Extrinsic nature of the broad photoluminescence in lead iodide-based Ruddlesden-Popper perovskites

Two-dimensional metal halide perovskites of Ruddlesden–Popper type have recently moved into the centre of attention of perovskite research due to their potential for light generation and for stabilisation of their 3D counterparts. It has become widespread in the field to attribute broad luminescence with a large Stokes shift to self-trapped excitons, forming due to strong carrier–phonon interactions in these compounds. Contrarily, by investigating the behaviour of two types of lead-iodide based single crystals, we here highlight the extrinsic origin of their broad band emission. As shown by below-gap excitation, in-gap states in the crystal bulk are responsible for the broad emission. With this insight, we further the understanding of the emission properties of low-dimensional perovskites and question the generality of the attribution of broad band emission in metal halide perovskite and related compounds to self-trapped excitons

Impact of two diammonium cations on the structure and photophysics of layered Sn-based perovskites

Layered metal-halide perovskites have shown great promise for applications in optoelectronic devices, where a large number of suitable organic cations give the opportunity to tune their structural and optical properties. However, especially for Sn-based perovskites, a detailed understanding of the impact of the cation on the crystalline structure is still missing. By employing two cations, 2,2′-oxybis(ethylammonium) (OBE) and 2,2′-(ethylenedioxy)bis(ethylammonium) (EDBE), we obtain a planar 〈100〉 and a corrugated 〈110〉-oriented perovskite, respectively, where the hydrogen bonding between the EDBE cations stabilises the corrugated structure. OBESnI4 exhibits a relatively narrow band gap and photoluminescence bands compared to EDBESnI4. In-depth analysis shows that the markedly different optical properties of the two compounds have an extrinsic origin. Interestingly, thin films of OBESnI4 can be obtained both in black and red colours. This effect is attributed to a second crystalline phase that can be obtained by processing the thin films at 100 °C. Our work highlights that the design of the crystal structure as obtained by ligand chemistry can be used to obtain the desired optical properties, whereas thin film engineering can result in multiple crystalline phases unique to Sn-based perovskites.</p

Elucidating the Structure and Photophysics of Layered Perovskites through Cation Fluorination

Optoelectronic devices based on layered perovskites containing fluorinated cations display a well-documented improved stability and enhanced performance over non-fluorinated cations. The effect of fluorination on the crystal structure and photophysics, however, has received limited attention up until now. Here, 3-fluorophenethylammonium lead iodide ((3-FPEA)(2)PbI4) single crystals are investigated and their properties to the non-fluorinated ((PEA)(2)PbI4) variant are compared. The bulkier 3-FPEA cation increases the distortion of the inorganic layers, resulting in a blue-shifted absorbance and photoluminescence. Temperature-dependent photoluminescence spectroscopy reveals an intricate exciton substructure in both cases. The fluorinated variant shows hot-exciton resonances separated by 12 to 15 meV, values that are much smaller than the 40 to 46 meV found for (PEA)(2)PbI4. In addition, high-resolution spectra show that the emission at lower energies consists of a substructure, previously thought to be a single line. With the analysis on the resolved photoluminescence, a vibronic progression is excluded as the origin of the emission at lower energies. Instead, part of the excitonic substructure is proposed to originate from bound excitons. This work furthers the understanding of the photophysics of layered perovskites that has been heavily debated lately

Double Perovskite Single-Crystal Photoluminescence Quenching and Resurge:The Role of Cu Doping on its Photophysics and Crystal Structure

Cs2AgBiBr6 is a potential lead-free double perovskite candidate for optoelectronic applications; however, its large and indirect band gap imposes limitations. Here, single crystals of Cs2AgBiBr6 are doped with Cu2+ cations to increase the absorption range from the visible region up to 0.5 eV in the near-infrared region. Inductively coupled plasma spectroscopy confirms the presence of 1.9% of copper in the Cs2AgBiBr6 structure. Structural and optical changes caused by Cu doping were studied by Raman spectroscopy combined with X-ray diffraction, heat capacity measurements, and low-temperature photoluminescence spectroscopy. Along with the 1.9 eV emission typical of the pristine Cs2AgBiBr6 single crystals, we report a novel low-energy emission at 0.9 eV related to deep defects. In the doped crystals, these peaks are quenched, and a new emission band at 1.3 eV is visible. This new emission band appears only above 120 K, showing that thermal energy is necessary to trigger the copper-related emission

Grain-Specific Transitions Determine the Band Edge Luminescence in Dion–Jacobson Type 2D Perovskites

The photophysics of 2D perovskites incorporating 1,4-phenylenedimethanammonium (PDMA) as spacer cations is studied. PDMAPbI4 and PDMASnI4 exhibit absorption and luminescence spectra dominated by excitonic transitions and an emission due to two different states. Low-temperature studies reveal a time-dependent red shift of 12 meV that is correlated with grain-specific luminescence spectra observed in optical micrographs. For the Pb-variant, grains of red-shifted and lower intensity band edge emission simultaneously exhibit a more pronounced luminescence from a broad defect-related band around 2 eV. This suggests the grain-specific emission to be related to local defects. These observations have important consequences for the understanding of luminescence of 2D perovskites, for which peak splitting of the band edge emission is a common, yet not completely resolved observation.</p

Correction to “Double Perovskite Single-Crystal Photoluminescence Quenching and Resurge: The Role of Cu Doping on Its Photophysics and Crystal Structure”

In our manuscript, we identified that the copper doping does not change the structure of Cs2AgBiBr6 double perovskite single crystals at room temperature but causes an increased NIR absorption with a photoluminescence band at 1.3 eV above 120 K. It is along these main findings that we also reported a photoluminescence peak at 0.9 eV in the pristine Cs2AgBiBr6. Additional measurements have identified that this 0.9 eV emission is a second-order diffraction peak of the main emission at 1.9 eV caused by the diffraction grating in our setup. Therefore, the origin of the 0.9 eV photoluminescence peak should be considered an experimental artifact instead of an emission stemming from deep defect levels as discussed in the manuscript. When measuring a very broad spectral range, longpass optical filters can be used to avoid artifacts due to the nonideal behavior of the diffraction grating.1 In the present work, an 850 nm long-pass filter was used to attenuate the 1.9 eV emission, resulting in absence of the 0.9 eV peak. We note that the central theme of this work regards the impact of copper doping on the structural and optical properties of Cs2AgBiBr6. Therefore, the main conclusions of the study are still intact, but the following corrections to the figures are needed. Figure 2 is adapted by replacing (a) with a measurement without the second-order diffraction peak and (b) with a schematic without the deep defect level that was accounting for the emission peak at 0.9 eV

CCDC 2204975: Experimental Crystal Structure Determination

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures

The Origin of Broad Emission in ⟨100⟩ Two-Dimensional Perovskites: Extrinsic vs Intrinsic Processes.

2D metal halide perovskites can show narrow and broad emission bands (BEs), and the latter's origin is hotly debated. A widespread opinion assigns BEs to the recombination of intrinsic self-trapped excitons (STEs), whereas recent studies indicate they can have an extrinsic defect-related origin. Here, we carry out a combined experimental-computational study into the microscopic origin of BEs for a series of prototypical phenylethylammonium-based 2D perovskites, comprising different metals (Pb, Sn) and halides (I, Br, Cl). Photoluminescence spectroscopy reveals that all of the compounds exhibit BEs. Where not observable at room temperature, the BE signature emerges upon cooling. By means of DFT calculations, we demonstrate that emission from halide vacancies is compatible with the experimentally observed features. Emission from STEs may only contribute to the BE in the wide-band-gap Br- and Cl-based compounds. Our work paves the way toward a complete understanding of broad emission bands in halide perovskites that will facilitate the fabrication of efficient narrow and white light emitting devices