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−1PbnI3n+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.
Highlights
Mixing the A-site cations leads to changes in bond angles in the structure, which leads to a lower bandgap as the angles near 180.32 The effects of the cations on the structure can be used to nd the ideal bandgap for devices
We report that the IAA n 1⁄4 2 is the rst discovered modulated 2D perovskite structure, with the layers exhibiting curvature and distorting in a long-range wave with a periodicity of eight PbI6 units
PbO and MACl were simultaneously dissolved in hydriodic acid (HI) and hydrophosphorous acid (H3PO2), to give a yellow solution which was heated to a boil
Summary
Hybrid halide perovskites have shown tremendous promise in photovoltaic devices with record efficiencies over 25% for optimized single junction solar cell devices.[1,2,3,4,5,6,7,8] Though the materials used for these devices have intrinsic stability issues, using two-dimensional (2D) halide perovskites leads to greater long-term stability.[9,10,11,12,13,14,15] Despite this, power conversion efficiencies using these materials are still lower, with few instances above 13% PCE for pure 2D materials.[16,17] Using 2D/3D composite heterostructures has allowed higher efficiencies of over 20% with proper cation choice, but still not on par with the 3D materials, emphasizing the need for more choices of 2D materials for optimization of photovoltaic devices.[18,19]Halide perovskites are a highly studied class of materials with the formula AMX3, in which A is a small cation, M is Queisser ideal bandgap for single-junction solar cells of 1.34 eV.[21]. IBA has already shown promise in solar cell devices because of its increased rate of charge transfer,[49,50] making the effect of bulky branched spacers on the structure and properties an important point to understand.
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