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

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.


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+1 X n T 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+1 AX 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 Ti 3 C 2 Tx MXene by etching the Al layers from the Ti 3 AlC 2 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, Ti 3 C 2 T 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-Ti 3 C 2 Tx MXene film as a counter electrode, surpassing that of a cell with Pt-based counter electrode (7.59%) [9].A comparative study of 2D-layerstructured Ti 3 C 2 Tx MXene and TiC particles was reported.The DSCs with a Ti 3 C 2 Tx-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)/Ti 3 C 2 Tx MXene composite-based counter electrode outperformed cells with PEDOT-or Ti 3 C 2 Tx MXene-based ones in PCE [11].
A conventional DSC is composed of a dye-adsorbed TiO 2 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 TiO 2 layer.The photoinjected electrons and the holes in the dye molecules are transported to the electrodes via the TiO 2 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 − /I 3 − , 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(NH 2 ) 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 TiO 2 semiconductor, the shift in the conduction band edge, and the interfacial charge recombination through being incorporating in small amounts [20].Ti 3 C 2 T 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 Ti 3 C 2 T x MXene to a quasi-solid-state electrolyte composed of an I − /I 3 − 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 Ti 3 C 2 T x to a quasi-solid-state electrolyte containing an I − /I 3 − redox couple, polyethylene oxide (PEO), and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) [22].
In this study, we report the effects of Ti 3 C 2 T x MXene addition to a liquid electrolyte on the photovoltaic performance of cells.Ti 3 C 2 T 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 Co 3+ and an ionic liquid (1-methyl-3-propylimidazolium iodine [MPII]) as a source of I − (iodide) were first prepared.Then, DSCs with Ti 3 C 2 T xdispersed Co 3+ /I − liquid electrolytes (redox mediators) were fabricated and their photovoltaic properties were compared with those of the reference cell without Ti 3 C 2 T x MXene.To the best of our knowledge, this is the first report on the effects of Ti 3 C 2 T x addition to Co complex (Co 3+ )/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 Ti 3 C 2 T x MXene are summarized in Table 1, including those of our devices with FK209 (Co 3+ )/MPII (I − )/Ti 3 C 2 T x -based liquid electrolytes.In a previous report, we revealed that, through the simple mixing of MPII and FK209, triiodide (I 3 − ) and Co 2+ (FK209) were produced via a chemical reaction (1) between the iodide (I − ) of MPII and Co 3+ (FK209), where Co 2+ (FK209) and Co 3+ (FK209) are the Co 2+ and Co 3+ ions of FK209, respectively.Since the Co 3+ (FK209) was almost fully converted into Co 2+ (FK209), the redox mediators contained both I 3 − and Co 2+ (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 − /I 3 − 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 − /I 3 − redox couple as I-DSCs.
Using the FK209 (Co 3+ )/MPII (I − ) redox mediators with or without Ti 3 C 2 T 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, while the raw data are presented in Table S1 in the Electronic Supplementary Materials (ESM).Through the incorporation of Ti 3 C 2 T x MXene into the Co 3+ /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 Ti 3 C 2 T x was very similar to that of the cells without Ti 3 C 2 T 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 Co 2+ (FK209) that was produced via reaction (1) reduced the oxidized dye (Dye + ), thereby regenerating Co 3+ (FK209) at the dye/redox mediator interface (chemical Equation ( 6)).The resulting Co 3+ (FK209) diffused to the counter electrode and then reduced to Co 2+ (FK209) through receiving an electron from the platinized FTO electrode (chemical Equation ( 7)), indicating that the Co 2+ /Co 3+ 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 Voc value of the Co 2+ /Co 3+ -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 Co 2+ /Co 3+ (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 Co 2+ /Co 3+ 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 Voc variations with the light exposure time of the I-DSCs.As shown in 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 Co 2+ (FK209) that was produced via reaction (1) reduced the oxidized dye (Dye + ), thereby regenerating Co 3+ (FK209) at the dye/redox mediator interface (chemical Equation ( 6)).The resulting Co 3+ (FK209) diffused to the counter electrode and then reduced to Co 2+ (FK209) through receiving an electron from the platinized FTO electrode (chemical Equation ( 7)), indicating that the Co 2+ /Co 3+ 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 Co 2+ /Co 3+ -based cell when compared with that of the I − /I 3 − -based cell, as the potential gap between the conduction band edge (CBE) of the TiO 2 and the redox potential (1.06 V versus a normal hydrogen electrode [NHE]) of the Co 2+ /Co 3+ (FK209) is wider than that (0.35 V versus a NHE) of the I − /I 3 − electrolyte [24][25][26].Here, we code the DSCs based on the Co 2+ /Co 3+ 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 Co 2+ /Co 3+ 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 Ti 3 C 2 T 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 TiO 2 's CBE and the Co 2+ /Co 3+ (FK209)'s redox potential, as mentioned above [Figure 1b].This suggests that the Co 2+ /Co 3+ (FK209) rather than the I − /I 3 − redox couple participates in the hole conduction (chemical Equations ( 6) and ( 7)) in the Co-DSCs. 2Co 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 Ti 3 C 2 T 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 Ti 3 C 2 T x MXene into Co 3+ /I − liquid redox mediators.Here, we denote the best-performing Co-DSCs with or without Ti 3 C 2 T x MXene as Ti 3 C 2 T x -Cob DSC or bare Cob DSC, respectively.Figure 2 presents the current density-voltage (J-V) curves of the Ti 3 C 2 T x -and bare Cob DSCs, while the cell performance is compared in Table 3.
or without Ti3C2Tx MXene were exposed to the AM 1.5 condition for 150 min and their photovoltaic performance was measured.As a reference, the Voc 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 Co 2+ /Co 3+ (FK209)'s redox potential, as mentioned above [Figure 1b].This suggests that the Co 2+ /Co 3+ (FK209) rather than the I − /I3 − redox couple participates in the hole conduction (chemical Equations ( 6) and ( 7)) in the Co-DSCs.
Through the addition of MXene, a substantial enhancement in the Jsc and a slight decrement in the Voc values were observed in the Co-DSCs with Ti3C2Tx 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 Jsc overcame the decrement in the Voc 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 Ti3C2Tx MXene into Co 3+ /I − liquid redox mediators.Here, we denote the best-performing Co-DSCs with or without Ti3C2Tx MXene as Ti3C2Tx-Co-bDSC or bare Co-bDSC, respectively.Figure 2 presents the current density-voltage (J-V) curves of the Ti3C2Tx-and bare Co-bDSCs, while the cell performance is compared in Table 3.Through the incorporation of Ti 3 C 2 T x MXene into the Co 3+ /I − redox mediator, the J sc value (18.45 mA/cm 2 ) of the Ti 3 C 2 T x -Cob DSC was substantially improved from that of the bare Cob DSC (14.41 mA/cm 2 ).It is believed that the conductive Ti 3 C 2 T x MXene present in the Co 2+ /Co 3+ (FK209) electrolyte reduced the charge transfer resistance and improved the electrocatalytic performance between the Co 3+ (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 Ti 3 C 2 T x -Cob DSC [21,22,27].To confirm this, impedance spectroscopic (EIS) analysis was performed for the bare and Ti 3 C 2 T x -Cob DSCs. 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 (R 1 ) of the redox reaction at the platinized FTO/electrolyte interface in the high-frequency region, the electron trans-Molecules 2024, 29, 1340 6 of 14 fer resistance (R 2 ) at the TiO 2 /dye/electrolyte interface in the medium-frequency region, and the ionic diffusion resistance (R 3 ) within the electrolyte, were observed.The fitted resistances are compared in Table 4.A smaller R 1 value (3.32 Ω) was measured in the Ti 3 C 2 T x -Cob DSC when compared with that of the bare Cob DSC (4.28 Ω).This indicates that the incorporation of Ti 3 C 2 T x MXene can lead to an improvement in the electrocatalytic performance, causing an increase in the hole collection efficiency, when considering that the R 1 value is related to the reduction of the Co 3+ (FK209) by the Pt catalyst (chemical Equation ( 7)) [21,22,27].In addition, it is considered that the reduced resistance (R 1 ) associated with chemical reaction (7) can effectively produce Co 2+ (FK209), resulting in the promotion of dye regeneration (chemical Equation ( 6)) [27], and thereby lowering the R 2 value.The R 3 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 Ti 3 C 2 T x -Cob DSCs (6.24 Ω), indicating that the presence of Ti 3 C 2 Tx MXene in the redox mediator did not hinder the diffusion of the Co 2+ /Co 3+ redox couple.
Jsc value of the Ti3C2Tx-Co-bDSC [21,22,27].To confirm this, impedance spectroscopic (EIS) analysis was performed for the bare and Ti3C2Tx-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 (Rs) 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.A smaller R1 value (3.32 Ω) was measured in the Ti3C2Tx-Co-bDSC when compared with that of the bare Co-bDSC (4.28 Ω).This indicates that the incorporation of Ti3C2Tx 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 Co 3+ (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 Co 2+ (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 Ti3C2Tx-Co-bDSCs (6.24 Ω), indicating that the presence of Ti3C2Tx MXene in the redox mediator did not hinder the diffusion of the Co 2+ /Co 3+ redox couple.Furthermore, a shift in the TiO 2 '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 Ti 3 C 2 T x -Cob DSCs.The onset potential of the dark current for the bare Cob DSC was estimated to be approximately 0.717 V, whereas the onset potential for the Ti 3 C 2 T x -Cob DSC was shifted to approximately 0.661 V. Through the incorporation of Ti 3 C 2 T x MXene, a lower onset potential was recorded, indicating that the potential difference (∆P MXene ) between the work function of the FTO and the TiO 2 's CBE in the Ti 3 C 2 T x -Cob DSC was smaller than that (∆P Bare ) in the bare Cob DSC (i.e., ∆P Bare > ∆P MXene ) as illustrated in Figure 5.This suggests that the TiO 2 's CBE in the Ti 3 C 2 T x -Cob DSC was located at a more positive potential than that in the bare Cob DSC.It is thought that the Ti 3 C 2 T x MXene particles are adsorbed on the TiO 2 surface, causing a surface dipole to be formed and resulting in a positive shift in the TiO 2 's CBE.This positive shift (away from the vacuum level) in the CBE in the Ti 3 C 2 T x -Cob DSC may lead to the more favorable injection of photoexcited electrons from the dye into the TiO 2 .It is because the potential difference (∆E MXene ) between the dye's lowest unoccupied molecular orbital level (LUMO) and the TiO 2 's CBE in the Ti 3 C 2 T x -Cob DSC was larger than that (∆E Bare ) in the bare Cob DSC (i.e, ∆E Bare < ∆E 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 TiO 2 's CBE in the Ti 3 C 2 T x -Cob DSC yielded a higher electron injection efficiency than the bare Cob DSC.A similar result was reported in relation to the incorporation of Ti 3 C 2 T x MXene into the mesoporous TiO 2 layer, which induced a positive shift in the TiO 2 's CBE, leading to an enhancement of the electron injection efficiency [33].
Co-bDSC (i.e., ΔPBare > ΔPMXene) as illustrated in Figure 5.This suggests that the TiO2's CBE in the Ti3C2Tx-Co-bDSC was located at a more positive potential than that in the bare Co-bDSC.It is thought that the Ti3C2Tx 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 Ti3C2Tx-Co-bDSC may lead to the more favorable injection of photoexcited electrons from the dye into the TiO2.It is because the potential difference (ΔEMXene) between the dye's lowest unoccupied molecular orbital level (LUMO) and the TiO2's CBE in the Ti3C2Tx-Co-bDSC was larger than that (ΔEBare) in the bare Co-bDSC (i.e, ΔEBare < ΔEMXene 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 Ti3C2Tx-Co-bDSC yielded a higher electron injection efficiency than the bare Co-bDSC.A similar result was reported in relation to the incorporation of Ti3C2Tx 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].the Ti3C2Tx-Co-bDSC was located at a more positive potential than that in the bare Co-bDSC.It is thought that the Ti3C2Tx 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 Ti3C2Tx-Co-bDSC may lead to the more favorable injection of photoexcited electrons from the dye into the TiO2.It is because the potential difference (ΔEMXene) between the dye's lowest unoccupied molecular orbital level (LUMO) and the TiO2's CBE in the Ti3C2Tx-Co-bDSC was larger than that (ΔEBare) in the bare Co-bDSC (i.e, ΔEBare < ΔEMXene 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 Ti3C2Tx-Co-bDSC yielded a higher electron injection efficiency than the bare Co-bDSC.A similar result was reported in relation to the incorporation of Ti3C2Tx 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 Ti 3 C 2 T x MXene particles can affect the lightharvesting efficiency of cells, and thereby the J sc value.To confirm this, UV-visible absorption spectra of Co 3+ /I − redox mediators with or without Ti 3 C 2 T x MXene were measured, and compared with the absorption spectrum of N719 dye.As displayed in Figure S2 of the ESM, the Co 3+ /I − /Ti 3 C 2 T 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 Co 3+ /I − /Ti 3 C 2 T 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 Ti 3 C 2 T x -Cob DSC 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 TiO 2 's CBE.

Effects of Ti 3 C 2 T x Incorporation on the V oc of Co-DSCs
The V oc value (0.760 V) of the Ti 3 C 2 T x -Cob DSC was decreased when compared with that of the bare Cob DSC (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 B T; 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 (n 0 ), and free electron density of the TiO 2 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 TiO 2 , 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 Ti 3 C 2 T x -Cob DSCs.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 Ti 3 C 2 T x -Cob DSCs, 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 Ti 3 C 2 T x -Cob DSCs, respectively.A shortened lifetime on the part of the electrons injected from the photoexcited dyes was observed for the Ti 3 C 2 T x -Cob DSC relative to that of the control cell (bare Cob DSC), indicating that the BET reaction (chemical Equation ( 10)) in the Ti 3 C 2 T x -Cob DSC occurred faster than that in the bare Cob DSC.
Co 3+ (FK209) + e − (TiO 2 ) → Co 2+ (FK209) (10) To further confirm that the BET reaction was faster in the Ti 3 C 2 T x -Cob DSC, 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 TiO 2 under external applied voltage and then transferred to the electrolyte.Thus, the R 2 ' value of the Nyquist plot measured in the dark corresponds to the resistance of the BET reaction between the Co 3+ (FK209) in the electrolyte and the electrons injected into the TiO 2 conduction band (chemical Equation ( 10)).It was observed that the arc of the impedance component R 2 ' (48.22 Ω) in the Ti 3 C 2 T x -Cob DSC was smaller than that in the bare Cob DSC (51.21 Ω).The smaller semicircle in the R 2 ' suggests that the BET reaction at the TiO 2 /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 Ti 3 C 2 T x -Cob DSCs, 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].
the TiO2/dye/electrolyte interface was faster [38,39].Moreover, from the peak frequencies (fmax) 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 Ti3C2Tx-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 kB is the Boltzmann constant, T is the temperature, e is the electron charge, and dVoc/dt is the derivative of the Voc transient [30,31,36], we could calculate the electron lifetime (τ) of the bare and Ti3C2Tx-Co-bDSCs.As shown in Figure 7b, the electron lifetimes recorded for the Ti3C2Tx-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 Voc of the Ti3C2Tx-Co-bDSC based on Equation (8).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 Ti 3 C 2 T x -Cob DSCs.As shown in Figure 7b, the electron lifetimes recorded for the Ti 3 C 2 T x -Cob DSC were shorter than those for the reference cell (bare Cob DSC), 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 Ti 3 C 2 T x -Cob DSC based on Equation (8).
(fmax) 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 Ti3C2Tx-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 kB is the Boltzmann constant, T is the temperature, e is the electron charge, and dVoc/dt is the derivative of the Voc transient [30,31,36], we could calculate the electron lifetime (τ) of the bare and Ti3C2Tx-Co-bDSCs.As shown in Figure 7b, the electron lifetimes recorded for the Ti3C2Tx-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 Voc of the Ti3C2Tx-Co-bDSC based on Equation (8).Furthermore, the reduced V oc value in the Ti 3 C 2 T x -Cob DSC can be explained by the positive shift in the TiO 2 's CBE, as discussed above (Figures 4 and 5).The lower n value can cause the positioning of the TiO 2 's CBE to shift in a positive direction, lowering the V oc value.It is because the potential gap (∆V MXene ) between the TiO 2 's CBE and the redox potential of the electrolyte in the Ti 3 C 2 T x -Cob DSC was narrower than that (∆V Bare ) in the bare Cob DSC (i.e., ∆V Bare > ∆E 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 Ti 3 C 2 T x MXene on the TiO 2 surface.More specifically, the electronically conductive Ti 3 C 2 T x MXene particles could provide pathways for chemical reaction (10)-that is, the BET reaction between the photoinjected electrons and the Co 3+ (FK209) in the redox mediator.Meanwhile, it was known that pure Ti 3 C 2 (or non-terminated Ti 3 C 2 ) MXene had a metallic character, indicating its band gap energy (Eg) was zero (or Eg < 0.2 eV for the Ti 3 C 2 T x MXene) [42,43].It was also reported that the work function of Ti 3 C 2 T 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 TiO 2 '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 Ti 3 C 2 T x MXene can take place in the Ti 3 C 2 T x -Cob DSC 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 Ti 3 C 2 T x -Cob DSC 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.We compared the long-term stability of the bare and Ti 3 C 2 T x -Cob DSCs 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 Ti 3 C 2 T 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 Ti 3 C 2 T x mXene into the redox mediator did not affect the devices' stability.

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.(Gyeonggi-do, Republic of Korea).Colloidal suspension of single-layer Ti3C2 in acetonitrile (2 mg Ti3C2/mL) (BK2020082105-08) was purchased from Beijing Beike New Material Technology Co., Ltd.(Suzhou, China).All the chemicals used for DSC fabrication

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 Ti 3 C 2 in acetonitrile (2 mg Ti 3 C 2 /mL) (BK2020082105-08) was purchased from Beijing Beike New Material Technology Co., Ltd.(Suzhou, China).All the chemicals used for DSC fabrication were exploited without further purification.The single-layer Ti 3 C 2 Tx MXene structure is illustrated in Figure S4   The Ti 3 C 2 T x -dispersed liquid electrolyte based on a Co 3+ /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 Ti 3 C 2 in acetonitrile (2 mL).For the purpose of comparison, Co 3+ /I − -based liquid electrolytes without Ti 3 C 2 Tx were also prepared by simply replacing the Ti 3 C 2 Tx colloid with acetonitrile solvent (2 mL).

Fabrication of DSCs
Similar procedures to those described in our previous work were utilized to prepare the working (glass/FTO/TiO 2 :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 Co 3+ /I − -based liquid electrolytes with or without Ti 3 C 2 T 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 mm 2 active area.Detailed procedures are mentioned in the ESM.

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.

Conclusions
The photovoltaic properties of DSCs with or without Ti 3 C 2 Tx MXene in Co 3+ /I −based redox mediators were investigated in this study.Through the incorporation of Ti 3 C 2 T x MXene into the Co 3+ /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 I 3 − 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 Ti 3 C 2 T x was very similar to that of the cells without Ti 3 C 2 T x .To obtain Co-DSCs based on the Co 2+ /Co 3+ redox couple, the I-DSCs were exposed to light for 150 min.Through the addition of Ti 3 C 2 Tx MXene into the Co 3+ /I − -based redox mediators, the hole collection, dye regeneration, and electron injection efficiencies of the Ti 3 C 2 Tx-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 Ti 3 C 2 Tx MXene, as a J sc -improver, is a good additive for improving the PCE of Co-DSCs.

Figure 1 .
Figure 1.Performance comparison of the I-and Co-DSCs with or without Ti 3 C 2 T x MXene: (a) J sc , (b) V oc , (c) FF, and (d) PCE measured under AM 1.5 irradiation.

Figure 2 .
Figure 2. J-V characteristics of the best-performing cells-that is, the bare Co-bDSC and Ti3C2Tx-Co-bDSC.

Figure 2 .
Figure 2. J-V characteristics of the best-performing cells-that is, the bare Cob DSC and Ti 3 C 2 T x -Cob DSC.

Figure 3 .
Figure 3. Nyquist (a) and Bode (b) plots of the EIS spectra for the bare and Ti3C2Tx-Co-bDSCs, as measured under open-circuit conditions under the illumination of simulated AM 1.5 solar light.

Figure 3 .
Figure 3. Nyquist (a) and Bode (b) plots of the EIS spectra for the bare and Ti 3 C 2 T x -Cob DSCs, as measured under open-circuit conditions under the illumination of simulated AM 1.5 solar light.

Figure 4 .
Figure 4. Dark current curves of the best-performing cells-that is, bare Co-bDSC and Ti3C2Tx-Co-bDSC.

Figure 4 .
Figure 4. Dark current curves of the best-performing cells-that is, bare Cob DSC and Ti 3 C 2 T x -Cob DSC.

Figure 4 .
Figure 4. Dark current curves of the best-performing cells-that is, bare Co-bDSC and Ti3C2Tx-Co-bDSC.

Figure 5 .
Figure 5. Schematic energy band diagram for the bare Cob DSC (a) and Ti 3 C 2 T x -Cob DSC (b), showing variations in the energy differences (∆P Bare > ∆P MXene , ∆E Bare < ∆E MXene and ∆V Bare > ∆V MXene ) by the positive shift in the TiO 2 's CBE.

Figure 6 .
Figure 6.Nyquist (a) and Bode (b) plots of the EIS spectra for the bare-and Ti3C2Tx-Co-bDSCs, as measured at −0.7 V in the dark.

Figure 7 .
OCVD (a) and electron lifetime versus voltage (b) curves for the bare and Ti3C2Tx-Co-bDSCs.

Figure 6 .
Figure 6.Nyquist (a) and Bode (b) plots of the EIS spectra for the bare-and Ti 3 C 2 T x -Cob DSCs, as measured at −0.7 V in the dark.

Figure 6 .
Figure 6.Nyquist (a) and Bode (b) plots of the EIS spectra for the bare-and Ti3C2Tx-Co-bDSCs, as measured at −0.7 V in the dark.

Figure 7 .
OCVD (a) and electron lifetime versus voltage (b) curves for the bare and Ti3C2Tx-Co-bDSCs.

Figure 7 .
Figure 7. OCVD (a) and electron lifetime versus voltage (b) curves for the bare and Ti 3 C 2 T x -Cob DSCs.

2. 4 .
Long-Term Stability of the Bare and Ti 3 C 2 T x -Cob DSCs

Figure 8 .
Figure 8. Variations in the photovoltaic performance over time: normalized Jsc (a), Voc (b), FF (c), and PCE (d) of the bare and Ti3C2Tx-Co-bDSCs stored at room temperature in the dark.

Figure 8 .
Figure 8. Variations in the photovoltaic performance over time: normalized J sc (a), V oc (b), FF (c), and PCE (d) of the bare and Ti 3 C 2 T x -Cob DSCs stored at room temperature in the dark.
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 Co 3+ /I − -Based Liquid Electrolytes with or without Ti 3 C 2 T x

Table 1 .
Cont.Photovoltaic Performance of DSCs Based on Ti 3 C 2 T x -Incorporated Co 3+ /I − Redox Mediators

Table 2 .
Averages and standard deviations of the cell performances measured using four I-and Co-DSCs with or without Ti 3 C 2 T x MXene.

Table 2 .
Averages and standard deviations of the cell performances measured using four I-and Co-DSCs with or without Ti3C2Tx MXene.

Table 3 .
Photovoltaic performance of the best-performing cells-that is, the bare Co-bDSC and Ti3C2Tx-Co-bDSC.

Table 3 .
Photovoltaic performance of the best-performing cells-that is, the bare Cob DSC and Ti 3 C 2 T x -Cob DSC.

Best-Performing Cells J sc (mA/cm 2 ) V oc (V) FF (%) PCE (%)
x Incorporation on the J sc of Co-DSCs

Table 4 .
Resistances for the Nyquist plots of the bare and Ti3C2Tx-Co-bDSCs.

Table 4 .
Resistances for the Nyquist plots of the bare and Ti 3 C 2 T x -Cob DSCs.