Superior Phonon-Limited Exciton Mobility in Lead-Free Two-Dimensional Perovskites.

Tin-based two-dimensional (2D) perovskites are emerging as lead-free alternatives in halide perovskite materials, yet their exciton dynamics and transport remain less understood due to defect scattering. Addressing this, we employed temperature-dependent transient photoluminescence (PL) microscopy to investigate intrinsic exciton transport in three structurally analogous Sn- and Pb-based 2D perovskites. Employing conjugated ligands, we synthesized high-quality crystals with enhanced phase stability at various temperatures.

Thickness control of organic semiconductor-incorporated perovskites.

Two-dimensional organic semiconductor-incorporated perovskites are a promising family of hybrid materials for optoelectronic applications, owing in part to their inherent quantum well architecture. Tuning their structures and properties for specific properties, however, has remained challenging. Here we report a general method to tune the dimensionality of phase-pure organic semiconductor-incorporated perovskite single crystals during their synthesis, by judicious choice of solvent.

Suppressing phase disproportionation in quasi-2D perovskite light-emitting diodes.

Electroluminescence efficiencies and stabilities of quasi-two-dimensional halide perovskites are restricted by the formation of multiple-quantum-well structures with broad and uncontrollable phase distributions. Here, we report a ligand design strategy to substantially suppress diffusion-limited phase disproportionation, thereby enabling better phase control. We demonstrate that extending the π-conjugation length and increasing the cross-sectional area of the ligand enables perovskite thin films with dramatically suppressed ion transport, narrowed phase distributions, reduced defect densities, and enhanced radiative recombination efficiencies.

Ligand-Driven Grain Engineering of High Mobility Two-Dimensional Perovskite Thin-Film Transistors.

Controlling grain growth is of great importance in maximizing the charge carrier transport for polycrystalline thin-film electronic devices. The thin-film growth of halide perovskite materials has been manipulated via a number of approaches including solvent engineering, composition engineering, and post-treatment processes. However, none of these methods lead to large-scale atomically flat thin films with extremely large grain size and high charge carrier mobility.

Lead-Free Organic-Perovskite Hybrid Quantum Wells for Highly Stable Light-Emitting Diodes.

Two-dimensional perovskites that could be regarded as natural organic-inorganic hybrid quantum wells (HQWs) are promising for light-emitting diode (LED) applications. High photoluminescence quantum efficiencies (approaching 80%) and extremely narrow emission bandwidth (less than 20 nm) have been demonstrated in their single crystals; however, a reliable electrically driven LED device has not been realized owing to inefficient charge injection and extremely poor stability. Furthermore, the use of toxic lead raises concerns.

Highly Efficient Halide Perovskite Light-Emitting Diodes via Molecular Passivation.

Metal halide perovskites are promising for applications in light-emitting diodes (LEDs), but still suffer from defects-mediated nonradiative losses, which represent a major efficiency-limiting factor in perovskite-based LEDs (PeLEDs). Reported here is a strategy to synthesize molecular passivators with different anchoring groups for defects passivation. The passivated perovskite thin films exhibit improved optoelectronic properties as well as reduced grain size and surface roughness, thus enable highly efficient PeLEDs with an external quantum efficiency of 15.

Protecting hot carriers by tuning hybrid perovskite structures with alkali cations.

Successful implementation of hot carrier solar cells requires preserving high carrier temperature as carriers migrate through the active layer. Here, we demonstrated that addition of alkali cations in hybrid organic-inorganic lead halide perovskites led to substantially elevated carrier temperature, reduced threshold for phonon bottleneck, and enhanced hot carrier transport. The synergetic effects from the Rb, Cs, and K cations result in ~900 K increase in the effective carrier temperature at a carrier density around 10 cm with an excitation 1.

Phenethylammonium Functionalization Enhances Near-Surface Carrier Diffusion in Hybrid Perovskites.

Understanding semiconductor surface properties and manipulating them chemically are critical for improving their performance in optoelectronic devices. Hybrid halide perovskites have emerged as an exciting class of highly efficient solar materials; however, their device performance could be limited by undesirable surface properties that impede carrier transport and induce recombination. Here we show that surface functionalization of methylammonium lead iodide (MAPbI) perovskite with phenethylammonium iodide (PEAI), a commonly employed spacer cation in two-dimensional halide perovskites, can enhance carrier diffusion in the near-surface regions and reduce defect density by more than 1 order of magnitude.

Long-lived charge separation in two-dimensional ligand-perovskite heterostructures.

Rational design of heterojunctions using nanostructured materials is a useful strategy for achieving efficient interfacial charge separation in photovoltaics. Heterojunctions can be constructed between the organic ligands and the inorganic layers in two-dimensional perovskites, taking advantage of their highly programmable structures. Here, we investigate charge transfer and recombination at the interface between the thiophene-based semiconducting ligands and the lead halide inorganic sublattices using time-resolved photoluminescence and transient reflection spectroscopy in single two-dimensional perovskite crystals.

Long-range exciton transport and slow annihilation in two-dimensional hybrid perovskites.

Two-dimensional hybrid organic-inorganic perovskites with strongly bound excitons and tunable structures are desirable for optoelectronic applications. Exciton transport and annihilation are two key processes in determining device efficiencies; however, a thorough understanding of these processes is hindered by that annihilation rates are often convoluted with exciton diffusion constants. Here we employ transient absorption microscopy to disentangle quantum-well-thickness-dependent exciton diffusion and annihilation in two-dimensional perovskites, unraveling the key role of electron-hole interactions and dielectric screening.

A Perylene-Based Polycyclic Aromatic Hydrocarbon Electron Donor for a Highly Efficient Solar Cell Dye.

The continuing efforts to develop novel polycyclic aromatic hydrocarbons and exploit them as building blocks to create organic donor-acceptor (D-A) dyes with impressive excited-state features should offer an excellent means by which to improve the power conversion efficiency (PCE) of dye-sensitized solar cells (DSSCs). To this end, a nonacyclic aromatic hydrocarbon, N-annulated benzoindenopentaphene (NBIP) was tethered with multiple solubilizing groups, including NBIPs with one 2-hexyldecyl, with one 2-hexyldecyloxy, and with four 4-hexylphenyl substituents. The side- and end-chain-functionalized NBIPs can be conveniently prepared in excellent yield, and further cross-coupled with 4-(7-ethynylbenzo[c][1,2,5]thiadiazol-4-yl)benzoic acid to afford a metal-free D-A dye, which achieves a high power conversion efficiency of 12.