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Optimized dye-sensitized solar cells: A comparative study with different dyes, mordants and construction parameters
Results in Physics 12 (2019) 2026–2037
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Optimized dye-sensitized solar cells: A comparative study with different dyes, mordants and construction parameters
T
Domingo Rangela, Juan Carlos Gallegosa, Susana Vargasa, Francisco Garcíaa,b, ⁎ Rogelio Rodrígueza, a b
Centro de Física Aplicada y Tecnología Avanzada, Universidad Nacional Autónoma de México, Boulevard Juriquilla 3001, Querétaro 76230, Mexico Instituto Tecnológico de Querétaro, Av. Tecnologico s/n, Centro, Querétaro 76000, Mexico
ARTICLE INFO
ABSTRACT
Keywords: DSSC Optimum thickness Diffusion properties Cochineal Mordant
Cochineal and β-carotene were the sensitizers used in the preparation of these solar cells; potassium alum and citric acid were used, separately, as mordant; acetyl acetone was the medium. For comparison purposes, another solar cell was prepared using a dye obtained from achiote (Bixa Orellana) with acetyl acetone. The mordants provide a stabilization of the pigment by improving the adhesion with the substrate (the mesoporous layer), increasing the lifetime of the solar cell; additionally. The use of mordants allows some changes in the dye color optimizing the harvest of the light by the solar cell. The dependence of the mesoporous thickness with the charge density was determined resulting 36 µm the optimum value. The diffusion properties of the dyes into the mesoporous were also determined. Comparisons with other cells are reported.
Introduction From all clean green technologies based on renewable energy sources, the one based on solar energy is the most confident and accessible because, in different ways and places, it is accessible to all living creatures on earth: the sun luminous energy which give places, in addition to the photosynthesis process for the growth and development of plants, to the possibility to generate electric energy completely necessary for the actual life style of the human kind. The solar energy can be converted, directly, in forms more usable like electricity: a lot of effort has been addressed to transform solar radiation in electric energy as cheap and green as possible; this is, precisely, the goal of photovoltaic cells. The dye sensitized solar cell (DSSC), corresponding to the third generation of photovoltaic cell, as attracted considerable attention by several reasons: low cost of manufacture, easiness of fabrication and ecological benefits; for these reasons the use of natural pigments represents an attractive alternative to convert solar radiation into electric energy [1–5]. However, these cells have, comparatively, low efficiency and short lifetime due to the pigment degradation; this effect has been reduced by chemical means or using combinations synthetic-natural dyes. On the other side, the cell construction has also an important contribution to enhance the cell efficiency; key parameters as thickness [6] and porosity of the mesoporous layer affect the performance of the
⁎
cell [7]. Ne and interesting dyes has been developed to increase the efficiency of the solar cells [8–10] Even when the solar cells are electrically evaluated by measuring some parameters (short circuit current, open circuit voltage, fill factor, maximum voltage and current, output power, etc.), in the case of natural sensitizers, other parameters are also important: cost, environmental impact, dye stability, lifetime, efficiency, accessibility to the raw materials, easiness in the fabrication, etc. [11–20]. When the photons of the sunlight impinge on dye molecules, these loose electrons (oxidation process). These excited electrons are injected into the conduction band of nano-porous titania semiconductor film [21] and travel toward the transparent electrode where they are conducted to a resistor to provide the required electrical energy. Finally, the electrons move back to the counter electrode completing the circuit [15]. Consequently, the characteristics of the titania layer are important in the performance (efficiency) of the cell [22,23]. The DSSC has two transparent electrodes: an anode (working electrode), and a cathode (counter electrode); the anode is coated with a layer of porous titania impregnated with the sensitizing dye. It is required a chemical link between the titania and the dye to reduce the dye degradation by the solar radiation and to increase its stability; for this reason it is recommendable to use mordants as a coupling agent to produce the chemical links [12,24]. The porous titania has two important properties: a) large surface
Corresponding author. E-mail address: [email protected] (R. Rodríguez).
https://doi.org/10.1016/j.rinp.2019.01.096 Received 28 November 2018; Received in revised form 28 January 2019; Accepted 31 January 2019 Available online 05 February 2019 2211-3797/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/BY/4.0/).
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Table 1 Electrical parameters using different dyes and relaxation times for two illumination conditions. Dye
Fill Factor
VOC [V]
ISC [mA]
η [%]
β-Carotene
0.039
0.123
0.162
0.0001
Carminic Acid
0.033
0.160
0.350
0.0003
Bixa Orellana
0.002
0.372
0.443
0.00005
Carminic-Orellana
0.024
0.370
0.365
0.0054
Eq. Circuit 2
Rsol = 547.5 Ωcm Rct = 920.0 Ωcm2 Cdl = 116.8 µF Rsol = 224.2 Ωcm2 Rct = 128.3 Ωcm2 Cdl = 40.9 µF Rsol = 293.8 Ωcm2 Rct = 282.7 Ωcm2 Cdl = 100.5 µF Rsol = 2.089 Ωcm2 Rct = 2.089 Ωcm2 Cdl = 261.7 µF
Relaxation time 1-8-1
Relaxation time 1-2-7
0.39 s
0.40 s
0.10 s
0.12 s
0.16 s
0.19 s
0.29 s
0.31 s
Table 2 Electrical parameters for DSSC at different mesoporous thickness and dye adsorption times. Thickness [µm]
Density Carriers 13
−2
mm
5.8
1.8 × 10
10.6
1.8 × 1013 mm−2
16.7
1.9 × 1013 mm−2
34.7
2.4 × 1013 mm−2
56.9
1.8 × 1013 mm−2
70.4
1.8 × 1013 mm−2
Eq. Circuit
Adsorption time [h] 2
Rsol = 313.5 Ωcm Rct = 244.8 Ωcm2 Cdl = 23.61 µF Rsol = 412.8 Ωcm2 Rct = 215.9 Ωcm2 Cdl = 49.78 µF Rsol = 417.5 Ωcm2 Rct = 199.0 Ωcm2 Cdl = 87.04 µF Rsol = 202.8 Ωcm2 Rct = 802.6 Ωcm2 Cdl = 146.1 µF Rsol = 285.8 Ωcm2 Rct = 610.7 Ωcm2 Cdl = 43.64 µF Rsol = 243.3 Ωcm2 Rct = 342.4 Ωcm2 Cdl = 97.17 µF
area to allow the chemical link with a large number of dye molecules, and b) the porosity improves the light penetration into the semiconductor layer reaching a large number of dye molecules which absorb the photons to initiate the releasing of electrons. The properties of this sensitive material have a strong influence in the electrical performance of the cell [25,26]. In this sense, the absorption time of the dye into the mesoporous titania is also important because allows to determine the required time for the dye to fill completely and homogenously the mesoporous: the absorption of the dye continuously blocks the pores in the mesoporous reducing the pore size and, consequently, the absorption rate [27]; it is important to determine the time required for a complete and homogeneous distribution of the dye into the mesoporous layer, because an inhomogeneous absorption alter the charge transport through the whole volume of the mesoporous reducing the density of electrons that can be transported. The electrolyte has important tasks [28]: to regenerate the dye and to provide the charge transport between electrodes; in this work, the electrolyte with the best performance reported in literature was selected to impregnate, uniform and completely, the titania internal surface to improve the transport of the electric charge [29]. The cathode completes the electrical circuit injecting electrons into the electrolyte to regenerate this; to improve the electric contact between the cathode and the electrolyte, several types of carbon coatings were tested and one was selected to cover the counter-electrode.
Density Carriers 13
mm
−2
0
2.9 × 10
12.5
2.0 × 1013 mm−2
24.5
1.5 × 1013 mm−2
37.5
1.6 × 1013 mm−2
48
1.6 × 1013 mm−2
61
1.6 × 1013 mm−2
Eq. Circuit Rsol = 248.2 Ωcm2 Rct = 209.3 Ωcm2 Cdl = 44.87 µF Rsol = 410.2 Ωcm2 Rct = 264.2 Ωcm2 Cdl = 93.29 µF Rsol = 261.5 Ωcm2 Rct = 120.0 Ωcm2 Cdl = 17.05 µF Rsol = 204.7 Ωcm2 Rct = 161.3 Ωcm2 Cdl = 58.65 µF Rsol = 394.0 Ωcm2 Rct = 239.9 Ωcm2 Cdl = 74.50 µF Rsol = 471.0 Ωcm2 Rct = 191.1 Ωcm2 Cdl = 74.11 µF
The advantages of DSSC (low cost and environmental friendly) are not enough to compensate for the disadvantages (low efficiency and short lifetime) in order to become these as commercial products [13]. However, it has been observed that, changing appropriated some structural parameters, the efficiency can be improved: thickness of the titania layer, dyes absorption time, type of dyes, adhesion dye-titania, etc. As mentioned, the thickness of the semiconductor plays an important role in the surface charge density and, consequently, in the output electric current; the contact resistance, capacitance and the electrolyte resistance have a strong dependence with the titania thickness. In this work several DSSCs were prepared with different semiconductor thickness to obtain the thickness-surface charge density dependence. Additionally, it was determined the time required to diffuse the pigment (together with the mordant) into the semiconductor in order to obtain a complete and homogeneous distribution of the dye into the mesoporous. Theoretical details The principal electronic transport media for DSSC is diffusion, however there exist electronic recombinations with the several compounds present the cell: electrodes, titania mesoporous, and electrolyte. Due this, the Ficks laws have to be modified to include these effects. The recombinations effects can be included by adding, to the Fickśs 2027
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Fig. 3. Superficial charge density as a function of the mesoporous thickness impregnated with carminic acid. The experimental data (discrete points) were fitted using a Gaussian curve.
Fig. 1. Power spectrum of: (halogen lamp) + (hot mirror).
Fig. 2. UV–Vis spectra of β-Carotene, Bixa Orellana and carminic acid. Fig. 4. Electrolyte resistance Rsol plotted as function of the semiconductor thickness and the dye absorbance time.
laws, the term: k(n – no): a recombination produces a reduction in the conductiońs electrons that diffuse in the medium (electrolyte) with a velocity k(n – no) where k is a constant (with the appropriated units) and no is the electronic concentration in equilibrium; with the inclusion of this term, the Ficḱs laws take the shape:
n = t
J x
k (n
no ) and
2n n =D 2 t x
k (n
Ln is a parameter called length of diffusion and defined as: Ln = (Dτn)1/2 and τn = k−1 is the lifetime of the electrons. This differential equation can be solved taken first the Laplace transform
no )
fusion-recombination model can be written as:
x2
x2
( )
s + k 1/2 d
C=0
and, after solving this, it is possible to obtain the Bisquert equation:
the last equation is the diffusion-recombination equation frequently used to describe the diffusion of ionic species in homogeneous reactions. In steady state, the excess of electrons concentration, in the dif2n
2C
(n
no)
Ln2
n (x , t ) =
= 0 where
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x
e
x 2 + kt 4Dt
2 D t3
(1)
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Fig. 5. Plots of: a) contact resistance Rct and b) capacitance Cdl, as function of the semiconductor thickness and the dye absorption time.
Experimental Two commercial ITO conductive glass plates of In2O3:SnO2 were used as anode and cathode electrodes [14]. The titania-based mesoporous semiconductor was prepared as reported elsewhere [30]: 2 g of commercial TiO2 nanoparticles of 25 nm (Aldrich) were mixed with 0.6 mL of acetic acid; this paste was spread onto the anode with different thickness: 5.8, 10.6, 16.7, 34.7, 56.9, 70.4 µm to find the optimal value; these thickness were obtained using strip spacers of different thickness placed at the narrow end of the ITO glass. Once the TiO2 layer was obtained with a specific thickness, the spacers were removed and the glass plate with the semiconductor was heated at 350 °C for 30 min to consolidate the mesoporous layer. The thickness was measured using a thickness tester apparatus Mitutoyo Absolute ID-C112EBS Digimatic Digital Indicator Thickness Gauge model 543-252BS. The dye-intomesoporous impregnation experiments were performed only with the carminic dye: the anode with the mesoporous was immersed in a carminic acid solutions during different times: from 0 to 60 h at room temperature for impregnation; after this, it was removed from the dye solution, dried at room conditions and used to determine the photoelectric properties. The chemicals used for the electrolyte preparation were reactive grade from Sigma-Aldrich. 0.128 g of I2 was mixed with 10 mL of acetonitrile under agitation. Separately, 0.15 g of LiI was mixed, also under agitation, with 0.5 mL of 3-metoxypropionitrile. Both solutions were mixed together with agitation for 9 min.
Fig. 6. Superficial charge density as a function of the dye diffusion time.
Dyes preparation and cell construction
require high temperature (close to boiling point of water) to carry on the adhesion; additionally, the diffusion time of the dye-direct into the mesoporous is significantly higher. Then, the use of directs makes the cell construction process cumbersome. For these reasons, low molecular weight mordant (typically between 192 and 258) were used; this improves the diffusion of the dye-mordant into the mesoporous titania and the process is carry out at room temperature, making the dye distribution homogeneous, complete and faster. Different cochineal-based dye solutions were prepared grinding, in an agate mortar and separately, 0.5 g of cochineal powder in presence of 0.035 g of citric acid and 0.25 g of potassium alum, compound used as mordant. Each of the dye-mordant compounds were mixed with
The natural carmine color is produced by the cochineal insect as a protection mechanism; the dye molecule is the carminic acid (C22H20O13) that consists of a core of anthraquinone linked to a glucose sugar unit. Mordants (dye fixative) link, chemically, dyes on different types of substrates (fabric, tissue, cell, etc.) through a coordination complex; these can change and intensify the stains depending on the substrate and the mordant. Even when the mordant has been displaced by “directs” which are also coupling agents to adhere the dye with the substrate by non-ionic forces, these are molecular structures with high molecular weight (typically between 900 and 1000 Daltons); these 2029
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8.0 mL of boiling water and the suspensions were homogenized using an ultrasonic bath (Branson 2510) for 40 min at room temperature; finally, the systems were filtered using a Whatman paper No. 40 to remove clumps. To prepare the Bixa Orellana pigment solutions, achiote seeds were ground in an agate mortar and, separately, 1.3 g of achiote was taken and mixed with 6 mL of acetyl acetone. The suspension was homogenized for 10 min with a magnetic stirrer, followed by the use of an ultrasonic bath (Brason 2510) for 5 min at room temperature; finally, the system was filtered using Whatman No. 1 paper [31]. β-carotene dye solutions were prepared using 0.01 g of β-carotene powder (Sigma Aldrich) in presence of 0.009 g of citric acid and 0.005 g of potassium alum that is used as mordant. Each of the dye-mordant compounds were mixed with 10 mL of acetyl acetone and the suspensions were homogenized using a magnetic stirrer for 5 min at 45 °C. Once the titania mesoporous layer was impregnated with the dyemordant during different times, a parafilm frame was placed around the cell as a container in order to fill the mesoporous with 0.4 mL of the electrolyte solution; this has an electrical conductivity of 8.88 mS. The upper ITO counter-electrode was covered with a thin coating of vegetal
carbon grounded and sieved at 74 µm. Once the ITO plate covered with the mesoporous coating containing the dye-mordant and the electrolyte are in place, the upper carbon-coated ITO counter-electrode glass was placed on top, pressed again the spacers, seal with parafilm and clamped firmly in a sandwich configuration; additional parafilm strips were used to improve the sealing of the whole sandwich to avoid leaks and to reduce evaporation. No leaks were detected during the assembling and operation of the cell. Electric wires were placed in the appropriated places to perform the electric measurements. For the cochineal-mordant system, the content of sugar of the components was low enough, reason why it was not necessary to remove it; this produces a reduction in the cell construction cost. The illumination of the solar cell was as reported elsewhere [30]: a halogen lamp of 50 W was used as light source together with a hot mirror to reduce the heat produced by the lamp; this reduces the temperature and, consequently, the evaporation of the electrolyte. The intensity was determined with a power meter. The light spectra of the halogen lamp were measured using a USB spectrometer (Ocean Optics 2000). Two different illumination profiles, dark-light–dark, were used: 1-8-1 and 1-2-7, all in seconds.
Fig. 7. Photocurrent profiles for different illumination conditions (1-8-1 and 1-2-7) and the relaxation times light-to-dark for: a) β-carotene, b) carminic acid, c) Bixa Orellana and d) (carminic acid)-(Bixa Orellana).
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Fig. 7. (continued)
Electric and Electrochemical characterization
30 kHz with amplitude of 5 mV. The positive electrodes of the cell was connected the work electrode of the apparatus, while the negative electrode of the cell was connected to the reference and auxiliary electrodes; all determinations were obtained using the option AC impedance.
The current–voltage determinations of all prepared cells were performed using a Keithley 2400 Source-Meter with KUSB-488B interface and a LabTracer 2.0 software based on LabView program. The time–current curves were measured using the same source-meter as before and the data were plotted and analyzed with software based on LabView v.2014 and synchronized with the dark-light-dark illumination sequence; the sampling rate was set to 100 ms. The output electric power was determined using resistors from 100 Ω to 100 MΩ and measuring the voltage with a 6½ digits FLUKE 8846A multimeter, while the current was determined with a Tektronix True RMS multimeter TX3. The Electrochemical Impedance Spectroscopy (EIS) results were performed in a Potenciostate GillAC 1144, ACM Instruments equipped with a software v. 5 which allows the option of EIS for the analysis of the circle fitting to obtain the equivalent circuit parameters: using the Nyquist diagram it is possible to determine Rsol (resistance of the electrolytic solution) and Rct (charge transfer resistance), on the other side, the Bode diagram allows to determine the Cdl (double layer capacitance) making use of Cdl = (2πfmaxRct)−1. These values are reported in Tables 1 and 2. The frequency was changed from 0.01 Hz to
Results and discussion The spectrum of the halogen lamp with the hot mirror is reported in Fig. 1. This halogen lamp has, by itself, an emission that goes from 350 to 950 nm; this means that a considerable contribution of the spectrum is in the IR region, i.e. a considerable amount of heat is produced. The hot mirror reduces the emission to the range from 350 to 750 nm and, consequently, the heat production, reducing the celĺs temperature and the evaporation. The dyes were characterized using UV–Vis spectroscopy and the results are reported in Fig. 2. The absorbance was determined from 350 to 900 nm. For β-Carotene the absorbance is high at 350 nm but at 500 nm is drastically reduced reaching almost a zero value. For Bixa Orellana the absorbance increases from 350 nm reaching its high value at 380 nm [32] but is reduced to zero at 550 nm [33]. For Carminic acid the absorbance shows a minimum at 390 nm and a maximum at 500 nm 2031
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Fig. 7. (continued)
and is reduced to zero near 600 nm, i.e. the carminic acid shows an extended range in red color absorption. The thickness of the mesoporous layer plays an important role in the electric response of the DSSC. In Fig. 3 it is plotted the superficial charge density as a function of the mesoporous thickness for carminic acid. The charge density is related to the current that is possible to obtain from the cell. The points represent the experimental data and the continuous line the fitting using a Gaussian model; the maximum of the Gaussian curve occurs at 34.5 µm. As can be noticed, the electric response of the cell depends strongly with the semiconductor thickness, decaying rapidly to zero for thickness smaller than 17 µm and larger than 57 µm. The concentration of electrons can be written, according to Bisquert et al model [34] in the injection limit, as:
n (x , t ) =
x
e
n (x = L , t ) =
L2 + kt 4Dt
(3)
Using the values of the fitting parameters reported in Fig. 3 it is possible to obtain the numerical values for β and D using Eq. (3): β = (78.32 ± 4.1) × 1013 m−1 and D = (4.76 ± 1.6) × 10−4 cm2/s. In addition to the titania thickness, the absorption time required for a fully impregnation of the dye-mordant into the mesoporous layer is also important because, to obtain a good performance of the cell, it is required a complete and uniform distribution of the dye-mordant into the titania layer. The absorption (or impregnation) time of the dyemordant into the titania layer is directly related to the layer thickness because thick titania layer requires longer times of diffusion to reach a complete and uniform distribution of the dye-mordant into the layer; due this, the cell parameters [14] were determined as a function of these two variables: the dye-mordant absorption time and the titania thickness. In Table 1 is reported the electrical parameters (fill factor, Voc, Isc,η) and the relaxation times for two illumination conditions (1-8-1 and 1-2-7). In Table 2 is reported the density of carriers for different mesoporous thickness and dye adsorption times. In Fig. 4 it is possible
x 2 + kt 4Dt
2 D t3
2 Dt 3
e
(2)
where n(x, t) is the superficial charge density, D the diffusion coefficient, k the dye absorption constant without solvent and β a fitting parameter. This equation can also be written as a function of the mesoporous thickness L as: 2032
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Fig. 7. (continued)
to see the electrolyte solution resistance Rsol as a function of, simultaneously, the titania thickness (down scale) and the dye absorption time (up scale); Rsol involves specifically the mesoporous layer because it is the resistance of the electrolyte solution contained into the mesoporous. Due this it is possible to observe, for Rsol, a strong correlation between these two parameters (thickness and diffusion time): the Rsol-thickness curve shows a maxima between 10 and 16 µm and a minimum at 34.5 µm, while for Rsol-adsorption the curve shows a maximum between 12 and 14 h and a minimum between 35 and 36 h; these two curves have similar shapes. In Fig. 5a and b are reported the contact resistance Rct and the double layer capacitance Cdl as a function of thickness (down scale) and as a function of absorption time (up scale). Rct is more susceptible to temporal shifts due to the intrinsic characteristics of the R-C circuit; for this reason there is not a equivalence between these variables: thickness and adsorption time. For a thickness of 35 µm Rct (Fig. 5a) reaches a maximum, while for the absorption time the maximum is between 46 and 52 h. The minimum of Rct is at 10 µm of thickness, while for the absorption the minimum happens at 24 µm. The same effect happens also for the capacitance Cdl (Fig. 5b) where the maximum is reached for thickness of 35 µm while Cdl reaches a maximum between 10 and 12 h.
The minimum of Cdl is for thickness of 57 µm and for an adsorption time of 24 h. The dependence of the superficial charge density as a function of the dye impregnation time is reported Fig. 6. Here it is possible to see that the charge density is reduced practically in a linear way during the first 24 h; after this time the charge density remains nearly constant until 60 h. This means that, under these conditions, after 24 h the dye was uniformly distributed in the whole volume of the mesoporous layer; shorter times produces superficial charge density no-uniformly distributed into the titania layer. Using this plot, it is possible to optimize the time the mesoporous must be immersed into the dye solution for a good impregnation. In Fig. 7 it is reported photocurrent profiles for: a) -carotene, b) carminic acid, c) Bixa Orellana and d) (carminic acid)-(Bixa Orellana), as a function of time for two-illumination conditions: 1-8-1 and 1-2-7 [35]. In these profiles it is possible to observe small unstable oscillations that appears on top of the photocurrent signal; these oscillations have reasonably the same amplitude and frequency for all dyes reported here and appears due to the rapid transition dark-to-light, i.e. the oscillations are a non-linear response of cell when is shifted, abruptly, far from equilibrium. The frequencies of these oscillations are practically 2033
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Fig. 8. Maximum photocurrent for different illumination conditions for all dyes.
identical for all samples and conditions, resulting: f = (5.00 ± 0.02) Hz. In these figures it is also reported the light-to-dark relaxation profiles (two plots in the bottom part of these figures) for both illumination conditions; in all cases the relaxation was modeled using a simple exponential decay function. The maximum photocurrents obtained for different illumination conditions and for all dyes considered here, are reported in Fig. 8. The maximum photocurrent was obtained for Bixa Orellana and the mixture (carminic acid)-(Bixa Orellana) [36], followed by the carminic acid; the minimum photocurrent was for -Carotene. It is interesting to note that the maximum photocurrent is practically independent of the illumination conditions (1-8-1 and 1-2-7), meaning that the recovery time for these cells is bellow one second. The relaxation times for different
illumination conditions and for all dye-mordant, are reported in Fig. 9 for both the decaying and the rising process. The largest relaxation times, for decaying and rising, corresponds to -Carotene followed by the mixture (Carminic acid)-(Bixa Orellana); in both cases, the minimum relaxation times for rising and decaying is for carminic acid. In Fig. 10 it is reported the typical I-V curves for a sample containing carminic acid, for two different illumination conditions: dark and light. For darkness conditions there is a soft transition (change in the slope) in the I-V curve that happens at (0.69 V, −6.0 × 10−4 A), while for illumination condition the change in the slope happens at (1.03 V, −1.4 × 10−3 A). Es importante resaltar que de las curvas I-V es possible cuantificar gráficamente, a partir del cambio en la pendiente, la resistencia de salida serie (Rserie) y la Resistencia en
Fig. 9. Relaxation times for: different illumination conditions, different dyes and for charging and discharging process.
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Fig. 10. I-V plots for different light conditions: dark and illumination.
derivación (Rshunt), las cuales representan la Resistencia de salida de la celda solar cuando se transfiere la potencia generada a una carga eléctrica. A typical result from the Electrochemical Impedance Spectroscopy for the sample containing carminic acid is shown in Fig. 11. This is a Nyquist diagram where it is plotted the imaginary part Z” versus the real part Ź of the impedance; with this information it was possible to determine Rsol and Rct for each sample; these values are reported in both Tables. Typical output power profiles as a function of voltage are reported in Fig. 12 for: a) carminic acid and b) (carminic acid)-(Bixa Orellana). The output power for different dyes is plotted in Fig. 12c; the highest output power corresponds to the mixture (carminic acid)-(Bixa Orellana) with 3.3 µW, followed by the carminic acid with 1.9 µW.
Conclusions Several dyes sensitized solar cells based on different natural dyes were prepared. The DSSĆs were fabricated using different thickness in the mesoporous titania layers. The impregnation times of the dyemordant into the mesoporous titania were determined. Unstable oscillations were found in the photocurrent profiles that are due to the abrupt change in the illumination conditions from dark-to-light; these oscillations have the same amplitude and frequency. Dependence of superficial charge density, electrolyte resistance, contact resistance and capacitance with the titania thickness and the dye impregnation time were obtained. The plots show a strong dependence of these variables with geometrical and diffusion parameters.
Fig. 11. Nyquist diagram (plot of the imaginary Z” versus the real part Ź of the impedance) allows to determine Rsol and Rct.
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Fig. 12. Typical power profiles as a function of the voltages for: a) carminic acid, b) (carminic acid)-(Bixa Orellana). The output power for different dyes is reported in (c).
References [1] Martín C, Ziółek M, Douhal A. Ultrafast and fast charge separation processes in real dye-sensitized solar cells. J Photochem Photobiol C 2016;26:1–30. 101016/j.jphotochemrev.2015.12.001. [2] Misra R, Maragani R, Arora D, Sharma A, Sharma GD. Positional isomers of pyridine linked triphenylamine-based donor-acceptor organic dyes for efficient dye-sensitized solar cells. Dyes Pigments 2016;126:38–45. https://doi.org/10.1016/j.dyepig. 2015.11.008. [3] Zhang H, Fan J, Iqbal Z, Kuang DB, Wang L, Meier H, et al. Novel dithieno [3,2b:2’,3’-d] pyrrole-based organic dyes with high molar extinction coefficient for dyesensitized solar cells. Org Electron 2013;14:2071–81. https://doi.org/10.1016/j. orgel.2013.04.046. [4] Wang P, Wang J, Yu H, Zhao L, Yu J. Hierarchically macro–mesoporous TiO2 film via self-assembled strategy for enhanced efficiency of dye sensitized solar cells. Mater Res Bull 2016;74:380–6. 101016/j.materresbull.2015.11.004. [5] Calogero G, Citro I, Di Marco G, Minicante SA, Morabito M, Genovese G. Brown seaweed pigment as a dye source for photoelectrochemical solar cells. Spectrochim Acta A 2014;117:702–6. https://doi.org/10.1016/j.saa.2013.09.019. [6] Shin I, Seo H, Son MK, Kim JK, Prabakar K, Kim HJ. Analysis of TiO2 thickness effect on characteristic of a dye sensitized solar cell by using electrochemical impedance spectroscopy. Curr Appl Phys 2010;10:S422–4. https://doi.org/10.1016/j. cap.2009.12.039. [7] Salavati-Niasari M, Davar F, Mir N. Preparation of ZnO nanoflowers and Zn
[8] [9]
[10] [11] [12]
[13] [14]
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glycerolate nanoplates using inorganic precursors via a convenient rout and application in dye sensitized solar cells. Chem Eng J 2012;181–182:779–89. https://doi. org/10.1016/j.cej.2011.11085. Qian X, Yan R, Xu C, Shao L, Li H, Hou L. New efficient organic dyes employing indeno[1,2-b]indole as the donor moiety for dye-sensitized solar cells. J Power Sources 2016;332:103–10. https://doi.org/10.1016/j.jpowsour.2016.09.138. Qian X, Wang X, Shao L, Li H, Yan R, Hou L. Molecular Engineering of D-D-π-A type organic dyes incorporating indoloquinoxaline and phenothiazine for highly efficient dyesensitized solar cells. J Power Sources 2016;326:129–36. https://doi.org/10. 1016/j.jpowsour.2016.06.127. Qian X, Lan X, Yan R, He Y, Huang J, Hou L. T-shaped (D)2–A–π–A type sensitizers incorporating indoloquinoxaline and triphenylamine for organic dye-sensitized solar cells. Electrochim Acta 2017;232:377–86. Syafinar R, Gomesh N, Irwanto M, Fareq M, Irwan YM. Chlorophyll pigments as nature-based dye for dye-sensitized solar cell (DSSC). Energy Procedia 2015;79:896–902. https://doi.org/10.1016/j.egypro.2015.11.584. Al-Alwani MAM, Mohamad AB, Kadhum AAH, Ludin NA. Effect of solvents on the extraction of natural pigments and adsorption onto TiO2 for dye-sensitized solar cell applications. Spectrochim Acta A 2015;138:130–7. https://doi.org/10.1016/j. saa.2014.11.018. Shalini S, Balasundara Prabhu R, Prasanna S, Mallick TK, Senthilarasu S. Review on natural dye sensitized solar cells: operation, materials and methods. Renew Sustain Energy Rev 2015;51:1306–25. https://doi.org/10.1016/j.rser.2015.07.052. Wang XF, Zhan CH, Maoka T, Wada Y, Koyama Y. Fabrication of dye-sensitized solar cells using chlorophylls c1 and c2 and their oxidized forms c′1 and c′2 and from
Results in Physics 12 (2019) 2026–2037
D. Rangel, et al.
[15] [16]
[17] [18]
[19]
[20]
[21] [22]
[23] [24]
[25]
Undaria pinnatifida (Wakame). Chem Phys Lett 2007;447:79–85. https://doi.org/ 10.1016/j.cplett.2007.08.097. Lei BX, Zeng LL, Zhang P, Qiao HK, Sun ZF. Sugar apple-shaped TiO2 hierarchical spheres for highly efficient dye-sensitized solar cells. J Power Sources 2014;253:269–75. https://doi.org/10.1016/j.jpowsour.2013.12.035. Ananth S, Vivek P, Arumanayagam T, Murugakoothan P. Natural dye extract of lawsonia inermis seed as photo sensitizer for titanium dioxide-based dye sensitized solar cells. Spectrochim Acta A 2014;128:420–6. https://doi.org/10.1016/j.saa. 2014.02.169. Panahi-Kalamuei M, Mousavi-Kamazani M, Salavati-Niasari M. Facile hydrothermal synthesis of tellurium nanostructures for solar cells. J Nanostruct 2014;4(4):459–65. https://doi.org/10.7508/JNS.2014.04.008. Panahi-Kalamuei M, Salavati-Niasari M, Hosseinpour-Mashkani SM. Facile microwave synthesis, characterization, and solar cell application of selenium nanoparticles. J Alloy Compd 2014;617:627–32. https://doi.org/10.1016/j.jallcom. 2014.07.174. Amiri O, Salavati-Niasari M, Sabet M, Ghanbari D. Synthesis and characterization of CuInS2 microsphere under controlled reaction conditions and its application in lowcost solar cells. Mater Sci Semicon Proc 2013;16(6):1485–94. https://doi.org/10. 1016/j.mssp.2013.04.026. Hosseinpour-Mashkani SM, Salavati-Niasari M, Mohandes F, Venkateswara-Rao K. CuInS2 nanoparticles: microwave-assisted synthesis, characterization, and photovoltaic measurements. Mater Sci Semicon Proc 2013;16(2):390–402. https://doi. org/10.1016/j.mssp.2012.09.005. Berhe SA, Gobeze HB, Pokharel SD, Park E, Youngblood WJ. Solid-state photogalvanic dye-sensitized solar cells. ACS Appl Mater Inter 2014;6:10696–705. https://doi.org/10.1021/am502473q. Sabet M, Salavati-Niasari M, Amiri O. Using different chemical methods for deposition of CdS on TiO2 surface and investigation for their influences on the dyesensitized solar cell performance. Electrochim Acta 2014;117:504–20. https://doi. org/10.1016/j.electacta.2013.11.176. Amiri O, Mir N, Ansari F, Salavati-Niasari M. Design and fabrication of a high performance inorganic tandem cell with 11.5% conversion efficiency. Electrochim Acta 2017;252:315–21. https://doi.org/10.1016/j.electacta.2017.09.013. Sobhani-Nasab A, Hosseinpour-Mshkani SM, Salavati-Niasari M, Taqriri H, Bagheri S, Saberyan K. Synthesis, characterization and photovoltaic application of NiTiO3 nanostructures via two-step sol-gel method. J Mater Sci: Mater Electron 2015;26(8):5735–42. https://doi.org/10.1007/s10854-015-3130-0. Mir N, Salavati-Niasari M. Preparation of TiO2 nanoparticles by using tripodal
[26] [27]
[28] [29] [30] [31] [32] [33]
[34] [35]
[36]
2037
tetraamine ligands as complexing agent via two-step sol-gel method and their application in dye-sensitized solar cells. Mater Res Bull 2013;48(4):1660–7. https:// doi.org/10.1016/j.materresbull.2013.01.006. Mir N, Salavati-Niasari M. Photovoltaic properties of corresponding dye sensitized solar cells: effect of active sites of growth controller on TiO2 nanostructures. Sol Energy 2012;86(11):3397–404. https://doi.org/10.1016/j.solener.2012.08.016. Hosseinpour-Mashkani SM, Mohandes F, Salavati-Niasari M, Venkateswara-Rao K. Microwave-assisted synthesis and photovoltaic measurements of CuInS2 nanoparticles prepared by using metal-organic precursors. Mater Res Bull 2012;47(11):3148–59. https://doi.org/10.1016/j.materresbull.1012.08.017. Hoshikawa T, Ikebe T, Kikuchi R, Eguchi K. Effects of electrolyte in dye-sensitized solar cells and evaluation by impedance spectroscopy. Electrochim Acta 2006;51:5286–94. https://doi.org/10.1016/j.electacta.2006.01.053. Koyyada G, Kumar P, Salvatori P, Marotta G, Lobello MG, Bizzarri O, et al. New terpyridine-based ruthenium complexes for dye sensitized solar cells applications. Inorg Chim Acta 2016;442:158–66. https://doi.org/10.1016/j.ica.2015.11.031. Rangel D, Navarro JA, Vargas S, González M, Castaño VM, Rodríguez R. A novel dual mechanism in dye-sensitized solar cells. Int J Energy Res 2017;41:1164–70. https://doi.org/10.1002/er.3700. Tamil SA, Aravindhan R, Madhan B, Raghava RJ. Studies on the application of natural dye extract from Bixa orellana seeds for dyeing and finishing of leather. Ind Crop Prod 2013;43:84–6. https://doi.org/10.1016/j.indcrop.2012.07.015. Barrozo MAS, Santos KG, Cunha FG. Mechanical extraction of natural dye extract from bixa orellana seeds in spouted bed. Ind Crop Prod 2013;45:279–82. https:// doi.org/10.1016/j.indcrop.2012.12.052. De Araujo Vilar D, De Araujo Vilar MS, De Lima e Moura TF, Raffin FN, De Oliveira MR, Franco CF, De Athayde-Filho PF, Melo Diniz MFF, Barbosa-Filho JM. Traditional uses, chemical constituents, and biological activities of bixa orellana L.: a review. Sci World J 2014;2014:1–11. https://doi.org/10.1155/2014/857292. –. Bisquert J, Mora-Sero I, Fabregat-Santiago F. Diffusion-recombination impedance model for solar cells with disorder and nonlinear recombination. Chemelectrochem 2014;1:289–96. https://doi.org/10.1002/c elc.201300091. Gómez-Ortíz NM, Vázquez-Maldonado IA, Pérez-Espadas AR, Mena-Rejón GJ, Azamar-Barrios JA, Oskam G. Dye-sensitized solar cells with natural dyes extracted from achiote seeds. Sol Energy Mater Sol C 2010;94:40–4. https://doi.org/10.1016/ j.solmat.2009.05.013. Pooran K. Photogalvanic cells: comparative study of various synthetic dye and natural photo sensitizers present in spinach extract. RSC Adv 2014;4:46194–202. https://doi.org/10.1039/C4RA08373C.
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Optimized dye-sensitized solar cells: A comparative study with different dyes, mordants and construction parameters
T
Domingo Rangela, Juan Carlos Gallegosa, Susana Vargasa, Francisco Garcíaa,b, ⁎ Rogelio Rodrígueza, a b
Centro de Física Aplicada y Tecnología Avanzada, Universidad Nacional Autónoma de México, Boulevard Juriquilla 3001, Querétaro 76230, Mexico Instituto Tecnológico de Querétaro, Av. Tecnologico s/n, Centro, Querétaro 76000, Mexico
ARTICLE INFO
ABSTRACT
Keywords: DSSC Optimum thickness Diffusion properties Cochineal Mordant
Cochineal and β-carotene were the sensitizers used in the preparation of these solar cells; potassium alum and citric acid were used, separately, as mordant; acetyl acetone was the medium. For comparison purposes, another solar cell was prepared using a dye obtained from achiote (Bixa Orellana) with acetyl acetone. The mordants provide a stabilization of the pigment by improving the adhesion with the substrate (the mesoporous layer), increasing the lifetime of the solar cell; additionally. The use of mordants allows some changes in the dye color optimizing the harvest of the light by the solar cell. The dependence of the mesoporous thickness with the charge density was determined resulting 36 µm the optimum value. The diffusion properties of the dyes into the mesoporous were also determined. Comparisons with other cells are reported.
Introduction From all clean green technologies based on renewable energy sources, the one based on solar energy is the most confident and accessible because, in different ways and places, it is accessible to all living creatures on earth: the sun luminous energy which give places, in addition to the photosynthesis process for the growth and development of plants, to the possibility to generate electric energy completely necessary for the actual life style of the human kind. The solar energy can be converted, directly, in forms more usable like electricity: a lot of effort has been addressed to transform solar radiation in electric energy as cheap and green as possible; this is, precisely, the goal of photovoltaic cells. The dye sensitized solar cell (DSSC), corresponding to the third generation of photovoltaic cell, as attracted considerable attention by several reasons: low cost of manufacture, easiness of fabrication and ecological benefits; for these reasons the use of natural pigments represents an attractive alternative to convert solar radiation into electric energy [1–5]. However, these cells have, comparatively, low efficiency and short lifetime due to the pigment degradation; this effect has been reduced by chemical means or using combinations synthetic-natural dyes. On the other side, the cell construction has also an important contribution to enhance the cell efficiency; key parameters as thickness [6] and porosity of the mesoporous layer affect the performance of the
⁎
cell [7]. Ne and interesting dyes has been developed to increase the efficiency of the solar cells [8–10] Even when the solar cells are electrically evaluated by measuring some parameters (short circuit current, open circuit voltage, fill factor, maximum voltage and current, output power, etc.), in the case of natural sensitizers, other parameters are also important: cost, environmental impact, dye stability, lifetime, efficiency, accessibility to the raw materials, easiness in the fabrication, etc. [11–20]. When the photons of the sunlight impinge on dye molecules, these loose electrons (oxidation process). These excited electrons are injected into the conduction band of nano-porous titania semiconductor film [21] and travel toward the transparent electrode where they are conducted to a resistor to provide the required electrical energy. Finally, the electrons move back to the counter electrode completing the circuit [15]. Consequently, the characteristics of the titania layer are important in the performance (efficiency) of the cell [22,23]. The DSSC has two transparent electrodes: an anode (working electrode), and a cathode (counter electrode); the anode is coated with a layer of porous titania impregnated with the sensitizing dye. It is required a chemical link between the titania and the dye to reduce the dye degradation by the solar radiation and to increase its stability; for this reason it is recommendable to use mordants as a coupling agent to produce the chemical links [12,24]. The porous titania has two important properties: a) large surface
Corresponding author. E-mail address: [email protected] (R. Rodríguez).
https://doi.org/10.1016/j.rinp.2019.01.096 Received 28 November 2018; Received in revised form 28 January 2019; Accepted 31 January 2019 Available online 05 February 2019 2211-3797/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/BY/4.0/).
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Table 1 Electrical parameters using different dyes and relaxation times for two illumination conditions. Dye
Fill Factor
VOC [V]
ISC [mA]
η [%]
β-Carotene
0.039
0.123
0.162
0.0001
Carminic Acid
0.033
0.160
0.350
0.0003
Bixa Orellana
0.002
0.372
0.443
0.00005
Carminic-Orellana
0.024
0.370
0.365
0.0054
Eq. Circuit 2
Rsol = 547.5 Ωcm Rct = 920.0 Ωcm2 Cdl = 116.8 µF Rsol = 224.2 Ωcm2 Rct = 128.3 Ωcm2 Cdl = 40.9 µF Rsol = 293.8 Ωcm2 Rct = 282.7 Ωcm2 Cdl = 100.5 µF Rsol = 2.089 Ωcm2 Rct = 2.089 Ωcm2 Cdl = 261.7 µF
Relaxation time 1-8-1
Relaxation time 1-2-7
0.39 s
0.40 s
0.10 s
0.12 s
0.16 s
0.19 s
0.29 s
0.31 s
Table 2 Electrical parameters for DSSC at different mesoporous thickness and dye adsorption times. Thickness [µm]
Density Carriers 13
−2
mm
5.8
1.8 × 10
10.6
1.8 × 1013 mm−2
16.7
1.9 × 1013 mm−2
34.7
2.4 × 1013 mm−2
56.9
1.8 × 1013 mm−2
70.4
1.8 × 1013 mm−2
Eq. Circuit
Adsorption time [h] 2
Rsol = 313.5 Ωcm Rct = 244.8 Ωcm2 Cdl = 23.61 µF Rsol = 412.8 Ωcm2 Rct = 215.9 Ωcm2 Cdl = 49.78 µF Rsol = 417.5 Ωcm2 Rct = 199.0 Ωcm2 Cdl = 87.04 µF Rsol = 202.8 Ωcm2 Rct = 802.6 Ωcm2 Cdl = 146.1 µF Rsol = 285.8 Ωcm2 Rct = 610.7 Ωcm2 Cdl = 43.64 µF Rsol = 243.3 Ωcm2 Rct = 342.4 Ωcm2 Cdl = 97.17 µF
area to allow the chemical link with a large number of dye molecules, and b) the porosity improves the light penetration into the semiconductor layer reaching a large number of dye molecules which absorb the photons to initiate the releasing of electrons. The properties of this sensitive material have a strong influence in the electrical performance of the cell [25,26]. In this sense, the absorption time of the dye into the mesoporous titania is also important because allows to determine the required time for the dye to fill completely and homogenously the mesoporous: the absorption of the dye continuously blocks the pores in the mesoporous reducing the pore size and, consequently, the absorption rate [27]; it is important to determine the time required for a complete and homogeneous distribution of the dye into the mesoporous layer, because an inhomogeneous absorption alter the charge transport through the whole volume of the mesoporous reducing the density of electrons that can be transported. The electrolyte has important tasks [28]: to regenerate the dye and to provide the charge transport between electrodes; in this work, the electrolyte with the best performance reported in literature was selected to impregnate, uniform and completely, the titania internal surface to improve the transport of the electric charge [29]. The cathode completes the electrical circuit injecting electrons into the electrolyte to regenerate this; to improve the electric contact between the cathode and the electrolyte, several types of carbon coatings were tested and one was selected to cover the counter-electrode.
Density Carriers 13
mm
−2
0
2.9 × 10
12.5
2.0 × 1013 mm−2
24.5
1.5 × 1013 mm−2
37.5
1.6 × 1013 mm−2
48
1.6 × 1013 mm−2
61
1.6 × 1013 mm−2
Eq. Circuit Rsol = 248.2 Ωcm2 Rct = 209.3 Ωcm2 Cdl = 44.87 µF Rsol = 410.2 Ωcm2 Rct = 264.2 Ωcm2 Cdl = 93.29 µF Rsol = 261.5 Ωcm2 Rct = 120.0 Ωcm2 Cdl = 17.05 µF Rsol = 204.7 Ωcm2 Rct = 161.3 Ωcm2 Cdl = 58.65 µF Rsol = 394.0 Ωcm2 Rct = 239.9 Ωcm2 Cdl = 74.50 µF Rsol = 471.0 Ωcm2 Rct = 191.1 Ωcm2 Cdl = 74.11 µF
The advantages of DSSC (low cost and environmental friendly) are not enough to compensate for the disadvantages (low efficiency and short lifetime) in order to become these as commercial products [13]. However, it has been observed that, changing appropriated some structural parameters, the efficiency can be improved: thickness of the titania layer, dyes absorption time, type of dyes, adhesion dye-titania, etc. As mentioned, the thickness of the semiconductor plays an important role in the surface charge density and, consequently, in the output electric current; the contact resistance, capacitance and the electrolyte resistance have a strong dependence with the titania thickness. In this work several DSSCs were prepared with different semiconductor thickness to obtain the thickness-surface charge density dependence. Additionally, it was determined the time required to diffuse the pigment (together with the mordant) into the semiconductor in order to obtain a complete and homogeneous distribution of the dye into the mesoporous. Theoretical details The principal electronic transport media for DSSC is diffusion, however there exist electronic recombinations with the several compounds present the cell: electrodes, titania mesoporous, and electrolyte. Due this, the Ficks laws have to be modified to include these effects. The recombinations effects can be included by adding, to the Fickśs 2027
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Fig. 3. Superficial charge density as a function of the mesoporous thickness impregnated with carminic acid. The experimental data (discrete points) were fitted using a Gaussian curve.
Fig. 1. Power spectrum of: (halogen lamp) + (hot mirror).
Fig. 2. UV–Vis spectra of β-Carotene, Bixa Orellana and carminic acid. Fig. 4. Electrolyte resistance Rsol plotted as function of the semiconductor thickness and the dye absorbance time.
laws, the term: k(n – no): a recombination produces a reduction in the conductiońs electrons that diffuse in the medium (electrolyte) with a velocity k(n – no) where k is a constant (with the appropriated units) and no is the electronic concentration in equilibrium; with the inclusion of this term, the Ficḱs laws take the shape:
n = t
J x
k (n
no ) and
2n n =D 2 t x
k (n
Ln is a parameter called length of diffusion and defined as: Ln = (Dτn)1/2 and τn = k−1 is the lifetime of the electrons. This differential equation can be solved taken first the Laplace transform
no )
fusion-recombination model can be written as:
x2
x2
( )
s + k 1/2 d
C=0
and, after solving this, it is possible to obtain the Bisquert equation:
the last equation is the diffusion-recombination equation frequently used to describe the diffusion of ionic species in homogeneous reactions. In steady state, the excess of electrons concentration, in the dif2n
2C
(n
no)
Ln2
n (x , t ) =
= 0 where
2028
x
e
x 2 + kt 4Dt
2 D t3
(1)
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Fig. 5. Plots of: a) contact resistance Rct and b) capacitance Cdl, as function of the semiconductor thickness and the dye absorption time.
Experimental Two commercial ITO conductive glass plates of In2O3:SnO2 were used as anode and cathode electrodes [14]. The titania-based mesoporous semiconductor was prepared as reported elsewhere [30]: 2 g of commercial TiO2 nanoparticles of 25 nm (Aldrich) were mixed with 0.6 mL of acetic acid; this paste was spread onto the anode with different thickness: 5.8, 10.6, 16.7, 34.7, 56.9, 70.4 µm to find the optimal value; these thickness were obtained using strip spacers of different thickness placed at the narrow end of the ITO glass. Once the TiO2 layer was obtained with a specific thickness, the spacers were removed and the glass plate with the semiconductor was heated at 350 °C for 30 min to consolidate the mesoporous layer. The thickness was measured using a thickness tester apparatus Mitutoyo Absolute ID-C112EBS Digimatic Digital Indicator Thickness Gauge model 543-252BS. The dye-intomesoporous impregnation experiments were performed only with the carminic dye: the anode with the mesoporous was immersed in a carminic acid solutions during different times: from 0 to 60 h at room temperature for impregnation; after this, it was removed from the dye solution, dried at room conditions and used to determine the photoelectric properties. The chemicals used for the electrolyte preparation were reactive grade from Sigma-Aldrich. 0.128 g of I2 was mixed with 10 mL of acetonitrile under agitation. Separately, 0.15 g of LiI was mixed, also under agitation, with 0.5 mL of 3-metoxypropionitrile. Both solutions were mixed together with agitation for 9 min.
Fig. 6. Superficial charge density as a function of the dye diffusion time.
Dyes preparation and cell construction
require high temperature (close to boiling point of water) to carry on the adhesion; additionally, the diffusion time of the dye-direct into the mesoporous is significantly higher. Then, the use of directs makes the cell construction process cumbersome. For these reasons, low molecular weight mordant (typically between 192 and 258) were used; this improves the diffusion of the dye-mordant into the mesoporous titania and the process is carry out at room temperature, making the dye distribution homogeneous, complete and faster. Different cochineal-based dye solutions were prepared grinding, in an agate mortar and separately, 0.5 g of cochineal powder in presence of 0.035 g of citric acid and 0.25 g of potassium alum, compound used as mordant. Each of the dye-mordant compounds were mixed with
The natural carmine color is produced by the cochineal insect as a protection mechanism; the dye molecule is the carminic acid (C22H20O13) that consists of a core of anthraquinone linked to a glucose sugar unit. Mordants (dye fixative) link, chemically, dyes on different types of substrates (fabric, tissue, cell, etc.) through a coordination complex; these can change and intensify the stains depending on the substrate and the mordant. Even when the mordant has been displaced by “directs” which are also coupling agents to adhere the dye with the substrate by non-ionic forces, these are molecular structures with high molecular weight (typically between 900 and 1000 Daltons); these 2029
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8.0 mL of boiling water and the suspensions were homogenized using an ultrasonic bath (Branson 2510) for 40 min at room temperature; finally, the systems were filtered using a Whatman paper No. 40 to remove clumps. To prepare the Bixa Orellana pigment solutions, achiote seeds were ground in an agate mortar and, separately, 1.3 g of achiote was taken and mixed with 6 mL of acetyl acetone. The suspension was homogenized for 10 min with a magnetic stirrer, followed by the use of an ultrasonic bath (Brason 2510) for 5 min at room temperature; finally, the system was filtered using Whatman No. 1 paper [31]. β-carotene dye solutions were prepared using 0.01 g of β-carotene powder (Sigma Aldrich) in presence of 0.009 g of citric acid and 0.005 g of potassium alum that is used as mordant. Each of the dye-mordant compounds were mixed with 10 mL of acetyl acetone and the suspensions were homogenized using a magnetic stirrer for 5 min at 45 °C. Once the titania mesoporous layer was impregnated with the dyemordant during different times, a parafilm frame was placed around the cell as a container in order to fill the mesoporous with 0.4 mL of the electrolyte solution; this has an electrical conductivity of 8.88 mS. The upper ITO counter-electrode was covered with a thin coating of vegetal
carbon grounded and sieved at 74 µm. Once the ITO plate covered with the mesoporous coating containing the dye-mordant and the electrolyte are in place, the upper carbon-coated ITO counter-electrode glass was placed on top, pressed again the spacers, seal with parafilm and clamped firmly in a sandwich configuration; additional parafilm strips were used to improve the sealing of the whole sandwich to avoid leaks and to reduce evaporation. No leaks were detected during the assembling and operation of the cell. Electric wires were placed in the appropriated places to perform the electric measurements. For the cochineal-mordant system, the content of sugar of the components was low enough, reason why it was not necessary to remove it; this produces a reduction in the cell construction cost. The illumination of the solar cell was as reported elsewhere [30]: a halogen lamp of 50 W was used as light source together with a hot mirror to reduce the heat produced by the lamp; this reduces the temperature and, consequently, the evaporation of the electrolyte. The intensity was determined with a power meter. The light spectra of the halogen lamp were measured using a USB spectrometer (Ocean Optics 2000). Two different illumination profiles, dark-light–dark, were used: 1-8-1 and 1-2-7, all in seconds.
Fig. 7. Photocurrent profiles for different illumination conditions (1-8-1 and 1-2-7) and the relaxation times light-to-dark for: a) β-carotene, b) carminic acid, c) Bixa Orellana and d) (carminic acid)-(Bixa Orellana).
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Fig. 7. (continued)
Electric and Electrochemical characterization
30 kHz with amplitude of 5 mV. The positive electrodes of the cell was connected the work electrode of the apparatus, while the negative electrode of the cell was connected to the reference and auxiliary electrodes; all determinations were obtained using the option AC impedance.
The current–voltage determinations of all prepared cells were performed using a Keithley 2400 Source-Meter with KUSB-488B interface and a LabTracer 2.0 software based on LabView program. The time–current curves were measured using the same source-meter as before and the data were plotted and analyzed with software based on LabView v.2014 and synchronized with the dark-light-dark illumination sequence; the sampling rate was set to 100 ms. The output electric power was determined using resistors from 100 Ω to 100 MΩ and measuring the voltage with a 6½ digits FLUKE 8846A multimeter, while the current was determined with a Tektronix True RMS multimeter TX3. The Electrochemical Impedance Spectroscopy (EIS) results were performed in a Potenciostate GillAC 1144, ACM Instruments equipped with a software v. 5 which allows the option of EIS for the analysis of the circle fitting to obtain the equivalent circuit parameters: using the Nyquist diagram it is possible to determine Rsol (resistance of the electrolytic solution) and Rct (charge transfer resistance), on the other side, the Bode diagram allows to determine the Cdl (double layer capacitance) making use of Cdl = (2πfmaxRct)−1. These values are reported in Tables 1 and 2. The frequency was changed from 0.01 Hz to
Results and discussion The spectrum of the halogen lamp with the hot mirror is reported in Fig. 1. This halogen lamp has, by itself, an emission that goes from 350 to 950 nm; this means that a considerable contribution of the spectrum is in the IR region, i.e. a considerable amount of heat is produced. The hot mirror reduces the emission to the range from 350 to 750 nm and, consequently, the heat production, reducing the celĺs temperature and the evaporation. The dyes were characterized using UV–Vis spectroscopy and the results are reported in Fig. 2. The absorbance was determined from 350 to 900 nm. For β-Carotene the absorbance is high at 350 nm but at 500 nm is drastically reduced reaching almost a zero value. For Bixa Orellana the absorbance increases from 350 nm reaching its high value at 380 nm [32] but is reduced to zero at 550 nm [33]. For Carminic acid the absorbance shows a minimum at 390 nm and a maximum at 500 nm 2031
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Fig. 7. (continued)
and is reduced to zero near 600 nm, i.e. the carminic acid shows an extended range in red color absorption. The thickness of the mesoporous layer plays an important role in the electric response of the DSSC. In Fig. 3 it is plotted the superficial charge density as a function of the mesoporous thickness for carminic acid. The charge density is related to the current that is possible to obtain from the cell. The points represent the experimental data and the continuous line the fitting using a Gaussian model; the maximum of the Gaussian curve occurs at 34.5 µm. As can be noticed, the electric response of the cell depends strongly with the semiconductor thickness, decaying rapidly to zero for thickness smaller than 17 µm and larger than 57 µm. The concentration of electrons can be written, according to Bisquert et al model [34] in the injection limit, as:
n (x , t ) =
x
e
n (x = L , t ) =
L2 + kt 4Dt
(3)
Using the values of the fitting parameters reported in Fig. 3 it is possible to obtain the numerical values for β and D using Eq. (3): β = (78.32 ± 4.1) × 1013 m−1 and D = (4.76 ± 1.6) × 10−4 cm2/s. In addition to the titania thickness, the absorption time required for a fully impregnation of the dye-mordant into the mesoporous layer is also important because, to obtain a good performance of the cell, it is required a complete and uniform distribution of the dye-mordant into the titania layer. The absorption (or impregnation) time of the dyemordant into the titania layer is directly related to the layer thickness because thick titania layer requires longer times of diffusion to reach a complete and uniform distribution of the dye-mordant into the layer; due this, the cell parameters [14] were determined as a function of these two variables: the dye-mordant absorption time and the titania thickness. In Table 1 is reported the electrical parameters (fill factor, Voc, Isc,η) and the relaxation times for two illumination conditions (1-8-1 and 1-2-7). In Table 2 is reported the density of carriers for different mesoporous thickness and dye adsorption times. In Fig. 4 it is possible
x 2 + kt 4Dt
2 D t3
2 Dt 3
e
(2)
where n(x, t) is the superficial charge density, D the diffusion coefficient, k the dye absorption constant without solvent and β a fitting parameter. This equation can also be written as a function of the mesoporous thickness L as: 2032
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Fig. 7. (continued)
to see the electrolyte solution resistance Rsol as a function of, simultaneously, the titania thickness (down scale) and the dye absorption time (up scale); Rsol involves specifically the mesoporous layer because it is the resistance of the electrolyte solution contained into the mesoporous. Due this it is possible to observe, for Rsol, a strong correlation between these two parameters (thickness and diffusion time): the Rsol-thickness curve shows a maxima between 10 and 16 µm and a minimum at 34.5 µm, while for Rsol-adsorption the curve shows a maximum between 12 and 14 h and a minimum between 35 and 36 h; these two curves have similar shapes. In Fig. 5a and b are reported the contact resistance Rct and the double layer capacitance Cdl as a function of thickness (down scale) and as a function of absorption time (up scale). Rct is more susceptible to temporal shifts due to the intrinsic characteristics of the R-C circuit; for this reason there is not a equivalence between these variables: thickness and adsorption time. For a thickness of 35 µm Rct (Fig. 5a) reaches a maximum, while for the absorption time the maximum is between 46 and 52 h. The minimum of Rct is at 10 µm of thickness, while for the absorption the minimum happens at 24 µm. The same effect happens also for the capacitance Cdl (Fig. 5b) where the maximum is reached for thickness of 35 µm while Cdl reaches a maximum between 10 and 12 h.
The minimum of Cdl is for thickness of 57 µm and for an adsorption time of 24 h. The dependence of the superficial charge density as a function of the dye impregnation time is reported Fig. 6. Here it is possible to see that the charge density is reduced practically in a linear way during the first 24 h; after this time the charge density remains nearly constant until 60 h. This means that, under these conditions, after 24 h the dye was uniformly distributed in the whole volume of the mesoporous layer; shorter times produces superficial charge density no-uniformly distributed into the titania layer. Using this plot, it is possible to optimize the time the mesoporous must be immersed into the dye solution for a good impregnation. In Fig. 7 it is reported photocurrent profiles for: a) -carotene, b) carminic acid, c) Bixa Orellana and d) (carminic acid)-(Bixa Orellana), as a function of time for two-illumination conditions: 1-8-1 and 1-2-7 [35]. In these profiles it is possible to observe small unstable oscillations that appears on top of the photocurrent signal; these oscillations have reasonably the same amplitude and frequency for all dyes reported here and appears due to the rapid transition dark-to-light, i.e. the oscillations are a non-linear response of cell when is shifted, abruptly, far from equilibrium. The frequencies of these oscillations are practically 2033
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Fig. 8. Maximum photocurrent for different illumination conditions for all dyes.
identical for all samples and conditions, resulting: f = (5.00 ± 0.02) Hz. In these figures it is also reported the light-to-dark relaxation profiles (two plots in the bottom part of these figures) for both illumination conditions; in all cases the relaxation was modeled using a simple exponential decay function. The maximum photocurrents obtained for different illumination conditions and for all dyes considered here, are reported in Fig. 8. The maximum photocurrent was obtained for Bixa Orellana and the mixture (carminic acid)-(Bixa Orellana) [36], followed by the carminic acid; the minimum photocurrent was for -Carotene. It is interesting to note that the maximum photocurrent is practically independent of the illumination conditions (1-8-1 and 1-2-7), meaning that the recovery time for these cells is bellow one second. The relaxation times for different
illumination conditions and for all dye-mordant, are reported in Fig. 9 for both the decaying and the rising process. The largest relaxation times, for decaying and rising, corresponds to -Carotene followed by the mixture (Carminic acid)-(Bixa Orellana); in both cases, the minimum relaxation times for rising and decaying is for carminic acid. In Fig. 10 it is reported the typical I-V curves for a sample containing carminic acid, for two different illumination conditions: dark and light. For darkness conditions there is a soft transition (change in the slope) in the I-V curve that happens at (0.69 V, −6.0 × 10−4 A), while for illumination condition the change in the slope happens at (1.03 V, −1.4 × 10−3 A). Es importante resaltar que de las curvas I-V es possible cuantificar gráficamente, a partir del cambio en la pendiente, la resistencia de salida serie (Rserie) y la Resistencia en
Fig. 9. Relaxation times for: different illumination conditions, different dyes and for charging and discharging process.
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Fig. 10. I-V plots for different light conditions: dark and illumination.
derivación (Rshunt), las cuales representan la Resistencia de salida de la celda solar cuando se transfiere la potencia generada a una carga eléctrica. A typical result from the Electrochemical Impedance Spectroscopy for the sample containing carminic acid is shown in Fig. 11. This is a Nyquist diagram where it is plotted the imaginary part Z” versus the real part Ź of the impedance; with this information it was possible to determine Rsol and Rct for each sample; these values are reported in both Tables. Typical output power profiles as a function of voltage are reported in Fig. 12 for: a) carminic acid and b) (carminic acid)-(Bixa Orellana). The output power for different dyes is plotted in Fig. 12c; the highest output power corresponds to the mixture (carminic acid)-(Bixa Orellana) with 3.3 µW, followed by the carminic acid with 1.9 µW.
Conclusions Several dyes sensitized solar cells based on different natural dyes were prepared. The DSSĆs were fabricated using different thickness in the mesoporous titania layers. The impregnation times of the dyemordant into the mesoporous titania were determined. Unstable oscillations were found in the photocurrent profiles that are due to the abrupt change in the illumination conditions from dark-to-light; these oscillations have the same amplitude and frequency. Dependence of superficial charge density, electrolyte resistance, contact resistance and capacitance with the titania thickness and the dye impregnation time were obtained. The plots show a strong dependence of these variables with geometrical and diffusion parameters.
Fig. 11. Nyquist diagram (plot of the imaginary Z” versus the real part Ź of the impedance) allows to determine Rsol and Rct.
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Fig. 12. Typical power profiles as a function of the voltages for: a) carminic acid, b) (carminic acid)-(Bixa Orellana). The output power for different dyes is reported in (c).
References [1] Martín C, Ziółek M, Douhal A. Ultrafast and fast charge separation processes in real dye-sensitized solar cells. J Photochem Photobiol C 2016;26:1–30. 101016/j.jphotochemrev.2015.12.001. [2] Misra R, Maragani R, Arora D, Sharma A, Sharma GD. Positional isomers of pyridine linked triphenylamine-based donor-acceptor organic dyes for efficient dye-sensitized solar cells. Dyes Pigments 2016;126:38–45. https://doi.org/10.1016/j.dyepig. 2015.11.008. [3] Zhang H, Fan J, Iqbal Z, Kuang DB, Wang L, Meier H, et al. Novel dithieno [3,2b:2’,3’-d] pyrrole-based organic dyes with high molar extinction coefficient for dyesensitized solar cells. Org Electron 2013;14:2071–81. https://doi.org/10.1016/j. orgel.2013.04.046. [4] Wang P, Wang J, Yu H, Zhao L, Yu J. Hierarchically macro–mesoporous TiO2 film via self-assembled strategy for enhanced efficiency of dye sensitized solar cells. Mater Res Bull 2016;74:380–6. 101016/j.materresbull.2015.11.004. [5] Calogero G, Citro I, Di Marco G, Minicante SA, Morabito M, Genovese G. Brown seaweed pigment as a dye source for photoelectrochemical solar cells. Spectrochim Acta A 2014;117:702–6. https://doi.org/10.1016/j.saa.2013.09.019. [6] Shin I, Seo H, Son MK, Kim JK, Prabakar K, Kim HJ. Analysis of TiO2 thickness effect on characteristic of a dye sensitized solar cell by using electrochemical impedance spectroscopy. Curr Appl Phys 2010;10:S422–4. https://doi.org/10.1016/j. cap.2009.12.039. [7] Salavati-Niasari M, Davar F, Mir N. Preparation of ZnO nanoflowers and Zn
[8] [9]
[10] [11] [12]
[13] [14]
2036
glycerolate nanoplates using inorganic precursors via a convenient rout and application in dye sensitized solar cells. Chem Eng J 2012;181–182:779–89. https://doi. org/10.1016/j.cej.2011.11085. Qian X, Yan R, Xu C, Shao L, Li H, Hou L. New efficient organic dyes employing indeno[1,2-b]indole as the donor moiety for dye-sensitized solar cells. J Power Sources 2016;332:103–10. https://doi.org/10.1016/j.jpowsour.2016.09.138. Qian X, Wang X, Shao L, Li H, Yan R, Hou L. Molecular Engineering of D-D-π-A type organic dyes incorporating indoloquinoxaline and phenothiazine for highly efficient dyesensitized solar cells. J Power Sources 2016;326:129–36. https://doi.org/10. 1016/j.jpowsour.2016.06.127. Qian X, Lan X, Yan R, He Y, Huang J, Hou L. T-shaped (D)2–A–π–A type sensitizers incorporating indoloquinoxaline and triphenylamine for organic dye-sensitized solar cells. Electrochim Acta 2017;232:377–86. Syafinar R, Gomesh N, Irwanto M, Fareq M, Irwan YM. Chlorophyll pigments as nature-based dye for dye-sensitized solar cell (DSSC). Energy Procedia 2015;79:896–902. https://doi.org/10.1016/j.egypro.2015.11.584. Al-Alwani MAM, Mohamad AB, Kadhum AAH, Ludin NA. Effect of solvents on the extraction of natural pigments and adsorption onto TiO2 for dye-sensitized solar cell applications. Spectrochim Acta A 2015;138:130–7. https://doi.org/10.1016/j. saa.2014.11.018. Shalini S, Balasundara Prabhu R, Prasanna S, Mallick TK, Senthilarasu S. Review on natural dye sensitized solar cells: operation, materials and methods. Renew Sustain Energy Rev 2015;51:1306–25. https://doi.org/10.1016/j.rser.2015.07.052. Wang XF, Zhan CH, Maoka T, Wada Y, Koyama Y. Fabrication of dye-sensitized solar cells using chlorophylls c1 and c2 and their oxidized forms c′1 and c′2 and from
Results in Physics 12 (2019) 2026–2037
D. Rangel, et al.
[15] [16]
[17] [18]
[19]
[20]
[21] [22]
[23] [24]
[25]
Undaria pinnatifida (Wakame). Chem Phys Lett 2007;447:79–85. https://doi.org/ 10.1016/j.cplett.2007.08.097. Lei BX, Zeng LL, Zhang P, Qiao HK, Sun ZF. Sugar apple-shaped TiO2 hierarchical spheres for highly efficient dye-sensitized solar cells. J Power Sources 2014;253:269–75. https://doi.org/10.1016/j.jpowsour.2013.12.035. Ananth S, Vivek P, Arumanayagam T, Murugakoothan P. Natural dye extract of lawsonia inermis seed as photo sensitizer for titanium dioxide-based dye sensitized solar cells. Spectrochim Acta A 2014;128:420–6. https://doi.org/10.1016/j.saa. 2014.02.169. Panahi-Kalamuei M, Mousavi-Kamazani M, Salavati-Niasari M. Facile hydrothermal synthesis of tellurium nanostructures for solar cells. J Nanostruct 2014;4(4):459–65. https://doi.org/10.7508/JNS.2014.04.008. Panahi-Kalamuei M, Salavati-Niasari M, Hosseinpour-Mashkani SM. Facile microwave synthesis, characterization, and solar cell application of selenium nanoparticles. J Alloy Compd 2014;617:627–32. https://doi.org/10.1016/j.jallcom. 2014.07.174. Amiri O, Salavati-Niasari M, Sabet M, Ghanbari D. Synthesis and characterization of CuInS2 microsphere under controlled reaction conditions and its application in lowcost solar cells. Mater Sci Semicon Proc 2013;16(6):1485–94. https://doi.org/10. 1016/j.mssp.2013.04.026. Hosseinpour-Mashkani SM, Salavati-Niasari M, Mohandes F, Venkateswara-Rao K. CuInS2 nanoparticles: microwave-assisted synthesis, characterization, and photovoltaic measurements. Mater Sci Semicon Proc 2013;16(2):390–402. https://doi. org/10.1016/j.mssp.2012.09.005. Berhe SA, Gobeze HB, Pokharel SD, Park E, Youngblood WJ. Solid-state photogalvanic dye-sensitized solar cells. ACS Appl Mater Inter 2014;6:10696–705. https://doi.org/10.1021/am502473q. Sabet M, Salavati-Niasari M, Amiri O. Using different chemical methods for deposition of CdS on TiO2 surface and investigation for their influences on the dyesensitized solar cell performance. Electrochim Acta 2014;117:504–20. https://doi. org/10.1016/j.electacta.2013.11.176. Amiri O, Mir N, Ansari F, Salavati-Niasari M. Design and fabrication of a high performance inorganic tandem cell with 11.5% conversion efficiency. Electrochim Acta 2017;252:315–21. https://doi.org/10.1016/j.electacta.2017.09.013. Sobhani-Nasab A, Hosseinpour-Mshkani SM, Salavati-Niasari M, Taqriri H, Bagheri S, Saberyan K. Synthesis, characterization and photovoltaic application of NiTiO3 nanostructures via two-step sol-gel method. J Mater Sci: Mater Electron 2015;26(8):5735–42. https://doi.org/10.1007/s10854-015-3130-0. Mir N, Salavati-Niasari M. Preparation of TiO2 nanoparticles by using tripodal
[26] [27]
[28] [29] [30] [31] [32] [33]
[34] [35]
[36]
2037
tetraamine ligands as complexing agent via two-step sol-gel method and their application in dye-sensitized solar cells. Mater Res Bull 2013;48(4):1660–7. https:// doi.org/10.1016/j.materresbull.2013.01.006. Mir N, Salavati-Niasari M. Photovoltaic properties of corresponding dye sensitized solar cells: effect of active sites of growth controller on TiO2 nanostructures. Sol Energy 2012;86(11):3397–404. https://doi.org/10.1016/j.solener.2012.08.016. Hosseinpour-Mashkani SM, Mohandes F, Salavati-Niasari M, Venkateswara-Rao K. Microwave-assisted synthesis and photovoltaic measurements of CuInS2 nanoparticles prepared by using metal-organic precursors. Mater Res Bull 2012;47(11):3148–59. https://doi.org/10.1016/j.materresbull.1012.08.017. Hoshikawa T, Ikebe T, Kikuchi R, Eguchi K. Effects of electrolyte in dye-sensitized solar cells and evaluation by impedance spectroscopy. Electrochim Acta 2006;51:5286–94. https://doi.org/10.1016/j.electacta.2006.01.053. Koyyada G, Kumar P, Salvatori P, Marotta G, Lobello MG, Bizzarri O, et al. New terpyridine-based ruthenium complexes for dye sensitized solar cells applications. Inorg Chim Acta 2016;442:158–66. https://doi.org/10.1016/j.ica.2015.11.031. Rangel D, Navarro JA, Vargas S, González M, Castaño VM, Rodríguez R. A novel dual mechanism in dye-sensitized solar cells. Int J Energy Res 2017;41:1164–70. https://doi.org/10.1002/er.3700. Tamil SA, Aravindhan R, Madhan B, Raghava RJ. Studies on the application of natural dye extract from Bixa orellana seeds for dyeing and finishing of leather. Ind Crop Prod 2013;43:84–6. https://doi.org/10.1016/j.indcrop.2012.07.015. Barrozo MAS, Santos KG, Cunha FG. Mechanical extraction of natural dye extract from bixa orellana seeds in spouted bed. Ind Crop Prod 2013;45:279–82. https:// doi.org/10.1016/j.indcrop.2012.12.052. De Araujo Vilar D, De Araujo Vilar MS, De Lima e Moura TF, Raffin FN, De Oliveira MR, Franco CF, De Athayde-Filho PF, Melo Diniz MFF, Barbosa-Filho JM. Traditional uses, chemical constituents, and biological activities of bixa orellana L.: a review. Sci World J 2014;2014:1–11. https://doi.org/10.1155/2014/857292. –. Bisquert J, Mora-Sero I, Fabregat-Santiago F. Diffusion-recombination impedance model for solar cells with disorder and nonlinear recombination. Chemelectrochem 2014;1:289–96. https://doi.org/10.1002/c elc.201300091. Gómez-Ortíz NM, Vázquez-Maldonado IA, Pérez-Espadas AR, Mena-Rejón GJ, Azamar-Barrios JA, Oskam G. Dye-sensitized solar cells with natural dyes extracted from achiote seeds. Sol Energy Mater Sol C 2010;94:40–4. https://doi.org/10.1016/ j.solmat.2009.05.013. Pooran K. Photogalvanic cells: comparative study of various synthetic dye and natural photo sensitizers present in spinach extract. RSC Adv 2014;4:46194–202. https://doi.org/10.1039/C4RA08373C.