8
8
L. Dimesso et al. / Materials Research Bulletin 85 (2017) 80–89
0
0
0
solution in chlorobenzene of 2,2 ,7,7 -tetrakis(N,N -di-p-methox-
measurements, revealed a lower content of iodine in the samples
after the annealing. This can be explained by the formation of a
liquid phase and consequent evaporation of iodine (containing)
vapors by annealing in nitrogen and by the replacement of iodine
ions by oxygen ions with the formation of Sn(II)-O, Sn(IV)-O and Pb
(II)-O binds during the annealing in air.
0
yphenylamine)9,9 -spirobifluorene (spiro-OMeTAD), with addi-
tives of lithium bis-(trifluoromethanesulfonyl)imide (added in
acetonitrile solution) and 4-tert-butylpyridine. Finally gold
electrodes were thermally evaporated under vacuum. Examples
of currentꢀvoltage (J-V) characteristics measured under simu-
ꢀ
2
lated sunlight (AM1.5, 100 mW cm ) illumination with full
device parameters extracted are shown in Fig. 10. The J-V curves
under illumination (black curves in Fig. 10a for x = 0.1 and
Fig. 10b for x = 0.3) show a semiconducting behavior of the
devices which confirms the photoluminescence profiles observed
Optical absorption measurements indicate that the Cs-Sn(Pb)-I
systems behave as direct-gap semiconductors with absorbance
energy edges above wavelengths of 1
in nitrogen at T = 270 C, the absorbance edge values of the
m
m. Indeed, after annealing
ꢂ
CsSn1ꢀxPb
x = 0.1 respectively whereas at higher Pb-content (x = 0.3,
set = 526 nm and x = 1.0, on-set = 474 nm) a blue shift of the
x 3
I systems are 1106 nm for x = 0.0 and 1052 nm for
during our measurements. The ability of CsSnI
near-IR PL at room temperature is a rare property and also
important since it is free of rare-earth metals. The additional Pb
ions in the system, even in low amount (10% mol), improved
3
to emit strong
l
on-
l
2
absorption edges has been observed. On other hand, an increase
of the adsorption edge values (1132 nm for x = 0.0, 1064 nm for
x = 0.1 and 997 nm for x = 0.3 respectively) has been noted after the
+
3
greatly the chemical stability of the CsSnI phase. This study
ꢂ
opens up opportunities to discover visible and IR light-emitting
materials at room temperature, which is highly desirable in
achieving breakthrough applications in photovoltaics, various
radiation detectors, and light-emitting diodes.
annealing treatments at higher temperature (T = 550 C). The re-
crystallization process from the liquid phase improved dramati-
cally the optical properties of the prepared materials possibly due
to the better crystallinity of the prepared materials.
2 6
After annealing in air, the adsorption edge of the Cs SnI phase
4
. Conclusions
shifts up 1098 nm. For x = 0.1, the optical profiles revealed the
presence of two absorbance steps at 1056 nm and 526 nm whereas,
for x = 0.3, one step only can be observed at 526 nm. The optical
measurements confirm the substitution of the Pb-ions for the
x 3
In this work, the properties of the CsSn1ꢀxPb I systems with
different Pb-content have been investigated. The powders have
been prepared by self-organizing processes in aqueous solutions
then annealed under nitrogen or air.
Sn-ions in the cubic half-perovskite Cs
2
SnI
6
, the formation of
phase.
orthorhombic structure similar to the CsPbI
3
After annealing in nitrogen, the X-ray powder diffractog-
ramms for x = 0.1 and x = 0.3 reveal the formation of ortho-
The Tauc plot revealed a direct band gap value of 1.22 eV after
annealing in air at T = 150 C. The band gap values increase after
ꢂ
rhombic crystalline structure (space group Pnma) and
dependence of the structural properties as a function of the
Pb-content. Indeed for x = 0.1, B- -CsSnI is the major ortho-
rhombic crystalline phase whereas, for x = 0.3, the X-ray pattern
is very similar to CsPbI phase possibly due to expansion of
CsSnI lattice when Sn atoms are replaced by Pb atoms. The
a
the addition of Pb-ions up to 2.44 eV (for x ꢃ 0.3). After annealing
ꢂ
in nitrogen at T = 550 C energy band gaps of 1.21 eV for x = 0.0,
g
3
1.23 eV for x = 0.1 and 1.29 eV for x = 0.3 respectively has been
explained by the presence of the liquid phase and consequent re-
crystallization of the B-phase from the liquid. The direct-gap
semiconducting behavior of the annealed Cs-Sn(Pb)-I systems has
3
3
2
+
morphological analysis confirmed the influence of the Pb -ions.
been confirmed by the PL-emission measurements (lexc = 500
ꢂ
By annealing at T = 500 C, the addition of Pb-ions increases the
nm) in the 950–1100 nm wavelength range. In nitrogen, for
0.0 ꢁ x ꢁ 0.3 a band peaking at ꢄ1014 nm can be observed and
additional two bands peaking at ꢄ1057 nm, ꢄ1084 nm respec-
tively have been detected. The PL-emission peaks are slightly
amount of the liquid leading to the partial dissolution of the
crystallites and the formation of flakes (solidified liquid) even at
lower concentration (x = 0.1) has been observed. At lower
ꢂ
2+
annealing temperature (T = 270 C) the stab-like habit of the
shifted towards higher wavelength values by adding Pb ions in
crystallites is retained however they assumed a polyhedral
3
the B-g-CsSnI phase. Moreover the relative intensity of the PL-
morphology and the presence of particles with flat surface,
emission decreases dramatically by increasing the Pb-content in
the system.
similar to those of the CsPbI
After annealing in air the XRD patterns for x = 0.1 and x = 0.3
showed a remarkable resemblance to the pattern of the CsPbI
3
phase.
After annealing in air, broad PL-emissions peaking at very
similar wavelength values (ꢄ1015 nm, ꢄ1060 nm and ꢄ1084 nm
3
2
+
phase. This has been explained by the increase of the miscibility
between the Sn- and Pb-containing phases which is dramatically
favored by the formation of the cubic Cs SnI phase. The
2 6
respectively) have been detected. However the addition of Pb
ions increases the intensity of the PL-emissions and shifts the
emission peaks to slightly lower values.
morphological analysis on the samples annealed in air support
the diffraction data. The SEM images confirmed the growth of stab-
like crystallites and revealed the formation of coarse particles with
x 3
The CsSn1ꢀxPb I powders were used as precursors to prepare
layers deposited on FTO coated glass substrates by spin coating.
The measurements revealed a semiconducting behavior of the
devices. This behavior has been explained by the formation of a
high density of Sn cation vacancy defects, which serve as p-type
dopants and recombination centres. Indeed, theoretical predic-
tions and experimental data suggest that PL originates from the Sn
defect centers rather than electron transfer between conduction
and valence bands.
flat surface particularly for x = 0.3. On the other hand, the CsPbI
3
phase (x = 1.0) is partially decomposed and the formation of PbO
has been detected.
The XPS analysis suggested the formation of Cs-I bind and that
the Pb4f binding energy peaks indicate the 2+ valence state of the
Pb-ions. On the other hand, a change of the oxidation state of the
Sn-ions has been observed. Indeed, the Sn3d binding energy of the
sample annealed in air peaks at higher value (487.6 eV) in
comparison to the binding energy of the Sn3d peak after annealing
in nitrogen (486.8 eV). The shift to higher binding energy can be
Acknowledgements
Many thanks are owed to Mr. J.-C. Jaud for the XRD analysis and
Mrs. K. Lakus-Wollny for technical assistance in SEM analysis. The
authors thank the Federal Ministry of Research and Development
(BMBF) (Project “Perosol” Nr. 03SF0483B) for the financial support
during this work.
explained by the (partial) oxidation of Sn2 -ions to Sn -ions as
+
4+
confirmed by the fitting of the Sn3d peak with different
components. The chemical composition (% at) of CsSn0.9Pb0.1
I
3
after annealing in nitrogen and air, determined by XPS