In order to improve the efficiency of perovskite solar cells (PSCs), careful device design and tailored interface engineering are needed to enhance optoelectronic properties and the charge extraction process at the selective electrodes. Here, we use two-dimensional transition metal carbides (the MXene Ti3C2TX) with various termination groups (TX) to tune the work function (WF) of the perovskite absorber and the TiO2 electron transport layer (ETL), and to engineer the perovskite/ETL interface. Ultraviolet photoemission spectroscopy measurements and Density Functional Theory calculations show that the addition of Ti3C2TX to halide perovskite and TiO2 layers permits to tune the materials' WFs, without affecting other electronic properties. Moreover, the dipole induced by the Ti3C2TX at the perovskite/ETL interface can be used to change the band alignment between these layers. The combined action of WF tuning and interface engineering can lead to substantial performance improvements in MXene-modified PSCs, as shown by the 26% increase of power conversion efficiency and hysteresis reduction with respect to reference cells without Mxene.

wileyonlinelibrary.comthe theoretical values, [ 9 ] further work is needed to increase the fi ll factor (FF) that today remains the bottleneck for the PCE enhancement. In this context, the optimization of interfaces between absorber, carrier transport layers, and electrode contact layers is crucial for an effi cient carrier transport. Thus, to improve the PCE of the PSCs, three main aspects need to be taken into account: the band alignment, the interface structure, and its passivation. [ 9 ] Furthermore, it must be taken into account that the injection times of electrons and holes in PSCs have been measured to be 0.4 and 0.6 ns, respectively, which are still orders of magnitude longer than the hot carrier cooling (or thermalization) time (≈0.4 ps). [ 10 ] Thus, a large amount of the converted photon energy is wasted in the thermalization process and in the carrier trapping. In this context, several materials have been proposed to facilitate electron/ hole extractions, including fullerene, [ 11 ] graphene, [ 12 ] or core/ shell metal nanoparticles. [ 13 ] Although graphene-based materials have been demonstrated to improve the performance and stability of organic and hybrid photovoltaic devices as recently reviewed by Singh and Nalwa, [ 14,15 ] a direct evidence is yet to be obtained regarding their role as a fast electron injector rather than just an electron trapping center. [ 16 ] In this work, to improve the electron extraction from the perovskite absorber into the mesoporous TiO 2 , an additional lithium-neutralized graphene oxide (GO-Li) layer as electron transporting Layer (ETL) is used. GO-Li has been already used as ETL in organic photovoltaic devices by Kakavelakis et al. [ 17 ] The authors demonstrated that the replacement of H atoms in the carboxyl groups of GO by Li atoms can effectively reduce the working function (WF) of GO from 4.9 to 4.3 ± 0.1 eV. Due to low electronegativity and low WF, Li atoms lose their valence electrons to the GO plane, and the resulting positive Li + induces dipoles. This transfer of charge from the metal to the GO leads to a shift in the Fermi level toward the vacuum and a consequent decrease in WF. [ 18 ] Since the WF of GO-Li displays a good match with the Lowest Unoccupied Molecular Orbital (LUMO) level of mesoporous TiO 2 , in this work a new effi cient perovskite solar cell structure is proposed which includes the GO-Li as an interlayer between the TiO 2 and the perovskite harvester. The resulting PSCs exhibit enhanced J SC and FF and reduced Effi ciency and Stability Enhancement in Perovskite Solar Cells by Inserting Lithium-Neutralized Graphene Oxide as Electron Transporting LayerAntonio Agresti , Sara Pescetelli , Lucio Cinà , Dimitrios Konios , George Kakavelakis , Emmanuel Kymakis , and

Interface engineering of organic-inorganic halide perovskite solar cells (PSCs) plays a pivotal role in achieving high power conversion efficiency (PCE). In fact, perovskite photoactive layer needs to work synergistically with the other functional components of the cell, such as charge transporting/active buffer layers and electrodes. In this context, graphene and related twodimensional materials (GRMs) are promising candidates to tune "on demand" the interface properties of PSCs. In this work, we fully exploit the potential of GRMs by controlling the optoelectronic properties of hybrids between molybdenum disulfide (MoS2) and reduced graphene oxide (RGO) as hole transport layer (HTL) and active buffer layer (ABL) in mesoscopic methylammonium lead iodide (CH3NH3PbI3) perovskite (MAPbI3)-based PSC. We show that zero-dimensional MoS2 quantum dots (MoS2 QDs), derived by liquid phase exfoliated MoS2 flakes, provide both holeextraction and electron-blocking properties. In fact, on the one hand, intrinsic n-type doping-induced intra-band gap states effectively extract the holes through an electron injection mechanism. On the other hand, quantum confinement effects increase the optical band gap of MoS2 (from 1.4 eV for the flakes to > 3.2 for QDs), raising the minimum energy of its conduction band (from -4.3 eV for the flakes to -2.2 eV for QDs) above the one of conduction band of MAPbI3 (between -3.7 and -4 eV) and hindering electron collection. The van der Waals hybridization of MoS2 QDs with functionalized reduced graphene oxide (f-RGO), obtained by chemical silanization-induced linkage between RGO and (3-mercaptopropyl)trimethoxysilane, is effective to homogenize the deposition of HTLs or ABLs onto the perovskite film, since the two-dimensional (2D) nature of RGO effectively plug the pinholes of the MoS2 QDs films. Our "graphene interface engineering" (GIE) strategy based on van der Waals MoS2 QD/graphene hybrids enable MAPbI3-based PSCs to achieve PCE up to 20.12% (average PCE of 18.8%). The possibility to combine quantum and chemical effects into GIE, coupled with the recent success of graphene and GRMs as interfacial layer, represents a promising approach for the development of next-generation PSCs. Figure 1. (a) Sketch of mesoscopic MAPbI3-based PSC exploiting MoS2 QDs:f-RGO hybrids as both HTL and ABL. (b) Scheme of the energy band edge positions of the materials used in the different components of the assembled mesoscopic MAPbI3-based PSC. The energy band edge positions of MoS2 flakes and MoS2 QDs were determined from OAS and UPS measurements detailed along the text, while those of the other materials were taken from literature: FTO, 52 TiO2, 52 MAPbI3, 134-139 spiro-OMeTAD 52 and Au 52 . (c) State-of-the-art and predicted PCE evolution for PSCs, highlighting the synergistic potential of GIE and the formulation of advanced perovskite chemistries. The RGO flakes are effective to plug the pinholes MoS2 QDs films, thus to homogenize the HTL. The choice of the functionalization for RGO relies on the bifunctional r...

Interfaces between perovskite solar cell (PSC) layer components play a pivotal role in obtaining high-performance premium cells and large-area modules. Graphene and related two-dimensional materials (GRMs) can be used to “on-demand” tune the interface properties of PSCs. We successfully used GRMs to realize large-area (active area 50.6 cm2) perovskite-based solar modules (PSMs), achieving a record high power conversion efficiency of 12.6%. We on-demand modulated the photoelectrode charge dynamic by doping the mesoporous TiO2 (mTiO2) layer with graphene flakes. Moreover, we exploited lithium-neutralized graphene oxide flakes as interlayer at the mTiO2/perovskite interface to improve charge injection. Notably, prolonged aging tests have shown the long-term stability for both small- and large-area devices using graphene-doped mTiO2. Furthermore, the possibility of producing and processing GRMs in the form of inks opens a promising route for further scale-up and stabilization of the PSM, the gateway for the commercialization of this technology.

Small area hybrid organometal halide perovskite\ud based solar cells reached performances comparable to the multicrystalline\ud silicon wafer cells. However, industrial applications\ud require the scaling-up of devices to module-size. Here, we report\ud the first fully laser-processed large area (14.5 cm2) perovskite solar\ud module with an aperture ratio of 95% and a power conversion\ud efficiency of 9.3%. To obtain this result, we carried out thorough\ud analyses and optimization of three laser processing steps required\ud to realize the serial interconnection of various cells. By analyzing\ud the statistics of the fabricated modules, we show that the error\ud committed over the projected interconnection dimensions is sufficiently\ud lowto permit even higher aperture ratios without additional\ud efforts

Interface engineering is performed by the addition of graphene and related 2 D materials (GRMs) into perovskite solar cells (PSCs), leading to improvements in the power conversion efficiency (PCE). By doping the mesoporous TiO layer with graphene flakes (mTiO +G), produced by liquid-phase exfoliation of pristine graphite, and by inserting graphene oxide (GO) as an interlayer between the perovskite and hole-transport layers, using a two-step deposition procedure in air, we achieved a PCE of 18.2 %. The obtained PCE value mainly results from improved charge-carrier injection/collection with respect to conventional PSCs. Although the addition of GRMs does not influence the shelf life, it is beneficial for the stability of PSCs under several aging conditions. In particular, mTiO +G PSCs retain more than 88 % of the initial PCE after 16 h of prolonged 1 sun illumination at the maximum power point. Moreover, when subjected to prolonged heating at 60 °C, the GO-based structures show enhanced stability with respect to mTiO +G PSCs, as a result of thermally induced modification at the mTiO +G/perovskite interface. The exploitation of GRMs in the form of dispersions and inks opens the way for scalable large-area production, advancing the possible commercialization of PSCs.

In this work, we demonstrate the successful application of two-dimensional (2D) materials, i.e., graphene and functionalized MoS 2 , in perovskite solar cells (PSCs) by interface engineering the standard mesoscopic n−i−p structure. The use of 2D materials has the dual role to improve both the stability and the overall power conversion efficiency (PCE) of the PSCs compared to standard devices. The application of 2D materials is successfully extended to large-area perovskite solar modules (PSMs), achieving PCEs of 13.4% and 15.3% on active areas of 108 cm 2 and 82 cm 2 , respectively. This performance results in record-high active area-indexed aperture PCE (AIAPCE) of 1266.5% cm 2. In addition, the 2D materials-based PSMs show a stability under a prolonged (>1000 h) thermal stress test at 65°C (ISOS-D2), representing a crucial advancement in the exploitation of perovskite photovoltaic technology. I n recent years, lead-halide perovskite solar cells (PSCs) have catalyzed the attention of the scientific community, with power conversion efficiency (PCE) exceeding 20%, by using cost-effective and potentially scalable solution processing approaches. 1,2 In particular, the global research effort boosted the PCE of PSCs up to 24.2% for singlejunction 3 and 27.3% for tandem perovskite/silicon 4 solar cells. Despite these important achievements, long-term stability 5 and scalability 6 are still the major constraints for the market entry of the perovskite photovoltaic technology. 7,8 In fact, the photoactive lead-halide perovskites typically lack stability due 34 to their hygroscopicity and propensity to back-convert into 35 their precursors during exposure to moisture, 9 oxygen, 10,11 and 36 light illumination. 10,12 Moreover, they experience a tetragonal-37 to-cubic phase transition at the temperature reached during 38 typical solar cell operation (>80°C), 13 resulting in making 39 them unfit for standard solar module certifications. 14,15 In 40 addition, the deposition of both pin-hole-free homogeneous

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