Elucidating the Methylammonium (MA) Conformation in MAPbBr₃ Perovskite with Application in Solar Cells

Abstract

Hybrid organic-inorganic perovskites, MAPbX (X = halogen), containing methylammonium (MA: CH-NH) in the large voids conformed by the PbX octahedral network, are the active absorption materials in the new generation of solar cells. CHNHPbBr is a promising member with a large band gap that gives rise to a high open circuit voltage. A deep knowledge of the crystal structure and, in particular, the MA conformation inside the perovskite cage across the phase transitions undergone below room temperature, seems essential to establish structure-property correlations that may drive to further improvements. The presence of protons requires the use of neutrons, combined with synchrotron XRD data that help to depict subtle symmetry changes undergone upon cooling. We present a consistent picture of the structural features of this fascinating material, in complement with photocurrent measurements from a photodetector device, demonstrating the potential of MAPbBr in optoelectronics.

Dynamic Disorder Restriction of Methylammonium (MA) Groups in Chloride‐Doped MAPbBr3 Hybrid Perovskites: A Neutron Powder Diffraction Study

“…All the samples exhibited a high purity; only for MAPbCl 3 minor amounts of MACl were identified. Figure shows the XRD patterns of the obtained samples in comparison with MAPbBr 3 . As shown, the diffraction lines exhibit a systematic shift at high 2 θ values as x increases, evidencing the decrease of the unit‐cell size due to chloride introduction into the crystalline network.…”

“…A singular case was found in the parent MAPbBr 3 described from NPD and SXRD data, in which the MA units are delocalized along [110] at RT; the reduction in size, driven by cooling the sample down to 240 K, still described in the cubic Pm m space group, forced the MA units to delocalize along the [100] directions . This nice example shows that the reduction in size of the PbX 6 network, either by a decrease of the temperature or by chemical doping, leads to an orientation evolution of the organic group.…”

“…All the patterns can be indexed in a cubic symmetry with space group Pm m . Previous reports for MAPbX 3 with X=Br or Cl also described these phases in this cubic space group at room temperature . A recent temperature‐dependent synchrotron XRD study was useful to identify different phase transitions depending on the Cl contents .…”

“…Although methylammonium lead iodide (MAPbI 3 ) remains the most widely studied perovskite due to its ability to absorb broadband light below its bandgap of 1.6 eV, the degradation in a humid atmosphere has remained a major obstacle for commercialization . MAPbBr 3 is a promising alternative to MAPbI 3 with a large bandgap of 2.2 eV, which gives rise to a high open circuit voltage (Voc≈1.2–1.5 V) . Their long exciton diffusion length (>1.2 μm) enables good charge transport in devices.…”

“…[7] MAPbBr 3 is ap romising alternative to MAPbI 3 with al arge bandgapo f2 .2 eV,w hich gives rise to ah igh open circuit voltage (Voc % 1.2-1.5 V). [8] Their long excitond iffusion length (> 1.2 mm) enables good charge transport in devices. In addition, MAPbBr 3 demonstrates higher stabilityt owardsa ir andm oisture due to its stable cubic phase and low ionic mobility relative to the pseudocubic MAPbI 3 .…”