Effects of Ti₃C₂Tx MXene Addition to a Co Complex/Ionic Liquid-Based Electrolyte on the Photovoltaic Performance of Solar Cells

Abstract

Redox mediators comprising I⁻, Co³⁺, and Ti₃C₂Tx MXene were applied to dye-sensitized solar cells (DSCs). In the as-prepared DSCs (I-DSCs), wherein hole conduction occurred via the redox reaction of I⁻/I₃⁻ ions, the power conversion efficiency (PCE) was not altered by the addition of Ti₃C₂Tx MXene. The I-DSCs were exposed to light to produce Co²⁺/Co³⁺-based cells (Co-DSCs), wherein the holes were transferred via the redox reaction of Co²⁺/Co³⁺ ions. A PCE of 9.01% was achieved in a Co-DSC with Ti₃C₂Tx MXene (Ti₃C₂Tx-Co-DSC), which indicated an improvement from the PCE of a bare Co-DSC without Ti₃C₂Tx MXene (7.27%). It was also found that the presence of Ti₃C₂Tx MXene in the redox mediator increased the hole collection, dye regeneration, and electron injection efficiencies of the Ti₃C₂Tx-Co-DSC, leading to an improvement in both the short-circuit current and the PCE when compared with those of the bare Co-DSC without MXene.

1. Introduction

Two-dimensional (2D) materials are typically crystalline solids consisting of a single layer of atoms. These materials are considered promising for various applications, which explains why they remain the focus of research. MXenes with a general formula of M_n+1X_nT_x (where n = 1–3; M denotes a transition metal; X is either carbon or nitrogen; and T_x indicates surface terminal groups such as −OH, −F, −Cl, and/or −O−) were created by selectively etching the “A” layers from layered MAX phases (M_n+1AX_n, where A is usually any element from among Cd, Al, Si, P, S, Ga, Ge, As, In, Sn, Tl, and Pb [groups 12–16]) and can be easily solution-processed in aqueous or polar organic solvents due to their hydroxyl- or oxygen-terminated surfaces [1,2,3,4]. Following the production of multilayered Ti₃C₂Tx MXene by etching the Al layers from the Ti₃AlC₂ MAX phase in 2011 at Drexel University [1], numerous related research results have been reported in the fields of energy storage, sensors, light-emitting diodes, electromagnetic shielding, and environmental applications [2,3,4]. In addition, MXenes have been extensively studied in relation to applications concerning solar cells, given their metallic conductivity, excellent charge carrier mobility, high optical transmittance, and tunable work function [5,6,7,8]. Among the various MXenes, Ti₃C₂T_x is the most commonly studied in terms of third-generation solar cells, such as dye-sensitized solar cells (DSCs) [9,10,11], perovskite solar cells [12,13,14], and polymer solar cells [15,16]. Di and Qin reported that a power conversion efficiency (PCE) of 8.08% was achieved in a DSC with TiN@Ni-Ti₃C₂Tx MXene film as a counter electrode, surpassing that of a cell with Pt-based counter electrode (7.59%) [9]. A comparative study of 2D-layer-structured Ti₃C₂Tx MXene and TiC particles was reported. The DSCs with a Ti₃C₂Tx-based counter electrode achieved a PCE of 9.57%, much higher than that of the counterpart device with a TiC particle-based one (7.37%) [10]. It was also reported that DSCs with a poly(3,4-ethylene dioxythiophene)(PEDOT)/Ti₃C₂Tx MXene composite-based counter electrode outperformed cells with PEDOT- or Ti₃C₂Tx MXene-based ones in PCE [11].

A conventional DSC is composed of a dye-adsorbed TiO₂ layer on a transparent electrode (i.e., a working electrode), a liquid electrolyte, and a Pt catalytic layer on a conductive electrode (i.e., a Pt counter electrode). Light absorption in dye molecules leads to the formation of excitons (electron–hole pairs), and the excited electrons are injected into the TiO₂ layer. The photoinjected electrons and the holes in the dye molecules are transported to the electrodes via the TiO₂ layer and the electrolyte, respectively. Finally, the electrons and holes are collected in the electrodes, allowing electron flows through the external circuits to occur [17,18]. The hole-conducting electrolyte comprises redox couples and electrical additives [19,20]. The redox couples, such as I⁻/I₃⁻, Co⁺²/Co⁺³, Cu⁺¹/Cu⁺², and Ni⁺³/Ni⁺⁴, are reduced near the Pt counter electrode and oxidized near the excited dye molecules, thereby allowing for hole collection and dye regeneration, respectively. Electrical additives such as 4-tert-butylpyridine (TBP) and cations (lithium [Li⁺] or guanidinium [C(NH₂)³⁺]) represent another important ingredient in a liquid electrolyte for enhancing the photovoltaic parameters of cells. These additives can control the potential of the redox couple, the surface state of the TiO₂ semiconductor, the shift in the conduction band edge, and the interfacial charge recombination through being incorporating in small amounts [20]. Ti₃C₂T_x MXene was introduced as an additive for electrolytes to improve the photovoltaic performance of quasi-solid-state DSCs [21,22]. Sun et al. reported that, via the addition of Ti₃C₂T_x MXene to a quasi-solid-state electrolyte composed of an I⁻/I₃⁻ redox couple and a melamine-formaldehyde (MF) sponge, the average PCE of the DSCs under a room light condition (1000 lux) was improved by 26.92% from that of the reference cell without MXene (23.35%) [21]. It was also reported that a PCE of 29.94% under a condition of 1000 lux was achieved through the incorporation of both reduced graphene oxide (rGO) and Ti₃C₂T_x to a quasi-solid-state electrolyte containing an I⁻/I₃⁻ redox couple, polyethylene oxide (PEO), and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) [22].

In this study, we report the effects of Ti₃C₂T_x MXene addition to a liquid electrolyte on the photovoltaic performance of cells. Ti₃C₂T_x-dispersed liquid electrolytes based on a metal complex (tris(2-(1H-pyrazol-1-yl)-4-tert-butylpyridine)cobalt(III) tri[bis(trifluoromethane)sulfonimide] [FK209]) as a source of Co³⁺ and an ionic liquid (1-methyl-3-propylimidazolium iodine [MPII]) as a source of I⁻ (iodide) were first prepared. Then, DSCs with Ti₃C₂T_x-dispersed Co³⁺/I⁻ liquid electrolytes (redox mediators) were fabricated and their photovoltaic properties were compared with those of the reference cell without Ti₃C₂T_x MXene. To the best of our knowledge, this is the first report on the effects of Ti₃C₂T_x addition to Co complex (Co³⁺)/ionic liquid (I⁻)-based redox mediators. The reported photovoltaic parameters (short-circuit current [J_sc], open-circuit voltage [V_oc], and fill factor [FF]) of the DSCs with Ti₃C₂T_x MXene are summarized in

Table 1. Photovoltaic performances of DSCs with Ti₃C₂T_x-MXene-incorporated electrolytes.

Device	Measurement Condition	Redox Mediator		Jsc (mA/cm2)	Voc (V)	FF (%)	PCE (%)	Ref.
Quasi-solid-state DSC	AM 1.5	MF-sponge-based I−/I3−	Without Ti3C2Tx	14.979 ± 0.175	0.778 ± 0.004	65.3 ± 0.3	7.610 ± 0.106	[21]
			With Ti3C2Tx	15.085 ± 0.188	0.781 ± 0.003	66.4 ± 0.6	7.822 ± 0.092	[21]
	1000 lux	MF-sponge-based I−/I3−	Without Ti3C2Tx	0.177 ± 0.001	0.569 ± 0.007	70.3 ± 0.5	23.35 ± 0.43	[21]
			With Ti3C2Tx	0.196 ± 0.003	0.579 ± 0.004	71.9 ± 0.4	26.92 ± 0.43	[21]
	AM 1.5	PEO/PVDH-HFP-based I−/I3−	Without rGO/Ti3C2Tx	-	-	-	-	-
			With rGO/Ti3C2Tx	15.170 ± 0.203	0.783 ± 0.002	69.5 ± 0.5	8.255 ± 0.109	[22]
	1000 lux	PEO/PVDH-HFP-based I−/I3−	Without Ti3C2Tx	-	-	-	-	-
			With rGO	0.189 ± 0.002	0.544 ± 0.002	76.1 ± 0.4	23.22 ± 0.43	[22]
			With rGO/Ti3C2Tx	0.223 ± 0.001	0.561 ± 0.004	75.7 ± 0.1	29.94 ± 0.49	[22]
Liquid electrolyte DSC	AM 1.5	Co3+/ I− (FK209/MPII)	Without Ti3C2Tx	15.46 ± 1.04	0.760 ± 0.023	61.33 ± 3.70	7.18 ± 0.11	This study
			With Ti3C2Tx	18.09 ± 0.94	0.746 ± 0.014	63.66 ± 2.69	8.58 ± 0.30	This study

, including those of our devices with FK209 (Co³⁺)/MPII (I⁻)/Ti₃C₂T_x-based liquid electrolytes.

2. Results and Discussion

2.1. Photovoltaic Performance of DSCs Based on Ti₃C₂T_x-Incorporated Co³⁺/I⁻ Redox Mediators

In a previous report, we revealed that, through the simple mixing of MPII and FK209, triiodide (I₃⁻) and Co²⁺(FK209) were produced via a chemical reaction (1) between the iodide (I⁻) of MPII and Co³⁺(FK209), where Co²⁺(FK209) and Co³⁺(FK209) are the Co²⁺ and Co³⁺ ions of FK209, respectively. Since the Co³⁺(FK209) was almost fully converted into Co²⁺(FK209), the redox mediators contained both I₃⁻ and Co²⁺(FK209) as well as I⁻ (originating from non-reacted MPII) [23]. Therefore, hole conduction occurs through reactions (2)–(5) when as-prepared DSCs are exposed to the AM 1.5 condition, indicating that the I⁻/I₃⁻ redox couple is involved in the dye regeneration (chemical Equation (4)) and hole collection (chemical Equation (5)). Here, we code the as-prepared DSCs based on the I⁻/I₃⁻ redox couple as I-DSCs.

3I⁻ + 2Co³⁺(FK209) → I₃⁻ + 2Co²⁺(FK209)

(1)

2Dye + hν → 2Dye*

(2)

2Dye* → 2Dye⁺ + 2e⁻ (TiO₂)

(3)

3I⁻ + 2Dye⁺ → I₃⁻ + 2Dye

(4)

I₃⁻ + 2e⁻ (Pt) → 3I⁻

(5)

Using the FK209 (Co³⁺)/MPII (I⁻) redox mediators with or without Ti₃C₂T_x MXene, DSCs were fabricated and the variations in the photovoltaic parameters were investigated. The average photovoltaic performance measured using the four I-DSCs (i.e., the as-prepared DSCs) is compared in Figure 1 and

Table 2. Averages and standard deviations of the cell performances measured using four I- and Co-DSCs with or without Ti₃C₂T_x MXene.

Devices		Jsc (mA/cm2)	Voc (V)	FF (%)	PCE (%)
I-DSC	Without Ti3C2Tx	18.84 ± 1.18	0.677 ± 0.018	57.70 ± 2.55	7.35 ± 0.39
	With Ti3C2Tx	19.95 ± 0.78	0.651 ± 0.010	57.29 ± 2.59	7.43 ± 0.40
Co-DSC	Without Ti3C2Tx	15.46 ± 1.04	0.760 ± 0.023	61.33 ± 3.70	7.18 ± 0.11
	With Ti3C2Tx	18.09 ± 0.94	0.746 ± 0.014	63.66 ± 2.69	8.58 ± 0.30

, while the raw data are presented in Table S1 in the Electronic Supplementary Materials (ESM). Through the incorporation of Ti₃C₂T_x MXene into the Co³⁺/I⁻ liquid redox mediators, the average J_sc value of the I-DSCs was enhanced, whereas the average V_oc value was decreased when compared with the values of the device without MXene, as shown in Figure 1a,b, respectively. As a result, the average PCE of the I-DSCs with Ti₃C₂T_x was very similar to that of the cells without Ti₃C₂T_x.

Moreover, it was also found that chemical reactions (6) and (7) occurred through the exposure of the I-DSCs to light for a certain time [23]. Under illumination, the Co²⁺(FK209) that was produced via reaction (1) reduced the oxidized dye (Dye⁺), thereby regenerating Co³⁺(FK209) at the dye/redox mediator interface (chemical Equation (6)). The resulting Co³⁺(FK209) diffused to the counter electrode and then reduced to Co²⁺(FK209) through receiving an electron from the platinized FTO electrode (chemical Equation (7)), indicating that the Co²⁺/Co³⁺ redox couple is related to the dye regeneration (chemical Equation (6)) and hole collection (chemical Equation (7)). This may lead to an increase in the V_oc value of the Co²⁺/Co³⁺-based cell when compared with that of the I⁻/I₃⁻-based cell, as the potential gap between the conduction band edge (CBE) of the TiO₂ and the redox potential (1.06 V versus a normal hydrogen electrode [NHE]) of the Co²⁺/Co³⁺(FK209) is wider than that (0.35 V versus a NHE) of the I⁻/I₃⁻ electrolyte [24,25,26]. Here, we code the DSCs based on the Co²⁺/Co³⁺ redox couple as Co-DSCs, which were transformed from the I-DSCs via exposure to AM 1.5 light. To determine the time taken to convert the I-DSCs into Co-DSCs, we measured the V_oc variations with the light exposure time of the I-DSCs. As shown in Figure S1, the V_oc values improved with an increasing exposure time and then saturated after over a period of 150 min. This indicates that hole conduction mainly occurred through the action of the Co²⁺/Co³⁺ redox couple after exposure of the I-DSCs to light for over 150 min. To investigate effects of MXene incorporation in Co-DSCs, the I-DSCs with or without Ti₃C₂T_x MXene were exposed to the AM 1.5 condition for 150 min and their photovoltaic performance was measured. As a reference, the V_oc values of the Co-DSCs were sharply increased when compared with those of the I-DSCs due to the wider potential gap between the TiO₂’s CBE and the Co²⁺/Co³⁺(FK209)’s redox potential, as mentioned above [Figure 1b]. This suggests that the Co²⁺/Co³⁺(FK209) rather than the I⁻/I₃⁻ redox couple participates in the hole conduction (chemical Equations (6) and (7)) in the Co-DSCs.

2Co²⁺(FK209) + 2Dye⁺ → 2Co³⁺(FK209) + 2Dye

(6)

2Co³⁺(FK209) + 2e⁻ (Pt) → 2Co²⁺(FK209)

(7)

Through the addition of MXene, a substantial enhancement in the J_sc and a slight decrement in the V_oc values were observed in the Co-DSCs with Ti₃C₂T_x when compared with those of the reference cell without MXene, as shown in Figure 1a,b, respectively. There was no meaningful variation in the FF value, as demonstrated in Figure 1c. As a consequence, an improvement in the PCE was recorded in the MXene-incorporated Co-DSCs because the increase in the J_sc overcame the decrement in the V_oc value, as shown in Figure 1d. Among the four cells, we focused on the best-performing cells to reveal the origins of the improvement in the PCE via the incorporation of Ti₃C₂T_x MXene into Co³⁺/I⁻ liquid redox mediators. Here, we denote the best-performing Co-DSCs with or without Ti₃C₂T_x MXene as Ti₃C₂T_x-Co-_bDSC or bare Co-_bDSC, respectively. Figure 2 presents the current density–voltage (J–V) curves of the Ti₃C₂T_x- and bare Co-_bDSCs, while the cell performance is compared in

Table 3. Photovoltaic performance of the best-performing cells—that is, the bare Co-_bDSC and Ti₃C₂T_x-Co-_bDSC.

Best-Performing Cells	Jsc (mA/cm2)	Voc (V)	FF (%)	PCE (%)
Bare Co-bDSC	14.41	0.780	64.66	7.27
Ti3C2Tx-Co-bDSC	18.45	0.760	64.23	9.01

.

2.2. Effects of Ti₃C₂T_x Incorporation on the J_sc of Co-DSCs

Through the incorporation of Ti₃C₂T_x MXene into the Co³⁺/I⁻ redox mediator, the J_sc value (18.45 mA/cm²) of the Ti₃C₂T_x-Co-_bDSC was substantially improved from that of the bare Co-_bDSC (14.41 mA/cm²). It is believed that the conductive Ti₃C₂T_x MXene present in the Co²⁺/Co³⁺(FK209) electrolyte reduced the charge transfer resistance and improved the electrocatalytic performance between the Co³⁺(FK209) and the platinized FTO electrode, thereby leading to a reduction in the internal resistances and, therefore, an increase in the J_sc value of the Ti₃C₂T_x-Co-_bDSC [21,22,27]. To confirm this, impedance spectroscopic (EIS) analysis was performed for the bare and Ti₃C₂T_x-Co-_bDSCs. Figure 3a shows the Nyquist plots of the EIS spectra of the Co-DSCs, as measured at the open-circuit condition under AM 1.5 one-sun illumination, providing the series (R_s) and internal resistances. Three typical arcs, corresponding to the resistance (R₁) of the redox reaction at the platinized FTO/electrolyte interface in the high-frequency region, the electron transfer resistance (R₂) at the TiO₂/dye/electrolyte interface in the medium-frequency region, and the ionic diffusion resistance (R₃) within the electrolyte, were observed. The fitted resistances are compared in

Table 4. Resistances for the Nyquist plots of the bare and Ti₃C₂T_x-Co-_bDSCs.

EIS Measurement Condition	Devices	Rs (Ω)	R1 (Ω)	R2 (Ω)	R3 (Ω)
Open-circuit	Bare Co-bDSC	10.24	4.28	30.88	6.79
	Ti3C2Tx-Co-bDSC	10.77	3.32	22.02	6.24
Dark	Bare Co-bDSC	11.40	5.42	51.21	3.16
	Ti3C2Tx-Co-bDSC	10.82	3.44	48.22	3.14

. A smaller R₁ value (3.32 Ω) was measured in the Ti₃C₂T_x-Co-_bDSC when compared with that of the bare Co-_bDSC (4.28 Ω). This indicates that the incorporation of Ti₃C₂T_x MXene can lead to an improvement in the electrocatalytic performance, causing an increase in the hole collection efficiency, when considering that the R₁ value is related to the reduction of the Co³⁺(FK209) by the Pt catalyst (chemical Equation (7)) [21,22,27]. In addition, it is considered that the reduced resistance (R₁) associated with chemical reaction (7) can effectively produce Co²⁺(FK209), resulting in the promotion of dye regeneration (chemical Equation (6)) [27], and thereby lowering the R₂ value. The R₃ values, named as Warburg diffusion resistance (Ws) arising from the ionic transport in the redox mediator [28], were almost similar in both the bare (6.79 Ω) and Ti₃C₂T_x-Co-_bDSCs (6.24 Ω), indicating that the presence of Ti₃C₂Tx MXene in the redox mediator did not hinder the diffusion of the Co²⁺/Co³⁺ redox couple.

Furthermore, a shift in the TiO₂’s CBE could be estimated from the dark current curves of the DSCs [29,30,31]. Figure 4 presents the dark current–voltage curves of the bare and Ti₃C₂T_x-Co-_bDSCs. The onset potential of the dark current for the bare Co-_bDSC was estimated to be approximately 0.717 V, whereas the onset potential for the Ti₃C₂T_x-Co-_bDSC was shifted to approximately 0.661 V. Through the incorporation of Ti₃C₂T_x MXene, a lower onset potential was recorded, indicating that the potential difference (ΔP_MXene) between the work function of the FTO and the TiO₂’s CBE in the Ti₃C₂T_x-Co-_bDSC was smaller than that (ΔP_Bare) in the bare Co-_bDSC (i.e., ΔP_Bare > ΔP_MXene) as illustrated in Figure 5. This suggests that the TiO₂’s CBE in the Ti₃C₂T_x-Co-_bDSC was located at a more positive potential than that in the bare Co-_bDSC. It is thought that the Ti₃C₂T_x MXene particles are adsorbed on the TiO₂ surface, causing a surface dipole to be formed and resulting in a positive shift in the TiO₂’s CBE. This positive shift (away from the vacuum level) in the CBE in the Ti₃C₂T_x-Co-_bDSC may lead to the more favorable injection of photoexcited electrons from the dye into the TiO₂. It is because the potential difference (ΔE_MXene) between the dye’s lowest unoccupied molecular orbital level (LUMO) and the TiO₂’s CBE in the Ti₃C₂T_x-Co-_bDSC was larger than that (ΔE_Bare) in the bare Co-_bDSC (i.e, ΔE_Bare < ΔE_MXene in Figure 5), thereby resulting in an improvement in the electron injection efficiency [32]. Thus, it is considered that the positive shift in the TiO₂’s CBE in the Ti₃C₂T_x-Co-_bDSC yielded a higher electron injection efficiency than the bare Co-_bDSC. A similar result was reported in relation to the incorporation of Ti₃C₂T_x MXene into the mesoporous TiO₂ layer, which induced a positive shift in the TiO₂’s CBE, leading to an enhancement of the electron injection efficiency [33].

Meanwhile, the light scattered by the Ti₃C₂T_x MXene particles can affect the light-harvesting efficiency of cells, and thereby the J_sc value. To confirm this, UV-visible absorption spectra of Co³⁺/I⁻ redox mediators with or without Ti₃C₂T_x MXene were measured, and compared with the absorption spectrum of N719 dye. As displayed in Figure S2 of the ESM, the Co³⁺/I⁻/Ti₃C₂T_x redox mediator showed strong and weak absorption at 200–500 nm and 500–1100 nm, respectively, indicating that incident light of 500–1100 nm can be in part scattered by the MXene particles. When considering that the absorption range of the N719 dye is 200–700 nm, the scattered light of 500–700 nm can be absorbed by the N719 dye and thus increase the light-harvesting efficiency. However, it was considered that the degree of increase in the light harvesting efficiency was not high, because the light of 500–700 nm was partly absorbed by the MXene particles. Actually, as shown in Figure S3 of the ESM, the deep brown-colored Co³⁺/I⁻/Ti₃C₂T_x redox mediator was due to the strong absorption at 200–500 nm and weak absorption at 500–700 nm. Overall, we attributed the enhanced J_sc value in the Ti₃C₂T_x-Co-_bDSC to the increases in both the hole collection and the dye regeneration efficiency, which resulted from the reduced internal resistances, as well as to the improvement in the electron injection efficiency due to the positive shift in the TiO₂’s CBE.

2.3. Effects of Ti₃C₂T_x Incorporation on the V_oc of Co-DSCs

The V_oc value (0.760 V) of the Ti₃C₂T_x-Co-_bDSC was decreased when compared with that of the bare Co-_bDSC (0.780 V). As expressed in Equation (8), the V_oc value of the DSCs under constant illumination can be expressed as a function of the quasi-Fermi level of the semiconductor (E_Fn) with respect to the dark value (E_F0), which equals the electrolyte redox energy (E_F0 = E_redox). Therefore, it can be written with the thermal energy (k_BT; 4.11 × 10⁻²¹ J at 25 °C), Boltzmann constant (k_B), absolute temperature (T), positive elementary charge (e; 1.602 × 10⁻¹⁹ C), concentration in the dark (n₀), and free electron density of the TiO₂ photoelectrode (n) [34,35,36]. Equation (8) indicates that the V_oc is affected by n, which is closely related to the back electron transfer (BET) reaction that occurs between the photoinjected electrons and the ions in the electrolyte. As the BET reaction decreases the n value, suppression of the BET reaction is necessary to increase the V_oc. The n value can be estimated by measuring the lifetime of the electrons photoinjected into the TiO₂, where a longer electron lifetime can increase the n value and, therefore, the V_oc. Figure 3b shows Bode phase plots of the EIS spectra of the bare and Ti₃C₂T_x-Co-_bDSCs. Using the peak frequencies (f_max) of 38.2 Hz and 45.5 Hz obtained from the EIS Bode phase plots of the bare and Ti₃C₂T_x-Co-_bDSCs, respectively, the electron lifetime (τ_n) was estimated using Equation (9) [30,37]. The calculated electron lifetimes were 4.16 ms and 3.50 ms for the bare and Ti₃C₂T_x-Co-_bDSCs, respectively. A shortened lifetime on the part of the electrons injected from the photoexcited dyes was observed for the Ti₃C₂T_x-Co-_bDSC relative to that of the control cell (bare Co-_bDSC), indicating that the BET reaction (chemical Equation (10)) in the Ti₃C₂T_x-Co-_bDSC occurred faster than that in the bare Co-_bDSC.

$$V_{oc}=\frac{E_{Fn}-E_{F0}}{e}=\frac{k_{B}T}{e}\ln (\frac{n}{n_0})$$

(8)

$${\mathsf{\tau }}_n=\frac{1}{2\pi f_{max}}$$

(9)

Co³⁺(FK209) + e⁻ (TiO₂) → Co²⁺(FK209)

(10)

To further confirm that the BET reaction was faster in the Ti₃C₂T_x-Co-_bDSC, Nyquist plots of the EIS spectra measured at −0.7 V in the dark were obtained, as shown in Figure 6a. When the EIS measurement is performed in the dark, electrons are injected from the FTO into the TiO₂ under external applied voltage and then transferred to the electrolyte. Thus, the R₂’ value of the Nyquist plot measured in the dark corresponds to the resistance of the BET reaction between the Co³⁺(FK209) in the electrolyte and the electrons injected into the TiO₂ conduction band (chemical Equation (10)). It was observed that the arc of the impedance component R₂’ (48.22 Ω) in the Ti₃C₂T_x-Co-_bDSC was smaller than that in the bare Co-_bDSC (51.21 Ω). The smaller semicircle in the R₂’ suggests that the BET reaction at the TiO₂/dye/electrolyte interface was faster [38,39]. Moreover, from the peak frequencies (f_max) given in Figure 6b and Equation (9), the lifetime of the electrons injected from the FTO was calculated to be 11.9 ms and 8.4 ms for the bare and Ti₃C₂T_x-Co-_bDSCs, respectively. The same tendency in terms of a shortened electron lifetime was observed in the measurements under illumination [Figure 3b] and dark conditions [Figure 6b].

Another method to estimate electron lifetime in cells is open-circuit voltage decay (OCVD) measurements. Using the results of OCVD measurements [Figure 7a] and Equation (11), where k_B is the Boltzmann constant, T is the temperature, e is the electron charge, and dV_oc/dt is the derivative of the V_oc transient [30,31,36], we could calculate the electron lifetime (τ) of the bare and Ti₃C₂T_x-Co-_bDSCs. As shown in Figure 7b, the electron lifetimes recorded for the Ti₃C₂T_x-Co-_bDSC were shorter than those for the reference cell (bare Co-_bDSC), which suggests that the incorporation of the MXene boosted the BET reaction between the photoinjected electrons and the redox mediators. Overall, a faster BET reaction resulted in a lower n value, leading to a decrease in the V_oc of the Ti₃C₂T_x-Co-_bDSC based on Equation (8).

$$\mathsf{\tau }=-\frac{k_{B}T}{e}{\left(\frac{dVoc}{dt}\right)}^{-1}$$

(11)

Furthermore, the reduced V_oc value in the Ti₃C₂T_x-Co-_bDSC can be explained by the positive shift in the TiO₂’s CBE, as discussed above (Figure 4 and Figure 5). The lower n value can cause the positioning of the TiO₂’s CBE to shift in a positive direction, lowering the V_oc value. It is because the potential gap (ΔV_MXene) between the TiO₂’s CBE and the redox potential of the electrolyte in the Ti₃C₂T_x-Co-_bDSC was narrower than that (ΔV_Bare) in the bare Co-_bDSC (i.e., ΔV_Bare > ΔE_MXene in Figure 5) [40,41]. It is considered that this decrease in the V_oc (or n) value originated from the adsorption of the Ti₃C₂T_x MXene on the TiO₂ surface. More specifically, the electronically conductive Ti₃C₂T_x MXene particles could provide pathways for chemical reaction (10)—that is, the BET reaction between the photoinjected electrons and the Co³⁺(FK209) in the redox mediator. Meanwhile, it was known that pure Ti₃C₂ (or non-terminated Ti₃C₂) MXene had a metallic character, indicating its band gap energy (Eg) was zero (or Eg < 0.2 eV for the Ti₃C₂T_x MXene) [42,43]. It was also reported that the work function of Ti₃C₂T_x MXene varied from 3.9 to 4.8 eV with annealing temperature [44], which was positioned between the dye’s LUMO level (around 3.64 eV) and the TiO₂’s CBE (around 5.11 eV). Thus, due to a lower position of the MXene’s work function than the dye’s LUMO level, another type of BET reaction between the photoexcited electrons in dyes and the Ti₃C₂T_x MXene can take place in the Ti₃C₂T_x-Co-_bDSC if the MXene particle comes into contact with the dye molecule. This can decrease the n value, and therefore the V_oc value.

As a reference, the shorter electron lifetime observed in the Ti₃C₂T_x-Co-_bDSC may decrease the electron collection on the FTO and, therefore, the J_sc value. In this study, it is believed that the enhanced hole collection, dye regeneration, and electron injection efficiencies surpassed the decreased electron collection efficiency.

2.4. Long-Term Stability of the Bare and Ti₃C₂T_x-Co-_bDSCs

We compared the long-term stability of the bare and Ti₃C₂T_x-Co-_bDSCs by evaluating their photovoltaic properties over time. Here, the fabricated devices were additionally sealed using hot-melt glue sticks to minimize electrolyte leakage. Prior to the measurement of the photovoltaic performance, the I-DSCs with or without Ti₃C₂T_x MXene were converted into Co-DSCs through exposing them to the AM 1.5 condition for 150 min. Figure 8 compares the time-dependent performance variations in the cells stored at room temperature in the dark. Similar decay curves were noted in both devices, indicating that the incorporation of Ti₃C₂T_x mXene into the redox mediator did not affect the devices’ stability.

3. Materials and Methods

3.1. Materials

To fabricate DSCs, the same materials as those used in our previous report were utilized [23]. Their detailed information is provided in the ESM. The acetonitrile solvent used to prepare the liquid electrolytes was procured from Daejung Chemicals and Metals Co., Ltd. (Siheung, Republic of Korea). Colloidal suspension of single-layer Ti₃C₂ in acetonitrile (2 mg Ti₃C₂/mL) (BK2020082105-08) was purchased from Bei**g Beike New Material Technology Co., Ltd. (Suzhou, China). All the chemicals used for DSC fabrication were exploited without further purification. The single-layer Ti₃C₂Tx MXene structure is illustrated in Figure S4 in the ESM. The chemical structures of the main components (FK209 and MPII) and additives (LiTFSI and TBP) of the electrolyte are shown in Figure S5 in the ESM.

3.2. Preparation of Co³⁺/I⁻-Based Liquid Electrolytes with or without Ti₃C₂T_x

The Ti₃C₂T_x-dispersed liquid electrolyte based on a Co³⁺/I⁻ redox mediator was prepared by dissolving MPII (0.6 M, 302.5 mg), FK209 (0.015 M, 15.2 mg), TBP (0.11 M, 32.4 mg), and LiTFSI (0.04 M, 22.8 mg) in the colloidal suspension of single-layer Ti₃C₂ in acetonitrile (2 mL). For the purpose of comparison, Co³⁺/I⁻-based liquid electrolytes without Ti₃C₂Tx were also prepared by simply replacing the Ti₃C₂Tx colloid with acetonitrile solvent (2 mL).

3.3. Fabrication of DSCs

Similar procedures to those described in our previous work were utilized to prepare the working (glass/FTO/TiO₂:dye) and counter (glass/FTO/Pt) electrodes for the DSCs [23]. A 25 μm thick hot-melt adhesive was placed between the working and counter electrodes and then annealed for 10 min at 120 °C to seal the two electrodes. The Co³⁺/I⁻-based liquid electrolytes with or without Ti₃C₂T_x MXene were injected into the cells through one of the two small holes predrilled into the counter electrodes. By sealing the two holes, we were able to fabricate DSCs with a 25 mm² active area. Detailed procedures are mentioned in the ESM.

3.4. Measurements

Photovoltaic performance measurements, EIS analyses, and dark current studies were carried out. UV-visible absorbance and OCVD measurements were also performed. All the measurements were performed under ambient conditions at room temperature. Detailed information for the measuring instruments is presented in the ESI.

4. Conclusions

The photovoltaic properties of DSCs with or without Ti₃C₂Tx MXene in Co³⁺/I⁻-based redox mediators were investigated in this study. Through the incorporation of Ti₃C₂T_x MXene into the Co³⁺/I⁻ liquid redox mediators, the average J_sc value of the I-DSCs, in which hole conduction occurred via the redox reaction of the I⁻ and I₃⁻ ions, was enhanced, whereas the average V_oc value was decreased when compared with that of the device without the MXene. As a result, the average PCE of the I-DSCs with Ti₃C₂T_x was very similar to that of the cells without Ti₃C₂T_x. To obtain Co-DSCs based on the Co²⁺/Co³⁺ redox couple, the I-DSCs were exposed to light for 150 min. Through the addition of Ti₃C₂Tx MXene into the Co³⁺/I⁻-based redox mediators, the hole collection, dye regeneration, and electron injection efficiencies of the Ti₃C₂Tx-Co-DSCs were all increased, leading to an improvement in both the J_sc and PCE when compared with those of the bare Co-DSCs without MXene. These results indicate that Ti₃C₂Tx MXene, as a J_sc-improver, is a good additive for improving the PCE of Co-DSCs.