Over 21% Efficiency Stable 2D Perovskite Solar Cells

Quasi-2D Ruddlesden-Popper Lead Halide Perovskites: How Edge Matters

A band-mapping technique is introduced to investigate the formation of low-energy edge states in quasi-2D Ruddlesden-Popper (RP) perovskites, (BA)₂(MA)_n-1Pb_nI_3n+1, through a localized mode of measurement, namely, scanning tunneling spectroscopy. The local band structures measured at different points reveal the formation of 3D CH₃NH₃PbI₃ (MAPbI₃) at the edges of the perovskite nanosheets; for thin films, the 3D phase (n = ∞) could be seen to form at grain boundaries. The presence of MAPbI₃ at the edges or grain boundaries of the perovskites has led to self-forming type-II band alignment in BA₂MA₂Pb₃I₁₀ (n = 3). The rationale behind achieving a high-efficiency solar cell based on the material, which has a large exciton binding energy, has been inferred. Kelvin probe force microscopy studies under illumination have yielded a higher surface photovoltage at the edges compared to the interior and supported the inference of exciton dissociation due to internal type-II band alignment in the quasi-2D RP perovskites.

Using the Diamagnetic Coefficients to Estimate the Reduced Effective Mass in 2D Layered Perovskites: New Insight from High Magnetic Field Spectroscopy

In this work, the current state of research concerning the determination of the effective mass in 2D layered perovskites is presented. The available experimental reports in which the reduced effective mass μ has been directly measured using magneto-absorption spectroscopy of interband Landau levels are reviewed. By comparing these results with DFT computational studies and various other methods, it is concluded that depending on the approach used, the μ found spans a broad range of values from as low as 0.05 up to 0.3 me. To facilitate quick and reliable estimation of μ, a model is proposed based solely on the available experimental data that bypass the complexity of interband Landau level spectroscopy. The model takes advantage of the μ value measured for (PEA)2PbI4 and approximates the reduced effective mass of the given 2D layered perovskites based on only two experimental parameters—the diamagnetic coefficient and the effective dielectric constant. The proposed model is tested on a broad range of 2D layered perovskites and captures well the main experimental and theoretical trends.

Spray-coated perovskite hemispherical photodetector featuring narrow-band and wide-angle imaging

Sphere imagers featuring specific wavelength recognition and wide-angle imaging are required to meet the fast development of modern technology. However, it is still challenging to deposit high-quality photosensitive layers on sphere substrates from low-cost solution processes. Here we report spray-coated quasi-two-dimensional phenylethylammonium/formamidinium lead halide (PEA₂FA_n-1Pb_nX_3n+1) perovskite hemispherical photodetectors. The crystallization speed is manipulated by perovskite compositions, and the film thickness can be controlled by spray-coating cycles and solution concentration from tens of nanometers to hundreds of micrometers with a fast velocity of 1.28 × 10^-4 cm³ s^-1. The lens-free hemispherical photodetectors allow light response at a wide incident angle of 180°. Simultaneously, the wavelength selective response from visible to the near-infrared range is achieved with full width at half maximums (FWHMs) of ~20 nm, comparable to single-crystal devices. Wide-angle and wavelength-selective imaging are also demonstrated, which can find potential applications in intelligent recognition and intraoperative navigated surgery.

Multidimensional Perovskites for High Detectivity Photodiodes

Low-dimensional perovskites attract increasing interest due to tunable optoelectronic properties and high stability. Here, it is shown that perovskite thin films with a vertical gradient in dimensionality result in graded electronic bandgap structures that are ideal for photodiode applications. Positioning low-dimensional, vertically-oriented perovskite phases at the interface with the electron blocking layer increases the activation energy for thermal charge generation and thereby effectively lowers the dark current density to a record-low value of 5 × 10^-9  mA cm^-2 without compromising responsivity, resulting in a noise-current-based specific detectivity exceeding 7 × 10¹² Jones at 600 nm. These multidimensional perovskite photodiodes show promising air stability and a dynamic range over ten orders of magnitude, and thus represent a new generation of high-performance low-cost photodiodes.

Fast A‐Site Cation Cross‐Exchange at Room Temperature: Single‐to Double‐ and Triple‐Cation Halide Perovskite Nanocrystals

We report here fast A‐site cation cross‐exchange between APbX₃ perovskite nanocrystals (NCs) made of different A‐cations (Cs (cesium), FA (formamidinium), and MA (methylammonium)) at room temperature. Surprisingly, the A‐cation cross‐exchange proceeds as fast as the halide (X=Cl, Br, or I) exchange with the help of free A‐oleate complexes present in the freshly prepared colloidal perovskite NC solutions. This enabled the preparation of double (MACs, MAFA, CsFA)‐ and triple (MACsFA)‐cation perovskite NCs with an optical band gap that is finely tunable by their A‐site composition. The optical spectroscopy together with structural analysis using XRD and atomically resolved high‐angle annular dark‐field scanning transmission electron microscopy (HAADF‐STEM) and integrated differential phase contrast (iDPC) STEM indicates the homogeneous distribution of different cations in the mixed perovskite NC lattice. Unlike halide ions, the A‐cations do not phase‐segregate under light illumination.

Novel PHA Organic Spacer Increases Interlayer Interactions for High Efficiency in 2D Ruddlesden-Popper CsPbI3 Solar Cells

The two-dimensional (2D) Ruddlesden-Popper (RP) CsPbI₃ with hydrophobic organic spacers can significantly improve the environmental and phase stability of photovoltaic devices by suppressing ion migration and inducing steric hindrance. However, due to the multiple-quantum-well structure, these spacer cations lead to weak interactions in 2D RP CsPbI₃, which seriously affect the carrier transport. Here, a novel N-H-group-rich phenylhydrazine spacer, namely, PHA, was developed for 2D RP CsPbI₃ perovskite solar cells (PSCs). A series of characterizations confirm that the 2D perovskites using PHA spacers enhanced the N-H···I hydrogen-bonding interaction between the organic spacer cations and the [PbI₆]^4- inorganic layer and accelerated the crystallization rate of the perovskite film, which was beneficial to the preparation of high-quality films with preferred vertical orientation, large grain size, and dense morphology. Meanwhile, the trap state density of the as-prepared 2D RP perovskite films is significantly reduced to enable efficient charge carrier transport. As a result, the (PHA)₂Cs₄Pb₅I₁₆ PSCs achieved a performance of 16.23% with good environmental stability. This work provides a simple organic spacer selection scheme to realize interaction optimization in 2D RP CsPbI₃ to develop efficient and stable PSCs.

Recognizing the Importance of Fast Nonisothermal Crystallization for High-Performance Two-Dimensional Dion-Jacobson Perovskite Solar Cells with High Fill Factors: A Comprehensive Mechanistic Study

Two-dimensional (2D) Dion-Jacobson (DJ) perovskite solar cells (PSCs), despite their advantage in versatility of n-layer variation, are subject to poor photovoltaic efficiency, particularly in the fill factor (FF), compared to their three-dimensional counterparts. To enhance the performance of DJ PSCs, the process of growing crystals and hence the corresponding morphology of DJ perovskites are of prime importance. Herein, we report the fast nonisothermal (NIT) crystallization protocol that is previously unrecognized for 2D perovskites to significantly improve the morphology, orientation, and charge transport of the DJ perovskite films. Comprehensive mechanistic studies reveal that the NIT effect leads to the secondary crystallization stage, forming network-like channels that play a vital role in the FF's leap-forward improvement and hence the DJ PSC's performance. As a whole, the NIT crystallized PSCs demonstrate a high power conversion efficiency and an FF of up to 19.87 and 86.16%, respectively. This research thus provides new perspectives to achieve highly efficient DJ PSCs.

PEDOT:PSS/CuCl Composite Hole Transporting Layer for Enhancing the Performance of 2D Ruddlesden-Popper Perovskite Solar Cells

Poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) is a popular hole transport layer (HTL) in 2D Ruddlesden-Popper (RP) perovskite solar cell (PSCs) due to its highly conductive, transparent, and solution-processable characteristics. However, fundamental questions such as its strong acidity or mismatched energy level with the 2D RP photoactive layer often restrict the performance and stability of devices. Herein, copper chloride (CuCl), a common direct band gap semiconductor, is doped into PEDOT:PSS, lowering the acidity and tuning the work function of PEDOT:PSS. Due to the improved wettability and the existing chloride in the PEDOT:PSS/CuCl composite substrate, the coated 2D perovskite films exhibit uniform morphology, vertically oriented crystal growth, and enhanced crystallinity. In comparison with controlled devices, the PEDOT:PSS/CuCl based inverted 2D RP PSCs show a maximum power conversion efficiency of 13.36% and long-term stability. The modified PEDOT:PSS overcomes intrinsic imperfections by doping CuCl, indicating that it has a lot of promise for mass production in electrical devices.

Highly Efficient and Stable 2D Dion Jacobson/3D Perovskite Heterojunction Solar Cells

Heterostructures involving two-dimensional/three-dimensional (2D/3D) perovskites have recently attracted increased attention due to their ability to combine the high photovoltaic performance of 3D perovskites with the increased stability of 2D perovskites. Here we report ammonium thiocyanate (NH₄SCN) passivated 3D methylammonium lead triiodide (MAPbI₃) perovskite active layer and deposition of 2D perovskite capping layer using xylylene diammonium iodide (XDAI) organic cation. The 2D/3D perovskite heterojunction formation is probed by using FESEM and UPS spectroscopy. The NH₄SCN passivated MAPbI₃ perovskite has shown 19.6% PCE compared to the 17.18% PCE of pristine MAPbI₃ perovskite solar cells (PSCs). Finally, the champion 2D/3D perovskite heterojunction based solar cells have achieved the remarkable PCE of 20.74%. The increased PCE in 2D/3D PSCs is mainly attributed to the reduced defect density and suppressed nonradiative recombination losses. Moreover, the hydrophobic 2D capping layer endows the 2D/3D heterojunction perovskites with exceptional moisture, thermal and UV stability, highlighting the promise of highly stable and efficient 2D/3D PSCs.

Rearranging the Phase Distribution of Quasi-2D Perovskite for Efficient and Narrow Emission Perovskite Light-Emitting Diodes

Quasi-2D perovskite light-emitting diodes (PeLEDs) have attracted significant attention for their promising light-emitting applications. However, quasi-2D perovskite films typically consist of a broad phase distribution and small grains with a large surface area to volume ratio, leading to inferior color purities and higher defect densities. Herein, a bifunctional additive ((l)-tryptophan bromide, l-TrpBr) was introduced into a quasi-2D perovskite film. The C═O moiety of l-TprBr formed hydrogen bonds with S-MBA⁺, retarding the coordination between S-MBABr and [PbBr₆]^4- and suppressing the formation of small-n phases. The C═O moiety also coordinated with unsaturated Pb²⁺ sites to passivate the defects. Finally, the PeLEDs with l-TrpBr exhibited a significantly improved EQE of 14.32% compared to the control devices (7.88%) and the narrowest fwhm (17 nm) for green quasi-2D PeLEDs reported to date. Our work provides a practical approach to controlling the phase distribution and passivating the defects in quasi-2D perovskite films, toward high-efficiency and color-pure quasi-2D PeLEDs.

Elucidating the Unique Hot Carrier Cooling in Two-Dimensional Inorganic Halide Perovskites: The Role of Out-of-Plane Carrier-Phonon Coupling

Two-dimensional (2D) halide perovskites represent the natural semiconductor quantum wells (QWs), which hold great promise for optoelectronics. However, due to the hybrid structure of Ruddlesden-Popper 2D perovskites, the intrinsic nature of hot-carrier kinetics remains shielded within. Herein, we adopt CsPbBr₃ nanoplates as a model system to reveal the intrinsic carrier dynamics in inorganic perovskite QWs. Interestingly, we revealed an ultrafast and hot-phonon-bottleneck (HPB)-free carrier cooling in monodisperse CsPbBr₃ QWs, which is in sharp contrast to the bulk and nanocrystalline perovskites. The absence of HPB was attributed to the efficient out-of-plane triplet-exciton-LO-phonon coupling in 2D perovskites because of the structural anisotropy. Accordingly, the HPB can be activated by shutting down the out-of-plane energy loss route through forming the layer-stacked perovskite superlattice. The controllable on and off of HPB may provide new possibilities in optoelectronic devices and these findings deepen the understanding of a hot-carrier cooling mechanism in 2D perovskites.

Insight into the Enhanced Charge Transport in Quasi-2D Perovskite via Fluorination of Ammonium Cations for Photovoltaic Applications

Fluorinated spacer cations in quasi-2D (Q-2D) perovskites have recently been demonstrated to improve the Q-2D perovskite solar cell (PSC) performance. However, the underlying mechanism of fluorination of organic cations on the improvement is still unclear. Here, using fluorinated benzylammonium (named F-BZA) as a spacer cation in Q-2D Ruddlesden-Popper (RP) perovskites, we deeply investigate the effect of fluorination of organic cations on perovskite crystallization and intermolecular interactions for improving the charge transport and device performance. It is found that fluorination of spacer cations can slow down the crystallization rate of perovskites, resulting in vertically aligned large grains. Moreover, the interaction between the adjacent spacer cations is further enhanced, constructing a new faster charge-transport channel with a lifetime of 77 ps. Accordingly, the carrier mobility is improved by an order of magnitude and a power conversion efficiency (PCE) of 16.82% is achieved in much more stable F-BZA-based Q-2D RP PSCs, 35% higher than that of BZA-based devices (12.39%). Our results elucidate the mechanism and its importance of fluorinating spacer cations for high-performance Q-2D PSC development.

Mixed-halide copper-based perovskite R2Cu(Cl/Br)4 with different organic cations for reversible thermochromism

Mechanically exfoliated flakes of mixed-halide Cu-based perovskite crystals, R2Cu(Cl/Br)4, with three alkyl chains exhibit reversible thermochromic behavior with differences in crystal lattice behavior depending on the organic spacer used.