Dye sensitized solar cell with lawsone dye using a ZnO photoanode: experimental and TD-DFT study

Abstract

The spectral features of lawsone (2-hydroxy-1,4-naphthoquinone), an active component of the natural dye henna, are analyzed in ethanol using experimental and computational methods. The calculated UV-Vis absorption spectrum from the time-dependent density functional theory (TD-DFT) approach is compared with the experimental results, allowing a detailed assignment of the UV-Vis spectral features based on molecular orbitals. Further, we have analyzed the light intensity dependent J–V characteristics and electrochemical impedance spectrum of a dye sensitized solar cell fabricated with lawsone and a ZnO photoanode. The photovoltaic data of the sensitizer adsorbed on ZnO films exhibited a reasonable power conversion efficiency, i.e. 0.68% at 26 mW cm⁻² light intensity.

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1. Introduction

Solar energy, with its unlimited quantity, is expected to be one of the most promising alternative energy sources for the future generation. Research and development of solar cells with low costs, high conversion efficiencies and low feedstock consumptions are currently required. To date, technology using silicon solar cells dominates the commercial market due to its high performance and good stability as compared to thin film solar cell technology.^1–3 The dye sensitized solar cell (DSSC), a low cost, biocompatible, non-toxic and readily available manufacturing technology, is considered to be an economically viable and environment friendly alternative to nanocrystalline silicon solar cell devices. A conventional DSSC (Grätzel cell) consists of a self-assembled monolayer of molecular dye at the interface between a mesoporous wide band gap semiconductor oxide and a liquid electrolyte.⁴

ZnO is a promising alternative photoanode to other semiconductors like TiO₂, SnO₂, etc.^5,6 It exhibits a unique combination of potentially interesting properties such as high bulk electron mobility (205–300 cm² V⁻¹ s⁻¹) and the richest variety of nanostructures, based on a very wide range of synthetic routes. These combined properties open wide possibilities in DSSC design. ZnO showed the first experimental evidence of irreversible electron injection from organic molecules into the conduction band of a wide band gap semiconductor.⁷

2-Hydroxy-1,4-naphthoquinone, commonly known as lawsone, is a component of a natural dye (henna), extracted from the leaves of Lawsonia alba and Lawsonia inermis. Lawsone has been used as a dye in the cosmetics industry to colour hair, nails and skin. It is also being used as a fingerprint detecting reagent.⁸ Lawsone can be used as an electron mediator in a biochemical fuel cell.⁹ Molecules of lawsone form a polymeric structure through O–H⋯O hydrogen bonding. It is a redox active molecule and can be reversibly converted into naphthosemiquinone and catechol by the acceptance or removal of electron(s). The most important feature of the lawsone molecule is that it absorbs visible light between 400–600 nm. It has been shown theoretically that there is percolation of electron density in lawsone molecules through intermolecular hydrogen bonding.¹⁰ Thus, the above properties could suggest lawsone as a good potential candidate for light harvesting materials. Recently, Jasim et al.¹¹ fabricated a TiO₂ DSSC using Bahraini and Yemeni henna extract, in which lawsone is one of the active components, as a sensitizer. The authors have observed a 0.45% power conversion efficiency by using 8.0 g Bahraini lawsone extract.

During the functioning of a DSSC, when the cell is illuminated, a dye molecule can absorb an incident photon and get promoted to an excited state. This excited dye molecule injects an electron to the conduction band edge of the semiconductor. Oxidized dye molecules are regenerated by the reduced redox species I⁻, returning the dye to its ground state and allowing it to absorb another photon. Photoelectrode films with a nanostructured semiconductor can significantly enhance the solar cell performance by offering a large surface area for dye adsorption, direct transport pathways for photoexcited electrons and efficient scattering centers for enhanced light harvesting efficiency.¹²

To assess the UV-Vis spectral features of many organic dye molecules, the time-dependent density functional theory (TD-DFT) approach has been extensively applied in the literature.^13–18 Recently, various dye sensitizers for DSSC have also been investigated using the TD-DFT method.^19,20 Further, TD-DFT calculations have successfully explored the solvent effect for the accurate prediction of transition energies of excited state molecules.¹³ The main idea behind the present computational study is to gain better insights into the electronic and spectral features of the lawsone.

Perpète et al.¹⁶ have studied various derivatives of 1,4-naphthoquinone (1,4-NQ) dye using the TD-DFT method. They have investigated the absorption spectra of 1,4-NQ dyes considering various basis sets and revealed that the 6-311+G(2d,p) basis set is adequate for the theoretical treatment of these dyes. It has also been demonstrated that PBE0 provides quite a good description of the absorption energies^15–18 of the systems. Further, to represent the solvent effect, the conductor polarizable continuum model (C-PCM) has been found to be quite promising.^15,16

Henna dye has been previously investigated in TiO₂ based dye sensitized solar cells.¹¹ Since this dye absorbs light in the UV-Visible range at three different wavelengths, viz. 287, 330 and 458 nm, it can potentially act as a light absorbing dye and thus can lead to an extension of the photo-response. In the present investigation, we have successfully fabricated a lawsone (one of the components of henna dye) sensitized ZnO photoanode and studied the device performance. In addition to this, the spectral features of lawsone have been explored using TD-DFT in vacuum as well as in ethanol.

2. Experimental procedure

2.1 Experimental details

ZnO photoanodes are constructed on fluorine-doped tin oxide (F:SnO₂) substrates that are first cleaned thoroughly using acetone and iso-propyl alcohol and by sonicating for 15 min. using absolute ethanol. The seed layer is prepared using chemical solution deposition. 0.1 M zinc nitrate (Zn(NO₃)₂.2H₂O, Thomas Baker) and 20% ammonia solution (SRL, AR grade) are used as precursors to carry out the experiment. Deposition is carried out at 65–70 °C for 10 min. ZnO paste is prepared by modification of a procedure given by Wong et al.²¹ It is prepared using 0.2 g ZnO powder (Sigma Aldrich, APS < 30 nm), 0.45 g ethyl cellulose (Loba chemie chemicals, India) and 3.5 g α-terpineol (Kemphasol). The ZnO slurry of all the above ingredients is pasted on the substrates using a doctor blade. The films are annealed at 450 °C for 1 h.

ZnO DSSCs are prepared by adsorbing lawsone dye onto the surface of nanorods grown on F:SnO₂ substrates. Lawsone is obtained from Sigma Aldrich and recrystallized from dry methanol before use. The performance of the nanostructured ZnO photoelectrode DSSC as a function of residence time in the dye solution has been reported in our group’s earlier work.²² We have explained the UV-Visible absorption spectrum of lawsone in ethanol solution. Experimentally, three UV peaks of lawsone in ethanol at 287, 330, 458 nm have been observed. In the case of photovoltaic performance, a 0.51% efficiency for a 20 h dye load time has been observed. It has been also shown that the device works more efficiently than with the 5 and 14 h dye loading times. Therefore, in the present work the films have been immersed in a 1 × 10⁻⁴ M dye solution in ethanol for 20 h. An electrolyte solution containing 0.5 M LiI, 0.05 M iodine and 0.5 M tert-butyl pyridine in acetonitrile is prepared. Lawsone sensitized ZnO films are studied for DSSC properties under three different intensities. The effective area of the device is 0.16 cm².

2.2 Characterization

The surface morphology of the films is observed by using scanning electron microscopy (SEM, JEOL-JSM 6360A). The UV-Vis studies for the dye solution and samples are carried out using a JASCO V-670 spectrophotometer in the wavelength range 287–700 nm. The fluorescence spectrum of the lawsone compound is recorded on a JASCO spectrofluorometer FP-8300. The fluorescence spectrum has been recorded in the range 350–650 nm upon excitation at 327 nm. The performance of the DSSCs is measured using different light intensities. The electrochemical impedance spectroscopy (EIS) measurements are made at room temperature using an Autolab PGSTAT30 (Eco-Chemie). The impedance measurement of the sample has been performed in order to check the resistance at different interfaces in the solar cell assembly. The frequency range chosen for the measurement is from 10⁻² Hz to 10⁶ Hz with an AC amplitude of 10 mV. The above measurements have been recorded using an electrochemical impedance analyzer equipped with a frequency response analyzer (FRA).

2.3 Computational study

The ground state geometry optimization of lawsone in a vacuum is performed with DFT using the hybrid functional PBE1PBE/6-311+G(d,p).^15,16,23 The frequency calculation is carried out with optimized geometry to ensure the obtained structure is minimum on the potential energy surface. Further, this optimized geometry is used to determine the UV-Vis spectrum using the non-equilibrium TD-DFT/PBE1PBE/6-311+G(d,p) method. The solvent effect is considered implicitly through the C-PCM model. The mapping of molecular orbitals in 3-dimensions is done using the PBE1PBE/6-311+G(d,p) basis set and visualized with UNIVIS software.²⁴ The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies are calculated at the B3LYP/6-311+G(d,p) level of theory in a vacuum and in ethanol. Moral et al.²⁵ have observed that for the calculation of HOMO/LUMO energies of 3,6-diphenyl-s-tetrazine derivatives, the B3LYP functional is adequate.²⁶ All the calculations in the present work are carried out with the Gaussian 09 programme.²⁷

3. Results and discussion

Fig. 1 shows the experimental UV-Vis spectrum with the molar extinction coefficient (ε) of lawsone. The ε measures how strongly a molecular moiety absorbs light at a given wavelength. The spectrum shows three main features, marked as I, II and III in order of decreasing absorbance of lawsone dye in ethanol at 287 (ε: 70 000 mol⁻¹ cm⁻¹), 332 (ε: 18 000 mol⁻¹ cm⁻¹) and 462 (ε: 12 000 mol⁻¹ cm⁻¹) nm.

Fig. 1 UV-Vis spectrum of lawsone.

The frequency calculation shows that the optimized geometry of lawsone is a minimum on the potential energy surface with zero imaginary frequencies. The theoretically calculated absorption wavelengths of lawsone are observed at 276, 321, 378 and 430 nm in vacuum with oscillator strength (f) values of 0.2001, 0.0642, 0.0208 and 0.0000, respectively. A singlet excited state with a non-zero f value is considered as an allowed transition. In ethanol, the absorptions are observed at 284, 332, 387 and 407 nm, having f values of 0.3014, 0.0914, 0.0097 and 0.0000, respectively (see Table 1). The calculated HOMO–LUMO gap of lawsone in ethanol turns out to be 3.81 eV, which is slightly lower than that calculated in vacuum, 3.85 eV. As seen in Table 1, the transitions for lawsone in vacuum as well as in ethanol are qualitatively similar in nature. The shapes of the molecular orbitals are also qualitatively similar (cf. Fig. 2) in both cases. Considering the absorption wavelengths of lawsone in ethanol, the former two bands at 284 and 332 nm, possessing higher f values, are in good agreement with the observed experimental wavelengths 287 and 332 nm, respectively, so that all peaks correspond to higher ε values. The latter two bands at 387 and 407 nm having negligible f values (i.e. 0.0097 and 0.0000 respectively) are reflected by quite low experimental ε values, with a shallow broad band at 462 nm. Khan et al.²⁸ have also studied the absorption spectrum of lawsone in cyclohexane with the semiempirical ZINDO/S method. With these calculations they have obtained absorption wavelengths of 221.5, 228.5, 229.9, 250.5, 288.6, 337.4 and 405.3 nm. In the paper²⁸ the authors have also discussed the experimental electronic absorption spectrum of lawsone reported by Morton²⁹ in ethanol with bands at 244, 276, 331, 395 and 460 nm (cf. Table 1). Jacquemin et al.¹⁵ have studied the transition energies of naphthoquinone derivatives with TD-DFT calculations. In this study, they have obtained absorption wavelengths of lawsone in chloroform at 283 and 328 nm and in methanol at 285 and 332 nm. Our results are in agreement with these experimental and computational results. The present work focuses on the TD-DFT approach for the analysis of the absorption bands of lawsone in ethanol and insights into charge transfer from the detailed analysis of molecular orbitals.

Table 1 Theoretical UV-Vis absorption wavelengths of lawsone calculated at the PBE1PBE/6-311+G(d,p) level of TD-DFT theory (H: HOMO, L: LUMO) along with experimental values. The f values are oscillator strengths

	Theoretical					Experimental ethanol
	Vacuum		Ethanol		Cyclohexane^a
	λ (nm)	f	λ (nm)	f	λ (nm)	λ (nm)	λ^b (nm)
a Values reported in ref. 28.b Values reported in ref. 29.
1.	276 (H-3 → L)	0.2001	284 (H-3 → L)	0.3014	288.6	287	276
2.	321 (H-2 → L)	0.0642	332 (H-2 → L)	0.0914	337.4	332	331
3.	378 (H → L)	0.0208	387 (H → L)	0.0097	—	—	395
	430 (H-1 → L)	0.0000	407 (H-1 → L)	0.0000	405.3	462	460

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The absorption at 284 nm, with the highest f value, is observed as a HOMO-3 → LUMO transition. Fig. 2(a) shows that HOMO-3 is mainly centered on the benzenoid ring, whereas the LUMO (Fig. 2(e)) is observed over the quinoid ring. This shows that the π → π* transition involves electron delocalization from the benzenoid ring to the quinoid ring of lawsone for the absorption. The second theoretically observed transition at 332 nm is attributed to HOMO-2 → LUMO. The electrons of HOMO-2 (Fig. 2(b)) are localized more on the benzenoid ring. Therefore, in this transition electrons are moving from the benzenoid ring to the quinoid ring, again with a π → π* transition. Both transition bands are observed in our experiment at the values 287 and 332 nm, respectively. The third absorption is observed theoretically at 387 nm with a negligible f value, which is not observed in our experiment. However, the band at 395 nm has been reported by Morton.²⁹ This absorption mainly involves a HOMO → LUMO transition with C–C and C–O regions of the quinoidal ring of lawsone. The next observed wavelength in the visible region is 407 nm with a zero f value, which arises from a HOMO-1 → LUMO transition. Fig. 2(c) shows that HOMO-1 is mainly localized on the oxygen atom of the quinoid ring. Therefore, this transition is attributed to n → π*. It should be noted here that the UV-Vis absorption band in the 450 nm region is substantially shallow, broad and very weak (cf. Fig. 1). This might be a cause of the difference between the experimentally observed value and the calculated (407 nm) one. In view of this, we have carried out the calculations by considering the hydrogen-bond interactions of lawsone with explicit ethanol as well as with another lawsone molecule, to check the role of the interactions on the calculated longest wavelength absorption band. The corresponding obtained band values are observed not to be enhanced towards the experimental one (see Tables S1 and S2 in the ESI†).

Fig. 2 Molecular orbital diagrams of 2-hydroxy-1,4-naphthaquinone in vacuum and ethanol with their iso-surfaces obtained using the TD-DFT method with the basis set PBE1PBE/6-311+G(d,p) [the respective iso-surface values are given in brackets].

In general, for favorable electron transfer in a DSSC, the LUMO of a dye should lie above the conduction band energy of the semiconductor, and for electron recombination of the dye, the LUMO should be under the reduction potential of the electrolyte. The theoretically observed LUMO energy of lawsone in ethanol is −3.58 eV, which lies above −4.45 eV, the conduction band energy of ZnO,³⁰ and the HOMO of lawsone is −7.39 eV, which lies under the reduction potential energy, −4.80 eV, of the electrolyte I⁻/I₃⁻.³¹ This reveals that lawsone is a suitable dye candidate for DSSC.

The fluorescence spectrum of lawsone at a concentration of 1 × 10⁻⁴ M is recorded at room temperature, and is shown in Fig. 3. The fluorescence spectrum of lawsone shows three peaks, one at 362 nm and two broad peaks with maxima at 412 and 432 nm. The bands in the visible region show the device-forming ability of this dye.

Fig. 3 Fluorescence spectrum of lawsone in ethanol.

With the above experimental and theoretical insights, we have successfully fabricated a DSSC using lawsone as a sensitizer. The photovoltaics performances are depicted in Fig. 4. To confirm the correct magnitude of these parameters, we determined the intensity dependent J–V characteristics of the solar cells. For a light intensity of 100 mW cm⁻², an increase in the photocurrent up to 1.80 mA cm⁻² is observed. Electron transport becomes more difficult with an increase in photocurrent, attributed to the increased recombination between the electrons and the oxidized dye or redox mediator in the electrolyte.³² Table 2 shows the photovoltaic parameters of the DSSC viz. short circuit current density (J_sc), open circuit voltage (V_oc), fill factor (FF) and power conversion efficiency (η) under low and high light intensities (P_in). The results are quoted after triplicate experiments. With decreasing illumination, the V_oc is enhanced and hence improves the FF. For a 26 mW cm⁻² intensity, the device shows V_oc at 0.49 V, J_sc at 0.59 mA cm⁻², FF at 0.60 and η at 0.68%. A donor–π–acceptor dye, zinc-porphyrin, designed with YD2-o-c8 and Y123 using a Co(II/III) tris(bipyridyl)-based redox electrolyte conductor, has led to a record power conversion efficiency of 13.1% under 50.8 mW cm⁻² intensity according to Yella et al.³³ It has been observed that when reducing the illumination intensity, the power conversion efficiency increases. Hence our results are in accordance with the work by Yella et al. As the light intensity decreases, the bias point and current passing through the solar cell also decreases. Further, many attempts have been made with various organic dyes as sensitizers for ZnO, such as D149, D358 and N719, respectively.^34–36

Fig. 4 J–V curve of DSSCs using lawsone dye with different light intensities. The η denotes efficiency.

Table 2 Detailed photovoltaic parameters of the devices using lawsone dye at different light intensities. See the text for details

Pin (mW cm^−2)	Jsc (mA cm^−2)	Voc (V)	FF	η (%)
26	0.59	0.49	0.60	0.68
100	1.80	0.52	0.62	0.56

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The EIS spectrum provides information about the chemical capacitance, electron recombination resistance and electron transport resistance of the DSSC.³⁷ The EIS spectrum is studied using Zview software.³⁸ The impedance spectrum of DSSC, represented as a Nyquist plot (Fig. 5), mainly shows three semicircles attributed to the interface between Pt|electrolyte (Z₁), ZnO|dye|electrolyte (Z₂) and diffusion of I₃⁻ (Z₃) in the electrolyte, respectively. The EIS fitted parameters are shown in Table 3. The observed series resistance is 44.6 Ω, while the charge transfer resistance is 15 Ω. The effective diffusion length (L_n)³⁹ is calculated using equation

where the diffusion coefficient (D_n) is 4.2 × 10⁻³ cm² s⁻¹. The electron lifetime (τ_n) is found to be 0.088 ms, a substantially low value. Therefore, there are more chances to increase the recombination rate. The shorter electron lifetime indicates that the electron transfer rate from the conduction band to the I₃⁻ dye is faster than that of ZnO and thus a substantial number of photo-induced electrons are lost during their travel to the F:SnO₂.

Fig. 5 Nyquist plot of the DSSC. The inset is an equivalent circuit impedance model. R_s is the series resistance accounting for the transport resistance of the TCO. R_ct is the charge transfer resistance for electron recombination at the FTO/ZnO/electrolyte interfaces, while CPE is the constant phase element representing capacitance at the ZnO/electrolyte interface. W is the Warburg impedance describing the diffusion of I₃⁻ in the electrolyte. C1 and C2 are the capacitance of the film.

Table 3 Results of EIS fitting parameters (see the text and Fig. 5 for details)

Dye	Rs (Ω)	Rct (Ω)	Ln (μm)	Dn (cm^2 s^−1)	τn (ms)
Lawsone	44.6	15	6.08	4.2 × 10^−3	0.088

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4. Conclusions

In summary, the spectral characteristics of lawsone, a photosensitizer used to fabricate DSSCs, is studied using experimental and computational approaches. The simulated absorption spectrum of lawsone dye in ethanol using the TD/CPCM/PBE1PBE/6-311+G(d,p) approach is in good agreement with our experimental values and those reported by Morton.²⁹ The study reveals the solvent effect of ethanol, due to which the absorption spectrum of lawsone shifts towards longer wavelengths and lowers its band gap. The analysis of molecular orbitals leads to the detailed assignments of the spectral features of lawsone, which show the dominance of π → π* transitions. The theoretically calculated HOMO and LUMO energies of lawsone in ethanol are in favor with the conduction band energy of the ZnO semiconductor and the reduction potential energy of the I⁻/I₃⁻ electrolyte for electron transfer in the DSSC. Further, we have successfully fabricated a DSSC with lawsone dye and a ZnO photoanode. We observed a power conversion efficiency up to 0.68%, under 26 mW cm⁻² light intensity. From the results of EIS fitting, we can conclude that there is a loss of photo-induced electrons during their travel towards the F:SnO₂ photoanode, which suggests more chances for their recombination. Thus, the study reveals that the 1,4-naphthoquinone class of compounds can provide the basis for the design of novel compounds for DSSC applications to enhance their efficiency. Further, the extension of metal complex systems with different donor groups is expected to significantly increase the molar extinction coefficient and may show a panchromatic response. We are currently working in these directions and trying to further enhance the performance of the device.

Acknowledgements

Authors gratefully acknowledge financial support from the PU/ISRO-STC/1466 and UPE Phase-II budgets. We are grateful to Professor Shridhar R. Gadre for his critical reading of this manuscript.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra14620d

‡ Authors have equally contributed.

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