Synchronous Elimination of Excess Photoinstable PbI2 and Interfacial Band Mismatch for Efficient and Stable Perovskite Solar Cells.

Eliminating the undesired photoinstability of excess lead iodide (PbI2 ) in the perovskite film and reducing the energy mismatch between the perovskite layer and heterogeneous interfaces are urgent issues to be addressed in the preparation of perovskite solar cells (PVSCs) by two-step sequential deposition method. Here, the 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4 ) is employed to convert superfluous PbI2 to more robust 1D EMIMPbI3 which can withstand lattice strain, while forming an interfacial dipole layer at the SnO2 /perovskite interface to reconfigure the interfacial energy band structure and accelerate the charge extraction. Consequently, the unencapsulated PVSCs device attains a champion efficiency of 24.28 % with one of the highest open-circuit voltage (1.19 V). Moreover, the unencapsulated devices showcase significantly improved thermal stability, enhanced environmental stability and remarkable operational stability accompanied by 85 % of primitive efficiency retained over 1500 h at maximum power point tracking under continuous illumination.

Monodisperse Adducts-Induced Homogeneous Nucleation Towards High-Quality Tin-Based Perovskite Film.

The classic solvent system can't sufficiently separate one-dimensional edge-sharing SnI2 crystals in solution, which severely restricts the fabrication of high-quality tin-based perovskite film. Herein, a strong Lewis base (hexamethylphosphoramide, HMPA) has been introduced to coordinate Sn2+ to modulate solvation behaviours on perovskite precursor and regulate crystallization kinetics. The large molecular volume of HMPA and stronger bind energy of SnI2 ⋅ 2HMPA (-0.595 eV vs -0.118 eV for SnI2 ⋅ 2DMSO) change the solvation structure of SnI2 from edge-sharing cluster to monodisperse adduct, which contributes to uniform nucleation sites and prolongs crystal growth process. Delightfully, a fully-covered perovskite film is formed on the large-area substrate and tin-based perovskite solar cells processed with HMPA exhibit an excellent efficiency of 13.46 %. This research provides novel insights and directions for the solution preparation of smooth and uniform large-area tin-based perovskite film.

Multifunctional Metal-Organic Frameworks Capsules Modulate Reactivity of Lead Iodide toward Efficient Perovskite Solar Cells with UV Resistance.

The two-step sequential deposition process is demonstrated as a reliable technology for the fabrication of efficient perovskite solar cells (PVSCs). However, the complete conversion of dense PbI2 to perovskite in planar PVSCs is tough without mesoporous titanium dioxide as support. Herein, multifunctional capsules consisting of zeolitic imidazolate framework-8 (ZIF-8) encapsulant and formamidinium iodide (FAI) are introduced between tin oxide (SnO2 ) and lead iodide (PbI2 ) layer. Intriguingly, the capsule dopant interlayer benefits the formation of porous PbI2 film due to the porous nanostructure of ZIF-8 that is favorable for the subsequent intercalation reaction. Furthermore, the constituent of the perovskite precursor in ZIF-8 pores can convert into the crystal nuclei of perovskite by reacting with PbI2 first, thereby promoting further perovskite crystallization. Significantly, the incorporation of ZIF-8 can enhance the resistance of perovskite against UV illumination due to down-conversion effect. Consequently, the modified device achieves a champion power conversion efficiency (PCE) of 24.08% and displays enhanced UV stability, which can sustain 83% of its original PCE under 365 nm UV illumination for 300 h. Moreover, the unencapsulated device maintains 90% of initial PCE after 1500 h storage in dark ambient conditions with a relative humidity range of 50%-70%.

Defect Passivation Effect of Chemical Groups on Perovskite Solar Cells.

Defect passivation is a key strategy to prepare high-performance perovskite solar cells (PVSCs). Even though abundant passivation molecules have been applied, the absence of detailed researches with regard to different functional groups in polymer additives may inevitably impede the establishment of passivation molecules selection rules. In this work, three passivation molecules including poly(vinyl alcohol) (PVA), polymethyl acrylate (PMA), and poly(acrylic acid) (PAA) are employed to systematically analyze the passivation effect from hydroxyl, carbonyl, and carboxyl groups. In general, PVA (-OH) can form hydrogen bonds with perovskite and PMA (-C═O) can complex with uncoordinated Pb2+. Specifically, PAA (-COOH) can interact selectively with MA+ and I- ions via hydrogen bonding and complex with uncoordinated Pb2+ to passivate defects more effectively. Hence, the PAA-incorporated PVSCs based on MAPbI3 achieve the champion power conversion efficiency (PCE) of 20.29% with open-circuit voltage up to 1.13 V. In addition, PAA cross-linking perovskite grains can relieve mechanical stress, as well as occupy the major channels to suppress ion migration and water/oxygen erosion. The corresponding unencapsulated devices demonstrate a superior light soaking stability, retaining more than 80% of the original PCE under one sun illumination for 1000 h.

Diammonium Molecular Configuration-Induced Regulation of Crystal Orientation and Carrier Dynamics for Highly Efficient and Stable 2D/3D Perovskite Solar Cells.

The effects from the molecular configuration of diammonium spacer cations on 2D/3D perovskite properties are still unclear. Here, we investigated systematically the mechanism of molecular configuration-induced regulation of crystallization kinetic and carrier dynamics by employing various diammonium molecules to construct Dion-Jacobson (DJ)-type 2D/3D perovskites to further facilitating the photovoltaic performance. The minimum average Pb-I-Pb angle leads to the smallest octahedral tilting of [PbX6 ]4- lattice in optimal diammonium molecule-incorporated DJ-type 2D/3D perovskite, which enables suitable binding energy and hydrogen-bonding between spacer cations and inorganic [PbX6 ]4- cages, thus contributing to the formation of high-quality perovskite film with vertical crystal orientation, mitigatory lattice distortion and efficient carrier transportation. As a consequence, a dramatically improved device efficiency of 22.68 % is achieved with excellent moisture stability.

Ionic Liquid-Induced Ostwald Ripening Effect for Efficient and Stable Tin-Based Perovskite Solar Cells.

Tin-based perovskite solar cells (PVSCs) are regarded as the most promising alternative among lead-free PVSCs. However, the rapid crystallization for tin-based perovskite tends to cause inferior film morphology and abundant defect states, which make poor photovoltaic performance. Here, 1-butyl-3-methylimidazolium bromide (BMIBr) ionic liquids (ILs) with strong polarity and a low melting point are first employed to produce the Ostwald ripening effect and obtain high-quality tin-based perovskite films with a large grain size. Meanwhile, the non-radiative recombination ascribed from defect states can also be effectively reduced for BMIBr-treated perovskite films. Consequently, a photoelectric conversion efficiency (PCE) of 10.09% for inverted tin-based PVSCs is attained by the Ostwald ripening effect. Moreover, the unencapsulated devices with BMIBr retain near 85% of the original PCE in a N2 glovebox beyond 1200 h and about 40% of the original PCE after exposure to air for 48 h.

Understanding the Mechanism between Antisolvent Dripping and Additive Doping Strategies on the Passivation Effects in Perovskite Solar Cells.

Perovskite polycrystalline films contain numerous intrinsic and interfacial defects ascribed to the solution preparation process, which are harmful to both the photovoltaic performance and the stability of perovskite solar cells (PVSCs). Although various passivators have been proved to be promising materials for passivating perovskite films, there is still a lack of deeper understanding of the effectiveness of the different passivation methods. Here, the mechanism between antisolvent dripping and additive doping strategies on the passivation effects in PVSCs is systematically investigated with a nonfullerene small molecule (F8IC). Such a passivated effect of F8IC is realized via coordination interactions between the carbonyl (C═O) and nitrile (C-N) groups of F8IC with Pb2+ ion of MAPbI3. Interestingly, F8IC antisolvent dripping can effectively passivate the surface defects and thus inhibit the nonradiative charge recombination on the upper part of the perovskite layer, whereas F8IC additive doping significantly reduces the surface and bulk defects and produces a compact perovskite film with denser crystal grains, thus facilitating charge transmission and extraction. Therefore, these benefits are translated into significant improvements in the short-circuit current density (Jsc) to 21.86 mA cm-2 and a champion power conversion efficiency of 18.40%. The selection of an optimal passivation strategy should also be considered according to the energy level matching between the passivators and the perovskite. The large energetic disparity is unsuitable for additive doping, whereas it is expected in antisolvent dripping.