Superhalogens as building blocks of two-dimensional organic-inorganic hybrid perovskites for optoelectronics applications.

Organic-inorganic hybrid perovskites, well known for their potential as the next generation solar cells, have found another niche application in optoelectronics. This was demonstrated in a recent experiment (L. Dou, et al., Science, 2015, 349, 1518) on atomically thin (C4H9NH3)2PbBr4, where, due to quantum confinement, the bandgap and the exciton binding energy are enhanced over their corresponding values in the three-dimensional bulk phase. Using density functional theory we show that when halogen atoms (e.g. I) are sequentially replaced with superhalogen molecules (e.g. BH4) the bandgap and exciton binding energy increase monotonically with the superhalogen content with the exciton binding energy of (C4H9NH3)2Pb(BH4)4 approaching the value in monolayer black phosphorus. Lead-free admixtures (C4H9NH3)2MI4-x(BH4)x (M = Sn and Ge; x = 0-4) also show a similar trend. Thus, a combination of quantum confinement and compositional change can be used as an effective strategy to tailor the bandgap and the exciton binding energy of two-dimensional hybrid perovskites, making them promising candidates for optoelectronic applications.