The Effect of Nematic Liquid Crystal on the Performance of Dye-Sensitized Solar Cells

Abstract

The motivation for increasing the efficiency of renewable energy sources is the basic problem of ongoing research. Currently, intensive research is underway in technology based on the use of dye-sensitized solar cells (DSSCs). The aim of this work is to investigate the effect of modifying the iodide electrolyte with liquid crystals (LCs) known for the self-organization of molecules into specific mesophases. The current–voltage (I-V) and power–voltage (P-V) characteristics were determined for the ruthenium-based dyes N3, Z907, and N719 to investigate the influence of their structure and concentration on the efficiency of DSSCs. The addition of a nematic LC of 4-n-pentyl-4-cyanobiphenyl (5CB) to the iodide electrolyte influences the I-V and P-V characteristics. A modification of the I-V characteristics was found, especially a change in the values of short circuit current (I_SC) and open circuit voltage (V_OC). The conversion efficiency for cells with modified electrolyte shows a complex dependence that first increases and then decreases with increasing LC concentration. It may be caused by the orientational interaction of LC molecules with the titanium dioxide (TiO₂) layer on the photoanode. A too high concentration of LC may lead to a reduction in total ionic conductivity due to the insulating effect of the elongated polar molecules.

1. Introduction

Renewable energy sources have become particularly important in the era of problems with access to energy produced from non-renewable sources [1]. Solar energy is usually converted into an electric current using photovoltaic cells based on semiconductor properties [2]. They are classified as first-generation photovoltaic cells, which use monocrystalline silicon produced by the Czochralski method or polycrystalline silicon, which is obtained by melting and appropriately forming silicon [3,4]. The efficiency of these cells is in the range of 18–25.6% [5]. The second-generation cells are thin-film systems (layer thickness 1–3 μm) built from monolayers of cadmium telluride (CdTe), a mixture of copper, indium, gallium, and selenium (CIGS), or amorphous silicon (a-Si). The efficiency changes from about 8% for a-Si cells to 17% for CdTe modules and 16% for CIGS [6]. In addition, third-generation photovoltaic cells based on a reversible photochemical process, known as dye-sensitized solar cells (DSSCs), are being intensively studied [7,8,9,10,11].

LCs, due to their intermediate properties between liquids and solid crystals and the ability of molecules to self-organize into variously ordered mesophases, are finding increasing applications not only in information imaging devices [12,13,14,15,16]. In particular, their unique optical and electro-optical properties and ease of deformation and reorientation by means of electric, magnetic, mechanical, and other external fields arouse constant interest in these materials. In many modern devices designed for energy conversion or storage—such as DSSCs, fuel cells, Li-ion batteries, and supercapacitors—an organic electrolyte is a key component. Its stability and physicochemical properties directly translate into the efficiency of these systems [17,18,19,20,21,22,23,24,25]. The essence of the concept of using LCs is that properly ordered structures on the nanometer scale contain an ionophilic and ionophobic part, leading to two functions, namely the formation of conductive channels and insulating moieties. Moreover, insulating parts fulfill the role of stabilizing the system mechanically and thermally [16]. Other advantages of using LCs include their ability to self-organize into systems with anisotropic properties, leading to an increase in ionic conductivity in a specific direction. Moreover, temperature induction of phase transitions enables switching of functions in a given electrolytic structure [13].

The aforementioned LC properties are employed in the design and synthesis of functional materials. They provide high ionic conductivity of electrolytes while maintaining safety, chemical resistance, and electrochemical stability. Additionally, they enhance the mechanical robustness of devices and help lower production costs [20]. A promising class of compounds is ionic liquid crystals (ILCs) that combine the properties of ionic liquids and LCs. ILCs consist of rigid aromatic molecular cores influencing long-range ordering and ionic centers and alkyl chains enabling modulation of charge mobility [20,21,24,26,27,28]. Long alkyl chains and spontaneous self-organization processes favor the formation of ordered mesophases on the macroscopic scale. Consequently, the system most often transitions into a smectic phase. Applications of ILCs involve the use of the organized, anisotropic, and partially ordered structure of ionic liquids to increase the conductivity of matter and charge, e.g., in DSSCs. For example, a binary electrolyte composed of a carbonate-functionalized mesogen and an iodide ionic liquid nanosegregates into lamellar structures exhibits more than four times higher ionic conductivity parallel rather than perpendicular to the smectic layers. By using this method, a thermally stable and durable LC-electrolyte is obtained, enabling the redox reaction I⁻/I₃⁻ over a wide temperature range and higher conversion rates. This can be attributed to longer electron lifetimes and higher electron densities in the photoelectrodes [29,30]. The I⁻/I₃⁻ redox couple plays a crucial role in regenerating photoexcited dye molecules in DSSCs. The process includes oxidizing iodide (I⁻) to triiodide (I₃⁻) by the oxidized dye molecules. The triiodide then diffuses to the counter electrode, where it is reduced back to iodide. Moreover, a method based on doping the electrolyte with LC E7 [17] was developed. After adding LC to the electrolyte prepared on the basis of 3-methoxypropionitrile, no mesophase was detected in the resulting mixture. The results show that small amounts of LC reduce the short circuit current density for DSSC because LC reduces the rate of electrochemical reaction between the DSSC counter electrode and the electrolyte. However, the LC dopant delays the rate of DSSC degradation due to the interaction between cyano-groups and organic solvent in the liquid electrolyte. Due to molecular interactions, LCs increase the viscosity and stability of the electrolyte, thus inhibiting the rate of its evaporation. It has also been shown that mesogenic molecules can form strong coordination bonds with titanium atoms on the surface of the porous TiO₂ layer, improving the stability and photovoltaic efficiency of the DSSC cell [31].

The use of electrolytes based on volatile organic solvents in DSSCs raises great concerns related to their commercialization due to flammability, instability, evaporation, or leakage tendency. Alternative solutions based on solid or quasi-solid materials, such as composites, polymers, hole-conducting materials, and LCs, are attracting increasing attention, especially the latter, which are promising solutions as a charge transport layer in DSSCs.

Taking into account the above-mentioned properties of LCs in the context of the conducted research, the following questions should be answered: (i) How does the presence of LC molecules in DSSC cells affect their basic parameters such as short circuit current, open circuit voltage, fill factor, maximum power, or conversion efficiency? (ii) What is the role of LCs in electrolyte stabilization and durability of DSSC cells? The aim of this work is to provide at least a partial answer to the questions posed and problems indicated above.

2. Materials and Methods

2.1. DSSC Cell Preparation Procedure

The measurement of DSSC cells involves a multilayer system consisting of a photoanode (glass with a conductive layer coated with TiO₂ with an absorbed dye), an electrolyte, and a cathode coated with a platinum layer as a redox reaction catalyst (Figure 1). Most of the materials and reagents necessary for the construction of the test DSSC cells were purchased from Solaronix [32]. The measurement of DSSC cells involved using glass plates with a size of 20 × 20 mm covered with a transparent conductive layer of fluorine-doped tin oxide (FTO). In addition, the porous layer of TiO₂ with an area of about 6 × 6 mm was applied on the anode surface.

Preparation for the measurement of DSSC cells consisted of several stages. First, solutions of a specific concentration were prepared by dissolving the dyes in spectrally pure ethanol. The concentration is determined by weighing specific masses of the components of the solution. A Mettler Toledo balance was used for this purpose, and each solution was placed in a tightly closed dark glass bottle. Before closing the bottles, the solutions were mixed using a glass rod until the dye crystals were completely dissolved. The solutions prepared in this way were left for several hours to ensure that the dye had completely dissolved. Then, the dye solution prepared in this way was poured into a small Petri dish, and a previously prepared anode with a porous TiO₂ layer was placed in it. Before that, the anode should be heated to remove water and other impurities. The dish was sealed with wax paper and placed for 12 h in a shaded desiccator to absorb the dye (anode sensitization). The experience of other authors shows that the time of dye sensitization is important for the efficiency of DSSC cells [33,34].

After the sensitization process, the anode was removed, washed with ethanol, and dried to eliminate excess solvent with the dye. Then, a 60 μm thick Meltronix polymer film spacer (DuPont Surlyn^®, Wilmington, DE, USA) (size: 14 × 14 mm; aperture: 8 × 8 mm) is applied to the anode with a porous TiO₂ layer to seal the system and prevent electrode short-circuiting. Sealing of the system occurred as a result of the melting of the gasket at a certain temperature (about 373 K) and pressure. The cathode was a glass plate covered with FTO and a platinum layer, which is a redox catalyst. A hole drilled in the cathode to fill the cell with electrolyte is positioned above the TiO₂ layer. The sandwich was assembled in such a way as to leave edges to which measuring electrodes can be connected. The resulting cell was filled with iodide electrolyte using capillary forces through a hole in the cathode. After filling the cell with electrolyte, the hole in the cathode was closed using a glass plate and Meltronix polymer film, which glues the system together under the influence of temperature. The prepared samples were stored in low humidity and in the absence of lighting. A dark glass desiccator with a hygroscopic insert was used.

2.2. Dyes and Electrolytes

As mentioned above, the DSSC with porous TiO₂ electrodes was sensitized using ruthenium-based dyes purchased at Solaronix [32]. In this experiment the following ruthenium dyes were used: cis-diisothiocyanato-bis (2,2′-bipyridil-4,4′-dicarboxylic acid), ruthenium (II) known as N3, cis-diisothiocyanato-bis (2,2′-bipyrida-4,4′-dicarboxylato), ruthenium (II) bis (tetrabutylammonium) marked as N719, and the amphiphilic ruthenium dye cis-diisothiocyanato-(2,2′-bipyridil-4,4′-dicarboxylic acid)-(2,2′-bipyridil-bipipyridil-4,4′-dinonyl) ruthenium (II) known in literature as Z907 (Figure 2). The spectral properties of N3, N719, and Z907 dyes and their use in the DSSC cells were investigated, e.g., in the works [35,36]. The low-viscosity electrolyte (iodolyte AN-50), contained as a redox couple iodide/triiodide and a redox concentration of 50 mM, was used. The electrolyte was prepared on the basis of acetonitrile solvent with the addition of ionic liquid, lithium salt, and pyridine derivative [32]. Acetonitrile is a low-reactive organic solvent with a relatively low viscosity of approximately 0.369 mPa·s at 298 K. The dipole moment of the acetonitrile molecule is about 3.92 D. The evaporation temperature of this solvent is 354.6 K. The electrolyte was stored in tight bottles in a fume hood. All manipulations were performed using sterile Pasteur pipettes.

2.3. Preparation of Samples with Liquid Crystal

The LC 5CB has a nematic phase below 308 K but crystallizes at 295.7 K. 5CB is formed by rod-like molecules with a rigid polarizable core of two phenyl rings with a cyano-group and a flexible aliphatic end group formed by a pentyl chain. The dipole moment of this polar molecule is approximately 4.8 D [37]. For this anisotropic material, the perpendicular viscosity is 129 mPa·s and the parallel viscosity is 45 mPa·s at 294 K [38]. In order to check the effect of 5CB on the operating parameters of the DSSC cells, several solutions of AN-50 iodide electrolyte (Solaronix, Aubonne, Switzerland) with an admixture of LC were prepared (10% wt, 15% wt). After weighing the appropriate portion of electrolyte and adding the LC to it, the solution was maintained at a temperature above 308 K (5CB isotropic phase) to obtain a homogeneous mixture. An ultrasonic washer with regulated water temperature was used for this purpose. The process lasted about 15 min at a temperature of about 313 K. Additionally, in order to obtain a homogeneous mixture, the solution was stirred using a glass rod. The DSSC cells were filled with the electrolyte prepared in this way as a result of capillary forces.

Solutions at 10% wt and 15% wt of 5CB in AN-50 electrolyte were examined under a polarizing microscope in both thin and thick cells. With crossed polarizers, the field of view was black. This suggests that the electrolyte prepared in this way behaves as an isotropic liquid without the presence of a nematic phase. The 5CB molecules dispersed in the electrolyte are incapable of forming a nematic phase at the concentrations discussed. However, polar LC molecules can interact with ions present in solution and influence their mobility.

2.4. The Current–Voltage Characteristics

Two Keysight 34465A multimeters were used to measure the current–voltage (I-V) characteristics of DSSC cells, along with dedicated Keysight BenchVue 2020 Update 1.0 software, which allows data collection in MS Excel files. Integration of the measurement system with the Origin 2023 (OriginLab) environment allowed for detailed analysis of the results and determination of basic cell parameters. The PET “Photo Emission Tech” sun simulator Cell Tester Model #CT150AAA was used as a source of light radiation. The optimal intensity of the simulator is 10³ W/m². The load used in the tests was a resistor consisting of seven decades with an accuracy class of 0.05 Ω and the possibility of changing the load from 0 to 1 MΩ. The temperature control was applied at the sample measurement site. The measurements were carried out under conditions of stable temperature of about 305 K.

In the case of an ideal photovoltaic cell, the I-V characteristic should have the shape of a rectangle with coordinates I_SC and V_OC (Figure 3). The maximum power of such a cell, P₀ = I_SCV_OC, is always greater than the power of the real cell, P_max = I_mV_m, where I_m and V_m are the coordinates of the P_max. Figure 3 also illustrates how the P_max value can be determined from the power–voltage (P-V) characteristics.

The I-V characteristic of a real cell deviates from the rectangular shape and can be approximately described by the simplified dependency [39,40,41]:

$$I=I_{ph}-I_0\left[\exp \left({\displaystyle \frac{qV}{n{k}_{B}T}}\right)-1\right]$$

(1)

where I_ph represents solar cell current, I₀ is a diode saturation current, q is an electron charge (1.6 × 10⁻¹⁹ C), k_B is a Boltzmann constant (1.38 × 10⁻²³ J/K), n is an ideality factor (from 1 to 2), and T is an absolute temperature. The second term in Equation (1) represents the known Shockley’s diode current formula for semiconductor materials [42].

An important parameter for a PV cell is the efficiency factor η of converting light energy into electric current, which is defined as the ratio of the P_max to the power of light L incident on the cell surface S:

$$\eta ={\displaystyle \frac{P_{max}}{LS}}\times 100\%$$

(2)

The next important parameter is the fill factor FF, which is defined as follows:

$$FF={\displaystyle \frac{P_{max}}{I_{SC}{V}_{OC}}}$$

(3)

By substituting (3) into (2), the nominal value of the conversion factor can be defined as follows:

$$\eta ={\displaystyle \frac{I_{SC}{V}_{OC}FF}{LS}}\times 100\%$$

(4)

Taking into account Equation (1) and using the definition of power, the power dissipated in the DSSC cell can be described by the following function:

$$P=V\left\{I_{ph}-I_0\left[\exp \left({\displaystyle \frac{qV}{n{k}_{B}T}}\right)-1\right]\right\}$$

(5)

The optimal load value is calculated using the following equation:

$$R={\displaystyle \frac{V_m}{I_m}}$$

(6)

3. Results

Figure 4a–c show the I-V characteristics of DSSC cells sensitized by three different ruthenium-based dyes, namely N3, Z907, and N719, for selected light intensities. The characteristics were obtained for a dye concentration of about 0.05% wt in ethanol. The dependence of the I_SC on the light intensity is typical of photovoltaic cells—changes in I_SC are proportional to changes in the light intensity.

It can be seen that with the increase in light intensity, the V_OC slightly increases. The solid lines marked on the figures were obtained by fitting Equation (1) to the experimental points. Although this function describes the photovoltaic phenomenon in a rather simplified way, the obtained fits are rather good. Moreover, Figure 4d,e, and f show P-V characteristics of the investigated DSSC cells for the three mentioned above dyes. The experimental points are fitted with Equation (5) in order to find the P_max of the DSSC cells. In the nonlinear part of the I-V or P-V characteristics, the fits with Equations (1) and (5) deviate more from the measurement points for the higher light intensities illuminating the sample. The fitting parameters are summarized in

Table 1. The DSSC cell parameters for N3 dye.

N3
L [W/m2]	53	121	254
ISC [A]	1.85 × 10−4	4.63 × 10−4	9.69 × 10−4
VOC [V]	0.61	0.65	0.69
P0 [W]	1.13 × 10−4	3.01 × 10−4	6.69 × 10−4
Pmax [W]	0.80 × 10−4	2.05 × 10−4	4.07 × 10−4
FF	0.71	0.68	0.61
η [%]	4.19	4.70	4.43
Iph [A]	(1.826 ± 0.004) × 10−4	(4.64 ± 0.01) × 10−4	(9.69 ± 0.11) × 10−4
I0 [A]	(2.19 ± 0.37) × 10−9	(7.33 ± 1.27) × 10−8	(2.39 ± 0.58) × 10−6
n	18.5 ± 0.3	13.5 ± 0.3	8.8 ± 0.4

,

Table 2. The DSSC cell parameters for N719 dye.

N719
L [W/m2]	53	121	254
ISC [A]	1.00 × 10−4	2.56 × 10−4	5.04 × 10−4
VOC [V]	0.60	0.65	0.67
P0 [W]	0.60 × 10−4	1.66 × 10−4	3.40 × 10−4
Pmax [W]	0.41 × 10−4	1.12 × 10−4	2.11 × 10−4
FF	0.68	0.67	0.62
η [%]	2.15	2.57	2.30
Iph [A]	(1.004 ± 0.002) × 10−4	(2.560 ± 0.003) × 10−4	(5.04 ± 0.02) × 10−4
I0 [A]	(4.84 ± 0.66) × 10−9	(2.00 ± 0.17) × 10−8	(5.90 ± 0.68) × 10−7
n	16.5 ± 0.2	14.7 ± 0.1	10.0 ± 0.2

and

Table 3. The DSSC cell parameters for Z907 dye.

Z907
L [W/m2]	53	121	254
ISC [A]	1.29 × 10−4	3.10 × 10−4	6.57 × 10−4
VOC [V]	0.61	0.64	0.67
P0 [W]	0.79 × 10−4	1.98 × 10−4	4.40 × 10−4
Pmax [W]	0.54 × 10−4	1.32 × 10−4	2.82 × 10−4
FF	0.68	0.67	0.64
η [%]	2.83	3.03	3.07
Iph [A]	(1.278 ± 0.003) × 10−4	(3.105 ± 0.003) × 10−4	(6.59 ± 0.03) × 10−4
I0 [A]	(1.24 ± 0.27) × 10−9	(4.81 ± 0.34) × 10−8	(5.27 ± 0.96) × 10−7
n	19.04 ± 0.4	13.7 ± 0.1	10.7 ± 0.3

. The parameters such as I_SC, V_OC, P₀, and P_max increase with increasing light intensity, but their nominal values depend on the dye used. Interestingly, the FF parameter increases slightly with decreasing light intensity, whereas the efficiency η exhibits a maximum for L = 121 W/m². The observed effect of light intensity on the DSSC parameters is consistent with theoretical predictions and results of other experiments [43,44]. Moreover, the I_ph obtained from the fitting is in good accordance with the values of I_SC read from the I-V characteristics. However, the obtained value of the ideality factor n needs a special elucidation because it differs significantly from the theoretical value. The Shockley–Read–Hall recombination theory predicts that the value of n should be equal to or less than 2. One explanation for this deviation is that the recombination current does not flow uniformly throughout the cell but always in local sites, e.g., in extended defects [45].

Figure 5a shows a comparison of I-V characteristics for different dyes at a selected constant sample illumination. The highest I_SC is shown by the sample sensitized with N3 dye, then with Z907 dye, and finally with N719 dye. Additionally, a slight shift of V_OC towards higher values is observed. The efficiency of DSSC cells depends to a large extent on the photosensitive dyes used and, in particular, on their molecular structure and the location of the HOMO/LUMO energy levels in relation to the energy levels of the TiO₂ semiconductor applied to the anode. The N3 and N719 dye molecules are quite large and compact molecular systems differing from each other by the tetrabutylammonium groups (Figure 2). However, the Z907 dye molecule has a structure similar to the N3 but has two long alkyl chains attached, which makes it an amphiphilic molecule. The discussed modifications of ruthenium-based molecules can affect the efficiency of DSSC cells sensitized with these molecules. On the other hand, Figure 5b shows the dependence of power dissipated in the DSSC cell on the voltage for the three dyes mentioned above.

The solid lines marked in the figures were obtained as before by fitting the measurement results with Equations (1) and (5). This procedure allows for determining the maximum power as well as other basic parameters of DSSC cells, which are listed in

Table 4. Comparison of the DSSC cells’ parameters for different dyes (L = 254 W/m²).

Sample	ISC [A]	VOC [V]	P0 [W]	Pmax [W]	FF	η [%]	Iph [A]	I0 [A]	n
N3	9.54 × 10−4	0.687	6.55 × 10−4	4.07 × 10−4	0.62	4.4	(9.69 ± 0.11) × 10−4	(2.39 ± 0.58) × 10−6	8.8 ± 0.4
Z907	6.60 × 10−4	0.665	4.39 × 10−4	2.82 × 10−4	0.64	3.1	(6.59 ± 0.03) × 10−4	(5.27 ± 0.96) × 10−7	10.7 ± 0.3
N719	5.04 × 10−4	0.674	3.40 × 10−4	2.11 × 10−4	0.62	2.3	(5.04 ± 0.02) × 10−4	(5.90 ± 0.68) × 10−7	10.0 ± 0.2

. As can be seen from Table 4, almost all parameters of DSSC cells sensitized with dyes in the order of N3, Z907, and N719 decrease. However, it is worth noting that the V_OC for the cell with the Z907 dye lies between the values for cells with N3 and N719 dyes. This leads to a situation where the FF for the cell with the Z907 dye is the highest. It should be noted that the efficiency of the cells is not very high and varies from 4.4% to 2.3%.

Figure 6 presents a summary of measurements taken for DSSC cells containing ruthenium N3 dye at different concentrations. As can be seen in Figure 6a, with the increase in dye concentration for a given light intensity, the I_SC increases, while the V_OC does not change much. However, the shift in V_OC for higher voltages is observed for 0.07% wt concentration. On this basis, it can be concluded that the efficiency of the cell increases with the increase in dye concentration. This relationship seems to be nonlinear. This could be expected because increasing the amount of dye in the porous TiO₂ anode increases the efficiency of converting light into electric current—a larger number of dye molecules are in contact with the semiconductor surface, which facilitates the release of electrons to the external circuit. It should be noted that despite small changes in dye concentration, the changes in the value of the I_SC, especially for the concentration of 0.09% wt, seem to be significant.

It should be noted that for sensitization of DSSC cells, it is unfavorable to use too low or too high dye concentrations, which is associated with too low a level of light conversion in the first case or increased agglomeration of the dye and reduced its efficiency in the second case [35,46].

Table 5. Parameters of the DSSC cells for different N3 dye concentrations (L = 121 W/m²).

N3 Dye [% wt]	ISC [A]	VOC [V]	P0 [W]	Pmax [W]	FF	η [%]	Iph [A]	I0 [A]	n
0.05	2.93 × 10−4	0.627	1.84 × 10−4	1.41 × 10−4	0.77	3.2	(2.932 ± 0.004) × 10−4	(4.42 ± 0.74) × 10−11	25.0 ± 0.3
0.07	3.20 × 10−4	0.665	3.16 × 10−4	1.67 × 10−4	0.53	3.8	(3.199 ± 0.009) × 10−4	(4.75 ± 1.76) × 10−11	23.6 ± 0.5
0.09	4.75 × 10−4	0.635	3.02 × 10−4	2.22 × 10−4	0.74	5.1	(4.750 ± 0.008) × 10−4	(1.13 ± 0.23) × 10−9	20.5 ± 0.3

shows the results obtained by fitting the experimental data with Equations (1) and (5) for three selected N3 dye concentrations. As can be seen, the P_max, I_SC, and η increase with the increase in dye concentration. However, V_OC and FF change slightly. Moreover, the above parameters somewhat differ from the observed trend for the 0.07% wt concentration, especially for V_OC and FF.

Figure 7a presents the I-V characteristics for a DSSC cell with an N3 dye at a weight concentration of about 0.05% wt. The characteristics of three DSSC cells differing in the preparation of the electrolyte were compared. The results marked with asterisks were obtained for cells filled with unmodified AN-50 electrolyte, while the remaining results refer to DSSC cells with an electrolyte modified with 5CB at different concentrations. The obtained characteristics show a rather complex and unexpected course. Figure 7b shows the power dependence on the voltage for various concentrations of 5CB in the electrolyte. Based on the presented relationship, P_max for the compared DSSC cells was determined. It can be noticed that the P_max of the DSSC cell with electrolyte without an LC is the smallest, while the P_max of the cell with a lower concentration of 5CB is higher compared to the other two. Based on the characteristics shown in Figure 7, the basic parameters of the studied DSSC cells were determined. The results are presented in

Table 6. Parameters of the DSSC cells for 5CB-electrolyte (L = 254 W/m²).

Sample	ISC [A]	VOC [V]	P0 [W]	Pmax [W]	FF	η [%]	Iph [A]	I0 [A]	n
N3	1.08 × 10−3	0.652	7.04 × 10−4	4.2 × 10−4	0.60	4.6	(1.080 ± 0.004) × 10−3	(5.45 ± 0.61) × 10−6	8.1 ± 0.2
10% wt 5CB	1.20 × 10−3	0.667	8.01 × 10−4	5.61 × 10−4	0.70	6.1	(1.200 ± 0.004) × 10−3	(8.76 ± 1.26) × 10−8	14.3 ± 0.2
15% wt 5CB	0.94 × 10−3	0.709	6.69 × 10−4	4.89 × 10−4	0.73	5.3	(0.944 ± 0.004) × 10−3	(6.33 ± 1.80) × 10−9	16.8 ± 0.4

.

For a 10% wt 5CB concentration in the electrolyte, the I_SC is greater than both the I_SC in the unmodified cell and for the cell filled with an electrolyte with a 5CB concentration of about 15% wt (Table 6). Instead, for a cell with a 15% wt concentration of 5CB in the electrolyte, the I-V characteristic shows I_SC lower than both previously discussed characteristics. The obtained results suggest that at not too high concentrations of 5CB in the electrolyte, the presence of elongated molecules promotes increased transport of charge carriers as a result of the formation of conduction channels facilitating ion migration [17,47]. It is also possible that 5CB molecules interact with the TiO₂ surface, forcing their homeotropic orientation, which may facilitate carrier mobility in this region. The result of this process is an increase in the I_SC. On the other hand, increasing the concentration of polar molecules in the electrolyte can inhibit ion transport. The interaction of polar molecules and ions can result in a decrease in their mobility [48,49]. Moreover, excessive LC concentrations near the photoanode can block carrier flow and reduce the DSSC efficiency. This translates into a reduction in I_SC, even compared to cells with an unmodified electrolyte. It has been observed that as the alkyl chain length increases from 5CB to 10CB, ionic conductivity decreases. This is because the longer the alkyl chain, the better the insulating properties of the material. Long alkyl chains can impede ionic conductivity due to their entanglement. Furthermore, shorter alkyl chains result in lower viscosity of the LC material [49].

However, as 5CB concentration increases, the increase in V_OC is observed—the greater the LC concentration, the greater the value of V_OC (Table 6). This may be due to enhanced molecular packing in the active layer and increased charge dissociation. Interestingly, the FF also increases with the increase in concentration. This is probably the effect of changes in both I_SC and V_OC, whereas the η of converting light energy into electric current changes depending on LC concentration and is the largest for some optimal concentration, in this case 10% wt 5CB. A greater spreading of the I-V characteristics is also observed with the increase in light intensity. This is the result of the increased conversion of photons to electrons as a result of the increased efficiency of this process in the dye. The described results require further detailed studies using LCs with different alkyl chain lengths (e.g., homologous series of n-cyanobiphenyls) or forming other LC phases, e.g., smectic or columnar phases. It should be noted that increasing the light intensity can lead to a change in the temperature at which the measurement is performed. Measurements should be performed under stable temperature conditions and within a short time frame to minimize the impact on measurement results. The influence of temperature on photovoltaic cell parameters is a well-known issue [50,51].

4. Conclusions

The I-V characteristics obtained for DSSC dye cells show typical features for photovoltaic cells. The observed I_SC are small, of the order of several or tenths of a milliampere, while the V_OC reach values of the order of 0.7 V. The parameters I_SC, U_OC, P₀, and P_max change proportionally to the changes in the light intensity. However, the parameter FF slightly increases with decreasing light intensity, but η shows some kind of maximum. The obtained value of the ideality factor n for all samples differs significantly from the theoretical value predicted by the Shockley–Read–Hall recombination theory, which suggests that current does not flow uniformly throughout the cells, but always in local sites, e.g., in extended defects. As the dye concentration increases, the I_SC also increases, and it is crucial that the changes in dye concentration are neither too large nor too small. This significantly affects the efficiency of DSSC cells. The type of dye and its molecular structure are important for the efficiency of the tested DSSC cells. The measurements showed that the highest efficiency was achieved by DSSC cells sensitized with the N3 dye. In contrast, the remaining two dyes, N719 and Z907, were characterized by a lower level of light conversion. Adding the nematic LC 5CB to the iodide electrolyte significantly modifies the efficiency of the DSSC cell. It was observed that for the lower concentrations of 5CB in the electrolyte (of the order of 10% wt), the Isc, and consequently the efficiency of the cell, increases. However, higher concentrations of 5CB in the electrolyte lead to a decrease in efficiency due to the inhibition of charge carriers’ mobility. The highest value of 6.1% was obtained for DSSC cells containing 10% wt 5CB in the electrolyte. Also, the presence of terminal cyano-groups of 5CB increases the intermolecular interactions with acetonitrile molecules, which improve the thermal stability of the electrolyte. Further studies are needed to find the optimal LC concentrations, the influence of the type and structure of mesophases (smectic phase, discotic phase, etc.), and the molecular structure of LC molecules (e.g., alkyl chain length) on the conversion efficiency in DSSC cells. The obtained results will allow for the optimization of the preparation and operating conditions of DSSC cells.