Engineering Band-Type Alignment in CsPbBr3 Perovskite-Based Artificial Multiple Quantum Wells

Engineering Spacer Conjugation for Efficient and Stable 2D/3D Perovskite Solar Cells and Modules

Incorporating two-dimensional (2D) perovskite in 3D perovskite absorber holds great potential to improve the stability and efficiency of perovskite solar cells (PSCs). However, the bulky-cation-based 2D structures often exhibit poor charge transport and are prone to formation of charge-extraction barrier that impedes efficient device operation. We address these issues by introducing aromatic spacers with molecular conjugation into 2D perovskites locating between 3D perovskites and electron charge transport layers. Among our tested aromatic spacers, the pyrenylbutanamine (PyBA) spacer was shown to endow 2D perovskites with superior charge transport properties and efficient charge extraction from the bulk perovskite in 2D/3D PSCs, due to the highest degree of conjugation. As a result, we achieved a power conversion efficiency (PCE) of up to 25.3 % in a 0.16-cm2 single cell and 21.0 % in a 24.8-cm2 module. Moreover, the incorporated PyBA substantially raised the resistance of 2D/3D PSCs against moisture and ion migration, resulting in enhanced environmental, thermal, and operational stability. Notably, the PyBA-based devices retained over 90 % of their initial PCE after 2000 hours at 25 °C and 80 % relative humidity, or 1000 hours at 85 °C and 85 % humidity, or 3000 hours of operation under continuous 1-Sun illumination at 40 °C, showcasing their exceptionally high stability compared to previously reported 2D/3D PSCs.

Luminescence and Degeneration Mechanism of Perovskite Light-Emitting Diodes and Strategies for Improving Device Performance

Perovskite light-emitting diodes (PeLEDs) can be a promising technology for next-generation display and lighting applications due to their excellent optoelectronic properties. However, a systematical overview of luminescence and degradation mechanism of perovskite materials and PeLEDs is lacking. Therefore, it is crucial to fully understand these mechanisms and further improve device performances. In this work, the fundamental photophysical processes of perovskite materials, electroluminescence mechanism of PeLEDs including carrier kinetics and efficiency roll-off as well as device degradation mechanism are discussed in detail. In addition, the strategies to improve device performances are summarized, including optimization of photoluminescence quantum yield, charge injection and recombination, and light outcoupling efficiency. It is hoped that this work can provide guidance for future development of PeLEDs and ultimately realize industrial applications.

Quantum Tunneling Effect in CsPbBr3 Multiple Quantum Wells

Two-dimensional (2D) lead halide perovskites (LHPs) have garnered incredible attention thanks to their exciting optoelectronic properties and intrinsic strong quantum confinement effect. Herein, we carefully investigate and decipher the charge carrier dynamics at the interface between CsPbBr₃ multiple quantum wells (MQWs) as the photoactive layer and TiO₂ and Spiro-OMeTAD as electron and hole transporting materials, respectively. The fabricated MQWs comprise three monolayers of CsPbBr₃ separated by 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) as barriers. By varying the BCP thickness, we show that charge carrier extraction from MQWs to the corresponding extracting layer occurs through a quantum tunneling effect, as elaborated by steady-state and time-resolved photoluminescence measurements and further verified by femtosecond transient absorption experiments. Ultimately, we have investigated the impact of the barrier-thickness-dependent quantum tunneling effect on the photoelectric behavior of the synthesized QW photodetector devices. Our findings shed light on one of the most promising approaches for efficient carrier extraction in quantum-confined systems.

Pressure-Induced Indirect-Direct Bandgap Transition of CsPbBr3 Single Crystal and Its Effect on Photoluminescence Quantum Yield

Despite extensive study, the bandgap characteristics of lead halide perovskites are not well understood. Usually, these materials are considered as direct bandgap semiconductors, while their photoluminescence quantum yield (PLQY) is very low in the solid state or single crystal (SC) state. Some researchers have noted a weak indirect bandgap below the direct bandgap transition in these perovskites. Herein, application of pressure to a CsPbBr3 SC and first-principles calculations reveal that the nature of the bandgap becomes more direct at a relatively low pressure due to decreased dynamic Rashba splitting. This effect results in a dramatic PLQY improvement, improved more than 90 times, which overturns the traditional concept that the PLQY of lead halide perovskite SC cannot exceed 10%. Application of higher pressure transformed the CsPbBr3 SC into a pure indirect bandgap phase, which can be maintained at near-ambient pressure. It is thus proved that lead halide perovskites can induce a phase transition between direct and indirect bandgaps. In addition, distinct piezochromism is observed for a perovskite SC for the first time. This work provides a novel framework to understand the optoelectronic properties of these important materials.

Optical Properties of Perovskite-Organic Multiple Quantum Wells

A comprehensive study of the optical properties of CsPbBr₃ perovskite multiple quantum wells (MQW) with organic barrier layers is presented. Quantum confinement is observed by a blue-shift in absorption and emission spectra with decreasing well width and agrees well with simulations of the confinement energies. A large increase of emission intensity with thinner layers is observed, with a photoluminescence quantum yield up to 32 times higher than that of bulk layers. Amplified spontaneous emission (ASE) measurements show very low thresholds down to 7.3 µJ cm^-2 for a perovskite thickness of 8.7 nm, significantly lower than previously observed for CsPbBr₃ thin-films. With their increased photoluminescence efficiency and low ASE thresholds, MQW structures with CsPbBr₃ are excellent candidates for high-efficiency perovskite-based LEDs and lasers.

Plasmonic Nb2CTx MXene-MAPbI3 Heterostructure for Self-Powered Visible-NIR Photodiodes

The ability of MXenes to efficiently absorb light is greatly enriched by the surface plasmons oscillating at their two-dimensional (2D) surfaces. Thus far, MXenes have shown impressive plasmonic absorptions spanning the visible and infrared (IR) regimes. However, their potential use in IR optoelectronic applications, including photodiodes, has been marginally investigated. Besides, their relatively low resistivity has limited their use as photosensing materials due to their intrinsic high dark current. Herein, heterostructures made of methylammonium lead triiodide (MAPbI₃) perovskite and niobium carbide (Nb₂CT_x) MXene are prepared with a matching band structure and exploited for self-powered visible-near IR (NIR) photodiodes. Using MAPbI₃ has expanded the operation range of the MAPbI₃/Nb₂CT_x photodiode to the visible regime while suppressing the relatively large dark current of the NIR-absorbing Nb₂CT_x. In consequence, the fabricated MAPbI₃/Nb₂CT_x photodiode has responded linearly to white light illumination with a responsivity of 0.25 A/W and a temporal photoresponse of <4.5 μs. Furthermore, when illuminated by NIR laser (1064 nm), our photodiode demonstrates a higher on/off ratio (∼10³) and faster response times (<30 ms) compared to that of planar Nb₂CT_x-only detectors (<2 and 20 s, respectively). The performed space-charge-limited current (SCLC) and capacitance measurements reveal that such an efficient and enhanced charge transfer depends on the coordinate bonding between the surface groups of the MXene and the undercoordinated Pb²⁺ ions of the MAPbI₃ at the passivated MAPbI₃/Nb₂CT_x interface.