Dye-sensitized solar cells based on ZnO nanotetrapods

Abstract

In this paper, we reviewed recent systematic studies of using ZnO nanotetrapods for photoanodes of dye-sensitized solar cells (DSSCs) in our group. First, the efficiency of power conversion was obtained by more than 3.27% by changes of conditions of dye loading and film thickness of ZnO nanotetrapod. Short-circuit photocurrent densities (J_sc) increased with the film thickness, J_sc would not be saturation even the film thickness was greater than 35 μm. The photoanode architecture had been charactered by good crystallinity, network forming ability, and limited electron-hopping interjunctions. Next, DSSCs with high efficiency was devised by infiltrating SnO₂ nanoparticles into the ZnO nanotetrapods photoanodes. Due to material advantages of both constituents described as above, the composite photoanodes exhibited extremely large roughness factors (RFs), good charge collection, and tunable light scattering properties. By varying the composition of the composite photoanodes, we had achieved an efficiency of 6.31% by striking a balance between high efficiency of charge collection for SnO₂ nanoparticles rich films and high light scattering ability for ZnO nanotetrapods rich films. An ultrathin layer of ZnO was found to form spontaneously on the SnO₂ nanoparticles, which primarily was responsible for enhancing open-circuit photovoltage (V_oc). We also identified that recombination in SnO₂/ZnO composite films was mainly determined by ZnO shell condition on SnO₂, whereas electron transport was greatly influenced by the morphologies and sizes of ZnO crystalline additives. Finally, we applied the composite photoanodes of SnO₂ nanoparticles/ZnO nanotetrapods to flexible DSSCs by low temperature technique of “acetic acid gelation-mechanical press-ammonia activation.” The efficiency has been achieved by 4.91% on ITO-coated polyethylenenaphtalate substrate. The formation of a thin ZnO shell on SnO₂ nanoparticles, after ammonia activation, was also found to be critical to boosting V_oc and to improving inter–particles contacts. Mechanical press, apart from enhancing film durability, also significantly improved charge collection. ZnO nanotetrapods had been demonstrated to be a better additive than ZnO particles for the improvement of charge collection in SnO₂/ZnO composite photoanodes regardless of whether they were calcined.

Keywords

dye-sensitized solar cell (DSSC) / metal oxides / nanostructure / ZnO nanotetrapod / photoanode / flexible solar cell

1 Introduction

Recently, great efforts have been devoted to develop efficient solid-state emitters for their various potential applications in organic light emitting diodes (OLEDs) [1], organic lasers [2], fluorescent sensors [3], etc. Many conventional π-conjugated fluorophores, however, suffer from the notorious aggregation-caused quenching (ACQ) effect: they emit strongly in the dilute solutions but become faintly fluorescent in the condense phase due to the formation of detrimental species such as excimers [4], which has obstructed their high-technological applications because the fluorophoric molecules are commonly used as solid films or nanoparticles in the real-world applications. To solve this ACQ problem, various molecular engineering approaches and device fabrication techniques had been proposed, but these attempts often ended with only limited success and even led to some side effects in many cases [5,6].

In 2001, an abnormal phenomenon, termed as aggregation-induced emission (AIE), was reported by Tang’s group [7], which had been proved to be an effective approach to solve the ACQ problem. The AIE fluorophores are almost non-luminescent in dilute solutions but exhibit efficient emissions in the aggregated state. A series of designed experiments and the theoretical calculations were performed, and restriction of intramolecular rotations (RIR) was rationalized to be the main working mechanism behind this novel photophysical phenomenon [8]. Among the typical AIE fluorophores, tetraphenylethene (TPE) and many of its derivatives enjoy the advantages of easy synthesis and outstanding AIE characteristic. Plenty of TPE derivatives can fluoresce intensely in solid state, and have been extensively used to fabricate efficient non-doped OLEDs [9-11].

As a typical heterocyclic molecule, imidazole has several substitution positions (N1, C2, C4 and C5), and many of its derivatives have been used extensively in OLEDs with high electroluminescence (EL) efficiencies, and good CIE coordinates in EL spectra [12-14]. In this work, we designed and successfully synthesized three new fluorophores (1-3) (Scheme 1) by melting the 1, 2, 4, 5-tetraphenyl-1H-imidazole (TPI) group and TPE unit in different patterns. The photoluminescence (PL) and EL properties of these new fluorophores were investigated. The OLEDs using the fluorophores as light-emitting layers were fabricated, which showed varied EL emission color from bluish green to deep blue, with moderate efficiencies.

Fig.1 Scheme 1 Molecular structures and synthesis of the new fluorophores

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2 Experimental

2.1 Materials and instruments

Tetrahydrofuran (THF) was distilled from sodium benzophenone ketyl under dry nitrogen immediately prior to use. Compounds 4-(1,2,2-triphenylvinyl)benzaldehyde (5) [15] and 4-(1,2,2-triphenylvinyl)aniline (7) [16] were prepared according to literature methods. All other chemicals and reagents were purchased from J&K Scientific Ltd., and used as received without further purification. ¹H and ¹³C nuclear magnetic resonance (NMR) spectra were measured on a Bruker AV 500 or 400 spectrometer in deuterated chloroform using tetramethylsilane (TMS; δ = 0) as internal reference. UV-vis absorption spectra were measured on a Shimadzu UV-2600 spectrophotometer. PL spectra were recorded on a Horiba Fluoromax-4 spectrofluorometer. Cyclic voltammetry (CV) was performed at room temperature by using a standard three-electrode electrochemical cell. The working and reference electrodes are platinum and saturated calomel electrode (SCE), respectively, with 0.1 M<FootNote>

1M= 1 mol·L^-1

</FootNote> tetrabutylammonium hexafluorophosphate in acetonitrile solution as the supporting electrolyte at a scan rate of 50 mV· s^-1. The ground-state molecular geometrics were fully optimized and the electronic structures were investigated in THF solvent at the B3LYP/6-31G(d,p) level using the Gaussian 09 program. The polarizable continuum model (PCM) was employed for taking the solvent effect into account.

2.2 Synthesis

1,4,5-Triphenyl-2-(4-(1,2,2-triphenylvinyl)phenyl)-1H-imidazole (1): A mixture of 9,10-phenanthrenequinone (4) (0.42 g, 2 mmol), 5 (0.72 g, 2 mmol), aniline (6) (0.18 mL, 2 mmol) and ammonium acetate (1.28 g, 10 mmol) in glacial acetic acid (50 mL) was heated at 140°C under a nitrogen atmosphere for 12 h. After cooling to room temperature, the reaction mixture was poured into a methanol solution with stirring. The separated solid was filtered off, washed with methanol, and dried to give the raw product, which was further purified by silica-gel column chromatography using petroleum ether/dichloromethane as eluent. White solid was obtained in 93% yield. ¹H NMR (500 MHz, CDCl₃), δ (TMS, ppm<FootNote>

1 ppm= 1×10^-6

</FootNote>): 7.61 (d, J = 7.5 Hz, 2H), 7.27–7.19 (m, 11H), 7.15–7.09 (m, 11H), 7.03–7.01 (m, 8H), 6.93 (d, J = 8.5 Hz, 2H). ¹³C NMR (125 MHz, CDCl₃), δ (TMS, ppm): 146.81, 143.59, 143.54, 143.33, 141.46, 140.41, 137.00, 131.43, 131.35, 131.30, 131.14, 130.79, 128.99, 128.41, 128.36, 128.29, 128.16, 128.00, 127.69, 127.66, 127.46, 126.54, 126.51. HRMS (C₄₇H₃₄N₂): m/z 626.2609 (M⁺, calcd 626.2722).

2,4,5-Triphenyl-1-(4-(1,2,2-triphenylvinyl)phenyl)-1H-imidazole (2): The procedure was analogous to that described for 1. Pale brown solid, yield 85%. ¹H NMR (400 MHz, CDCl₃), δ (TMS, ppm): 7.60 (d, J = 6.8 Hz, 2H), 7.44 (d, J = 6.0 Hz, 2H), 7.27–7.21 (m, 9H), 7.09 (br, 11H), 6.99–6.91 (m, 8H), 6.77 (d, J = 8.0 Hz, 2H). ¹³C NMR (100 MHz, CDCl₃), δ (TMS, ppm): 146.72, 144.19, 143.26, 143.15, 143.03, 141.91, 139.58, 135.02, 132.11, 131.31, 131.19, 131.14, 130.83, 129.02, 128.39, 128.26, 128.20, 128.04, 127.97, 127.83, 127.77, 127.48, 126.79, 126.74. HRMS (C₄₇H₃₄N₂): m/z 626.2708 (M⁺, calcd 626.2722).

4,5-Diphenyl-1,2-bis(4-(1,2,2-triphenylvinyl)phenyl)-1H-imidazole (3): The procedure was analogous to that described for 1. Green solid, yield 80%. ¹H NMR (500 MHz, CDCl₃), δ (TMS, ppm): 7.65 (br, 2H), 7.35–7.27 (m, 6H), 7.18–7.02 (m, 32H), 6.95 (d, J = 7.0 Hz, 6H), 6.73 (d, J = 8.0 Hz, 2H). ¹³C NMR (125 MHz, CDCl₃), δ (TMS, ppm): 131.42, 131.38, 131.29, 131.18, 131.13, 127.83, 127.71, 126.73. HRMS (C₆₇H₄₈N₂): m/z 880.3798 (M⁺, calcd 880.3817).

2.3 Device fabrication

The devices were fabricated on 80 nm Indium Tin Oxide (ITO)-coated glass with a sheet resistance of 25Ω/□. Prior to loading into the pretreatment chamber, the ITO-coated glass was soaked in ultrasonic detergent for 30 min, followed by spraying with de-ionized water for 10 min, soaking in ultrasonic de-ionized water for 30 min, and oven-baking for 1 h. The cleaned samples were treated by perfluoromethane plasma with a power of 100 W, gas flow of 50 sccm, and pressure of 0.2 Torr for 10 s in the pretreatment chamber. The samples were transferred to the organic chamber with a base pressure of 7 × 10^-7 Torr for the deposition of NPB, emitter, and TPBi, which served as hole-transport, light-emitting, and electron-transport layers, respectively. The samples were then transferred to the metal chamber for cathode deposition which composed of lithium fluoride (LiF) capped with aluminum (Al). The light-emitting area was 4 mm². The current density-voltage characteristics of the devices were measured by a HP4145B semiconductor parameter analyzer. The forward direction photons emitted from the devices were detected by a calibrated UDT PIN-25D silicon photodiode. The luminance and external quantum efficiencies of the devices were inferred from the photocurrent of the photodiode. The electroluminescence spectra were obtained by a PR650 spectrophotometer. All measurements were carried out under air at room temperature without device encapsulation.

3 Results and discussion

3.1 Optical property

Figure 1(a) shows the absorption spectra of these new fluorophores in THF solutions. 1 and 2 exhibit absorption maxima at 337 and 290 nm, respectively, indicating that 2 possesses shorter effective conjugation length compared to 1. Although the maximum absorption peak of 3 is located at 307 nm, its absorption spectral profile presents a long and strong tail at the longer wavelength region, which makes 3 and 1 have the same optical bandgaps (E_opt = 3.20 eV, Table 1) estimated from the onset wavelength of their absorption spectra, indicative of the similar conjugation length of both molecules.

The emissions of these new fluorophores are very weak in solutions. Their PL spectra in dilute THF solutions (10^-5 M) only exhibit noisy signals without discernible peaks. And the fluorescence quantum yields (Φ_F) of 1, 2 and 3 are merely 0.1%, 1.3% and 1.0% (Table 1), respectively, manifesting that they are indeed weak emitters when molecularly dissolved in good solvent. However, they emit very strong fluorescence when fabricated into thin solid films. As shown in Fig. 1(b), the emission maxima of 1 and 3 in solid films are located at 490 and 482 nm, being red-shifted relative to that of 2 in solid film (469 nm) due to their better conjugations. The Φ_F values of 1, 2 and 3 in solid films are increased to 72.8%, 37.0% and 50.7%, respectively, revealing that they are AIE-active and excellent solid-state light emitters. This makes them promising candidates for the fabrication of efficient OLED devices. Understandably, owing to the presence of TPE units, the intramolecular rotation is active in these fluorophores when they are molecularly dissolved in THF solutions, which effectively deactivates the excited state via a nonradiative relaxation channel. The intramolecular rotations, however, are restricted by steric constraint in solid films, and thus the nonradiative decay channel is blocked, rendering the molecules emissive in solid films.

Fig.2 (a) Absorption spectra in THF solutions (10-5 M) and (b) PL spectra in solid films of the new fluorophores

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Tab.1 Optical properties and energy levels of the new fluorophores

	λabsa)/nm	λemb)/nm	ΦFc)/%	(HOMO/LUMO)d) /eV	Eopte)/eV
soln	film
1	337	490	0.1	72.8	-5.51 (-5.27)/-2.31 (-1.48)	3.20
2	290	469	1.3	37.0	-5.78 (-5.44)/-2.30 (-1.48)	3.48
3	307	482	1.0	50.7	-5.53 (-5.26)/-2.33 (-1.50)	3.20

Notes: a) In THF solution (10-5 M). b) Spin-coated film. c) Fluorescence quantum yield determined by a calibrated integrating sphere. d) Experimental data determined by cyclic voltammetry and calculated values given in parentheses. e) Optical bandgap energy determined from the onset of absorption spectra. HOMO, highest occupied molecular orbital; LUMO: lowest unoccupied molecular orbital

3.2 Electrochemical property

The electrochemical properties of these new fluorophores were investigated by CV. The voltammograms are presented in Fig. 2 and the obtained energy levels are summarized in Table 1. 1 and 3 exhibit oxidation onset potentials ($E_{\mathrm{o}\mathrm{n}\mathrm{s}\mathrm{e}\mathrm{t}}^{\mathrm{o}\mathrm{x}}$) at 1.11 and 1.13 V, respectively, both of which are lower than that of 2 (1.38 V). According to the following equation: HOMO= - (4.4+ $E_{\mathrm{o}\mathrm{n}\mathrm{s}\mathrm{e}\mathrm{t}}^{\mathrm{o}\mathrm{x}}$) eV, the HOMO energy levels are calculated to be -5.51, -5.78 and -5.53 eV for 1, 2 and 3, respectively. Their LUMO energy levels [LUMO= - (HOMO+ E_opt) eV] could be determined from HOMO and E_opt values. As a result, these three fluorophores give similar LUMO energy levels: - 2.31 eV for 1, - 2.30 eV for 2 and - 2.33 eV for 3.

Fig.3 CV curves of the new fluorophores in acetonitrile solution with 0.1 M tetrabutylammonium hexafluorophosphate as the supporting electrolyte at a scan rate of 50 mV·s-1

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3.3 Theoretical calculation

To further study the photophysical properties of these new fluorophores, theoretical calculations were performed using the Gaussian 09 program. The molecular geometrics were fully optimized and electronic structures were investigated at the ground state (S₀) in THF solvent at the B3LYP/6-31G(d,p) level [17]. The PCM was employed for taking the solvent effect into account. As we all know, the HOMO and LUMO are two very important orbitals for determining the photophysical properties of the fluorophores. Hence, the electron density contours of HOMOs and LUMOs for these new fluorophores are illustrated in Fig. 3 and the corresponding orbital energy levels are summarized in Table 1. We found that the calculated values are basically consistent with the experimental data, which indicates this computational method is reasonable. The optimized molecular structures reveal that the torsion angles between TPE and TPI vary greatly in 1 (29.4°) and 2 (84.8°). As indicated in Scheme 1, when TPE is located at the 1-position of imidazole ring, the torsion angles are much larger than those when TPE are linked to the 2-positions of imidazole ring. This is indicative of a better conjugation of 1 and 3 than 2. As shown in Fig. 3, the HOMOs of 1 and 3 are mainly located on the central imidazole core and 4,5-positioned phenyl rings and 2-positioned TPE unit, which endows them with similar HOMO energy levels. The HOMO of 2, however, allows the electrons to be only delocalized over the central imidazole core and 2,4,5-positioned phenyl rings. Obviously, 1 and 3 have more extended conjugation than 2, resulting in a much lower HOMO energy level of 2 than those of 1 and 3. The LUMOs of the 1, 2 and 3 are mainly localized on TPE unit(s). Based on above reasonable explanations, the much redder absorption and emission maxima of 1 and 3 relative to 2 become understandable. In comparison with TPE, TPI appears to be an electron-donating group. To confirm this, the energy levels of TPE and TPI are further calculated individually. According to the calculation results, the TPI unit has a higher HOMO energy level (-5.22 eV) than TPE (-5.33 eV), while TPE possesses a lower LUMO energy level (-1.22 eV) than TPI (-0.85 eV). Therefore, TPI and TPE units should serve as electron-donor and acceptor in these molecules, respectively.

Fig.4 Calculated molecular orbital amplitude plots of HOMOs and LUMOs for the new fluorphores

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3.4 Electroluminescence

The efficient emissions of 1, 2 and 3 films encourage us to study their EL properties. Non-doped OLED devices with a configuration of ITO/NPB (60 nm)/EML (20 nm)/TPBi (40 nm)/LiF (1 nm)/Al (100 nm) were fabricated, where N,N′-di(1-naphthyl)-N,N′-diphenyl-benzidine (NPB) acted as a hole-transporting layer (HTL), 2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1H-benzimidazole) (TPBi) served as an electron-transporting layer (ETL) and the new fluorophores functioned as light-emitting layers (EML). Figure 4 shows the EL spectra, luminescence-voltage-current density characteristics and current efficiency versus current density curves of the OLEDs based on 1, 2 and 3. And the EL performance data are also summarized in Table 2. The EL devices based on 1, 2 and 3 emit at 467, 445 and 495 nm with CIE chromaticity coordinates of (0.17, 0.22) (sky blue), (0.16, 0.15) (deep blue) and (0.21, 0.38) (bluish green), respectively. The EL peak of 3 is only slightly red-shifted from the PL peak (482 nm) of its solid film, confirming that the EL is indeed from the emitting layer. The EL spectra of 1 and 2, however, are obviously blue-shifted from their PL spectra in solid films, implying that partial crystallization may happen in the emitting layer of the devices of 1 and 2, because crystallization will cause blue-shifted emission in many TPE derivatives [18-20]. The devices of 1 and 3 show better performances than that of 2 due to the more efficient solid-state emission of 1 and 3. The device of 1 is turned on at a low voltage of 3.6 V and exhibits a maximum luminance (L_max) of 5560 cd·m^-2, a maximum current efficiency (η_C,max) of 3.12 cd·A^-1, a maximum power efficiency (η_P,max) of 2.72 lm·W^-1 and a maximum external quantum efficiency (η_ext,max) of 1.77%. The device based on 3 shows a similar performance compared to that of 1, with turn-on voltage (V_on), L_max, η_C,max, η_P,max and η_ext,max of 4.4 V, 5110 cd·m^-2, 3.97 cd·A^-1, 2.43 lm·W^-1, 1.58%, respectively. Since the conjugation of 2 is disrupted, its OLED device emits at deep blue region and shows inferior EL data (3620 cd·m^-2, 0.96 cd·A^-1, 0.75 lm·W^-1 and 0.72%). According to the above results, the EL emission is tuned from bluish green to deep blue, realizing the control of EL emission and efficiency through a very simple strategy of minor structural modification.

Fig.5 (a) EL spectra; (b) current density-voltage-luminance characteristics; and (c) changes in current efficiency with the current density in multilayer EL devices of these new fluorophores [ITO/NPB (60 nm)/EML (20 nm)/TPBi (40 nm)/LiF (1 nm)/Al (100 nm)]

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Tab.2 EL performance of the new fluorophoresa)

EML	λEL/nm	Von/V	Lmax/(cd·m-2)	ηC/(cd·A-1)	ηP/(lm·W-1)	EQE/%	CIE(x, y)
1	467	3.6	5560	3.12	2.72	1.77	(0.17, 0.22)
2	445	3.9	3620	0.96	0.75	0.72	(0.16, 0.15)
3	495	4.4	5110	3.97	2.43	1.58	(0.21, 0.38)

Notes: a) With a device configuration of ITO/NPB/EML/TPBi/LiF/Al. Abbreviation: EML= light-emitting layer, λEL = EL maximum, Von = turn-on voltage at 1 cd·m-2, Lmax = maximum luminance, ηC = maximum current efficiency, ηP = maximum power efficiency, and EQE= maximum external quantum efficiency

4 Conclusions

In summary, three new fluorophores consisting of TPE and TPI units were prepared and their optical properties were investigated. They show very weak emissions when molecularly dissolved in dilute solutions, but they are induced to emit intensely in the solid films, demonstrating that they possess AIE characteristics. 1 and 3 exhibit longer effective π-conjugation lengths than 2, which results in redder PL emissions and higher fluorescence quantum yields in the solid state. Non-doped EL devices of were fabricated using these fluorophores as emitters, and blue EL emissions with varied EL efficiencies were obtained. The impacts of a minor structural alternation on the PL and EL properties of the fluorophores have been demonstrated.

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Acknowledgements

This work was supported by the Research Grants Council of Hong Kong under the General Research Funds (No. 604809). We wish to thank all those who contributed to the work reviewed here.

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