Double dye cubic-sensitized solar cell based on Förster resonant energy transfer

Abstract

To extend the solar spectral response range of dye-sensitized nanocrystalline semiconductor thin film solar cells, eosin Y (EY) and Rhodamine B (RB) were chosen to cubic-sensitize nanocrystalline ZnO thin film. A hybrid layer containing EY and ZnO (EY/ZnO) was first electrodeposited on a nanocrystalline ZnO thin film (n-ZnO) to form a structure of n-ZnO/EY/ZnO, and then RB was sensitized to form a double dye cubic-sensitized nanocrystalline ZnO thin film with a structure of n-ZnO/EY/ZnO/RB. The absorption spectra of the two organic dyes are complementary and the emission spectrum of EY overlaps with the absorption spectrum of RB. A new dye-sensitized solar cell based on the n-ZnO/EY/ZnO/RB thin film was fabricated, in which EY as an energy-relay dye simultaneously transfers both electrons and holes to the sensitizing dye of RB by the Förster resonant energy transfer (FRET) process. Thus the spectral response range of the fabricated dye-sensitized solar cell was extended and the energy conversion efficiency was improved. The thickness of the EY/ZnO hybrid layer is the key factor in affecting the conversion efficiency of the new solar cells, and it was optimized by changing the deposition time of the hybrid layer.

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1. Introduction

Dye-sensitized solar cells (DSSCs) have been considered as potential candidates for the conventional silicon based solar cells due to their low-cost production, simple fabrication process and relatively high energy conversion efficiency.^1–3 Although the conversion efficiency of DSSCs based on liquid-electrolyte has achieved 13% after having been investigated for the past two decades,⁴ this conversion efficiency is not enough for cost-effective commercial production and further improvements are still necessary. Many attempts have been made to improve the conversion efficiency for the practical use of this device to make it commercially attractive. At present, one of the further enhancement limiting factors to the conversion efficiency is the fact that the sensitizers cannot absorb light over a broad region of the solar spectrum efficiently. So one way to further improve the conversion efficiency of DSSCs is to overcome the bottleneck of narrow absorption range of the sensitizers to match with the solar spectrum better.

Much attention has been devoted to broaden the absorption spectrum responds of DSSCs, such as tandem structure solar cell,^5,6 and co-sensitization of two complementary sensitizers^7–13 and so on. For tandem DSSCs, however, the problem is that on the one hand, a strict adjustment of the transmittance of the top cells is required in the tandem structure. On the other hand, the short circuit current density and open circuit voltage of the two cells also need to be adjusted respectively to fabricate tandem DSSCs successfully,⁵ which leads to some difficulties to some extent. For co-sensitization of two sensitizers, because of the competitive adsorption between two organic dyes, under the fixed specific surface area of the semiconductor nanocrystalline thin film, adsorption of another kind of dye decreases the dye loading of the dye having better performance. As a result, the energy conversion efficiency of two kinds of dyes co-sensitization is lower than that of single organic dye sensitized solar cell. Unfavorable competitive adsorption and interaction between two sensitizers cannot be avoided, causing a decrease in the energy conversion efficiency of DSSCs.¹⁴

Recently, Förster resonance energy transfer (FRET) has been investigated as a promising method to extend the solar spectral response of DSSCs.^15–19 FRET is based on dipole–dipole coupling between different sensitizers, in which one sensitizer (donor) nonradiatively transfers its excitation energy including both electron and hole to another sensitizer (acceptor) separated by a short distance.^20,21 It can be seen that most previous efforts on this mechanism have been concentrated on the hybrid structure sensitized solar cells including two kinds of quantum dots or the quantum dots and organic dye serving as a donor and an acceptor, respectively.^22–27 For FRET process of two organic dyes applied in DSSCs, donor dye was put in the electrolyte other than on the nanocrystalline thin film.^28–32 The research on two dyes located on the nanocrystalline thin film as a donor and an acceptor to realize FRET process was not reported up to now.

In this paper, we employed eosin Y (EY) and rhodamine B (RB) to fabricate a double dyes cubic-sensitized nanocrystalline ZnO thin film solar cell. The hybrid layer of EY/ZnO was deposited on the nanocrystalline ZnO thin film (n-ZnO) by one-step electrodeposition method in the bath containing EY and Zn(NO₃)₂.^33–35 A new hybrid structure of n-ZnO/EY/ZnO/RB was obtained by sensitization of RB on the structure of n-ZnO/EY/ZnO. In this structure, EY transmits energy to RB according to the FRET process. Due to the complementary absorption spectra of EY and RB, the dye bilayer-sensitized solar cell broadens the spectral response range and then improves the energy conversion efficiency.

2. Experimental

2.1 Materials

Zinc oxide nanoparticles with the mean diameter of 20 nm and 30 nm were purchased from Nanjing Haitai nano materials Co. Ltd. The dyes of EY and RB were purchased from Kanto without further purification. Zinc nitrate hexahydrate (99%), triton X-100 (99%), n-butanol (99.5%), lithium iodide (99%), iodine (99.8%) and 3-methoxypropionitrile (99%) were purchased from Alfa Aesar Inc. China. Fluorine-doped tin oxide (FTO, 10 Ω sq⁻¹, Nippon Sheet Glass Co., Ltd. Japan) was used as the substrate.

2.2 Preparation of double dyes cubic-sensitized nanocrystalline ZnO thin film

FTO substrates were ultrasonically washed with deionized water, acetone and ethanol successively for 15 min and immersed in isopropyl alcohol for 24 h. ZnO powder paste was prepared by grinding the mixture of 2 g ZnO nanoparticles powder (mass ratio for two kinds of ZnO nanoparticle powders: 1 : 1) in n-butanol with several drops of triton X-100 for 3 h at room temperature. Then the doctor-blade method was employed to form porous ZnO thin film on FTO substrate. The films were further sintered at 450 °C in air for 30 min. The morphologies of the ZnO thin film were observed using a scanning electron microscope (SEM, S-4800, Hitachi). On the nanocrystalline ZnO thin film, a hybrid layer of EY and ZnO was deposited by the one-step electrodeposition method.^33–35 In the one-step electrodeposition, EY can be deposited inside ZnO or on the surface of ZnO according to the deposition conditions. The electrodeposition procedure was carried out using a computerized voltammetry system (Hokuto Denko HSV-110) in a three-electrode cell under potentiostatic condition of −1.0 V, in which the nanocrystalline ZnO thin film served as the working electrode, a pure zinc wire used as the counter electrode and a saturated calomel electrode (SCE) used as the reference electrode. The deposition solution is an aqueous solution of 0.2 M Zn(NO₃)₂, 30 mM EY and 0.2 M KNO₃. The bath temperature was controlled at 70 °C and the solution was stirred continuously using a magnetic stirrer. Then the ZnO/EY hybrid thin layer with EY inside ZnO crystals was coated on the ZnO nanocrystalline thin films to form a structure of n-ZnO/EY/ZnO. The thickness of the resulted hybrid layer could be controlled by changing the deposition time. The hybrid layer thickness was determined using a transmission electron microscope (TEM, Tecnai G2 F20).

The resulted thin film of n-ZnO/EY/ZnO was immersed in a RB absolute alcohol solution of 0.5 mM for 12 h. After the dye adsorption process, the sensitized thin film was washed with absolute alcohol before drying it at room temperature. Then a new hybrid structure of n-ZnO/EY/ZnO/RB was obtained. RB sensitized nanocrystalline ZnO thin films with deposition of a pure ZnO layer (n-ZnO/ZnO/RB) and without deposition of the hybrid layer (n-ZnO/RB) were also prepared for comparison. UV-Vis absorption spectra of samples were measured on a spectrophotometer (UV-2600). Fluorescence spectra of the thin films were recorded using a Cary Eclipse fluorescence spectrophotometer.

In order to determine the dye loading amount, RB sensitized thin films were soaked in a NaOH solution at pH 10.5 overnight to desorb RB. The adsorption value of the solution containing the desorbed RB was measured by an UV-Vis spectrophotometer (UV-2600) using 105 536 L mol⁻¹ cm⁻¹ at 554 nm as the molar extinction coefficient. The dye loading was calculated based on the initial film surface area, the absorbance value and the solution volume.

2.3 Assembly and photoelectrochemical measurement of DSSCs

A Pt sheet was used as the counter electrode. An I⁻/I₂ redox electrolyte used in DSSCs contains 0.5 M I₂ and 0.05 M LiI dissolved in 3-methoxypropionitrile. The photocurrent–voltage (J–V) curves of DSSCs were recorded using a potentiostat (CHI 660E). A 500 W xenon light source was calibrated to 100 mM cm⁻² coupled with an AM 1.5G filter. A mask with an aperture of 0.2 cm² was used to define the active illumination area of DSSCs. The incident photon-to-current conversion efficiency (IPCE) of DSSCs was measured using a xenon lamp, which was focused through a double monochromator (Zolix DCS300PA). The number of incident photons on the device was calculated for each wavelength using a calibrated Si-cell.

3. Results and discussion

The FRET process involves dipole–dipole coupling of two chromophores, known as the donor and acceptor, through an electric field.³⁶ The key parameter for the FRET process is the FRET radius (R_o), which is defined as the distance at which the probability of FRET between donor and acceptor is 50%. In addition, it needs an overlap between the emission spectrum of the donor and the absorption spectrum of the acceptor. According to the FRET mechanism, we designed a double dyes cubic-sensitized nanocrystalline semiconductor thin film solar cell. Fig. 1 displayed the perform mechanism of the double dyes cubic-sensitized thin film electrode with the structure of n-ZnO/EY/ZnO/RB. Higher energy photons are absorbed by the energy-relay dye (EY), which simultaneously transfers an electron and a hole to the sensitizing dye (RB) through the FRET process. Lower energy photons are absorbed by the sensitizing dye (RB) to excite electrons. And both the excited electrons and the electrons transferred from the energy-relay dye were injected into the nanocrystalline ZnO thin film, and holes including the holes transferred from the energy-relay dye were transferred to the counter electrode through the redox electrolyte. The structure of n-ZnO/EY/ZnO/RB was prepared by depositing a hybrid layer of EY/ZnO on nanocrystalline ZnO thin film prior to sensitization of RB. The EY/ZnO hybrid layer was electrodeposited by the one-step electrodeposition developed by Yoshida.^33–35 EY can be deposited inside nanocrystals or on the surface of nanocrystals depending on the electrodeposition conditions. Here, EY was controlled to be deposited inside the ZnO nanocrystals layer as shown in Fig. 1. It is important to note if the loaded EY molecules in the electrodeposited film could be desorbed almost completely without dissolving ZnO by dipping the film in dilute aqueous KOH solution, EY was loaded on the surface of ZnO.³⁵ In our case, EY cannot be extracted from the hybrid EY/ZnO layer as the n-ZnO/EY/ZnO thin film was dipped in the alkaline solution, indicating that EY molecules are actually present below the surface of ZnO, namely EY molecules were deposited inside the ZnO hybrid layer and they cannot be accessible from the redox electrolyte in the case of DSSCs.

Fig. 1 The mechanism processes of double dyes cubic-sensitized nanocrystalline ZnO thin film based on Förster resonant energy transfer (FRET).

Fig. 2 displayed the typical SEM images of nanocrystalline ZnO thin film on the FTO substrate. The surface morphology (Fig. 2a) shows the uniform porous structure formed by the nanoparticles with an average particle size of 20–30 nm and some aggregations. The formed aggregations can make the large pores, which will leave enough space for deposition of the EY/ZnO hybrid layer and sensitization of RB. The cross-sectional image (Fig. 2b) indicates the film thickness is about 10 μm.

Fig. 2 SEM surface (a) and cross section (b) morphologies of nanocrystalline ZnO thin film.

Fig. 3 showed the UV-Vis absorption spectra of EY sensitized nanocrystalline ZnO thin film, RB sensitized nanocrystalline ZnO thin film and the n-ZnO/EY/ZnO/RB thin film. The normalized absorption spectra were also shown in Fig. 3 for easy observing the shape difference of their absorption curves. The EY sensitized thin film shows a maximum absorption park at 518 nm, while the maximum absorbance of the RB sensitized one is located at 554 nm. Compared to the EY and RB sensitized thin films, the n-ZnO/EY/ZnO/RB hybrid thin film significantly expanded its absorption spectrum in the visible region. By comparing the normalized absorption spectra, it is clear that the n-ZnO/EY/ZnO/RB absorption spectrum is contributed by both EY and RB. Can they also contribute the photoelectrochemical responses of DSSCs based on the n-ZnO/EY/ZnO/RB thin film electrode? To answer this question, the J–V curves and IPCE of DSSCs based on different dye sensitized nanocrystalline thin films were measured. And the corresponding photovoltaic performance parameters, such as the short circuit photocurrent (J_sc), open circuit photovoltage (V_oc), fill factor (ff) and light-to-electric conversion efficiency (η), were collected in Table 1.

Fig. 3 UV-Visible absorption spectra (a) and normalized spectra (b) of the EY sensitized nanocrystalline ZnO thin film (dash line), RB sensitized nanocrystalline ZnO thin film (dot line) and n-ZnO/EY/ZnO/RB thin film (solid line).

Table 1 Photovoltaic performance parameters of DSSCs based on RB sensitized nanocrystalline ZnO thin films with the EY/ZnO hybrid layer prepared under different deposition times

Deposition time (min)	Jsc (mA cm^−2)	Voc (mV)	ff	η (%)
a With deposition of the pure ZnO layer.
0	1.46	330	0.41	0.20 ± 8%
1^a	1.56	340	0.32	0.17 ± 7%
0.5	1.62	400	0.48	0.31 ± 8%
1	2.29	370	0.44	0.37 ± 7%
2	1.83	340	0.36	0.22 ± 12%

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Without deposition of the EY/ZnO hybrid thin layer or with deposition of only ZnO thin layer, as shown in Table 1, J_sc of RB sensitized nanocrystalline ZnO thin film solar cells is 1.46 mA cm⁻² and 1.56 mA cm⁻², respectively. With deposition of the EY/ZnO hybrid thin layer, the J_sc value was enhanced. The enhancement of J_sc may be contributed by the more RB sensitization or EY in the EY/ZnO hybrid layer. The RB loading amount on the nanocrystalline ZnO thin film was measured as 2.51 × 10⁻⁹ N mol⁻¹ cm⁻² and the loading amount was decreased to 2.45 × 10⁻⁹ N mol⁻¹ cm⁻² as the EY/ZnO hybrid thin layer was deposited for 2 min prior to sensitization of RB. This small difference can be ignored and the RB loading amount was considered not to be changed due to the EY/ZnO hybrid layer deposition. Therefore, the J_sc enhancement was attributed to EY inside the EY/ZnO hybrid thin layer.

To confirm the contribution of RB, IPCE spectra of DSSCs based on dye sensitized nanocrystalline ZnO thin films with different structures were further measured and shown in Fig. 4. For comparison, the normalized IPCE curves were also shown in Fig. 4. IPCE of the RB sensitized solar cell is red shifted compared to that of the EY sensitized one. Also we observed broaden of the IPCE curve of the n-ZnO/EY/ZnO/RB solar cell compared with the single dye sensitized one. At the IPCE part at the longer wavelength region (>550 nm), the ZnO/EY/ZnO/RB solar cell increases in intensity due to the contribution of RB. The intensity ratio of two peaks of IPCE curves at about 520 nm and 500 nm for the n-ZnO/EY/ZnO/RB solar cell and for the EY sensitized nanocrystalline ZnO thin film solar cell is similar. This reveals the EY has contribution to the n-ZnO/EY/ZnO/RB solar cell. Therefore, both the EY and RB contribute to the overall cell performance.

Fig. 4 IPCE (a) and the normalized IPCE (b) of DSSCs based on the EY sensitized nanocrystalline ZnO thin film (dash line), RB sensitized nanocrystalline ZnO thin film (dot line) and the n-ZnO/EY/ZnO/RB thin film (solid line).

Since EY presents inside ZnO crystals, it cannot contact with electrolyte and the holes will not be transferred, thus electrons cannot be injected to ZnO nanocrystals. The left possible way for EY to transfer electrons is through the FRET process. To certificate the possibility of FRET process from EY to RB, the fluorescence spectrum of the n-ZnO/EY/ZnO thin film was measured. The fluorescence spectrum of the n-ZnO/EY/ZnO thin film overlaps partly with the absorption spectrum of RB sensitized nanocrystalline ZnO thin film as shown in Fig. 5, which indicates the FRET process from EY to RB is allowed. The fluorescence spectra of n-ZnO/EY/ZnO and n-ZnO/EY/ZnO/RB thin films were shown in Fig. 6. The emission intensity of n-ZnO/EY/ZnO thin film at 550 nm was decreased as RB was sensitized on the n-ZnO/EY/ZnO film. Therefore, fluorescence resonant energy transfer process can be realized from EY to RB on the n-ZnO/EY/ZnO/RB structure. In DSSC based on the n-ZnO/EY/ZnO/RB structure, EY simultaneously transfers both an electron and a hole to RB according to the FRET mechanism and then RB dye transfers electrons to the inner ZnO nanocrystals. If so, all these processes are affected by the thickness of the outer EY/ZnO hybrid thin layer directly. The hybrid thin layer thickness can be controlled by the electrodeposition time. The thickness of the outer ZnO layer was estimated from TEM morphologies of single particles. Fig. 7 showed TEM of ZnO nanoparticles scraped from the n-ZnO/EY/ZnO thin films with deposition of the EY/ZnO hybrid layer for 1 min and 2 min, respectively. 1 min deposition can grow the ZnO layer with thickness of about 3.0 nm, and it becomes about 10.0 nm after 2 min deposition. The influence of the outer ZnO layer deposition time on the performance of the solar cells was investigated and the photovoltaic parameters were collected in Table 1. For the deposition time of 0 min, namely no EY/ZnO hybrid layer, J_sc of the cell based on RB sensitized nanocrystalline ZnO thin film is 1.46 mA cm⁻². For three different deposition times, J_sc of the cells was enhanced overall. By utilizing two sensitized dye layers, the energy conversion efficiency can be improved compared with the single dye sensitized solar cell. With the increase of deposition time, the outer ZnO hybrid layer containing EY becomes thicker. J_sc of the solar cell was increased from 1.62 mA cm⁻² to 2.29 mA cm⁻² (+41.4%), then decreased to 1.83 mA cm⁻². 1 min of deposition time achieved the best cell performance. Because occurring of the FRET process needs a proper distance between the two kinds of dyes. The proper distance is generally about 1–10 nm.¹⁵ For the deposition time less than 1 min, the transferring energy process by FRET from EY to RB is weak, because less EY molecules are loaded. For a longer deposition time, the outer ZnO hybrid layer becomes thicker. The thick outer ZnO layer suppressed the transferring energy process from EY to RB, which will decrease the photovoltaic performance of the cell. Therefore, it is very important to optimize the deposition time to achieve the highest energy conversion efficiency. In our case, the deposition time of the EY/ZnO hybrid thin layer was optimized to be 1 min and the appropriate thickness of the outer ZnO layer is about 3.0 nm.

Fig. 5 Absorption spectrum of RB sensitized nanocrystalline ZnO thin film and fluorescence spectrum of the n-ZnO/EY/ZnO thin film. The excitation wavelength is 470 nm.

Fig. 6 Fluorescence spectra of the n-ZnO/EY/ZnO thin film (solid line) and the n-ZnO/EY/ZnO/RB thin film (dash line). The excitation wavelength is 470 nm.

Fig. 7 TEM images of samples scraped from the n-ZnO/EY/ZnO thin films with deposition of the EY/ZnO hybrid layer for 1 min (a) and 2 min (b).

The conversion efficiency of double dyes cubic-sensitized solar cells is not satisfied. The low efficiency is due to several interfaces presenting in the n-ZnO/EY/ZnO/RB structure, in which there are many recombination centers. These recombination centers will decrease the transferring efficiency of charges in the n-ZnO/EY/ZnO/RB structure and finally leading to a lower conversion efficiency of the new structure DSSCs. For further improving the conversion efficiency, the interface modification techniques are needed to passivate each interface in the n-ZnO/EY/ZnO/RB structure, and to select two more matching dyes having the better complementary absorption spectra is also necessary. Although DSSC based on the n-ZnO/EY/ZnO/RB thin film electrode showed lower conversion efficiency, it opens a way to expand the solar spectrum responding range and offers a possible way to further increase the conversion efficiency of DSSCs in future.

4. Conclusions

We have demonstrated the ability to widen the spectral response of DSSCs based on Förster resonant energy transfer in double dyes cubic-sensitized nanocrystalline ZnO thin film solar cell. One-step electrodeposition method was employed to grow the EY/ZnO hybrid layer on nanocrystalline ZnO thin film prior to sensitization of RB to form the n-ZnO/EY/ZnO/RB structure. EY and RB located in different layers of dye sensitized nanocrystalline ZnO thin film. In the n-ZnO/EY/ZnO/RB structure, EY was used as a donor dye to transfer energy including both hole and electron to acceptor dye of RB. Compared with the single dye-sensitized solar cell, the spectrum responding range of DSSCs based on the n-ZnO/EY/ZnO/RB thin film was expanded and the corresponding energy conversion efficiency was increased. The thickness of the outer EY/ZnO hybrid thin layer was adjusted by changing the deposition time, which can affect the FRET process between EY and RB. The deposition time of the outer ZnO hybrid thin layer was optimized to be 1 min.

Acknowledgements

This work was financially supported by the National Nature Science Foundation of China (Grant nos 21273160 and 20873162), the Program for Innovative Research Team in University of Tianjin (TD12-5038), the Nature Science Foundation of Tianjin (Grant no. 14JCYBJC18000) and the Doctoral Foundation of Tianjin Normal University (52XB1111). J. Zhang thanks Prof. Yoshida in Yamagata University, Japan for his kind supervision in electrodeposition of ZnO/dye hybrid thin films.

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