Hybrid Organic-Inorganic Coordination Complexes as Tunable Optical Response Materials

Article abstract of DOI:10.1021/acs.inorgchem.5b02749

Novel lead and bismuth dipyrido complexes have been synthesized and characterized by single-crystal X-ray diffraction, which shows their structures to be directed by highly oriented π-stacking of planar fully conjugated organic ligands. Optical band gaps are influenced by the identity of both the organic and inorganic component. Density functional theory calculations show optical excitation leads to exciton separation between inorganic and organic components. Using UV-vis, photoluminescence, and X-ray photoemission spectroscopies, we have determined the materials' frontier energy levels and show their suitability for photovoltaic device fabrication by use of electron- and hole-transport materials such as TiO₂ and spiro-OMeTAD respectively. Such organic/inorganic hybrid materials promise greater electronic tunability than the inflexible methylammonium lead iodide structure through variation of both the metal and organic components.

Full text of DOI:10.1021/acs.inorgchem.5b02749

Article
pubs.acs.org/IC
Hybrid Organic−Inorganic Coordination Complexes as Tunable
Optical Response Materials
Will Travis,† Caroline E. Knapp,† Christopher N. Savory,‡ Alex M. Ganose,‡,§ Panagiota Kafourou,†
Xingchi Song,† Zainab Sharif,† Jeremy K. Cockcroft,† David O. Scanlon,‡,§ Hugo Bronstein,†
and Robert G. Palgrave*,†
‡
†Department of Chemistry and Kathleen Lonsdale Materials Chemistry, Department of Chemistry, University College London, 20
Gordon Street, London, WC1H 0AJ, United Kingdom
§Diamond Light Source Ltd., Diamond House, Harwell Science and Innovation Campus, Didcot, Oxfordshire OX11 0DE, United
Kingdom
S
* Supporting Information
ABSTRACT: Novel lead and bismuth dipyrido complexes have been synthesized and
characterized by single-crystal X-ray diffraction, which shows their structures to be
directed by highly oriented π-stacking of planar fully conjugated organic ligands. Optical
band gaps are influenced by the identity of both the organic and inorganic component.
Density functional theory calculations show optical excitation leads to exciton separation
between inorganic and organic components. Using UV−vis, photoluminescence, and X-
ray photoemission spectroscopies, we have determined the materials’ frontier energy
levels and show their suitability for photovoltaic device fabrication by use of electron- and
hole-transport materials such as TiO2 and spiro-OMeTAD respectively. Such organic/
inorganic hybrid materials promise greater electronic tunability than the inflexible
methylammonium lead iodide structure through variation of both the metal and organic
components.
and rapidly degrades when held at operating temperatures,
forming methylammonium iodide and PbI2.12
INTRODUCTION
■
Solar energy is regarded as perhaps the most promising
alternative to fossil fuels.1 In recent years, solar cells based on
hybrid organic−inorganic absorbers with perovskite structure
have shown potential as an alternative to silicon solar cells.2
Perovskite solar cells already show efficiencies surpassing those
of dye-sensitized, organic, and amorphous silicon solar
modules.3 The archetypal perovskite solar cell absorber material
is methylammonium lead(II) iodide (CH3NH3PbI3 or MAPI)
and has significant advantages over competing commercial and
emerging technologies: synthesis is achieved from available and
cheap starting materials via solution processes4 or scalable
vapor-phase deposition methods,5 making possible large-scale
industrial production.6 Furthermore, efficiencies of MAPI solar
cells have exceeded 20%.4b,7 Such high efficiencies have been
achievable due to MAPI’s high optical absorption coefficient,
excellent defect tolerance,8 ambipolar charge transport,9 and
very long electron−hole diffusion lengths10 which result from
high minority-carrier lifetime (τ) and mobility (μ).10b,11 An
important factor in each of these properties is thought to be the
three-dimensional (3D) connectivity of the inorganic (lead
iodide) sublattice in the perovskite structure.
The perovskite structure, with the general chemical formula
ABX3, has very many well-studied examples among oxides and
fluoride compounds (X = O, F), and is synonymous in those
fields with versatility of composition: typically in oxides and
fluorides a very wide range of metal cations can be substituted
onto the A and B sites, providing that simple geometrical
conditions are met.13 This allows materials to be finely tuned by
varying composition to elicit a wide range of properties from
magnetism to ionic conductivity to catalysis.
Thus, to overcome the disadvantages of MAPI, it would
initially seem that an approach of partially or completely
substituting A, B, or X ions in the MAPI perovskite structure,
may be successful. Attempts to replace Pb(II) on the B site
have used the other divalent post-transition group 14 ions
Sn(II) and Ge(II);14 however, the stability of the 2+ oxidation
state decreases in the lighter group 14 elements, such that Sn
and Ge favor the 4+ oxidation state, which cannot be
accommodated in the halide perovskite structure.14 Thus,
though removal of Pb is possible while maintaining the
perovskite structure, doing so exacerbates the problem of
instability. Substitutions on the A site have been similarly
limited in success. While, on geometric grounds, various A-site
Despite these highly positive characteristics, there remain
two important drawbacks to MAPI and related materials. First
is the toxicity of lead, the inclusion of which in consumer
electronics is undesirable due to the chance of extensive
environmental release. Second, MAPI is unstable in moist air
Received: December 2, 2015
© XXXX American Chemical Society
A
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cations have been proposed to form cubic lead iodide ABX3
arrays (hydroxylammonium, hydrazinium, azetidinium, imida-
zolium, and guanidinium),15 only methylammonium (MA) and
formamidinium are currently known to do so.11a,16 For
instance, replacing the MA cations of the MAPI structure
with either ethylammonium or ammonium results in a
structural change from cubic perovskite to a 2H hexagonal
perovskite with one-dimensional (1D) lead iodide chains, a
significant loss of dimensionality leading to a wider band gap
and inferior transport properties.17 Since only small A-site
organic components are permissible within the 3D perovskite
structure, optically or electronically active groups, such as
conjugated π-systems, cannot be included. Lower-dimensional
classes of organic haloplumbate compounds that can
accommodate larger organic units are known. These consist
of perovskite units separated by larger organic cations, such as
A2PbX4, A3PbX5, and A4PbX6 classes, which yield two-, one-,
and zero-dimensional (0D) inorganic sublattices, respectively.18
Finally, substitution of iodide with lighter halides in the X site
leads to an increase in the band gap (2.3 eV for MAPbBr3),
hindering theoretical solar cell efficiency.4a,19
Hence, despite the promising foundation of the perovskite
structure, which is highly tunable in oxides and fluorides, it has
proved impossible to meet the challenges of overcoming the
toxicity and instability of MAPI through a “materials design”
approach of substituting the A, B, or X ions.20 Indeed, the well-
established idea of tuning individually the inorganic and organic
components of hybrid optoelectronic materials21 is virtually
impossible within the cubic perovskite structure for the reasons
mentioned. Instead, progress in MAPI devices has largely been
made not in discovery of new compositions but rather in
advancements in fundamental understanding of the material,22
device characterization, and manufacturing processes.20,23
Here we consider alternative inorganic−organic materials as
photoactive agents for potential use in photovoltaic (PV)
devices. As already stated, when the A site of hybrid perovskites
is altered, a major drawback is that a reduction in inorganic
dimensionality appears unavoidable, and this leads to a
widening of the band gap and lowering of the conductivity.24
To counter this, we propose construction of materials with
mixed organic and inorganic connectivity, where the organic
component allows charge transport in one or more dimensions.
We envisage that use of planar aromatic organic ligands that
favor highly oriented π-stacking conformations with structure-
directing effects will produce conjugated pathways suitable for
electron and hole transport.25 These structures would
compensate for reduced inorganic dimensionality with
increased organic connectivity, lifting the stringent require-
ments of the perovskite structure and opening a much larger
compositional space for exploration and design of hybrid solar
materials.26 As a demonstration of this approach, here we
report synthesis and characterization of novel Bi and Pb
dipyrido iodide coordination complexes. We choose Pb(II) and
Bi(III) compounds since these metals have lone pair 6s2
configurations thought necessary to aid charge carrier
mobility.20 Pb(II) and Bi(III) are also more resistant to
oxidation compared to their lighter analogues Sn(II) and
Sb(III). By studying compounds of two related conjugated
ligands, we show the dimensionality of the inorganic portion
can be controlled, and furthermore, the optical and electronic
properties are promising for photovoltaic application. Specifi-
cally, we combine lead and bismuth iodide with dipyrido[3,2-
a:2′,3′-c]phenazine (dppz) and benzo[i]dipyrido[3,2-a:2′,3′-
c]phenazine (dppn) (see Figure S1 in Supporting Information).
We have characterized the structural configurations, electronic
band positions, theoretical partial charges, and band structures
and show they have potential as a class of highly tunable
organic−inorganic photoactive materials with long-lived charge
separation lifetimes and chemical stability.
RESULTS AND DISCUSSION
■
Synthesis of the lead and bismuth iodide dppz and dppn
complexes was achieved through solution-phase methods. Both
organic ligands dppz and dppn can be synthesized through non-
air-sensitive, catalyst-free processes using ethanolic solutions
from easily available starting materials. Single-crystal synthesis
of the PbI2 and BiI3 dppz and dppn complexes was achieved
through solvent layering by use of either N-methyl-2-
pyrrolidone (NMP) or dimethylformamide (DMF) to dissolve
the reagents and a layer of methanol to induce crystallization.
Structural Characterization. Lead(II) iodide heterocyclic
compounds [Pb(dppz)I2]n, also referred to as PbI2-dppz (1),
and [Pb(dppn)2I2] or PbI2-dppn (2), and bismuth(III) iodide
heterocyclic compounds [Bi(dppz)I3]2 or BiI3-dppz (3) and
[Bi(dppn)I3.NMP] or BiI3-dppn (4), were structurally
determined by X-ray crystallography (Figure 1; Table S1 in
Supporting Information). Compound 1 crystallized in the
monoclinic space group I2/a (Figure 1a,e; Figure S2). The
asymmetric unit of 1 consists of a lead diiodide unit
coordinated to one dppz ligand through two nitrogen atoms.
A one-dimensional chain is formed from Pb−I bridges between
[Pb(dppz)I2] monomeric units, each lead center is coordinated
Figure 1. Molecular structures of (a) PbI2-dppz (1), (b) PbI2-dppn
(2), (c) BiI3-dppn (4), and (d) BiI3-dppz (3). Packed extended
structures are shown for (e) PbI2-dppz viewed to show the Pb−I 1D
linear chains and overlapping dppz ligands, (f) PbI2-dppn, (g) BiI3-
dppz, and (h) BiI3-dppn viewed showing the eclipsed organic moieties.
B
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by four iodine atoms and two nitrogen atoms from the dppz
ligand, resulting in a distorted octahedral geometry.
expected octahedral geometry is still observed (Table S2). This
is likely, in part, due to the small bite angle of the dppn ligand
[64.95(12)°], which is slightly larger than the bite angle of the
same ligand in 2 [62.94(17)°]; however, when the change in
ionic radius when moving from lead to bismuth is considered,
this is as expected.
The infinite chain along the c-axis of alternating lead and
iodine atoms is very similar to the previously reported
compounds [Pb(2,2′-bipy)I2] and [Pb(1,10-phen)I2],27 with
dppz ligands lying on alternate sides of each chain; bond
lengths about the metal center are similar to those reported
previously. As in [Pb(2,2′-bipy)I2] and [Pb(1,10-phen)I2], 1 is
consistent with Brown’s model of two strong Pb−N bonds
[2.519(6) and 2.574(7) Å] and two intermediate [3.215(7) and
3.192(7) Å] and two weak [3.320(7) Å] Pb−I bonds.28 The
I(1)−Pb(1)−I(2) bond angle deviates from the expected 180°
considerably [168.13(9)°] and may be attributed to the
tendency for the Pb(II) 6s electron pair to hybridize with the
6p orbitals such that it occupies a position in the coordination
sphere, leading to hemidirected coordination.29 Furthermore,
the N−M−I bond angles also deviate from the expected 90°
(mean value 83.86°); this is likely due to the rigidity of the
dppz ligand as it stacks up in the 1D array. Static disorder is
observed throughout the structure, with the ligand and iodine
atoms fractionally displaced giving two parts, both with 50%
occupancy; see Experimental Section for further discussion.
Compound 2 is mononuclear, crystallizes in the monoclinic
space group C2/c, and has a distorted octahedral geometry
(Figure 1b,f; Figure S3). The crystal structure of 2 has been
reported previously.30 The lead atom is surrounded by four
nitrogen atoms, forming strong bonds with each of the two
dppn ligands (Pb−N average distance 2.59(4) Å) and two
iodine atoms (Pb−I average distance 3.22(1) Å), which is
indicative of an intermediate bonding interaction. The bite
angle of the ligand [N(1)−Pb(1)−N(2), 62.94(17)°; N(3)−
Pb(1)−N(4), 63.33(17)°] in 2 is smaller than in comparable
complexes,27 which is also observed in the structurally
analogous [Pb(dppz)2I2]30 and leads to a range in cis angles
[62.94(17)°−107.47(12)°] differing from the expected 90°.
This results in mononuclear molecular formation organized
through π-stacking of the highly conjugated aromatic dppn
ligand forming staggered nonbonding sheets in the ac-plane as
shown in Figure 1f, which is viewed down the c-axis through
the eclipsed overlapping organic units.
Polycrystalline samples of 1−4 were measured by powder X-
ray diffraction (PXRD) (Figure 2 a,b). The measured data are
Figure 2. Measured (black) and simulated (green) PXRD patterns for
(a) PbI2-dppz and PbI2-dppn and (b) BiI3-dppz and BiI3-dppn. (c)
UV−vis absorbance spectra of dppz and dppn ligands and Pb and Bi
dipyrido complexes. (d) Photoluminescence (PL) spectra of Pb and Bi
dipyrido complexes. Band gaps derived from Tauc plots are
highlighted with vertical lines.
in good agreement with the simulated patterns derived from
single-crystal structures. After 2 months exposure to air, repeat
PXRD measurements (Figure S6) showed the materials to be
robust with no degradation observed. Conversely, MAPI
decomposes, turning yellow (with formation of PbI2), within
24 h of exposure to air.
Optical and Electronic Characterization. Optical spectra
of polycrystalline samples 1−4 and of the ligands, dppn and
dppz, in the solid state were measured in diffuse reflection
mode by UV−vis spectrophotometer (Figure 2c). The as-
recorded reflectance values were transformed with the
Kubelka−Munk function, F(R), to give spectral intensity
proportional to the absorbance.
The unbound ligands dppn and dppz show principal
absorption features at wavelengths below 450 nm (Figure
2c). Compounds 1−4 all show optical absorption at
significantly longer wavelengths than the unbound organic
ligands. The absorption spectra of 1−4 all contain an
absorption edge, and from this feature the optical band gaps
for the complexes were obtained by constructing Tauc plots of
[F(R)E]2 (Figure S7).32 For 1 and 2, band gaps were
determined as 2.52 and 2.13 eV, respectively, and for 3 and 4
they were 2.24 and 2.05 eV, respectively. For comparison, the
binary halides PbI2 and BiI3 have band gaps at room
Compound 3 crystallized in the triclinic space group P1 and
̅
consists of a dimeric structure bridged by two iodine atoms
(Figure 1d,g; Figure S4).31 The centrosymmetric four-
membered Bi2I2 ring is planar, with the remainder of the
coordination sphere of the bismuth atoms being filled with two
terminal iodine atoms and the two nitrogen atoms of the dppz
ligand.
The Bi−N bonds are indicative of strong bonds (mean value
2.501 Å) and the difference between the bridging Bi−I
(3.1367(5), 3.2637(5) Å) and terminal Bi−I (2.8988(5) Å)
bond lengths is obvious. Each bismuth center in 3 adopts a
distorted octahedral six-coordinate geometry (see bond angles
in Table S2), and can be assigned in part to the smaller bite
angle of the ligand [N(1)−Pb(1)−N(2) 66.53(17)°].
Compound 4 is made up of a BiI3 unit coordinated to a dppn
ligand via nitrogen atoms in two positions, completing the
octahedral sphere of this mononuclear compound is a molecule
of solvent, NMP (Figure 1c,h; Figure S5). Compound 4
crystallized in the triclinic space group P1 with typical Bi−N
̅
bond lengths observed (average 2.54(4) Å), and in the absence
of dimerization or formation of a 1D chain, as in 1 and 3, there
is an average Bi−I bond length of 2.98(4) Å. Despite the lack of
steric constraints seen in 1−3, a large deviation from the
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temperature of 2.35 eV (λ = 527 nm)33 and 1.96 eV (λ = 632
nm).34
Pb−I angle 168.13°) than for PbI2-dppn (I−Pb−N angle
152.37(13)°), with the former having a larger band gap (2.52 vs
2.13 eV). Octahedral distortion is also less for BiI2-dppz (I−
Bi−I angle 166.48°; Eg = 2.24 eV) than for BiI2-dppn (I−Bi−O
angle 162.43(9)°; Eg = 2.05 eV). That the organic components
are electronically active in these materials, while the
formamidinium and methylammonium from previous studies
are not, goes some way to explain these differences.11a
In summary, 1−4 show optical spectra characteristic of
semiconductors with band gaps in the range 2.05−2.52 eV,
reasonably close to that required for efficient photovoltaic
absorbers.37 The trend in band gap within this limited range of
compounds seems to indicate that both the size of the organic
conjugated system and the identity of the metal play important
roles in determining the optical properties. These results
support the hypothesis that not only through modification of
the inorganic component but also by varying the organic ligand
can the optical properties of these materials be tuned, leading to
a rich parameter space for further exploration.
Figure 2d shows the normalized photoluminescence (PL)
spectra for the novel hybrid materials. The PL of the free
ligands is shown in Figure S8 (Supporting Information). BiI3-
dppn has a PL maximum at 630 nm, at slightly lower energy
than the band gap (indicated by a vertical line in Figure 2d).
PbI2-dppn in contrast shows a PL emission peak with a
maximum almost exactly coincident with the band gap energy.
BiI3-dppz showed very weak photoluminescence, with peak
emission at 560 nm, again at slightly lower energy than the
band gap. The close coincidence between the PL maximum and
band gap energy in these three compounds suggests that the
defect concentration in the band gap is relatively low. PbI2-dppz
showed a much larger difference between band gap and PL
emission maximum. This may be related to the tailing seen in
the PbI2-dppz optical absorption spectrum (Figure 2c) below
the absorption edge, indicative of defect states within the band
gap. It is notable that the solution-processed materials reported
here are both highly crystalline (PXRD analysis, Figure 2a,b and
Figure S6) and PL-active. Conversely, Kanatzidis and co-
workers11a found that hybrid perovskite materials synthesized
by similar solution-processing methods and of high crystallinity
are PL-inactive. The fact that 1−4 are concurrently crystalline
and PL-active may be explained by the PL activity of the ligands
with the greater extinction coefficient of the dppn ligand giving
rise to more active complexes.
Band energy positions (Figure 3) for the materials were
determined by depositing 1−4 as thin films by spray deposition
The optical band gap of PV absorber materials is critical to
their function. In the hybrid PV field to date, it has been noted
that band gaps tend to decrease with increasing dimensionality
of the inorganic sublattice and with increasing mass of the
halide anion.35 For example, in the series RNH3PbI3, when R =
methyl (MAPI), the structure has 3D connectivity of the
inorganic sublattice and a band gap of 1.5 eV. Increasing the
size of the A-site cation results in lower inorganic
dimensionality: for example, when R = ethyl or propyl, both
adopt the hexagonal perovskite structure with 1D connectivity
of the inorganic sublattice and show considerably larger band
gaps of 2.20 and 2.40 eV, respectively.17a,24
Figure 3. Band alignment for Pb and Bi dipyrido complexes. The
positions of typical functional layer materials used in photovoltaic
devices [e.g., conduction band of typical ETL TiO2 (−4.0 eV)6 and
valence band level of common HTL PEDOT:PSS (5.20 eV)] are
shown for comparison.
In the Pb(II) and Bi(III) systems studied here, the larger
conjugated organic ligand (dppn) induces a smaller band gap,
contrary to the trend seen for the simple aliphatic organic ions
mentioned above. This is the case even in the two Pb(II)
complexes that show different inorganic dimensionality: the
smaller dppz ligand induces 1D lead iodide chains, whereas the
larger dppn ligand results in dimeric, 0D inorganic clusters.
Thus the relationship observed in RNH3PbI3 compounds, that
greater inorganic dimensionality results in a smaller band gap, is
not observed in this case, which indicates that for these systems
band gaps can be tuned independently of inorganic
connectivity, or that lower inorganic dimensionality can be
compensated for by increased organic dimensionality.
In the hybrid perovskites, shorter average M−I bond lengths
and reduced octahedral distortion corresponds to smaller band
gap materials.36 This trend can also be observed here, with BiI3-
dppn having the lowest band gap of 2.05 eV and lowest average
M−I bond length; by contrast, PbI2-dppz has the largest band
gap of 2.52 eV and longest average M−I bond length (Table
S2). Conversely, the M−N bond lengths cannot be correlated
to band gap variances. While octahedral distortion has been
observed for wider band gap perovskite materials, here the
opposite is apparent. Distortion from the ideal bond angle of
180° in octahedral geometry is much less for PbI2-dppz (I−
of the respective metal halide and organic ligand in DMF or
NMP solution onto heated substrates, and the films were
analyzed by XPS (Figure S9 and Table S3). The energy of the
valence band maximum relative to the Fermi level was
determined via linear extrapolation of the valence band onset
to the spectral background. Work functions for the materials
were obtained by applying a sample bias of 30 V and measuring
the low kinetic energy threshold of the XPS spectrum.38
Determination of the energy levels allows prediction of suitable
junction fabrications using typical electron transport (ETL) and
hole transport layers (HTL); for example, the BiI3-dppz energy
levels suggest a TiO2/BiI3-dppz/PEDOT:PSS junction would
be well-matched for charge separation (Figure 3).39 The
materials are easily solution-processed and films can be readily
fabricated as shown in Figure S10.
Theoretical Analysis. Each structure was optimized by use
of the PBEsol functional initially, and then the hybrid functional
HSE06 was applied in each case to obtain an accurate electronic
band structure. A comparison of relaxed theoretical structures
and experimental results is presented in Table S4, demonstrat-
ing the accuracy of the PBEsol functional at describing these
structures, with differences attributable to thermal effects. Band
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structures for each of the four compounds are also depicted
(Figure S11). The effect of spin−orbit coupling on the
electronic structures was found to be negligible, as shown by
the PBEsol and PBEsol+SOC band structures (Figure S12−
S15). As such, we expect the HSE06 band gap values to provide
a reasonable estimate of the band gap magnitude, without the
additional spin−orbit perturbation usually required in hybrid
lead halide perovskites.40 In all cases, the compounds were
found to be medium to wide band gap semiconductors, with
both valence and conduction bands showing negligible
dispersion.
localized on the organic ligand, therefore not facilitating exciton
separation; rationalizing these structural and electronic differ-
ences may therefore be a fruitful area of study.
The calculated carrier effective masses (Table S5) are almost
an order of magnitude higher than those of MAPI,43 indicating
significantly poorer charge transport of both holes and
electrons. The key comparison to be made between
experimental and theoretical results is in the band gap trend;
the indirect and direct band gaps of each compound are listed
in Table S6. The dppn systems have significantly smaller band
gaps than the dppz compounds, in agreement with
experimentally observed trends. Underestimation of the
calculated band gaps compared to experiment, when the
HSE06 functional is used, has previously been observed in
other semiconductors.44
Of particular interest are the characters of the valence and
conduction band edges. Figures 4 and 5 show the partial
CONCLUSIONS
■
Four post-transition metal iodide dipyrido coordination
complexes have been synthesized and structurally characterized.
These materials demonstrate promising tunability of their
optical, structural, and electronic properties and show promise
for potential applications as PV absorbers. As such, they
represent a possible alternative to the perovskite structure,
which offers very little compositional flexibility.
EXPERIMENTAL SECTION
■
Figure 4. Partial charges of VBM and CBM for lead iodide complexes
by use of HSE06 functional.
1,10-Phenanthroline, o-phenylenediamine, 2,3-diaminonaphthalene,
lead(II) nitrate, potassium iodide, and bismuth(III) iodide were all
purchased from Sigma−Aldrich and used without further purification.
Caution! The lead compounds utilized are toxic, and measures must be
taken to avoid exposure of workers. An especially hazardous step is the
spray coating of thin films, as without proper controls it may result in
inhalation of lead-containing chemicals and toxic solvents.
Reagent Synthesis. Synthesis of 1,10-Phenanthroline-5,6-dione.
Potassium bromide (1.999 g, 16.80 mmol) and 1,10-phenanthroline
(2.220 g, 11.20 mmol) were added to an ice-cold solution of sulfuric
acid (98%, 20 mL) and nitric acid (68%, 10 mL). The reaction was
heated at reflux for 5 h before the warm solution was poured over ice
and neutralized with sodium hydroxide. 1,10-Phenanthroline-5,6-dione
was extracted with chloroform, anhydrous magnesium sulfate was
added, and the solution was filtered and then dried in vacuo. 1H NMR
(600 MHz, CDCl3) 9.14 (dd, J = 4.7, 1.7 Hz, 2H), 8.52 (dd, J = 7.9,
1.5 Hz, 2H), 7.60 (m, 2H) ppm.
Synthesis of Dppz. Ethanolic solutions of 1,10-phenanthroline-5,6-
dione (0.500 g, 2.379 mmol) and o-phenylenediamine (0.386 g, 3.568
mmol) were combined and refluxed for 6 h before being cooled to
obtain a solid. The solid was filtered off and recrystallized from
ethanol. Dppz: 1H NMR (600 MHz, CDCl3): δH 9.71 (dd, J = 8.0, 1.5
Hz, 2H), 9.32 (d, J = 3.0 Hz 2H), 8.39 (dd, J = 6.5, 3.4 Hz, 2H), 7.96
(dd, J = 6.5, 3.3 Hz, 2H), 7.85 (dd, J = 8.0, 4.5 Hz, 2H).
Figure 5. Partial charges of VBM and CBM for bismuth iodide
Synthesis of Dppn. Ethanolic solutions of 1,10-phenanthroline-5,6-
dione (0.500 g, 2.379 mmol) and 2,3-diaminonaphthalene (0.377 g,
2.386 mmol) were combined and refluxed for 5 h before being cooled
to obtain a solid. The solid was filtered off and recrystallized from
ethanol. Dppn: 1H NMR (600 MHz, CDCl3): δH 9.68 (dd, J = 8.1, 1.6
Hz, 2H), 9.30 (d, J = 2.9 Hz, 2H), 8.97 (s, 2H), 8.22 (dd, J = 6.4, 3.2
Hz, 2H), 7.83 (dd, J = 8.0, 4.4 Hz, 2H), 7.65 (dd, J = 6.6, 3.0 Hz, 2H).
Synthesis of Lead(II) Iodide. Separate 50 mL aqueous solutions of
lead(II) nitrate (14.240 g, 0.043 mol) and potassium iodide (14.289 g,
0.086 mol) were prepared. Upon mixing together, lead(II) iodide
formed immediately as a yellow precipitate, which was separated via
filtration and washed thoroughly with deionized water.
complexes by use of HSE06 functional.
charges for valence band maximum (VBM) and conduction
band minimum (CBM) energy levels. In BiI3-dppz and PbI2-
dppn, the valence band lies on the inorganic portion of the
complex, composed of lead or bismuth s states with iodine p
states (Figures S12 and S15). Alternatively the conduction
band is localized on the organic ligand, constituting carbon s
and p, nitrogen p, and hydrogen states. This gives rise to
exciton charge separation across the inorganic/organic
components and the potential for long-lived charge-separated
states derived from an active organic cation species,41 which
may lead to the reduction of recombination losses.42 In BiI3-
dppn and PbI2-dppz, both conduction and valence bands are
Polycrystalline Sample Synthesis. Polycrystalline forms of the
materials were synthesized through solvent evaporation: DMF was
used for the dppz complexes, while NMP was required for preparation
of the larger dppn-containing species.
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PbI2-dppz (1). Dppz (0.050 g, 0.177 mmol) was dissolved in a
minimum amount of methanol by heating. In addition, concentrated
methanolic solutions of sodium iodide (0.053 g, 0.355 mmol) and
lead(II) acetate trihydrate (0.132 g, 0.177 mmol) were prepared. Upon
addition of the three solutions together, a yellow precipitate was
formed, which was separated by filtration.
PbI2-dppn (2). Dppn (0.050 g, 0.150 mmol) and lead(II) iodide
(0.069 g, 0.150 mmol) were separately dissolved in minimum amounts
of NMP. The solutions were combined and left under continuous air
flow for several days before being dried in air at 170 °C.
BiI3-dppz (3). Dppz (0.050 g, 0.177 mmol) was dissolved in
anhydrous DMF (1 mL) by heating and then added cold to a solution
of bismuth(III) iodide (0.104 g, 0.177 mmol) in DMF (1 mL). The
resultant orange/brown solution was heated under continuous air flow
at 125 °C until a dry solid was obtained.
BiI3-dppn (4). Dppn (0.050 g, 0.150 mmol) and bismuth(III) iodide
(0.089 g, 0.150 mmol) were separately dissolved in minimum amounts
of NMP and prepared as for 2.
Single-Crystal Synthesis. Single crystals were synthesized
through solvent layering, with DMF used for 1 and 3 and NMP for
2 and 4, as exemplified below.
Dppz (0.050 g, 0.177 mmol) was dissolved with heating in
anhydrous DMF (1 mL) and then added cold to a solution of
bismuth(III) iodide (0.104 g, 0.177 mmol) in DMF (1 mL). The
resultant orange/brown solution was layered under methanol in an
NMR tube. After several days, orange crystals had grown and were
used for structure determination.
Sample Characterization. Powder X-ray diffraction (PXRD) data
were collected on a Stoe StadiP diffractometer using Cu Kα1 radiation,
λ = 1.540 56 Å. Capillaries (0.5 mm) were filled with powdered
samples and data were collected over the 2θ range 2−60° in steps of
0.5° at 10 s/step.
Single-crystal X-ray diffraction data sets of 1−4 were collected on a
SuperNova (dual source) Atlas diffractometer with Cu Kα radiation (λ
= 1.541 84 Å). Single crystals of 1−4 were mounted on Hampton
Research 20 μm nylon loops (⌽ ≈ 0.3 mm) and kept at 150 K during
data collection with the exception of 1, which was collected in addition
at 100 K. A summary of crystal data, data collection, and refinement
for crystallographically characterized compounds is given in Table S1.
By use of Olex2,45 structures were solved for 1 and 3 with the
ShelXT46 structure solution program using Direct Methods and
refined with the ShelXL47 refinement package using least-squares
minimization. Also by use of Olex2,45 structures for 2 and 4 were
solved with the Superflip48 structure solution program using charge
flipping and refined with the ShelXL47 refinement package using least-
squares minimization.
Data for 1 were initially collected at 150 K, but due to suspected
static disorder, the collection was repeated at 100 K, which confirmed
static and not dynamic disorder. In 1, the dppz ligand (point symmetry
2/m) does not lie with the 2-fold axis of the molecule along the 2-fold
axis of the space group. The disorder in the structure can be modeled
with the 2-fold axis of the ligand skewed away from the crystallo-
graphic 2-fold axis, resulting in two positions for the ligand with 50%
occupancy. As a consequence of the disorder, a thermal constraint
(EADP) was placed on pairs of atoms (e.g., N1 with N2, C1 with C10,
C5 with C6 etc.) throughout the ligand, with isotropic displacement
parameters deemed to give the best fit to the data.
While the disorder in 1 is observed with respect to the ligand, use of
the dppn ligand in 2 with its larger conjugated π system and stronger
π−π stacking interaction between ligands results in the disorder being
manifest in the inorganic PbI2 component (Figure 1f). This leads to
two different conformations about each lead cation with a refined
occupancy ratio 4:1.
Following the synthesis of 3, single-crystal data were collected at
150 K. This structure has been recently published31 with the data
published collected at room temperature with a final R1 = 0.0834 and
wR2 = 0.1947 for 2749 observed reflections with I > 2σ(I). Due to the
significant difference in cell parameters and anisotropic displacement
parameters, the data collected at 150 K are included here for
comparison (R1 = 0.0342, wR2 = 0.0834 for 4789 observed reflections
with I > 2σ(I)] and have been submitted to the Cambridge Structural
Database.
UV−vis data was recorded on a Lambda 950 spectrophotometer
equipped with an integrating sphere at ambient temperature. Powders
were fixed to carbon tape and run in diffuse reflectance mode with a
data collection step of 1 nm. Photoluminescence spectroscopy was
recorded on a Horiba FluoroMax 4 with a 150 W xenon lamp source
and excitation radiation wavelength of 450 nm. The powder samples
were ground and sandwiched between float glass microscope slides, a
glass slide background was removed, and the data were normalized.
Nuclear magnetic resonance was recorded on a Bruker Axis 600
MHz. XPS and work function measurements were carried out on a
Thermo Scientific Al−Kα.
Theoretical Methods. All calculations were performed via density
functional theory (DFT) through the Vienna ab initio simulation
package (VASP).49 Within VASP, the projector-augmented wave
(PAW) method was used to describe interactions between the core
and valence electrons.50 The two functionals used in the calculations
were PBEsol,51 which is the Perdew−Burke−Ernzerhof (PBE)52
generalized gradient approximation (GGA) functional adapted for
solids, and HSE06,53 a hybrid functional that includes 25% screened
Hartree−Fock exchange with correlation and 75% exchange from PBE,
with a screening parameter of ω = 0.11 bohr−1. PBEsol has been
successful at describing the structure of hybrid lead halides,22a,54,55 so it
was used here for structure optimization, while HSE06 has been
shown to accurately reproduce experimental band gaps for semi-
conductors.56,57 A cutoff energy of 400 eV and a k-point mesh of 3 × 2
× 2, were found to be sufficient for the bismuth and PbI2-dppz
compounds. For PbI2-dppn, a cutoff of 400 eV and a k-point mesh of 2
× 2 × 1 was sufficient. In all cases, the structures were considered
converged when the forces on each atom totaled less than 0.01 eV·Å−1.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.5b02749.
Fifteen figures and six tables showing powder XRD of
samples after exposure to air, photographs of films of the
reported compounds, band structure calculations, and
partial densities of states derived from DFT calculations
(PDF)
Crystallographic file for 2 (CIF)
Crystallographic file for 4 (CIF)
Crystallographic file for 1 (CIF)
Crystallographic file for 3 (CIF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: r.palgrave@ucl.ac.uk.
■
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
C.N.S. and A.M.G. contributed equally.
Funding
We acknowledge funding from the EPSRC SUPERGEN
Supersolar Hub (EP/J017361/1). Z.S. acknowledges a Royal
Society of Chemistry Summer Bursary. This work made use of
the ARCHER UK National Supercomputing Service (http://
www.archer.ac.uk), via the membership of the UK's HPC
Materials Chemistry Consortium, which is funded by EPSRC
(EP/L000202). This work was supported by EPSRC (EP/
N01572X/1). C.N.S. acknowledges the UCL Department of
Chemistry for providing a DTA studentship. A.M.G. acknowl-
F
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Article
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edges Diamond Light Source for the co-sponsorship of a
studentship on the EPSRC Centre for Doctoral Training in
Molecular Modelling and Materials Science (EP/L015862/1).
A.M.G. acknowledges Diamond Light Source and the M3S
CDT at UCL for the cosponsorship of a studentship.
Notes
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
R.G.P. and D.O.S. acknowledge the Materials Design Network.
D.O.S., C.N.S., and A.M.G. acknowledge the use of the Archer
U.K. National Supercomputing Service (http://www.archer.ac.
uk), via our membership in the U.K.’s HEC Materials
Chemistry Consortium, which is funded by EPSRC (EP/
L000202).
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