ORIGINAL PAPER
Double-layer TiO
2inverse opal-based quantum dot-sensitized solar
cells
Jingsha Jin1&Lingran Zhao1&Yuyu Liu1&Shufang Gao1&Xiangxiang Yu1&Yan Xiong1 Received: 9 May 2020 / Revised: 10 August 2020 / Accepted: 12 August 2020
# Springer-Verlag GmbH Germany, part of Springer Nature 2020 Abstract
The operation of dye-sensitized solar cells and quantum dot-sensitized solar cells (QDSSCs) depends strongly on the photoanode material employed. This is addressed in the present work by developing photoanodes based on a double-layer TiO2inverse opal
material with different interconnected pore sizes in the bottom and upper layers for use in QDSSCs. The proposed photoanode material leads to better infiltration of the sensitizers and the hole transporting material through the entire depth of the TiO2layer.
Double-layer TiO2inverse opal-based QDSSCs are demonstrated to facilitate the greater absorbance of quantum dots and obtain
higher photocurrent and power conversion efficiency than QDSSCs adopting single-layer TiO2inverse opal photoanodes. Various
QDSSCs employing double-layer TiO2inverse opal photoanodes with different pore sizes in the layers are tested. The CdS/CdSe
co-sensitized solar cell adopting the optimum photoanode configuration and thickness provided the highest QDSSC conversion efficiency of 5.79%.
Keywords TiO2inverse opal . Quantum dot . Quantum dot-sensitized solar cells
Introduction
Dye-sensitized solar cells (DSSCs) are a third-generation solar cell technology that has been demonstrated to be one of the most promising technologies for realizing efficient photoelec-tric conversion since it was first developed in 1991 [1]. Moreover, DSSCs have attracted considerable attention owing to a number of additional advantages such as low-cost, simple, and environmentally friendly fabrication processes, and ease of large-area fabrication [1–3]. A DSSC mainly consists of a photoanode, an electrolyte, and a counter electrode. The photoanode is mainly composed of an electrically conductive transparent substrate, such as fluorine-doped tin oxide (FTO)-coated glass, with an overlying semiconductor oxide film, such as TiO2or ZnO, that adsorbs a sensitizer, such as a
ruthenium dye, that generates photoelectrons under illumina-tion [4]. Accordingly, the morphology of the photoanode di-rectly affects the scattering of the incident light, the collection and transmission of electrons, and the adsorption of dye
sensitizers in the cell, and therefore has a significant effect on the photoelectric conversion efficiency of a DSSC [5].
Double-layer photoanodes have been widely used in DSSCs because of their effective scattering of visible light. The structure of double-layer photoanodes consists generally of TiO2nanoparticles with different diameters in the upper
and lower layers [6–8]. In addition, the photoelectric conver-sion efficiency of DSSCs can be increased by altering the morphology of the photoanode materials, such as through the use of TiO2nanorods and spherical nanocrystals [9,10].
Accordingly, the photovoltaic (PV) performance of DSSCs can be effectively improved by controlling the morphology of TiO2in the photoanodes [11–19].
Considerable progress has been made recently by replacing the organic dyes in DSSCs with semiconductor quantum dots (QDs) as sensitizers, and the resulting QD-sensitized solar cells (QDSSCs) have been demonstrated to exhibit unique advantages such as the quantum size effect, multi-exciton effect, large ab-sorption coefficient, and easy matching of energy levels between electron donor and acceptor materials [4]. However, QDSSCs are more sensitive to the morphology of the photoanodes because the deposition of QDs is more difficult to adjust than N719 dyes. This was addressed by the Toyoda group through the develop-ment of photoanodes based on TiO2inverse opal [20–22]. TiO2
inverse opal electrode with ordered periodic mesoporous * Yan Xiong
1 School of Physics and Optoelectronic Engineering, Yangtze
University, Jingzhou 434023, Hubei, People’s Republic of China
https://doi.org/10.1007/s10008-020-04806-9
structure is useful for QDSSCs because of better penetration of electrolyte than conventional nanoparticulate structure [23,24]. Moreover, this structure has the possibility of enhancing the light-harvesting efficiency [25]. Semiconductor QDs such as CdS, CdSe, and CdTe have been coupled with TiO2inverse opal
to fabricate QDSSCs [22,26]. The unique properties of the QDs make the band gap of TiO2to be easily tuned in a wide range,
and the photogenerated electrons to be effectively transferred. Much efforts have been devoted to surface modification and doping of TiO2inverse opal to improve the performance of
DSCCs and QDSSCs based on it. However, the power conver-sion efficiency (PCE) of QDSSCs based on TiO2inverse opal
photoanode is not more than 5%.
The present work improves on the contributions made in past studies by developing photoanodes based on double-layer TiO2
inverse opal materials for use in QDSSCs with cadmium sulfide (CdS) and cadmium selenide (CdSe) QD sensitizers. Recently, different kinds of quantum dots (QDs) have frequently been used to co-sensitize photoelectrodes for QDSSC application, due to widening absorption spectrum and the stepwise band edge struc-ture constructed in the co-sensitizer system [22,27,28]. For QD co-sensitizers, CdS/CdSe is a promising combination that has been reported to exhibit good performance. The band edges for the TiO2/CdS/CdSe QDSSC device are inferred to have a
step-wise structure [28]. The TiO2inverse opal materials are prepared
using polystyrene (PS) microsphere templates. The pore size of the TiO2layers is determined by the PS microsphere diameter,
and the resulting layer thickness is determined by the concentra-tion of PS microspheres used during TiO2layer fabrication.
Therefore, we first investigate the effect of pore size and layer thickness on the performance of QDSSCs based on single-layer TiO2inverse opal photoanodes by varying the PS microsphere
diameter and the concentration of PS microspheres used during anode fabrication. Then, double-layer TiO2inverse opal anodes
are fabricated with different pore diameters in the upper and lower layers, and the effects of different pore size combinations on the PV performance of the resulting QDSSCs are investigat-ed. The physical and PV properties of the photoanodes and resulting QDSSCs are investigated according to their photocur-rent density-voltage characteristics, dye adsorption-desorption characteristics, scanning electron microscopy (SEM) images, and electrochemical impedance spectroscopy (EIS) results. The QDSSC adopting the optimum photoanode configuration and thickness provided the highest PCE of 5.79%.
Experimental
Materials
Experimental materials are listed in Table1. All of the chemicals were reagent grade and used without further purification.
Preparation of TiO
2photoanodes
Liquid phase deposition techniques were adopted for prepar-ing sprepar-ingle-layer and double-layer TiO2inverse opal films
using PS microsphere templates, as reported elsewhere [16]. Briefly, a 0.1 wt% aqueous solution of monodisperse PS mi-crospheres (100, 350, 500, or 750 nm in diameter) with 0.003 wt% IGEPAL CO-520 surfactant was subjected to son-ication for 30 min. The conductive FTO-coated glass sub-strates were cleaned by sonication in water followed by etha-nol and then rinsed with water and dried in an oven. The clean FTO substrates were then immersed in the PS microsphere solution and placed in an oven at 55 °C for approximately 3 days until evaporated. After evaporation, the PS microspheres were seeded with TiO2by soaking the films in a 1.2% (w/v)
ethanol solution of titanium (IV) isopropoxide and 0.12% (w/v) HNO3for 5 min. After soaking, the films were left to
dry for 20 min. The PS microspheres were then infiltrated by immersing the films for 30 min in a combined aqueous solu-tion of 0.2 M (NH4)2TiF6and 0.25 M HBO3at 51 °C and a pH
adjusted to 2.9 with HCl. After rinsing the films with water and drying in air, the PS microspheres in the films were re-moved by calcination in air at 400 °C for 8 h [16,17]. This process yielded TiO2frameworks with highly ordered pore
structures [22]. Double-layer TiO2films were fabricated by
repeating the above process twice with different PS micro-sphere diameters and PS micromicro-sphere solution concentrations in the lower and upper layers. In this case, the corresponding PS microsphere solution concentrations for both layers were selected to obtain double-layer TiO2films of roughly
equiva-lent thicknesses.
Deposition of QDs
The CdS QDs were deposited on the TiO2films through a
two-step dipping process. Here, a TiO2film was first dipped
into an ethanol solution containing 0.1 M Cd(NO3)2for 1 min
and rinsed with ethanol. Then, the film was dipped for another 1 min into a 0.1 M Na2S methanol solution and rinsed with
methanol. This two-step process is regarded as a single ionic layer adsorption and reaction (SILAR) cycle, and the number of CdS QDs incorporated into a TiO2film can be increased by
repeating the number of SILAR cycles [22,27]. A total of 12 SILAR cycles were conducted, after which the films were dried in air. Next, CdSe QDs were deposited on the CdS-infiltrated TiO2films by a chemical bath deposition (CBD)
method. Here, CdSe deposition was achieved using NTA as a complex and selenosulfate as an Se source. First, NTA, and KOH were mixed to prepare a K3NTA solution that served as
the complex solution. Then, an aqueous Na2SeSO3solution
was freshly prepared as the Se source by refluxing 0.2 M Se powder in an aqueous solution of 0.5 M Na2SO3at 70 °C for
mixing 80 mM CdSO4, 160 mM K3NTA, and 80 mM
Na2SeSO3, and the CdS-infiltrated TiO2films were placed
in the solution at room temperature in the dark for 4 h to promote CdSe QD adsorption [27]. Our previous work shows that QDs could be well deposited on the TiO2inverse opal
film [22].
Preparation of counter electrodes
A compact Cu2S film was used as the counter electrode. Here,
a thin sheet of brass was cleaned and dipped in a 1 M HCL solution for 10 min. The brass sheet was then dipped in water and methanol (1:1 by volume) solution with 0.1 M Na2S,
0.1 M S, and 0.2 M KCl for 10 s to generate Cu2S on the
surface.
QDSSC device assembly and characterization
The electrolyte was composed of 1 M Na2S, 0.1 M S, and
0.2 M KCl in a water/methanol (1:1 by volume) solution and was injected between the photoanode and counter elec-trode through the siphon effect. The PV performances, which include the short-circuit current density (Jsc), open-circuit
voltage (Voc), fill factor (FF), and maximum PCE (η), of the
cells were examined by measuring the current density-voltage (J-V) characteristics of the cells using a Keithley 2450 source meter under a light intensity of 100 mW cm−2provided by a solar simulator (Newport, 911023A). The optical absorption spectra of the photoanodes were measured using a spectropho-tometer (Shimadzu, UV-2450). The TiO2 morphology was
analyzed by SEM (JEOL, JSM7100F), and EIS measurements were obtained using an electrochemical workstation (CorrTest, CS 350H) under a light intensity of 100 mW cm−2.
Results and discussion
The values ofη obtained for QDSSCs assembled with single-layer TiO2inverse opal photoanodes fabricated with different
PS microsphere diameters and solution concentrations are listed in Table 2. In addition, the J-V characteristics of the QDSSCs are presented in Fig.1a–dfor PS microsphere diam-eters of 100 nm, 350 nm, 500 nm, and 750 nm, respectively. We note from Table 2 that the value of η obtained for QDSSCs employing a photoanode fabricated with a given PS microsphere diameter increases with an increasing PS mi-crosphere solution concentration until achieving a maximum, Table 1 Experimental materials
Material Chemical formula Purity Manufacturer
Cadmium nitrate tetrahydrate Cd(NO3)2·4H2O ≥ 98.0% Sigma-Aldrich Chemical Co.
(St. Louis, MO, USA) Sodium sulfide nonahydrate Na2S·9H2O ≥ 98.0%
Selenium Se ≥ 99.5%
Sodium sulfite Na2SO3 ≥ 98.0%
Cadmium sulfate hydrate CdSO4·8/3H2O ≥ 99.0%
Nitrilotriacetic acid (NTA) C6H9NO6 ≥ 99.0%
Sulfur S ≥ 99.9%
Potassium chloride KCl ≥ 99.5%
Potassium hydroxide KOH ≥ 85.0%
Ammonium hexafluorotitanate (IV) (NH4)2TiF6 ≥ 99.9%
Boric acid H3BO3 ≥ 99.9%
IGEPAL® CO-520 (C2H4O)n·C15H24O
Titanium (IV) isopropoxide Ti[OCH(CH3)2]4 Reagent grade Sinopharm Chemical Reagent Co., Ltd.
(Shanghai, China)
Isopropyl alcohol (CH3)2CHOH
Methanol CH4O
Ethanol C2H6O
Acetone CH3COCH3
Hydrochloric acid HCL
Nitric acid HNO3
Conductive FTO-coated glass Yinkou OPV Tech New Energy Co. Ltd.
(Yinkou, China) Cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)
ruthenium (II) bis-tetrabutylammonium ruthenium dye
N719
Polystyrene latex microspheres 2.5 wt% dispersion in water
after whichη generally decreases with further increases in the solution concentration. These trends are also very clearly rep-resented in theJ-V curves shown in Fig.1. Here, an increasing PS microsphere solution concentration results in increased TiO2layer thickness, which enhances the photocurrent, and
thus increases the cell efficiency. However, the thickness of the TiO2layer also affects the transport of charge carriers,
which will eventually be hindered with increasing layer thick-ness. In addition, the pore size of the TiO2 inverse opal
photoanodes affects the deposition of QDs, the infiltration of
the electrolyte, and the transport of charge carriers. Accordingly, the diameter of the PS microspheres also im-pacts the performance of the QDSSCs, as shown by the results in Table2and Fig.1. Here, the QDSSC adopting the single-layer TiO2inverse opal photoanode fabricated with a PS
mi-crosphere diameter of 500 nm attains the highest cell efficien-cy of 4.80% at a PS microsphere solution concentration of 0.048%. Again, we note the presence of countervailing trends as the PS microsphere diameter increases to 750 nm, where the larger diameter begins to negatively affect the deposition of QDs, the infiltration of the electrolyte, and the transport of charge carriers, resulting in cell efficiencies among the worst obtained. However, theirJ-V curves are closer to ideal char-acteristics than those of the QDSSCs fabricated with PS mi-crosphere diameter of 100 nm.
The PV parameters of the QDSSCs assembled with double-layer TiO2inverse opal photoanodes fabricated with
different PS microsphere diameters and solution concentra-tions are listed in Table 3. The various combinations of PS microsphere diameters and solution concentrations resulted in a total of eight double-layer photoanodes of roughly equiva-lent thicknesses. As seen, most of the double-layer TiO2
in-verse opal photoanode-based QDSSCs achieve higher cell Table 2 Maximum power conversion efficiency (η) of QDSSCs based
on single-layer TiO2inverse opal photoanodes fabricated with various PS
microsphere diameters and solution concentrations PS sphere diameter (nm) Concentration (wt %)
0.024 0.048 0.072 0.096
100 2.55 2.64 2.89 1.90
350 1.90 2.49 2.56 2.63
500 3.80 4.80 4.30 3.90
750 1.96 2.35 2.82 2.17
Fig. 1 Current density-voltage (J-V) characteristics of QDSSCs employing single-layer TiO2inverse opal photoanodes fabricated with various PS
efficiency than the QDSSCs assembled with single-layer TiO2
inverse opal photoanodes. Here, QDSSCs assembled with photoanodes having a bottom layer fabricated with a PS mi-crosphere diameter of 350 nm and upper layers fabricated with PS microsphere diameters greater than 350 nm are denoted herein as QDSSC A, B, C, and D, as shown in Fig. 2. In contrast, QDSSCs assembled with photoanodes having an up-per layer fabricated with a PS microsphere diameter of 350 nm and bottom layers fabricated with PS microsphere diameters greater than 350 nm are denoted herein as QDSSC E, F, G, and H. In addition, theJ-V characteristics of QDSSCs A–D and E–H are presented in Fig.3a and b, respectively.
We note from Table3that QDSSC B achieves the highest η value of 5.79%, along with generally the highest values for
all PV performance parameters considered. This is relatively high efficiency for a QDSSC based on CdS/CdSe QDs with-out any further modification to the photoanode [28,29]. Interestingly, QDSSC F provides a substantially lowerη value of 5.07% even though the only difference between QDSSCs F and B is that the order of the upper and bottom layers is reversed.
These interesting results can be explained according to the schematic illustration shown in Fig.2. Here, the larger pores in the upper layer are expected to facilitate the entry of inci-dent light irradiation and the electrolyte solution and are also beneficial for facilitating the deposition of QDs. Moreover, the larger pores in the upper layer facilitate the collection of photoelectrons, which results in greater photocurrent and cell Table 3 Layer parameters and
photovoltaic parameters, including the short-circuit current density (Jsc), open-circuit voltage
(Voc), fill factor (FF), and
maxi-mum power conversion efficien-cy (η), of the QDSSCs assembled with double-layer TiO2inverse
opal photoanodes fabricated with different PS microsphere diame-ters and solution concentrations
Sample Bottom layer Upper layer Voc
(V) Jsc(mA/ cm2) FF η (%) Diameter (nm) Concentration (%) Diameter (nm) Concentration (%) A 350 0.048 500 0.024 0.59 19.1 0.482 5.43 B 350 0.048 750 0.024 0.58 19.9 0.499 5.79 C 350 0.024 500 0.048 0.56 13.5 0.471 3.56 D 350 0.024 750 0.048 0.58 17.2 0.452 4.60 E 500 0.024 350 0.048 0.57 22.3 0.415 5.28 F 750 0.024 350 0.048 0.57 17.0 0.515 5.07 G 500 0.048 350 0.024 0.57 16.1 0.522 4.81 H 750 0.048 350 0.024 0.57 16.4 0.524 4.91
Fig. 2 Schematic illustrating the structure of a double-layer TiO2
efficiency. In addition, the specific surface area of the TiO2
layer also has a pronounced effect on the light-scattering prop-erties of the layer, where the scattering effect increases with the increasing specific surface area [7–9]. Finally, we note that the denser TiO2structure of the bottom layer can be expected
to facilitate the transport of photoelectrons generated by the QD sensitizers. The structures of the photoanode applied in QDSSC B, as well as the corresponding PS microsphere tem-plate, are further clarified by the cross-sectional SEM image shown in Fig.4. We note that the TiO2framework and PS
microsphere template shown in Fig.4 agree well with the schematic illustration shown in Fig.2.
The dye adsorptive capacities of the double-layer TiO2
in-verse opal films applied in QDSSCs B and H (Table3) were investigated via adsorption-desorption analysis. Here, N719
dye was used instead of QDs to facilitate the examination of the desorption process. In addition, the dye adsorptive capacity of a single-layer TiO2inverse opal film fabricated with a PS
microsphere diameter of 350 nm and a solution concentration 0.096% was also measured and is denoted herein as sample S. This resulted in a single-layer film with a thickness roughly equivalent to the thicknesses of samples B and H. Each of the TiO2films was first immersed in a 0.5 mM N719 dye solution
in ethanol and stored at room temperature in the dark for 24 h for ensuring adequate adsorption of the dye molecules and then rinsed with ethanol. A 0.1 M NaOH solution was prepared in a water/methanol (1:1 by volume) solution to resolve N719 on the TiO2film. A 2 mL resulting solution was measured by
ultraviolet-visible (UV-vis) spectrophotometry, and the results are shown in Fig.5. As can be seen, the absorption peaks of samples B and H are both greater than the absorption peak of sample S, indicating that a greater concentration of dye mole-cules remained absorbed on the surfaces of samples B and H. Moreover, sample H, with larger pores in the bottom layer, absorbed dye molecules more effectively than sample B.
The effects of different photoanode structures on QDSSC performance were explored further using EIS measurements conducted from 1 to 105Hz under 100 mW cm−2illumination. Here, QDSSCs assembled using photoanode samples A, B, and H were tested. The Nyquist plots of the QDSSCs are shown in Fig.6a, where two semicircles are observed for all QDSSCs at high frequencies and middle frequencies. For each QDSSC, the large semicircle appearing at the middle frequen-cies represents the impedance at the photoanode/QD/electro-lyte interface, while the small semicircle appearing at the high frequencies represents the redox impedance at the electrolyte/ counter electrode interface. The semicircles can be represent-ed by the equivalent circuit shown in the inset in Fig.6a[30,
31]. The equivalent circuit is composed of a series resistance Rs, charge transfer resistancesRct1andRct2at the electrolyte/
counter electrode interface and at the TiO2/QD/electrolyte
in-terface, respectively, and the chemical capacitancesCPE1and
CPE2corresponding to the electrolyte/counter electrode and
TiO2/QD/electrolyte interfaces. A standard fitting process was
applied according to the equivalent circuit model [31,32]. The values ofRs,Rct1, andRct2obtained from the fitting process
are listed in Table4. The Bode diagrams of the QDSSCs are shown in Fig.6b. The lifetimeτ of the injected electrons in the TiO2 photoanode is related to the position of the
mid-frequency peakfmaxand is defined as follows [27,33]:
τ ¼ 2πf1
max
ð1Þ
The obtained values ofτ are listed in Table4as well. We note from Table4that the injected electron lifetimes obtained for the QDSSCs with double-layer TiO2inverse opal
Fig. 3 Current density-voltage (J-V) characteristics of QDSSCs employing double-layer TiO2inverse opal photoanodes fabricated with
photoanode structures conforming to Fig.2, i.e., small pores (350 nm) in the bottom layer and larger pores in the upper layer (QDSSCs A and B), are considerably greater (3.008 ms) than that obtained for the QDSSC photoanode with the re-versed structure (i.e., 1.194 ms for sample H). We note for QDSSCs A and B that, although the larger pores in the upper layer of the QDSSC B photoanode (750 nm) than those of QDSSC A (500 nm) result in a higher interfacial charge trans-fer resistanceRct2(15.13Ω) than that of QDSSC A (7.726 Ω),
its series resistanceRs (0.872 Ω) is much less than that of
QDSSC A (1.639 Ω). However, we also note that the QDSSC assembled with the single-layer TiO2 inverse opal
photoanode (QDSSC S) has a longer lifetime (3.804 ms) than those of all the QDSSCs based on the double-layer TiO2
in-verse opal photoanodes tested and also has moderate values of RsandRct2. These results demonstrate that the performances
of the photoanodes and corresponding QDSSCs are the result of comprehensive effects. For example, the photoanode employed in QDSSC H adsorbs dye better than the photoanode employed in QDSSC B, but the carrier transport of QDSSC B is better. In addition, while the double-layer TiO2inverse opal photoanodes reduce the charge carrier
life-time relative to that of the single-layer photoanode and may increase the interfacial resistances, a well-designed double-layer photoanode with an appropriate dual-double-layer configuration and layer thicknesses leads to better cell performance.
The double-layer TiO2inverse opal-based photoanode is
expected to be applicable to other QD systems, for example, ternary cadmium sulfoselenide (CdSxSe1-x) QDs. The ternary
alloyed CdSxSe1-xQDs has a tunable band gap between those
of CdSe and CdS without reducing the dimension and it was used to yield high PCE of QDSSCs [34,35]. A double-layer TiO2 inverse opal-based CdSSe QDSSC is expected to
achieve further enhanced cell efficiency, and more work would be carried out in the next step in our research group.
Conclusions
The present work proposed double-layer TiO2inverse
opal-based photoanodes for use in QDSSCs. The effects of the pore size obtained using PS microsphere templates and the film Fig. 4 Cross-sectional scanning
electron microscopy (SEM) im-age of the double-layer TiO2
in-verse opal film (a) and its PS mi-crosphere template (b) of QDSSC B
thickness on the performance of single-layer TiO2inverse
opal-based photoanodes were first investigated, and the highest cell efficiency of 4.80% was achieved for a PS micro-sphere diameter of 500 nm. The PV performances of various QDSSCs employing double-layer TiO2 inverse opal
photoanodes with different pore sizes in the layers were tested.
The QDSSC adopting the optimum photoanode configuration and thickness provided the highest QDSSC conversion effi-ciency of 5.79%.
Funding information This work was supported by the Grants No. 11804032 from the National Natural Science Foundation of China and No. 201801023A from the Intellectual Property Office of Hubei Province of China.
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