Perovskite solar cells have shown great potential in commercial applicationsdue to their high performance and easy fabrication. However, the electron transport layer (ETL) materials with good optoelectrical properties and energy levels matching that of the perovskite layer still need to be explored to meet the need of commercialization. In this work, 2D Nb 2 CT x MXene nanosheets are prepared and their work function (WF) is reduced from 4.65 to 4.32 eV to match the conduction band minimum of perovskite layer by replacing the surface -F groups with NH 2 groups through hydrazine (N 2 H 4 ) treatment. Besides, the N 2 H 4 treated (T-Nb 2 CT x ) MXene nanosheets with abundant NH 2 groups are incorporated into the perovskite precursor to retard the crystallization rate by forming hydrogen bond with iodine ions, which promotes the formation of high-quality and oriented growth perovskite films. Consequently, the PVSCs with T-Nb 2 CT x MXene ETLs and T-Nb 2 CT x MXene nanosheets additive exhibit the highest power conversion efficiency (PCE) of 21.79% and the corresponding flexible and large-area devices achieve the highest PCE of 19.15% and 18.31%. Meanwhile, the unencapsulated devices maintain 93% of the original PCEs after 1500 h of storage. This work demonstrates the considerable application prospects of 2D Nb 2 CT x MXene in photoelectric devices.

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The functional group is the main body in modifying the perovskite film, and different functional groups lead to different modification effects. Here, several conjugated triazine-based small molecules such as melamine (Cy-NH2), cyanuric acid (Cy-OH), cyanuric fluoride (Cy-F), cyanuric chloride (Cy-Cl), and thiocyanuric acid (Cy-SH) are used to modify perovskite films by mixing in antisolvent. The crystallizations of perovskites are optimized by these molecules, and the perovskite films with low trap density are obtained by forming Lewis adducts with these molecules (Pb2+ and electron-donating groups including −NH2, CN–, and CO; I– and electron-withdrawing groups including F, Cl, N–H, and O–H). Especially for the Cy-F and Cy-Cl, the heterojunction structure is formed in the perovskite layer by p-type modification, which is conducive to charge transfer and collection in PSCs. Compared with that of control devices, the performance of devices with trap passivation and heterojunction engineering is obviously improved from 18.49 to 20.71% for MAPbI3 and 19.27 to 21.11% for FA0.85Cs0.15PbI3. Notably, the excellent moisture (retaining 67%, RH: 50% for 20 days) and thermal (retaining 64%, 85 °C for 72 h) stability of PSCs are obtained by a kind of second modification (Cy-F/Cy-SH)spin-coating a few Cy-SH on the Cy-F-modified perovskite film surface. It also reduces Pb pollution because Cy-SH is a highly potent chelating agent. Therefore, this work also provides an effective method to obtain high-performance, stable, and low-lead pollution PSCs, combining trap passivation, heterojunction engineering, and surface treatment.

of methylammonium iodide (MAI) with formamidine iodide (FAI) improves the structure stability of perovskite crystal and maximizes the short-circuit current density (J sc ) of PVSCs due to the lower bandgap of FAPbI 3 perovskite film, which significantly enhances the device stability and efficiency. [9][10][11] Recently, many works focus on minimizing the interfacial nonradiative recombination and further improving the efficiency of PVSCs by introducing passivation layers since the defect states on the perovskite surface is much larger than that in the perovskite bulk film. [12][13][14] In addition to the improvement of efficiency, the passivation layers also improve the stability of PVSCs by protecting the functional layers from external water or oxygen erosion and retraining ions' migration inside the devices. [15][16][17] The synergetic development of composition engineering, additive engineering, and interface engineering facilitates the charge transportation and reduces nonradiative recombination in the device, which leads to the enhancement of certified PCE up to 25.5%. [4,18,19] However, this efficiency still lags behind the theoretical efficiency defined by the Shockley-Queisser theory due to the severe nonradiative recombination losses inside devices, thus strategies are still needed to be developed to suppress the nonradiative recombination losses and further improve the device efficiency, especially for the open-circuit voltage (V oc ) while the J sc and fill factor (FF) are approaching the theoretical limit. [20][21][22] The V oc of PVSC is determined by the internal quasi-Fermi level splitting (QFLS) in the absorber layer. [22] The QFLS will reduce compared with the QFLS of the neat perovskite film when the charge transport layers (CTLs) attach to the perovskite layer, which is possibly induced by the mismatched energy level alignment between different layers and the defects at the perovskite/CTL interfaces. [23] The reduction of QFLS should be minimized as possible for fabrication of high-performance PVSCs as it leads to the reduction of V oc and efficiency. Thus, it is urging to reduce nonradiative recombination losses aroused by defects and mismatched energy level alignment between the perovskite layer and the CTLs so as to reduce the V oc loss.Theoretical simulations and experimental data show that the perovskite materials possess self-doping characteristic by The severe nonradiative recombination losses limit the further improvement of open-circuit voltage (V oc ) and power conversion efficiency (PCE) of perovskite solar cells (PVSCs). In this work, the 4,4′-cyclohexylidenebis [N,N-bis(4-methylphenyl) benzene amine] is dissolved into the antisolvent to prepare perovskite films, which reduces defects, improves the crystallinity, and induces a p/p + homojunction on the top surface of perovskite film. Besides, the 2-thiophenemethylammonium iodide and 2-thiophenethylammonium iodide form interface electric field and passivate defects on the bottom surface of perovskite film. The p/p + homojunction and...

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The energy loss (Eloss) aroused by inefficient charge transfer and large energy level offset at the buried interface of p‐i‐n perovskite solar cells (PVSCs) limits their development. In this work, a BF4− anion‐assisted molecular doping (AMD) strategy is first proposed to improve the charge transfer capability of hole transport layers (HTLs) and reduce the energy level offset at the buried interface of PVSCs. The AMD strategy improves the carrier mobility and density of poly[bis(4‐phenyl) (2,4,6‐trimethylphenyl) amine] (PTAA) and poly[N,N′‐bis(4‐butilphenyl)‐N,N′‐bis(phenyl)‐benzidine] (Poly‐TPD) HTLs while lowering their Fermi levels. Meanwhile, BF4− anions regulate the crystallization and reduce donor‐type iodine vacancies, resulting in the energetics transformation from n‐type to p‐type on the bottom surface of perovskite film. The faster charge transfer and formed p–n homojunction reduce charge recombination and Eloss at the HTL/perovskite buried interface. The PVSCs utilizing AMD treated PTAA and Poly‐TPD as HTLs demonstrate a highest power conversion efficiency (PCE) of 24.26% and 22.65%, along with retaining 90.97% and 85.95% of the initial PCE after maximum power point tracking for 400 h. This work provides an effective way to minimize the Eloss at the buried interface of p‐i‐n PVSCs by accelerating charge transfer and forming p–n homojunctions.