Long periodic ripple in a 2D hybrid halide perovskite structure using branched organic spacers

Abstract

Two-dimensional (2D) halide perovskites have great promise in optoelectronic devices because of their stability and optical tunability, but the subtle effects on the inorganic layer when modifying the organic spacer remain unclear. Here, we introduce two homologous series of Ruddlesden-Popper (RP) structures using the branched isobutylammonium (IBA) and isoamylammonium (IAA) cations with the general formula (RA)₂(MA) _n-1Pb _n I_3n+1 (RA = IBA, IAA; MA = methylammonium n = 1-4). Surprisingly, the IAA n = 2 member results in the first modulated 2D perovskite structure with a ripple with a periodicity of 50.6 Å occurring in the inorganic slab diagonally to the [101] direction of the basic unit cell. This leads to an increase of Pb-I-Pb angles along the direction of the wave. Generally, both series show larger in-plane bond angles resulting from the additional bulkiness of the spacers compensating for the MA's small size. Larger bond angles have been shown to decrease the bandgap which is seen here with the bulkier IBA leading to both larger in-plane angles and lower bandgaps except for n = 2, in which the modulated structure has a lower bandgap because of its larger Pb-I-Pb angles. Photo-response was tested for the n = 4 compounds and confirmed, signaling their potential use in solar cell devices. We made films using an MACl additive which showed good crystallinity and preferred orientation according to grazing-incidence wide-angle scattering (GIWAXS). As exemplar, the two n = 4 samples were employed in devices with champion efficiencies of 8.22% and 7.32% for IBA and IAA, respectively.

Fig. 1. (a and b) Crystal structures of n = 1–4 for (a) IBA and (b) IAA. The structure for IAA n = 2 is shown without modulation. On the right is the view of the structures perpendicular to the layers of n = 3 for IBA and IAA showing the similar in-plane distortion of the octahedra. Different layers are shown in different colors. (c) Optical images of crystals of IBA (top) and IAA (bottom) in order of increase n from left to right.

Fig. 2. (a) The modulated IAA n = 2 structure viewed along the (1 0 1) axis showing a single wave in the structure. The minimum and maximum interlayer distances are labeled in red and green, respectively. (b) The diffraction of a single crystal showing the supercell reflections. (c) A view of the crystal structure looking perpendicular to a single layer. The areas colored in green indicate maximum interlayer spacing between two arbitrary layers, and red indicates minimum interlayer spacing.

Fig. 3. (a) The average equatorial angles for branched and straight-chain amines shows the overall increase in angle for the branched amines. For IAA n = 2, two points are shown. The top is for the direction along the wave modulation and the bottom for the direction perpendicular to it. (b) The interlayer spacing for branched and straight-chain amines shows the increase in spacing for the branched amines. Note that for IAA n = 2, both the minimum and maximum interlayer spacings were plotted here. (c) A comparison of interlayer spacing for BA (left) and IAA (right) with the bold line in the center highlighting the inability of the branch amines to cross.

Fig. 4. Differential scanning calorimetry (DSC) for (a) IBA and (b) IAA, normalized for clarity.

Fig. 5. (a) The reflectance spectra for each system transformed via the Kubelka–Munk equation. To find the bandgap, the linear portion of the spectra with energy above the bandgap (shown by purple arrows) was extrapolated to the x-axis. (b) Photoluminescence for each system. (c and d) A comparison of bandgap and PL, respectively, for the two systems, showing that the IAA is blue-shifted for all n values except the modulated structure, n = 2.

Fig. 6. Photoemission yield spectroscopy in air (PYSA) of (a) IBA and (b) IAA, compared to MAPbI₃ (n = inf).

Fig. 7. Resistivity measurements of IBA and IAA n = 4 crystals parallel to the layers, both in the dark and under white light. Note the strong photovoltaic response for each.

Fig. 8. (a and c) GIWAXS patterns for IAA and IBA n = 4 films using one-step spincoating. (b and d) The films with an MACl additive show less broadening along χ, indicative of more preferred orientation.

Fig. 9. (a) The architecture used for these devices. “2D Perovskite” refers to either IAA or IBA n = 4 with 2.5 wt% MACl added. (b and c) Representative device curves for each material, showing scans in both directions. (d) The reflectance data of each film transformed by the Kubelk–Munk equation, showing the appearance of various n peaks. (e) The EQE spectrum of representative devices for each material.