Broadband emission in lead iodide 2D perovskites has been alternately attributed to self-trapped excitons (STEs) or permanent structural defects and/or impurities. Here, we investigate six different multilayered (n > 1) 2D lead iodide perovskites as a function of sample temperature from 5 to 300 K. We distinguish shallow defect-associated emission from a broad near-infrared (NIR) spectral feature, which we assign to an STE through subgap photoexcitation experiments. When we varied the thickness (n = 2, 3, 4), A-site cation (methylammonium vs formamidinium), and organic spacer (butylammonium vs hexylammonium vs phenylethylammonium), we found that the temperature dependence of broad NIR emission was strongly correlated with both the strength of electron–phonon coupling and the extent of structural deformation of the ground-state lattice, strongly supporting the assignment of this spectral feature to an STE. However, the extent to which formation of these STEs is intrinsic versus defect-assisted remains open to debate.

We characterize exciton−exciton interactions in weakly confined CsPbBr 3 nanocrystals by combining fluence-dependent transient absorption spectroscopy with a robust spectral deconvolution method. This data-driven approach allows for the extraction of overlapping transient absorption spectra of exciton and biexciton states while making no assumptions about the spectral line shape. The ensemble spectrum of the biexciton state is found to be broader and blue-shifted from the spectrum of the exciton state, with both effects becoming more prominent as nanocrystal size decreases. We conclude that exciton−exciton interactions in CsPbBr 3 are net repulsive at room temperature, but redshifted optical gain from the biexciton state is still possible because the spectral broadening exceeds the blue-shifting. Finally, we extract size-dependent biexciton lifetimes of 35−200 ps, increasing with nanocrystal size. In contrast to the attractive exciton−exciton interactions found in most conventional semiconductor nanocrystals, repulsive exciton− exciton interactions may arise from polaron formation in perovskites. These observations provide important insight into lasing, high-flux emission, and multicarrier interactions in this class of materials.

Substitutional metal doping is a powerful strategy for manipulating the emission spectra and excited state dynamics of semiconductor nanomaterials. Here, we demonstrate the synthesis of colloidal manganese (Mn2+)-doped organic–inorganic hybrid perovskite nanoplatelets (chemical formula: L2[APb1–x Mn x Br3] n−1Pb1–x Mn x Br4; L, butylammonium; A, methylammonium or formamidinium; n (= 1 or 2), number of Pb1–x Mn x Br6 4– octahedral layers in thickness) via a ligand-assisted reprecipitation method. Substitutional doping of manganese for lead introduces bright (approaching 100% efficiency) and long-lived (>500 μs) midgap Mn2+ atomic states, and the doped nanoplatelets exhibit dual emission from both the band edge and the dopant state. Photoluminescence quantum yields and band-edge-to-Mn intensity ratios exhibit strong excitation power dependence, even at a very low incident intensity (<100 μW/cm2). Surprisingly, we find that the saturation of long-lived Mn2+ dopant sites cannot explain our observation. Instead, we propose an alternative mechanism involving the cross-relaxation of long-lived Mn-site excitations by freely diffusing band-edge excitons. We formulate a kinetic model based on this cross-relaxation mechanism that quantitatively reproduces all of the experimental observations and validate the model using time-resolved absorption and emission spectroscopy. Finally, we extract a concentration-normalized microscopic rate constant for band edge-to-dopant excitation transfer that is ∼10× faster in methylammonium-containing nanoplatelets than in formamidinium-containing nanoplatelets. This work provides fundamental insight into the interaction of mobile band edge excitons with localized dopant sites in 2D semiconductors and expands the toolbox for manipulating light emission in perovskite nanomaterials.

Silver phenylselenolate (AgSePh, also known as “mithrene”) and silver phenyltellurolate (AgTePh, also known as “tethrene”) are two-dimensional (2D) van der Waals semiconductors belonging to an emerging class of hybrid organic–inorganic materials called metal–organic chalcogenolates. Despite having the same crystal structure, AgSePh and AgTePh exhibit a strikingly different excitonic behavior. Whereas AgSePh exhibits narrow, fast luminescence with a minimal Stokes shift, AgTePh exhibits comparatively slow luminescence that is significantly broadened and red-shifted from its absorption minimum. Using time-resolved and temperature-dependent absorption and emission microspectroscopy, combined with subgap photoexcitation studies, we show that exciton dynamics in AgTePh films are dominated by an intrinsic self-trapping behavior, whereas dynamics in AgSePh films are dominated by the interaction of band-edge excitons with a finite number of extrinsic defect/trap states. Density functional theory calculations reveal that AgSePh has simple parabolic band edges with a direct gap at Γ, whereas AgTePh has a saddle point at Γ with a horizontal splitting along the Γ-N1 direction. The correlation between the unique band structure of AgTePh and exciton self-trapping behavior is unclear, prompting further exploration of excitonic phenomena in this emerging class of hybrid 2D semiconductors.

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