theoreticians have successfully applied this approach in the designing of photosensitizers [17–24].
2. Computer Simulation
Amsterdam Density Functional (ADF) program (2013.01) was used to perform the DFT calculations. Generalized gradient approximation (GAD) at OLYP level with triple-ζ polarization basis function was used to optimize structures of TiO2 and dyes. Relativistic effects were considered in all the calculations by applying zero order regular approximation in its scalar approximation [21, 22, 25].
3. Synthesis of Dyes
Novel dyes were synthesized according to Scheme 1.
Step 1 (see Scheme 2(a): synthesis of ester (1)). A solution of biphenyl-4-carboxylic acid (5 g) in methanol (25 mL) in the presence of a catalytic amount of sulfuric acid (0.5 mL) was heated under reflux for 24 h. The mixture was cooled to room temperature and then concentrated in vacuo. The resulting residue was diluted with water (50 mL) and extracted with ethyl acetate (2 × 50 mL). The combined organic layers were washed with saturated sodium bicarbonate solution followed by water. The organic layer was dried over sodium sulfate and concentrated under reduced pressure to give the pure ester 1 in quantitative yield.
Step 2 (see Scheme 2(b): synthesis of hydrazide (2)). To a stirred solution of methyl biphenyl-4-carboxylate (1, 5 g) in ethanol (50 mL), hydrazine monohydrate (100% solution) (24 g) was added. The resulting mixture was stirred at 80°C for 8 h and then concentrated under reduced pressure. The residue was diluted with ice cooled water (50 mL) and the solid was filtered and washed with water (2 × 50 mL). The title compound was obtained as a pale amorphous solid in 88% yield.
Step 3 (see Scheme 2(c): synthesis of 1,3,4-oxadiazole (3)). To a mixture of hydrazide (2, 3.5 g) and thiophene-2-carboxylic acid (2.1 g), POCl3 (20 mL) was added slowly at room temperature and the resulting mixture was stirred for 2 h at 110°C. After the solution was cooled, excess POCl3 was removed under reduced pressure. The residue was diluted with water and then neutralized with aqueous sodium bicarbonate. The white precipitate was collected by filtration and washed with water. The resulting solid (4.8 g) was further purified by column chromatography to give the pure oxadiazole (4 g).
Step 4 (see Scheme 2(d): formylation of thiophene ring by duff reaction (4)). A mixture of oxadiazole (3, 4 g), hexamethylenetetramine (8 g) in 40 mL of trifluoroacetic acid was heated under reflux (90–110°C). The progress of the reaction was monitored by TLC. Up on completion, the mixture was quenched with saturated sodium bicarbonate solution. The precipitated solids were filtered and then washed with water. The resulting solid was purified by column chromatography to give the desired aldehyde 4 (2.5 g).
Step 5 (see Scheme 2(e): synthesis of cyanoacrylic acid 5 through knoevenagel condensation (5)). To a solution of carboxaldehyde 4 (2 g) and ammonium acetate (0.1 equiv.) in acetic acid (20 mL), cyanoacetic acid was added (1.1 equiv.). The resulting mixture was heated under reflux for 4 h and the progress of the reaction was monitored by TLC. After completion, the mixture was cooled and diluted with ice water. The precipitate was filtered and washed with water. The resulting solid was recrystallized from toluene: methanol mixture (9 : 1) to afford the required dye 5 in pure form.
All other dyes (D1, D3, and D4) were prepared in the same manner. The structure and names of the dyes are shown in Figure 1.
4. Fabrication and Characterization of DSSCs
A 2 mM solution of each dye was prepared in chloroform. TiO2 paste was deposited on conductive glass substrate and then sintered at 450°C for 30 minutes. The electrodes were immersed in the dye solution for 24 hours. After sensitization, the samples were washed with ethanol to eliminate unanchored dye. A platinum coated counter electrode was prepared by depositing platinum paste (Plasticol T, Solaronix) on FTO glass substrate and then heated at 450°C for 10 minutes. Then, DSSCs were fabricated employing the sensitized anode, counter electrode, 60 μm sealing spacer (Meltonix 1170, Solaronix), and redox couple electrolyte prepared in methoxypropionitrile with a 50 mM redox concentration (Iodolyte Z-50, Solaronix). The visible spectra of dyes in methanol and anchored to TiO2 films at glass substrates were recorded with JASCO-670 UV/VIS spectrophotometer. Keithley 2400 Source Meter was used to measure the I-V characteristics of the DSSCs using IV-5 solar simulator (Sr #83, PV Measurement, Inc.) at AM1.5G (100 mWcm−2). The silicon solar cell was used as a reference for calibration. The EIS was measured in dark conditions of illumination via Bio-Logic SAS (VMP3, s/n: 0373), with an AC signal of 10 mV in amplitude, in the frequency range between 10 Hz and 500 KHz.
5. Results and Discussion
5.1. Energy Levels of Photosensitizers
The frontier molecular orbital (FMO) of dyes provides the necessary driving force for the charge transfer. We used a DFT technique to find the band gaps of TiO2 cluster and novel photosensitizers. The electron distribution of the HOMOs and LUMOs of D1, D2, D3, and D4 is shown in Figure 2. Clearly, the HOMOs of these compounds are the highest electron density located at donor moieties. The LUMOs are located in the anchoring group through the pi-bridge. Thus, the HOMO-LUMO excitation induced by light irradiation could move the electron distribution from the donor moieties to the anchoring unit through the pi-bridge segment. (TiO2)8 cluster was also simulated to find its conduction band (−4.0 eV) and band gap (3.18 eV).
Table 1 shows that the HOMOs of sensitizers are in the sequence of D3 (−4.689) > D4 (−5.180) > D1 (−5.657) > D2 (−5.682). The LUMOs energy levels are in the sequence of D3 (−3.694) > D4 (−3.840) > D2 (−3.867) > D1 (−3.956). The electron-donating moieties significantly affect the HOMO and LUMO energy levels of the dyes. Similarly, the of the dyes are in the order of D3 (1.175) < D4 (1.340) < D1 (1.701) < D2 (1.815). These results suggest that dyes 1–4 can inject electrons to the conduction band of titanium oxide.
5.2. Absorption Spectra of Dyes
The absorption spectra of D3 in chloroform and adsorbed on TiO2 are shown in Figure 3. Two distinct absorption bands of D3 in chloroform can be observed: one relatively weak band is in the region (380–400 nm) corresponding to the electron transitions of the conjugated molecules and the other is around 430–460 nm that can be assigned to an intramolecular charge transfer (ICT) between electron-donor and electron acceptor anchoring moieties. However, the absorption shifts to lower energy values when anchored to TiO2. This is due to the fact that on the electrode the carboxylate groups bind to the TiO2 surface in which acts as proton. The interaction between the carboxylate group and the surface ions may lead to increased delocalization of the orbital. The energy of the level is decreased by this delocalization, which explains the red shift for the absorption spectra.
5.3. Photovoltaic Performance
DSSCs were fabricated using D1, D2, D3, and D4 dyes having an effective area of 0.35 cm2. The photovoltaic parameters of DSSCs are recorded in Table 2 and the corresponding photocurrent voltage (J-V) characteristics are showed in Figure 4. Solar cell based on D3 sensitizer shows the best performance among the four dyes. It can be clearly seen that the electron donor moiety of dyes plays a key role in the photovoltaic performance. Thus, introducing the anthracene unit as an electron donor moiety improves the photocurrent of DSSC. Moreover, the higher HOMO and LUMO energy levels of D3 do not only generate charge separation but also accelerate the dye regeneration to avoid charge recombination.
5.4. EIS Analysis
EIS analysis is performed to investigate the interface resistances in DSSCs. Figure 5 shows the Nyquist plot of DSSCs which were assembled with novel photosensitizers. Generally, a normal impedance spectrum of DSSCs is represented by three arcs (semicircles). The first semicircle represents the resistance of electron transfer at counter electrode/electrolyte (), second signifies the resistance of charge transfer at the photoanode/electrolyte interface (), and third indicates the diffusion of in electrolyte () [26, 27]. Only second arc comes out in the Nyquist plot in Figure 5. It is probable that the other two arcs corresponding to and are overshadowed by large semicircle representing [28, 29]. The signifies the charge recombination rate; for example, a larger specifies a slower charge recombination. The value for DSSC assembled with D3 is greater that of D1, D2, and D4. The higher of D3 relative to D1, D2, and D3 could be attributed to the steric hindrance of bulky anthracene unit.
6. Conclusion
A series of novel dyes D1, D2, D3, and D4 have been prepared for DSSCs. The results indicate that the donor moiety in organic dyes strongly influences the performance of DSSCs. The incorporation of anthracene unit as a donor moiety improves the efficiency of DSSC as compared to other donor units, that is, biphenyl, naphthalene, and triphenylamine. Among the four photosensitizers synthesized, maximum efficiency of 2.58% was obtained with DSSCs based on D3 ( mA/cm2, mV, ). These results advocates that the photosensitizers based on anthracene unit are auspicious candidates for DSSCs.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
Acknowledgments
The authors would like to acknowledge the funding provided by King Abdulaziz City for Science and Technology (KACST) through Project no. 11-ENE1635-04 as part of the National Science, Technology and Innovation Plan. The authors would like to acknowledge the Center of Research Excellence for Renewable Energy at KFUPM.
Supplementary Materials
(E)-2-Cyano-3-(5-(5-(naphthalen-2-yl)-1,3,4-oxadiazol-2-yl)thiophen-2-yl)acrylic acid [D1].
(E)-3-(5-(5-([1,1′-Biphenyl]-4-yl)-1,3,4-oxadiazol-2-yl)thiophen-2-yl)-2-cyanoacrylic acid [D2].
(E)-3-(5-(5-(Anthracen-9-yl)-1,3,4-oxadiazol-2-yl)thiophen-2-yl)-2-cyanoacrylic acid [D3].
(E)-2-Cyano-3-(5-(5-(4-(diphenylamino)phenyl)-1,3,4-oxadiazol-2-yl)thiophen-2-yl)acrylic acid [D4].