Tuning the light emission properties by band gap engineering in hybrid lead halide perovskite

Article abstract of DOI:10.1021/ja511198f

We report about the relationship between the morphology and luminescence properties of methylammonium lead trihalide perovskite thin films. By tuning the average crystallite dimension in the film from tens of nanometers to a few micrometers, we are able to tune the optical band gap of the material along with its photoluminescence lifetime. We demonstrate that larger crystallites present smaller band gap and longer lifetime, which correlates to a smaller radiative bimolecular recombination coefficient. We also show that they present a higher optical gain, becoming preferred candidates for the realization of CW lasing devices.

Full text of DOI:10.1021/ja511198f

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Communication
Tuning the light emission properties by band
gap engineering in hybrid lead-halide perovskite
Valerio D’Innocenzo, Ajay Ram Srimath Kandada, Michele
De Bastiani, Marina Gandini, and Annamaria Petrozza
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja511198f • Publication Date (Web): 03 Dec 2014
Downloaded from http://pubs.acs.org on December 11, 2014
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Tuning the light emission properties by band gap engineering in
hybrid lead-halide perovskite
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Valerio D’Innocenzo , Ajay Ram Srimath Kandada *, Michele De Bastiani , Marina Gandini
1
and Annamaria Petrozza *
1
Center for Nano Science and Technology @Polimi, Istituto Italiano di Tecnologia, via Giovanni Pascoli 70/3, 20133,
Milan, Italy.
2
Dipartimento di Fisica, Politecnico di Milano, Piazza L. da Vinci, 32, 20133 Milano, Italy.
3
Dipartimento di Scienze Chimiche, Università degli studi di Padova, Via Marzolo 1, 35122, Padova, Italy.
Supporting Information Placeholder
by efficient radiative processes. This explains well the
ABSTRACT: We report about the relationship between
methylammonium lead tri-halide perovskite thin films
morphology and luminescence properties. By simply tuning
the average crystallite dimension in the film from tens of
nanometers to a few micrometers, we are able to tune the
optical band gap of the material along with its
photoluminescence lifetime. We demonstrate that larger
crystallites present smaller bandgap and longer lifetime
1
,5
increase in the PL quantum efficiency at higher fluences .
Another notable phenomenon is the incredibly long PL
lifetime found for the Cl doped MAPbI3, reaching up to
hundreds of ns, especially because it has been associated to
8
micrometer long photo-carriers diffusion length . Such a
long lifetime was initially associated to the presence of Cl
ions within the perovskite unit cell. However, recent
investigations have highlighted a major role of Chlorine
based precursor in the crystallization process of hybrid
perovskite. In particular, some of us have shown that Cl is
not present in detectable concentrations in the unit cell and
it is mainly expelled from the flat films. Nevertheless its
presence drives the crystallization dynamics, producing
which correlates to
a
smaller radiative bimolecular
recombination coefficient. We also show that they present a
higher optical gain, becoming preferred candidates for the
realization of lasing devices.
9
larger crystals with anisotropic shape . This highlights an
After revolutionising the field of photovoltaics, organo
metal halide perovskites are also emerging as viable materials
for light emitting devices, paving the way towards solution
important unexplored issue of the relationship between
structural and optoelectronic properties in these self-
assembled compounds. Hybrid perovskites are usually
deposited as polycrystalline thin-films with variable
mesoscale morphologies that depend on the growth
1
–3
processable and tunable electrically pumped lasers . This
increases the interest in the understanding of the optical
properties of these materials. Recent works have
demonstrated that the photoluminescence (PL) decay in
CH NH PbI (MAPbI ) thin films can be described by simple
1
0,11
conditions . The obtained grain size ranges from tens to
thousands of nm depending on the processing methods and
this seems to strongly influence the solar cell
3
3
3
3
10,12
rate equations involving carrier trapping and electron−hole
radiative recombination. In particular, according to Yamada
performances , though such observations have never been
rationalized.
4
5
6
et al. , Stranks et al. and Saba et al. the PL dynamics are
driven by radiative non geminate electron-hole
Here we investigate the influence of MAPbI3 thin film
morphology, from a molecular to mesoscopic level, on the
luminescence properties. In Figures 1.(a)-(d) we report the
top-view SEM images of four representative samples
investigated in this work, with an observed average size of
the crystallite dimension ranging from < 250 nm to > 2 µm.
They are fabricated using the two-step sequential deposition
a
7
recombination process , possibly also involving non radiative
de-excitation paths. Thus, the temporal evolution of the
excited carrier population n(t) can be satisfactorily modelled
by taking into account a bi-molecular intrinsic radiative
ꢈ
recombination coefficient, Brad (ꢀꢁꢂꢃ =ꢄꢅꢁꢂꢃ⁄ꢆ , where R
ꢇ
rad
1
3,14
is the radiative rate and ni is the intrinsic carrier
technique
(for a detailed description of the protocol refer
concentration) and a non-radiative monomolecular trapping
rate (A), i.e. ꢉꢆ⁄ꢉꢊ =ꢄ−ꢋꢆ − ꢀꢁꢂꢃꢆ . At low excitation
densities the charge-trapping pathways limit the
photoluminescence quantum efficiency, whereas at high
fluences the traps are predominantly filled and
recombination of the photogenerated species is dominated
to the Supporting Information). In the top panel of figure
1.(e) we report the position of the optical band edge obtained
from UV-Vis absorption spectra as well as the position of the
peak of PL of all the samples (associated SEM images and
spectra of the entire set of samples are shown in Figure S1
and Figure S2 respectively). We observe that as the poly-
ꢈ
1
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Figure 1: Top View SEM images of MAPbI (a) [MAI] = 0.063 M, T = 25 °C; (b) [MAI] = 0.045 M, T = 25 °C; (c) [MAI] =
3
0.031 M, T = 25 °C; (d) [MAI] = 0.045 M, T = 70 °C; scale bars: 2 µm. (e) Spectral position of the UV-Vis absorption band
edge, peak position of cw-PL (top) and PL lifetime (bottom) as function of the crystallite size (the SEM images of the
full set of samples is reported in Fig S1); (f) representative tr-PL dynamics for three different MAPbI crystal size
3
dimensions and the PL dynamics of Cl-doped MAPbI spin-coated on a glass substrate. Pumping wavelength 700 nm,
3
1
6
-3
initial excitation density ~4∙10 cm .
crystallite grows in size, the optical absorption edge shifts to
longer wavelengths (lower energies) along with the peak
position of the PL, still keeping the Stokes shift constant.
In Figure 1.(f) we show time resolved photoluminescence
mechanisms when compared to larger crystals (see Fig S4)
6
and Cl-doped MAPbI . This may be related to the presence
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1
9
of higher defect density in the smaller crystallite .
In Figure 2b we show the excitation density dependence of
the PL lifetimes. Two representative samples have been
chosen, the one studied in Figure 2.(a), with crystal
dimension < 250nm and MAPBI3 with crystallites > 1 µm,
corresponding to the samples shown in Figure 1.(a) and 1.(d)
respectively. In this framework the dependence of the PL
(
tr-PL) dynamics of the aforementioned series of samples.
The measured tr-PL decays are fitted with a stretched
exponential function to obtain the respective PL lifetimes
which are reported as a function of the average crystal size in
the bottom panel of Figure 1.(e) (details in the Supporting
Information). For the smallest crystals (< 250 nm) we obtain
a lifetime of about 2 ns. As the crystallite size is increased, we
observe an associated increase in the lifetime (Figure 1.(e),
bottom panel) with the largest crystallites (> 1 µm) showing a
lifetime greater than 100 ns, approaching values that in
previous works have been reported only for Cl-doped
lifetime on the initial excitation density (ꢆ ) can be written
ꢌ
4
,20
as
:
1
ꢍꢎꢏ =ꢄ ꢓ
(1).
ꢐꢋ ꢑ ꢀꢁꢂꢃ ∙ ꢆ ꢒꢄꢄ
ꢌ
By fitting Equation (1) to the experimental data we
1
5,16
estimate the intrinsic bi-molecular radiative recombination
coefficient for both the samples. We find that larger
crystallites exhibit lower radiative recombination coefficient,
MAPbI samples , reported in Figure 1.(f) for comparison
3
(
black dots).
Our results show that the PL lifetime critically depends on
-9 -1
3
Brad = (0.62±0.06)∙10 s cm , with respect to the smaller ones,
the specific morphology of the sample and thus on the
-9 -1
3
11,15–18
Brad = (3.7±0.2)∙10 s cm , and this indeed results in the
adopted fabrication protocol,
. At this stage, the most
-9
-1
3
longer PL lifetimes (Saba et al found Brad = 0.25∙10 s cm for
Cl-doped MAPbI3 flat film). Note that the monomolecular
intuitive conclusion may suggest an enhancement of the
non-radiative channels when the poly-crystallinity increases.
In order to shed light on such observations we first need to
validate the use of the model proposed by Yamada et al., also
for the set of samples considered here. Specifically, we need
to exclude any non-radiative bimolecular mechanisms such
as surface recombination. In Figure 2.(a), we show the
7
-1
7
trapping rate A is not very much affected, 1.3∙10 s to 0.72∙10
-
1
s as the crystallite dimension decreases. However this rate is
4,20,21
usually very sensitive to the sample production
. It may
be associated to localized defects in the crystals and thus it
may vary significantly from sample to sample due to
imperfect control of their fabrication.
As noticed above, the reduction in Brad is accompanied by
the red shift of the band edge, going from 1.63 eV in the case
of the small crystallites sample to 1.61 eV for big crystallites
6
excitation density dependence of the normalized PL yield
for the sample with the smallest crystallites, that is most
likely to present larger number of surface defects (see Figure
S4 for the larger crystals). The efficiency increases with
increasing excitation density, thus supporting a major role of
radiative recombination in the bimolecular recombination
process also in the small crystallites. It must be noted that
showing
a correlation between Eg and Brad. Intensity
dependent PL measurements, while lowering the sample
temperature, also corroborate this observation. It has been
previously observed that as the temperature is lowered
1
7
above a certain threshold (n > 10 ) density-dependent non-
0
(
whilst staying above the temperature for phase
radiative processes become more important and eventually
dominate, causing the decrease of the PL Yield. Saba et al
have modeled this behavior according to Auger-like
22,23
transition ) the optical band gap of the perovskite shifts to
lower energies as a consequence of lattice dilatation (Varshni
shift ). In this case we considered MAPbI film deposited by
3
24
3
mechanisms proportional to n . Interestingly, smaller
crystallites present a lower threshold for such non-radiative
the two-step procedure, on a 1 µm thick Al O mesoporous
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perceived by lowering the temperature, which notably affects
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the lattice strain, suggesting a direct relation between the
two.
Mitzi et al. experimentally demonstrated that, in 2D tin
based hybrid perovskite semiconductors, the relaxation of
the halide-metal-halide bonding angle, induced by different
arrangement of the organic, plays a fundamental role in the
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determination of the electronic structure of these materials .
26
In agreement, very recently, Filip et al. revealed that it is
possible to modulate the optical gap of MAPbI perovskites
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over almost an electron-volt, modulating the Pb-I-Pb bond
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angles using cations of different sizes and without altering
the metal-halide chemistry . These theoretical findings find
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support in the Raman analysis by Grancini et al which reveals
a stronger distortion in the Pb-I bond when moving from
large crystals grown on a flat substrate to smaller crystals
9,28
grown in a mesoporous scaffold . Thus we relate the
observed shrinking of the optical band-gap in the larger
perovskite crystallites to a change in the Pb-I bond stress. In
2
9
a recent work, Filippetti et al. show by ab initio calculation
that different orientation patterns of the organic cation
within the perovskite lattice correspond to distinct
equilibrium energy state associated to a shift of the energy
gap of the semiconductor. Importantly, they highlight a
d
e
p
e
n
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e
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c
e
o
f
B
o
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e
o
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perovskites. In particular they associate the reduction in Brad
to a reduction of the band-gap due to direct dependence of
the radiative recombination rate R and the intrinsic carrier
rad
3
0
concentration n on the band-gap
.
i
Thus, we conclude that by controlling the crystallization
procedure it is possible to engineer the semiconductor band-
gap and the intrinsic radiative lifetime of the compound.
Considering the potential employment of these materials for
lasing applications, it is interesting at this stage to
understand how one can exploit simple processing to
optimize the emissive properties of the thin film. A longer
lifetime would create larger steady state population under cw
illumination which can be beneficial for low threshold CW
lasing. However, another important parameter that
determines the lasing threshold is the gain coefficient of the
material that is related not only to the recombination rates,
but also to the non-radiative losses. To shed light on this
aspect, we performed optical gain measurements using the
Figure 2. (a) Normalized PL quantum yield for the sample
showed in Fig 1(a). It is calculated as the ratio between the
integrated PL spectrum and the incident power.(b) PL
lifetime as a function of the excitation density for small
(black dots) and big (red squares) crystallites MAPbI3
sample. Dashed lines represent the curves obtained fitting
the relation (1) to the data. In the inset the PL lifetime as a
function of initial excitation density for Meso-MAPbI3 at RT
(
black triangles), 220 K (red circles) and 150 K (green
squares) respectively. The error bars refer to the 95%
confidence interval of the fitted parameter values.
3
1
established variable stripe length method (details in the
Experimental section in the Supporting Information). Briefly,
the spatial profile of the photo-excitation is made into a
stripe and the emission is collected from one end of the
stripe. The output intensity is then related to the stripe
length l by:
scaffold (indicated as Meso MAPbI in the following). Here,
3
the presence of the scaffold assures a crystal size smaller than
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5
0 nm , a sample which allows us to exclude excitonic effects
22
even at lower temperatures . We observe a PL lifetime of
about 2 ns when the sample is excited with a density
comparable to the one used for obtaining the PL decay
ꢋꢐꢕꢒꢔꢗ
ꢔꢄ
ꢐꢕ, ꢖꢒ =ꢄ
ꢄꢐꢙꢚꢐꢛꢒꢜ − 1ꢒꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢄꢐ2ꢒ
16
-3
shown in Figure 1.(f) (~6∙10 cm ). At that initial excitation
density (i.e. fixed laser fluence), upon decreasing the
temperature, the PL lifetime increases and the spectra shifts
to longer wavelengths (Figure S3 in Supporting Information).
In the inset of Figure 2.(b) we show the PL lifetime versus the
excitation density at different temperatures. Considering
again the expression (1), looking at the slopes of emissive
recombination rate curves evaluated as 1/τPL (reported in
figure S5.(b) in the Supporting Information) it is evident that
Brad (which is proportional to that slope) decreases as the
temperature is lowered. Hence the correlation between Eg
and Brad observed by increasing the crystal dimension is also
ꢘꢐꢕꢒ
Where A is a constant related to the spontaneous emission
cross section, I is the intensity of the pump, g is the gain
p
coefficient and
λ is the wavelength of emission. The PL
intensity from the two samples with large and small crystal
respectively as a function of stripe length at three different
powers along with the fits obtained using the above equation
is reported in the Supporting Information (figure S6). In
Figure 3, we show the values of the gain coefficients obtained
as a function of incident pump intensity. The gain is
consistently higher in the case of larger crystals. This allows
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Figure 3. Optical gain for both the small crystallites
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ASSOCIATED CONTENT
Supporting Information
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Nano Lett. 2013, 13, 1810.
Experimental section, Top-View SEM, UV-Vis and cw-PL
spectra of all the investigated samples. This material is
available free of charge via the Internet at
http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
(
22) D’Innocenzo, V.; Grancini, G.; Alcocer, M. J. P.; Kandada, A. R.
srinivasa.srimath@iit.it; annamaria.petrozza@iit.it;
S.; Stranks, S. D.; Lee, M. M.; Lanzani, G.; Snaith, H. J.; Petrozza, A.
Nat. Commun. 2014, 5, 3586.
(23) Yamada, Y.; Nakamura, T.; Endo, M.; Wakamiya, A.;
Kanemitsu, Y. Japan Soc. Appl. Phys. 2014, 7, 032302.
Notes
The authors declare no competing financial interests.
(
(
24) Varshni, Y. P. Physica 1967, 34, 149.
25) Knutson, J. L.; Martin, J. D.; Mitzi, D. B. Inorg. Chem. 2005, 44,
ACKNOWLEDGMENT
4
699.
The authors acknowledge funding from the EU Seventh
Framework Program [FP7/2007-2013] under grant agreement
n° 604032 of the MESO project. The authors thank Dr F.
Scotognella, Dr F. Tassone, Dr G. Grancini and Prof G.
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