Perovskite light emitting diodes suffer from poor operational stability, exhibiting a rapid decay of external quantum efficiency within minutes to hours after turn-on. To address this issue, we explore surface treatment of perovskite films with phenylalkylammonium iodide molecules of varying alkyl chain lengths. Combining experimental characterization and theoretical modelling, we show that these molecules stabilize the perovskite through suppression of iodide ion migration. The stabilization effect is enhanced with increasing chain length due to the stronger binding of the molecules with the perovskite surface, as well as the increased steric hindrance to reconfiguration for accommodating ion migration. The passivation also reduces the surface defects, resulting in a high radiance and delayed roll-off of external quantum efficiency. Using the optimized passivation molecule, phenylpropylammonium iodide, we achieve devices with an efficiency of 17.5%, a radiance of 1282.8 W sr−1 m−2 and a record T50 half-lifetime of 130 h under 100 mA cm−2.

Owing to the low exciton dissociation energy, long charge carrier lifetime, high absorption coefficient, and the ease of large-area fabrication of hybrid perovskite thin films, perovskite solar cells (PSCs) have attracted extensive attention during the past decade. [1][2][3][4][5][6][7][8][9][10][11] To date, the power conversion efficiency (PCE) of the stateof-the-art perovskite solar devices has been boosted from 3.8% [12] to a certified 25.5%. [13] Except for the composition (the types of cations and anions) and the quality of the perovskite absorber, [7,[14][15][16][17][18][19] the photoelectrical properties of the carrier transport layers (CTLs) are another important factor that determines the PCE and longterm stability of the PSCs. The optical and electrical properties of the CTLs not only affect the fill factor (FF) of the PSCs, but also the short-circuit current density ( J SC ). [6,8] In addition, the atomistic details of the surface of the CTLs also bring the

degradation caused by heat and electric field during intensive current stressing remain detrimental for the performance of PeLEDs and hinder the commercialization of this technology.Red-emitting PeLEDs are a critical component for both display and biomedical applications. One of the most popular strategies to combat instability in red perovskite emitters is dimension regulation. Highly efficient red PeLEDs have been fabricated by employing perovskite quantum dots (QDs) and multiple quantum well (MQW) perovskites to manipulate emission wavelength and confine excitons. [3][4][5][6][7] However, the hot-injection synthesis and post treatment of QDs require complex processing and high efficiencies can only be achieved at a low current density. The quantum-well width in MQW perovskites also needs to be finely regulated to avoid small-n phases and guarantee efficient energy transfer. [8,9] 3D perovskites converted directly from precursor mixtures would be more desirable because of simple fabrication and efficient charge transport. A mixture of bromide and iodide ions is generally required to construct 3D perovskite lattices that emit red light. In this case the formation of defects during the 3D film formation is almost inevitable. Various passivation strategies have been developed to address this issue and improved both the device efficiency by removing nonradiative 3D mixed-halide perovskite-based red emitters combine excellent chargetransport characteristics with simple solution processing and good film formation; however, light-emitting diodes (LEDs) based on these emitters cannot yet outperform their nanocrystal counterparts. Here the use of diammonium halides in regulating the formation of mixed bromide-iodide perovskite films is explored. It is found that the diammonium cations preferentially bond to Pb-Br, rather than Pb-I, octahedra, promoting the formation of quasi-2D phases. It is proposed that the perovskite formation is initially dominated by the crystallization of the thermodynamically more favorable 3D phase, but, as the solution gets depleted from the regular A cations, thin shells of amorphous quasi-2D perovskites form. This leads to crystalline perovskite grains with efficiently passivated surfaces and reduced lattice strain. As a result, the diammonium-treated perovskite LEDs demonstrate a record luminance (10745 cd m −2 ) and half-lifetime among 3D perovskitebased red LEDs.

Despite the rapid progress in perovskite solar cells, their commercialization is still hindered by issues regarding long-term stability, which can be strongly affected by metal oxide-based charge extraction layers next to the perovskite material. With MoO 3 being one of the most successful hole transport layers in organic photovoltaics, the disastrous results of its combination with perovskite films came as a surprise but was soon attributed to severe chemical instability at the MoO 3 /perovskite interface. To discover the atomistic origin of this instability, we combine density functional theory (DFT) calculations and X-ray photoelectron spectroscopy (XPS) measurements to investigate the interaction of MoO 3 with the perovskite precursors MAI, MABr, FAI, and FABr. From DFT calculations we suggest a scenario that is based upon oxygen vacancies playing a key role in interface degradation reactions. Not only do these vacancies promote decomposition reactions of perovskite precursors, but they also constitute the reaction centers for redox reactions leading to oxidation of the halides and reduction of Mo. Specifically iodides are proposed to be reactive, while bromides do not significantly affect the oxide. XPS measurements reveal a severe reduction of Mo and a loss of the halide species when the oxide is interfaced with I-containing precursors, which is consistent with the proposed scenario. In line with the latter, experimentally observed effects are much less pronounced in case of Br-containing precursors. We further find that the reactivity of the MoO 3 substrate can be moderated by reducing the number of oxygen vacancies through a UV/ozone treatment, though it cannot be fully eliminated.

In recent years, metal halide perovskites (MHPs) for optoelectronic applications have attracted the attention of the scientific community due to their outstanding performance. The fundamental understanding of their physicochemical properties is essential for improving their efficiency and stability. Atomistic and molecular simulations have played an essential role in the description of the optoelectronic properties and dynamical behavior of MHPs, respectively. However, the complex interplay of the dynamical and optoelectronic properties in MHPs requires the simultaneous modeling of electrons and ions in relatively large systems, which entails a high computational cost, sometimes not affordable by the standard quantum mechanics methods, such as density functional theory (DFT). Here, we explore the suitability of the recently developed density functional tight binding method, GFN1-xTB, for simulating MHPs with the aim of exploring an efficient alternative to DFT. The performance of GFN1-xTB for computing structural, vibrational, and optoelectronic properties of several MHPs is benchmarked against experiments and DFT calculations. In general, this method produces accurate predictions for many of the properties of the studied MHPs, which are comparable to DFT and experiments. We also identify further challenges in the computation of specific geometries and chemical compositions. Nevertheless, we believe that the tunability of GFN1-xTB offers opportunities to resolve these issues and we propose specific strategies for the further refinement of the parameters, which will turn this method into a powerful computational tool for the study of MHPs and beyond.

The inverted p-i-n perovskite solar cells hold high promise for scale-up toward commercialization. However, the interfaces between the perovskite and the charge transport layers contribute to major power conversion efficiency (PCE) loss and instability. Here, we use a single material of 2-thiopheneethylammonium chloride (TEACl) to molecularly engineer both the interface between the perovskite and fullerene-C 60 electron transport layer and the buried interface between the perovskite and NiO x -based hole transport layer. The dual interface modification results in optimized band alignment, suppressed nonradiative recombination, and improved interfacial contact. A PCE of 24.3% is demonstrated, with open-circuit voltage (V oc ) and fill factor (FF) of 1.17 V and 84.6%, respectively. The unencapsulated device retains >97.0% of the initial performance after 1000 h of maximum power point tracking under illumination. Moreover, a PCE of 22.6% and a remarkable FF of 82.4% are obtained for a mini-module with an active area of 3.63 cm 2 .

Favorable optoelectronic properties and ease of fabrication make NiO a promising hole transport layer for perovskite solar cells. To achieve maximum efficiency, the electronic levels of NiO need to be optimally aligned with those of the perovskite absorber. Applying surface modifiers by adsorbing species on the NiO surface is one of the most widespread strategies to tune its energy levels. Alkali halides are simple inorganic surface modifiers that have been used extensively in organic optoelectronics, but rarely studied in perovskite solar cells. Using density functional theory calculations, we investigate the effect of single-layer adsorption of 20 different alkali halides on the electronic levels of NiO. Our results show that alkali halides can shift the position of the valence-band maximum (VBM) of NiO to a surprisingly large extent in both directions, from −3.10 to +1.59 eV. We interpret the direction and magnitude of the shift in terms of the surface dipoles, formed by the adsorbed cations and anions, where the magnitude of the VBM shift is a monotonic function of the surface coverage. Our results indicate that with alkali halide surface modifiers, the electronic levels of NiO can be tuned robustly and potentially match those of many perovskite compositions in perovskite solar cells.

Polycrystalline perovskite films fabricated on flexible and textured substrates often are highly defective, leading to poor performance of perovskite devices. Finding substrate-tolerant perovskite fabrication strategies is therefore paramount. Herein, we show that a small amount of Cadmium Acetate (CdAc2) in PbI2 precursor solution results in nano-hole array films and improves the diffusion of organic salts in PbI2, and promotes favorable crystal orientation and suppresses non-radiative recombination. Polycrystalline perovskite films on flexible substrate with ultra-long carrier lifetimes exceeding 6 μs are achieved. Eventually, a record PCE of 22.78% is obtained for single-junction flexible perovskite solar cells (FPSCs). Furthermore, we find this strategy is also demonstrated to be applicable for textured tandem solar cells. Our perovskite/silicon tandem solar cells (TSCs) with CdAc2 showed an efficiency of 29.25% (0.5003 cm2), with a certified value being 23.07% (11.8792 cm2), respectively, the highest value reported for solution-based TSCs. Un-encapsulated TSCs based on CdAc2 maintain 109.78% of their initial efficiency after operation at maximum power point under continuous one-sun illumination for 300 hours at 45 oC in N2 atmosphere. This strategy will provide facile access to high-efficiency perovskite-based solar cells.