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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

Reduced graphene oxide (rGO) is added in the [6,6]‐Phenyl‐C61‐butyric acid methyl ester (PCBM) electron transport layer (ETL) of planar inverted perovskite solar cells (PSCs), resulting in a power conversion efficiency (PCE) improvement of ≈12%, with a hysteresis‐free PCE of 14.5%, compared to 12.9% for the pristine PCBM based device. The universality of the method is demonstrated in PSCs based on CH3NH3PbI3−xClx and CH3NH3PbI3 perovskites, deposited through one step and two step spin coating process, respectively. After a comprehensive spectroscopic characterization of both devices, it is clear that the introduction of rGO in PCBM ETL results in an important increase of the ETL conductivity, together with reduced series resistance and surface roughness. As a result, a significant photoluminescence quenching of such perovskite/ETL is observed, confirming the increased measured short circuit current density. Transient absorption measurements reveal that in the rGO‐based device, the relaxation process of the excited electrons is significantly faster compared to the reference, which implies that the charge injection rate is significantly faster for the first. Furthermore, the light soaking effect is significantly reduced. Finally, aging measurements reveal that the rGO stabilizes the ELT/perovskite interface, which results in the stabilization of perovskite crystal structure after prolonged illumination.

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.