Engineering of the perovskite/electron-transporting layer interface with transition metal chalcogenides for improving the performance of inverted perovskite solar cells

Abstract

Layered two-dimensional (2D) transition-metal chalcogenides (TMCs) attract substantial interest across multiple disciplines due to their unique properties. In perovskite solar cells (PSCs), researchers have extensively explored the integration of 2D TMCs to enhance device power conversion efficiency (PCE) and stability. However, there is a research gap in understanding their impact on inverted (p–i–n) PSCs, especially at the perovskite/electron-transporting layer (ETL) interface. This study addresses this gap by investigating the effect of inserting InSe, MoSe₂, and SnS₂ nanosheets at the perovskite/ETL interface in inverted PSCs. The introduction of 2D TMC interlayers induces a downward shift in perovskite energy levels, optimizing the energy level alignment at the perovskite/ETL interface and substantially increasing the PCE. The SnS₂-incorporating PSCs exhibit the highest relative improvement of 5.05% (InSe and MoSe₂ nanosheets yield 3.37% and 2.5% PCE increase, respectively). This enhancement results in an absolute PCE of 18.5% with a fill factor exceeding 82%. Furthermore, the incorporation of InSe nanosheets eliminates the burn-in phase enhancing the long-term stability (T₇₀ of 250 h) of unencapsulated devices. This study underscores the significant improvement in PSCs' PCE and stability by selectively incorporating suitable TMCs at the perovskite/ETL interface. This research offers insights into the potential role of TMCs in advancing PSCs.

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Introduction

Transition metal chalcogenides (TMCs) represent a highly promising class of materials with a wide range of applications. These materials exhibit exceptional optical, electrical, catalytic, and mechanical properties, making them suitable for numerous fields, including batteries,^1,2 supercapacitors,³, (photo)catalysis,^4–8, (photo)transistors^9–11 and other optoelectronic devices.^12–17 One feature of TMCs is their band gap tunability, which makes them ideal for adjustable light absorption, e.g., serving as the photoactive layer in photovoltaic applications. However, photovoltaic devices using TMC-based photoactive layers currently exhibit insufficient power conversion efficiency (PCE) just above 5%.^18,19 When integrated with other photoactive layers as interlayers or additives, TMCs have been reported to enhance the performance of solar cells by finely tuning specific device characteristics, ultimately improving the overall performance.²⁰ For instance, MoS₂ has been successfully integrated into both Si-based^21,22 and organic solar cells,²³ improving significantly the cell performance. The role of TMCs depends specifically on photovoltaic technologies, for each of which a systematic investigation of the role of TMCs is required for reliable advancements in this field.²⁴

In this context, the incorporation of TMCs, typically in their two-dimensional (2D) forms (i.e., nanosheets), in perovskite solar cells (PSCs) has been investigated in various studies.^24–26 In particular, MoS₂ was deposited on the perovskite in normal (n–i–p) PSCs, acting as an interlayer between the perovskite and the hole-transporting layer (HTL) to enhance the device PCE and stability by improving the hole extraction processes and offering a physical barrier against ion migration from the metal contact to the perovskite and vice versa.^27–29 In a similar study, MoSe₂ nanosheets were also screened together with MoS₂ ones to form thin interlayers between the perovskite and the HTL in normal PSC structures, enhancing the performance of reference devices.^30–32 MoS₂ was also used in combination with graphene-incorporating electron transporting layers (ETLs) and MXene-incorporating perovskites, testifying the effectiveness of 2D material interface engineering.^33,34 Also, TMCs have been proposed as charge-transporting layers to completely replace conventional materials of PSCs.^24,35 Specifically, MoS₂ and WS₂ were used as HTLs in inverted PSCs showing similar PCEs to the references based on poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS).^36,37 SnS₂ was used as the ETL in PSCs, improving the PCE up to 20%.³⁸ Despite its n-type behavior, MoSe₂ was used as an HTL in an inverted PSC,³⁹ showing a high PCE of 18%. The mixing of PEDOT:PSS with MoS₂, as well as other dichalcogenides, was used to increase the PCE of inverted PSCs.^40–42 Dichalcogenides, including MoS₂, but also other TMCs, have also been proposed to modify the electron-transporting layer (ETL)/metal electrode interface, aiming at creating favorable energy level alignments to efficiently collect photogenerated electrons.^26,43–45

Despite the tremendous progress recently achieved by incorporating MoS₂ in PSCs, including the realization of the world's first perovskite solar farm operating for several months,⁴⁶ other representative TMCs for photovoltaic applications, based on the above consideration, are SnS₂, MoSe₂, as well as monochalcogenides, e.g., InSe.²⁴ While MoS₂ is mainly used to improve the hole extraction efficiency in n–i–p PSCs, there are other TMCs that may play a relevant role in promoting the electron extraction from the perovskite to the current collectors. In particular, SnS₂ is an n-type layered semiconductor with a tunable band gap ranging from 2.1 to 3.4 eV, depending on the number of its layers. In its bulk form, SnS₂ has already been established in photovoltaic applications.⁴⁷ Meanwhile, the exfoliation of SnS₂ into 2D few-layer nanosheets has been achieved through both chemical⁴⁸ and physical methods.⁴⁹ Extensive characterization of SnS₂ has revealed that its monolayer possesses an indirect bandgap equal to 2.033 eV, while its field-effect transistor (FET) mobility was estimated to be in the range of 5 cm² V⁻¹ s⁻¹ to 250 cm² V⁻¹ s⁻¹,⁵⁰ enabling the realization of ultrathin FETs and logic gates.⁴⁹ Also, SnS₂ shows a high electron density of ∼6 × 10⁻¹⁹ cm⁻³,⁴⁸ justifying its use as an effective dopant for PC₇₀BM in inverted PSCs.⁵¹ In this context, the high conductivity of 2D SnS₂ has been leveraged for the fabrication of ultra-thin ETLs in PSCs.^38,52 The valence band maximum (VB) and conduction band minimum (CB) of 2D SnS₂ have been estimated through ultraviolet photoelectron spectroscopy (UPS) to be −6.54 and −4.24 eV, respectively, indicating their potential ability to collect electrons from common perovskite active layers.³⁸

MoSe₂ shows a similar structure to that of MoS₂ (2H phase). MoSe₂ is a semiconductor with an indirect band gap of 1.41 eV.⁵³ However, few-layer MoSe₂ exhibits a direct band gap of 1.58 eV (ref. 53 and 54) and a FET electron mobility of 50–250 cm² V⁻¹ s⁻¹.⁵⁵ Thin-films of 2D MoSe₂ have been used in photovoltaics,^56,57 catalysis for the hydrogen evolution reaction,^58,59 and energy storage systems.^60,61

Finally, InSe is a III–VI layered compound made of stacked quaternary layers of Se–In–In–Se atoms that are held together by van der Waals interactions. Consequently, bulk InSe can be exfoliated into 2D forms by means of various methods, including scalable liquid-phase exfoliation (LPE) techniques.^62–64 InSe is generally considered as an n-type semiconductor and it was shown through experimental and theoretical studies that a band gap transition from direct to indirect occurs when InSe layer thickness reduces below 6 nm.⁶⁵ Also, its bandgap can increase from ∼1.2 eV for the bulk to 1.4 eV for few-layers InSe⁶⁶ and up to 2.1 eV for a monolayer of InSe.⁶⁷ The electron affinity was found to be −4.55 eV and the work function (W_F) to be 5.1 eV for bulk InSe.⁶⁸ The Hall mobility of InSe nanosheets was estimated to be ∼10³ cm² V⁻¹ s⁻¹ at room temperature (RT)⁶⁹ and the carrier density was measured to be ∼10¹³ cm⁻².⁷⁰ Pioneering studies also demonstrated n-type FETs based on 2D InSe, with an electron mobility at room temperature (RT) on the order of 10³ cm² V⁻¹ s⁻¹.⁷¹ This electron mobility is higher than those achieved by other high mobility 2D semiconducting materials, including MoS₂. Based on its properties, 2D InSe has been used in photovoltaic devices,^72,73 as well as for (photo)electrochemical water splitting reactions,⁶⁴ photodetectors,^63,74 sensors,⁷⁵ thermoelectric devices,⁷⁶ and spintronic devices.⁷⁷

Overall, the above discussed TMCs represent interesting 2D materials for the engineering of the perovskite/ETL interface, aiming at improving the device PCE. Most studies on incorporating TMCs in PSCs have been focused on the optimization of the perovskite/HTL interface. To the best of our knowledge, there is no systematic work on engineering the perovskite/ETL interface with TMCs. In this study, we aim to fill this research gap by studying representative TMCs, i.e., SnS₂, MoSe₂ and InSe, to optimize the perovskite/ETL interface in inverted PSCs. Selected TMCs, exfoliated through LPE, were deposited over the perovskite layer using solution processing, ensuring the scalability and industrialization feasibility of our interface engineering approach. The insertion of TMCs between the perovskite layer and the PC₇₀BM effectively aligns the energy levels of the materials, enabling an efficient electron extraction from the perovskite to the current collector. This research aims to contribute to the extension of current 2D material interface engineering strategies to PSCs, providing new insights into the application of TMCs in inverted PSCs. The findings of this study have implications on the development of high-performance solar cells and can boost the utilization of TMCs in large-scale solar energy production, as already demonstrated for 2D MoS₂.⁴⁶

Results

The methods of bulk TMC synthesis and exfoliation, and those for characterization of the exfoliated TMCs, are reported in the ESI.† In particular, bulk 2H-MoSe₂, 2H-SnS₂ and β-InSe were exfoliated through ultrasonication-assisted LPE in isopropyl alcohol (IPA),^78,79 followed by sedimentation-based separation (SBS) to remove unexfoliated crystals from the exfoliated material.^59,80Fig. 1a, d and g show the transmission electron microscopy (TEM) images of representative nanosheets of MoSe₂, SnS₂ and InSe, respectively. Fig. S1† reports the statistical analysis of the lateral size of the nanosheets. The lateral size data approximately follow log-normal distributions, with modes of 52.9 nm, 99.1 nm, and 143.9 nm for MoSe₂, SnS₂ and InSe nanosheets, respectively. Fig. 1b, e and h report the XRD patterns recorded for MoSe₂, SnS₂ and InSe nanosheets, respectively, comparing them with those measured for the bulk counterparts. The positions of the diffraction peaks of the exfoliated samples resemble those of the bulk counterparts, well-matching with the patterns of their hexagonal phases, i.e., 2H-MoSe₂, P6̄m2 space group, JCPDS card no. 029-0914; 2H-SnS₂, P6₃mc space group, JCPDS card no. 023-0677; β-InSe structure, P6₃/mmc space group, JCPDS card no. 034-1431. Bulk crystals show the diffraction peaks belonging to the {00l} family, which means that they are aligned along the (001) plane perpendicular to the c axis. Compared to the bulk materials, MoSe₂ and InSe nanosheets exhibit additional XRD peaks, likely due to their disordered arrangements. SnS₂ nanosheets, instead, preserve a placement perpendicular to their c axis. The broadening of the XRD peaks in the exfoliated materials indicate that their crystal sizes decreased after the LPE process. Fig. 1c, f and i report the Raman spectra of the bulk and exfoliated materials, exhibiting the characteristic Raman modes expected for each material by the group theory.^81–83 In MoSe₂ nanosheets, the redshift of the A_1g peak compared to the same peak of the bulk counterpart is explained by the softening of the vibrational mode with decreasing the crystal thickness.^84,85 The Raman spectrum of bulk SnS₂ shows two phonon modes at ∼313 cm⁻¹ (A_1g) and ∼203 cm⁻¹ (E_g), as well as a weak two-phonon scattering signal of A_1g-LA (M) (Fig. 1f).⁸⁶ After exfoliation, additional peaks appear and are likely associated with by-products, e.g., oxides.⁸⁷ Finally, bulk and exfoliated InSe exhibit various Raman peaks at similar positions, i.e., A¹_1g at ∼116 cm⁻¹, E²_1g at ∼177 cm⁻¹, E¹_2g at ∼199 cm⁻¹, A_2u at ∼208 cm⁻¹, and A²_1g at ∼227 cm⁻¹, which is consistent with previous studies.^63,64

Fig. 1 TEM images, XRD patterns and Raman spectra measured for (a–c) MoSe₂, (d–f) SnS₂ and (g–i) InSe nanosheets. The XRD patterns and Raman spectra of bulk counterparts are also shown for comparison.

The deposition of TMC nanosheets was carried out by spin coating their dispersion (formulated in chlorobenzene) onto the perovskite layer. Successive spin coating steps were applied to optimize the interlayer of TMC nanosheets at the perovskite/PC₇₀BM interface. The formation of ultrathin 2D TMC films on the surface of the perovskites was assessed through X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) measurements. As shown in Fig. S2,† the 3d In, Mo, and Sn XPS spectra, acquired on the perovskite surfaces treated with 2D TMCs, confirm the presence of InSe, MoSe₂ and SnS₂ nanosheets, respectively. Fig. S3† presents top-view SEM images of the perovskite surface treated with 2D TMCs. The images show scattered TMC nanosheets atop the perovskite surface, without the presence of large material aggregates. Importantly, the few-layer nature of the exfoliated TMCs permits nanometer-thick interlayers to be obtained without resulting in significant changes of the surface topography of the underlying perovskite film.

The effect of the 2D TMC interlayers between the perovskite and PC₇₀BM layers was evaluated by measuring the device performance estimated through the analysis of their J–V curves under 1 sun illumination. Fig. S4–S6† report the statistical analysis of the photovoltaic parameters, i.e., PCE, open circuit voltage (V_oc), fill factor (FF) and short circuit current (J_sc), for the investigated devices, produced with different 2D TMC interlayers and by varying the number of successive spin coating steps used for the deposition of 2D TMCs atop the perovskite. These data indicate that the number of spin coating steps has a significant impact on the final photovoltaic parameters of the devices. Specifically, exceeding two spin coating steps has a detrimental effect on the device performance. This negative effect is likely associated with the excessive exposure of the perovskite layer to the solvent of 2D TMC dispersions. Importantly, the incorporation of 2D TMCs into the device structure through one or two spin coating steps always increases the PCE of the device. In addition, different 2D TMC interlayers result in the improvement of specific photovoltaic parameters, indicating different beneficial effects of the investigated materials. To compare the devices that have different 2D TMC interlayers and to eliminate any uncontrollable effect between various fabrication batches, Fig. 2 plots the percentage relative change of PCE and of the other photovoltaic parameters measured for the most performing 2D TMC-incorporating PSCs compared to reference (without 2D TMCs) devices. Cells incorporating InSe and SnS₂ nanosheets exhibit a similar behavior, characterized by a substantial increase in their FF by 2.32% and 2.83%, respectively, along with a slight improvement in J_sc, and no change in V_oc, compared to the reference device. Differently, the device containing MoSe₂ nanosheets exhibits a decrease in FF by about 1.9% compared to the reference cell. Meanwhile the J_sc increases considerably by 2.95% and the V_oc increases by 1.42% compared to the reference cell. Notably, the devices incorporating SnS₂ nanosheets demonstrated the greatest relative PCE improvement of 5.05%, with a 2.83% increase in FF, resulting in a PCE as high as 18.5% and FF of 82%. Fig. S7† shows the external quantum efficiency spectrum of the best device.

Fig. 2 Percentage relative change of PCE, FF, V_oc, and J_sc of 2D TMC-incorporating PSCs compared to reference (without 2D TMCs) PSCs.

The optoelectronic properties of the perovskite surface treated with 2D TMCs were investigated to assess if they are correlated with the device photovoltaic parameters. The W_F and valence band (VB) maximum energy of the perovskite surface were estimated using ambient photoemission spectroscopy measurements. Fig. S8† illustrates the W_F measurements of the perovskite surface before and after the deposition of 2D TMCs, and the extrapolated W_F values are listed in Table 1. The W_F of the pristine perovskite was determined to be 4.15 eV and increased after depositing the 2D TMCs (i.e., 4.8 eV after SnS₂ nanosheet deposition). This behavior suggests the presence of a surface dipole that shifts the perovskite vacuum level to higher energies, with the negative part of the dipole facing outward from the surface (positive part facing toward the perovskite). The value of the interface dipole, Δ, for each case is listed in Table 1. Fig. S9† shows the VB measurements for the investigated samples. Interestingly, compared to the untreated perovskite, the presence of 2D TMCs shifts the perovskite VB to lower energy. Fig. 3 reports the sketch of the energy level diagram expected for the investigated cell configurations. The incorporation of 2D TMCs alters the energy level alignment at the interfaces between the perovskite and the PC₇₀BM. Specifically, the conduction band (CB) minimum energy and VB maximum energy shift to lower values. Thus, the CB minimum energy of the perovskite approaches that of PC₇₀BM, while the VB maximum energy of the perovskite moves away from that of PTAA. To quantify the offset of the energy levels, the energy difference between the CB minimum energies (conduction band offset-CBO) and the difference between the VB maximum energies (valence band offset-VBO) of the respective charge transporting layers and the perovskite were evaluated. In greater detail, the CBO and VBO can be calculated as:

CBO = χ_perovskite − χ_PCBM (1)

VBO = χ_PTAA + E_g,PTAA − (χ_perovskite + E_g,perovskite) (2)

in which χ is the electron affinity and E_g is the bandgap of the material indicated in the subscript.

Table 1 Energy level values of the perovskite without 2D TMCs (reference) and the with 2D TMC interlayers at the perovskite/PC₇₀BM interface. The CBO and VBO values are defined in the text and are calculated from eqn (1) and (2). Δ is the size of the interface dipole at the perovskite interface

Perovskite	W F (eV)	Δ (eV)	VB (eV)	CB (eV)	CBO (eV)	VBO (eV)
Reference	−4.15		−5.41	−3.82	−0.08	−0.01
InSe	−4.46	0.31	−5.49	−3.9	0.00	−0.09
SnS2	−4.82	0.67	−5.47	−3.88	−0.02	−0.07
MoSe2	−4.73	0.58	−5.52	−3.93	0.03	−0.12

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Fig. 3 Sketch of the energy level diagram of the investigated devices (energy values are not in scale). The dashed lines show the energy levels of the perovskite after the deposition of InSe, SnS₂, and MoSe₂ nanosheets.

The measured values for VB maximum energies and the calculated values for CBO and VBO are listed in Table 1. For the reference device, the CBO is negative (−0.08 eV), indicating that the CB minimum energy of PC₇₀BM is lower than that of the perovskite. This energy level alignment at the perovskite/PC₇₀BM interface is commonly indicated as a “cliff,” signifying the absence of an energy barrier for electron injection from the perovskite to the PC₇₀BM. Similarly, the VBO for the reference device is −0.01 eV, implying no injection barrier for holes at the PTAA/perovskite interface. However, the CBO and VBO change drastically upon the deposition of 2D TMCs atop the perovskite. The CBO is reduced to −0.02 eV after the deposition of SnS₂ nanosheets and becomes 0.00 eV after depositing InSe nanosheets. Conversely, the CBO is positive (0.03 eV) when MoSe₂ nanosheets are deposited atop the perovskite, resulting in the formation of a “spike” feature at the perovskite/PC₇₀BM interface.

In our previous work, we have demonstrated that the doping of the perovskite with BiTeI nanosheets leads to a modification of the energy levels of the perovskite.⁸⁸ In this context, increasing the CBO and decreasing the VBO have proven to be advantageous for device performance. Similar to our previous findings, tuning the CBO and VBO by incorporating TMC interlayers enhances the FF and, thus, the PCE of PSCs, as demonstrated for PSCs containing InSe and SnS₂ nanosheets. In the case of MoSe₂-containing PSCs, the formation of a “spike” level alignment at the interface is also expected to enhance the FF. However, a decrease in FF was observed together with an increase in V_oc, suggesting a different behavior of MoSe₂ nanosheets compared to SnS₂ and InSe nanosheets. The VBO of the reference sample (−0.01 eV) decreases to −0.12 eV with the incorporation of the MoSe₂ interlayer. Our previous work has demonstrated that lowering the VBO is advantageous for device performance, as it reduces nonradiative recombination at the interface between the HTL (PTAA) and the perovskite.⁸⁸ Moreover, experimental and theoretical studies have emphasized the importance of an absolute energy level offset of approximately 0.2 eV at the PTAA/perovskite interface.⁸⁹ Additionally, a VBO between 0 and −0.18 eV has been identified as the optimum range for efficient hole extraction.⁹⁰ Therefore, the observed improvement in V_oc for MoSe₂ can be attributed to the reduced recombination at the PTAA/perovskite interface, as the incorporation of MoSe₂ helps to tune the VBO closer to the theoretically optimum range. The improved PCE of devices incorporating InSe can be attributed to the tendency of this material to oxidize under ambient conditions, forming In₂O₃. Notably, the surface of InSe readily undergoes oxidation when subjected to air heat treatment.^91–93 The native oxide In₂O₃ is an n-type semiconductor with an optical band gap of 3.66 eV⁹² and a high carrier concentration of approximately 6 × 10¹⁹ cm⁻³,^68,94 which can improve device PCE. Theoretical studies have indicated that H₂O, N₂, and O₂ molecules physisorbed onto the 2D InSe surface, resulting in partial electron charge transfer from InSe to the adsorbed molecules.⁹⁵ Additionally, it has been shown that a monolayer of InSe readily physisorbs H₂O under ambient conditions, irrespective of the presence of In or Se vacancies. Depending on the vacancies, InSe can be doped with H₂O, leading to either p-type (In vacancies) or n-type (Se vacancies) behavior,⁹⁶ suggesting precaution to assess the actual role of the InSe interlayers. The interface dipole at the interface of perovskite/ETL could reduce interface recombination, by preventing electron carriers in the ETL from reaching the interface.

The incorporation of 2D TMCs atop the perovskite has been the subject of extensive research, and our findings supplement previous studies, mainly focused on dichalcogenides and normal PSCs. Indeed, we have observed a strong agreement between our results and a theoretical study investigating the perovskite/InSe interface,⁹⁷ in which it was shown that InSe plays a crucial role in facilitating electron charge transport, causing charge transfer from the perovskite towards the InSe. In line with this, we have observed that the W_F of the perovskite surface covered with InSe nanosheets is higher than that of the control sample, indicating the accumulation of negative charge on the top surface with InSe, forming an interface dipole, as well as for the other investigated 2D TMCs.⁹⁷ Furthermore, experimental evidence has shown that the interfaces of TMCs such as MoS₂, WSe₂, and MoSe₂ with inorganic perovskites exhibit ultrafast and efficient charge carrier transfer.^98,99 Based on these findings, it is expected that the presence of 2D TMCs on the perovskite will not impede charge transport, a hypothesis that is confirmed by our experimental results. Moreover, SnS₂ nanosheets have been utilized in the literature to form the ETL, enhancing charge extraction and passivating interfacial traps more efficiently compared to SnO₂.³⁸ Our results are aligned with this, further supporting the beneficial effects of SnS₂ as an ETL in PSCs. Consequently, our study provides additional insights on the incorporation of the 2D TMC interlayer at the perovskite/PC₇₀BM interface, highlighting their potential in improving charge extraction in PSCs.

Lifetime measurements

The stability of the PSCs incorporating TMC nanosheets was investigated following the International Summit on Organic Photovoltaic Stability (ISOS) L-2 protocol. Thus, the devices were subjected to continuous 1 sun illumination under ambient humidity at 65 °C.¹⁰⁰Fig. 4 illustrates the normalized PCE of the unencapsulated devices over time. The reference device exhibited a burn-in phase during the initial testing period and a fast degradation afterwards. This behavior can be attributed to the degradation of the Ag electrode caused by its reaction with iodine, which migrated from the perovskite layer.^101–103 The device incorporating SnS₂ nanosheets showed the worst stability, while the one containing MoSe₂ nanosheets exhibited similar behavior to the reference cell. On the other hand, the incorporation of InSe led to the disappearance of the burn-in phase, enhancing the cell stability. The InSe-incorporating PSC exhibited a T₇₀ of 250 h, i.e., the device retained 70% of its initial PCE after 250 h. The superior stability of the InSe-containing PSC compared to the other cells can be correlated with the morphological and chemical properties of InSe nanosheets. The photo-induced degradation of InSe towards In₂Se₃ and In₂O₃ under ambient conditions can commence a surface passivation interlayer at the perovskite/ETL interface. The formation of In₂Se₃ and In₂O₃ (ref. 104) does not seem to affect the PCE and enhances the stability of the device, according to our results. Moreover, the InSe nanosheets show the highest lateral size amongst the investigated flakes (Fig. S1†) and therefore, InSe can serve as an effective barrier, protecting the metal electrode from the degradation species of the perovskite, thus enhancing the device stability.

Fig. 4 Stability measurements of the PSCs incorporating InSe, MoSe₂, and SnS₂ interlayers under continuous 1 sun illumination, ambient humidity and 65 °C temperature (ISOS L-2).

Conclusions

The structure of inverted PSCs has been systematically optimized by screening the incorporation of various 2D TMCs in the form of an interlayer between the perovskite and the PC₇₀BM ETL. Specifically, bulk InSe, SnS₂, and MoSe₂ were exfoliated by means of ultrasonication-assisted LPE and deposited atop the perovskite surface by spin coating to form thin (nanometer-thick) films. The incorporation of TMC interlayers causes a shift in the energy levels towards lower energies, altering the energy level offset at perovskite/charge-transporting layer interfaces. In detail, the introduction of SnS₂ and InSe nanosheets reduces the energy offset at the perovskite/PC₇₀BM interface, resulting in an increase of the FF and PCE of the devices. The SnS₂- and InSe-incorporating devices exhibited relative improvement in PCE by 5.05% and 3.37%, respectively, compared to reference (without 2D TMCs) cells. The PSCs engineered with the SnS₂ interlayer showed the highest PCE, reaching 18.5% and FF of 82%. Conversely, the MoSe₂ interlayer introduces a small cliff arrangement at the interface of perovskite/PC₇₀BM. Compared to reference cells, the cells of MoSe₂-incorporating devices demonstrated improved PCE compared to reference cells, increasing the J_sc and V_oc. Moreover, to assess the long-term stability of the cells, the devices were subjected to continuous 1 sun illumination at 65 °C, following the ISOS-L2 ageing protocol. The results indicate different stability behaviors of the cells incorporating the investigated TMC interlayers, reflecting the different physico-chemical properties of the 2D TMCs. The incorporation of the MoSe₂ interlayer has no discernible effect on device stability, while the SnS₂ interlayer has a negative impact. On the other hand, InSe interlayer eliminates the burn-in degradation observed in the reference devices, significantly improving the overall cell stability. In summary, this work elucidates the potential of 2D TMCs to engineer the architecture of inverted PSCs.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

We thank the Microelectronic Research Group at the University of Crete and Aleka Manousaki for the SEM images of the perovskite films, and Dr Manolis Spanakis from the Materials Science and Technology Department of the University of Crete, for the XPS measurements. The work has been supported by the European Union's Horizon Europe Framework Programme for research and innovation under grant agreement no. 694101—2D-PRINTABLE and by the European Union's Horizon 2020 research and innovation program under the Project EMERGE. The EMERGE project has received funding under grant agreement no. 101008701. Z. S. was supported by the ERC-CZ program (project LL2101) from the Ministry of Education Youth and Sports (MEYS) and used large infrastructure from project reg. no. CZ.02.1.01/0.0/0.0/15_003/0000444 financed by the EFRR.

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Footnote

† Electronic supplementary information (ESI) available: Material synthesis, exfoliation, and characterization, PSC fabrication and characterization. Moreover, in the supplementary material are presented the TEM statistical analysis, XPS spectra, SEM images, box charts of PSC photovoltaic parameters, W_F measurements, and APS measurements. See DOI: https://doi.org/10.1039/d4se00212a

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