Electronic Structure and Trap States of Two-Dimensional Ruddlesden–Popper Perovskites with the Relaxed Goldschmidt Tolerance Factor

Two-dimensional
Ruddlesden–Popper perovskites (2D RPPs)
have been considered as promising building blocks for optoelectronic
applications owing to optical properties comparable to the ones of
3D perovskites, together with superior stability. In addition, the
more flexible structure adopted by such perovskites leads to a relaxation
of the Goldschmidt tolerance factor (τ) requirement. Herein,
we compare the crystalline and electronic structures, as well as the
photophysics of two 2D perovskite single crystals (n-BA)2(MA)2Pb3I10 (BMAPI)
and (n-BA)2(EA)2Pb3I10 (BEAPI) (n-BA = n-butylamine) containing small A-cations (MA, methylammonium) and
large A-cations (EA, ethylammonium), respectively. The latter presents
a relaxed τ (τEA > 1) compared with the
requirement
of a stable phase in 3D perovskites (τ < 1). Such relaxed
τ is beneficial from the structural flexibility of the long
organic cation bilayer and the pronounced lattice distortions in the
2D perovskite structures. We further elucidate how the greater lattice
distortions concurrently modulate the electronic structure as well
as trap densities in these 2D RPPs. The electronic band gap (Eg) of BEAPI (2.08 ± 0.03 eV) is ∼0.17
eV larger than the one of BMAPI (1.91 ± 0.03 eV). This is mainly
because of a shift in the valence band maximum associated with the
expansion of the Pb–I bond length in BEAPI. In addition, the
overall trap state densities for BMAPI and BEAPI are calculated to
be ∼2.18 × 1016 and ∼3.76 × 1016 cm–3, respectively, as extracted from
the time-resolved photoluminescence studies. The larger trap density
in BEAPI can be attributed to the stronger interfacial lattice distortion
that sets in when large EA cations are contained into the inorganic
crystal lattice.