High efficiency quasi 2D lead bromide perovskite solar cells using various barrier molecules†
This work reports on the high power conversion efficiency (PCE) and high open circuit voltage (Voc) of bromide-based quasi 2D perovskite solar cells. A Voc of more than 1.4 V and, at the same time, a PCE of 9.5% for cells with hole transport material (HTM) were displayed, whereas a Voc value of 1.37 V and a PCE of 7.9% were achieved for HTM-free cells. Bromide quasi 2D perovskites were synthesized using various long organic barriers (e.g., benzyl ammonium, BA; phenylethyl ammonium, PEA; and propyl phenyl ammonium, PPA). The influence of different barrier molecules on the quasi 2D perovskite's properties was studied using absorbance, X-ray diffraction, and scanning electron microscopy. No change was observed in the exciton binding energy as a result of changing the barrier molecule. Density functional theory (DFT) with spin–orbit coupling calculations showed that in the case of BA, holes are delocalized over the whole molecule, whereas for PEA and PPA, they are delocalized more at the phenyl ring. This factor influences the electrical conductivity of holes, which is highest for BA in comparison with the other barriers. In the case of electrons, the energy onset for the nonzero conductivity is lowest for BA. Both calculations support the better PV performance observed for the quasi 2D perovskite based on BA as the barrier. Finally, using contact angle measurements, it was shown that the quasi 2D perovskite is more hydrophobic than the 3D perovskite. Stability measurements showed that cells based on the quasi 2D perovskite are more stable than cells based on the 3D perovskite.