Nonvolatile chlorinated additives adversely influence CH3NH3PbI3 based planar solar cells

Article abstract of DOI:10.1039/c5ta01198a

We demonstrate that nonvolatile CaCl₂ additives can significantly improve the film morphology of CH₃NH₃PbI₃. Unlike those volatile chlorinated additives, a small portion of Cl anions from CaCl₂ seem to enter into the CH₃NH₃PbI₃ crystal, yet most insulating CaCl₂ remains within the perovskite film, which is detrimental to perovskite solar cells.

Full text of DOI:10.1039/c5ta01198a

Journal of
Materials Chemistry A
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Nonvolatile chlorinated additives adversely
influence CH NH PbI based planar solar cells†
3
3
3
Cite this: DOI: 10.1039/c5ta01198a
a
b
a
Yani Chen, Yixin Zhao and Ziqi Liang*
Received 13th February 2015
Accepted 13th March 2015
DOI: 10.1039/c5ta01198a
www.rsc.org/MaterialsA
5
We demonstrate that nonvolatile CaCl
improve the film morphology of CH NH
chlorinated additives, a small portion of Cl anions from CaCl
enter into the CH NH PbI crystal, yet most insulating CaCl
2
additives can significantly proved to yield a longer diffusion length of carriers (ꢀ1 mm)
24
3
3 3
PbI . Unlike those volatile and higher electrical conductivity. However, the function of
2
seem to chloride ions in the formation of perovskite lm remains a
remains mystery.
3
3
3
2
within the perovskite film, which is detrimental to perovskite solar
Herein, we prepare a dense and uniform perovskite lm by
adding nonvolatile compounds such as NaCl and CaCl addi-
tives to the standard CH NH PbI precursor solution consisting
NH I and PbI . It was found that
Cl had a signicant inuence on the lm morphology by
altering the kinetics of perovskite crystal growth. Interestingly,
XRD measurements verify the formation of an intermediate
phase preceding the crystallization, which helps to avoid
incomplete and uneven surface coverage. Unlike those volatile
cells.
2
3
3
3
of an equimolar mixture of CH
3
3
2
Hybrid organic–inorganic halide perovskites have attracted
signicant attention owing to their extraordinary characteristics
À
1
2
such as high absorption coefficient, tunable optical bandgaps,
3
high carrier mobilities, and long electron and hole diffusion
lengths. Within a very short timeframe, the power conversion
4,5
efficiency (PCE) of perovskite based solar cells has rocketed to a
additives such as CH
3 3
NH Cl and HI, most of the added insu-
6
7–12
certied 20.1% since they were rst introduced.
lating CaCl remains in the nal perovskite lm, which is
2
Currently, most of the signicant device enhancements have
been achieved by the realization of high-quality perovskite
detrimental to the device performance, despite the partial
inclusion into the CH NH PbI crystal. We therefore argue that
3
3
3

lms, suggesting that the lm morphology such as uniformity
good additives used in solution chemistry of the perovskite lm
should not only enable to improve the lm morphology but also
possess the removability. Note that other inorganic chloride
salts such as NaCl have similar effects on such lm formation.
We choose CaCl as a proof-of-concept due to its relatively large
2
solubility in DMF compared to NaCl.
and surface coverage is of prime importance for the high-effi-
13–16
ciency operation of perovskite solar cells.
The use of addi-
tives has been reported as a simple yet effective way of
controlling the lm morphology by successfully manipulating
the nucleation and growth of the perovskite crystals, thereby
improving the device performance. For instance, by adding HI,
All perovskite lms were prepared via the one-step solution
method on the quartz substrate from CH NH I : PbI : CaCl
2
1
,8-diiodooctane (DIO) or CH
standard precursor solutions, the continuous and uniform
HC(NH PbI , CH NH PbI3ÀxCl or CH NH PbI lms were
made, respectively. Most of these reported additives are
3 3
NH Cl to their corresponding
3
3
2
precursor mixtures with a molar ratio of 1 : 1 : 0.5, followed by
)
2
3
3
1
3
x
3
3
3
ꢁ
2
thermal annealing at 95 C. Fig. 1 compares optical spectra of
7–20
annealed perovskite lms over annealing time with the
then evaporated or sublimed during the thermal annealing
step.
starting lm before annealing. The unannealed perovskite
25

3 3 3
lm exhibits a typical absorption spectrum of CH NH PbI .
À
Moreover, recent studies showed that Cl had a great impact
With an increase in thermal annealing time, the annealed
perovskite lm shows an increase in the visible absorbance
across the entire spectral range, particularly in the range of
600–750 nm, and remains unchanged aer 20 min, indicating
that the conversion of the perovskite crystal structure is
18,20–23
on the crystallization dynamics.
The Cl incorporation has
a
Department of Materials Science, Fudan University, Shanghai 200433, China. E-mail:
zqliang@fudan.edu.cn
b
College of Environmental Science and Engineering, Shanghai Jiao Tong University, complete.
Shanghai 200240, China
Scanning electron microscopy (SEM) was conducted to
†
Electronic supplementary information (ESI) available: Experimental section and
explore the effect of CaCl
2
in precursor solution on the
Table S1. See DOI: 10.1039/c5ta01198a
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ionized from CaCl
2
in DMF solution plays a vital role in regu-
lating the crystallization dynamics to improve the lm topog-
raphy. Aer thermal annealing for 20 min, some white spots
show up. These tiny spots are probably the segregation of CaCl2,
which could be attributed to thermally induced recrystallization
of CaCl
spectroscopy (EDX) mapping of a fully annealed perovskite lm
Fig. 2d and e) discloses a homogeneous distribution of Pb, I, Ca
2
(Fig. 2e and f). In addition, energy dispersive X-ray
(
and Cl elements throughout the whole lm.
À
In order to further understand how Cl functions during the
formation of the perovskite lm, X-ray diffraction (XRD)
measurements were carried out. Fig. 3a displays the XRD
Fig.
CH
1
NH
Absorption spectra of the perovskite film obtained from patterns of perovskite lms obtained from CH
3
NH
3
I : PbI
2
-
3
3
I : PbI
2
: CaCl
2
precursor mixtures with 1 : 1 : 0.5 molar ratio
2
: CaCl precursor mixtures as a function of thermal annealing
ꢁ
with different thermal annealing times at 95 C.
time, in comparison with the spin-coated CaCl
2
lm. CaCl
2
ꢁ
exhibits an intense peak at 31.70 , while all perovskite lms
ꢁ
ꢁ
ꢁ
show strong diffraction peaks at 14.08 , 28.41 and 31.73 ,
assigned to the 110, 220, and 310 reections of the tetragonal
CH NH PbI phase, respectively, which are in good agreement
3
3
3
27,28
with other reports.
Upon heat treatment, all these annealed
perovskite lms present similar XRD patterns in which all
diffraction peaks become sharper and enhanced in intensity
with increasing thermal annealing time due to improved crys-
ꢁ
tallinity. It is worth noting that an additional peak of 10.70
appears in the unannealed perovskite lm (Fig. 3b), which
disappears aer thermal annealing of only 1 min. This could be
indicative of the formation of an instable intermediate phase
À
induced by Cl preceding the crystallization step. A possible
explanation is that this intermediate phase inhibits the
formation of elongated crystals and incomplete coverage, and
upon heat treatment it undergoes the rearrangement by
À
releasing Cl , which is consistent with the SEM observation in
ꢁ
Fig. 2. Moreover, the (310) diffraction peak at 31.73 of the CaCl
2
treated perovskite lm that is overlapped with the characteristic
ꢁ
peak of CaCl at 31.70 is down-shied and broadened with
2
prolonged thermal annealing time. This is presumed to relate to
the CaCl recrystallization.
2
Furthermore, both EDX and X-ray photoelectron spectros-
copy (XPS) results conrm the above assumption. The lead,
Fig. 2 SEM images of (a and b) the perovskite film prepared from
standard precursor solution of CH
and (e and f) annealed perovskite films at 95 C for 20 min obtained
3
NH
3
I and PbI
2
, (c and d) unannealed
ꢁ
3 3 2 2
from CH NH I : PbI : CaCl precursor mixtures with 1 : 1 : 0.5 molar
ratio. (g) EDX analysis of the annealed perovskite film: Pb map (red), I
map (green), Ca map (light blue) and Cl map (dark blue).
perovskite lm morphology. By using the standard precursor
solution of CH NH I and PbI , as displayed in Fig. 2a and b, a
3
3
2
poor lm morphology is formed showing needle-shaped crys-
tals and incomplete, uneven surface coverage, which is consis-
20,26
tent with previous reports.
2
By the addition of CaCl , closely
packed crystals are generated and the lm becomes uniform
with noticeably improved surface coverage as shown in Fig. 2c
2
Fig. 3 (a) XRD patterns of CaCl and perovskite films obtained from
2
3 3 2 2
and d. This morphological variation caused by CaCl is similar CH NH I : PbI : CaCl precursor mixtures showing the effects of
20
À
ꢁ
thermal annealing time at 95 C, respectively. Magnified XRD images of
graph (a) ranging from (b) 10–11.5 and (c) 29–35 .
to the case of CH NH Cl. It seems reasonable that free Cl
3
3
ꢁ
ꢁ
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CH
3
NH
3
PbI
3
is successfully generated out of the solid solu-
À
2+
tion, and most of the Cl and Ca precipitate out in the form
of CaCl while Cl is partially incorporated into CH NH PbI
À
2
3
3
3
crystals. However, the exact chemical pathways are very diffi-
cult to determine due to the high instability of the interme-
diate state.
Finally, in order to test the effect of nonvolatile CaCl
photovoltaic performance of perovskite solar cells based on
CH NH I : PbI : CaCl precursor mixtures with varied molar
2
on the
3
3
2
2
ratios, the solar cells were constructed with a typical congu-
ration of the ITO/PEDOT:PSS/perovskite/PC BM/Ca/Al device
6
1
2
with an active area of 0.11 cm . Fig. 5a–c show the photocurrent
density versus voltage (J–V) curves of the best-performing CaCl
2
treated perovskite planar photovoltaic cells under AM 1.5G
À2
simulated light with an intensity of 100 mW cm . The best
cells' characteristics are summarized in Table S1† and their
average values with a standard deviation of 10 individual
devices are presented in Table S2.†
With the increasing molar ratio of CaCl , the device
2
performs worse. Interestingly, the deterioration of device
performance mainly lies in the short-circuit current density (JSC
)
rather than the open-circuit voltage (VOC). This agrees well with
our other results, suggesting that the perovskite layer contains a
certain amount of insulating CaCl . Note that the exact impact
2
of the remaining Ca element remains unclear and needs further
investigation. In addition, all of these devices exhibit the
serious hysteresis. This could be explained by the fact that the
Fig. 4 (a) Atomic percentages of the main elements (Pb, Ca, I and Cl) perovskite layer here accumulates the charge carriers if the
revealed by EDX profiles in the perovskite films as a function of thermal
charges are not quickly transferred or transported into electron
annealing time. (b) XPS spectra (Cl 2p core level) of CaCl
and annealed perovskite films. (c) Possible chemical pathways among
CaCl , CH NH I and PbI during the formation of the perovskite film.
2
, unannealed
29,30
and hole conductors, respectively.
sentative external quantum efficiency (EQE) spectra of CaCl
treated perovskite cells, also showing a decrease of EQE with
increasing proportion of CaCl , which indicates the aggravated
photon-to-electron conversion.
Fig. 5d shows the repre-
2
2
3
3
2
2
calcium, chlorine and iodine elements are detected in the
perovskite lms from the EDX proles (Fig. 4a). The lead
amount shows a slight drop while the amounts of chlorine and
calcium exhibit a little increase as thermal annealing went on.
2
This indicates that the low-density CaCl agent may have
migrated to the upper portion of the perovskite lm, which
coincides with SEM and XRD results of the recrystallization
2
process of CaCl . To explore further where and how Cl is
located and bonded in the perovskite lm, XPS spectra were
acquired as shown in Fig. 4b. The unannealed perovskite lm
has a 2p3/2 signal of Cl at 198.8 eV, the same as CaCl , sug-
2
3 3 3
gesting the signicant absence of Cl in the CH NH PbI crys-
tals. Note that the Cl signal in the annealed perovskite lm has
an up-shi by 0.5 eV, implying the likelihood of a partial
À
inclusion of Cl into the CH
ions precipitate out as CaCl
preferred orientation of CH NH PbI crystal grains. In short,
3 3 3
NH PbI crystals. Likely, most Cl
2
in the lm aer inducing a
3
3
3
the possible chemical pathways among CaCl , CH NH I and
2
3
3
Fig. 5 Photocurrent density–voltage characteristics of regular ITO/
PbI during lm formation are shown in Fig. 4c. The mixture
2
PEDOT:PSS/perovskite/PC61BM/Ca/Al solar cells measured under light
of CaCl
2
, CH
3
NH
3
I and PbI
2
rst forms a kind of “solid solu-
À2
at an intensity of 100 mW cm at forward (black line) and reverse (red
tion”, which is unstable yet helpful in avoiding the formation
3 3 2 2
line) scans, respectively, prepared from CH NH I : PbI : CaCl
of large needle-shaped CH
incomplete surface coverage. Then aer heat treatment,
3
NH
3
PbI
3
crystals, resulting in the precursor mixtures with (a) 1 : 1 : 0.5, (b) 1 : 1 : 0.75, and (c) 1 : 1 : 1
molar ratio and (d) EQE spectrum of representative perovskite solar
cells.
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