A Review on Interface Engineering of MXenes for Perovskite Solar Cells

Highlights This review discusses the roles of MXenes in different positions/layers in perovskite solar cells. The issues in different layers/interfaces and their addressal with the incorporations of MXenes in perovskite solar cells are elaborately discussed.


Introduction
Solar technology converts solar energy directly into electrical energy through photovoltaic cells.Solar cells have attracted great attention from the energy community to meet the increasing demand for sustainable green, and clean energy sources.Apart from wafer-based Si solar cells that are widely available on the market, perovskite solar cells (PSCs) have achieved the highest power conversion efficiency (PCE) of 25.7% in 2021, which is inching towards the theoretical efficiency of about 31% [1][2][3].Furthermore, a recent study shows halide PSCs can surpass the Shockley-Queisser (SQ) PCE limit of 33.7% (predicted for a single-junction solar cell) [4].Most importantly, PSCs are in an advantageous position for ease of fabrication and their affordable cost coming from solution-based processing [5][6][7].
Perovskite materials, such as MAPbI 3, FAPbI 3 , and CsPbI 3 , have been spotlighted due to their excellent bandgap tunability, excellent light absorption co-efficient (> 10 5 cm −1 ), good charge carrier mobility (2-40 cm 2 V −1 s −1 ), and long carrier-diffusion length in the PSCs.Despite the stellar performance, the perovskite device remains a long-standing issue due to the structural and chemical instability originating from the degradation factors related to the intrinsic shortcomings of the perovskite materials as well as at their interfaces.Researchers have explored various pathways to improve stability and performance efficiency, for instance, the development of passivating materials, enlargement of the grain sizes or single crystals, solvent engineering, additive engineering, and interface engineering [6,[8][9][10][11][12][13].
In this review, we aim to provide a comprehensive understanding of MXenes and their roles as additives or themselves alone in different components (classified according to photoactive absorber layer, ETL, HTL, and electrodes) and interfaces of the PSCs.In Sect. 1, we begin by introducing a brief overview of the recent research trend of PSCs.In Sect.2, we have classified MXenes according to their roles as additives (into perovskite absorber layer, charge transport layers, and electrodes) and themselves alone in PSCs.In Sect.3, we discuss MXenes as an interfacial layer on each layer of PSCs, followed by a discussion of MXenes' roles in improving morphological features, electrical conductivity, mechanical stability, and WF tuning to impact electron transfer and extraction mechanism and thereby increasing the operational stability of PSCs.In Sect.4, we suggest the future direction of an interfacial design/engineering that will realize the advent of an overarching framework for advanced PSC technology.

Synthesis of MXenes
MXenes are 2D transition metal carbides, nitrides, and carbonitrides with the general formula M n+1 X n T x (n = 1-4), where 'M' represents an early transition metal (Sc, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, etc.), 'X' represents carbon and/ or nitrogen, and 'T x ' stands for the surface terminations including -O, -OH, -F, etc. [32][33][34].In 2011, Ti 3 C 2 T x MXene was first reported by Naguib et al. [35].MXenes with a distinct combination of M and X elements, number of elements at M or X sites (solid solutions at M or X sites, (M′,M′′) n+1 X n T x , where M′ and M′′ are two different metals), number of atomic layers (n), ordering (in-plane or i-MXene or out-of-plane or o-MXene) of atoms, and compositions 1 3 of surface functional groups (T x ) were produced, as represented in Fig. 1 [36].For certain combinations of transition metals, ordered MXenes are energetically more stable than their solid-solution counterparts.MXenes with monolayer or multilayer morphology are usually synthesized from the M n+1 AX n (MAX) phase precursors by eliminating monoatomic layers of the A element, which is from group 13 or 14 (e.g., Al, Ga, Si, or Ge).However, some of the MXenes are also synthesized from non-MAX phase precursors [37,38].The high reactivity of the A element and relatively weak strength of M-A bonds compared to M-X bonds allow the selective etching of A layers from the MAX phase, typically in aqueous solution using either direct hydrogen fluoride (HF) or in-situ HF (produced through LiF/HCl mixture) [33,39,40].Over the years, apart from HF-based methods, such as using HF/other acids, HF/oxidants, NH 4 HF 2 , HCl/fluoride salt as etchants, several new synthesis methods include Lewis acidic molten salts assisted etching, alkali treatment under hydrothermal condition, treatment with organic bases, etching in organic solvents, salt-solution-based acoustic synthesis, electrochemical etching, etc. [41][42][43][44][45][46][47][48][49][50][51][52][53][54][55][56].
The type and composition of surface functional groups of MXene depend significantly on etching process, and they are mainly electronegative in nature, enabling to pull electron Fig. 1 Schematic display for 2D MXene's structures having a general formula of M n+1 X n T x , (M = an early transition metal, X = carbon and/or nitrogen, and T x = surface terminations of the outer metal layers, n = 1-4, examples: Ti 2 CT x , Ti 3 C 2 T x , Nb 4 C 3 T x , and (Mo, V) 5 C 4 T x ) [37].One or more transition metal atoms may occupy the M sites of MXenes to create ordered structures or solid solutions.The ordered double transition metal MXenes can be divided into three types: in-plane ordered structures (i-MXenes), in-plane vacancy structures, and out-of-plane ordered structures (o-MXenes), where either one layer of M′′ transition metal or two layers of M′′ transition metal are sandwiched between two layers of M′ transition metal.Other configurations might be feasible, such as the bottom row of the M 5 X 4 structure with one, two, or three layers of M′′ sandwiched between layers of M′.Predicted structures like higher-order single M or o-MXenes and high-entropy MXenes that have not been empirically proven are represented by the faded images at the bottom of the schematic.Copyright © 2021 AAAS Publishing density away from the M atoms of MXenes (i.e., shifting the Fermi energy ( E F ) of MXenes to a lower energy).That is, the electronic structure and WF of MXenes can be easily tuned by controlling the surface functional groups' compositions [57,58].In addition to an E F shift, surface dipoles induced by surface functional groups play a significant role in tuning the WF of Mxenes [59,60].Computational calculations such as density functional theory (DFT) through altering the M and X elements and surface functional groups can also predict several other intriguing characteristics of MXenes, such as a wide range tunability of the WF between 1.6 and 8.0 eV, superconductivity, topological insulator behaviors, metal-to-semiconductor transition, and metalto-insulator transition [59,[61][62][63][64][65][66].Particularly, fine-tuning the WF brings a perfect energy offset between the perovskite active layer and the charge transport layer by adjusting the vacuum level and obtaining the optimal energy-level alignment at interfaces in PSCs applications.Optimizing energy offsets is necessary to achieve effective charge separation and/or collection.The various roles of MXenes as components themselves or additives into the components in PSCs, in terms of passivating defects, crystallinity, electrical conductivity, mechanical flexibility, and moisture resistance, are discussed in the following sections.

PSCs
PSCs have been perceived as the next-generation photovoltaic candidates for commercial applications due to their high PCEs, easy and scalable solution fabrication processes, and low fabrication cost [67,68].For high-performance PSCs, the perovskite absorber layer is the most important.The characteristics such as crystallinity, surface coverage, compactness, uniformity, phase purity, internal trap defect density, grain size distribution, and grain boundaries of the perovskite absorber layers are the pivot players in determining the PCE performance of PSCs [69,70].A typical planar (or mesoporous) n-i-p or inverted p-i-n PSC consists of a photoactive absorber layer, an ETL, a HTL, and two (back and front) electrodes.The front electrode is based on a glass/flexible substrate facing simulated solar light.The solar light is incident on an absorber layer, and the electrons excite from the valence band to the conduction band, forming electron-hole pairs [8,71].These photogenerated charge carriers are extracted by the respective ETL and HTL and then moved to the electrodes (Fig. 2a).These distinctive features determine the stability and current-voltage hysteresis.Several key factors such as bulk, surface defects and reduced charge extraction/transportation, moisture, humidity, heat, and electric field can impede the solar cell device performance and longevity [72].These impediments can be controlled via solvent engineering, additive engineering, compositional engineering, interface engineering, device engineering, and encapsulation (Fig. 2b) [8,[73][74][75][76].Furthermore, until now, the highest efficiency devices are Pbbased PSCs.Due to the Pb toxicity, several alternatives such as lead-free PSCs, double-PSCs, and 2D Ruddlesden-Popper (RP) and Dion-Jacobson (DJ) PSCs have been examined as promising candidates in the perovskite photovoltaic arena [77][78][79][80].We have excluded an insightful discussion on the topics mentioned above and suggest you refer to this paper [72].In this review, we discuss various roles of MXenes as components or parts in PSCs, as depicted in the schematic (Fig. 3).Thus, we categorized the functions of MXenes depending on the positions and the roles of MXenes as additives in the perovskite absorber layer/ETL/HTL/electrodes or as ETL/HTL/electrodes along with as interfacial layers.The discussion about their respective pros and cons is presented in the following sections.

MXenes as Additives in Perovskite Absorber Layers
First up, the PCE performance of a PSC depends on the light-absorbing perovskite film layer that generates the charger carriers (electrons and holes).The charge carriers are separated and collected at the respective electrodes [81].The quality of the perovskite film is determined by crystal sizes and grain boundaries.Large crystal sizes with fewer grain boundaries are essential for maximum charge transfer and high-performance [82,83].In 2018, Guo et al. introduced the Ti 3 C 2 T x MXene into a CH 3 NH 3 Pbl 3 (MAPbI 3 ) perovskite absorber layer [31].This is the first report on an MXene's application in a PSC.Ti 3 C 2 T x MXene slows the crystallization rate and enlarges the size of MAPbI 3 crystals with no pin holes.The O-H⋅⋅⋅I − van der Waals interaction between the methylammonium iodide (MAI) and the additive restricts uniform nucleation on the tin oxide (SnO 2 ) layer and rather generates lesser nuclei, implying slow crystal growth or retardation of the nucleation process [84].After annealing, the small crystals grow into large crystals.Perovskite films with Ti 3 C 2 T x MXene additives show an improvement in light absorption owing to enhanced scattering of incident light from the large crystals [85,86].PSC with a device structure of indium tin oxide (ITO)/SnO 2 / MAPbI 3 :(0.3wt%) Ti 3 C 2 T x /2, 2′, 7,7 ′-t etr aki s(N ,N-di-p-m eth oxy phe nyl -am ine )9,9′-spirobifluorene (Spiro-OMeTAD)/ Au achieved a PCE of 17.4%, which is 12% higher than the PCE (15.6%) of the control device.The device with the optimal amount of additives also showed an increased open-circuit voltage (V oc ), current density (J sc ), and fill factor (FF).The increase in V oc is ascribed to the passivation of the perovskites by Ti 3 C 2 T x , spurring the hole extraction and reduced recombination at the perovskite/HTL interface [87].A schematic of a PSC with Ti 3 C 2 T x additive and J-V curves with 0, 0.01, 0.02, 0.03, 0.5, and 1 wt% of Ti 3 C 2 T x MXene additives are shown in Fig. 4a-b.The optimal amount of additive is 0.03 wt%.On further increase in wt%, the additives form aggregates on the surface of the perovskite films, resulting in charge trapping centers.Hence, the device performance degrades with excessive additives.The charge transfer resistance of the perovskite film with additive also reduced from 7000 to 1800 Ω, indicating fast charge transfer, and hence the J sc was increased [31].All-inorganic CsPbI 3 perovskite has a near-ideal bandgap and thermal stability and is a suitable candidate for the development of perovskite/Si tandem solar cells.But when exposed to moisture, CsPbI 3 experiences a quick phase change.Especially, the preferred p-i-n structure for perovskite/Si tandems shows a large performance difference compared to other perovskite compounds.Heo et al. reported a surface-engineered CsPbI 3 layer with oxidized Ti 3 C 2 T x MXene (OMXene) nanoplates to form effective and stable p-i-n-structured CsPbI 3 perovskite solar cells [88].In addition to improving charge separation at the perovskite-electron transporting layer interface through an improved electric field, OMXene acts as a physical barrier against moisture.
As a result, the p-i-n devices based on CsPbI 3 /OMXene dispersion with 10 wt% concentration attained the PCEs of 19.7% for 0.096 cm 2 cells and 14.6% for 25 cm 2 in minimodules.The pristine cell has a PCE of 18.1% with a V oc , J sc , and FF of 1.18 V (1.21 V), 19.05 (19.85) mA cm −2 , and 80.3% (81.6%), respectively.Furthermore, the encapsulated minimodule showed good stability, maintaining 85% of the initial efficiency over 1000 h while being exposed to 85 °C at a relative humidity (RH) of 85% and 1-sun light soaking.[89].CsBr can be diffused into these pores to grow the perovskite grain with sizes compared to pre-expanded volume and thus result in a high-quality CsPbBr 3 perovskite film with large and compact grains.The residual stress in the soft perovskite lattice distorts the surface lattices, leading to low device performance.Introducing Ti 3 C 2 Cl x MXene removes the residual stress owing to a strong Pb-Cl binding energy (301 kJ mol −1 ), paving the way for a high device efficiency [90].In further work, Saranin et al. introduced MXene doping into the perovskite absorber layer in inverted perovskite solar cells [91].MXene doping in the absorber layer significantly decreases charge recombination caused by deep trap states.More charges are collected at the perovskite/phenyl-C 61 -butyric acid methyl ester (PCBM) interface compared to reference devices.The MXene-based tailored cells outperformed the pristine devices and exhibited PCEs of 18.6% and steady power output.2D RP PSCs have received significant interest owing to their exceptional stability against moisture.The bulky cations in 2D RP perovskites restrict the internal movement of ions and allow the passage of organic ions, thus resulting in the hydrophobic nature of the absorber layer [92,93].However, low crystallinity, disordered orientation, and inferior charge transport constrain the PCE of 2D PSCs [94,95].Jin et al. explored Ti 3 C 2 T x MXene nanosheets as additives in 2D RP (BA) 2 (MA) 4 Pb 5 I 16 perovskite absorber layer to fabricate the perovskite films with excellent electrical conductivity and mobility [96].A homogeneous and highly crystalline perovskite film with Ti 3 C 2 T x MXene nano-dopants was formed.After adding an optimal concentration of 0.3 mM Ti 3 C 2 T x nanosheets, the PCE of PSC with a configuration of ITO/SnO 2 /(BA) 2 (MA) 4 Pb 5 I 16 -Ti 3 C 2 MXene/Spiro-OMeTAD/Ag increased from 13.7% (without additive) to 15.7%.The Ti 3 C 2 T x nanosheets doped-PSC also exhibited an increased J sc of 20.87 mA cm −2 , V oc of 1.11 V, and FF of 67.8%.The schematic of the PSC, 2D RP layered structure, and J-V curves of the PSC with different concentrations are shown in Fig. 4c-d.The enhanced crystallinity, orientation, and passivated trap states led to an accelerated charge transfer process in the vertical direction and are responsible for this improved performance.The unencapsulated PSCs with Ti 3 C 2 T x nanosheets exhibited excellent stability in ambient settings with an RH of 55% [96].
In 2021, Bati et al. produced Ti 3 C 2 T x MXene nanosheets doped with cesium (Cs) and added them to a lead iodide (PbI 2 ) precursor solution for PSCs via a two-step deposition approach [97].The theoretical and practical study demonstrated that Cs is crucial for enhancing PCE.The perovskite crystallization with doped Ti 3 C 2 T x MXene results in larger crystal grains, long lifetime charge carriers, and reduced charge recombination.The PSCs combined with Cs-doped Ti 3 C 2 T x Mxene (ITO/SnO 2 /FA 1-x MA x PbI 3-x Br x :Cs-Ti 3 C 2 T x / Spiro-OMeTAD/Au) achieved a high PCE of 21.6% (19.0% for the pristine device) with increased V oc , J sc , and FF.The increased V oc is ascribed to the filling of Cs + ions into the surface MA + vacancy.Moreover, with the introduction of Cs + , the interaction between MXene and perovskite film increases with a binding energy of −0.016 eV Å −2 .In contrast, without Cs + , the binding energy between the perovskite film and MXene is −0.013 eV Å −2 (Fig. 4e-f).Additionally, the unencapsulated Cs-Ti 3 C 2 T x MXene-doped PSC device exhibited stability by retaining 65% of its initial PCE after 2000 h [97].
High photovoltaic performance and large-scale commercialization of planar PSCs depend on the perovskite layer having a high crystallinity and long-term stability against high humidity.In 2021, Liu et al. used one-step deposition to incorporate Ti 3 C 2 Cl x quantum dots (QDs) as additives in the perovskite precursor solution [98].During the film crystallization process, the strong interaction between the Cl terminations of Ti 3 C 2 Cl x QDs and Pb 2+ ions slowed down the crystallization rate and induced the preferred grain orientation.This resulted in a high-quality perovskite film with high crystallinity, fewer trap-states, and less residual tensile strain.Ti 3 C 2 Cl x QDs also quickened charge extraction and benefitted band alignment between the SnO 2 ETL and the perovskite layer due to their top-bottom rising gradient distribution.As a result, the PSC with 0.2 mg mL −1 Ti 3 C 2 Cl x QDs and a device structure of fluorine-doped tin oxide (FTO)/SnO 2 /Perovskite + Ti 3 C 2 Cl x QDs/Spiro-OMeTAD/ Ag) achieved an improved efficiency of 21.3% (18.8%) and V oc of 1.19 V with minimal hysteresis (Fig. 4g-h).The Clterminated Ti 3 C 2 QDs can also prevent potential deprotonation of protonated organic amine in the perovskite, improving the overall stability.The Ti 3 C 2 Cl x QDs (0.2 mg mL −1 ) device shows a higher integrated J sc of 21.60 mA cm −2 than the pristine device (20.64 mA cm −2 ).The calculated J sc is less than the observed J sc by less than 5% discrepancy (Fig. 4i).Additionally, the unencapsulated device demonstrates exceptional long-term humidity stability by retaining over 84% of its initial PCE after aging for 1000 h at 40% RH in the dark at room temperature [98].
The solution method is the most common route to fabricate the perovskite films.The solution method is categorized into one-step and two-step sequential processes [99][100][101].In the simple and facile one-step solution method, it is difficult to achieve uniform and compact perovskite films owing to the anisotropic growth of perovskites [102].In the two-step process, the PbI 2 layer is formed first, and then the MAI solution is dropped to react, and thus high-quality perovskite films without cracks and pinholes are formed.Crack and pinhole-free perovskite films are pertinent for highly efficient PSCs [103].However, low reactivity leads to incomplete conversion of PbI 2 , and hence compact films are not formed owing to the presence of residual PbI 2 .As a result, the device performances are degraded [104,105].In 2021, to increase the reactivity of PbI 2 by generating porous channels, Zhao et al. added 2D monolayer Ti 3 C 2 T x (T x = -O, -OH, and -F) MXene nanosheets into the PbI 2 layer [106].This modification enhanced the subsequent PbI 2 reaction with MAI to transform into MAPbI 3 completely.As a result, the quantity of remaining PbI 2 in the perovskite film was reduced, and the size of the perovskite grain increased (Fig. 5a).Additionally, Ti 3 C 2 T x MXene controls the perovskite WF, resulting in a better energy-level alignment that makes carrier extraction and injection easier (Fig. 5b).The functional groups on the surface of Ti 3 C 2 T x were found to interact with the poorly coordinated Pb 2+ in perovskites to passivate defects, having a significant impact on reducing hysteresis and inhibiting nonradiative recombination.Finally, by adding 0.03 wt% of the Ti 3 C 2 T x additive, the PSC with a device configuration of FTO/SnO 2 /MAPbI 3 :Ti 3 C 2 T x / Spiro-OMeTAD/Au achieved a PCE of 19.3%, demonstrating an improvement of about 18% over the control device (Fig. 5c) [106].
A promising contender for inorganic double perovskite solar cells without lead is Cs 2 AgBiBr 6 PSC.The reported PCE of Cs 2 AgBiBr 6 is approximately 3%, which limits its photovoltaic capability.Yang et al. used D149 indoline dye to make the TiO 2 ETL more sensitive and Ti 3 C 2 T x MXene nanosheets to Cs 2 AgBiBr 6 to improve the crystallization [107].Cs 2 AgBiBr 6 and D149 indoline dye both have the potential to influence the photocurrent.Furthermore, the DFT was used to calculate the interface properties and electron structure.Fermi level pinning is effectively reduced while the perovskite maintains its semiconductor properties, thanks to weak van der Waals forces at the interfaces of Cs 2 AgBiBr 6 and Ti 3 C 2 T x .The high WF of Ti 3 C 2 T x modifies the Fermi level of the valence band of Cs 2 AgBiBr 6 , which results in increased carrier mobility.The device showed enhanced long-term stability and PCEs of 4.5% under 1-sun illumination and 7.2% under 200 lx indoor light illumination.Furthermore, the unencapsulated devices based on Cs 2 AgBiBr 6 , Cs 2 AgBiBr 6 @ Ti 3 C 2 T x (0.01 mg mL −1 ), D149-Cs 2 AgBiBr 6 and D149-Cs 2 AgBiBr 6 @Ti 3 C 2 T x (0.01 mg mL −1 ) exhibited long-term stability by retaining 70%, 74%, 83%, and 86% of their initial PCEs after 1000 h of storage at 25 °C in air at about 20% RH (Fig. 5d-e) [107].Bykkam et al. used a 2D MXene as an additive in MAPbI 3 perovskite at concentrations ranging from 0 to 20 vol% with a 5 vol% increment [108].With an increase in the vol.% of the 2D MXene, the perovskite peak at 2θ = 14.2° shifts slightly towards the lower angle.The continuous change in peak position reflects the homogeneous distribution of strain during perovskite crystal formation and stresses induced by the 2D MXene additive.However, the perovskite film with 5 vol% additive has fewer defects and improves the photoresponse of the PSC.On further increase in additive concentration, stacking of 2D MXene occurs in the perovskite active layer, hindering the light propagation across the perovskite film and reducing the photogeneration of charge carriers.The best PSC device with 5 vol% 2D MXene additive attained the highest PCE of 13.6%, V oc of ~ 0.81 V, J sc ~ 27.6 mA cm −2 , and FF of ~ 61.1%.This is higher than the PCE of 11.4% in the PSC without 2D MXene [108].
Din et al. explored the synergistic effect of high-quality NiO x HTLs deposited by ion beam sputtering on ITO substrates and the Ti 3 C 2 T x MXene doping of MAPI perovskite layers to increase the PCE of p-i-n PSCs [109].The 18 nmthick NiO x films are pinhole-free and have large-scale uniform surface morphology.For non-stoichiometric NiO x , the grazing-incidence X-ray diffraction revealed a 0.75% enlargement of the face-centered cubic lattice.Atomic force microscopy studies revealed that doping increased the size of MAPI polycrystalline grains from 430 ± 80 to 620 ± 190 nm.The best PSC with 0.15 wt% MXene doping with a device structure of ITO/NiO x /MXene-doped MAPI/PC 61 BM/BCP/ Ag showed a PCE of 16% (14% for the undoped PSC), which is a 14.3% improvement.However, the band gap of the MXene-doped MAPI layer was found to have a one order of magnitude higher density of defect states (∼10 19 cm −3 eV −1 for the MXene-doped MAPI and ∼10 18 cm −3 eV −1 for the undoped-MAPI), which lowers the favorable effect of the total area of bigger MAPI grain boundaries, reducing the J sc of the MXene-doped devices.The WF drops from −5.26 to −5.32 eV with MXene doping, which raises the V oc and FF, and thus is credited with improving the PCEs of PSCs.Such different results are attributed to large and partly delaminated multilayer MXene sheets and an increased density of states in the bandgap [109].
To improve the PCE of PSCs using a one-step coating method, Li et al. added 2D Ti 3 C 2 T x and V 2 CT x MXene to a PbI 2 precursor solution to fabricate perovskite films [110].The addition of Ti 3 C 2 T x and V 2 CT x additives boosted the hydrophobicity of the perovskite film, showing water contact angles of 85.4° and 69.8°, respectively, as compared to 52.9° for the pristine film.Furthermore, the additives also enhanced the shape and grain size of the perovskite films.The PCEs of PSCs with Ti 3 C 2 T x and V 2 CT x additives attained 17.6% and 17.2%, respectively, compared to the device without additives (15.0%).Moreover, a distinct morphology with uniform grain size and organized layered crystal particles can be observed for perovskite films employing the V 2 CT x additive, indicating that the V 2 CT x additive controls the formation of perovskite crystal films.Due to the improved crystallinity and perovskite film quality, the V 2 CT x additive integrated devices can preserve 68.3% of the initial PCE after 15 days, which is greater than the devices without additives by 9.7% and with Ti 3 C 2 T x as additive by 47.2%.Thus, V 2 CT x in photovoltaics offers a practical way to enhance the performance of PSCs [110].

MXenes as ETLs
Yang et al. used Ti 3 C 2 T x MXene nanosheets as a novel kind of ETL in planar structured PSCs that have undergone lowtemperature processing [111].The metallic Ti 3 C 2 T x can be improved as an ETL by applying a UV-ozone treatment that boosts the surface Ti-O bonds without affecting the bulk properties, such as high electron mobility.The schematic and cross-sectional SEM image of the PSC device with a configuration of ITO/Ti 3 C 2 T x /MAPbI 3 /Spiro-OMeTAD/Ag are displayed in Fig. 6a-b, respectively.The thickness of the Ti 3 C 2 T x layer is found to be 18 ± 3 nm.Figure 6c 6d-f).After 1500 h of storage, the unencapsulated devices still retain 93% of the original PCEs.This research illustrates the wide range of potential applications for 2D Nb 2 CT x MXene in photoelectric devices [112].Despite many benefits of 2D Ti 3 C 2 T x MXene, including high transparency, high conductivity, variable WF, and solution processability, the performance of the MXene-based PSC is still subpar to that of the conventional TiO 2 -or SnO 2 -based equivalent.The MXene/perovskite interface has some critical problems that need to be resolved.Wang et al. adopted a room-temperature solution technique followed by oxygen plasma treatment to use Ti 3 C 2 T x MXene as ETL in PSCs [113].Oxygen plasma treatment was demonstrated to form abundant Ti-O bonds randomly distributed on MXene and disrupt portions of Ti-C bonds.In addition to reduced trap states and better electron transport along the interface, the surface change made MXene WFs variable.In addition, contact angle and topography measurements were used to extensively analyze the surface tension of MXene and the related perovskite morphology.The device stability was improved, thanks to the PbO contacts between perovskite and MXene.The champion device attained a high PCE of 18.9% with a steady state output J sc of 21.5 mA cm −2 .The unencapsulated PSC with plasma-treated ETL demonstrated long-term stability by retaining 90% of its initial PCE after 750 h of storage in ambient air at 25 °C and RH of ~ 50% [113].MXenes are appealing for use in PSCs due to their distinct features that result from surface functional groups and oxidation.Yang et al. oxidized Ti 3 C 2 T x hydrocolloid to adjust its characteristics for an ETL in low-temperature processed PSCs [114].The energy levels were calculated using the Vienna ab initio simulation package code, which is based on DFT.Ti 3 C 2 T x can be oxidized to produce Ti-O bonds and significantly minimize the macroscopic defects in a spincoated film.However, after substantial oxidation, the material changes from metallic to semiconductor.In the case of a hybrid of oxidized and pristine Ti 3 C 2 T x , better matching of energy levels between perovskite and ETL layer results in a champion PCE of 18.3%.The improved electron mobility in the ETL, which encourages electron transport and lowers electron-hole recombination, is responsible for improving the PCE.This work illustrates the significant potential of MXene-derived materials in low-temperature processed PSCs [114].

MXenes as Additives in ETLs
SnO 2 ETLs are extensively used in planar and flexible devices due to a lower annealing temperature (below 185 °C), and better electrical and optical properties [115,116].These ETLs have disadvantages of de-wetting property, low transmittance, and conductivity.On the other hand, TiO 2 ETL is also commonly used for high PSC devices and requires a high annealing temperature of 450-500 °C [117].Furthermore, the PSC with the SnO 2 nanoparticlemodified TiO 2 (SnO 2 @TiO 2 ) composite ETL processed at low temperature shows a high PCE of 21.3%.However, the amorphous nature of TiO 2 at low temperatures limits the PCE of the devices [118,119].In 2020, Huang et al. used a multi-dimensional conductive network (MDCN) heterojunction structure made of TiO 2 , SnO 2 , and Ti 3 C 2 T x MXene as the electron transport layer to fabricate stable and efficient planar PSCs [120].Based on an oxygen vacancy scrambling effect, the zero-dimensional anatase TiO 2 quantum dots are in-situ rooted on three-dimensional SnO 2 nanoparticles, forming nanoscale TiO 2 /SnO 2 heterojunctions, and are surrounded by 2D conductive Ti 3 C 2 T x sheets.The fabrication process for the (FAPbI 3 ) 0.97 (MAPbBr 3 ) 0.03 perovskite thin film with the MDCN ETL is carried out using a controlled low-temperature annealing technique first in an environment of air and later in N 2 (Fig. 7a).The optical quality, the crystallinity of the perovskite layer, and internal interfaces are all improved by the optimal MXene concentration of 0.02 wt% contributing more carriers with effective and speedy transfer in the device.The resulting PSCs with MDCN ETL-air and N 2 attained a champion PCE of 19.1%, compared to the PCE of 16.8% for the pristine device with SnO 2 ETL.Furthermore, the device with MDCN ETL retained nearly 85% of its initial performance for over 45 days in air with a humidity of 30-40%; in contrast, the pristine (control) device only retained about 75% of its initial performance [120].
Defect-free polycrystalline perovskite films are highly desirable for fabricating an effective and stable PSC.However, the use of molecular materials in conventional defect reduction methods suffers from their complicated procedures, poor durability, and limited effects.Zheng et al. used a hybrid film made of SnO 2 nanoparticles and Ti 3 C 2 T x MXene nanoflakes as ETL in a planar regular-structure PSC [121].The SnO 2 /MXene colloidal suspension is spun onto ITO substrates to form the ETL, and then the perovskite layer (MA 0.15 FA 0.85 PbI x Br 3-x ) is spin-coated on the previously coated ETL via a typical two-step deposition technique (Fig. 7b).The film properties of the top perovskite layers, such as compactness, crystal size, surface roughness, crystallinity, optical absorption, defect density, and so on, are changeable by varying the Ti 3 C 2 T x /SnO 2 ratios (0-2.2 wt%) in ETLs.Compared to a pristine device with SnO 2 ETL, the defect density in perovskite films with an optimized 1.4 wt% hybrid ETL is significantly reduced from 5.65 × 10 15 to 2.25 × 10 15 cm 3 .However, the electrical conductivity of the hybrid ETL is decreased, most likely because of the geometric configuration of the added MXene.As a result, the PCE of PSCs with 1.5 wt% Ti 3 C 2 T x /SnO 2 ratio is boosted significantly from 16.28 to 20.35%, along with J sc and FF increasing from 20.65 and 0.71 to 23.65 mA cm −2 and 0.76 (Fig. 7c).However, V oc (1.111 to 1.113 V) remained almost stable.Furthermore, the environmental stability of the unencapsulated devices is dramatically enhanced by retaining 74% of their initial PCE after 768 h of storage under an air environment at 25 °C with an RH of 30% [121].
To address the issue of the instability of the APbX 3 structure and lead toxicity, the inorganic Cs 2 AgBiBr 6 double perovskite structure is a viable development route for PSCs.However, the optoelectronic application is severely constrained by the low J sc and PCE owing to the low crystallization of Cs 2 AgBiBr 6 .Li et al. used a straightforward method to dope single-layered MXene nanosheets into titania (Ti 3 C 2 T x @TiO 2 ) to serve as a versatile ETL for stable and effective Cs 2 AgBiBr 6 double PSCs [122].In addition to considerably increasing TiO 2 electrical conductivity and electron extraction rate, single-layered MXene nanosheets also alter the surface wettability of the electron transport layer and promote the crystallization of Cs 2 AgBiBr 6 double perovskite in solar cell devices.The Cs 2 AgBiBr 6 double perovskite films based on both TiO 2 and Ti 3 C 2 T x @TiO 2 ETLs have a similar grain size of 200-500 nm (Fig. 7d-e).The TiO 2 -based Cs 2 AgBiBr 6 film contains large voids, which are pathways for current leakage.In contrast, the Ti 3 C 2 T x @ TiO 2 -based Cs 2 AgBiBr 6 film is smooth and void-free.As a result, compared to a device based on TiO 2 ETL, the PCE increased by more than 40% to 2.8%, and the hysteresis was significantly reduced.Additionally, the Ti 3 C 2 T x @TiO 2 device demonstrated long-term operational stability.The Ti 3 C 2 T x @TiO 2 device retained 93% of its initial PCE even after 15 days of storage in ambient air [122].Saranin et al. doped MXene in both MAPbI 3 perovskite absorber layer and PCBM layers in an inverted PSC (glass-ITO/NiO/Perovskite + MXenes/PCBM + MXenes/bathocuproine (BCP)/ Ag).The MXene doping improves the band alignment at the perovskite/charge transport layer owing to the WF shift, which facilitates charge extraction at the electrodes.Hence the inverted PSC attained a PCE of 19.20% [91].
To reduce recombination losses and enhance the PCE of PSCs, defect passivation and customizing the perovskite charge transport layer interfaces are crucial.Chava et al. tailored the electrical characteristics of the ETL and the ETL/ perovskite interface in inverted (pin) PSCs using Ti 3 C 2 T x MXene [123].A [6,6]-phenyl-C 61 -butyric acid methyl ester (MPC 61 BM)-based ETL with MXene doping has improved electrical conductivity and band alignment at the ETL/perovskite interface.The n-doping of PC 61 BM was confirmed by a red shift of the A g (2) peak in the Raman spectrum, and a localized upshift of the Fermi level measured using scanning Kelvin probe force microscopy (SKPFM).As a result, PSC devices using M-PC 61 BM as the ETL attained a higher PCE of over 18% than control devices using PC 61 BM as the ETL, with a PCE of 15.55%.Furthermore, the PSC with MXene as an interfacial layer between the perovskite and ETL with a structure of ITO/NiO x /MXene/MAPbI 3 / PC 61 BM/BCP/Ag attained a maximum PCE of 15.99% with a maximum V oc , J sc , and FF of 1.05 V, 20.74 mA cm −2 , and 74%, respectively.In comparison, the PSC with a standalone Ti 3 C 2 T x MXene ETL showed a maximum of 2.06%, with a maximum V oc , J sc , and FF of 0.72 V, 5.57 mA cm −2 , and 50%, respectively.The addition of MXene to PSCs has demonstrated diverse effects, including improvement in carrier transport, passivation and trap state reduction, and better interfacial energy alignment [123].
Yang et al. utilized SnO 2 -Ti 3 C 2 MXene (Ti 3 C 2 Mxene + SnO 2 ) nanocomposites with varying Ti 3 C 2 contents (0, 0.5, 1.0, 2.0, and 2.5 wt%) as ETLs and applied in planar-structured PSCs with an architecture of ITO/ SnO 2 + Ti 3 C 2 T x /MAPbI 3 /Spiro-OMeTAD/Ag (Fig. 8a) [124].The corresponding cross-sectional SEM image of the PSC and energy-level diagram of each component are shown in Fig. 8b-c.The lowest occupied molecular orbital reduces from −4.39 eV for pure SnO 2 ETL to −4.63 eV for SnO 2 + Ti 3 C 2 T x ETL, facilitating fast electron transfer.The PCE of the device fabricated with pure Ti 3 C 2 as the ETL increases from 17.2 to 18.3% when SnO 2 and 1.0 wt% Ti 3 C 2 are combined.The metallic Ti 3 C 2 MXene nanosheets demonstrated superior charge transfer pathways, improving electron extraction and mobility while lowering electron transfer resistance at the ETL/perovskite interface, resulting in higher photocurrents [124].
ETL materials with good optoelectrical properties and energy levels comparable to the perovskite layer are essential to meet the demand for commercialization.Niu et al. introduced Nb 2 C MXenes as an additive to SnO 2 ETL for the first time, which caused the SnO 2 grains to develop clearly [125].The incorporation of Nb 2 C MXenes leads to an increase in lattice spacing of ( 101) and ( 110) planes corresponding to SnO 2 ETL from 0.32 to 0.33 nm and 0.240 to 0.253 nm, respectively.Further, the roughness and average grain size increased from 3.76 and 179 nm for SnO 2 ETL to 11.1 and 256 nm for the SnO 2 -Nb 2 C ETL, respectively.In addition, the Nb 2 C inclusion in SnO 2 also reduces surface energy between the perovskite and the ETL, improving the surface wettability so that the perovskite solution can spread smoothly on the ETL.The light reflection also reduced, indicating improved light absorption and an improved fill factor.The electron mobility for the SnO 2 ETLs increased from 2.3 × 10 −5 to 1.39 × 10 −4 cm 2 V −1 s −1 for the SnO 2 -Nb 2 C ETLs, indicating faster electron transfer from perovskites to ETLs.Hence, the PSCs based on the architecture of ITO/ SnO 2 -Nb 2 C/Perovskite/Spiro-OMeTAD/Ag exhibited the best PCE of 22.86% (18.96% for the control device) with improved V oc , J sc and FF.The PSCs with the modified ETL maintain 98% of the original efficiencies after 40 days at 25 °C under a humidity of 40-60% [125].
Yin et al. utilized fluoroalkylsilane and dodecyltrimethoxysilane functionalized MXene nanosheets as dopants into the SnO 2 ETL to fabricate PSC devices with modified ETL layers, respectively, denoted as SnO 2 -MF and SnO 2 -MH [126].To fabricate high-performance devices, it is necessary to have an ETL with improved energy alignment and improved charge transfer, which will aid in the efficient extraction and transport of photogenerated carriers.The SnO 2 -MH ETL has improved band alignment, as evidenced by DFT calculations and ultraviolet photoelectron spectra measurements.In the meantime, functionalized MXene nanosheets exhibit strong electrical conductivity and mobility and may quickly and effectively establish a zero Schottky barrier heterojunction with SnO 2 .Finally, the appropriate surface energy attained by functionalized MXene additives can increase the grain size of the perovskite thin film.The schematic of PSC and the energy-level diagram of different SnO 2 , ITO and perovskite layers are sketched in Fig. 8d-e.The devices based on the SnO 2 -MH ETL with the device architecture of ITO/ SnO 2 -MH/FAPbI 3 /Spiro/MoO x /Ag significantly enhanced their PCE from 21% (with no additive) to 23.66%.The champion device exhibited a maximum PCE of 24.1% with a J sc of 25.49 mA cm −2 , a V oc of 1.121 V, and a FF of 84.4% (Fig. 8f).The PSCs also demonstrated better operational stability and moisture resistance [126].
Yang et al. fabricated a modified SnO 2 bottom layer to produce highly crystalline perovskite films to improve the photovoltaic performance of PSCs [127].To produce highly crystalline and durable perovskite films, perovskite crystallization mechanisms must be modulated.Ti 3 C 2 T x quantum dots (MQDs) were introduced into SnO 2 ETL to investigate the crystallization kinetics of the perovskite.It was discovered that perovskite nucleation from the precursor solution could be induced quickly by Ti 3 C 2 T x MQDs-modified SnO 2 (MQDs-SnO 2 ) ETL, resulting in the formation of an intermediate perovskite phase after anti-solvent treatment.As a result, the perovskite film crystal quality and phase stability are significantly improved.A steady-state PCE of 23.3%, with a J sc of 24.96 mA cm −2 , a V oc of 1.172 V, and an FF of 0.798 were attained for PSCs with a device structure of ITO/ SnO 2 -MQD/Perovskite/Spiro/MoO x /Au (Fig. 8g-h) by taking advantage of the excellent charge extraction capabilities of the MQDs-SnO 2 layer.The PSC with MQDs-SnO 2 ETL has higher incident photon-to-electron conversion efficiency (IPCE) over the entire visible region compared to the pristine device.The integrated J sc value of 24.39 mA cm −2 is very close to the observed J sc value of 24.96 mA cm −2 as observed by a solar simulator (Fig. 8i).The fabricated PSC also demonstrated exceptional stability against humidity and light soaking [127].

MXenes as HTLs
In PSCs, the HTL plays a vital role in controlling the crystallization of a perovskite film and the hole transfer efficiency at the perovskite/HTL interface.Although many successes in PSC by organic hole transport materials (HTMs) such as N,N'-bis(3-methylphenyl)-N,N'-bis(phenyl) benzidine, poly(trimethylene terephthalate-co-trimethylene isophthalate)-Terephthalic acid (PTTI-TPA), triphenylamine-based HTM incorporating pyridine core (coded as H-Pyr) and pyridine-based polymer semiconductor (PPY2), the organic HTLs fail to scale up the mass-production due to complexity of synthesis, purification and the manufacturing cost associated with the synthesis procedures [128][129][130].In contrast, the common inorganic HTM NiO x requires an annealing temperature above 200 °C and is unsuitable for flexible devices.Hence, low-cost and low-temperature processed HTMs with excellent electrical properties are necessary.Nb 2 CT x MXene is a promising HTM due to good conductivity and an adjustable WF [131,132].Furthermore, the Nb 2 CT x MXene has three atomic layers compared to the five atomic layers of the Ti 3 C 2 T x MXene (excluding atomic layers of surface functional groups).Hence Nb 2 CT x has a larger specific area, and consequently, the contact area with the perovskite layer is also larger.This promptly increased the charge transfer in PSCs.The charge transfer depends upon the WF of Nb 2 CT x MXene.However, the WF can be controlled depending on the type and quantity of the surfaceterminated functional groups [59,133].As observed from First-principles calculations, the WF of the pristine Nb 2 CT x nanosheets with -O, -OH, and -F surface functional groups is approximately 4.7 eV, which is not enough to transport holes across the Nb 2 CT x /MAPbI 3 interface.Commonly, O-terminated Nb 2 CT x shows a higher WF compared to the F-and OH-terminated counterparts.This WF difference arises due to the different surface dipole moments and a difference in charge transfer between the terminated functional groups and Nb atoms [59,61].In 2021, Zhang et al. synthesized the highly photoelectric Nb 2 CT x MXene by oxygen plasma treatment and used it as the HTL in inverted PSCs [134].The oxygen plasma-treated Nb 2 CT x MXene nanosheets showed enhanced conductivity and high transmittance.The oxygen plasma treatment modified the WF of Nb 2 CT x from 4.68 eV (pristine) to 5.04 eV by enhancing the O functional groups on the Nb 2 CT x surface.In other words, the high electronegative O atoms pull electrons from Nb atoms, thus reducing the Fermi level of Nb 2 CT x .At the same time, the O functional groups induce a dipole moment with O atoms facing toward Nb atoms, uplifting the vacuum level.The combined effect of the downshifted Fermi level and uplifted vacuum level increased the WF of the Nb 2 CT x .This high WF reduced energy offset/loss at the interface between the HTL/perovskite absorber layer and eased the hole transfer from the perovskite layer to HTL, leading to suppressed charge recombination and high J sc .Furthermore, the photoluminescence (PL) spectra from the perovskite film with the oxygen plasma treated Nb 2 CT x (Nb 2 CT x /MAPbI 3 ) indicated the most significant PL intensity quenching, suggesting an improved hole extraction ability of Nb 2 CT x .From time-resolved PL spectra, the average PL lifetime is found to decrease from 16.51 ns (glass/MAPbI 3 ) to 2.26 ns (ITO/MAPbI 3 ), 1.64 ns (ITO/Nb 2 CT x /MAPbI 3 ), and 0.72 ns (ITO/treated Nb 2 CT x /MAPbI 3 ), which further confirms the excellent hole transportability of Nb 2 CT x .The PSC with the plasma-treated Nb 2 CT x MXene HTL with a device structure of ITO/Nb 2 CT x /MAPbI 3 /PCBM/Ag exhibits a higher PCE of 20.74% with increased V oc and J sc than those with the pristine Nb 2 CT x (18.1%) and without Nb 2 CT x MXene HTLs (15.5%).The higher external quantum efficiency (EQE) values indicate the enhanced J sc , suggesting more efficient charge separation and collection efficiency.The PSCs with oxygen plasma-treated Nb 2 CT x HTLs attain the highest EQE and integrated J sc (22.75 mA cm −2 ), close to the J sc values obtained from the J-V measurements (Fig. 9c).Additionally, the flexible and large area (0.99 cm 2 ) PSCs with oxygen plasma treated Nb 2 CT x HTLs exhibit the highest PCE of 17.3% and 17.9%, respectively.Moreover, the Nb 2 CT x HTL-based PSCs show excellent long-term stability in the glovebox for 70 days and thermal stability at 85 °C [134].
A silane coupling agent (SCA) can connect various materials and enhance the interface properties [135,136].Furthermore, this coupling agent reduces the cost by increasing the corrosion resistance of the material [137,138].However, SCA is hard to use directly for bonding.Including hydroxyl groups in SCA is a possibility to improve the interface bonding.In 2022, Du et al. fabricated an HTL by spraying Ti 3 C 2 T x MXene nanosheets with varying concentrations of the SCA (vinyltris (2-methoxyethoxylsilane)) (0, 0.1, 0.3, 0.5, and 0.7 vol%) [139].As shown in the schematic of the spray deposition device (Fig. 9e), the ITO glass substrate is placed on the 60 °C hot stage, and the nozzle of the spray gun is set 20 cm above.The Ti 3 C 2 T x -SCA dispersion of different proportions was deposited uniformly on the ITO substrate, setting the spray speed of the spray gun to 2 mL min −1  .A flat, pin-hole-free Ti 3 C 2 T x film was produced with a mixing content of 0.3 V/V% by utilizing SCA to efficiently adjust the interface distribution of Ti 3 C 2 T x after film fabrication.The superior production of Ti 3 C 2 T x films enabled better hole transfer paths, improved hole extraction (hole mobility increases from 1.30 × 10 −6 to 3.08 × 10 −7 cm 2 V −1 s −1 ), and lower transfer resistance from 369.3 Ω to 194.1 Ω at the HTL/perovskite interface.The compared values are for the Ti 3 C 2 T x HTL and Ti 3 C 2 T x + 0.3% SCA HTL.As a result, the device with the structure of ITO/ Ti 3 C 2 T x -0.3SCA/MAPbI 3 /PC 61 BM/Ag exhibited an increased PCE from 11.1% (pristine Ti 3 C 2 T x ) to 13.7%.The schematic of the PSC and the J-V curves of PSCs with Ti 3 C 2 T x -SCA, having different concentrations of SCA, are shown in Fig. 9f-g.However, the V oc dropped by 11%.The voltage drop is attributed to stacking due to high surface roughness (root-mean-square (RMS) = 0.87 nm for only nanosheet to 4.07 nm for nanosheet and SCA) and good coverage of Ti 3 C 2 T x [38,39].Additionally, the calculated EQE (17.75 and 19.94 mA cm −2 .)are well consistent with the observed J sc variations (19.19 mA cm −2 and 21.30 mA cm −2 ) for the PSCs with or without SCA (Fig. 9h).Moreover, the PSC based on Ti 3 C 2 T x -SCA HTL exhibited excellent stability at ambient air humidity ~ 20% and room temperature, and retained about 80% of the initial PCE after 80 h of storage [139].

MXenes as Electrodes or Additives in Electrodes
The large-scale commercialization of PSCs is constrained by the high cost of commonly used HTM-(Spiro-OMeTAD, PTAA, NiO x , etc.) and noble metal electrodes (Au and Ag) [155,156].Low-cost coal-based carbon electrodes are considered reasonable replacements for both the HTMs and the noble metal electrodes in PSCs.However, the PCEs of PSCs with carbon electrodes are still lower than those of conventional devices.In 2019, Cao et al. were the first to use a more conductive 2D MXene material (Ti 3 C 2 ) with an energy level comparable to carbon materials as back electrodes in HTM and noble-metal-free MAPbI 3 PSCs [157].
Using a straightforward hot-pressing technique at 85 °C and 0.4 MPa, a seamless interfacial contact between the MAPbI 3 perovskite layer and Ti 3 C 2 material was achieved, as seen in Fig. 11a-b.The WF of the Ti 3 C 2 material was found to be 4.96 eV and matches well with the valence band (5.4 eV) of the MAPbI 3 layer.The hole charge carriers easily transfer from the perovskite layer to the Ti 3 C 2 electrode and the electrons from the TiO 2 layer to the FTO electrode.Furthermore, the square resistance decreased from 30.93 to 25.34 Ω sq −1 as the thickness of the Ti 3 C 2 electrodes increased from 280 to 330 mm.The Nyquist plot measured under the illumination of 100 mW cm −2 at a bias voltage of 0.60 V and a frequency range of 100 mHz to 1 MHz shows a similar series and decreased charge transport resistance.The decreased charge transport resistance means efficient hole extraction.However, as the thickness of the electrode increases further, transport resistance increases, and hence the PSC shows low photovoltaic performance.Because of the better conductivity and favorable interfacial contact between the optimally thick Ti 3 C 2 electrode and the perovskite layer, the champion PSC based on this electrode with the device structure of FTO/TiO 2 /MAPbI 3 /Ti 3 C 2 T x exhibited a PCE of 13.83% with a V oc of 0.95 V, a J sc of 22.97 mA cm −2 , and an FF of 63% (Fig. 11c).The resulting PCE value is 27% higher than that of the control PSC (10.87%) based on carbon electrodes.A 1 3 good reproducibility in terms of manufacturability without significant batch-to-batch variations was also observed in the fabrication of PSCs based on Ti 3 C 2 electrodes (Fig. 11d).
The device also demonstrated greater stability than the conventional FTO/compact (c) and mesoporous (m) -TiO 2 /Perovskite/Spiro-OMeTAD/Au device for 360 h when stored at room temperature at an RH of 30%.The Ti 3 C 2 electrode plays the role of an encapsulating layer, preventing the active or absorber layer from reacting with air and water [157].
As stated earlier, the high costs of HTMs and noble metals impede the scaling of commercialization of PSCs.The straightforward manufacturing processes, low cost, and great stability of carbon-based inorganic PSCs have shown excellent performance in photovoltaics.Although devices with carbon electrodes are less efficient than those with traditional structures, interest in their potential large-scale applications has grown.However, the commercially available carbon paste forms a point contact with the perovskite layer because of the point-to-point contact in the carbon electrode.As a result, many pinholes remain at the interface between the electrode and the perovskite layer, impeding the carrier transport [158].CNTs with a one-dimensional (1D) structure show high conductivity and direct transfer paths for charge carriers, and 2D Ti 3 C 2 T x MXene has high surface areas and shows high conductivity via accelerating the charge carriers [31,124,159,160].In 2020, Mi et al. incorporated commercial CNTs and 2D Ti 3 C 2 MXene into the carbon paste and formed a mixed carbon electrode for the inorganic CsPbBr 3 perovskite-based PSC [161].This mixed carbon electrode offers a network structure and a multi-dimensional charge transfer path due to a good interface with the perovskite absorber layer.MXenes nanosheets fill the voids of carbon powders.This synergistic structure significantly boosts the conductivity of the carbon electrode and carrier transport.The pure carbon electrode only shows point-to-point contact, and the device with the structure of FTO/c-TiO 2 /CsPbBr 3 / pure carbon exhibits a PCE of 5.9%.For the inorganic CsPbBr 3 PSC with the mixed carbon electrode, the champion device with the configuration of FTO/c-TiO 2 /CsPbBr 3 / mixed carbon achieves a decent PCE of 7.09% with a V oc of 1.357 V, a J sc of 7.16 mA cm −2 , and an FF of 72.97% (Fig. 11e-f).Furthermore, the devices with the mixed carbon electrodes show good reproducibility and excellent stability by retaining 80% of their initial PCEs after storage in air for one month [161].
Wearable and other flexible optoelectronic systems require flexible transparent electrodes (FTEs).The excellent conductivity and transparency properties of ITO have placed it as the most reported transparent electrode material among rigid photoelectric devices [162][163][164][165].However, poor mechanical stability (brittleness) and expensive manufacturing equipment costs of ITO electrodes prevent them being used in flexible electronics.Silver nanowires (AgNWs) have low sheet resistance and high transmittance and are solution-processable [166][167][168].They also show super-bending resistance and mechanical stability compared to ITO electrodes.However, AgNWs suffer from high roughness and low adhesion with the substrate and are prone to oxidation and high wire-wire junction resistance [169,170].Metal oxide nanoparticles and 2D materials such as electrochemically exfoliated graphene, graphene oxide, and reduced graphene oxide can modify the conductivity and roughness of AgNW networks [171][172][173].However, poor doping stability hinders the performance of graphenebased devices [174].Ti 3 C 2 T x MXene, a 2D material, can enhance the performance of AgNW networks due to its high electrical conductivity, carrier mobility, tunable WF, and superior mechanical properties [126,175].In 2022, Chen et al. combined 1D AgNWs and 2D Ti 3 C 2 T x MXene nanosheets to fabricate composite Ti 3 C 2 T x MXene FTEs for flexible solar cell devices [176].Under electrostatic interaction, a composite 1D:2D structure (AgNW:MXene) FTE is produced.The 1D AgNWs deposited on hydrophilic PET substrates are coated with the MXene nanosheets to form a conductive AgNW:MXene composite network, as shown in Fig. 12a.The MXene nanosheets weld the crossing junctions and the broken junctions.Using capillary force, 2D MXene nanosheets fill the gaps in the AgNW networks and join the wire-wire connections.These provide more continuous conductive paths, improving the conductivity of the electrodes.Additionally, the oxygen-containing functional groups of MXene nanosheets adhere strongly to the PET substrate via a strong hydrogen bonding interaction.The photoelectric performance of FTEs is determined by the figure of merit (FoM).The FTE fabricated with 5 mg mL −1 AgNWs dispersion deposited with a concentration of 0.5 mg mL −1 MXene shows an FoM by a low sheet resistance of 10.91 Ω sq −1 (12.09Ω sq −1 for AgNW without MXene) and a high transmittance of 82.84% (81.96%) at 550 nm.The roughness of the AgNW: MXene FTEs is also decreased, as observed from the RMS roughness of AgNW: MXene FTEs (13.6 nm) compared to the pristine AgNW FTEs (19.2 nm).The mechanical endurance of FTEs determines the ability of FTE to sustain a continuous conductive path to collect charge under an external mechanical stress [177].The PET/AgNW: MXene shows a surface Young's modulus of 0.78 GPa, compared to 0.96 and 1.15 GPa of PET/ITO and PET/AgNW, respectively.Additionally, the AgNW:MXene FTEs exhibit superior long-term stability at 60% RH and 120 °C for 240 h, as well as strong mechanical resilience after 1,000 cycles of bending tests at a 5 mm curvature radius.Furthermore, WF matching is important for accelerating the charge transfer.The WF of AgNW:MXene is calculated to be 4.81 eV, which lies in the required normal range for optoelectronic devices.As a result, the inverted PSC with a device structure of PET/AgNW:MXene/NiO x / Perovskite/PC 61 BM/Ag (Fig. 12b) and an area of 0.1 cm 2 show a high PCE of 20.22% (18.7% for the cell with pristine AgNWs only) with V oc of 1.06 V, J sc of 25.16 mA cm −2  , and FF of 75.5% (Fig. 12c) [176].

At the Interfaces Between Perovskites and Electrodes
MXenes are promising candidates in photovoltaics because of their high transmittance, tunable WF (2. (Fig. 13c), with a long-term stability of more than 1900 h in a moist environment and more than 600 h under heat circumstances [160].

At the Interfaces Between Perovskites and ETLs
Proper device design and specialized interface engineering are required to improve optoelectronic characteristics and the charge extraction process at the selective electrodes to increase the PCE of PSCs.In 2019, Agresti et al. used 2D Ti 3 C 2 T x MXene (T x = -O, -OH, and -F) to tune the WF of the perovskite absorber and engineered a perovskite/ETL interface.It was observed that the OH-terminated surface of MXene reduced the WF of the perovskite, while the O-terminated surface increased the WF.This nonlinear behavior is reported in the literature [178].Furthermore, light absorption was increased in MXene -doped perovskites.The addition of Ti 3 C 2 T x to halide perovskite and TiO 2 layers enables the adjustment of the materials' WFs without changing the other electronic characteristics.The PSC with MXene-doped perovskite active layer also showed higher PCE (17.4%) than that with the un-doped perovskite absorber layer (15.6%) [179].Additionally, the band alignment between these layers can be altered using the dipole induced by Ti 3 C 2 T x at the perovskite/ETL interface.WF tuning and interface engineering work together to significantly improve the PCE of MXene-modified PSCs to 20% with a V oc of 1.09 V, a J sc of 23.82 mA cm −2 and an FF of 77.6%, as well as a reduction in hysteresis compared to reference cells without MXene [179].Chen et al. used Ti 3 C 2 T x QDs (TQDs) to engineer a perovskite/TiO 2 ETL interface and perovskite absorber and introduced Cu 1.8 S nanocrystals to optimize the Spiro-OMeTAD HTL in PSCs (Fig. 13d) [180].
The QDs with a diameter of about 5.2 nm and a thickness of ~ 1 nm in the absorber layer significantly contribute to the improved crystalline quality of the perovskite film, and large grain sizes are formed.Furthermore, TQDs improve electron extraction and collection at the perovskite/ETL.Cu 1.8 S improves the hole extraction at perovskite/HTL interfaces.With the improved J sc , V oc , and FF because of the synergistic effect of both TQD/Cu 1.8 S, the hysteresis-free PCE of PSCs significantly increased from 18.3 to 21.6% (V oc of 1.14 V, J sc of 24.12 mA cm −2 , FF of 78.70%) (Fig. 13e).
The TQD and/or Cu 1.8 S nanocrystals doping also significantly improves the long-term ambient and light stability of PSCs by improving perovskite crystallization, reducing HTL film aggregation and crystallization, and preventing ETL from ultraviolet-induced photocatalysis.The results show that TQD and Cu 1.8 S can function as ultrafast electron and hole tunnels for optoelectronic devices.The trap-filled limit voltage (V TFL ) values of FTO/TiO 2 /Perovskite/PCBM/ Ag (control), FTO/TiO 2 /MQD/Perovskite/PCBM/Ag (S1), and FTO/TiO 2 /MQD/MQD-Perovskite/PCBM/Ag (S2) electron-only devices were found to be 0.82, 0.66, and 0.59 V, respectively.The electron trap-state densities of perovskites are 1.59 × 10 16 cm −3 (control), 1.28 × 10 16 cm −3 (S1), and 1.15 × 10 16 cm −3 (S2) (Fig. 13f).The large grain size and low grain boundary density in the perovskite film resulted in a lower trap density [180].Crystal deformations, such as lattice strain at the surfaces and grain boundaries, owing to the soft perovskite lattice, affect the charge extraction-transfer dynamics and recombination, leading to a low PCE.Zhou et al. added an inorganic 2D Ti 3 C 2 Cl x MXene to the bulk and surface of the CsPbBr 3 film [90].This method dramatically reduces the superficial lattice tensile strain.The expanded perovskite lattice is compressed and confined to act as lattice "tape", in which the Pb-Cl bond functions as "glue", and the 2D Ti 3 C 2 immobilizes the lattice.This compression and confinement result from the strong interaction between Cl atoms in Ti 3 C 2 Cl x and the under-coordinated Pb 2+ in the CsPbBr 3 lattice.Under light irradiation, the carrier transfer is affected because defects and strain are introduced to the perovskite film.Hence, a smaller number of holes is available at the perovskite top surface.The MXene interlayer, defective states, and grain boundaries are passivated, leading to the availability of charge carriers and thus enhanced photovoltage.The champion all-inorganic CsPbBr 3 PSC is finally able to attain the PCE as high as 11.08% with an ultrahigh V oc up to 1.702 V, which is the highest efficiency record for this type of PSCs to date.Additionally, the unencapsulated device exhibits almost unaltered performance at 85 °C for 30 days and at 80% RH for 100 days [90].High-efficiency photovoltaic (PV) devices can be produced by adding 2D MXenes to the ETL of PSCs.However, the oxidation that results from the ambient fabrication of the ETLs causes an inevitable decline in the electrical characteristics of MXene.To enhance the photovoltaic performance of PSCs, Bati et al. used metallic single-walled carbon nanotubes (m-SWCNTs) to make MXene/SWCNT composites [181].The champion device with a configuration of ITO/ SnO 2 :MXene/SWCNT(2:1)/(FAPbI 3 ) x (MAPbBr 3 ) 1−x /Spiro-OMeTAD/Au attained a maximum PCE of 21.42%, a J sc of 25.09 mA cm −2 , a V oc of 1.073 V and an FF of 80% with the optimum composition.The improved PL and reduced charge transfer resistance confirmed a low trap density and improved charge extraction and transport characteristics owing to the improved conductivity facilitated by the presence of carbon nanotubes and decreased oxygen vacancies on the surface of SnO 2 .The MXene/SWCNTs approach offers a promising route to realize high-performance PSCs [125,181].
As discussed earlier, the major causes of PSCs' instability and low PCE are defects at the interfaces of perovskite thin films.Wu et al. used a composite ETL made of SnO 2 -MXene to enhance interfacial contact and passivate defects at the SnO 2 /perovskite interface in PSCs [182].The newly developed MXene controls SnO 2 dispersion and allows the perovskite film to grow vertically.The Lewis acid-base interaction of the -OH functional groups on the MXene surface and Sn atoms in SnO 2 weakens the van der Waals interactions between neighboring crystals, impeding the accumulation of SnO 2 nanocrystals.The lattice matching between MXene and perovskite nullifies the generated interfacial stress and induces vertical growth of perovskites (Fig. 14a).The average grain size of SnO 2 -MXene/perovskite (~ 1 μm) increased compared to the grain size of the control SnO 2 /perovskite (∼356 nm).Additionally, the conductivity of SnO 2 increased from 1.85 × 10 to 9.62 × 10 S cm −1 .This resulted in a higher current in MXene-PSCs (25.07 mA cm −2 ) than in pristine PSCs (24.16 mA cm −2 ).The lifetime of SnO 2 /perovskite (τ 1 = 20.73 ns, τ 2 = 80.24 ns) is longer than that of charge carriers in SnO 2 -MXene/perovskite (τ 1 = 17.34 ns, τ 2 = 48.20 ns).Hence charges are extracted faster in the SnO 2 -MXene ETL than in the pristine SnO 2 ETL.Moreover, the MXene introduction also downshifts the valence band edge to −7.93 eV (SnO 2 -MXene) from −7.72 eV (SnO 2 ), which indicates the suppression of hole recombination and hole migration to the ITO electrode (Fig. 14b).The PCE of the SnO 2 -MXene-based device with a structure of ITO/SnO 2− MXene/Perovskite/Spiro-OMeTAD/Au attained a PCE of 23.07%, with a V oc of 1.13 eV, a J sc of 25.07 mA cm −2 , and an FF of 81.1%.The attained PCE is 15% higher than for the SnO 2 -based device (20.03%).Furthermore, even after 500 h of storage at 30-40% relative humidity in ambient air, the unencapsulated device still retained almost 90% of its initial PCE [182].
Another research group led by Liu et al. used Au@ Nb 2 CT x -MXene in PSCs to nullify charge accumulation and interfacial defects at SnO 2, leading to poor charge transfer from the perovskite layer to the charge transport layer [183].An Nb 2 CT x -MXene multilayer was obtained by Nb 2 AlC via selective etching of Al atoms by HF treatment, and a further MXene monolayer was derived from the multilayer via tetramethylammonium hydroxide (TMAOH) treatment.Further, Au nanoparticles were coated on MXenes to form Au@Nb 2 CT x -MXene (Fig. 14c).In PSCs, Nb 2 CT x -MXene increases conductivity (3.37 × 10 −6 S cm −1 ), thus lowering Sn vacancies in the SnO 2 layer's interstitial void and consequently lowering defect density and aligning the bandgap.With the inclusion of Au NPs, Au@Nb 2 CT x -MXene improves the perovskite film quality, controls the tensile tension of perovskites, and stifles Auger recombination.As a result, the Au@Nb 2 CT x -MXene-modified device exhibits a very high V oc of 1.215 V, a J sc of 24.28 mA cm −2 , an FF of 80.62%, and a PCE of 23.78%, compared to the control device with Nb 2 CT x -MXene (PCE of 20.86%) (Fig. 14d).In addition, the unencapsulated devices preserve 90% of their initial PCE values after being exposed to air with a relative humidity of 40% for 1000 h and continue to function at or above 80% of their initial efficiency after being exposed to sunlight for 500 h (Fig. 14e) [183].
As reported earlier by Zhou et al., 2D Ti 3 C 2 Cl x MXene was inserted into the bulk and surface of CsPbBr 3 film [90].The 2D Ti 3 C 2 Cl x MXene was deposited on the surface of SnO 2 ETL.Inclusion of MXenes into bulk and surface of the device results in decreased defects and strains and leaving more holes for larger voltage (Fig. 15a-b).The defect reduction can be quantified in terms of the reduced trap-filled limit voltage (V TFL ).The V TFL decreases from 0.78 (pristine) to 0.56 V for Ti 3 C 2 Cl x passivated perovskite film and the defect density (n t ) decreased from 5.93 × 10 15 to 1.92 × 10 15 cm −3 , respectively (Fig. 15c).

At the Interfaces Between Perovskites and HTLs
CsPbI 3 inorganic perovskites have garnered much interest for their outstanding thermal stability and suitable bandgap for tandem solar cells.However, due to nonradiative recombination, CsPbI 3 PSCs show low PCEs and a high V oc loss (0.5 vs. 0.3 V for hybrid perovskites) [184].To improve the functionality of CsPbI 3 PSCs, Xu et al. synthesized Ti 3 C 2 F x MQDs with abundant Ti-F groups by a liquid-phase exfoliation technology and used them as interface passivators [185].The Ti 3 C 2 F x MQDs function as efficient passivators primarily in three ways: 1) p-type Ti 3 C 2 F x MQDs can adjust the energy levels of perovskite films and provide a reliable pathway for hole transfer; 2) Ti 3 C 2 F x MQDs can effectively passivate defects and reduce interfacial nonradiative recombination; and  15d).The air stability of Ti 3 C 2 F x MQDs treated CsPbI 3 film is remarkably enhanced after 120 h of storage in air with an RH of 35%, as shown in the photographs of the films (Fig. 15e).In addition, after 600 h of storage in ambient air, the target device without encapsulation maintained 93% of its initial efficiency [185].
Even at ambient temperatures, the perovskites based on formamidinium (FA) suffer an unnecessary spontaneous yellow phase transition yet promise a high PCE in photovoltaics.This has inspired significant efforts, which pose a formidable challenge to the soft perovskite lattice's powerful anchoring.To effectively delay the lattice instability in FA-based perovskites, Guo et al. developed a rational design of interfacial ionic bonding between halogen (F, Cl, Br or I)-terminated nano-MXenes and perovskites [186].They used pulsed laser irradiation to produce the halogenterminated nano-MXenes using the green anti-solvent of ethyl acetate (Fig. 16a).The halognen-terminated nano-MXenes form heterointerfaces between nanocrystals and the perovskite (Fig. 16b-c).The strong heterointerface between perovskite and nano-MXenes also allows for efficient control of the WF of perovskite films, lowering of the interfacial charge transfer barrier, and effective modulation of deep-energy-level defects.The champion device with a structure of FTO/TiO 2 /FAPbI 3 /nano-Ti 3 C 2 T x /Spiro-OMeTAD/Au attained the highest PCE of 24.2%, a J sc of 25.84 mA cm −2 , a V oc of 1.132 V and an FF of 82.6%.This is the highest efficiency utilizing MXene in PSCs reported so far.The stabilized power outputs for the pristine and the modified PSCs are 22.07%and 23.79%, respectively (Fig. 16d).These advantages enable unencapsulated FAbased perovskite solar cells to maintain over 90% of their initial efficiency after operation at maximum power point under continuous illumination for 1000 h and more than 85% of their initial efficiency even after annealing for 1000 h at 85 °C in an inert atmosphere [186].

At the Interfaces Between ETLs and Electrodes
The high annealing temperature (> 450 °C), critical photoinstability of TiO 2 , and spontaneous aggregation of SnO 2 hinder the scale-up of the commercial application of PSCs [187][188][189].Furthermore, inadequate charge extraction and hysteresis are due to the energy-level mismatch between the conduction band minimum (CBM) of SnO 2 and the WF of FTO at the interface.Hence interface engineering is essential for charge extraction in PSCs.Wang et al. introduced a strong interface interacting with 2D carbide MXene to improve the electron mobility and charge transfer ability of SnO 2 ETL (Fig. 16e-f) [190].The MXene-modified SnO 2 ETL induced a preferable growth orientation route for perovskite films.It reduced the trap density and thereby reduced the non-radiative recombination.As shown in Fig. 16g, the MXene-modified device has a few localized defects, and the carrier transport is homogeneously enhanced, as reflected by the current transport efficiency (f T ) values over the active area of the entire device.Furthermore, the modified SnO 2 /perovskite interface provided a better platform for charge extraction and transfer.The PSC with MXene-modified SnO 2 and a device structure of FTO/(0.5 mg mL −1 MXene)/SnO 2 / Perovskite/Spiro-OMeTAD/Au achieved a PCE of 20.7% with ultralow saturated current density and negligible hysteresis.Moreover, the champion PSC demonstrated an excellent shelf lifetime by retaining 90% of its initial PCE after 3 months.However, no significant degradation is observed in the V oc of the PSC with an MXene interlayer (Fig. 16h) [190].
The electrical conductivity of 2D MXene can be adjusted by changing the termination groups and applying controlled oxidation, which has enormous promise for solar applications.Yang et al. added in-situ oxidized Ti 3 C 2 T x (O-Ti 3 C 2 T x ) MXene to create a nanoscale heterojunction with SnO 2 that was used as an ETL in a MAPbI 3 PSC with a device structure of FTO/O-Ti 3 C 2 T x / SnO 2 /MAPbI 3 /Spiro-OMeTAD/Ag [191].The properties of Ti 3 C 2 T x are modified to have their energy levels wellmatched with the perovskite by taking advantage of the change from metallicity to semiconductivity during the oxidation process.Additionally, the O-Ti 3 C 2 T x insertion increased the SnO 2 ETL electron mobility.As a result, the O-Ti 3 C 2 T x /SnO 2 heterojunction provided better electron extraction and decreased recombination between the ETL and the perovskite, increased PCE from 17.7 to 20.1%, and enhanced stability with preserving 80% of its initial PCE in air [191].

Prospects and Conclusions
MXenes have significantly impacted the device performance and stability of the PSCs.The roles and functions of MXenes in PSCs are summarized as shown in Fig. 17.As an application, a detailed overview of MXenes in PSCs starting from the first report in 2018 to recent advances, has been provided.MXenes play multiple roles as different components of PSC.The roles are broadly divided into five categories depending on their functions: additive in perovskite absorber layers, ETLs/additives in ETLs, HTLs/additives in HTL, electrodes, and interfacial layers.As additives in absorbers, MXenes retard the crystallization rate and enlarge the crystal sizes of perovskites by controlling the morphology.Furthermore, MXenes mitigate perovskite lattice instability and passivate the bulk and surface defects, prolonging the carrier lifetimes and reducing charge recombination.In addition, these materials induce hydrophobic features on the surface of the absorber layer and prevent deprotonation of protonated organic amine in perovskites.Most importantly, MXenes can tune the work function of the perovskite absorber layer for efficient charge transfer.The combined effect of these features improves the PCE and stability of the PSCs.Similarly, as/in ETL, MXenes tune the work functions of the ETLs for well-matched band alignment with the perovskite layer to facilitate smooth electron transport.These observations are further supported by the improved electron mobilities and the reduced interface recombination for the ETL/perovskite.In addition, MXenes mitigate the defects in the ETL layer, which is a prerequisite for improved crystallinity of the next perovskite layer.As electrodes, MXenes pose as emerging materials in transparent hybrid electrodes in PSCs.MXenes as/or in HTLs provide a superior hole transfer path, improve hole extraction, and decrease charge transfer resistance at the HTL/perovskite interface via tuning the WF of the HTLs.The presence of MXenes in electrodes provides improved adhesion to the substrates and superior continuous conductive paths for improved electrical conductivity.In addition, because of their low square resistance, MXenes act as promising electrodes.MXenes are also used as additional interfacial layers in ETL/electrodes, perovskite/electrode, perovskite/ETL, and perovskite/HTL layers.These layers help in efficient charge extractions and transfers via improved interfacial energy alignment, reduce recombination (trap defects) at the interfaces, provide a well-controlled growth platform for smooth perovskite films, and passivate perovskite grains for improved film quality.These essential features provide a promising route for highly efficient and stable PSCs.Different roles of MXenes and their photovoltaic performance in PSCs are summarized in Tables 1, 2, 3, 4, and 5. Until now, MXene-assisted PSCs have attained the highest PCE of 24.17% and the highest reported stability time reported so far is 2000 h.In addition, the PSCs with a large area of 0.99 cm 2 and the minimodules with the size of 25 cm 2 are reported so far.However, MXene's applications in perovskite solar cells are in their infancy, and many advancements are yet to come.Several distinct MXenes with varying compositions, surface terminations, band topologies, and WFs have been reported.However, only a limited number of MXenes such as Ti 3 C 2 T x (T x = -O, -OH, -F, -Cl, -Br, -I, -NH 2 ), V 2 CT x , Mo 2 C, and Nb 2 CT x have been reported in PSCs so far.Hence theoretical studies are required to further understand the charge transport/extraction behavior and WF modification of new MXene compositions, MXene/ perovskite, MXene/ETL, and MXene/HTL interfaces.A more detailed theoretical investigation is required to elucidate the optical, electrical, and mechanical properties of MXenes, effects of various termination groups of MXenes, surface modifications, variation of WF, and energy-level  alignment on the photovoltaic parameters of MXene-based PSCs.In addition, different new synthesis routes and posttreatments such as laser irradiation, UV-ozone treatment, etc. need to be explored to obtain high quality pure and functionalized MXenes.MXenes should be transparent in the photovoltaic response range of PSCs in addition to having strong flexibility and high electrical conductivity.In addition, the benefits that MXenes provide in terms of material characteristics and performance should ensure affordable price, environmental compatibility, long-term stability, and sustainable manufacturing in upscaling the production of MXene and its commercial uses in PSCs (small size and large size) and solar cell modules [192].Furthermore, the additional cost due to MXenes inclusion in PSCs needs to be evaluated.As a next step, a plethora of unexplored MXenes materials needs to be studied and optoelectronic properties are to be optimized to improve both PCE and stability of PSCs further.Apart from PSCs, the MXenes have drawn interest in other perovskite-based optoelectronic devices such as photodetectors, photodiodes, phototransistors, and plasmonic, etc. [193].MXenes are used in photodetectors (PDs) in various roles, such as transparent electrodes, Schottky contacts, light absorbers, and plasmonic materials [194].Ti 3 C 2 T x quantum dots (TQDs) are used to boost the charge carriers of 2D perovskite Ca 2 Nb 3 O 10 (CNO) nanosheets (NS) and enhance the performance of the TQD-modified 2D perovskite CNO NS PD.Other such examples are CsPbBr 3 NSs/Ti 3 C 2 T x MXene and CsPbBr 3 NCs/Ti 3 C 2 T x MXenes nanocomposite PDs [195].Similarly, plasmonic Nb 2 CTx MXene-MAPbI 3 heterostructure is reported for self-powered visible-NIR photodiodes [196].In recent years, perovskite solar cells have undergone significant scientific developments.Low-cost, low-temperature solution-based fabrication processes, advances in multi-cation and -anion perovskites, improvements in all-inorganic perovskites, Pb-free perovskites, application of 2D/3D perovskites, defects passivation via solvent

Fig. 2
Fig. 2 Schematics showing a stacking of different layers in a typical PSC and b the prominent ways for improving PCE and stability of PSCs

Fig. 3
Fig. 3 MXenes in PSCs: Various roles of MXenes as components or parts of the components in regular n-i-p PSCs.The device structure of Glass/TCO/ETL/Perovskite/HTL/Electrode with different ILs, positions of MXenes and numbers in the parenthesis indicate the Section numbers.IL stands for "interfacial layer," and TCO stands for "transparent conducting oxide"

Fig. 4 a
Fig. 4 a Schematic of MAPbI 3 planar PSCs with MXenes additives in the absorber layer.b J-V characteristics curves of PSCs with different amounts of Ti 3 C 2 T x [31].Copyright © 2018 WILEY-VCH Verlag GmbH & co.KGaA, Weinheim.c Schematic diagram of PSCs with the structure of Glass/ITO/SnO 2 /2D perovskite/Spiro-OMeTAD/Ag.d J-V curves of PSCs with different amounts of Ti 3 C 2 T x doping [96].Copyright © 2018 Springer.e The atomic structures showing the interface between the MAPbI 3 (100) surface and the Ti 3 C 2 O 2 MXene without Cs + and f with Cs + [97].Copyright © 2021 Cell Press.g Schematic illustration of the device structure.h Reverse and forward scans for the pristine and 0.2 mg mL −1 Ti 3 C 2 Cl x QDs-treated PSC devices.i EQE of the devices (the pristine and 0.2 mg mL −1 Ti 3 C 2 Cl x QDs-treated) [98].Copyright © 2022 Elsevier B.V

Fig. 5 a
Fig. 5 a Mechanism diagram illustrating the preparation of high-quality two-step-processed perovskite films assisted by Ti 3 C 2 T x additive.b Device configuration of the planar PSCs and its energy-level diagram with and without Ti 3 C 2 T x doping.c J-V curves of the champion PSCs with different contents of Ti 3 C 2 T x [106].Copyright © 2021 Elsevier B.V. d Schematic of the PSC with Ti 3 C 2 T x MXene additive in the perovskite absorber layer.e The stability of the PSC devices with and without D149 sensitized TiO 2 and/or Ti 3 C 2 T x doping in ambient air at RH ~ 20% without encapsulation at 25 °C [107].Copyright © 2022 Elsevier B.V shows a schematic illustration of the energy-level diagram for each component of the PSC device.The shift in WF of Ti 3 C 2 T x from − 5.52 to − 5.62 eV after UV-ozone treatment enables faster electron transport.The PCE of PSC increased to 5% in the case of Ti 3 C 2 T x ETL without UV-ozone treatment and to 17.2% with a Ti 3 C 2 T x ETL film after 30 min of UVozone treatment because of an improved electron transfer.The UV-ozone treated Ti 3 C 2 T x ETL suppressed recombination at the ETL/perovskite interface [111].Zhang et al. fabricated 2D Nb 2 CT x MXene nanosheets ETL and incorporated them into the perovskite absorber layer [112].The WF of Nb 2 CT x was tuned from 4.65 to 4.32 eV by replacing its -F groups with -NH 2 groups through hydrazine (N 2 H 4 ) treatment to match the conduction band minimum of the perovskite layer.Additionally, incorporating N 2 H 4 -treated (T-Nb 2 CT x ) MXene nanosheets with abundant NH 2 groups slows the crystallization rate of the perovskite precursor by forming a hydrogen bond with iodine ions, which encourages the creation of high-quality and oriented perovskite films.As a result, the PSC with a device configuration of ITO/Nb 2 CT x /FA 0.85 Cs 0.15 PbI 3 / Spiro-OMeTAD/Ag demonstrated a maximum PCE of 21.79% with T-Nb 2 CT x MXene ETLs and T-Nb 2 CT x MXene nanosheet additive.The highest PCE values of 19.2% and 18.3% were realized for the corresponding flexible and largearea devices (Fig.

Fig. 6 a
Fig. 6 a Device architecture of ITO/ETL/MAPbI 3 /Spiro-OMeTAD/Ag based on Ti 3 C 2 T x with/without UV-ozone treatment as ETL.b Crosssectional SEM image of the PSC device.c Schematic energy-level diagram of each layer [111].Copyright © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.d Schematic device structure of ITO/Nb 2 CT x /FA 0.85 Cs 0.15 PbI 3 /Spiro-OMeTAD/Ag, and.e J-V curves of the flexible and f the large area devices [112].Copyright © 2022 Wiley-VCH GmbH

Fig. 7 a
Fig. 7 a Fabrication process of perovskite layer on MDCN ETL [120].-Copyright © 2020 Springer.b Schematic illustration of the preparation processes of the ETL and the perovskite film.c Schematic diagram of PSC and MXene with Perovskite, efficiency, and defect density values with respect to Ti 3 C 2 T x /SnO 2 wt% [121].Copyright © 2021 American Chemical Society.d Schematic of Cs 2 AgBiBr 6 /Ti 3 C 2 T x @TiO 2 structure with scanning electron microscope (SEM) image illustrating film quality.e J-V curves of planar Cs 2 AgBiBr 6 PSCs based on TiO 2 and different weight percentage of Ti 3 C 2 T x @TiO 2 [122].Copyright © 2021 American Chemical Society

Fig. 8 a
Fig. 8 a Device architecture of ITO/ETL/MAPbI 3 /Spiro-OMeTAD/Ag based on representative SnO 2 -Ti 3 C 2 as the ETL.b Cross-sectional SEM image of the PSC device.c Schematic energy-level diagram of each layer [124].-Copyright © The Royal Society of Chemistry.d Cross-sectional image with ITO/SnO 2 -MH/Perovskite/Spiro/Ag structure.e Energy-level diagram of the ITO/ETL/AL structure based on SnO 2 , SnO 2 -M, SnO 2 -MF, and SnO 2 -MH as the ETLs.f J-V curves of the champion devices based on each ETL under reverse scans for the FAPbI 3 system [126].Copyright © 2022 Cell Press.g Schematic of the planar heterojunction structure used for the SnO 2 -based PSCs.h J-V curves of one of the best SnO 2 -based and MQDs-SnO 2 -based PSCs devices under both forward and reverse scans.i IPCE curves and integrated current density of these two devices [127].Copyright © 2021 The Royal Society of Chemistry

Fig. 9 a
Fig. 9 a A schematic diagram of the device structure and the structure of Nb 2 CT x MXene.b J-V curves of PVSCs measured under different scan directions.c External quantum efficiency (EQE) and integrated J sc curves of different PSCs.d J-V curves of the flexible PSCs utilizing oxygen plasma-treated Nb 2 CT x HTL [134].Copyright © 2021 AIP Publishing.e Schematic representation of spray coating process to form the Ti 3 C 2 T x -SCA film and structural formula of vinyl tris (2-methoxy ethoxy) silane.f Schematic diagram of a device with ITO/HTL/Perovskite/ PC 61 BM/Ag structure.g J-V curve of PSCs based on Ti 3 C 2 T x , with different volume fractions of SCA as HTL.h EQE spectrum and corresponding integrated current density [139].Copyright © 2022 Elsevier

Fig. 11 a
Fig. 11 a Schematic diagram showing the fabrication process of the Ti 3 C 2 electrode by the hot-pressing method.b Cross-sectional SEM image of the PSC based on Ti 3 C 2 electrode.c J-V curves of devices with different thicknesses of the Ti 3 C electrode.d PCE histogram of the PSCs obtained from the measurements of 20 devices [157].Copyright © 2019 The Royal Society of Chemistry.e Schematic structure of the CsPbBr 3 solar cell with mixed carbon electrode.f J-V curves of devices with different types of electrodes [161].Copyright © 2020 The Royal Society of Chemistry

14 - 5 .
65 eV), and metallic conductivity.Chen et al. introduced 2D Ti 3 C 2 MXene nanosheets as an interlayer into all-inorganic CsPbBr 3 PSCs for the first time [160].The MXene interlayer was formed on the FTO/c-TiO 2 /CsPbBr 3 substrate via spin-coating of the Ti 3 C 2 MXene nanosheet dispersion 1 3 solution (Fig. 13a-b).The selected area electron diffraction (SAED) pattern of Ti 3 C 2 MXene nanosheets shows a few highly crystalline layers with the device structure of FTO/TiO 2 /CsPbBr 3 /Ti 3 C 2 MXene/Carbon to achieve a better interfacial energy-level alignment, which aids in the elimination of the energy-level mismatch, speeds up hole extraction, and lowers recombination at the interface of the perovskite/carbon electrode.Additionally, the Ti 3 C 2 MXene nanosheets' surface functional groups offer powerful interactions between the MXene and under-coordinated Pb atoms.This significantly lessens the deep trap defects in the CsPbBr 3 films.The device with the Ti 3 C 2 -MXene interlayer displays an exceptional initial PCE of ~ 9%

Fig. 12 a
Fig. 12 a Schematic of the Ti 3 C 2 T x MXene and the fabrication procedure of AgNW:MXene flexible transparent conductive electrodes.b Flexible PSCs with the structure of PET/AgNW:MXene/NiO x /Perovskite/PC 61 BM/Ag.c J-V curves of the flexible PSCs based on ITO, AgNW, and AgNW:MXene electrodes [176].Copyright © 2022 The Royal Society of Chemistry

Fig. 14 a 3 3)
Fig. 14 a Schematic of PSCs and MXene-PSCs films, MXene induces vertical growth of perovskite and improves the interfacial contact between perovskite and SnO 2 .b Energy level diagram of the device SnO 2 -MXene interface illustrating hindrance of hole transport [182].Copyright © 2022 Wiley.c Structure of PSCs modified by Au@Nb 2 CT x -MXene.d J V curves of the control and Au@Nb 2 CT x -MXene-modified devices measured under the reverse scan.e Stability test of the control and Au@Nb 2 CT x -MXene-modified PSCs in ambient air under RH ∼ 40% for over 1000 h [184].Copyright © 2023 American Chemical Society

Fig. 15 a
Fig. 15 a Schematic diagram of full defect passivation in CsPbBr 3 film by Ti 3 C 2 Cl x MXene.b Illustration for the photogenerated carrier transfer in pristine and Ti 3 C 2 Cl x containing perovskite films.c Dark J-V curves for the hole-only devices with and without Ti 3 C 2 Cl x Mxene [90].Copyright © 2021 Wiley-VCH GmbH.d Schematic image of a CsPbI 3 PSC with the structure of FTO/TiO 2 /CsPbI 3 /Ti 3 C 2 F x /Spiro-OMeTAD/Au.e J-V curves of devices comprising pristine and Ti 3 C 2 F x QDs treated CsPbI 3 films.f Photographs of control and Ti 3 C 2 F x QDs-treated CsPbI 3 films aged in ambient air conditions (RH: ≈35%, T = 25 °C) [185].Copyright © 2022 Wiley

Fig. 16 a
Fig. 16 a Schematic diagram of nano-MXenes prepared via the pulsed laser irradiation.b Schematic of ionic-bonding heterointerface of hybrid perovskite via interfacial embedding of halogen-terminated nano-MXenes, Pb-T based ionic lattice anchoring and carrier dynamics modulation.c High-resolution transmission electron microscopy images and fast fourier transform patterns of nano-Ti 3 C 2 F x .d J-V curves of the champion PSCs based on FAPbI 3 films and the stabilized power outputs of the corresponding devices (insets) [187].Copyright © 2022 Wiley-VCH GmbH.e-f Device configuration and cross-sectional SEM image of the entire device.g Spatially resolved f T maps of MXene-modified devices (scale bar, 1 mm).h Shelf lifetime tracking of the optimized PSC [191].Copyright © 2020 American Chemical Society

Fig. 17
Fig. 17 Positions and functions of MXenes in PSCs Fig. 17 Positions and functions of MXenes in PSCs

Table 3
Role of MXenes as HTL/ additives in HTL in PSCs [The letter "C" in the bracket stands for the control (reference or pristine) device)] engineering, consistent improvements in the photovoltaic performance (PCE and stability) of tandem solar cells, solar concentrators incorporating scattering effects and spectral modification, and flexibility of perovskite solar cells hold the promise for scalability and large-scale commercialization of PSCs with high performance-to-cost ratios.In this review article, we have succinctly analyzed the incorporation of 2D MXene materials into the perovskite absorber layer, ETL/HTL, electrode, and interfaces.High optical transparency, wide tunable work function via functionalization of surface terminal groups, flexibility and superior mechanical properties of MXenes and their optimum concentrations are the key factors in producing high quality and uniform films, reducing trap states density and improving charge carrier extraction and collection capabilities in PSCs.These features present MXenes as tremendous potential candidates for PSC applications.This review highlights the recent advances of MXenes in PSCs that is simply a fractional part, and many key improvements in the MXene-modified PSCs are about to come in the near future.Development Program) (1415172732/20011410, Development of SPD smart film and service of aftermarket for energy saving in building and automobiles) funded By the Ministry of Trade, Industry & Energy (MOTIE, Korea).This research was also supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2018R1A6A1A03023788 and 2021R1I1A1A01055790).Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made.The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material.If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.To view a copy of this licence, visit http:// creat iveco mmons.org/ licen ses/ by/4.0/.