Ab initio investigation of hybrid organic-inorganic perovskites based on tin halides
Ivo Borriello,* Giovanni Cantele, and Domenico Ninno
Coherentia CNR-INFM and Università di Napoli “Federico II,” Dipartimento di Scienze Fisiche,
Complesso Universitario Monte Sant’Angelo, Via Cintia, I-80126 Napoli, Italy
Received 13 March 2008; revised manuscript received 7 May 2008; published 23 June 2008
The structural and electronic properties of both inorganic and hybrid organic-inorganic perovskites based on
tin halides are investigated from the first principles. In particular, we contrast the inorganic CsSnCl
3
and
CsSnI
3
to their hybrid counterparts CH
3
NH
3
SnCl
3
, CH
3
NH
3
SnI
3
, and NH
2
CH=NH
2
SnI
3
, which were
obtained by substituting the inorganic Cs cation with the methylammonium CH
3
NH
3
and the formamidinium
NH
2
CH=NH
2
cations. The comparison between the hybrid perovskites and the inorganic counterparts sheds
light on the effects of the filling molecule on the structural and electronic properties of the compound. We show
that the stability against the distortion of the perovskitic cage strongly depends on the embedded cation. The
electronic properties in particular, the band gap can be tuned by a suitable choice of the organic molecule,
and, in particular, of its size.
DOI: 10.1103/PhysRevB.77.235214 PACS numbers: 31.15.E-, 71.15.Mb, 71.20.Nr, 81.16.Dn
I. INTRODUCTION
Hybrid organic-inorganic compounds are an emerging
class of new materials that hold significant promise.
1–4
These
complex structures, based on a molecular scale composite of
organic and inorganic components, allow the combination of
properties of organic and inorganic elements in a unique ma-
terial. Inorganic compounds, typically characterized by cova-
lent and ionic interactions, provide a wide range of electronic
properties: high electrical mobility, wide range of band gaps
e.g., designing insulators, semiconductors, and metals, in-
teresting magnetic and dielectric properties, thermal stability,
and mechanical hardness. Organic compounds, which typi-
cally interact through weaker interactions van der Waals or
hydrogen bonding, offer the potential of high luminescence
efficiency, large polarizability, plastic mechanical properties,
and in some case exhibit conducting properties. Hybrid
organic-inorganic compounds are considered innovative ad-
vanced materials. Promising applications are expected in
many fields including optics, electronics, mechanics, protec-
tive coatings, catalysis, sensors, biology, and others.
1–18
The
tuning of the electronic structure of hybrid materials at nano-
scale can lead to unique electronic and optical properties that
are typical neither of the organic nor of the inorganic com-
ponent alone. This is the case of layered organic-inorganic
heterostructures such as multiple quantum wells
10
MQW,
exhibiting a spatially modulated electronic structure. In par-
ticular, hybrid organic-inorganic multilayers realizing
staggered
10,15
or type II MQW structures are of growing
interest for their potential practical applications in electronic
devices such as light-emitting diodes and photovoltaics.
11–18
The main working principle can be described as follows.
Typically, electrons and holes are photogenerated in the or-
ganic layers, which absorb light in the visible region. The
inorganic layers are selected to have high electron mobility,
larger band gap, and larger electron affinity compared to the
organic layers. Electrons and holes are, thus, separated out
at the organic-inorganic interface: the electrons transfer to
the inorganic conduction band due to the larger electron af-
finity and the high electron mobility reduces the recombina-
tion probability, producing high photoconductivity gain.
15
Hybrid perovskite compounds based on metal halides
4
are
a particular class of organic-inorganic materials. The basic
building component of the organic-inorganic perovskites is
the ABX
3
perovskite structure. This simple structure consists
of a network of corner-sharing BX
6
octahedra, where the B
atom is a metal cation typically Sn
2+
or Pb
2+
and X is a
monovalent anion typically Cl
-
, Br
-
, or I
-
; the A cation is
selected to neutralize the total charge and it can even be a
molecule. In this case, the organic cation must fit into a rigid
and relatively small cuboctahedral hole formed by the 12
nearest X atoms; thus, limiting the dimension of the selected
molecule. In fact, a tolerance factor t can be defined from the
relation R
A
+ R
X
= t
2R
B
+ R
X
, where R
A
, R
B
, and R
X
are
the ionic radii of the corresponding elements: by changing
R
A
, the tolerance factor t can be varied but only in a restrict
range of values around the unity t =1 corresponds to a per-
fectly packed perovskite structure to have a stable, even
distorted, three-dimensional perovskite structure.
19
Perov-
skite based hybrids can be synthesized with simple and
cheep techniques thanks to their self-assembling character.
20
The desired structural and electronic properties
21
can be en-
gineered through a suitable choice of both the inorganic cage
that is, the B and X elements forming the BX
6
octahedra
and the cation. Moreover, the organic-inorganic superlattices
are easily obtained by altering the combination of the organic
and inorganic components in the starting solution from
which the hybrids are crystallized. Therefore, the dimension-
ality can be used as a further degree of freedom for
tuning the material properties. Inorganic layers two-
dimensional 2D systems or multilayers, as well as inor-
ganic chains one dimensional 1D and dots zero
dimensional embedded in an organic matrix, have
been fabricated.
8,22,23
For example, Mitzi et al. have demon-
strated that layered perovskites with unit formula
NH
2
CI =NH
2
2
CH
3
NH
3
m
Sn
m
I
3m+2
can be synthesized to
realize 1D m =1, 2D m =2, or multilayer m 2 inor-
ganic structures, which have different conductivities.
8,23
For
all these and other reasons, metal in particular, lead and tin
halide hybrid perovskites have been extensively studied in
PHYSICAL REVIEW B 77, 235214 2008
1098-0121/2008/7723/2352149 ©2008 The American Physical Society 235214-1