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Hybrid dye sensitized solar cell based on single layer graphene quantum dots
Journal Pre-proof Hybrid dye sensitized solar cell based on single layer graphene quantum dots Farhad Jahantigh, S.M. Bagher Ghorashi, Amir Bayat PII:
S0143-7208(19)31020-4
DOI:
https://doi.org/10.1016/j.dyepig.2019.108118
Reference:
DYPI 108118
To appear in:
Dyes and Pigments
Received Date: 5 May 2019 Revised Date:
27 November 2019
Accepted Date: 7 December 2019
Please cite this article as: Jahantigh F, Ghorashi SMB, Bayat A, Hybrid dye sensitized solar cell based on single layer graphene quantum dots, Dyes and Pigments (2020), doi: https://doi.org/10.1016/ j.dyepig.2019.108118. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Hybrid dye sensitized solar cell based on single layer graphene quantum dots *
Farhad Jahantigha,b, S.M. Bagher Ghorashia ,Amir Bayat b a
Atomic and Molecular Group, Faculty of Physics, University of Kashan, Iran
b
Department of Physics, Faculty of Basic Science, Tarbiat Modares University
Abstract Single layer graphene quantum dots (SLGQDs, average size of ∼ 9 nm) were added into N719/TiO2 nanoparticles photoanode (prepared using a doctor blade method) as cosensitizer and photovoltaic properties were investigated for dye sensitized solar cell (DSSC) application. Low-cost and high-yield SLGQDs solution was synthesized with only glucose and DI water as precursor using a hydrothermal method. This method allows the tuning of optical properties and energy states by using appropriate precursors and synthesis conditions. Optical characterization reveals that the SLGQDs solution absorbs UV and visible light photons with wavelengths up to ∼ 700 nm. For engineering a suitable energy state, and then to obtain an efficient DSSC, lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) of the SLGQDs were determined against vacuum energy level by utilizing a cyclic voltammetry measurement. Electrical measurements indicated an improvement in power conversion efficiency for the DSSC fabricated based on SLGQDs and N719 as co-sensitizers. The experimental analysis shows that this improvement arises from enhancement of charge collection and separation due to the cascaded alignment of the energy levels between N719/SLGQDs/TiO2 interfaces. By addition of SLGQDs into N719/TiO2 nanoparticles photoanode, we were able to increase the short circuit current density and efficiency by 39% (from 14.47 to 20.03 mA/cm2) and 35% (from 6.57% to 8.92%), respectively.
Keywords: glucose, dye sensitized solar cell, hydrothermal, cyclic voltammetry *
Dr S.M. Bagher Ghorashi. Tel.: +98-315-591-2398; fax: +98-315-591-2570; e-mail: [email protected].
1. Introduction Dye-sensitized solar cells (DSSCs) present a promising third generation photovoltaic device and due to low-cost preparation, relatively high energy-conversion efficiency, and simplicity have been widely considered for the next class of low-cost solar cells [1-3]. Photoanode electrode is one of the most important components in DSSCs. In a DSSC, photoanode electrode influences both photovoltage and photocurrent, thus efficiency of the DSSCs depends mostly on photoanode electrode [4-6]. TiO2 is a n-type semiconductor and its anatase phase (band gap 3.2 eV) is specially preferred to utilize as a wide band gap material in DSSC. TiO2 nanostructure due to its special properties such as good electron transfer, high porosity, suitable energy level of the valence and conduction bands, and high refractive index [7], absorbs the dye molecules and helps in acceptance of electron from the dye [8]. In a typical DSSC based on TiO2 photoanode, for increasing charge collection efficiency of photogenerated electrons any recombination should be avoided. Actually, one of the main drawbacks is related to the charge recombination between dye molecules and TiO2 nanoparticles that reduces the efficiency of the solar cell[9, 10]. To reduce charge recombination and improve the charge collection efficiency, incorporation of carbon-based nanomaterials in the photoanode (specially based on TiO2 and ZnO) is a promising approach for reducing the photoelectrons’ loss[11, 12]. Among various carbon nanomaterials, addition of graphene quantum dots (GQDs) onto TiO2 nanostructures can enhance the performance of DSSC due to its excellent optical properties and low resistance. Due to special properties of GQDs such as non-zero band gap, excellent photostability, low cytotoxity, chemical inertness, cost effective preparation methods, GQDs have been used for supercapacitors, bio imaging, water splitting, and photovoltaic devices [13, 14]. Properties of GQDs such as band gap energy are tunable by changing surface functionalization and their size [15]. Tunable band gap of GQDs allows to designing an appropriate energy states for LUMO and HOMO that leads to a multistep cascade energy levels in the photoanode[16]. Therefore, a method to increase efficiency of the DSSCs is to use co-sensitizers containing both conventional dyes (N3 or N719) and GQDs leading to reducing the photoelectrons’ loss. Recently, carbon quantum dots have been successfully utilized as co-sensitizers in DSSCs [17]. Huang et. al reported nitrogen doped graphene quantum dots for DSSC application and have obtained efficiency of 0.13 for their optimum sample [18]. Radoi et. al reported GQDs/N3 dye sensitized TiO2 nanoparticles and efficiency
of 2.15 was obtained for their work [19]. Li et. al reported GQDs optimization of DSSCs and have obtained efficiency of 6.1 for their optimum sample [20]. Subramania et. al reported Graphene quantum dots decorated electrospun TiO2 nanofibers for DSSCs. They have used electrophoretic filling for decoration of TiO2 nanofiber with GQDs and efficiency of 6.22 was obtain for optimum sample[21]. To the best of our knowledge, single layer graphene quantum dots (SLGQDs) as hybrid co-sensitizers have never been reported. Here, we have used SLGQDs for sensitizing TiO2 nanoparticles (NPs) film. Highly porous TiO2 NPs film as a wide band gap semiconductor was obtained using a doctor blade method. The effect of engineered SLGQDs on the photovoltaic parameters of N719/TiO2 photoanode was investigated by means of current voltage (I-V) and electrochemical impedance spectroscopy (EIS) measurements. Explaining the photoelectron generation under illumination and electron transfer mechanism in N719/SLGQDs/TiO2 interfaces, could be informative for using SLGQDs in solar cells.
2. Experimental 2.1 Preparing of TiO2 paste
For preparation of TiO2 paste, 0.5 g of TiO2 nanoparticles powder (25 nm) was dispersed in 10 ml ethanol using sonication. After that, 0.9 g ethyl cellulose was added to the solution and stirred vigorously for 20 min. Then, after 20 min the amount of 3.5 g terpineol slowly was added to the composite on the stirrer. For evaporation of ethanol the prepared paste placed at 50 ˚C for 2 h. 2.2 Preparation of TiO2 nanoparticle (NPs) films
For preparing TiO2 NPs films, at first FTO glasses (1⨯2 cm2, 15 Ω/cm2) were cleaned using sonication with DI water, acetone and ethanol for 15 min, respectively. Afterwards a compact blocking layer of TiO2 was deposited onto FTO substrate by immersing FTO into 40 mM aqueous TiCl4 solution for 30 min at 70˚ C and then cleaned with DI water and ethanol. To fabricate TiO2 NPs films, a layer of TiO2 paste was deposited by doctor blade technique over blocking layer. After drying the films for 6 min at 125˚ C, the films were heated at 325˚ C for 5 min, at 375˚ C for 5 min, at 450˚ C for 15 min, and 500˚ C for 30 min. Finally, the films again were immersed in 40 mM aqueous TiCl4 solution and sintered at 500˚ C for 30 min.
2.3 Synthesis of SLGQDs SLGQDs solution prepared according to our previous work with some effective modifications[22]. At first, 50 mg of glucose (white powder) was dissolved in 50 ml of DI water to obtain a colorless solution. Then, one-step hydrothermal method in a Teflon-lined stainless-steel autoclave (75 ml) at 200 ˚C for 8 h was used. Final product was pale brown after synthesis of SLGQDs. This method allows to tuning the energy levels of HOMO and LUMO by employing appropriate precursors. 2.4 Sensitizing of TiO2 NPs films with SLGQDs and N719 To sensitize TiO2 NPs films with the SLGQDs and N719, TiO2 NPs films were immersed in a solution of SLGQDs with concentration of 1 g/l for 6 h, after that placed at temperature of 120˚ C for 12 h. Then, after absorption of SLGQDs on the TiO2 nanoparticles, the films were immersed into a 0.4 mM N719 dye solution in ethanol for 24 h. For comparison the simultaneous effect of SLGQDs-N719 on the efficiency of the cells, also the TiO2 photoanodes based on SLGQDs and N719 were prepared separately. 2.5 Preparation of Pt counter electrode For preparing platinum counter electrode, a hole with diameter about 1 mm was drilled on the FTO glasses to allow the electrolyte introduction later. Then the FTO glasses were cleaned with DI water and ethanol. Then, the FTO glasses were heated to 470 ˚C for 15 min. The thermal decomposition method was employed to deposit the Pt catalyst on the FTO glasses. For this purpose, a drop of 0.3 mM H2PtCl6 solution in ethanol was drop casted on the FTO substrates and annealed at 470 ˚C for 15 min. 2.6 Electrolyte The electrolyte I-/I3- consisted of 0.1 M LiI, 0.1 M I2, 0.5 M 4-tert-butylpyridine, and 0.6 M Tetrabutylammonium iodide in acetonitrile was used. 2.7 DSSC assembly The SLGQDs, N719 and SLGQDs-N719 loaded TiO2 nanoparticle photoelectrodes and the Pt counter electrodes were sealed together with a 30 µm Surlyn (Dyesol) spacer around the TiO2 active area (0.25 cm2). The liquid electrolyte (I-/I3-) was injected into assembled cells. At the end, the hole of counter electrode was covered by a glass (1cm⨯1cm) and sealed by a spacer. TiO2 NPs, SLGQDs/TiO2 NPs, N719/TiO2 NPs and (SLGQDs-
N719)/TiO2 NPs films were prepared for DSSCs measurements. The schematic illustration of the (SLGQDs-N719)/TiO2 NPs under illumination is shown in Fig. 1. 2.8 Characterization The structural morphology of the nanomaterials was investigated using a transmission electron microscope (TEM, CM30 300kV) and field emission scanning electron microscope (FESEM). UV-vis absorption measurement was performed using a spectrophotometer (Unico 4802). Atomic force microscopy (AFM, Veeco Autoprobe CP research) was used to investigate the height profile of the SLGQDs on mica as substrates. Raman technique (BRUKER, Germany, SENTERRA) was utilized to prove the formation of SLGQDs on the TiO2 nanoparticles. X-ray diffraction patterns (XRD, Co-Kα radiation source, Philips, X'Pert MPD) were utilized in order to study the lattice structures. The current-voltage (I-V) characteristics of the fabricated DSSCs were measured under solar simulator illumination of AM 1.5 (100 mW/cm2). The electrochemical impedance spectroscopy (EIS) measurements of the cells were carried out with an Iviumstat. Cyclic voltammetry (CV) measurement was carried out with a three electrode configuration. Ag/AgCl as the reference electrode, a platinum plate as the counter electrode, and SLGQDs/glassy carbon as the working electrode in an acetonitrile solution including 0.1 M Tetrabutylammonium hexafluorophosphate (TBAPF6) was used.
Fig. 1. Schematic illustration of the DSSC based on (SLGQDs-N719)/TiO2 NPs as photoanode.
For estimation of the amount of N719 on the N719/TiO2 and (SLGQDs-N719)/TiO2 photoanodes, the standard addition method [23, 24] was used. The concentration of absorbed dyes was estimated by first desorbing the dyes from the N719/TiO2 and (SLGQDsN719)/TiO2 photoanodes in 0.1 M NaOH aqueous solution and then analyzing by UV-Vis spectrophotometer. According to this analysis, the number of 8.21⨯1016 cm-2 and 9.12⨯1016 cm-2 was obtained for N719/TiO2 and (SLGQDs-N719)/TiO2 photoanodes, respectively. This enhancement shows that addition of SLGQDs on the TiO2 increases the effective surface area leading to more absorption of N719. Ratio of the N719-dye absorption after addition of SLGQDs was estimated about 1.1.
3. Results and discussion 3.1 Characterization study FESEM analyze was utilized to characterize the morphology of TiO2 NPs films and results are shown in Fig. 2. Top view FESEM image (Fig. 2-a) reveals that the entire surface of the FTO substrate is covered uniformly with TiO2 NPs with average particle size of 25 nm. Fig. 2-b shows the cross-section image of highly porous TiO2 NPs film with thickness of about 10 µm deposited using doctor blade method. According to this figure, TiO2 NPs photoanode has a nanoporous structure. This porosity has an extremely large specific surface area that enhances light harvesting as well as dye adsorption. The increased surface area leads to increase of the adsorbed SLGQDs and dye molecules (N719). Fig. 2-c shows XRD patterns of the highly porous TiO2 NPs film deposited on FTO substrates. TiO2 NPs film showed the peaks of (103), (004), (105), (211), (213) and (204), which corresponded to the pure anatase phase of TiO2 [25]. The diffraction patterns of TiO2 NPs films are consistent with the tetragonal structure of TiO2 (JCPDS Card No. 71–1166). In the XRD patterns, strong peaks indicate high crystallinity of the TiO2 NPs. The peaks of brookite and rutile phases were not detected, indicating the purity of the anatase phase. The red peaks (showed by star) originate from the FTO substrate.
Fig. 2. FESEM images of the TiO2 NPs film, top view (a) and crosss section (b). (c) XRD patterns of the anatase TiO2 NPs grown on the FTO substrate.
For AFM analysis, a drop of SLGQDs solution was drop casted on the mica surface and the result is shown in Fig. 3-a. As we know, graphene has a thickness of about ∼0.35 nm [26] that can be detected on a smooth substrate such as mica. As can be seen in Fig. 3-b, topographic height of the SLGQDs is less than 0.5 nm, suggesting that the obtained samples are single-layered. Fig. 3-c indicates the TEM image of the SLGQDs particles. This figure shows that the as-prepared particles have a uniform dispersion without any aggregation with an average width of about 9 nm. UV–vis absorption spectra of the SLGQDs, N719, and SLGQDs/N719 samples are shown in Fig. 3-d. Absorption spectrum of N719 dye solution shows three peaks within the range of 300–800 nm. These peaks are characteristics of N719 dye [27]. SLGQDs solution has a wide absorption curve over the visible light that originates from functional groups such as hydroxyl and carboxyl [14]. According to UV–vis absorption spectra, absorption region is recorded at all visible wavelengths. As can be seen, the SLGQDs/N719 solution has a stronger absorption curve related to the effective overlapping between SLGQDs and N719 that can be used for photovoltaic applications.
Fig. 3. (a) AFM image of the SLGQDs on the mica substrate, (b) height profile of the SLGQDs (line1), (c) TEM image of the SLGQDs and (d) shows the UV-Vis absorption spectra of SLGQDs, N719 and SLGQDs/N719, as well as the photograph of the SLGQDs solution.
To obtain an estimate for the band gap energy, Tauc plot method was used. By plotting the square of the absorption energy (αhv, where α is the absorbance and hv is the photon energy) versus hv, the band gap energy of TiO2 and TiO2/SLGQDs were determined about 3.1 and 2.9 eV (Fig. 4), respectively. As can be seen, SLGQDs decrease the band gap of TiO2 that can be used for higher absorption over the visible light.
Fig. 4. Tauc plots for TiO2/SLGQDs and TiO2 NPs showing estimated band gap energy.
Electrochemical method allows to determination the absolute positions of the energy levels, relative to a reference such as vacuum or Ag/AgCl, that is necessary in developing further photoelectrochemical experiments. Thus, cyclic voltammetry has been used to investigate the effects of quantum confinement on the LUMO and HOMO energy states. In order to elucidate the role of SLGQDs in mediating the charge transfer between N719 and TiO2 NPs, CV measurement was used. In our previous [22] work we comprehensively have studied the method for determination of LUMO and HOMO energy levels of the SLGQDs. Briefly, electrode for CV measurement was prepared by drop-casting of SLGQDs solution (5 µl) and nafion (0.5 µl, 5% w) onto a polished glassy carbon electrode, then the electrode dried naturally at room temperature during two days. The SLGQDs/glassy carbon electrode depicts an oxidation peak at +2.2 V and a reduction peak at −0.7 V versus Ag/AgCl as the reference. The values for ELUMO and EHOMO are ELUMO= −4.25 eV and EHOMO= −7.15 eV versus vacuum scale, respectively. Raman analysis was utilized to confirm the formation of graphitic Raman modes of SLGQDs and TiO2 anatase phase as well as existence of SLGQDs on the TiO2 NPs surfaces. Fig. 5 shows the Raman spectrum of SLGQDs decorated on the TiO2 NPs. In the spectrum only peaks corresponding to anatase active vibration modes are identified at 401 cm-1 (B1g(1)), 521 cm-1 (B1g(2) + A1g), 635 cm-1 (Eg(2)) [28]. Obviously, two additional peaks at 1354 cm-1 (for D band) and 1600 cm-1 (for G band) for the graphitic structures were observed. D band is corresponding to disorder graphitic structure while the G band is attributed to sp2 hybridized carbon [29, 30]. The Raman spectrum confirms the incorporation of SLGQDs on the surfaces of TiO2 NPs.
Fig. 5. Raman spectrum of the SLGQDs/TiO2 NPs.
3.2 Energy level study The schematic illustration of energy level diagram for anatase TiO2, SLGQDs and N719 dye were used as sensitizers are presented in Fig. 6. In the photoanodes, the N719-dye and SLGQDs sensitizers are interfaced with the TiO2 NPs. Under illumination, in a typical dye solar cell based on TiO2 as the base semiconductor, the electron excited from HOMO into the LUMO energy level is injected into the conduction band (CB) of TiO2, while, simultaneously, the oxidized sensitizer is regenerated by the redox couple (I-/I3-). Fig. 6 shows the energy level diagram of the various parts of the photoanode that evidences the compatibility of the N719 dye, with the prepared SLGQDs that act as additional light harvesters. Thus, while the LUMO and HOMO energy levels of the N719 are at -3.46 and -5.68 eV, respectively [31], the SLGQDs HOMO energy level is -7.15 and LUMO energy level is -4.25 eV. This order for alignment corresponds to a type II alignment of the energy states in hybrid systems. As explained, the LUMO energy level of SLGQDs is higher than the conduction band of the anatase TiO2 NPs (-4.4 eV [32]), which shows that the electronic transfer from the SLGQDs to the TiO2 nanoparticles takes place when the SLGQDs are excited under illumination. At this situation, effective separation of holes and electrons in the SLGQDs and the fast injection of photoelectron into the conduction band of TiO2 through the heterojunctions can be occurred. Furthermore, active holes move into the HOMO energy of N719, where reduce by electrolyte.
Because of mismatch between the energy level positions of the SLGQDs and N719, a cascaded band edge alignment is occurred. This effect leads to transfer of electrons from the LUMO level of the N719 to the TiO2 electrode through the SLGQDs, and consequently increases the charge separation and electron injection.
Fig. 6. Schematic illustration of energy level diagram for anatase TiO2, SLGQDs and N719 dye.
3.3 Current measurements Influence of SLGQDs and N719 on the amount of light harvesting in the prepared DSSC was systematically investigated in terms of fill factor (FF), short-circuit current (Jsc), open-circuit voltage (Voc), power conversion efficiency (PCE), and charge transfer resistance (Rct). To investigate the role of SLGQDs as efficient co-sensitizers, 4 cells were prepared: cells without sensitizing (only TiO2 NPs); cells sensitized only with SLGQDs (SLGQDs/TiO2 NPs); cells sensitized only with N719 (N719/TiO2 NPs); and cells with sensitizers consisting of SLGQDs and N719 dye (SLGQDs-N719)/TiO2 NPs. The cell without sensitizing consists of a TiO2 NPs film (thickness of 10 µm, see Fig. 2-b). The current density voltage (J–V) characteristics of the DSSCs are shown in Fig. 7 and the photovoltaic parameters obtained from J-V curves are listed in Table 1. All experiments were done for three times and for every cell we prepared three sample and reported average of them. Standard deviations of photovoltaic values were added in the Table 1. In the (SLGQDs-N719)/TiO2 NPs cell, N719 and SLGQDs sensitizers are interfaced with the TiO2 NPs. The position of conduction band (CB) for anatase TiO2 is -4.4 eV against vacuum scale [32]. In other hand, LUMO state of graphene quantum dots is higher than CB of anatase TiO2 (as can be seen in Fig. 6). Therefore, electron transfer from LUMO of SLGQDs to TiO2 CB is thermodynamically
favorable. When SLGQDs excited under illumination, generated electrons more efficiently transfer to conduction band of TiO2. The short circuit current density (Jsc) and open circuit voltage (Voc) are 20.03 mA/cm2 and 0.73 V, respectively for the (SLGQDs-N719)/TiO2 NPs cell, improved with respect to the bare TiO2 cells. Therefore, the dye solar cell fabricated using (SLGQDs-N719)/TiO2 NPs indicated an increased power conversion efficiency (PCE, η=8.92%) with respect to the DSSC fabricated using bare TiO2 NPs. A significant improvement of the Jsc (~39%) is observed for (SLGQDs-N719)/TiO2 NPs cell compared to N719/TiO2 NPs cell. The power conversion efficiency (η) increases with 35% versus N719/TiO2 NPs due to the larger Jsc. The charge transfer kinetics is thermodynamically faster for (SLGQDs-N719)/TiO2 NPs electrode versus N719/TiO2 NPs as proved by the higher generated charge versus applied potential. The cascaded energy level structure between the N719 and SLGQDs facilitates the electron injection into TiO2 NPs. This shows that the SLGQDs play a dual role in the fabricated hybrid (SLGQDs-N719)/TiO2 NPs DSSC. This role leads to the improving of the charge transfer properties and light harvesting efficiency. The high efficiency of the (SLGQDs-N719)/TiO2 NPs DSSC is due to the following effects; (i) Highly porous TiO2 NPs film allows more absorption of N719 molecules leading to generation of more number of photoelectrons; (ii) addition of SLGQDs increases the effective surface area for dye loading in a ratio of 1.1; (iii) due to presence of SLGQDs, photogenerated electrons transport faster from N719 to conduction band of TiO2. So these effects contributed to increase in the Jsc and PCE of the DSSC. As the results show, in a hybrid DSSCs, it is reasonable to assume that the SLGQDs are effective co-sensitizer for light harvesting. As shown by photoelectrical measurements, an improvement in PCE was achieved when SLGQDs were used as co-sensitizers together with N719.
Fig. 7. Current density–voltage characteristics of TiO2 NPs and sensitized TiO2 NPs as photoanodes in DSSCs.
3.4 Electrochemical impedance spectroscopy (EIS) For the analysis of ionic and electronic transport processes in the DSSCs EIS measurements were used [33]. To analyze the charge carrier dynamics of the photoanodes and investigate difference in the interfacial characteristics, EIS of the fabricated cells was performed at an applied bias of Voc and a frequency ranging from 0.01 Hz to 1 MHz with AC amplitude of 10 mV under illumination AM 1.5. Fig. 8 indicates the typical Nyquist plots of the fabricated DSSCs. Under illumination, there are two well-defined semicircles. The first semicircle in the high-frequency region represents the redox reaction of I-/I3- at the Pt/electrolyte interface, and the other semicircle in the low-frequency region denotes the electron transfer at the photoanode/electrolyte interface (Rct). As can be seen, at higher frequency, semicircles are identical prove that the addition of SLGQDs onto TiO2 NPs does not affect the charge transfer at Pt/electrolyte interface. The diameter (Rct) of the semicircle at low-frequency region corresponds to the resistance associated with the transfer of photogenerated electrons through the (N719SLGQDs)/TiO2/electrolyte interfaces. The Rct values for all cells are listed in Table 1. The values of Rct for the (N719-SLGQDs)/TiO2 cell at the photoanode/electrolyte interface decreases to some extent over all cells. This decrease in resistance at the (N719SLGQDs)/TiO2/electrolyte interface shows that the diffusion of electrons in this cell is faster and more effective than other cells. Presence of SLGQDs between N719 and TiO2 NPs
provides a fast and easy transport medium to charge carriers, which led to a decrease of 35% in Rct value compared with the cell based on N719/TiO2 NPs. The interface of (N719SLGQDs)/TiO2/electrolyte is significant as the interface controlling the photocurrent of the DSSC and the lower interfacial resistant is expected for high performance DSSC. Under excitation at AM 1.5, the second semicircle radius at low frequency in the Nyquist plot decreased after co-sensitizing with SLGQDs, and the values are in the order of (N719SLGQDs)/TiO2 < N719/ TiO2< SLGQDs/TiO2 < TiO2, which indicates an increase of charge transfer rate and a decrease of the electron transfer impedance at this interface.
Fig. 8. Nyquist Plots of sensitized DSSCs based on SLGQDs and N719. Inset shows the equivalent circuit.
Table 1. Photovoltaic values of DSSCs fabricated using bare TiO2 NPs and sensitized TiO2 NPs as photoanodes.
Sample
TiO2
SLGQDs/TiO2
N719/TiO2
(SLGQDs-N719)/TiO2
JSC (mA/cm2)
0.93±0.07
1.36±0.09
14.47±0.38
20.03±0.49
VOC (V)
0.34±0.03
0.46±0.03
0.71±0.01
0.73±0.01
FF
0.4±0.03
0.52±0.03
0.64±0.03
0.61±0.03
η (%)
0.12±0.01
0.33±0.03
6.57±0.32
8.92±0.49
R c t (Ω)
-
35±1
17±0.8
11±0.8
4. Conclusions In conclusion, a co-sensitized DSSC was designed based on N719 and low-cost SLGQDs. The fabricated solar cells exhibit significant improvement of Jsc and enhanced performance in the order of (N719-SLGQDs)/TiO2 > N719/ TiO2> SLGQDs/TiO2 > TiO2. Results showed that the cascaded energy level positions between the SLGQDs and N719 and high conductivity of SLGQDs facilitates the electron transfer and, consequently, a possible faster injection of electrons into the conduction band of TiO2. The (N719-SLGQDs)/TiO2 cosensitized dye solar cell yields the best overall efficiency of 8.92%, which is 35% greater than that of the dye solar cell containing only N719/TiO2. This study shows that the SLGQDs solution, as co-sensitizers is easily handled and inexpensive, and thereby deserves to be explored for the application in photovoltaic devices.
Acknowledgments
The research council of the University of Kashan is gratefully acknowledged for the financial support of this study.
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1. Using TiO2 nanoparticles film as a wide band gap semiconductor.
2. Utilization of single layer graphene quantum dots solution as an effective cosensitizer for DSSCs.
3. Obtaining maximum efficiency among all articles based on GQDs and TiO2.
Author Contributions Section Manuscript title: Hybrid dye sensitized solar cell based on single layer graphene quantum dots
Farhad Jahantigh: Data curation, Writing- Original draft preparation. S.M. Bagher Ghorashi: Supervision, Writing- Reviewing and Editing. Amir Bayat: Methodology, Formal analysis,
Investigation .
This statement is signed by all the authors to indicate agreement that the above information is true and correct.
Conflicts of Interest Statement Manuscript title: Hybrid dye sensitized solar cell based on single layer graphene quantum dots
This paper consists of original, unpublished work which is not under consideration for publication elsewhere, and that all co-authors have approved the contents of this manuscript and submission. Author names: Farhad Jahantigh, S.M. Bagher Ghorashi ,Amir Bayat
The authors whose names are listed immediately below certify that they have NO affiliations with or involvement in any organization or entity with any financial interest (such as honoraria; educational grants; participation in speakers’ bureaus; membership, employment, consultancies, stock ownership, or other equity interest; and expert testimony or patent-licensing arrangements), or non-financial interest (such as personal or professional relationships, affiliations, knowledge or beliefs) in the subject matter or materials discussed in this manuscript. Author names: Farhad Jahantigh, S.M. Bagher Ghorashi ,Amir Bayat
The authors whose names are listed immediately below report the following details of affiliation or involvement in an organization or entity with a financial or non-financial interest in the subject matter or materials discussed in this manuscript. Author names: Farhad Jahantigh, S.M. Bagher Ghorashi ,Amir Bayat
This statement is signed by all the authors to indicate agreement that the above information is true and correct.
S0143-7208(19)31020-4
DOI:
https://doi.org/10.1016/j.dyepig.2019.108118
Reference:
DYPI 108118
To appear in:
Dyes and Pigments
Received Date: 5 May 2019 Revised Date:
27 November 2019
Accepted Date: 7 December 2019
Please cite this article as: Jahantigh F, Ghorashi SMB, Bayat A, Hybrid dye sensitized solar cell based on single layer graphene quantum dots, Dyes and Pigments (2020), doi: https://doi.org/10.1016/ j.dyepig.2019.108118. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Hybrid dye sensitized solar cell based on single layer graphene quantum dots *
Farhad Jahantigha,b, S.M. Bagher Ghorashia ,Amir Bayat b a
Atomic and Molecular Group, Faculty of Physics, University of Kashan, Iran
b
Department of Physics, Faculty of Basic Science, Tarbiat Modares University
Abstract Single layer graphene quantum dots (SLGQDs, average size of ∼ 9 nm) were added into N719/TiO2 nanoparticles photoanode (prepared using a doctor blade method) as cosensitizer and photovoltaic properties were investigated for dye sensitized solar cell (DSSC) application. Low-cost and high-yield SLGQDs solution was synthesized with only glucose and DI water as precursor using a hydrothermal method. This method allows the tuning of optical properties and energy states by using appropriate precursors and synthesis conditions. Optical characterization reveals that the SLGQDs solution absorbs UV and visible light photons with wavelengths up to ∼ 700 nm. For engineering a suitable energy state, and then to obtain an efficient DSSC, lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) of the SLGQDs were determined against vacuum energy level by utilizing a cyclic voltammetry measurement. Electrical measurements indicated an improvement in power conversion efficiency for the DSSC fabricated based on SLGQDs and N719 as co-sensitizers. The experimental analysis shows that this improvement arises from enhancement of charge collection and separation due to the cascaded alignment of the energy levels between N719/SLGQDs/TiO2 interfaces. By addition of SLGQDs into N719/TiO2 nanoparticles photoanode, we were able to increase the short circuit current density and efficiency by 39% (from 14.47 to 20.03 mA/cm2) and 35% (from 6.57% to 8.92%), respectively.
Keywords: glucose, dye sensitized solar cell, hydrothermal, cyclic voltammetry *
Dr S.M. Bagher Ghorashi. Tel.: +98-315-591-2398; fax: +98-315-591-2570; e-mail: [email protected].
1. Introduction Dye-sensitized solar cells (DSSCs) present a promising third generation photovoltaic device and due to low-cost preparation, relatively high energy-conversion efficiency, and simplicity have been widely considered for the next class of low-cost solar cells [1-3]. Photoanode electrode is one of the most important components in DSSCs. In a DSSC, photoanode electrode influences both photovoltage and photocurrent, thus efficiency of the DSSCs depends mostly on photoanode electrode [4-6]. TiO2 is a n-type semiconductor and its anatase phase (band gap 3.2 eV) is specially preferred to utilize as a wide band gap material in DSSC. TiO2 nanostructure due to its special properties such as good electron transfer, high porosity, suitable energy level of the valence and conduction bands, and high refractive index [7], absorbs the dye molecules and helps in acceptance of electron from the dye [8]. In a typical DSSC based on TiO2 photoanode, for increasing charge collection efficiency of photogenerated electrons any recombination should be avoided. Actually, one of the main drawbacks is related to the charge recombination between dye molecules and TiO2 nanoparticles that reduces the efficiency of the solar cell[9, 10]. To reduce charge recombination and improve the charge collection efficiency, incorporation of carbon-based nanomaterials in the photoanode (specially based on TiO2 and ZnO) is a promising approach for reducing the photoelectrons’ loss[11, 12]. Among various carbon nanomaterials, addition of graphene quantum dots (GQDs) onto TiO2 nanostructures can enhance the performance of DSSC due to its excellent optical properties and low resistance. Due to special properties of GQDs such as non-zero band gap, excellent photostability, low cytotoxity, chemical inertness, cost effective preparation methods, GQDs have been used for supercapacitors, bio imaging, water splitting, and photovoltaic devices [13, 14]. Properties of GQDs such as band gap energy are tunable by changing surface functionalization and their size [15]. Tunable band gap of GQDs allows to designing an appropriate energy states for LUMO and HOMO that leads to a multistep cascade energy levels in the photoanode[16]. Therefore, a method to increase efficiency of the DSSCs is to use co-sensitizers containing both conventional dyes (N3 or N719) and GQDs leading to reducing the photoelectrons’ loss. Recently, carbon quantum dots have been successfully utilized as co-sensitizers in DSSCs [17]. Huang et. al reported nitrogen doped graphene quantum dots for DSSC application and have obtained efficiency of 0.13 for their optimum sample [18]. Radoi et. al reported GQDs/N3 dye sensitized TiO2 nanoparticles and efficiency
of 2.15 was obtained for their work [19]. Li et. al reported GQDs optimization of DSSCs and have obtained efficiency of 6.1 for their optimum sample [20]. Subramania et. al reported Graphene quantum dots decorated electrospun TiO2 nanofibers for DSSCs. They have used electrophoretic filling for decoration of TiO2 nanofiber with GQDs and efficiency of 6.22 was obtain for optimum sample[21]. To the best of our knowledge, single layer graphene quantum dots (SLGQDs) as hybrid co-sensitizers have never been reported. Here, we have used SLGQDs for sensitizing TiO2 nanoparticles (NPs) film. Highly porous TiO2 NPs film as a wide band gap semiconductor was obtained using a doctor blade method. The effect of engineered SLGQDs on the photovoltaic parameters of N719/TiO2 photoanode was investigated by means of current voltage (I-V) and electrochemical impedance spectroscopy (EIS) measurements. Explaining the photoelectron generation under illumination and electron transfer mechanism in N719/SLGQDs/TiO2 interfaces, could be informative for using SLGQDs in solar cells.
2. Experimental 2.1 Preparing of TiO2 paste
For preparation of TiO2 paste, 0.5 g of TiO2 nanoparticles powder (25 nm) was dispersed in 10 ml ethanol using sonication. After that, 0.9 g ethyl cellulose was added to the solution and stirred vigorously for 20 min. Then, after 20 min the amount of 3.5 g terpineol slowly was added to the composite on the stirrer. For evaporation of ethanol the prepared paste placed at 50 ˚C for 2 h. 2.2 Preparation of TiO2 nanoparticle (NPs) films
For preparing TiO2 NPs films, at first FTO glasses (1⨯2 cm2, 15 Ω/cm2) were cleaned using sonication with DI water, acetone and ethanol for 15 min, respectively. Afterwards a compact blocking layer of TiO2 was deposited onto FTO substrate by immersing FTO into 40 mM aqueous TiCl4 solution for 30 min at 70˚ C and then cleaned with DI water and ethanol. To fabricate TiO2 NPs films, a layer of TiO2 paste was deposited by doctor blade technique over blocking layer. After drying the films for 6 min at 125˚ C, the films were heated at 325˚ C for 5 min, at 375˚ C for 5 min, at 450˚ C for 15 min, and 500˚ C for 30 min. Finally, the films again were immersed in 40 mM aqueous TiCl4 solution and sintered at 500˚ C for 30 min.
2.3 Synthesis of SLGQDs SLGQDs solution prepared according to our previous work with some effective modifications[22]. At first, 50 mg of glucose (white powder) was dissolved in 50 ml of DI water to obtain a colorless solution. Then, one-step hydrothermal method in a Teflon-lined stainless-steel autoclave (75 ml) at 200 ˚C for 8 h was used. Final product was pale brown after synthesis of SLGQDs. This method allows to tuning the energy levels of HOMO and LUMO by employing appropriate precursors. 2.4 Sensitizing of TiO2 NPs films with SLGQDs and N719 To sensitize TiO2 NPs films with the SLGQDs and N719, TiO2 NPs films were immersed in a solution of SLGQDs with concentration of 1 g/l for 6 h, after that placed at temperature of 120˚ C for 12 h. Then, after absorption of SLGQDs on the TiO2 nanoparticles, the films were immersed into a 0.4 mM N719 dye solution in ethanol for 24 h. For comparison the simultaneous effect of SLGQDs-N719 on the efficiency of the cells, also the TiO2 photoanodes based on SLGQDs and N719 were prepared separately. 2.5 Preparation of Pt counter electrode For preparing platinum counter electrode, a hole with diameter about 1 mm was drilled on the FTO glasses to allow the electrolyte introduction later. Then the FTO glasses were cleaned with DI water and ethanol. Then, the FTO glasses were heated to 470 ˚C for 15 min. The thermal decomposition method was employed to deposit the Pt catalyst on the FTO glasses. For this purpose, a drop of 0.3 mM H2PtCl6 solution in ethanol was drop casted on the FTO substrates and annealed at 470 ˚C for 15 min. 2.6 Electrolyte The electrolyte I-/I3- consisted of 0.1 M LiI, 0.1 M I2, 0.5 M 4-tert-butylpyridine, and 0.6 M Tetrabutylammonium iodide in acetonitrile was used. 2.7 DSSC assembly The SLGQDs, N719 and SLGQDs-N719 loaded TiO2 nanoparticle photoelectrodes and the Pt counter electrodes were sealed together with a 30 µm Surlyn (Dyesol) spacer around the TiO2 active area (0.25 cm2). The liquid electrolyte (I-/I3-) was injected into assembled cells. At the end, the hole of counter electrode was covered by a glass (1cm⨯1cm) and sealed by a spacer. TiO2 NPs, SLGQDs/TiO2 NPs, N719/TiO2 NPs and (SLGQDs-
N719)/TiO2 NPs films were prepared for DSSCs measurements. The schematic illustration of the (SLGQDs-N719)/TiO2 NPs under illumination is shown in Fig. 1. 2.8 Characterization The structural morphology of the nanomaterials was investigated using a transmission electron microscope (TEM, CM30 300kV) and field emission scanning electron microscope (FESEM). UV-vis absorption measurement was performed using a spectrophotometer (Unico 4802). Atomic force microscopy (AFM, Veeco Autoprobe CP research) was used to investigate the height profile of the SLGQDs on mica as substrates. Raman technique (BRUKER, Germany, SENTERRA) was utilized to prove the formation of SLGQDs on the TiO2 nanoparticles. X-ray diffraction patterns (XRD, Co-Kα radiation source, Philips, X'Pert MPD) were utilized in order to study the lattice structures. The current-voltage (I-V) characteristics of the fabricated DSSCs were measured under solar simulator illumination of AM 1.5 (100 mW/cm2). The electrochemical impedance spectroscopy (EIS) measurements of the cells were carried out with an Iviumstat. Cyclic voltammetry (CV) measurement was carried out with a three electrode configuration. Ag/AgCl as the reference electrode, a platinum plate as the counter electrode, and SLGQDs/glassy carbon as the working electrode in an acetonitrile solution including 0.1 M Tetrabutylammonium hexafluorophosphate (TBAPF6) was used.
Fig. 1. Schematic illustration of the DSSC based on (SLGQDs-N719)/TiO2 NPs as photoanode.
For estimation of the amount of N719 on the N719/TiO2 and (SLGQDs-N719)/TiO2 photoanodes, the standard addition method [23, 24] was used. The concentration of absorbed dyes was estimated by first desorbing the dyes from the N719/TiO2 and (SLGQDsN719)/TiO2 photoanodes in 0.1 M NaOH aqueous solution and then analyzing by UV-Vis spectrophotometer. According to this analysis, the number of 8.21⨯1016 cm-2 and 9.12⨯1016 cm-2 was obtained for N719/TiO2 and (SLGQDs-N719)/TiO2 photoanodes, respectively. This enhancement shows that addition of SLGQDs on the TiO2 increases the effective surface area leading to more absorption of N719. Ratio of the N719-dye absorption after addition of SLGQDs was estimated about 1.1.
3. Results and discussion 3.1 Characterization study FESEM analyze was utilized to characterize the morphology of TiO2 NPs films and results are shown in Fig. 2. Top view FESEM image (Fig. 2-a) reveals that the entire surface of the FTO substrate is covered uniformly with TiO2 NPs with average particle size of 25 nm. Fig. 2-b shows the cross-section image of highly porous TiO2 NPs film with thickness of about 10 µm deposited using doctor blade method. According to this figure, TiO2 NPs photoanode has a nanoporous structure. This porosity has an extremely large specific surface area that enhances light harvesting as well as dye adsorption. The increased surface area leads to increase of the adsorbed SLGQDs and dye molecules (N719). Fig. 2-c shows XRD patterns of the highly porous TiO2 NPs film deposited on FTO substrates. TiO2 NPs film showed the peaks of (103), (004), (105), (211), (213) and (204), which corresponded to the pure anatase phase of TiO2 [25]. The diffraction patterns of TiO2 NPs films are consistent with the tetragonal structure of TiO2 (JCPDS Card No. 71–1166). In the XRD patterns, strong peaks indicate high crystallinity of the TiO2 NPs. The peaks of brookite and rutile phases were not detected, indicating the purity of the anatase phase. The red peaks (showed by star) originate from the FTO substrate.
Fig. 2. FESEM images of the TiO2 NPs film, top view (a) and crosss section (b). (c) XRD patterns of the anatase TiO2 NPs grown on the FTO substrate.
For AFM analysis, a drop of SLGQDs solution was drop casted on the mica surface and the result is shown in Fig. 3-a. As we know, graphene has a thickness of about ∼0.35 nm [26] that can be detected on a smooth substrate such as mica. As can be seen in Fig. 3-b, topographic height of the SLGQDs is less than 0.5 nm, suggesting that the obtained samples are single-layered. Fig. 3-c indicates the TEM image of the SLGQDs particles. This figure shows that the as-prepared particles have a uniform dispersion without any aggregation with an average width of about 9 nm. UV–vis absorption spectra of the SLGQDs, N719, and SLGQDs/N719 samples are shown in Fig. 3-d. Absorption spectrum of N719 dye solution shows three peaks within the range of 300–800 nm. These peaks are characteristics of N719 dye [27]. SLGQDs solution has a wide absorption curve over the visible light that originates from functional groups such as hydroxyl and carboxyl [14]. According to UV–vis absorption spectra, absorption region is recorded at all visible wavelengths. As can be seen, the SLGQDs/N719 solution has a stronger absorption curve related to the effective overlapping between SLGQDs and N719 that can be used for photovoltaic applications.
Fig. 3. (a) AFM image of the SLGQDs on the mica substrate, (b) height profile of the SLGQDs (line1), (c) TEM image of the SLGQDs and (d) shows the UV-Vis absorption spectra of SLGQDs, N719 and SLGQDs/N719, as well as the photograph of the SLGQDs solution.
To obtain an estimate for the band gap energy, Tauc plot method was used. By plotting the square of the absorption energy (αhv, where α is the absorbance and hv is the photon energy) versus hv, the band gap energy of TiO2 and TiO2/SLGQDs were determined about 3.1 and 2.9 eV (Fig. 4), respectively. As can be seen, SLGQDs decrease the band gap of TiO2 that can be used for higher absorption over the visible light.
Fig. 4. Tauc plots for TiO2/SLGQDs and TiO2 NPs showing estimated band gap energy.
Electrochemical method allows to determination the absolute positions of the energy levels, relative to a reference such as vacuum or Ag/AgCl, that is necessary in developing further photoelectrochemical experiments. Thus, cyclic voltammetry has been used to investigate the effects of quantum confinement on the LUMO and HOMO energy states. In order to elucidate the role of SLGQDs in mediating the charge transfer between N719 and TiO2 NPs, CV measurement was used. In our previous [22] work we comprehensively have studied the method for determination of LUMO and HOMO energy levels of the SLGQDs. Briefly, electrode for CV measurement was prepared by drop-casting of SLGQDs solution (5 µl) and nafion (0.5 µl, 5% w) onto a polished glassy carbon electrode, then the electrode dried naturally at room temperature during two days. The SLGQDs/glassy carbon electrode depicts an oxidation peak at +2.2 V and a reduction peak at −0.7 V versus Ag/AgCl as the reference. The values for ELUMO and EHOMO are ELUMO= −4.25 eV and EHOMO= −7.15 eV versus vacuum scale, respectively. Raman analysis was utilized to confirm the formation of graphitic Raman modes of SLGQDs and TiO2 anatase phase as well as existence of SLGQDs on the TiO2 NPs surfaces. Fig. 5 shows the Raman spectrum of SLGQDs decorated on the TiO2 NPs. In the spectrum only peaks corresponding to anatase active vibration modes are identified at 401 cm-1 (B1g(1)), 521 cm-1 (B1g(2) + A1g), 635 cm-1 (Eg(2)) [28]. Obviously, two additional peaks at 1354 cm-1 (for D band) and 1600 cm-1 (for G band) for the graphitic structures were observed. D band is corresponding to disorder graphitic structure while the G band is attributed to sp2 hybridized carbon [29, 30]. The Raman spectrum confirms the incorporation of SLGQDs on the surfaces of TiO2 NPs.
Fig. 5. Raman spectrum of the SLGQDs/TiO2 NPs.
3.2 Energy level study The schematic illustration of energy level diagram for anatase TiO2, SLGQDs and N719 dye were used as sensitizers are presented in Fig. 6. In the photoanodes, the N719-dye and SLGQDs sensitizers are interfaced with the TiO2 NPs. Under illumination, in a typical dye solar cell based on TiO2 as the base semiconductor, the electron excited from HOMO into the LUMO energy level is injected into the conduction band (CB) of TiO2, while, simultaneously, the oxidized sensitizer is regenerated by the redox couple (I-/I3-). Fig. 6 shows the energy level diagram of the various parts of the photoanode that evidences the compatibility of the N719 dye, with the prepared SLGQDs that act as additional light harvesters. Thus, while the LUMO and HOMO energy levels of the N719 are at -3.46 and -5.68 eV, respectively [31], the SLGQDs HOMO energy level is -7.15 and LUMO energy level is -4.25 eV. This order for alignment corresponds to a type II alignment of the energy states in hybrid systems. As explained, the LUMO energy level of SLGQDs is higher than the conduction band of the anatase TiO2 NPs (-4.4 eV [32]), which shows that the electronic transfer from the SLGQDs to the TiO2 nanoparticles takes place when the SLGQDs are excited under illumination. At this situation, effective separation of holes and electrons in the SLGQDs and the fast injection of photoelectron into the conduction band of TiO2 through the heterojunctions can be occurred. Furthermore, active holes move into the HOMO energy of N719, where reduce by electrolyte.
Because of mismatch between the energy level positions of the SLGQDs and N719, a cascaded band edge alignment is occurred. This effect leads to transfer of electrons from the LUMO level of the N719 to the TiO2 electrode through the SLGQDs, and consequently increases the charge separation and electron injection.
Fig. 6. Schematic illustration of energy level diagram for anatase TiO2, SLGQDs and N719 dye.
3.3 Current measurements Influence of SLGQDs and N719 on the amount of light harvesting in the prepared DSSC was systematically investigated in terms of fill factor (FF), short-circuit current (Jsc), open-circuit voltage (Voc), power conversion efficiency (PCE), and charge transfer resistance (Rct). To investigate the role of SLGQDs as efficient co-sensitizers, 4 cells were prepared: cells without sensitizing (only TiO2 NPs); cells sensitized only with SLGQDs (SLGQDs/TiO2 NPs); cells sensitized only with N719 (N719/TiO2 NPs); and cells with sensitizers consisting of SLGQDs and N719 dye (SLGQDs-N719)/TiO2 NPs. The cell without sensitizing consists of a TiO2 NPs film (thickness of 10 µm, see Fig. 2-b). The current density voltage (J–V) characteristics of the DSSCs are shown in Fig. 7 and the photovoltaic parameters obtained from J-V curves are listed in Table 1. All experiments were done for three times and for every cell we prepared three sample and reported average of them. Standard deviations of photovoltaic values were added in the Table 1. In the (SLGQDs-N719)/TiO2 NPs cell, N719 and SLGQDs sensitizers are interfaced with the TiO2 NPs. The position of conduction band (CB) for anatase TiO2 is -4.4 eV against vacuum scale [32]. In other hand, LUMO state of graphene quantum dots is higher than CB of anatase TiO2 (as can be seen in Fig. 6). Therefore, electron transfer from LUMO of SLGQDs to TiO2 CB is thermodynamically
favorable. When SLGQDs excited under illumination, generated electrons more efficiently transfer to conduction band of TiO2. The short circuit current density (Jsc) and open circuit voltage (Voc) are 20.03 mA/cm2 and 0.73 V, respectively for the (SLGQDs-N719)/TiO2 NPs cell, improved with respect to the bare TiO2 cells. Therefore, the dye solar cell fabricated using (SLGQDs-N719)/TiO2 NPs indicated an increased power conversion efficiency (PCE, η=8.92%) with respect to the DSSC fabricated using bare TiO2 NPs. A significant improvement of the Jsc (~39%) is observed for (SLGQDs-N719)/TiO2 NPs cell compared to N719/TiO2 NPs cell. The power conversion efficiency (η) increases with 35% versus N719/TiO2 NPs due to the larger Jsc. The charge transfer kinetics is thermodynamically faster for (SLGQDs-N719)/TiO2 NPs electrode versus N719/TiO2 NPs as proved by the higher generated charge versus applied potential. The cascaded energy level structure between the N719 and SLGQDs facilitates the electron injection into TiO2 NPs. This shows that the SLGQDs play a dual role in the fabricated hybrid (SLGQDs-N719)/TiO2 NPs DSSC. This role leads to the improving of the charge transfer properties and light harvesting efficiency. The high efficiency of the (SLGQDs-N719)/TiO2 NPs DSSC is due to the following effects; (i) Highly porous TiO2 NPs film allows more absorption of N719 molecules leading to generation of more number of photoelectrons; (ii) addition of SLGQDs increases the effective surface area for dye loading in a ratio of 1.1; (iii) due to presence of SLGQDs, photogenerated electrons transport faster from N719 to conduction band of TiO2. So these effects contributed to increase in the Jsc and PCE of the DSSC. As the results show, in a hybrid DSSCs, it is reasonable to assume that the SLGQDs are effective co-sensitizer for light harvesting. As shown by photoelectrical measurements, an improvement in PCE was achieved when SLGQDs were used as co-sensitizers together with N719.
Fig. 7. Current density–voltage characteristics of TiO2 NPs and sensitized TiO2 NPs as photoanodes in DSSCs.
3.4 Electrochemical impedance spectroscopy (EIS) For the analysis of ionic and electronic transport processes in the DSSCs EIS measurements were used [33]. To analyze the charge carrier dynamics of the photoanodes and investigate difference in the interfacial characteristics, EIS of the fabricated cells was performed at an applied bias of Voc and a frequency ranging from 0.01 Hz to 1 MHz with AC amplitude of 10 mV under illumination AM 1.5. Fig. 8 indicates the typical Nyquist plots of the fabricated DSSCs. Under illumination, there are two well-defined semicircles. The first semicircle in the high-frequency region represents the redox reaction of I-/I3- at the Pt/electrolyte interface, and the other semicircle in the low-frequency region denotes the electron transfer at the photoanode/electrolyte interface (Rct). As can be seen, at higher frequency, semicircles are identical prove that the addition of SLGQDs onto TiO2 NPs does not affect the charge transfer at Pt/electrolyte interface. The diameter (Rct) of the semicircle at low-frequency region corresponds to the resistance associated with the transfer of photogenerated electrons through the (N719SLGQDs)/TiO2/electrolyte interfaces. The Rct values for all cells are listed in Table 1. The values of Rct for the (N719-SLGQDs)/TiO2 cell at the photoanode/electrolyte interface decreases to some extent over all cells. This decrease in resistance at the (N719SLGQDs)/TiO2/electrolyte interface shows that the diffusion of electrons in this cell is faster and more effective than other cells. Presence of SLGQDs between N719 and TiO2 NPs
provides a fast and easy transport medium to charge carriers, which led to a decrease of 35% in Rct value compared with the cell based on N719/TiO2 NPs. The interface of (N719SLGQDs)/TiO2/electrolyte is significant as the interface controlling the photocurrent of the DSSC and the lower interfacial resistant is expected for high performance DSSC. Under excitation at AM 1.5, the second semicircle radius at low frequency in the Nyquist plot decreased after co-sensitizing with SLGQDs, and the values are in the order of (N719SLGQDs)/TiO2 < N719/ TiO2< SLGQDs/TiO2 < TiO2, which indicates an increase of charge transfer rate and a decrease of the electron transfer impedance at this interface.
Fig. 8. Nyquist Plots of sensitized DSSCs based on SLGQDs and N719. Inset shows the equivalent circuit.
Table 1. Photovoltaic values of DSSCs fabricated using bare TiO2 NPs and sensitized TiO2 NPs as photoanodes.
Sample
TiO2
SLGQDs/TiO2
N719/TiO2
(SLGQDs-N719)/TiO2
JSC (mA/cm2)
0.93±0.07
1.36±0.09
14.47±0.38
20.03±0.49
VOC (V)
0.34±0.03
0.46±0.03
0.71±0.01
0.73±0.01
FF
0.4±0.03
0.52±0.03
0.64±0.03
0.61±0.03
η (%)
0.12±0.01
0.33±0.03
6.57±0.32
8.92±0.49
R c t (Ω)
-
35±1
17±0.8
11±0.8
4. Conclusions In conclusion, a co-sensitized DSSC was designed based on N719 and low-cost SLGQDs. The fabricated solar cells exhibit significant improvement of Jsc and enhanced performance in the order of (N719-SLGQDs)/TiO2 > N719/ TiO2> SLGQDs/TiO2 > TiO2. Results showed that the cascaded energy level positions between the SLGQDs and N719 and high conductivity of SLGQDs facilitates the electron transfer and, consequently, a possible faster injection of electrons into the conduction band of TiO2. The (N719-SLGQDs)/TiO2 cosensitized dye solar cell yields the best overall efficiency of 8.92%, which is 35% greater than that of the dye solar cell containing only N719/TiO2. This study shows that the SLGQDs solution, as co-sensitizers is easily handled and inexpensive, and thereby deserves to be explored for the application in photovoltaic devices.
Acknowledgments
The research council of the University of Kashan is gratefully acknowledged for the financial support of this study.
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1. Using TiO2 nanoparticles film as a wide band gap semiconductor.
2. Utilization of single layer graphene quantum dots solution as an effective cosensitizer for DSSCs.
3. Obtaining maximum efficiency among all articles based on GQDs and TiO2.
Author Contributions Section Manuscript title: Hybrid dye sensitized solar cell based on single layer graphene quantum dots
Farhad Jahantigh: Data curation, Writing- Original draft preparation. S.M. Bagher Ghorashi: Supervision, Writing- Reviewing and Editing. Amir Bayat: Methodology, Formal analysis,
Investigation .
This statement is signed by all the authors to indicate agreement that the above information is true and correct.
Conflicts of Interest Statement Manuscript title: Hybrid dye sensitized solar cell based on single layer graphene quantum dots
This paper consists of original, unpublished work which is not under consideration for publication elsewhere, and that all co-authors have approved the contents of this manuscript and submission. Author names: Farhad Jahantigh, S.M. Bagher Ghorashi ,Amir Bayat
The authors whose names are listed immediately below certify that they have NO affiliations with or involvement in any organization or entity with any financial interest (such as honoraria; educational grants; participation in speakers’ bureaus; membership, employment, consultancies, stock ownership, or other equity interest; and expert testimony or patent-licensing arrangements), or non-financial interest (such as personal or professional relationships, affiliations, knowledge or beliefs) in the subject matter or materials discussed in this manuscript. Author names: Farhad Jahantigh, S.M. Bagher Ghorashi ,Amir Bayat
The authors whose names are listed immediately below report the following details of affiliation or involvement in an organization or entity with a financial or non-financial interest in the subject matter or materials discussed in this manuscript. Author names: Farhad Jahantigh, S.M. Bagher Ghorashi ,Amir Bayat
This statement is signed by all the authors to indicate agreement that the above information is true and correct.