Synthesis of perylene monoimide derivative and its use for quasi-solid-state dye-sensitized solar cells based on bare and modified nano-crystalline ZnO photoelectrodes

Abstract

A novel perylene monoimide derivative, PCA, with a strongly electron-donating cyclohexylimide segment, bulky alkylphenoxy substituents at the 1,7-bay positions of the perylene core and an acid anhydride as the strong coupling group was synthesized and used as sensitizer for dye-sensitized solar cells (DSSCs). PCA was readily soluble in common organic solvents, decomposed above 390 °C and had glass transition temperature of 64 °C. The absorption curves had maximum at 505–512 nm, with thin film absorption onset at 643 nm corresponding to an optical band gap of 1.93 eV. We have fabricated the quasi-solid-state DSSCs with ZnO nano-crystalline films deposited from different values of pH. It is found that the power conversion efficiency (PCE) is high when the pH value of sol gel is about 9. The PCE of DSSC fabricated with the gold-coated ZnO photoanode is higher than that for bare ZnO photoanode. This enhancement is attributed to the formation of Schottky barrier at the Au/ZnO interface that blocks the electron transfer from ZnO to dye and electrolyte, increasing the electron density in the conduction band of ZnO. The incorporation of TiO₂ nanoparticles in polymer gel electrolyte further increases the PCE of DSSC up to 3.0%. The enhancement is primarily explained by studying the dark reaction, diffusion coefficient of tri-iodide and exchange current density in the interface of electrolyte/Pt counter electrolyte, and lifetime of electrons in the anode film.

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Broader context

The conversion of solar energy into electricity using dye sensitized solar cells (DSSCs) represents a most promising method for future large scale production, because of its low cost and high efficiency about 10–11%, at present. In the DSSCs, all components used in the fabrication, play important role in the optimization of its photovoltaic response. We describe the fabrication of quasi-solid-state DSSC based on a perylene monoimide derivative as sensitizer, Au nanoparticle-modified ZnO photoelectrode and polymer gel electrolyte with TiO₂ particles. We have achieved power conversion efficiency about 3.0%, which is quite respectable for the DSSCs with polymer gel electrolyte and a low cost photosensitizer.

Introduction

Energy demands and environmental pollution resulting in global warming have led to an intense search all over the world for alternative renewable energy sources, over the past decades. Since the first report in 1991 by Grätzel and coworkers,¹ dye sensitized solar cells (DSSCs) based on wide band gap nano-crystalline TiO₂ semiconductors loaded with dyes have attracted significant attention as low cost alternatives to conventional inorganic solar cells.^2–6 At present, DSSCs based on large variety of Ru-complex photo-sensitizers have been explored and power conversion efficiency (PCE) about 9–12% have been reported.^7–11 However, in view of their high cost, environmental problems, and difficult routes of synthesis, metal free organic photo-sensitizers are strongly desired. Pure organic dyes, compared with metal complexes, have higher absorption coefficients, which could save the amount of dyes used and reduce the thickness of the semiconductor film. Moreover, metal free organic dyes are easier to prepare and cost less compared with those of metal Ru based complexes. Metal free organic molecular photosensitizers such as squaraine, triphehylamine and xanthene organic dyes have been developed and show promising power conversion efficiencies. Recently, organic dye sensitized TiO₂ solar cells have made great progress, and highest overall yield of solar cells sensitized by pure organic dyes has exceeded 6.3%.¹² So organic dyes are promising and are expected to be a new type of sensitizer for DSSCs.¹²

Perylenediimides (PDIs) represent one of the most widely studied classes of organic semiconductors with possible applications in photovoltaic cells, electrophotography, laser dyes and organic light-emitting diodes.¹³ They are inexpensive and easily accessible, attractive materials owing to intense, high fluorescence quantum yield and general photostability but suffer from poor solubility in common organic solvents. Synthesis of highly soluble PDIs is very important for the preparation of their thin films to be used in photoelectronic applications.^14–16 They exhibit singlet energy transfer over unusually long distances. Most organic conducting materials can be described as p-type semiconductors; in contrast, PDIs are described as n-type semiconductors in which the major charge carriers are in their conduction band. Such materials are employed as electron accepting materials in all organic photovoltaic solar cells. Additionally, macroscopic oxide films are under intensive investigation due to their use in optoelectronic applications such as DSSCs, electrochromic and electroluminescent displays.

Recently, two near infrared absorbing perylene dyes containing benzo[e]indole have been used for DSSCs.¹⁷ Perylene monoimide dyes have been also used for DSSCs with overall conversion efficiency of 1.61%¹⁸ and 2.6%.¹⁹ Soluble perylene derivative has been used to enhance the photoelectric efficiency of the hybrid poly(3-hexylthiophene)/ZnO bulk-heterojunction solar cells.²⁰ Finally, two soluble perylene–anthracene and perylene–pyrene compounds²¹ have been synthesized in our laboratory and used for quasi-solid-state DSSCs with PCE up to 3.42%.

Many investigators have made great efforts to enhance the performance of the DSSCs by improving the constituent's properties. These efforts are largely classified into four categories of developments: sensitizers,^22–25 anodic materials,^26–29 electrolytes^30–32 and modification of anodic materials^33–35 especially TiO₂. Due to chemical stability and moderate charge capability of the TiO₂, it has been most frequently used as a photoanode material in DSSCs. However, the electrons injected into the TiO₂ layer are sometimes able to return to the sensitizer or the electrolyte due to electron recombination phenomena, causing a reduction in PCE.

Zinc oxide (ZnO) has attracted attention as a fascinating alternative to TiO₂ in DSSCs because ZnO and TiO₂ exhibit similar lowest conduction band edges and electron injection processes from excited dyes in DSSCs.^36,37 Moreover, the lifetime of carriers in ZnO is significantly longer that that in TiO₂.³⁸ However, apart from some basic investigations, few studies have been made on high efficiency DSSCs incorporating ZnO, because their conversion efficiency of around 2–3% under 100 mW/cm² illumination^39–45 is much lower than that of DSSCs with TiO₂. A serious problem for ZnO based DSSCs is the aggregation of dye which generates the Zn²⁺–dye aggregates. Such aggregates lower the electron injection efficiencies and filling of the nanopores of ZnO electrode, resulting in reduction in PCE of the DSSC. Therefore, novel processing of ZnO electrode is needed to overcome this problem and increase the PCE of the ZnO based DSSCs.

The present investigation describes the synthesis, characterization, the photophysics of a novel perylene monoimide, PCA, which was successfully used as sensitizer for DSSCs. PCA contains an electron-donating and solubilizing cyclohexyl ring which was connected with the perylene core via the imide nitrogen. Moreover, PCA carries bulky alkylphenoxy groups at the 1,7-bay positions of the perylene, the alkyl chains of which enhanced the solubility of the compound. Finally, PCA contains an acid anhydride as anchoring unit on the ZnO surface. The nature of the anchoring unit and the electron coupling between the perylene core and the ZnO surface would influence the cell performance. We have designed the quasi-solid-state DSSCs using PCA as sensitizer and ZnO nano-crystalline coated on fluorine doped tin oxide (FTO) glass substrate and polymer gel electrolyte. We found that the PCE of the device is highest when the photoelectrode with ZnO crystalline film deposited for ZnO sol gel having pH value about 9. The effect of gold nanoparticles deposited on the ZnO surface and incorporation of TiO₂ particles into the polymer gel electrolyte has been discussed in detail.

Experimental

Characterization methods

IR spectra were recorded on a Perkin-Elmer 16PC FT-IR spectrometer with KBr pellets. ¹H NMR (400 MHz) spectra were obtained using a Brucker spectrometer. Chemical shifts (δ values) are given in parts per million with tetramethylsilane as an internal standard. UV-vis spectra were recorded on a Beckman DU-640 spectrometer with spectrograde THF. TGA was performed on a DuPont 990 thermal analyzer system. Ground samples of about 10 mg each were examined by TGA and the weight loss comparisons were made between comparable specimens. Dynamic TGA measurements were made at a heating rate of 20 °C/min in atmospheres of N₂ at a flow rate of 60 cm³/min. Thermomechanical analysis (TMA) was recorded on a DuPont 943 TMA using a loaded penetration probe at a scan rate of 20 °C/min in N₂ with a flow rate of 60 cm³/min. The TMA experiments were conducted at least in duplicate to ensure the accuracy of the results. The TMA specimens were pellets of 10 mm diameter and ∼1 mm thickness prepared by pressing powder of sample for 3 min under 8 kp/cm² at ambient temperature. The T_g is assigned by the first inflection point in the TMA curve and it was obtained from the onset temperature of this transition during the second heating. Elemental analyses were carried out with a Carlo Erba model EA1108 analyzer.

Preparation of compounds

1,7-Dibromo-3,4,9,10-perylenetetracarboxylic dianhydride (1). Compound 1 was prepared according to a reported method.⁴⁶

1,7-Bis(4-[1,1,3,3-tetramethylbutyl]phenoxy)perylene-3,4:9,10-tetracarboxylic acid dianhydride (2). A flask was charged with a mixture of 1 (0.18 g, 0.33 mmol), 4-(1,1,3,3-tetramethylbutyl)-phenol (0.17 g, 0.84 mmol), K₂CO₃ (0.12 g, 0.87 mmol) and N,N-dimethylformamide (DMF) (20 mL). The mixture was refluxed for 12 h under N₂. Then it was concentrated under reduced pressure and the concentrate was poured into water. The precipitate was filtered off, washed thoroughly with hot water to dissolve the excess of phenol and dried to afford 2 (0.20g, 75%).

FT-IR (KBr, cm⁻¹): 2954, 2900, 1803, 1764, 1592, 1506, 1364, 1230, 1176, 1014, 832.

¹H NMR (CDCl₃) ppm: 8.56–8.04 (m, 6H, perylene); 7.42 (m, 4H, phenylene meta to oxygen); 7.06 (m, 4H, phenylene ortho to oxygen); 1.40–1.50 (m, 16H, C(CH₃)₂CH₂C(CH₃)₃); 1.22 (s, 18H, C(CH₃)₂CH₂C(CH₃)₃).

Anal. Calcd. for C₅₂H₄₈O₈: C, 77.98; H, 6.04. Found: C, 76.27; H, 5.86.

1,7-Bis(4-[1,1,3,3-tetramethylbutyl]phenoxy)perylene-N-cyclohexyl-3,4:9,10-tetracarboxylic acid dianhydride (PCA). A flask was charged with a mixture of 2 (0.12 g, 0.15 mmol), cyclohexylamine (0.015 g, 0.15 mmol), DMF (15 mL) and glacial acetic acid (0.02 g, 0.37 mmol). The mixture was heated at 150 °C for 12 h. It was subsequently concentrated under reduced pressure and the concentrate was poured into water (50 mL) containing hydrochloric acid (3 mL). The precipitate was filtered off, washed thoroughly with water and dried to afford PCA (0.10g, 75%).

FT-IR (KBr, cm⁻¹): 2950, 2865, 1800, 1766, 1654, 1592, 1504, 1364, 1216, 1176, 1014, 832.

¹H NMR (CDCl₃) ppm: 8.58–8.06 (m, 6H, perylene); 7.43 (m, 4H, phenylene meta to oxygen); 7.05 (m, 4H, phenylene ortho to oxygen); 2.57 (m, 1H, cyclohexyl proton close to the nitrogen); 2.04–1.84 (m, 26H, C(CH₃)₂CH₂C(CH₃)₃ and other cyclohexyl protons); 1.24 (s, 18H, C(CH₃)₂CH₂C(CH₃)_3.).

Anal. Calcd. for C₅₈H₅₉NO₇: C, 78.97; H, 6.74; N, 1.59. Found: C, 77.21; H, 6.26; N, 1.40.

Synthesis of ZnO nanoparticles

The preparation of ZnO colloids or nanocrystals from decomposition or hydrolysis of zinc salts is an established route.⁴⁷ We have adopted such an approach to prepare the ZnO nanocrystals from the hydrolysis as described below.

The ZnO sol gel nano-crystalline power was prepared by dissolving zinc acetate dehydrate (CH₃COO)₂ Zn 2H₂O) in methanol at room temperature. A clear solution of 0.5M was obtained by magnetic stirring at 30 °C for 2 h. The sol gel prepared was found stable and transparent. ZnO nano-crystalline powers were prepared by varying the pH value of solution from 5 to 11. The pH value of the solutions was adjusted to the desired value using sodium hydroxide (0.1N NaOH) solution. The modified solutions were again stirred for 1 h at room temperature. The clear solution was subsequently filtered through micron filter paper. The resulting transparent filtrate was kept for 3 h to complete the gelation and hydrolysis process. During this period of time, while ZnO precipitates were slowly crystallized and settled down in the bottom of the flask. The white precipitate was filtered and washed out with methanol to remove the staring materials and then dried at 120 °C for 2 h. The X-ray diffraction (XRD) of the samples was obtained using XRD diffractrometer. The crystallite size was determined by XRD peak broadening. The absorbance of the ZnO nanocrystalline power was measured using UV-visible spectrophotometer.

DSSCs were fabricated using the synthesized nano-crystalline ZnO powder. The nano-crystalline ZnO films were deposited on the fluorine doped tin oxide (FTO) glass substrates by the doctor blade technique. The substrates were cleaned using accetone, methanol and distilled water in sequence and then dried. The ZnO nano-crystalline powder ground by a mortar and pestle with methanol and prepared a paste. The paste was spread on the surface of FTO substrate with glass rod, using an adhesive tape as spacer. After drying in air, the films were sintered for 30 min at 350 °C in air.

To incorporate the gold (Au) nanoparticles on the surface of nano-crystalline ZnO films, the ZnO coated films were then immersed in a chloroauric acid (HAuCl₄ 1 mM) solution with a pH value between 8 and 9 being adjusted by sodium citrate (Na₃C₆H₅O₇ 2H₂O). The Au ions (Au³⁺) were reduced to neutral atoms attached to ZnO nano-crystalline surfaces.

Above electrodes were immersed in 5 × 10⁻⁴ M PCA solution in THF overnight at room temperature. The polymer gel quasi-solid-state electrolyte was prepared as follows. The appropriate amount of LiI (0.5 M)/(0.05 M I₂), was dissolved in binary organic mixture of propylene carbonate (5 mL), poly(ethylene oxide) (PEO) (0.25 g) and 4-tert-butyl pyridine (TBP) (0.05 mL) in acetonitrile (5 mL) solvent. To incorporate the nano-filler in the polymer electrolyte different wt% TiO₂ powder was added to it. The resulting mixture was heated at 60 °C under vigorous stirring until a viscous gel was formed, followed by cooling down to room temperature. After the sensitization, a quasi-solid-state polymer electrolyte spread on the top of PCA sensitized ZnO photoelectrode by the doctor blade technique. Pt coated FTO electrode was used as counter electrode. The DSSCs were made by clamping the photoanode consisting of polymer electrolyte and counter electrode.

The current voltage (J–V) characteristics in dark and under illumination intensity of 100 mW/cm² were obtained with Keithley electrometer equipped with built in power supply. The intensity of the illumination was calibrated for AM 1.5 and measured using a lux meter. The incident photon to current efficiency of the devices was measured using a monochromator and the resulting current was measured employing Keithley electrometer under short circuit condition. The electrochemical impedance spectra of the DSSCs were carried out under illumination, by applying bias voltage equal to the open circuit voltage of DSSC, using Autolab Potentiostat-10 equipped with frequency response analyzer (FRA).

Results and discussion

Synthesis and characterization

Scheme 1 outlines the synthesis of compound PCA. Particularly, the bromination of 3,4,9,10-perylenetetracarboxylic dianhydride using bromine and a catalytic amount of iodine in sulfuric acid afforded compound 1.⁴⁶ The 1,7-dibromo derivative was the predominant isomer which was obtained as reaction product.⁴⁸ The latter reacted subsequently with 4-(1,1,3,3-tetramethylbutyl)-phenol in DMF in the presence of K₂CO₃ to yield the diether 2. Finally, 2 reacted with an equimolar amount of cyclohexamine in DMF in the presence of acetic acid to afford PCA. Compound PCA was soluble in THF, chloroform, dichloromethane and other common organic solvents due to the presence of the aliphatic segments.

Scheme 1 Synthesis of PCA

The structure of PCA was confirmed by FT-IR and ¹H NMR spectroscopy (see Experimental). The IR spectrum showed characteristic absorption bands at 2950, 2865 (C–H stretching of aliphatic segments); 1800, 1766 (anhydride); 1654 (carbonyl stretching); 1592, 1504 (aromatic breathing modes) and 1216, 1176 cm⁻¹ (ether bond). The ¹H NMR spectrum of PCA displayed an upfield multiplet at 8.58–8.06 ppm assigned to perylene protons. The phenylene protons meta and ortho to oxygen resonated at 7.43 and 7.07 ppm, respectively. Finally, the aliphatic moieties gave signals at the region of 2.57–1.24 ppm.

The thermal characterization of PCA was accomplished by TGA and TMA. The decomposition temperature (T_d) and the char yield (Y_c) at 800 °C, by TGA, as well as the glass transition temperature (T_g), by TMA, are listed in Table 1. Even though PCA contained a large fraction of aliphatic moieties, it exhibited a satisfactory thermal stability. Specifically, it had T_d of 390 °C and Y_c of 42%. The T_g was relatively low (64 °C) owing to the monomeric nature of the compound as well as the cyclohexylamine ring and the aliphatic side chains.

Table 1 Thermal, optical and electrochemical properties of PCA

a Decomposition temperature corresponding to 5% weight loss in N2 determined by TGA. b Char yield at 800 °C in N2 determined by TGA. c Glass transition temperature determined by TMA. d λa,max: The absorption maxima from the UV-vis spectra in THF solution or in thin film. e Eg^opt Optical band gap determined from the absorption onset in thin film. f Eg^el Electrochemical band gap determined from cyclic voltammetry.
Td^a (°C)	390
Yc^b (%)	42
Tg^c (°C)	64
λa,max^d in solution (nm)	505
λa,max^d in thin film (nm)	512
Eg^opt^e(eV)	1.93
HOMO (eV)	−5.9
LUMO (eV)	−4.0
Eg^el^f(eV)	1.90

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Photophysical properties

The photophysical characteristics of PCA are summarized in Table 1. Fig. 1 depicts the normalized UV-vis absorption spectra of PCA in 10⁻⁵ M THF solution and thin film. The latter was prepared on quartz substrate from THF solution by spin-coating. The absorption curves had maximum (λ_a,max) around 500 nm, were broad and extended about up to 700 nm. Thus PCA had very nice absorption coverage over the visible and near infrared region which is desirable for photovoltaic applications. The thin film absorption onset was located at 643 nm corresponding to an optical band gap (E_g^opt) of 1.93 eV. This value of E_g^opt is comparable with that (2.05 eV) of two symmetrical compounds based on perylene–anthracene and perylene–pyrene, which have been used for quasi-solid-state DSSCs.²¹ Perylene polyimides containing p–n diblocks, which have been used for sensitization of TiO₂ solar cells, showed band gap energies of 2.16–2.25 eV.⁴⁹ Finally, polyether derivatives of perylenedimides adsorbed on monocrystalline metal oxide films exhibited E_g^opt of ∼2.20 eV.⁵⁰

Fig. 1 Normalized UV-vis absorption spectra of PCA in THF solution and thin film.

Cyclic voltammetry (CV) measurements were performed on Autolab PGSTAT-30 electrochemical workstation equipped with three electrode system.The three electrode cell is composed of a gold working electrode, a platinum counter electrode and a SCE reference electrode calibrated against Fc/Fc+couple (+0.09V vs SCE). The CV characteristic of PCA was conducted in a distilled DMF containing 10⁻³ M of PCA and 0.5 M aqueous KCl as supporting electrolyte. The CV measurements were performed at a scan rate of 60 mV/s. The HOMO and LUMO energy levels are estimated from the onset oxidation potential and reduction potential observed in CV measurements and they are listed in Table 1.

Characterization of ZnO nano-crystalline films

We have recorded the XRD patterns of all ZnO samples obtained with different pH values. It is observed that the powders showed crystalline nature with peaks corresponding to (100), (002), (101) planes. The preferred orientation corresponding to the (101) plane is observed for ZnO powders. The spacing values and relative intensities of the peak coincide with the JCPDS card no. 36-1451 for ZnO powder. The crystallite size D was obtained by measurements of the broadening of diffraction lines and using Debye–Scherrer formula:
where λ is the wavelength of CuK_α irradiations (1.54 Å), β is the full width at half maximum of the peak corresponding to the plane (101) and θ is the angle obtained from 2θ value corresponding to maximum intensity peak in the XRD pattern. The diameter of crystallite size obtained ZnO nanoparticles varied from 8 to 16 nm. The largest size of a crystallite ZnO nanocrystal is about 16 nm, derived from the sol gel with pH value of 9. No sharp peak was observed for ZnO sol gel having pH value 6, which indicates the amorphous nature. The ZnO sol gel obtained at pH 9 has most intense peaks, which is thus most crystalline and has maximum crystallite size of 16 nm. The intensity of the reflected peak was found to decrease for the ZnO sol gel obtained at pH value greater than 9 and can be attributed to the higher reaction rate, when precipitate starts to dissolve.⁵¹

The growth of ZnO from the zinc acetate dihydrate as precursor using sol gel process generally undergo with four steps such as solvation, hydrolysis, polymerization and transformation into ZnO. The zinc acetate dehydrate precursor was first solvated in methanol, and then hydrolyzed, regarded as removal of the intercalated acetate ions and results in a colloidal gel of zinc hydroxide according to reaction (A). The size and activity of solvent influences the reactivity progress and final product. Methanol has small size and more active –OH and –OCH₃ groups. Methanol can react more easily to form a polymer precursor with high polymerization degree, which is required to convert sol into gel.⁵² These zinc hydroxide splits into Zn²⁺ cation and OH⁻ anion according to reaction (B) and followed by polymerization of hydroxyl complex to form Zn–O–Zn bridge and finally transformed into ZnO (reaction (C)).⁵³

Zn(CH₃COO)₂·2H₂O + 2NaOH → Zn(OH)₂ + 2CH₃COONa + 2H₂O (A)

Zn(OH)₂ + 2H₂O → Zn(OH)₄²⁻ + 2H⁺ (B)

Zn(OH)₄²⁻ ↔ ZnO + H₂O + 2OH⁻ (C)

We have observed that the growth of ZnO nanoparticles does not proceed, when the pH value of solution is below 6, because of the lack of Zn(OH)₂ formation in the solution. Since pH value of solution controls the rate of ZnO formation, it affects the size and stability. The largest size was observed, when the pH of solution was about 9. Further increase in the pH value of sol gel leads to a decrease in the size of the crystallite nanoparticles. This may be attributed to the dissolution of ZnO (back reaction equation (C)). When ZnO reacts with OH⁻, the dissolution of ZnO occurs.

Photovoltaic properties of DSSCs

Fig. 2 shows the current–voltage characteristics of the DSSCs fabricated with ZnO nanocrystalline ZnO powder having different pH values, under illumination intensity of 100 mW/cm². The photovoltaic parameters estimated from these curves are summarized in Table 2. It can be seen from the table that the open circuit voltage (V_oc) is the same for all devices, whereas the short circuit current (J_sc), fill factor and PCE for the devices fabricated with ZnO nano-crystalline photoanode having pH 9 are higher than other devices. The better solar cell performance observed for this device may be attributed to the high quality nanocrystalline ZnO powder obtained from the solution having pH value of 9 and can be understood through the growth mechanism of nanocrystalline ZnO , as discussed above.

Table 2 Effect of pH value of ZnO sol gel on the photovoltaic performance of the quasi-solid-state DSSCs

pH value of ZnO	Short circuit photocurrent (Jsc) (mA/cm^2)	Open circuit voltage (Voc) (V)	Fill factor (FF)	Power conversion efficiency (η) (%)
7	1.38	0.60	0.45	0.38
8	3.0	0.61	0.48	0.88
9	4.22	0.61	0.52	1.34
10	3.76	0.61	0.48	1.11

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Fig. 2 Current–voltage characteristics of DSCs using photoelectrode of ZnO synthesized at (A) pH 7 (B) pH 8 (C) pH 9 and (D) pH 10.

The incident photon to current efficiency of the DSSCs have been estimated using the following equation:

where J_sc (mA/cm²) is photocurrent density under short circuit condition, λ (nm) and P_in (W/m²) are wavelength and incident illumination intensity of monochromatic light. The IPCE spectra of the DSSCs fabricated on ZnO nano-crystalline thin films derived from the sol gel having pH values 7 and 9 are shown in Fig. 3. It can be seen from this figure that the IPCE value is higher for the pH 9 than that for 7. The IPCE for a DSSC can also be expressed as

IPCE(λ) = LHE(λ)ϕ_inη_c

where LHE (λ) is the light harvesting efficiency of the photoelectrode, ϕ_in is the charge injection efficiency from the excited state of dye into conduction band of the nano-crystalline semiconductor, and η_c is the collection efficiency of the injected electron by the electrode. The crystallite size of ZnO nano-crystalline film deposited from the sol gel having pH 9 is higher than that for pH 7. Therefore, the PCA loading for former electrode is higher, resulting enhancement in the LHE. We conclude that the enhancement in LHE and the injection efficiency of electrons from the excited state of PCA are responsible for the increase in IPCE.

Fig. 3 IPCE spectra of the DSSCs fabricated on ZnO electrode with pH 7 and pH 9.

Effect of gold nanoparticles layer deposited on the surface of nano-crystalline ZnO electrode

To further increase the photovoltaic performance of the DSSCs, we have used ZnO (pH = 9) photoanode coated with gold nanoparticles. Gold nanoparticles (NP) are one of the nano-systems in which light absorption in the visible region depends on the NP size due to the collective electron oscillations at the surface.⁵⁴ Recently gold nanoparticles have been investigated for their photocatalytic,⁵⁵ and potential photovoltaic applications.⁵⁶ Tetsu et al., studied the enhancement of anodic photocurrent induced by visible light irradiation in a device structure based on gold nanoparticles.⁵⁷

The current–voltage characteristics under illumination intensity of 100 mW/cm² are shown in Fig. 4. It was found that upon illumination, the device with ZnO surface coated with Au nanoparticles yields a V_oc of 0.68 V and a J_sc of 5.2 mA/cm² which are significantly higher than that for cell consisting of only bare ZnO photoanode. As a result the PCE was increased from 1.34% to 1.91%. The increase in the PCE can be understood by the following discussion.

Fig. 4 Current–voltage characteristics under illumination for DSSCs with (A) bare ZnO and (B) ZnO coated with Au nanoparticles.

We assume that the electron transfer from the excited dye into ZnO nano-crystal surface occurs via a combination of two possible mechanisms. In the first mechanism the electrons from lowest unoccupied molecular orbital of dye are transported to the conduction band ZnO via tunneling effect across a thin layer of Au nanoparticles. The second mechanism involves two steps: First the FTO/ZnO/Au/dye electrode, upon excitation with visible light being predominantly absorbed by the dye, injects electrons into the gold nanoparticles as indicated in Fig. 5. We assumed that the injected electrons are accumulated in the gold nanoparticles, raising their Fermi levels towards more negative potentials until the matching with the Fermi level of ZnO leads to a quick transport of electrons from gold to ZnO.⁵⁸ The electrons transferred to the ZnO surface are collected by FTO electrode, and thereby generate the photocurrent. Additionally, the Schottky barrier formed at the Au–ZnO interface reduces the charge recombination by blocking the transfer of electrons from ZnO to the dye and/or electrolyte thus improving the PCE of the DSSC. The generation of photocurrent in FTO/ZnO/Au/dye electrode in electrolyte can be illustrated as follows:

Fig. 5 Energy level diagram and mechanism of photocurrent generation in the DSSC with FTO/ZnO/Au/PCA/polymer electrolyte.

The improvement in the PCE can also be attributed to the suppression of charge recombination by the gold nanoparticle layer. The thin Au nanoparticle layer blocks the electron flow in the back direction (tri-iodides of redox electrolyte and cation of the dye), which contributes to the increase in the V_oc.

Effect of nano-fillers in the polymer electrolyte

We have investigated the effect of TiO₂ nano-fillers in the electrolyte on photovoltaic response of DSSC fabricated with Au coated ZnO (pH 9) photoelectrode. Fig. 6 demonstrates the dark current–voltage characteristics curves of DSSC as a function of nanoparticles concentration in electrolyte. The dark current results from the reduction of the tri-iodide by the conduction band electrons of the nano-crystalline wide band semiconductor used in the photoanode. The onset of the dark current of the DSSC with the various electrolytes occurs at 0.25, 0.28, 0.32 and 0.38 V for 0%, 2%, 6% and 8% wt, respectively. The onset of the dark current of DSSC with 8 wt% of NP occurs at the highest forward bias, which indicates that the 8 wt% NP can most efficiently suppress the dark reaction.

Fig. 6 Effect of concentration of nano-filler in electrolyte on the dark current–voltage characteristics of DSSCs.

Fig. 7 shows the J–V characteristics of the DSSC under illumination intensity of 100 mW/cm² as a function of nanoparticle content in electrolyte. The photovoltaic characteristics are summarized in Table 3. It is found that the PCE (η) has been significantly enhanced by the composite electrolyte. With the increase of NP content to 8 wt%, the J_sc and V_oc are both enhanced, reaching their maximum of 0.74 and 7.0 mA/cm² and 0.74 V, respectively. As the content of NP increases above 8 wt%, both V_oc and J_sc decrease.

Table 3 Effect of TiO₂ nano-filler in the polymer gel electrolyte on photovoltaic performance of DSSCs

wt% of TiO2	Short circuit photocurrent (Jsc) (mA/cm^2)	Open circuit voltage (Voc) (V)	Fill factor (FF)	Power conversion efficiency (η) (%)
0	5.2	0.68	0.54	1.91
5	5.7	0.70	0.55	2.2
8	7.0	0.74	0.58	3.0
10	6.35	0.70	0.55	2.44

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Fig. 7 Current–voltage characteristics of the DSSCs with polymer electrolyte having different concentration of TiO₂ nano-fillers.

According to the electrochemical impedance spectroscopy (EIS) model of DSSC,⁵⁹ the electron life time (τ_e) in the nano-crystalline thin film can be obtained from the characteristics angle frequency (ω_mid) of the mid frequency (f_mid) peak in the EIS bode phase plots, by the expression τ_e = 1/ω_mid = 1/2πf_mid.

We have measured bode phase plots for DSSCs with different concentrations of NP in electrolyte. It has been observed that the mid frequency peak in bode phase plots shifted towards the lower frequencies as the concentration of NP increased in the electrolyte. The bode phase plots of DSSC with 8% NP contents and without NP in electrolyte under illumination are shown in Fig. 8. The DSSC fabricated with composite electrolyte with 8 wt% of NP has minimum f_mid 12 Hz as compared to without NP (f_mid = 126 Hz), which corresponds to the maximum electron life time about 13 ms. The f_mid of the DSSC with 8 wt% NP decreased by a factor of 10.5 as compared to the DSSC without NP, suggesting the τ_e is lengthened significantly.

Fig. 8 Bode plots of EIS spectra for DSSCs with (8 wt%) and without TiO₂ nanoparticles in polymer electrolyte.

To understand the influence of NP on the photovoltaic properties of DSSC, the electrochemical properties of the composite electrolyte were studied according to the thin layer cell model.⁶⁰Fig. 9 shows the dependence of the diffusion coefficient of tri-iodide (D_I3⁻) and exchange current density (J_o) on the NP content. D_I3⁻ is derived from the limiting current measurement⁶⁰ by the expression

D_I3⁻ = J_limd/2nFC_I3⁻

where J_lim is the limiting current density, n is the number of electrons transferred in each reaction (here n = 2), d is the distance between the two electrodes (here d = 50 µm), and C_I3⁻ is the concentration of I₃⁻ (here C_I3⁻ = 0.05 M). J_o can be obtained from the charge transfer resistance (R_ct) by the expression

J_o = RT/nFR_ct

where R is the gas constant, T is the Kelvin temperature (here T = 300 K) and F is the Faraday constant.

Fig. 9 Dependence of diffusion coefficient of tri-iodide and exchange current density on TiO₂ content.

Fig. 9 shows that both the diffusion coefficient and J_o of the composite electrolyte with NP additive are significantly higher than that of pure ionic liquid electrolyte. When the content of NP is 8 wt %, diffusion coefficient and J_o are both up to maximum 1.56 · 10⁻⁷ cm²/s and 0.68 mA/cm². The intercalation behavior of TBP molecule into NP can be responsible for the enhancement of J_o and diffusion coefficient. The enhancement of the J_o can be due to the increase of the active area of electrolyte/NP interface after the intercalation of TBP molecule into NP. The confinement effect by the nano-channels in the interlayer of NP may cause the enhancement of diffusion coefficient.

For a regenerative photoelectrochemical system, the V_oc can be expressed as⁶¹

where I_inj is the flux of injected charge, n_cb is the concentration of electron at the surface of nano-crystalline photoanode and k_et is the rate constant for I₃⁻ reduction. The enhancement of J_o and diffusion coefficient decreases the C_I3⁻, which therefore, leads to an increase in V_oc.

Under illumination, short circuit photocurrent of the DSSC can be given by equation J_sc = J_inj − J_r, where J_inj is the flux of injected electron and J_r is the I₃⁻ reduction current. The J_r can also be expressed as:

Here, k_r is the rate constant of the reduction reaction, and the exponents γ and β are the dark reaction orders, respectively.⁶² The decrease of C_I⁻3 which is derived by the enhancement of J_o and D_I⁻3, can depress the dark reaction, and then lengthen electron life time. Hence, the suppression of the dark reaction can efficiently reduce J_r and hence improve J_sc.

Conclusions

The perylene monoimide PCA was successfully synthesized by a three-step synthetic route. The cyclohexyl ring and the aliphatic chains enhanced the solubility of PCA. It showed satisfactory thermal stability and relatively low glass transition temperature. The absorption curves of PCA were broad with maximum around 500 nm and optical band gap of 1.93 eV. We have fabricated the quasi-solid-state DSSCs using ZnO anode deposited from the different values of pH sol. It was found that the highest PCE is for the DSSCs which were fabricated on ZnO nano-crystalline film having pH value of 8. Since the ZnO nanoparticles derived from the sol with pH 9 show highest crystallite size of 16 nm, as more dye has been adsorbed by them.

The enhancement in photovoltaic performance of DSSC fabricated with gold coated ZnO photoanode is probably associated with injection of electrons from gold nanoparticles into ZnO and simultaneous tunneling of electrons from dye PCA to ZnO. We have also reported the effect of the addition of TiO₂ nanoparticles in polymer gel electrolyte on the photovoltaic performance of DSSCs. The intercalation behavior of polymer gel with TiO₂ NP can contribute to the enhancement of exchange charge density and diffusion coefficient of tri-iodide, which will depress the concentration of I₃⁻. Furthermore, the polymer gel electrolyte with TiO₂ can efficiently suppress the dark reaction. Hence, J_sc, V_oc and corresponding PCE of DSSC are significantly enhanced. The increase in the electron lifetime also supports the increase in PCE.

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