Tin Dioxide-Based Photoanodes Integrated Into the Dye Sensitized Solar Cells Structure

Abstract

This academic article investigates the performance of dye-sensitized solar cells (DSSCs), which are affected by various defects like charge recombination, high impedance, and low power conversion efficiency due to complex physicochemical phenomena. The study explores different materials and processes to enhance DSSC performance, with a focus on photoanode development. Tin dioxide, tin chloride II dihydrate, copper oxide, boric acid, and sodium phosphate are utilized to fabricate the photoanodes. A thermal treatment at 450°C for 30 minutes precedes dye assembly on the photoanode surface. Results show that the tin chloride II dihydrate-modified photoanode outperforms others, achieving higher open-circuit voltage (0.50V), current density (9.58 mAcm^-2), fill factor (0.37), and power conversion efficiency (1.77%). The study suggests that tin salt particles positively impact voltage, current, electron transport, and charge separation, enhancing overall cell performance. Charge recombination significantly affects power conversion efficiency and impedance, but accurately quantifying it remains challenging. Lower recombination levels favor efficiency, while higher levels reduce it.

Keywords:
Photoanodes; solar cells; tin dioxide

1. Introduction

Dye-sensitized solar cells (DSSCs) feature a sandwich-type architecture comprising a dye-sensitized photoanode, an electrolyte, and a counter electrode structure¹-³⁸. Within DSSCs, the photoanode governs photocurrent via the quantity of immobilized dye and voltage through the potential gradient between the Fermi level at the interface between the photoanode and the electrolyte¹⁴. The observed low voltage and electrical current in DSSCs are attributed to charge recombination occurring within the cell, influenced by the layers in the sandwich-type architecture. The values for charge recombination vary across the layers during the photovoltaic process in DSSCs, making their calculation difficult. However, the effect of this variation can be analyzed through the electrical parameters of current, voltage, fill factor, and efficiency. Generally, lower levels of charge recombination enhance these electrical parameters, while higher levels reduce them. Additionally, some authors represent solar cell power generation using a coordinate system where current density and/or voltage values are negative, while others use positive values. However, the study of coordinate systems for representing solar cell power generation is beyond the scope of this work.

Among the different components and processes in DSSCs, the selection of the photoanode type is crucial due to its significant impact on the resulting performance. Various materials have been considered for the development of photoanodes, including TiO2/GO/Ag nanofibers²⁸, TiO2/MnO₂ bilayer-assembled photoanode³⁰, spinel oxide-incorporated photoanode³¹, ruthenium-based double dyes incorporated photoanode³², reduced graphene oxide and carbon nanotubes in ZnO nanostructures³³, lithium incorporation into TiO₂ photoanode³⁴, rGO-incorporated TiO₂ photoanode³⁵, and CuO:TiO₂ nanocomposites photoanode³⁶. These endeavors have significantly contributed to the advancement of photoanode research for DSSCs, providing valuable insights for future investigations. However, the challenge of producing photoanodes capable of minimizing charge recombination remains a hurdle to be addressed.

Titanium dioxide (TiO₂), a semiconductor with a large bandgap, has long served as the traditional photoanode material for DSSCs. In a pioneering study, O'Regan and Grätzel utilized TiO₂ nanoparticles modified by titanium trichloride, followed by thermal treatment at 450°C for 30 minutes, achieving a remarkable 7.9% power conversion efficiency (PCE)¹. This marked a significant milestone as, up until that point, it represented the highest reported efficiency for DSSCs, igniting widespread interest and research contributions from scientists worldwide.

As of the time of writing, the highest reported PCE for DSSCs stands at 15.2%³⁷, marking a substantial improvement. According to the authors, this enhanced performance is attributed to the precise control of dye molecule packing facilitated by a monolayer of a hydroxamic acid derivative pre-adsorbed onto the surface of nanocrystalline mesoporous TiO₂, which promotes efficient charge generation.

As charge recombination within the photoanode significantly impacts the voltage and electrical photocurrent, leading to decreased DSSC efficiency, considerable efforts are being made to find efficient photoanodes. Various photoanode architectures have been explored to enhance electron transfer, light conversion, and ion diffusion at the semiconductor-electrolyte interface within DSSCs. Among these architectures, the use of dye-sensitized tin dioxide (SnO₂) semiconductor as a photoanode has been reported in the literature as an alternative material to titanium dioxide TiO₂¹⁴. SnO₂ photoanodes have the potential to enhance DSSCs due to their larger band gap of 3.8 eV compared to 3.2 eV for TiO₂. This difference can result in the generation of fewer oxidative holes in the valence band under ultraviolet radiation, thereby reducing the rate of dye degradation in DSSCs⁶. In studies of electronic structure, the Kubelka-Munk method has been reported as a useful tool for determining the SnO₂ band gap from optical reflectance³⁸, while Raman spectroscopy has been utilized to investigate the crystal structure³⁹,⁴⁰.

The high recombination of charges in SnO₂-based photoanodes poses a challenge for their development and application in DSSCs. To address this issue, various parameters have been adjusted in the development of SnO₂-based photoanodes. SnO₂ powder was synthesized from tin chloride through sintering at 500°C. Subsequently, the SnO2 layer was treated at 450°C for 30 minutes to serve as a photoanode, resulting in DSSCs with a current density (J_sc) of 3.975 mAcm^-2, an open-circuit voltage (V_oc) of 0.32 V, a fill factor (FF) of 0.3559, and a power conversion efficiency (PCE) of 0.3592%¹¹. The authors did not report the type of dye used; instead, their focus was on the preparation method of SnO₂ nanoparticles using a tin Schiff base complex.

Various structures of SnO₂ photoanodes have been incorporated into DSSC structures using commercially available ruthenium-based dyes to enhance photogenerated current density and cell voltage. The performance of N3-dye sensitized solar cells (N3-DSSC) varied depending on the photoanode structure: SnO₂ (J_sc = -8.0 mAcm^-2, V_oc =0.36 V, FF =48%), SnO₂:ZnO(J_sc = - 9.2 mAcm^-2, V_oc = 0.58 V, FF =67%) and SnO₂:ZnO:Nb₂O₅ (J_sc = -7.1 mAcm^-2, V_oc =0.58 V, FF =69%)⁸. The authors observed that the amount of SnO₂, ZnO, and Nb₂O₅ in photoanode affects the solar cell performance. Similar behavior was observed in N719-DSSC with photoanodes of SnO₂ (J_sc = 2.21 mAcm^-2, V_oc =0.54993 V, FF = 57.38%), SnO₂: (1wt%)Li-ZnO (J_sc = 4,73mAcm^-2, V_oc =0.63015 V, FF = 69.33%) and SnO₂: (0.5t%)Li-ZnO (J_sc = 4,73mAcm^-2, V_oc =0.63015 V, FF = 69.33%)¹⁶. Also, in N719-DSSC with photoanodes of SnO₂:TiO₂ (J_sc = 5.735 mAcm^-2, V_oc =0.64 V, FF =45%) SnO₂:TiO₂:(0.45wt%)grapheme oxide(J_sc = 10.185 mAcm^-2, V_oc =0.67 V, FF =46%) and SnO₂:TiO₂:(4.75wt%)grapheme oxide(J_sc = 10.185 mAcm^-2, V_oc =0.67 V, FF =46%)¹⁸.

Combinations of SnO₂ with chemical elements have been utilized to enhance the electrical potential gradient between modified SnO2 and the electrolyte14. When SnO2 was paired with nickel oxide (NiO) as a photoanode and [N-bis(2-20-bipyridyl-4,40-dicarboxylic acid)] Ru(II) dye, the resulting DSSC generated Jsc = 8.6 mAcm^-2, Voc = 0.51 V, and PCE = 2.7%. In contrast, DSSCs without the modified photoanode exhibited Jsc = 2.8 mAcm^-2, Voc = 0.33 V, and PCE = 0.5%². The superior performance was attributed to the semiconductor nature of NiO, which is of p-type, thereby facilitating the creation of a local p-n junction. This enhancement resulted in an over five-fold increase in cell conversion efficiency.

Dye-sensitized solar cells (DSSCs) employing SnO₂ nanosheets and zinc oxide (ZnO) as the photoanode, treated at 400°C, exhibited a photocurrent density (Jsc) of 2.65 mAcm^-2, an open-circuit voltage (Voc) of 0.62 V, and a power conversion efficiency (PCE) of 0.54%. The elevated voltage is attributed to the presence of ZnO. DSSCs employing SnO₂ and calcium carbonate (CaCO₃) as the photoanode, along with N719 dye, demonstrated efficiencies ranging from 2.3% to 5.4%. The impact of varying efficiency was elucidated as a function of the amount of CaCO₃, while a PCE of 1.7% was achieved for the DSSC utilizing unmodified SnO₂³. Additionally, the influence of the dye on DSSCs employing the SnO₂ photoanode was investigated. In the last two works, the authors adopted a coordinate system in which current is negative, with more negative values associated with greater charge generation.

Despite substantial work by the community to advance DSSCs, there is currently no published data, to the author’s knowledge, on SnO₂-based photoanode arrays modified with tin chloride II dehydrate (SnCl₂.2H₂O) obtained from natural sedimentation and subsequent thermal treatment, and with the addition of copper oxide (CuO), boric acid (HBO₃), and sodium phosphate (Na₂HPO₄) powder layers before sealing the DSSCs. The photovoltaic characteristics of DSSCs with modified SnO₂ photoanodes were investigated using current-voltage density (J-V) and photoelectrochemical impedance spectroscopy. In this work, the authors utilized SnO₂-based photoanodes as a starting point to examine the compatibility of electrical parameters as a tool for the indirect analysis of charge recombination throughout the layers during the photovoltaic process in DSSCs.

2. Experimental

2.1. Preparation of the photoanode

The fabrication process for the photoanodes began with the cleaning of transparent conductive oxides (TCOs) containing fluorine-doped tin oxide coating, with a sheet electrical resistance of 17.60 ohm/sq. The cleaning procedure involved subjecting the TCOs to an ultrasound bath for 10 minutes, using a mixture of water and ethyl alcohol in equal volumes. Two different processes were employed for the assembly of SnO₂-based photoanodes.

In the first process, 0.084g of SnO₂ and an amount of tin salt equivalent to 16 percent of the SnO₂ mass were added to approximately 10 mL of distilled water. The mixture underwent ultrasonic treatment for 30 minutes with stirring. The resulting suspension was then added to a reactor containing a transparent conductive oxide (TCO) substrate measuring 1.00 cm x 2.5 cm. The setup was left for evaporation by natural convection for 7 days, after which the photoanodes were thermally treated at 450°C for 30 minutes. Following thermal treatment, the photoanodes were coated with N719 dye, completing the assembly process. This assembly is referred to as S:S. In the absence of tin salt, a similar procedure was followed, and the resulting photoanode is referred to as S.

In the second process, a powder layer of CuO (S:C), HBO₃ (S:B), and Na₂HPO₄ (S:NP) was deposited on top of each SnO₂ photoanode (S). The amount of these materials was equivalent to 16 percent of the mass of SnO₂. The deposition onto S was performed using a 72 micrometer hole sieve, without subsequent thermal treatment.

2.2. DSSCs assembly

The open cell sandwich structure (photoanode/dye/electrolyte/counter electrode) with an active area of 0.50 cm² was adopted. For dye immobilization onto the photoanodes to obtain the S and S:S photoanodes, they were immersed in equal volumes of an ethyl alcohol solution containing 3.0 × 10^-4 M of Ruthenizer 535-bisTBA (N719) dye and 0.3 × 10^-4 M of chenodeoxycholic acid additive. Commercial counter electrodes containing fluorine-doped tin oxide (FTO) with platinum coating were used. The FTO type TCO, counter electrodes, and Iodolyte AN-50 electrolyte were purchased from Solaronix (Switzerland).

2.3. Characterizations

The diffuse reflectance spectra of the photoanodes were measured using a UV-2600 Shimadzu spectrophotometer equipped with an integrating sphere ISR-2600Plus. The band gap of the photoanodes was determined using the Kubelka-Munk method with diffuse reflectance data³⁸.

For X-ray diffraction measurements, the Panalytical X’Pert PRO diffractometer - model MPD - was used, using cobalt radiation (λ = 1.78896Å) and operating at 40 kV and 40 mA. The geometry used was parallel beam using a monochromator and a 1/4° slit. The sample holder was mounted on a spinner and the geometry of the diffracted beam consisted of a 0.02° soller slit. The range was from 6° to 100° with a step of 0.013°.

The Raman Raman Spectroscopy analyses were performed using an Alpha 300 system from Witec (Ulm, Germany) working with a 20x Nikon objective, and with Nd:YAG (532 nm) laser excitation. The Raman signal analysed throught a high-sensitivity, back illuminated CCD located behind a 600 l/mm grating. For each sample, the Raman spectrum was accquired throught 3 accumulations of 10 seconds each with a laser intensity of 5 mW.

The AFM analyses were performed using a Asylum MFP 3D (Oxford instruments) working in non-contact (tapping) mode with a silicon tip (NCHR-W, Nano World) mounted in a cantilever of 4 µm thicknes, 125 µm lenght, 30 µm width, 320 kHz resonance frequency and 42 N/m force constant. For each sample, it was sccaned a 30 µm x 30 µm area.

The cells were characterized using a potentiostat/galvanostat PGSTAT302N (Metrohm, Switzerland) to obtain current density (J) versus voltage (V) curves and electrochemical impedance spectroscopy (EIS) characteristics. Both J-V and EIS measurements were conducted under simulated solar illumination using white LED light sources with an intensity pattern of 100 mW/cm². For impedance measurements, the conditions included a frequency interval from 100 kHz to 1 Hz, a signal amplitude of 10 mV, and the open-circuit voltage (V_oc).

3. Results and Discussion

3.1. Structural characterization of prepared materials

Figure 1 and Figure 2 depict X-ray diffraction (XRD) patterns of the S and S:S photoanodes, respectively, after calcination for 30 minutes at 450°C, with diffraction patterns indexed to Sn-based clusters.

Figure 1
X-ray diffraction (XRD) patterns of S photoanodes.

Figure 2
X-ray diffraction (XRD) patterns of S:S photoanodes.

Taking Figure 1, the XRD patterns⁴¹-⁴⁵ match well with the superlattice (SnO₂)₁(SnO₂)₁ (COD 1000062), which is the dominant phase identified in the SnO₂-based photoanodes. However, the identification of a secondary phase represented by (SnO₂)₁(SnO.12O₂)₁ (COD 1533658) is observed as a byproduct of the material's calcination.

On the other hand, in Figure 2, all diffraction peaks were perfectly indexed to the superlattice (SnO₂)1(SnO₂)1 (COD 1534785) without the presence of other phases. Possibly, the addition of tin salt contributes to the absence of a secondary phase in the S:S photoanode

Raman spectroscopy measurements were performed for all prepared samples (Figure 3).

Figure 3
Raman spectra of the SnO2, SnO2:NaHPO4, SnO2:SnCl2, SnO2:CuO, and SnO2:H3BO3 samples.

In the SnO₂ sample, the typical spectrum of this material is observable⁴⁶-⁴⁹, with the presence of bands at 476 cm^-1 (Eg), 635 cm^-1 (A_1g), and 775 cm^-1 (B_2g). These bands correspond to the fundamental phonon modes Eg, A_1g, and B_2g, respectively, in good agreement with those observed in pure SnO₂. These active modes are in good agreement with the standard rutile tetragonal SnO₂ structure as reported in earlier literature, where the non-degenerate Raman modes A_1g and B_2g denote the contraction and expansion of the vibrational mode of Sn – O bonds, while the doubly degenerate Eg mode is related to the vibration of oxygen in the oxygen plane.

In the SnO₂:NaHPO₄ sample, the formation of bands at 345 and 731 cm^-1 is observable, while in the SnO₂:SnCl₂, SnO₂:CuO, and SnO₂:H₃BO₃ samples, a shift of the 627 cm^-1 band present in the pure SnO₂ sample is observed, indicating that in all synthesized samples, an interaction of the SnO₂ compound with the present salts was observed.

It is important to emphasize that the incorporation of these salts in powder form and their distinct granulometries can alter the behavior of the formed cells. High porosity may lead to the infiltration of a high concentration of dye into the inner layer, promoting electrical resistance, while low porosity may result in dye accumulation in superficial regions of the cell, also acting as regions of electrical resistance.

However, the images obtained by Atomic Force Microscopy (AFM) (Figure 4) of the analyzed samples⁴⁷,⁵⁰ reveal the formation of a porous final material with minimal differences in porosity among them, indicating that the insertion of salts did not cause significant changes in their surface porosities.

Figure 4
Atomic Force Microscopy (AFM) images for all prepared materials.

3.2. Photovoltaic performance by J-V

Figure 5 depicts the current density-voltage values for DSSCs with various compositions of SnO₂-based photoanodes under an illumination intensity of 100 mW/cm^2. Notably, among the cells, only DSSC S:S (as listed in Table 1) exhibits both high V_oc and J_sc, attributed to enhanced charge separation facilitated by particles generated from SnCl₂.2H₂O.

Figure 5
J-V characteristics for the DSSCs with SnO2-based photoanodes.

Table 1
Photovoltaic characteristics for the SnO₂-based Photoanodes DSSCs.

Photoanodes	DSSC	Voc(V)	Jsc(mA/cm2 )	FF	PCE (%)	Rseries (Ω.cm2)	Rshunt (Ω.cm2)
SnO2	S	0.26	3.71	0.31	0.30000	1248.80	990.60
SnO2:SnCl2	S:S	0.50	9.58	0.37	1.77000	1226.00	998.20
SnO2: CuO	S:C	0.67	2.93	0.40	0.79000	1053.60	998.10
SnO2:H3BO 3	S:B	0.09	2.27	0.27	0.05600	2166.70	974.00
SnO2:Na2HPO4	S:NP	0.03	0.12	0.29	0.00087	3146.80	878.70

Table 1 lists the main parameters (J_sc, V_oc, FF, PCE, parallel (R_shunt), and series (_series) resistances) studied in the literature as tools to investigate the photovoltaic performance of solar cells.

R_series represents all the ohmic resistances contributing to energy loss in the cell as heat, while R_shunt represents potential charge recombinations within the cell. When comparing the R_series and R_shunt values listed in Table 1, it becomes evident that cells with higher R_series values tend to exhibit lower efficiency due to increased energy loss. Additionally, the comparison between R_shunt and Rseries values for the S:S cell and other cells highlights a direct relationship between R_series loss and R_shunt gain, indicating a dependency on the materials present in the photoanode.

The photovoltaic parameters (V_oc, J_sc, and fill factor) of the S, S:S, and S:C cells exhibit similarities with parameters reported in the literature for DSSCs incorporating tin dioxide-based photoanodes through various strategies. For instance, DSSCs with SnO₂ and N719 dye as the photoanode have been reported to exhibit R_series = 19 Ω.cm² and Rshunt = 114 Ω.cm², with Jsc = 4.74 mA cm^-2, Voc = 0.36 V, FF = 0.3140, and PCE = 0.53%²⁴. Similarly, for DSSCs with SnO₂-ZnO-Nb₂O₅ and N719 dye as the photoanode and iodide/triiodide electrolyte, higher values were reported, with Jsc = -8.1 mA cm^-2, Voc = 0.54 V, and FF = 0.65⁸. The enhanced V_oc and FF in the latter case were attributed to the inclusion of ZnO and Nb₂O₅.

Replacing the N719 dye with the SnO₂-ZnO-Nb₂O₅ and N3 ruthenium-based dye as the photoanode resulted in changes in the photovoltaic parameters of the DSSCs. Specifically, the values became Jsc = -5.2 mA cm^-2, Voc = 0.53 V, and FF = 0.54⁸. Similarly, Chen and collaborators produced DSSCs based on SnO2 and cis-bis(thiocyanato)-N,N-bis(2,2-bipyridyl-4,4-dicarboxylate) ruthenium (II) or N3 dye as photoanodes, with an electrolyte iodide/triiodide couple, resulting in J_sc = 2.96 mA cm^-2, V_oc = 0.299 V, FF = 0.32, and PCE = 0.28%. Moreover, DSSCs with the SnO₂ photoanode and ruthenium-based dye were reported with photovoltaic parameters of V_oc = 0.32 V, J_sc = 3.795 mA cm^-2, FF = 0.32, and PCE = 0.3952%¹¹. These findings highlight the influence of all layers in the sandwich structure on the performance of the DSSC.

The use of SnCl₂.2H₂O as an additive for modifying SnO₂-based photoanodes has notably improved the photovoltaic performance of the cell, as evident in Table 1 and Figure 5. This improvement can be attributed to the effective complementarity between the reduction in charge recombination, as indicated by the increase in Rshunt, and the decrease in ohmic resistance induced by the additive on the SnO₂-based photoanode, particularly in the S:S cell. This relationship significantly influences all current and voltage data, resulting in the observed plots in Figure 5.

Due to the lower Fermi level of SnO₂ compared to TiO₂ semiconductor, DSSCs based on pure SnO₂ photoanodes activated with iodide/triiodide redox electrolyte typically exhibit V_oc values below 0.5 V²-⁴,⁸,¹¹,¹⁴, whereas TiO₂ photoanodes yield Voc around 0.7 V¹⁴. Consequently, extensive research has been conducted to enhance the Voc of SnO₂-based photoanode DSSCs. Methods explored include the addition of organic and inorganic additives, alterations in dye composition, and modifications to the electrolyte. In this study, high voltage was achieved through the incorporation of SnCl₂ and CuO as inorganic additives, while N719 acted as the organic additive to enhance photocurrent gain.

Initially, under luminous energy, solar cells generate electrons, and as these electrons migrate (photocurrent) within the cell, a layer begins to form and captures the free electrons. This leads to the formation of a junction with layers of opposite charges, which is responsible for generating the cell voltage. In DSSCs, recombination occurs at the interface of the photoanode with the immobilized dye and the electrolyte. To achieve higher values of luminous energy to photovoltaic electricity conversion, the increase in voltage must be balanced by the generation of free electrons with high energy. The behavior observed, particularly in the S:S cell (Table 1), suggests appropriate generation of high potential at the interface of the modified SnO₂ and the electrolyte, albeit with a high concentration of electrons.

In the S:C DSSC, the high V_oc value of 0.67 V and the low J_sc value of 2.93 mA cm^-2 indicate an incompatibility between voltage increase and the generation of free electrons with high energy. While the presence of copper atoms (Cu) contributes to the increase in cell voltage, the high capture of free electrons leads to a decrease in J_sc. Conversely, for the S:NP and S:B cells, modification of the SnO² photoanodes results in a decrease in cell voltage and high capture of free electrons. However, the opposite effect is observed for the S:S cell, leading to higher efficiency. This suggests that the modification in the S:S configuration yields a balance between voltage increase and generation of free electrons, resulting in improved cell performance.

The fill factor (FF) serves as an indicator of the stability of solar cells in generating electrical energy under illumination. It represents the cell's resistance to energy loss when subjected to an electric field induced by the accumulation of charge in the presence of luminous energy. This stability can be illustrated through a simple experiment: when a photovoltaic cell is connected to a data collector focused on luminous energy, the product of each current density (J) and its associated voltage (V) is calculated (Figure 5). Among these values, one stands out as the maximum energetic product, representing the peak energy produced by the cell. However, beyond a certain voltage threshold, the energy product (J x V) sharply decreases from the maximum energy product (J x V)_max (Figure 5).

Beyond a specific voltage threshold, less stable cells exhibit electrons with lower energy than the electric field produced by the accumulation of charge (EAC). Consequently, these electrons are captured from the EAC rather than being accelerated. The fill factor (FF) is determined by the ratio of the maximum energy product (J x V)_max to the ideal energetic product (J_sc x V_oc). A FF closer to unity indicates higher cell stability in the presence of luminous energy. The lower FF values presented in Table 1 indicate a high concentration of electrons with energy below that of the electric fields induced in SnO₂-based photoanode cells, both modified and unmodified. However, these FF values are consistent with those reported in the literature for SnO₂ photoanodes, whether modified or unmodified.

The superior photovoltaic performance of the S:S DSSC, characterized by a PCE of 1.77% and J_sc of 9.58 mA.cm^-2, suggests an optimal concentration of tin (Sn) on the modified SnO₂ photoanode. Notably, the Jsc value of 9.58 mA.cm^-2 surpasses that of other DSSCs, while the Voc value of 0.50 V is second only to that associated with the S:C DSSC. These results can be attributed to the effective capture of excess electrons in the conduction range of SnO₂. As these electrons accumulate, there is an upward and forward shift in the conduction band fringe at the SnO₂-SnO₂ modified grain boundary. This shift generates a high potential at the interface of the modified SnO₂ and the electrolyte, facilitating the injection of electrons from the energy level of the dye to the energy level of the SnO₂ modified with tin atoms, thereby promoting charge separation. The observed differences in open circuit voltage among the experimental samples were further investigated using spectroscopy of electrochemical impedance (EIS) applied to DSSCs under illumination.

3.3. Electrons transport characteristics in the DSSC estimated by EIS

In studying the behavior of photoelectron injection and charge recombination in DSSCs, electrochemical impedance spectroscopy (EIS) is a valuable tool applied to solar cells⁹,¹⁰,¹²,¹³,¹⁶,¹⁷,²³,²⁷. This analysis focuses on the impedance of the cell, which comprises several components. Specifically, the total impedance (Z) encompasses the impedance at the counter electrode/electrolyte interface (Z_ce) and the diffusion and recombination processes occurring at the photoanode (Z_ph). These processes are influenced by factors such as the diffusion of triiodide in the electrolyte and the charge transfer resistance associated with electron recombination (Rel).

In particular, Z_ph is typically linked with the resistance between electron transport within the photoanode and the charge transfer resistance associated with electron recombination in the electrolyte. By examining these impedance components, researchers can gain insights into the mechanisms governing photoelectron injection and charge recombination dynamics within DSSCs.

The parameters associated with electron transfer in DSSCs were assessed through EIS measurements, and the values obtained for the electrical circuits are listed in Table 2. Figure 6 depicts the experimental Nyquist plots spectrum. In the Nyquist plots (Figure 6), the semicircle near R_el represents the Z_ce impedance. Both the S:S and S DSSCs exhibit two distinct arcs. Moving from left to right, these arcs correspond to charge transport at the counter electrode/electrolyte interface and the interface between the photoanode-dye and the electrolyte, respectively. However, for the S:S DSSC, only one arc is observed, indicating high impedance at the counter electrode/electrolyte interface. The discrepancy in the arcs positions relative to the origin is attributed to the high electrolyte resistance, likely influenced by solvent properties, the composition of electroactive species, and electrode characteristics.

Table 2
Equivalent circuit parameters of electronic transport characteristics estimated by EIS for SnO₂-based DSSCs photoanodes.

DSSC	Equivalent Circuit
S:S
S
S:C

Figure 6
Nyquist graph of the DSSCs: S, S:S and S:C.

The equivalent circuit parameters listed in Table 2 reveal that Z_ph comprises resistance (R_ph) and capacitance (C_p), while Z_ce comprises capacitance (C_c) associated with R_ce. The variation in series resistance (R_el) is influenced by the electrical contacts of each photovoltaic device.

The charge transport resistance (R_sh) increases while R_ce decreases, resulting in an increase in electron density in the conduction band of the photoanode and a reduction in inter-resistance. As shown in Table 2, the calculated EIS parameters indicate that the S:S DSSC exhibits low R_el and R_ce values compared to other cells, contributing to the increased PCE of the S:S DSSC (Table 1). The lower R_sh value is reflected in the enhanced PCE of the S:S DSSC.

The equivalent circuit depicted in Table 2 aligns with existing literature on DSSCs, where the values of resistive and capacitive elements are influenced by the type of materials and DSSC configuration. In this study, the R_ph value, associated with electron recombination at the photoanode-dye/electrolyte interface, was found to be higher for the S:C DSSC compared to the other cells, resulting in lower efficiency in converting luminous energy to photovoltaic electricity (Table 1).

Although the Rph value for the S:S DSSC is higher than that for the S DSSC, this is compensated by the decrease in R_el and Z_ce. However, this compensation does not occur for the S:C DSSC sample. The dynamics exhibited by the S:S DSSC account for its higher efficiency (PCE = 1.77%) compared to the S and S:C DSSCs. However, the higher value for C_ph indicates increased recombination at the photoanode, leading to an increase in Z_ph, which in turn reduces cell efficiency.

The lower Jsc value of 2.93 mA.cm^-2 for the S:C DSSC compared to the S and S:S DSSCs suggests the possibility of increased electron recombination at the SnO₂-CuO grain boundary. Conversely, the higher J_sc observed in the S:S DSSC indicates that the high recombination at the interface of the SnO₂ photoanode-dye/electrolyte, integrated into the cell, was counteracted by the increase in open-circuit voltage. This increase provided the necessary energy for photoelectron acceleration and reduced electron recombination in the modified SnO₂-SnO₂ grain boundary. Consequently, this led to a decrease in R_el and Z_ce due to improved ion diffusion in the electrolyte, as shown in Table 2.

The S:S DSSC demonstrates superior efficiency in converting luminous energy into photovoltaic electricity compared to the S, S:B, S:C, and S:NP DSSCs. This is attributed to the favorable electron transport in the conduction band, leading to increased energy conversion efficiency. The presence of numerous oxygen vacancies in the SnO₂ further enhances conductivity in the SnO₂-based photoanodes, resulting in higher J_sc. Additionally, the higher R_ph value in the S:S DSSC contributes to an increase in V_oc compared to the S DSSC. Despite the increase in V_oc and R_ph, the higher generated photocurrent density offsets these effects in the S:S DSSC.

The photoexcited electrons from the N719 dye molecules are injected into the conduction band of the modified SnO₂ with tin atoms during photoexcitation. These electrons are then transferred to the unmodified SnO₂. This process leads to an excess of electrons in the modified SnO₂ with tin atoms and a downward shift towards the conduction band edge in the grain boundary. As a result, photoexcited electron charge separation is enhanced, leading to improvements in photocurrent and voltage.

The DSSC utilizing the S:S photoanode demonstrates an increase in PCE compared to the S photoanode. To further understand these two photoanodes, Raman spectroscopy and the Kubelka-Munk method were employed. Figure 7 illustrates the Kubelka-Munk plots of the S and S:S photoanodes, derived from diffuse reflectance spectroscopy. The estimated band gap values are 3.83 eV for the S photoanode and 3.82 eV for the S:S photoanode. These values closely align with those reported by Souza and colleagues for pure SnO₂ photoanodes treated at 450°C³⁸.

Figure 7
Kubelka and Munk plots: (a) S photoanode and (b) S:S photoanode.

The results indicate that it is not feasible to directly correlate the band gap of the S and S:S photoanodes with the tin oxide phases³⁹,⁴⁰.

4. Conclusion

The comparison of efficiencies among the DSSCs manufactured in this study highlights the superior performance of the DSSC utilizing a modified SnO₂ photoanode, achieved through the addition of tin salt and subsequent heat treatment at 450°C for 30 minutes. This modification facilitates enhanced charge separation, leading to elevated values of both open circuit voltage (V_oc) and short circuit current density (J_sc), thereby improving the overall efficiency of the DSSC.

Moving forward, potential future research could involve further characterization of the SnO₂ photoanodes by varying the amount of tin salt and subsequent heat treatment to investigate their effects on the properties and performance of DSSCs. This could provide deeper insights into the optimal conditions for enhancing the efficiency of DSSCs and advancing their potential for practical applications in renewable energy technologies.

5. Acknowledgments

The research received financial support from the following funding bodies: Fundação Cearense de Apoio ao Desenvolvimento Científico e Tecnológico (FUNCAP) and Fundação de Desenvolvimento Tecnológico do Ceará (FUNDETEC). The authors would like to express their gratitude to the Center for Technology and Industrial Quality of Ceará (NUTEC), the Analytical Center, and the Research Group on Characterization of Materials by X-Ray Diffraction, both from the Federal University of Ceará (UFC), for providing the necessary facilities for the completion of this work.

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Publication Dates

• Publication in this collection
  08 July 2024
• Date of issue
  2024

History

• Received
  30 Jan 2024
• Reviewed
  08 May 2024
• Accepted
  20 May 2024