Fabry-Perot Oscillation and Resonance Energy Transfer: Mechanism for Ultralow-Threshold Optically and Electrically Driven Random Laser in Quasi-2D Ruddlesden-Popper Perovskites.

The recently emerged metal-halide hybrid perovskite (MHP) possesses superb optoelectronic features, which have obtained great attention in solid-state lighting, photodetection, and photovoltaic applications. Because of its excellent external quantum efficiency, MHP has promising potential for the manifestation of ultralow threshold optically pumped laser. However, the demonstration of an electrically driven laser remains a challenge because of the vulnerable degradation of perovskite, limited exciton binding energy (Eb), intensity quenching, and efficiency drop by nonradiative recombinations. In this work, based on the paradigm of integration of Fabry-Perot (F-P) oscillation and resonance energy transfer, we observed an ultralow-threshold (∼250 µWcm-2) optically pumped random laser from moisture-insensitive mixed dimensional quasi-2D Ruddlesden-Popper phase perovskite microplates. Particularly, we demonstrated an electrically driven multimode laser with a threshold of ∼60 mAcm-2 from quasi-2D RPP by judicious combination of a perovskite/hole transport layer (HTL) and an electron transport layer (ETL) having suitable band alignment and thickness. Additionally, we showed the tunability of lasing modes and color by driving an external electric potential. Performing finite difference time domain (FDTD) simulations, we confirmed the presence of F-P feedback resonance, the light trapping effect at perovskite/ETL, and resonance energy transfer contributing to laser action. Our discovery of an electrically driven laser from MHP opens a useful avenue for developing future optoelectronics.

Layered perovskite materials: key solutions for highly efficient and stable perovskite solar cells.

Metal halide perovskites having three-dimensional crystal structures are being applied successfully in various optoelectronic applications. To address their most challenging issues-instability and toxicity-without losing efficiency, lower-dimensional perovskites appear to be promising alternatives. Recently, two-dimensional (2D) perovskite solar cells have been developed exhibiting excellent photostability and moisture-stability, together with moderate device efficiency. This review summarizes the photophysical properties and operating mechanisms of 2D perovskites as well as recent advances in their applications in solar cell devices. Also presented is an agenda for the next-stage development of stable perovskite materials for solar cell applications, highlighting the issues of stability and toxicity that require further study to ensure commercialization.

Modulating Performance and Stability of Inorganic Lead-Free Perovskite Solar Cells via Lewis-Pair Mediation.

Fully inorganic perovskites based on Bi3+ and Sb3+ are emerging as alternatives that overcome the toxicity and low stability of their Pb-based perovskite counterparts. Nevertheless, the thin film fabrication of Pb-free perovskites remains a struggle, with poor morphologies and incomplete conversions greatly inhibiting device performance. In this study, we modulated the crystallization of an all-inorganic dimer phase of a Sb perovskite (d-Cs3Sb2I9) through gradual increase in the annealing temperature, accompanied by the use of Lewis bases for adduct formation. Here, the role of Lewis pairing in the crystallization of the resulting Pb-free Cs3Sb2I9 thin films has been investigated. Both, "S-donor" (thiourea) and "O-donor" [N-methylpyrrolidone (NMP)] Lewis bases are examined for their abilities to form adducts with Cs+ and Sb3+ cations. Furthermore, density functional theory has been used to estimate the binding energies of these Lewis bases with the Cs3Sb2I9 lattice. Temperature-dependent photoluminescence spectroscopy revealed the nature of the band gap of d-Cs3Sb2I9. The efficiency of the resulting perovskite solar cells was enhanced to 1.8%, with excellent stability observed, when using NMP to form the adduct film. To the best of our knowledge, this is the best solar cell efficiency for the dimer phase of the inorganic Sb-based perovskite. The effects of both S- and O-donors are studied under various environmental stresses to reveal the stability responses of the devices.

Top Illuminated Hysteresis-Free Perovskite Solar Cells Incorporating Microcavity Structures on Metal Electrodes: A Combined Experimental and Theoretical Approach.

Further technological development of perovskite solar cells (PSCs) will require improvements in power conversion efficiency and stability, while maintaining low material costs and simple fabrication. In this Research Article, we describe top-illuminated ITO-free, stable PSCs featuring microcavity structures, wherein metal layers on both sides on the active layers exerted light interference effects in the active layer, potentially increasing the light path length inside the active layer. The optical constants (refractive index and extinction coefficient) of each layer in the PSC devices were measured, while the optical field intensity distribution was simulated using the transfer matrix method. The photocurrent densities of perovskite layers of various thicknesses were also simulated; these results mimic our experimental values exceptionally well. To modify the cavity electrode surface, we deposited a few nanometers of ultrathin MoO3 (2, 4, and 6 nm) in between the Ag and poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) layers provide hydrophobicity to the Ag surface and elevate the work function of Ag to match that of the hole transport layer. We achieved a power conversion efficiency (PCE) of 13.54% without hysteresis in the device containing a 4 nm-thick layer of MoO3. In addition, we fabricated these devices on various cavity electrodes (Al, Ag, Au, Cu); those prepared using Cu and Au anodes displayed improved device stability of up to 72 days. Furthermore, we prepared flexible PSCs having a PCE of 12.81% after incorporating the microcavity structures onto poly(ethylene terephthalate) as the substrate. These flexible solar cells displayed excellent stability against bending deformation, maintaining greater than 94% stability after 1000 bending cycles and greater than 85% after 2500 bending cycles performed with a bending radius of 5 mm.

Bifacial Perovskite Solar Cells Featuring Semitransparent Electrodes.

Inorganic-organic hybrid perovskite solar cells (PSCs) are promising devices for providing future clean energy because of their low cost, ease of fabrication, and high efficiencies, similar to those of silicon solar cells. These materials have been investigated for their potential use in bifacial PSCs, which can absorb light from both sides of the electrodes. Here, we fabricated bifacial PSCs featuring transparent BCP/Ag/MoO3 rear electrodes, which we formed through low-temperature processing using thermal evaporation methods. We employed a comprehensive optical distribution program to calculate the distributions of the optical field intensities with constant thicknesses of the absorbing layer in the top electrode configuration. The best PSC having a transparent BCP/Ag/MoO3 electrode achieved PCEs of 13.49% and 9.61% when illuminated from the sides of the indium tin oxide and BCP/Ag/MoO3 electrodes, respectively. We observed significant power enhancement when operating this PSC using mirror reflectors and bifacial light illumination from both sides of the electrodes.