Highlights
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This review discusses the roles of MXenes in different positions/layers in perovskite solar cells.
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The issues in different layers/interfaces and their addressal with the incorporations of MXenes in perovskite solar cells are elaborately discussed.
Abstract
With an excellent power conversion efficiency of 25.7%, closer to the Shockley–Queisser limit, perovskite solar cells (PSCs) have become a strong candidate for a next-generation energy harvester. However, the lack of stability and reliability in PSCs remained challenging for commercialization. Strategies, such as interfacial and structural engineering, have a more critical influence on enhanced performance. MXenes, two-dimensional materials, have emerged as promising materials in solar cell applications due to their metallic electrical conductivity, high carrier mobility, excellent optical transparency, wide tunable work function, and superior mechanical properties. Owing to different choices of transition elements and surface-terminating functional groups, MXenes possess the feature of tuning the work function, which is an essential metric for band energy alignment between the absorber layer and the charge transport layers for charge carrier extraction and collection in PSCs. Furthermore, adopting MXenes to their respective components helps reduce the interfacial recombination resistance and provides smooth charge transfer paths, leading to enhanced conductivity and operational stability of PSCs. This review paper aims to provide an overview of the applications of MXenes as components, classified according to their roles as additives (into the perovskite absorber layer, charge transport layers, and electrodes) and themselves alone or as interfacial layers, and their significant importance in PSCs in terms of device performance and stability. Lastly, we discuss the present research status and future directions toward its use in PSCs.
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1 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 MAPbI3, FAPbI3, and CsPbI3, have been spotlighted due to their excellent bandgap tunability, excellent light absorption co-efficient (> 105 cm−1), good charge carrier mobility (2–40 cm2 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].
Recently, MXenes, as emerging two-dimensional (2D) layered materials, have drawn interest in various applications, including photocatalysts, light emitting diodes, transparent electrodes, nanofiltration, water purification, electromagnetic interference shielding, antibacterial activity, supercapacitors, sensors, and more [14,15,16,17,18,19,20,21,22]. 2D nanomaterials such as graphene, g-C3N4, WS2, MoS2, and black phosphorous have been demonstrated as potential additives in the perovskite absorber layer in PSCs owing to their excellent electrical and optical properties [23,24,25,26,27,28]. MXenes have been used as all-round materials starting from additives, electron transport layers (ETL), hole transport layers (HTL), and interfacial layers to electrodes in PSCs due to their excellent electrical conductivity (2 × 104 S cm−1), high charge carrier density (3.8 × 1022 cm−3), high mobility (1 cm2 V−1 s−1), high transparency, and tunable work function (WF) through controlled surface chemistry [29, 30]. With the introduction of Ti3C2Tx MXene into MAPbI3-based perovskite solar cells in 2018, there has been a tremendous surge in the use of MXenes in the photovoltaic community [31].
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.
2 MXenes in Perovskite Solar Cells
2.1 Synthesis of MXenes
MXenes are 2D transition metal carbides, nitrides, and carbonitrides with the general formula Mn+1XnTx (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 ‘Tx’ stands for the surface terminations including –O, –OH, –F, etc. [32,33,34]. In 2011, Ti3C2Tx 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+1XnTx, 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 of surface functional groups (Tx) 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 Mn+1AXn (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, NH4HF2, 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 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 metal-to-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.
2.2 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 Pb-based 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.
2.3 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 Ti3C2Tx MXene into a CH3NH3Pbl3 (MAPbI3) perovskite absorber layer [31]. This is the first report on an MXene’s application in a PSC. Ti3C2Tx MXene slows the crystallization rate and enlarges the size of MAPbI3 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 (SnO2) 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 Ti3C2Tx 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)/SnO2/MAPbI3:(0.3 wt%) Ti3C2Tx/2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenyl-amine)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 (Voc), current density (Jsc), and fill factor (FF). The increase in Voc is ascribed to the passivation of the perovskites by Ti3C2Tx, spurring the hole extraction and reduced recombination at the perovskite/HTL interface [87]. A schematic of a PSC with Ti3C2Tx additive and J–V curves with 0, 0.01, 0.02, 0.03, 0.5, and 1 wt% of Ti3C2Tx 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 Jsc was increased [31]. All-inorganic CsPbI3 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, CsPbI3 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 CsPbI3 layer with oxidized Ti3C2Tx MXene (OMXene) nanoplates to form effective and stable p-i-n-structured CsPbI3 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 CsPbI3/OMXene dispersion with 10 wt% concentration attained the PCEs of 19.7% for 0.096 cm2 cells and 14.6% for 25 cm2 in minimodules. The pristine cell has a PCE of 18.1% with a Voc, Jsc, 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. Zhou et al. reported an inorganic Cl-terminated Ti3C2 (Ti3C2Clx) MXene to passivate the surface defect states and grain boundaries of CsPbBr3 film. The addition of Ti3C2Clx MXene enlarges the crystal sizes. The solvent volatilization produced the Ti3C2Clx-tailored PbBr2 film with a high porosity [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 CsPbBr3 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 Ti3C2Clx 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-C61-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 Ti3C2Tx MXene nanosheets as additives in 2D RP (BA)2(MA)4Pb5I16 perovskite absorber layer to fabricate the perovskite films with excellent electrical conductivity and mobility [96]. A homogeneous and highly crystalline perovskite film with Ti3C2Tx MXene nano-dopants was formed. After adding an optimal concentration of 0.3 mM Ti3C2Tx nanosheets, the PCE of PSC with a configuration of ITO/SnO2/(BA)2(MA)4Pb5I16-Ti3C2 MXene/Spiro-OMeTAD/Ag increased from 13.7% (without additive) to 15.7%. The Ti3C2Tx nanosheets doped-PSC also exhibited an increased Jsc of 20.87 mA cm−2, Voc 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 Ti3C2Tx nanosheets exhibited excellent stability in ambient settings with an RH of 55% [96].
In 2021, Bati et al. produced Ti3C2Tx MXene nanosheets doped with cesium (Cs) and added them to a lead iodide (PbI2) 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 Ti3C2Tx MXene results in larger crystal grains, long lifetime charge carriers, and reduced charge recombination. The PSCs combined with Cs-doped Ti3C2Tx Mxene (ITO/SnO2/FA1-xMAxPbI3-xBrx:Cs-Ti3C2Tx/Spiro-OMeTAD/Au) achieved a high PCE of 21.6% (19.0% for the pristine device) with increased Voc, Jsc, and FF. The increased Voc 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- Ti3C2Tx 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 Ti3C2Clx quantum dots (QDs) as additives in the perovskite precursor solution [98]. During the film crystallization process, the strong interaction between the Cl terminations of Ti3C2Clx QDs and Pb2+ 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. Ti3C2Clx QDs also quickened charge extraction and benefitted band alignment between the SnO2 ETL and the perovskite layer due to their top–bottom rising gradient distribution. As a result, the PSC with 0.2 mg mL−1 Ti3C2Clx QDs and a device structure of fluorine-doped tin oxide (FTO)/SnO2/Perovskite + Ti3C2Clx QDs/Spiro-OMeTAD/Ag) achieved an improved efficiency of 21.3% (18.8%) and Voc of 1.19 V with minimal hysteresis (Fig. 4g–h). The Cl-terminated Ti3C2 QDs can also prevent potential deprotonation of protonated organic amine in the perovskite, improving the overall stability. The Ti3C2Clx QDs (0.2 mg mL−1) device shows a higher integrated Jsc of 21.60 mA cm−2 than the pristine device (20.64 mA cm−2). The calculated Jsc is less than the observed Jsc 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 PbI2 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 PbI2, and hence compact films are not formed owing to the presence of residual PbI2. As a result, the device performances are degraded [104, 105]. In 2021, to increase the reactivity of PbI2 by generating porous channels, Zhao et al. added 2D monolayer Ti3C2Tx (Tx = –O, –OH, and –F) MXene nanosheets into the PbI2 layer [106]. This modification enhanced the subsequent PbI2 reaction with MAI to transform into MAPbI3 completely. As a result, the quantity of remaining PbI2 in the perovskite film was reduced, and the size of the perovskite grain increased (Fig. 5a). Additionally, Ti3C2Tx 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 Ti3C2Tx were found to interact with the poorly coordinated Pb2+ in perovskites to passivate defects, having a significant impact on reducing hysteresis and inhibiting nonradiative recombination. Finally, by adding 0.03 wt% of the Ti3C2Tx additive, the PSC with a device configuration of FTO/SnO2/MAPbI3:Ti3C2Tx/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 Cs2AgBiBr6 PSC. The reported PCE of Cs2AgBiBr6 is approximately 3%, which limits its photovoltaic capability. Yang et al. used D149 indoline dye to make the TiO2 ETL more sensitive and Ti3C2Tx MXene nanosheets to Cs2AgBiBr6 to improve the crystallization [107]. Cs2AgBiBr6 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 Cs2AgBiBr6 and Ti3C2Tx. The high WF of Ti3C2Tx modifies the Fermi level of the valence band of Cs2AgBiBr6, 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 Cs2AgBiBr6, Cs2AgBiBr6@ Ti3C2Tx (0.01 mg mL−1), D149-Cs2AgBiBr6 and D149-Cs2AgBiBr6@Ti3C2Tx (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 MAPbI3 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%, Voc of ~ 0.81 V, Jsc ~ 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 NiOx HTLs deposited by ion beam sputtering on ITO substrates and the Ti3C2Tx MXene doping of MAPI perovskite layers to increase the PCE of p-i-n PSCs [109]. The 18 nm-thick NiOx films are pinhole-free and have large-scale uniform surface morphology. For non-stoichiometric NiOx, 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/NiOx/MXene-doped MAPI/PC61BM/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 (∼1019 cm−3 eV−1 for the MXene-doped MAPI and ∼1018 cm−3 eV−1 for the undoped-MAPI), which lowers the favorable effect of the total area of bigger MAPI grain boundaries, reducing the Jsc of the MXene-doped devices. The WF drops from −5.26 to −5.32 eV with MXene doping, which raises the Voc 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 Ti3C2Tx and V2CTx MXene to a PbI2 precursor solution to fabricate perovskite films [110]. The addition of Ti3C2Tx and V2CTx 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 Ti3C2Tx and V2CTx 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 V2CTx additive, indicating that the V2CTx additive controls the formation of perovskite crystal films. Due to the improved crystallinity and perovskite film quality, the V2CTx 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 Ti3C2Tx as additive by 47.2%. Thus, V2CTx in photovoltaics offers a practical way to enhance the performance of PSCs [110].
2.4 MXenes as ETLs or Additives in ETL
2.4.1 MXenes as ETLs
Yang et al. used Ti3C2Tx MXene nanosheets as a novel kind of ETL in planar structured PSCs that have undergone low-temperature processing [111]. The metallic Ti3C2Tx 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/Ti3C2Tx/MAPbI3/Spiro-OMeTAD/Ag are displayed in Fig. 6a–b, respectively. The thickness of the Ti3C2Tx layer is found to be 18 ± 3 nm. Figure 6c shows a schematic illustration of the energy-level diagram for each component of the PSC device. The shift in WF of Ti3C2Tx 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 Ti3C2Tx ETL without UV-ozone treatment and to 17.2% with a Ti3C2Tx ETL film after 30 min of UV-ozone treatment because of an improved electron transfer. The UV-ozone treated Ti3C2Tx ETL suppressed recombination at the ETL/perovskite interface [111].
Zhang et al. fabricated 2D Nb2CTx MXene nanosheets ETL and incorporated them into the perovskite absorber layer [112]. The WF of Nb2CTx was tuned from 4.65 to 4.32 eV by replacing its –F groups with –NH2 groups through hydrazine (N2H4) treatment to match the conduction band minimum of the perovskite layer. Additionally, incorporating N2H4-treated (T-Nb2CTx) MXene nanosheets with abundant NH2 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/Nb2CTx/FA0.85Cs0.15PbI3/Spiro-OMeTAD/Ag demonstrated a maximum PCE of 21.79% with T-Nb2CTx MXene ETLs and T-Nb2CTx MXene nanosheet additive. The highest PCE values of 19.2% and 18.3% were realized for the corresponding flexible and large-area devices (Fig. 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 Nb2CTx MXene in photoelectric devices [112].
Despite many benefits of 2D Ti3C2Tx 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 TiO2- or SnO2-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 Ti3C2Tx 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 Jsc 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 Ti3C2Tx 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. Ti3C2Tx can be oxidized to produce Ti–O bonds and significantly minimize the macroscopic defects in a spin-coated film. However, after substantial oxidation, the material changes from metallic to semiconductor. In the case of a hybrid of oxidized and pristine Ti3C2Tx, 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].
2.4.2 MXenes as Additives in ETLs
SnO2 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, TiO2 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 SnO2 nanoparticle-modified TiO2 (SnO2@TiO2) composite ETL processed at low temperature shows a high PCE of 21.3%. However, the amorphous nature of TiO2 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 TiO2, SnO2, and Ti3C2Tx 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 TiO2 quantum dots are in-situ rooted on three-dimensional SnO2 nanoparticles, forming nanoscale TiO2/SnO2 heterojunctions, and are surrounded by 2D conductive Ti3C2Tx sheets. The fabrication process for the (FAPbI3)0.97(MAPbBr3)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 N2 (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 N2 attained a champion PCE of 19.1%, compared to the PCE of 16.8% for the pristine device with SnO2 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 SnO2 nanoparticles and Ti3C2Tx MXene nanoflakes as ETL in a planar regular-structure PSC [121]. The SnO2/MXene colloidal suspension is spun onto ITO substrates to form the ETL, and then the perovskite layer (MA0.15FA0.85PbIxBr3-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 Ti3C2Tx/SnO2 ratios (0–2.2 wt%) in ETLs. Compared to a pristine device with SnO2 ETL, the defect density in perovskite films with an optimized 1.4 wt% hybrid ETL is significantly reduced from 5.65 × 1015 to 2.25 × 1015 cm3. 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% Ti3C2Tx/SnO2 ratio is boosted significantly from 16.28 to 20.35%, along with Jsc and FF increasing from 20.65 and 0.71 to 23.65 mA cm−2 and 0.76 (Fig. 7c). However, Voc (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 APbX3 structure and lead toxicity, the inorganic Cs2AgBiBr6 double perovskite structure is a viable development route for PSCs. However, the optoelectronic application is severely constrained by the low Jsc and PCE owing to the low crystallization of Cs2AgBiBr6. Li et al. used a straightforward method to dope single-layered MXene nanosheets into titania (Ti3C2Tx@TiO2) to serve as a versatile ETL for stable and effective Cs2AgBiBr6 double PSCs [122]. In addition to considerably increasing TiO2 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 Cs2AgBiBr6 double perovskite in solar cell devices. The Cs2AgBiBr6 double perovskite films based on both TiO2 and Ti3C2Tx@TiO2 ETLs have a similar grain size of 200–500 nm (Fig. 7d–e). The TiO2-based Cs2AgBiBr6 film contains large voids, which are pathways for current leakage. In contrast, the Ti3C2Tx@TiO2-based Cs2AgBiBr6 film is smooth and void-free. As a result, compared to a device based on TiO2 ETL, the PCE increased by more than 40% to 2.8%, and the hysteresis was significantly reduced. Additionally, the Ti3C2Tx@TiO2 device demonstrated long-term operational stability. The Ti3C2Tx@TiO2 device retained 93% of its initial PCE even after 15 days of storage in ambient air [122]. Saranin et al. doped MXene in both MAPbI3 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 Ti3C2Tx MXene [123]. A [6, 6]-phenyl-C61-butyric acid methyl ester (MPC61BM)-based ETL with MXene doping has improved electrical conductivity and band alignment at the ETL/perovskite interface. The n-doping of PC61BM was confirmed by a red shift of the Ag (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-PC61BM as the ETL attained a higher PCE of over 18% than control devices using PC61BM 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/NiOx/MXene/MAPbI3/PC61BM/BCP/Ag attained a maximum PCE of 15.99% with a maximum Voc, Jsc, and FF of 1.05 V, 20.74 mA cm−2, and 74%, respectively. In comparison, the PSC with a standalone Ti3C2Tx MXene ETL showed a maximum of 2.06%, with a maximum Voc, Jsc, 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 SnO2-Ti3C2 MXene (Ti3C2 Mxene + SnO2) nanocomposites with varying Ti3C2 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/SnO2 + Ti3C2Tx/MAPbI3/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 SnO2 ETL to −4.63 eV for SnO2 + Ti3C2Tx ETL, facilitating fast electron transfer. The PCE of the device fabricated with pure Ti3C2 as the ETL increases from 17.2 to 18.3% when SnO2 and 1.0 wt% Ti3C2 are combined. The metallic Ti3C2 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 Nb2C MXenes as an additive to SnO2 ETL for the first time, which caused the SnO2 grains to develop clearly [125]. The incorporation of Nb2C MXenes leads to an increase in lattice spacing of (101) and (110) planes corresponding to SnO2 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 SnO2 ETL to 11.1 and 256 nm for the SnO2-Nb2C ETL, respectively. In addition, the Nb2C inclusion in SnO2 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 SnO2 ETLs increased from 2.3 × 10−5 to 1.39 × 10−4 cm2 V−1 s−1 for the SnO2-Nb2C ETLs, indicating faster electron transfer from perovskites to ETLs. Hence, the PSCs based on the architecture of ITO/SnO2-Nb2C/Perovskite/Spiro-OMeTAD/Ag exhibited the best PCE of 22.86% (18.96% for the control device) with improved Voc, Jsc 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 SnO2 ETL to fabricate PSC devices with modified ETL layers, respectively, denoted as SnO2-MF and SnO2-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 SnO2-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 SnO2. 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 SnO2, ITO and perovskite layers are sketched in Fig. 8d–e. The devices based on the SnO2-MH ETL with the device architecture of ITO/SnO2-MH/FAPbI3/Spiro/MoOx/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 Jsc of 25.49 mA cm−2, a Voc 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 SnO2 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. Ti3C2Tx quantum dots (MQDs) were introduced into SnO2 ETL to investigate the crystallization kinetics of the perovskite. It was discovered that perovskite nucleation from the precursor solution could be induced quickly by Ti3C2Tx MQDs-modified SnO2 (MQDs-SnO2) 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 Jsc of 24.96 mA cm−2, a Voc of 1.172 V, and an FF of 0.798 were attained for PSCs with a device structure of ITO/SnO2-MQD/Perovskite/Spiro/MoOx/Au (Fig. 8g–h) by taking advantage of the excellent charge extraction capabilities of the MQDs-SnO2 layer. The PSC with MQDs-SnO2 ETL has higher incident photon-to-electron conversion efficiency (IPCE) over the entire visible region compared to the pristine device. The integrated Jsc value of 24.39 mA cm−2 is very close to the observed Jsc 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].
2.5 MXenes as HTLs or Additives in HTLs
2.5.1 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 NiOx 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. Nb2CTx MXene is a promising HTM due to good conductivity and an adjustable WF [131, 132]. Furthermore, the Nb2CTx MXene has three atomic layers compared to the five atomic layers of the Ti3C2Tx MXene (excluding atomic layers of surface functional groups). Hence Nb2CTx 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 Nb2CTx MXene. However, the WF can be controlled depending on the type and quantity of the surface-terminated functional groups [59, 133]. As observed from First-principles calculations, the WF of the pristine Nb2CTx nanosheets with –O, –OH, and –F surface functional groups is approximately 4.7 eV, which is not enough to transport holes across the Nb2CTx/MAPbI3 interface. Commonly, O-terminated Nb2CTx 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 Nb2CTx MXene by oxygen plasma treatment and used it as the HTL in inverted PSCs [134]. The oxygen plasma-treated Nb2CTx MXene nanosheets showed enhanced conductivity and high transmittance. The oxygen plasma treatment modified the WF of Nb2CTx from 4.68 eV (pristine) to 5.04 eV by enhancing the O functional groups on the Nb2CTx surface. In other words, the high electronegative O atoms pull electrons from Nb atoms, thus reducing the Fermi level of Nb2CTx. 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 Nb2CTx. 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 Jsc. Furthermore, the photoluminescence (PL) spectra from the perovskite film with the oxygen plasma treated Nb2CTx (Nb2CTx/MAPbI3) indicated the most significant PL intensity quenching, suggesting an improved hole extraction ability of Nb2CTx. From time-resolved PL spectra, the average PL lifetime is found to decrease from 16.51 ns (glass/MAPbI3) to 2.26 ns (ITO/MAPbI3), 1.64 ns (ITO/Nb2CTx/MAPbI3), and 0.72 ns (ITO/treated Nb2CTx/MAPbI3), which further confirms the excellent hole transportability of Nb2CTx. The PSC with the plasma-treated Nb2CTx MXene HTL with a device structure of ITO/Nb2CTx/MAPbI3/PCBM/Ag exhibits a higher PCE of 20.74% with increased Voc and Jsc than those with the pristine Nb2CTx (18.1%) and without Nb2CTx MXene HTLs (15.5%). The higher external quantum efficiency (EQE) values indicate the enhanced Jsc, suggesting more efficient charge separation and collection efficiency. The PSCs with oxygen plasma-treated Nb2CTx HTLs attain the highest EQE and integrated Jsc (22.75 mA cm−2), close to the Jsc values obtained from the J–V measurements (Fig. 9c). Additionally, the flexible and large area (0.99 cm2) PSCs with oxygen plasma treated Nb2CTx HTLs exhibit the highest PCE of 17.3% and 17.9%, respectively. Moreover, the Nb2CTx 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 Ti3C2Tx 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 Ti3C2Tx-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 Ti3C2Tx film was produced with a mixing content of 0.3 V/V% by utilizing SCA to efficiently adjust the interface distribution of Ti3C2Tx after film fabrication. The superior production of Ti3C2Tx films enabled better hole transfer paths, improved hole extraction (hole mobility increases from 1.30 × 10−6 to 3.08 × 10−7 cm2 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 Ti3C2Tx HTL and Ti3C2Tx + 0.3% SCA HTL. As a result, the device with the structure of ITO/ Ti3C2Tx-0.3SCA/MAPbI3/PC61BM/Ag exhibited an increased PCE from 11.1% (pristine Ti3C2Tx) to 13.7%. The schematic of the PSC and the J–V curves of PSCs with Ti3C2Tx-SCA, having different concentrations of SCA, are shown in Fig. 9f–g. However, the Voc 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 Ti3C2Tx [38, 39]. Additionally, the calculated EQE (17.75 and 19.94 mA cm−2.) are well consistent with the observed Jsc 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 Ti3C2Tx-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].
2.5.2 MXenes as Additives in HTLs
As stated earlier, apart from the complexity of synthesis and purification, the poor conductivity, low carrier mobility, resistance to processing solvents, and low transmittance in the UV–vis range of the inorganic HTMs such as CuOx, Fe2O3, CuSCN, and NiOx and the organic-based polymers poly (3-hexylthiophene) P3HT, poly(triaryl amine) (PTAA), modified fluorene–dithiophene (FDT), and poly[2,5-bis(2-decyldodecyl)-pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione-(E)-1,2-di(2,2′-bithiophen-5-yl)-ethene] (PDPPDBTE) limit the commercial applications as HTLs in PSCs [140,141,142,143,144,145,146]. Solution-processed poly (3,4-ethylene dioxythiophene):poly- (styrenesulfonate) (PEDOT:PSS) is used as a common HTL due to its high conductivity, appropriate WF, and high transparency. However, its acidic and hygroscopic nature leads to a low efficiency and stability of PSCs [147].
In contrast, transition metal carbide (Mo2C) and carbon nanotubes (CNTs) are promising HTL candidates owing to their high metallic conductivity, higher charge extraction probability, and tunable WF [148,149,150,151,152]. Hussain et al. combined Mo2C and CNTs to form a conductive Mo2C-CNT hybrid network for the first time with a PEDOT:PSS HTL in PSCs with a device configuration of ITO/Mo2C-CNT@PEDOT:PSS/MAPbI3/PCBM/LiF [153]. As seen from the energy-level diagram in Fig. 10a, the absorbed light produces electron–hole pairs as the light is incident on the active layer. The pairs get separated and move toward the ETL and HTL. The modified PEDOT:PSS with Mo2C-CNT act as exciton dissociation centers and facilitate fast charge separation between MAPbI3 and ITO and fast charge transfer to the anode. The schematic of the fabricated PSC with the modified HTL, 1.5 wt% Mo2C-CNT@PEDOT:PSS HTL, of PEDOT:PSS and Mo2C-CNT is shown in Fig. 10b. The PSC with the modified HTL exhibits the highest PCE of 12% compared to 9.2% (Pure PEDOT:PSS), 9.82% (Mo2C@PEDOPT:PSS) and 10.61% (CNT@PEDOPT:PSS) under AM 1.5 G illumination at 100 mW cm−2 (Fig. 10c). Additionally, the pure and Mo2C, CNT, and Mo2C-CNT blended PEDOT:PSS HTLs showed a conductivity of 410.12, 581.73, 604.25, and 712.34 S cm−1, respectively, thus evidencing high charge carrier collection/extraction and low internal resistance [148]. In further work, Hussain et al. decorated WO3 nanoparticles on 2D conductive Ti3C2Tx MXene sheets to fabricate an MXene/WO3 hybrid structure and then blended HTL with PEDOT:PSS for PSCs using a simple solution process [154]. The n-type WO3 semiconducting material is a promising HTL candidate owing to high electron mobilities (10–20 cm2 V−1 s−1), a tunable bandgap (2.7–3.9 eV), inexpensiveness, high stability against moisture, and possible fabrication at room temperature [6]. On the other hand, Ti3C2Tx MXene possesses high electrical conductivity (2 × 104 S cm−1) and high mobility of 1 cm2 V−1 s−1 [28, 30]. An MXene/WO3 hybrid structure with 1, 2, and 3 wt% was used with PEDOT:PSS to change the perovskite hybrid module's HTL for a highly efficient planar solar cell with a configuration of glass/ITO/MXene/WO3@PEDOPT:PSS/MAPbI3/PCBM/LiF/Al. The corresponding energy band diagram and schematic are shown in Fig. 10d–e. The energy band level of MXene/WO3 is well matched with the perovskite active layer and forms good interfacial contact. The fabricated perovskite solar cell using the optimal device configuration of 2 wt% MXene/WO3/PEDOT:PSS HTL achieves the highest PCE of 12.26 ± 0.12%. The PCEs of PSCs with pure PEDOT:PSS, WO3/PEDOT:PSS, and WO3/PEDOT:PSS are 9.19 ± 0.12%, 10.13 ± 0.14% and 11.42 ± 0.13%, respectively. Furthermore, the PSCs with pure MXene, WO3, and MXene/WO3 nanostructure as HTLs show low PCEs of 6.51%, 6.68%, and 6.87%, respectively. Additionally, the RMS surface roughness of the MXene/WO3 composite with PEDOT:PSS decreases to 21.71 nm for WO3 (35.79 nm) and MXene (34.23 nm) blended PEDOT:PSS, indicating better adaptable surface behavior with finely dispersed nanoparticles and reduced domain sizes, which results in enhanced interfaced characteristics. The MXene/WO3 heterostructure also showed high crystallinity, low shunt resistance ~ 5004 ± 68 Ω cm2, and series resistance ~ 157 ± 2 Ω cm2. These results suggest better exciton separation and passivation of trapping centers by the densely blended smooth MXene/WO3 thin films. These blended nanostructures efficiently tune the HTL/perovskite interface through their active interface for charge transfer and collection and suitably alter energy band alignment, resulting in facile charge extraction and the shortest charge carrier lifetime [154].
2.6 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, NiOx, 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 (Ti3C2) with an energy level comparable to carbon materials as back electrodes in HTM and noble-metal-free MAPbI3 PSCs [157]. Using a straightforward hot-pressing technique at 85 °C and 0.4 MPa, a seamless interfacial contact between the MAPbI3 perovskite layer and Ti3C2 material was achieved, as seen in Fig. 11a–b. The WF of the Ti3C2 material was found to be 4.96 eV and matches well with the valence band (5.4 eV) of the MAPbI3 layer. The hole charge carriers easily transfer from the perovskite layer to the Ti3C2 electrode and the electrons from the TiO2 layer to the FTO electrode. Furthermore, the square resistance decreased from 30.93 to 25.34 Ω sq−1 as the thickness of the Ti3C2 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 Ti3C2 electrode and the perovskite layer, the champion PSC based on this electrode with the device structure of FTO/TiO2/MAPbI3/Ti3C2Tx exhibited a PCE of 13.83% with a Voc of 0.95 V, a Jsc 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 good reproducibility in terms of manufacturability without significant batch-to-batch variations was also observed in the fabrication of PSCs based on Ti3C2 electrodes (Fig. 11d). The device also demonstrated greater stability than the conventional FTO/compact (c) and mesoporous (m) -TiO2/Perovskite/Spiro-OMeTAD/Au device for 360 h when stored at room temperature at an RH of 30%. The Ti3C2 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 Ti3C2Tx 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 Ti3C2 MXene into the carbon paste and formed a mixed carbon electrode for the inorganic CsPbBr3 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-TiO2/CsPbBr3/pure carbon exhibits a PCE of 5.9%. For the inorganic CsPbBr3 PSC with the mixed carbon electrode, the champion device with the configuration of FTO/c-TiO2/CsPbBr3/mixed carbon achieves a decent PCE of 7.09% with a Voc of 1.357 V, a Jsc 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 from 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 graphene-based devices [174]. Ti3C2Tx 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 Ti3C2Tx MXene nanosheets to fabricate composite Ti3C2Tx 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 (12.09 Ω 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/NiOx/Perovskite/PC61BM/Ag (Fig. 12b) and an area of 0.1 cm2 show a high PCE of 20.22% (18.7% for the cell with pristine AgNWs only) with Voc of 1.06 V, Jsc of 25.16 mA cm−2, and FF of 75.5% (Fig. 12c) [176].
3 MXenes as Interfacial Layers
3.1 At the Interfaces Between Perovskites and Electrodes
MXenes are promising candidates in photovoltaics because of their high transmittance, tunable WF (2.14–5.65 eV), and metallic conductivity. Chen et al. introduced 2D Ti3C2 MXene nanosheets as an interlayer into all-inorganic CsPbBr3 PSCs for the first time [160]. The MXene interlayer was formed on the FTO/c-TiO2/CsPbBr3 substrate via spin-coating of the Ti3C2 MXene nanosheet dispersion solution (Fig. 13a–b). The selected area electron diffraction (SAED) pattern of Ti3C2 MXene nanosheets shows a few highly crystalline layers with the device structure of FTO/TiO2/CsPbBr3/Ti3C2 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 Ti3C2 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 CsPbBr3 films. The device with the Ti3C2-MXene interlayer displays an exceptional initial PCE of ~ 9% (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].
3.2 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 Ti3C2Tx MXene (Tx = –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 Ti3C2Tx to halide perovskite and TiO2 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 Ti3C2Tx 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 Voc of 1.09 V, a Jsc 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 Ti3C2Tx QDs (TQDs) to engineer a perovskite/TiO2 ETL interface and perovskite absorber and introduced Cu1.8S 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. Cu1.8S improves the hole extraction at perovskite/HTL interfaces. With the improved Jsc, Voc, and FF because of the synergistic effect of both TQD/Cu1.8S, the hysteresis-free PCE of PSCs significantly increased from 18.3 to 21.6% (Voc of 1.14 V, Jsc of 24.12 mA cm−2, FF of 78.70%) (Fig. 13e). The TQD and/or Cu1.8S 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 Cu1.8S can function as ultrafast electron and hole tunnels for optoelectronic devices. The trap-filled limit voltage (VTFL) values of FTO/TiO2/Perovskite/PCBM/Ag (control), FTO/TiO2/MQD/Perovskite/PCBM/Ag (S1), and FTO/TiO2/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 × 1016 cm−3 (control), 1.28 × 1016 cm−3 (S1), and 1.15 × 1016 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 Ti3C2Clx MXene to the bulk and surface of the CsPbBr3 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 Ti3C2 immobilizes the lattice. This compression and confinement result from the strong interaction between Cl atoms in Ti3C2Clx and the under-coordinated Pb2+ in the CsPbBr3 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 CsPbBr3 PSC is finally able to attain the PCE as high as 11.08% with an ultrahigh Voc 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/SnO2:MXene/SWCNT(2:1)/(FAPbI3)x(MAPbBr3)1−x/Spiro-OMeTAD/Au attained a maximum PCE of 21.42%, a Jsc of 25.09 mA cm−2, a Voc 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 SnO2. 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 SnO2-MXene to enhance interfacial contact and passivate defects at the SnO2/perovskite interface in PSCs [182]. The newly developed MXene controls SnO2 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 SnO2 weakens the van der Waals interactions between neighboring crystals, impeding the accumulation of SnO2 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 SnO2-MXene/perovskite (~ 1 μm) increased compared to the grain size of the control SnO2/perovskite (∼356 nm). Additionally, the conductivity of SnO2 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 SnO2/perovskite (τ1 = 20.73 ns, τ2 = 80.24 ns) is longer than that of charge carriers in SnO2- MXene/perovskite (τ1 = 17.34 ns, τ2 = 48.20 ns). Hence charges are extracted faster in the SnO2-MXene ETL than in the pristine SnO2 ETL. Moreover, the MXene introduction also downshifts the valence band edge to −7.93 eV (SnO2-MXene) from −7.72 eV (SnO2), which indicates the suppression of hole recombination and hole migration to the ITO electrode (Fig. 14b). The PCE of the SnO2-MXene-based device with a structure of ITO/SnO2− MXene/Perovskite/Spiro-OMeTAD/Au attained a PCE of 23.07%, with a Voc of 1.13 eV, a Jsc of 25.07 mA cm−2, and an FF of 81.1%. The attained PCE is 15% higher than for the SnO2-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@Nb2CTx-MXene in PSCs to nullify charge accumulation and interfacial defects at SnO2, leading to poor charge transfer from the perovskite layer to the charge transport layer [183]. An Nb2CTx-MXene multilayer was obtained by Nb2AlC 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@Nb2CTx-MXene (Fig. 14c). In PSCs, Nb2CTx- MXene increases conductivity (3.37 × 10−6 S cm−1), thus lowering Sn vacancies in the SnO2 layer's interstitial void and consequently lowering defect density and aligning the bandgap. With the inclusion of Au NPs, Au@Nb2CTx-MXene improves the perovskite film quality, controls the tensile tension of perovskites, and stifles Auger recombination. As a result, the Au@Nb2CTx-MXene-modified device exhibits a very high Voc of 1.215 V, a Jsc of 24.28 mA cm−2, an FF of 80.62%, and a PCE of 23.78%, compared to the control device with Nb2CTx- 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 Ti3C2Clx MXene was inserted into the bulk and surface of CsPbBr3 film [90]. The 2D Ti3C2Clx MXene was deposited on the surface of SnO2 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 (VTFL). The VTFL decreases from 0.78 (pristine) to 0.56 V for Ti3C2Clx passivated perovskite film and the defect density (nt) decreased from 5.93 × 1015 to 1.92 × 1015 cm−3, respectively (Fig. 15c).
3.3 At the Interfaces Between Perovskites and HTLs
CsPbI3 inorganic perovskites have garnered much interest for their outstanding thermal stability and suitable bandgap for tandem solar cells. However, due to nonradiative recombination, CsPbI3 PSCs show low PCEs and a high Voc loss (0.5 vs. 0.3 V for hybrid perovskites) [184]. To improve the functionality of CsPbI3 PSCs, Xu et al. synthesized Ti3C2Fx MQDs with abundant Ti–F groups by a liquid-phase exfoliation technology and used them as interface passivators [185]. The Ti3C2Fx MQDs function as efficient passivators primarily in three ways: 1) p-type Ti3C2Fx MQDs can adjust the energy levels of perovskite films and provide a reliable pathway for hole transfer; 2) Ti3C2Fx MQDs can effectively passivate defects and reduce interfacial nonradiative recombination; and 3) Ti3C2Fx MQDs form a barrier layer. As a result, the champion CsPbI3 PSC treated with Ti3C2Fx MQDs (FTO/TiO2/CsPbI3/Ti3C2Fx MQDs/Spiro-OMeTAD/Au) displays excellent PCE of 20.4% (18.4% for the pristine) with high Voc of 1.22 V, Jsc of 20.59 mA cm−2, and FF of 81.5% (Fig. 15d). The air stability of Ti3C2Fx MQDs treated CsPbI3 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 halogen-terminated 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/TiO2/FAPbI3/nano-Ti3C2Tx/Spiro-OMeTAD/Au attained the highest PCE of 24.2%, a Jsc of 25.84 mA cm−2, a Voc 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 FA-based 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].
3.4 At the Interfaces Between ETLs and Electrodes
The high annealing temperature (> 450 °C), critical photo-instability of TiO2, and spontaneous aggregation of SnO2 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 SnO2 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 SnO2 ETL (Fig. 16e–f) [190]. The MXene-modified SnO2 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 (fT) values over the active area of the entire device. Furthermore, the modified SnO2/perovskite interface provided a better platform for charge extraction and transfer. The PSC with MXene-modified SnO2 and a device structure of FTO/(0.5 mg mL−1 MXene)/SnO2/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 Voc 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 Ti3C2Tx (O-Ti3C2Tx) MXene to create a nanoscale heterojunction with SnO2 that was used as an ETL in a MAPbI3 PSC with a device structure of FTO/O-Ti3C2Tx/SnO2/MAPbI3/Spiro-OMeTAD/Ag [191]. The properties of Ti3C2Tx are modified to have their energy levels well-matched with the perovskite by taking advantage of the change from metallicity to semiconductivity during the oxidation process. Additionally, the O-Ti3C2Tx insertion increased the SnO2 ETL electron mobility. As a result, the O-Ti3C2Tx/SnO2 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].
4 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 cm2 and the minimodules with the size of 25 cm2 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 Ti3C2Tx (Tx = –O, –OH, –F, –Cl, –Br, –I, –NH2), V2CTx, Mo2C, and Nb2CTx 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 post-treatments 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]. Ti3C2Tx quantum dots (TQDs) are used to boost the charge carriers of 2D perovskite Ca2Nb3O10 (CNO) nanosheets (NS) and enhance the performance of the TQD-modified 2D perovskite CNO NS PD. Other such examples are CsPbBr3 NSs/Ti3C2Tx MXene and CsPbBr3 NCs/Ti3C2Tx MXenes nanocomposite PDs [195]. Similarly, plasmonic Nb2CTx MXene-MAPbI3 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 engineering, compositional engineering, interface engineering, 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.
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Acknowledgements
This research was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (No. 2022M3J7A1062940 and 2022033777) and supported by Korea Electric Power Corporation (Grant Number: R21XO01-5). This work was supported by the Technology Innovation Program (or Industrial Strategic Technology 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).
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Palei, S., Murali, G., Kim, CH. et al. A Review on Interface Engineering of MXenes for Perovskite Solar Cells. Nano-Micro Lett. 15, 123 (2023). https://doi.org/10.1007/s40820-023-01083-9
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DOI: https://doi.org/10.1007/s40820-023-01083-9