Despite high photo-conversion efficiency, the short long-term stability and toxicity issues have prevented lead-based perovskites from becoming the standard in high efficiency solar cells. Hence, lead-free perovskites with low inorganic...
We provide a deep atomistic understanding of the structural, energetic, electronic, and optical properties of the CH3NH3 (methylammonium, MA), CH3PH3 (methylphosphonium, MP) and CH(NH2)2 (formamidinium, FA) compounds based on DFT calculations.
Zero-dimensional
(0D) perovskites have received increased attention
in the last years due to their wide range of compositions and suitable
optoelectronic properties for several technological applications.
Here, we performed density functional theory calculations to investigate
the role of halide anion (X = Cl, Br, I) and trivalent cation (B =
Bi, Sb) substitution in the structural, energetic stability, optoelectronic,
and excitonic properties of lead-free 0D perovskites using the C20H20N4B2X10 and
C30H25N6BX6·2H2O compounds as prototype structures. We found a crucial role
of the replacement of the halide species in the investigated physical
properties; that is, most properties change (decrease or increase)
almost linearly from Cl → Br → I, while the effects
induced by B are less pronounced. For example, 0D perovskites with
smaller halide species have higher energetic stability. The valence
band is dominated by the X p-states, in which the electron binding
decreases from 3p → 4p → 5p, and hence, it yields a
reduction of the band gap energy as Cl is replaced by Br and then
by I. The electronic band structures are very flat, which can be explained
by the 0D nature of the crystal structures (i.e., isolated inorganic
octahedra). As expected, spin–orbit coupling is more significant
for I-based systems. As a consequence of quantum confinement, the
exciton binding energies of 0D perovskites (up to 456.95 meV) are
about 2 orders of magnitude higher than their conventional 3D counterparts.
Finally, we found power conversion efficiencies to be less than 14%
for all systems.
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