Two-step dipping technique for the preparation of organic-inorganic perovskite thin films

A convenient two-step dipping technique for preparing high-quality thin films of a variety of perovskites is provided by the invention. Thin films of Mi.sub.2 (M=Pb, Sn) were first prepared by vacuum-depositing MI.sub.2 onto ash glass or quart substrates, which were subsequently dipped into a solution containing the desired organic ammonium cation for a short period of time. Using this technique, thin films of different layered organic-inorganic perovskites (RNH.sub.3).sub.2 (CH.sub.3 NH.sub.3).sub.n-1 M.sub.n I.sub.3n+1 (R=butyl, phenethyl; M=Pb, Sn; and n=1, 2) and three-dimensional perovskites CH.sub.3 NH.sub.3 MI.sub.3 (M=Pb, Sn; i.e. n=.infin.) were successfully prepared at room temperature. The lattice constants of these dip-processed perovskites are very similar to those of the corresponding compounds prepared by solution-growth or by solid state reactions. The layered perovskite thin films possess strong photoluminescence, distributed uniformly across the film areas. Similar results are achieved starting from spin-coated MI.sub.2 films, which were dipped into appropriate solutions of the organic ammonium cations. The process of the invention can be used for a variety of organics and inorganics, even if they have incompatible solubility characteristics or even if the organic component is susceptible to thermal decomposition on heating. Thin perovskite films prepared by the method are attractive candidates for emitter materials in electroluminescent devices.

BACKGROUND OF THE INVENTION 
Organic-inorganic perovskites have recently attracted much attention due to 
their unique electrical, magnetic, and optical properties, as well as 
their excellent film processability. Layered perovskites, 
(RNH.sub.3).sub.2 (CH.sub.3 NH.sub.3).sub.n-1 M.sub.n I.sub.3n+1 (M=group 
IVB metal), naturally form a quantum-well structure in which a 
two-dimensional semiconductor layer of corner-sharing MI.sub.6 octahedra 
and an organic ammonium layer are alternately stacked. The excitons, 
resulting from the low-dimensionality of these semiconductor sheets, have 
a binding energy of several hundred milli-electron-volts, which enables 
strong emission even at room temperature. Their strong room temperature 
photoluminescence, along with significant photoluminescence wavelength 
tunability make the organic-inorganic perovskites attractive candidates 
for emitter materials in electroluminescent devices. Era et al., in Appl. 
Phys. Lett., V. 65, p. 676, have recently (1994) reported an 
electroluminescent device using the layered perovskite (C.sub.6 H.sub.5 
C.sub.2 H.sub.4 NH.sub.3).sub.2 PbI.sub.4 as an emitter material. At 
liquid nitrogen temperature, an electroluminescent intensity of 10,000 
cd/m.sup.2 was obtained at a current density of 2 A cm.sup.-2, using an 
applied voltage of 24 V. 
Proper processing is essential in order to generate the material quality 
and morphology required to observe strong luminescence or to build 
devices. Single crystals and deposited thin films are two of the most 
useful forms for studies of optical and electrical properties. In general, 
when high-quality single crystals are required, crystal growth from the 
melt phase is often used. However, as a result of the organic ammonium 
cations, which decompose at relatively low temperature (&lt;250.degree. C.), 
organic-inorganic perovskites are typically made using solution chemistry 
techniques. Crystals resulting from solution methods are often, however, 
too small or of insufficient quality to be useful in device applications. 
For the preparation of thin films, the spin-coating technique is suitable 
for processing many organic-inorganic perovskites because they are often 
substantially soluble in conventional organic solvents. Spin-coating can 
be considered a special case of solution crystal growth. It allows the 
formation of perovskites on a substrate, while the solvent is evaporating 
off. Using this method, high-quality, highly oriented layered perovskite 
thin films can often be obtained. However, control of film thickness, 
uniformity, and surface morphology is difficult using spin-coating. In 
addition, while simple organic ammonium salts are soluble in a range of 
organic solvents, including those which can dissolve the inorganic 
MI.sub.2 salt, for more complex organic cations, the choice of solvent 
becomes more limited. Furthermore, solvent techniques are not always 
compatible with the MI.sub.2 salt, due to problems with solubility, strong 
solvent coordination or the stability of the metal valence state. 
Vacuum evaporation techniques have also recently been employed to grow 
oriented thin films of layered perovskites through a dual-source vapor 
deposition process. The benefits of this technique are that it is possible 
to precisely control the thickness and smoothness of the thin film 
surfaces. However, the preparation of various perovskites using different 
organic components is limited because each organic component easily 
contaminates the inside of the evaporation equipment. In addition, in some 
cases, the organic salt might not be thermally stable up to the 
temperatures required for evaporation, making this approach impractical 
for these systems. Even when it is possible to evaporate the organic salt, 
it is often difficult to balance the organic and inorganic rates, an 
important criterion for achieving the correct compositions of the 
resulting perovskite films. It is even more problematic that for each new 
organic-inorganic system, a re-establishment of the rates has to be 
carried out empirically. 
SUMMARY OF THE INVENTION 
A convenient two-step dipping technique for preparing high-quality thin 
films of a variety of perovskites is provided by the invention. Thin films 
of MI.sub.2 (M=Pb, Sn) were first prepared by vacuum-depositing MI.sub.2 
onto ash glass or quartz substrates, which were subsequently dipped into a 
solution containing the desired organic ammonium cation for a short period 
of time. Using this technique, thin films of different layered 
organic-inorganic perovskites (RNH.sub.3).sub.2 (CH.sub.3 
NH.sub.3).sub.n-1 (R=butyl, phenethyl; M=Pb, Sn; and n=1, 2) and 
three-dimensional perovskites CH.sub.3 NH.sub.3 MI.sub.3 (M=Pb, Sn; i.e. 
n=.infin.) were successfully prepared at room temperature. The lattice 
constants of these dip-processed perovskites are very similar to those of 
the corresponding compounds prepared by solution-growth or by solid state 
reactions. The layered perovskite thin films possess strong 
photoluminescence, distributed uniformly across the film areas. Similar 
results are achieved starting from spin-coated MI.sub.2 films, which were 
dipped into appropriate solutions of the organic ammonium cations. The 
process of the invention can be used for a variety of organics and 
inorganics, even if they have incompatible solubility characteristics or 
even if the organic component is susceptible to thermal decomposition on 
heating. Thin perovskite films prepared by the new method are attractive 
candidates for emitter materials in electroluminescent devices.

DETAILED DESCRIPTION OF THE INVENTION 
Vacuum evaporation of the MI.sub.2 films (M=Pb, Sn) was performed using an 
evaporation chamber. The MI.sub.2 powder (PbI.sub.2, Alfa Aesar, 99.999%; 
SnI.sub.2, APL Engineered Materials, 99.999%) was loaded into a quartz 
crucible, placed into the chamber, and the system was pumped down to a 
base pressure of about 4.times.10.sup.-7 Torr before beginning the 
evaporation. The metal iodide was evaporated and deposited onto ash glass 
or quartz substrates, which were maintained at room temperature, achieving 
a pressure of approximately 1.times.10.sup.-6 Torr during the deposition. 
During the deposition, the deposition rate and film thickness were 
monitored using a quartz crystal balance, with the deposition rates 
typically kept in the range of 60 to 70.ANG./min. The MI.sub.2 films were 
immediately transferred into a nitrogen-filled drybox after deposition. 
The resultant films were transparent, with the PbI.sub.2 film having a 
light yellow color and Snl.sub.2 a greenish-yellow color. The thin films 
were uniform and smooth, as indicated by the profiles measured on an 
Alpha-Step 200 and an AFM, and the final film thicknesses were in the 
range of 2000 to 3000.ANG.. X-ray diffraction indicated that they 
consisted of grains which were primarily c-axis oriented with respect to 
the substrate. 
In addition to the evaporated metal(II) iodide films, spin-coated films 
were also used as a starting point for the dipping process. Spin-coating 
was performed in a nitrogen-filled drybox and the spin rate was 
electronically controlled to be approximately 1600 rpm. Saturated methanol 
solutions of metal(II) iodides were employed for the spin-coating. As was 
the case for the evaporated films, the spin-coated MI.sub.2 films were 
primarily c-axis oriented. 
Preparation of the organic salt solutions was performed as follows. The 
solutions of organic ammonium iodides were prepared by dissolving 107 mg 
of butylammonium iodide (C.sub.4 H.sub.9 NH.sub.2 .multidot.HI) or 268 mg 
of phenethylammonium iodide (C.sub.6 H.sub.5 C.sub.2 H.sub.4 NH.sub.2 
.multidot.HI) in 1 ml of 2-propanol (Aldrich, anhydrous). The 2-propanol 
(C.sub.3 H.sub.8 O) solution was then diluted with 6 ml of toluene 
(Aldrich, anhydrous). The molar concentrations for the butylammonium 
iodide and the phenethylammonium iodide solutions were 38 mM (millimolar) 
and 77 mM, respectively. Due to the limited solubility in the above mixed 
solvent, 7 ml of pure 2-propanol was used to dissolve 140 mg of 
methylammonium iodide (CH.sub.3 NH.sub.2 .multidot.HI) to prepare a 127 mM 
solution of this organic salt. A solution containing 2 equivalents of 
butylammonium iodide, and 1 equivalent of methylammonium iodide was 
prepared by completely dissolving 80 mg of butylammonium iodide, or 0.40 
millimole (mmol), and 32 mg of methylammonium iodide (0.20 mmol) in 5 ml 
of 2-propanol, and then diluting this solution with 25 ml of toluene 
(C.sub.7 H.sub.8). All of the solutions were kept in a nitrogen-filled 
drybox. 
The butylammonium iodide used in the above solutions was prepared by 
bubbling hydrogen iodide (Matheson Gas Products) into a butylamine 
(Aldrich) chloroform solution. Phenethylammonium iodide and methylammonium 
iodide were previously prepared in this laboratory by mixing aqueous 
solutions of the organic amine and hydriodic acid and drying the product 
under vacuum. 
To prepare the films by dipping, each MI.sub.2 thin film was immersed into 
a solution (in a nitrogen-filled drybox) containing the desired organic 
ammonium iodide for a selected period of time. After the film was taken 
out from the dipping solution, it was immediately immersed in a rinse 
solution, which had the same composition as the dipping solution solvent 
(without the dissolved organic ammonium salt), for 5-10 seconds, to remove 
any excess organic ammonium salt. Before the thin film was transferred out 
of the drybox for measurements, it was pumped in the drybox loading lock 
for 10 minutes to ensure that any remaining solvent was removed. The 
specific films were prepared as follows: 
(C.sub.4 H.sub.9 H.sub.3).sub.2 PbI.sub.4. Upon immersing an evaporated 
PbI.sub.2 thin film into the 38 mM butylammonium iodide solution, it 
immediately turned an opaque yellow. Although there were no significant 
changes in appearances for the thin films prepared with different dipping 
duration, ranging from 5 seconds to 10 minutes, X-ray diffraction showed 
that it took about 1 to 3 minutes to complete the reaction of converting 
all the PbI.sub.2 into the perovskite compounds. In terms of 
photoluminescence, it was of little benefit to extend the dipping duration 
beyond 2 minutes. In fact, it sometimes caused the formed perovskite films 
to partially come off the substrate if the immersion period was longer 
than 30 minutes, resulting in films with rough surfaces. 
Spin-coated films of PbI.sub.2 also formed (C.sub.4 H.sub.9 N.sub.3).sub.2 
PbI.sub.4 when they were dipped into a butylammonium iodide solution. 
Typically, a period longer than 1 minute was used for dipping the 
spin-coated films in the solution of butylammonium iodide. The films 
showed yellowish color after the dipping process and they were still 
transparent (in contrast to the films prepared from evaporated PbI.sub.2). 
The absorption spectrum showed that a dipped spin-coated film of PbI.sub.2 
had an strong absorption peak at 524 nm, in accordance with the absorption 
peak associated with free excitons in single crystals of (C.sub.4 H.sub.9 
NH.sub.3).sub.2 PbI.sub.4..sup.10 
(C.sub.6 H.sub.5 C.sub.2 H.sub.4 NH.sub.3)PbI.sub.4 : The procedure of 
dipping PbI.sub.2 films into phenethylammonium iodide solution was similar 
to the butylammonium iodide analog. The resultant films of (C.sub.6 
H.sub.5 C.sub.2 H.sub.4 NH.sub.3).sub.2 PbI.sub.4 were yellow and not 
transparent. In the dipping process, a 1 to 3 minute period was generally 
required to complete the reaction between the PbI.sub.2 films and 
phenethylammonium iodide in the solution. Longer than 20 minutes of 
dipping duration usually led to rough films. 
CH.sub.3 NH.sub.3 PbI.sub.3 : The three-dimensional perovskites were also 
formed when methylammonium iodide was used in the dipping solution. As 
soon as a PbI.sub.2 film was introduced into a methylammonium iodide 
2-propanol solution the film turned black, indicating the formation of a 
surface layer of CH.sub.3 NH.sub.3 PbI.sub.3. To complete the 
transformation of the film into the three-dimensional perovskite required 
a period of 1 to 3 hours, much longer than for the synthesis of the 
layered perovskite films. The resulting CH.sub.3 NH.sub.3 PbI.sub.3 
perovskite films were black and uniform. 
(C.sub.4 H.sub.9 NH.sub.3).sub.2 (CH.sub.3 NH.sub.3)Pb.sub.2 I.sub.7. The 
preparation of thin films of the bilayer (i.e. n=2) perovskite, (C.sub.4 
H.sub.9 NH.sub.3).sub.2 (CH.sub.3 NH.sub.3)Pb.sub.2 I.sub.7, was carried 
out stepwise. First, a PbI.sub.2 film was dipped into the 38 mM 
butylammonium iodide solution for 10 seconds, resulting in a film of 
(C.sub.4 H.sub.9 NH.sub.3).sub.2 PbI.sub.4. It was then dipped for 3 
minutes into the mixed solution containing 2 equivalents of butylammonium 
iodide and 1 equivalent of methylammonium iodide. The resulting film 
appeared bright red and was not transparent. Using these dipping 
parameters, No evidence for lower order (i.e., n=1) or higher order (n&gt;2) 
members of the perovskite family could be detected using X-ray 
diffraction. The (C.sub.4 H.sub.9 NH.sub.3).sub.2 (CH.sub.3 
NH.sub.3)Pb.sub.2 I.sub.7 film changed little for samples with dipping 
periods up to 3 hours. However, a 12 hour dipping duration eventually led 
to a dark gray film, which was identified by X-ray diffraction as 
primarily CH.sub.3 NH.sub.3 PbI.sub.3. 
The order of the procedure is vital for the preparation of the n =2 
compound. If a PbI.sub.2 thin film was directly immersed into a C.sub.4 
H.sub.9 NH.sub.2 .multidot.HI--CH.sub.3 NH.sub.2 .multidot.HI (2:1, molar 
ratio) solution, the reaction did not lead to the formation of (C.sub.4 
H.sub.9 NH.sub.3).sub.2 (CH.sub.3 NH.sub.3)Pb.sub.2 I.sub.7. Instead, 
CH.sub.3 NH.sub.3 PbI.sub.3 was formed, as shown by the X-ray diffraction 
pattern. When a PbI.sub.2 thin film was first immersed into a 
methylammonium iodide solution, the resulting CH.sub.3 NH.sub.3 PbI.sub.3 
material did not convert into (C.sub.4 H.sub.9 NH.sub.3).sub.2 (CH.sub.3 
NH.sub.3)Pb.sub.2 I.sub.7 dipping it into either the C.sub.4 H.sub.9 
NH.sub.2 .multidot.HI--CH.sub.3 NH.sub.2 .multidot.HI or the C.sub.4 
H.sub.9 NH.sub.2 .multidot.HI 2-propanol solution, even for periods of up 
to 12 hours. 
(C.sub.4 H.sub.9 NH.sub.3).sub.2 SnI.sub.4 : The dipping process for 
(C.sub.4 H.sub.9 NH.sub.3).sub.2 Snl.sub.4 films was similar to that of 
the lead(II) analog, although the reactions were found to proceed at a 
faster rate. A Snl.sub.2 film was first immersed into a 70 mM 
butylammonium iodide 1:16 volume ratio solution of 2-propanoltoluene 
(1:16, v/v) for 2 seconds and then rinsed with toluene, yielding an opaque 
red-brown film. It emitted a nominally orange light under excitation from 
a 366 nm hand-held light. 
(CH.sub.6 C.sub.2 H.sub.4 NH.sub.3)SnI.sub.4 : The film of (C.sub.6 H.sub.5 
C.sub.2 H.sub.4 NH.sub.3).sub.2 SnI.sub.4 was prepared by first dipping a 
Snl.sub.2 film into a saturated phenethylammonium iodide 
2-propanol/toluene (1/16, v/v) solution for 3 seconds and then taking it 
out of the solution to dry in a nitrogen atmosphere. This procedure was 
repeated twice, and the resulting film was rinsed with the same mixed 
solvent in the absence of phenethylammonium iodide. The resulting film was 
red-brown and not transparent. An orange emission was seen when it was 
exposed to 366 nm light. 
CH.sub.3 NH.sub.3 SnI.sub.3 : The films of CH.sub.3 NH.sub.3 SnI.sub.3 were 
also black, similar to those of CH.sub.3 NH.sub.3 PbI.sub.3. They were 
prepared by dipping the SnI.sub.2 films into an 88 mM methylammonium 
iodide 2-propanol solution for 30 seconds. The resulting films were rinsed 
with 2-propanol. While the reaction between the SnI.sub.2 films and methyl 
ammonium iodide was slower than for the corresponding two-dimensional 
butylammonium or phenethylammonium compounds, it was much faster than for 
the formation of the Pb analog. 
Room temperature X-ray diffraction patterns were collected over the range 
of 2.degree..ltoreq.2.theta..ltoreq.60.degree. for each MI.sub.2 thin film 
and each perovskite thin film, using a Siemens D5000 diffractometer (Cu 
K.alpha. radiation). All the samples were measured directly under ambient 
conditions, except the CH.sub.3 NH.sub.3 SnI.sub.3 films, which were cut 
into 1.times.2 cm stripes and contained in an air-tight cell filled with 
nitrogen from the drybox. The indexing of the diffraction peaks was 
performed for the perovskite films based on a comparison with the patterns 
from the corresponding materials prepared using a solution chemistry 
method or solid state reactions. The lattice constants were refined using 
the Siemens WIN-METRIC program after removing the background and the 
K.alpha.2 component from the diffraction pattern. 
Absorption spectra were recorded on a Hewlett Packard UV-Vis 8543 
spectrophotometer. Excitation and emission spectra were taken on a Spex 
Fluorolog-2 spectrophotometer using a front-face geometry. The incident 
light was from a xenon arc lamp which was passed through a SPEX 1680 0.22 
m double spectrometer. The emission was detected using a SPEX 1911F 
detector after it was passed through a similar double spectrometer. 
The emission spectra of each sample were collected with three different 
excitation wavelengths (350 nm, 385 nm, and 480 nm, for example, for the 
(C.sub.4 H.sub.9 NH.sub.3).sub.2 PbI.sub.4 films) to ensure that the 
emission spectra were independent of excitation wavelength. 
The room temperature AFM images of the various thin films were recorded on 
a Nanoscope III (DFM-5000) (Digital Instruments, Calif.). The scan area 
was typically 10.times.10 .mu.m for preliminary scans and 3.times.3 .mu.m 
for more detailed scans. 
The selection of the solvents for dissolving the organic ammonium iodides 
was found to be critical for obtaining high-quality thin films of the 
perovskites using the new dipping technique. The criteria for choosing the 
solvents are that they must be good solvents for the organic ammonium 
iodides, but poor solvents for the metal(II) iodides and the resulting 
perovskite compounds. While both hexane and toluene do not dissolve 
metal(II) iodides and the corresponding perovskites, they are not 
appropriate solvents because the organic ammonium salts that were used are 
only slightly soluble in them. For example, PbI.sub.2 thin films only 
minimally reacted after being immersed in a saturated butylammonium 
iodide/toluene solution for 12 hours. In contrast, 2-propanol is a good 
solvent for organic ammonium salts as well as for the layered perovskites. 
When a PbI.sub.2 thin film was dipped into a butylammonium 
iodide/2-propanol solution, it dissolved into the solution in a few 
seconds. In order to balance these two extremes, a mixed solvent was 
chosen, with a composition of 1 portion of 2-propanol to 6 portions of 
toluene by volume for A.sub.2 PbI.sub.4, and 1 portion of 2-propanol to 16 
portions of toluene for A.sub.2 SnI.sub.4 (A=C.sub.4 H.sub.9NH.sub.3 and 
C.sub.6 H.sub.5 C.sub.2 H.sub.4 NH.sub.3). The smaller ratio of 2-propanol 
in the case of A.sub.2 SnI.sub.4 helped to obtain better films because of 
higher solubility for the tin(II) compounds compared to the corresponding 
lead(II) materials. In the case where methylammonium iodide was used to 
yield the three-dimensional perovskites, pure 2-propanol was used, as 
methylammonium iodide was only slightly soluble in the above mixed-solvent 
system. More importantly, the resulting CH.sub.3 NH.sub.3 MI.sub.3 
perovskite films did not degrade during the synthetic process (a period of 
time up to 3 hours), enabling the complete reaction of the MI.sub.2 thin 
films. 
The reaction between the organic ammonium iodides and the MI.sub.2 films 
was very fast, especially for the layered perovskites. The formation of 
the perovskites was seen as soon as the MI.sub.2 thin films were immersed 
into the organic ammonium solutions. In fact, a PbI.sub.2 thin film, 
dipped in a butylammonium iodide solution for shorter than 5 seconds, had 
comparable photoluminescence intensity with the one which was dipped in 
the same solution for 5 minutes or longer. As X-ray diffraction patterns 
demonstrated, PbI.sub.2 was sometimes detected in the perovskite films 
which were made using a dipping duration of less than 1 minute. A 1 to 3 
minute period appeared to be necessary for all the lead(II) iodide in the 
films to be completely converted into perovskite. Tin(II) iodide thin 
films were found to react at a faster rate than the corresponding lead 
iodide films, with the reactions often being completed within a few 
seconds. 
Surprisingly, the reactions of the invention are completed very quickly, 
even for relatively thick films. The reactions, therefore, are believed to 
occur through the organic ammonium iodides diffusing to the 
perovskite/metal(II) iodide interface, and subsequently reacting with the 
metal(II) iodides to form more of the perovskite. Layered perovskites 
consist of alternate inorganic layers and organic bilayers. This 
presumably allows organic ammonium iodide molecules to more easily diffuse 
into the structure along the van der Waals gap formed between organic 
layers of the structure. 
Compared to the layered perovskites, A.sub.2 MI.sub.4, a much longer time 
is required to complete the reaction between metal(II) iodides and 
methylammonium iodide, which results in the formation of the 
three-dimensional perovskites, CH.sub.3 NH.sub.3 MI.sub.3. In a 
methylammonium iodide solution, MI.sub.2 rapidly reacts and forms a thin 
layer of the perovskite at the solution-film interface, as indicated by a 
color change for the film. In contrast to the layered perovskites, 
however, this surface layer does not aid in the diffusion of 
methylammonium iodide from the solution into the interior of the film, 
since the three-dimensional structure lacks a van der Waals gap. 
The importance of thermodynamics for the reactions between organic ammonium 
salts and PbI.sub.2 was revealed by the formation process for the (C.sub.4 
H.sub.9 NH.sub.3).sub.2 (CH.sub.3 NH.sub.3)Pb.sub.2 I.sub.7 films. 
CH.sub.3 NH.sub.3 PbI.sub.3 is found to be thermodynamically more stable 
relative to the other members of the layered perovskite family. This 
accounts for the fact that dipping a lead(II) iodide-based film into a 
C.sub.4 H.sub.9 NH.sub.2 .multidot.HI--CH.sub.3 NH.sub.2 HI (2:1, molar 
ratio) solution eventually gave rise to CH.sub.3 NH.sub.3 PbI.sub.3, 
regardless of the starting film. (C.sub.4 NH.sub.3).sub.2 (CH.sub.3 
NH.sub.3)Pb.sub.2 I.sub.7 is apparently just an intermediate for the 
conversion of (C.sub.4 H.sub.9 NH.sub.3).sub.2 PbI.sub.4 into CH.sub.3 
NH.sub.3 PbI.sub.3 in the presence of methylammonium iodide. Fortunately, 
the kinetics are slow enough to enable the n=2 material to form as a 
metastable intermediate. 
The vacuum-deposited PbI.sub.2 thin films were mainly c-axis oriented, as 
indicated, for example, by the X-ray diffraction pattern shown in FIG. 
1(A). Higher order 001! diffraction peaks were detected, suggesting that 
these PbI.sub.2 films were reasonably well-ordered. Upon dipping into the 
butylammonium iodide solution, the c-axis oriented PbI.sub.2 films 
resulted in predominantly c-axis oriented thin films of the layered 
perovskite, (C.sub.4 H.sub.9 NH.sub.3).sub.2 PbI.sub.4, as shown in FIG. 
1(B). It can be seen that the c-axis diffraction peaks overwhelmingly 
dominate the diffraction pattern, with 7 observed 001! peaks. The 
presence of higher order peaks indicates that the formed perovskite film 
is well-crystallized. Although they are also c-axis oriented, the thin 
films of the layered perovskite (C.sub.6 H.sub.5 C.sub.2 H.sub.4 
NH.sub.3).sub.2 PbI.sub.4, prepared using dual source vapor deposition, 
appeared to be less organized, as indicated by the absence of higher order 
diffraction peaks. 
While the strongest reflections from the thin films were 001! peaks, 
several off-axis reflections (which tended to be weak, but varied in 
intensity depending on preparation conditions) could also be detected for 
some of the films, enabling the determination of the lattice constants for 
the perovskite films. All of the 13 diffraction peaks for the (C.sub.4 
H.sub.9 NH.sub.3).sub.2 PbI.sub.4 film were successfully indexed (all of 
them were singly indexed) and the refined orthorhombic lattice parameters 
were, a=8.882(2).ANG., b=8.696(3).ANG., c=27.631(5).ANG.. This is in good 
agreement with the parameters, a =8.886(2).ANG., b=8.698(2).ANG., 
c=27.637(5).ANG. for samples prepared by a solid state reaction at 
160.degree. C., and a =8.863(2).ANG., b=8.682(1) A, c =27.570(2) A for 
single crystals prepared by a solution crystal growth technique. 
The diffraction patterns of (C.sub.6 H.sub.5 C.sub.2 H.sub.4 
NH.sub.3).sub.2 PbI.sub.4 thin films showed that they were typically 
c-axis oriented, as for (C.sub.4 H.sub.9 NH.sub.3).sub.2 PbI.sub.4. FIG. 2 
demonstrates a good agreement between the diffraction pattern of a 
(C.sub.6 H.sub.5 C.sub.2 H.sub.4 NH.sub.3).sub.2 PbI.sub.4 film made using 
the dipping method, FIG. 2A, and that of ground crystals prepared by 
diffusing a methanol solution of C.sub.6 H.sub.5 CH.sub.4 NH.sub.2 
.multidot.HI into an aqueous HI solution of PbI.sub.2, FIG. 2B. 
The (C.sub.4 H.sub.9 NH.sub.3).sub.2 (CH.sub.3 NH.sub.3)Pb.sub.2 I.sub.7 
(n=2) films also exhibited an X-ray diffraction pattern suggesting 
primarily c-axis orientation, as seen in FIG. 3. The long cell dimension 
along c-axis was 39.37.ANG., an increase of 11.78.ANG. from the n=1 
structure, (C.sub.4 H.sub.9 NH.sub.3).sub.2 PbI.sub.4. For the tin(II) 
counterpart, the long unit-cell dimension perpendicular to the perovskite 
sheets increased from 27.576(2).ANG. for (C.sub.4 H.sub.9 NH.sub.3).sub.2 
SnI.sub.4 (n=1) to 39.395(5).ANG. for (C.sub.4 H.sub.9 NH.sub.3).sub.2 
(CH.sub.3 NH.sub.3)Sn.sub.2 I.sub.7 (n=2), an increase of 11.819.ANG.. The 
increase observed in both of these systems is the result of inserting two 
more CH.sub.3 NH.sub.3 MI.sub.3 perovskite layers per unit cell. 
Dip-processed CH.sub.3 NH.sub.3 PbI.sub.3 thin films exhibited very similar 
diffraction patterns to those of CH.sub.3 NH.sub.3 PbI.sub.3 prepared by a 
solid state reaction, as shown in FIG. 4. The refined tetragonal lattice 
parameters, a=8.874(1).ANG. and c=12.670(3).ANG., are virtually identical 
to the values, a=8.874.ANG. and c=12.669.ANG., for a sample prepared by 
solid state synthesis. 
For the dip-processed tin(II) systems, the X-ray diffraction patterns of 
(C.sub.4 H.sub.9 NH.sub.3).sub.2 SnI.sub.4 and (C.sub.6 H.sub.5 C.sub.2 
H.sub.4 NH.sub.3) .sub.2 SnI.sub.4 films were similar to those of 
corresponding crystals obtained by solution chemistry. Higher order 001 ! 
peaks were observed, indicating c-axis orientations of the films and 
reasonable crystalline order. The diffraction peaks for the CH.sub.3 
NH.sub.3 SnI.sub.3 film reflected a cubic unit cell, with the refined 
lattice parameter a=6.239(1).ANG. (compared to a=6.2397(5).ANG. for 
samples prepared by precipitation from an aqueous hydriodic acid 
solution). 
Two-dimensional AFM topology and phase images, from the surfaces of a 
thermally deposited PbI.sub.2 film and several (C.sub.4 NH.sub.3).sub.2 
PbI.sub.4 films, prepared using dipping times varying from 10-180 sec, are 
shown in FIG. 5. The surface of the evaporated PbI.sub.2 film, FIG. 5A, 
had small grains evenly distributed over the film area and had a mean 
roughness of 9 nm. After this film had been dipped in a butylammonium 
iodide solution, the surface morphology dramatically changed. As shown in 
FIG. 5(B), small, well-defined, randomly distributed grains were observed 
for a film prepared by dipping for 10 seconds. The grains appeared to be 
plate-like, and the mean roughness of the film was about 97 nm. Compared 
to the undipped lead iodide film, the grains appeared to be substantially 
larger, with typical in-plane dimensions on the order of 0.6(2) .mu.m and 
an average thickness of around 0.2(1) .mu.m. X-ray diffraction indicates 
that a substantial fraction of the PbI.sub.2 remained unreacted in this 
sample. For a film with a dipping duration of 60 seconds, in which most of 
the PbI.sub.2 had reacted to form the corresponding perovskite, the grain 
size appeared larger and the mean roughness of the film increased to 121 
nm. Although macroscopically the clusters were still randomly distributed, 
they were now often composed of two or three plate-like grains which were 
packed together in parallel, as shown in both the topology and phase 
images (FIG. 5(C)). FIG. 5(D) shows the images of a film dipped for 180 
seconds in a butylammonium iodide solution. In this case the PbI.sub.2 had 
completely reacted, as indicated by its X-ray diffraction pattern. The 
mean roughness of this film (113 nm) changed little relative to that for 
the 60 second dipping duration. However, the plate-like grains appeared to 
have grown ether relative to the shorter dipping duration samples, with 
many grains having in-plane dimensions larger than 1 .mu.m. These results 
indicate that the arrangement and size of the perovskite grains strongly 
depend upon the dipping duration. 
The structure of the layered perovskites is highly anisotropic and crystal 
growth tends to occur most easily along the plane of the perovskite sheets 
and more slowly along the perpendicular direction to the sheets. 
Consequently, crystals of these materials tend to grow as plate-like 
crystals, with the in-plane dimension corresponding to the plane of the 
perovskite sheets. In examining FIG. 5, the plate-like crystals appear to 
be approximately randomly oriented with respect to the substrate. This 
would suggest some a-b orientation to the films, rather than pure c-axis 
orientation. However, while there were typically off-axis reflections 
observed in the X-ray diffraction patterns for these films, the c-axis 
pattern 
The images of spin-coated PbI.sub.2 films from a methanol solution show 
that they are smooth and homogeneous, with a mean roughness of 1.7 nm. 
After it had been dipped in a butylammonium iodide solution for 1 minute, 
the X-ray diffraction pattern showed that all the PbI.sub.2 had reacted, 
and the surface roughness increased, with plate-like domains forming on 
the surface (FIG. 6). While the surface of the dip-processed sample from 
the evaporated PbI.sub.2 film was uniformly rough, with crystallites 
sticking out of the surface over the entire area of the film, the film 
resulting from the spin-coated PbI.sub.2 had a substantial amount of 
smooth area, with however sizable crystals growing out of the surface over 
some interval. This difference in surface morphology presumably at least 
partially accounts for the difference in optical transparency between the 
organic-inorganic films prepared from spin-coated PbI.sub.2 as opposed to 
those prepared from evaporated PbI.sub.2. 
In contrast to the films prepared from the two-step dipping process, the 
thin films prepared by spin-coating from an acetonitrile solution of 
(C.sub.4 H.sub.9 NH.sub.3).sub.2 PbI.sub.4 yielded a much smoother film, 
with mean roughness of only 1.1 nm. There were small dark spots 
homogeneously distributed over the film surface. Presumably these are tiny 
crystals of (C.sub.4 H.sub.9 NH.sub.3).sub.2 PbI.sub.4 formed in the 
course of spin-coating. While the dip-coated films appear substantially 
rougher, the surface morphology is expected to depend on the choice of the 
solvent system used during the dipping process. The fall range of 
potential solvents has only begun to be explored. Furthermore, it may be 
possible to control the orientation of the perovskite films using the 
dipping process. Finally, it should be easier to control thickness 
uniformity over larger areas using dip-processing. 
Single crystals of organic-inorganic layered perovskites, (RNH.sub.2).sub.2 
MI.sub.4 (M=Pb, Sn) exhibit strong exciton absorption and sharp exciton 
emission in the visible range at room temperature. The luminescence 
originates from electronic transitions within the inorganic perovskite 
layer. Due to their natural quantum-well structure, it is possible to 
tailor optical properties by varying either the organic or the inorganic 
components of the structure. 
When the thin films of (C.sub.4 H.sub.9 NH.sub.3).sub.2 PbI.sub.4, prepared 
using the dipping method, were exposed to a hand-held ultraviolet light 
(366 nm), they emitted strong green luminescence, which was evenly 
distributed across the whole film surface (approximately 1.5 cm.sup.2). 
FIG. 7(A) shows the emission spectra for this sample. A strong, sharp peak 
was observed at 537 nm, with full width at half height of 20 nm, when the 
film was excited at 480 nm. The emission peak position was independent of 
the excitation wavelength. It was found that there was no explicit 
relation between emission spectrum and the dipping duration. Both the peak 
profile and width were virtually the same in all thin films with dipping 
duration ranging from 5 seconds to 10 minutes. 
Compared to the emission spectra of single crystals samples, the spectra of 
(C.sub.4 H.sub.9 NH.sub.3).sub.2 PbI.sub.4 films, prepared from evaporated 
PbI.sub.2, were red-shifted by about 10 nm. While it was not possible to 
measure the absorption spectra of these films since they were not 
transparent, the excitation spectrum did show a sharp peak at 523 nm, 
which corresponds to the absorption of the exciton state within the 
inorganic perovskite sheets. For comparison, the absorption spectrum of a 
thin film of (C.sub.4 H.sub.9 NH.sub.3).sub.2 PbI.sub.4, prepared from a 
spin-coated PbI.sub.2 film (which was transparent), exhibited a sharp 
exciton peak at 524 nm. The luminescence peak in these films was at 525 
nm, which matches the results from the solution grown crystals. Because 
the emission peak arises from an exciton state in the inorganic sheets of 
the layered perovskite structure, it is possible that subtle changes in 
the crystal lattice of these films relative to the single crystal samples 
(perhaps due to organic cation disordering) may significantly impact the 
luminescence properties and account for the 10 nm red-shift for the films 
produced from the evaporated PbI.sub.2 films. It is also possible that 
differences in grain size and crystallographic orientation in these films, 
as suggested from the AFM results, could contribute to the shift in 
wavelength. Perhaps the most likely explanation arises from states near 
the band edge of the perovskite layer, which could be due to defects in 
the crystal structure or surface states. 
The thin films of the perovskite, (C.sub.6 H.sub.5 C.sub.2 H.sub.4 
NH.sub.3).sub.2 PbI.sub.4, prepared from evaporated PbI.sub.2, showed 
similar photoluminescence to the butylammonium analog. Upon exciting at 
480 nm, an emission peak at 537 nm, with full width at half height of 14 
nm was observed. This emission was also red-shifted about 10 nm relative 
to the emission of single crystals of (C.sub.6 H.sub.5 C.sub.2 H.sub.4 
NH.sub.3).sub.2 PbI.sub.4, similar to the case for (C.sub.4 H.sub.9 
NH.sub.3).sub.2 PbI.sub.4. The photoluminescence characteristics of 
(C.sub.4 H.sub.9 NH.sub.3).sub.2 PbI.sub.4 and (C.sub.6 H.sub.5 C.sub.2 
H.sub.4 NH.sub.3).sub.2 PbI.sub.4 were very similar, presumably because 
they have similar structures within the inorganic sheets. Furthermore, 
this similarity suggests that the interactions between adjacent inorganic 
sheets are insignificant in (C.sub.6 H.sub.5 C.sub.2 H.sub.4 
NH.sub.3).sub.2 PbI.sub.4 and (C.sub.4 H.sub.9 NH.sub.3).sub.2 PbI.sub.4, 
because the former has a substantially longer distance between adjacent 
inorganic sheets (16.316.ANG. for (C.sub.6 H.sub.5 C.sub.2 H.sub.4 
NH.sub.3).sub.2 PbI.sub.4 vs. 13.794.ANG. for (C.sub.4 H.sub.9 
NH.sub.3).sub.2 PbI.sub.4). 
The red films of (C.sub.4 H.sub.9 NH.sub.3).sub.2 (CH.sub.3 
NH.sub.3)Pb.sub.2 I.sub.7 emitted at 618 nm when excited at 480 nm (FIG. 
7(b)), with full width at half height of approximately 30 nm. The emission 
wavelength was red-shifted by about 83 nm relative to that of (C.sub.4 
H.sub.9 NH.sub.3).sub.2 PbI.sub.4. Although the absolute efficiency was 
not determined, the photoluminescence from the n=2 thin films was 
significantly weaker than for the (C.sub.4 H.sub.9 NH.sub.3).sub.2 
PbI.sub.4 films. In contrast to (C.sub.4 H.sub.9 NH.sub.3).sub.2 
PbI.sub.4, the inorganic sheets of (C.sub.4 H.sub.9 NH.sub.3).sub.2 
(CH.sub.3 NH.sub.3)Pb.sub.2 I.sub.7 have a bilayer structure. It has been 
previously reported that in the family, (C.sub.6 H.sub.5 C.sub.2 H.sub.4 
NH.sub.3).sub.2 (CH.sub.3 NH.sub.3).sub.n-1 Pb.sub.3n+1 I.sub.3, the 
band-gap energy, the lowest exciton energy, and the exciton binding energy 
all decrease with increasing sheet thickness (increasing n) as a result of 
quantum confinement or dimensionality effects. As a consequence, the 
exciton state in layered perovskites with thicker sheets is progressively 
less stable, and the emission associated with this exciton state 
correspondingly red-shifts. This accounts for the red-shifted emission and 
relatively low emissive efficiency for the thin films of (C.sub.4 H.sub.9 
NH.sub.3).sub.2 (CH.sub.3 NH.sub.3)Pb.sub.2 I.sub.7, relative to the n=1 
member of the (C.sub.4 H.sub.9 NH.sub.3).sub.2 (CH.sub.3 NH.sub.3)Pb.sub.2 
I.sub.3n+1 family. As the n=.infin. member, the thin film of CH.sub.3 
NH.sub.3 PbI.sub.3 emitted even more weakly, with a further red-shift of 
the emission peak to 780 nm (FIG. 7(C)). The emission peak was broad, with 
fall width at half height of approximately 40 nm. The absorption spectrum 
of these films had a step-shaped profile with the absorption edge at 
approximately 1.6 eV, which is in good agreement with the reported bandgap 
of CH.sub.3 NH.sub.3 PbI.sub.3. 
The films of (C.sub.4 H.sub.9 NH.sub.3).sub.2 SnI.sub.4 and (C.sub.6 
H.sub.5 C.sub.2 H.sub.4 NH.sub.3).sub.2 SnI.sub.4 emitted at 639 nm and 
629 nm, respectively. Qualitatively, their emissions were weaker relative 
to their Pb counterparts, which is in agreement with the conclusion drawn 
from a comparison with the corresponding solution-grown crystals. The 
CH.sub.3 NH.sub.3 SnI.sub.3 films, however, were not emissive under 
similar conditions at room temperature. 
The emission intensities of thin films of the layered perovskites were 
comparable to those of the corresponding single crystal samples. It has 
been found that the luminescence of these layered perovskites is very 
sensitive to defects in the structure. For (C.sub.4 H.sub.9 
NH.sub.3).sub.2 PbI.sub.4, poorly crystallized samples, finely ground 
crystals, or pressed pellets prepared by solid state synthesis, exhibit 
very little, if any, photoluminescence at room temperature. The fact that 
thin films of the layered perovskites made by the dipping technique have 
strong photoluminescence indicates that they are well-crystallized and 
have few non-radiative decay centers for the excitons. 
As a potential emitter material for electroluminescent devices, 
organic-inorganic layered perovskites offer many potential advantages over 
conventional organic or inorganic materials. In addition to their strong 
photoluminescence, they have higher mobility relative to pure organic 
emitter materials, and much more processing flexibility and luminescence 
tunability compared to inorganic emitter materials. The quality of the 
perovskite thin films is very important for retaining their luminescent 
properties. Spin-coating enables simple processing of well-crystallized 
thin films, which can locally be very smooth. However, control over film 
thickness and uniformity over large areas is difficult, and this technique 
would not be suitable for inorganic and organic constituents which have 
incompatible solubility characteristics. Vacuum deposition is another 
promising technique which allows precise control of the thickness and 
smoothness of the films, but involves the often difficult to control 
process of evaporating the organic salt. 
The two-step dipping technique of this invention can be considered a hybrid 
between the vacuum deposition and solution chemistry techniques. It not 
only takes advantage of the vacuum deposition technique with respect to 
managing film thickness and uniformity, but also retains the benefits of 
the solution chemistry methods in the sense that well-crystallized films 
can be generated without heating the organic component for evaporation. In 
addition to using evaporated MX.sub.2 films as a first step, spin-coated 
films can also be used if this is more compatible with the system under 
consideration. 
The convenience of this novel technique lies in its stepwise nature. The 
MX.sub.2 films can be prepared in advance and can then be used to form the 
desired perovskites just before they are needed. The fact that only the 
evaporation of MX.sub.2 is required significantly simplifies the 
preparation process, and furthermore enables syntheses in which two 
organic salts are used. In addition, single source evaporation of MX.sub.2 
helps to maintain a clean deposition environment. For the dipping 
technique, the main factors influencing the quality of the films are the 
choices of solvent for dissolving organic ammonium salts and the dipping 
duration. These two factors are relatively easy to control. As a 
consequence, high-quality thin films can now be conveniently prepared. For 
applications which require patterning, the dipping technique may also 
provide a promising pathway, since prior to the dipping step the surface 
of the film can be coated with a resist so that only selected areas are 
exposed to the organic cation in solution. 
The versatility of the dipping technique has also been demonstrated in this 
work by preparing a variety of perovskites, (RNH.sub.3).sub.2 (CH.sub.3 
NH.sub.3).sub.n=1 M.sub.n X.sub.3n+1, with n=1, . . . 2, . . . , .infin., 
using virtually the same dipping procedures. When n.fwdarw..infin., the 
resulting structure is three dimensional and is no longer layered. When n 
is not equal to infinity, the layered structure of the preceding formula 
is commonly referred to as the &lt;100&gt; oriented (in crystallographic 
nomenclature) family because the perovskite sheets have &lt;100&gt; orientation. 
Also, the methylammonium cation (CH.sub.3 NH.sub.3.sup.+) may be replaced 
by a monovalent alkali metal cation (e.g., Cs.sup.+, Rb.sup.+, K.sup.+). 
For a given metal, "M", the family of perovskites can be prepared by 
simply varying the organic ammonium cations or by using mixed cations in 
the dipping solution. While we have demonstrated here the group fourteen 
metal iodide based systems (using tin (II) and lead (II), the other 
possibility being Ge(II)) with relatively simple organic cations, a 
variety of other perovskites, with various metals, halogens, or more 
complex organic ammonium cations (i.e., RNH.sub.3.sup.+) are expected to 
form by dipping MX.sub.2 (X=Cl, Br, I) thin films into the corresponding 
organic ammonium halide solutions. These other metals might also include 
the divalent rare-earth metals and transition metals. The general criteria 
for selecting the organic ammonium iodides for the dipping solution are 
that they have one or more ammonium groups (NH.sub.3) that are 
appropriately oriented on the organic molecule to allow for effective 
hydrogen bonding between the organic ammonium cation and the metal halide 
layers of the perovskite structure. In addition, the organic molecule must 
be chosen such that it is crystallographically compatible with the layered 
perovskite structure (i.e., will "fit" into the structure). Finally, it is 
expected that members of the &lt;110&gt; oriented family of perovskites can be 
formed by the method of the invention. Such &lt;110&gt; oriented perovskites are 
also denoted by the general formula (NH.sub.2 C(I)=NH.sub.2).sub.2 
(CH.sub.3 NH.sub.3).sub.n M.sub.n X.sub.3n+2 with n=1, 2, . . . .infin..