Subphthalocyanine as hole transporting material for perovskite solar cells

Article abstract of DOI:10.1039/c5ra12004g

Non planar 14-π aromatic subphthalocyanine has been introduced for the first time as hole transporting material for organometal halide perovskite solar cells and achieved a power conversion efficiency of 6.6%. Cells stored in the dark under ambient conditions underwent an incubation period of nine days during which, we observed an increase in efficiency followed by slow progressive deterioration. However, Raman spectral analysis of pristine perovskite deposited on titania revealed a much faster degradation thus indicating that the subphthalocyanine layer provides a temporary protection to the underlying perovskite layer.

Full text of DOI:10.1039/c5ra12004g

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Subphthalocyanine as Hole Transporting Material for Perovskite
Solar Cells.
Georgia Sfyria,b, Challuri Vijay Kumara*, Gokulnath Sabapathic, Lingamallu Giribabuc,
Konstantinos S. Andrikopoulosa, Elias Stathatosd and Panagiotis Lianosa,e*
aFORTH/ICEꢀHT, P.O. Box 1414, 26504 Patras, Greece
bPhysics Department, University of Patras, 26500 Patras, Greece
cInorganic and Physical Chemistry Division, Indian Institute of Chemical Technology,
Hyderabad,500 007, India.
dElectrical Engineering Department, Technological–Educational Institute of Western Greece,
26334 Patras, Greece
eDepartment of Chemical Engineering, University of Patras, 26500 Patras, Greece
Key words: perovskite, solar cells, hole transport material, subphthalocyanine.
CORRESPONDING AUTHORS: challuri.iict@gmail.com and lianos@upatras.gr Tel:
++30 2610 997513 Fax: ++30 2610 997803
Abstract
Non planar 14 –π aromatic subphthalocyanine has been introduced for the first time as
hole transporting material for organometal halide perovskite solar cells and achieved a power
conversion efficiency of 6.6%. Cells stored in the dark under ambient conditions underwent
an incubation period of nine days during which, we observed an increase in efficiency
followed by slow progressive deterioration. However, Raman spectral analysis of pristine
perovskite deposited on titania revealed a much faster degradation thus indicating that the
subphthalocyanine layer provides a temporary protection to the underlying perovskite layer.

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INTROUDUCTION
Hybrid organic–inorganic solar cells based on organo lead halide perovskites have
attracted significant attention over the last three years thanks to their impressive conversion
efficiencies and the intriguing intrinsic properties of organo lead halide perovskites.1ꢀ6
Efficient lightꢀabsorbing sensitizers and inorganic semiconducting metal oxides must be
paired with an efficient organic hole transporting material (HTM) in order to establish
appropriate regenerative cycles for the holes left in the oxidized sensitizers after photon
absorption and electron injection. Currently, the most popular HTM is the single molecule
spiro–OMeTAD, which has offered power conversion efficiencies (PCE) of over 15%, based
on various device structures4,7. Most recent developments have reached the impressive PCE
values of ~20%.8, 9, 10
Despite offering the best performance yet achieved in an HTM, spiroꢀOMeTAD
suffers from high synthesis cost, low hole mobility and low conductivity, which limit its
potential for massive future applications of perovskite solar cells. Therefore, the development
of other efficient and inexpensive molecular HTMs with optimal electronic properties
remains an attractive goal. Recently reported single molecule HTMs, such as 3,4ꢀ
ethylenedioxythiophene11, pyrene12, linear πꢀconjugated13, butadiene14, a spiroꢀOMeTAD
derivative15 and starꢀshaped HTMs16 resulted in high conversion efficiencies of 11–16%. p ꢀ
19,20
Type inorganic semiconductors (i.e., CuI, CuSCN, and NiO)17,18 and polymeric HTMs
have also been extensively studied.
We have recently reported copper phthalocyanine as hole transporting material for
perovskite solar cells and achieved a PCE of 5%21. Torres, Nazeeruddin and coworkers also
22
recently published a nonꢀaggregated Zn(II)octa(2,6ꢀdiphenylphenoxy) phthalocyanine
(TT80), which was used as HTM for solution processed perovskite solar cells with
(bis(trifluoromethane) sulfonimide lithium salt (LiTFSI) and tertbutyl pyridine additives and
achieved PCE of 6.7%.
Boron subphthalocyanines (SubPc) have excellent thermal stability and are highly
attractive as photon absorbing materials in organic solar cells because their absorbance
maximum overlaps almost perfectly with the part of the solar irradiation spectrum with the
highest intensity at the earth’s surface. The symmetrical 14ꢀπ electron configuration of SubPc
is of paramount importance for its application as active material in photodynamic therapy23,
non linear optics24, OLEDs25, and OPVs26. In continuation of our research work on HTM
alternatives of spiroꢀOMETAD, we presently report vacuum processed boron

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subphthalocyanine as hole transport material for perovskite solar cells. The PCE achieved
with this material reached 6.6% and there is ground for further improvement.
EXPERIMENTAL SECTION
Materials
All materials were purchased from Aldrich, unless otherwise specified, and they were
used as received. FTO glasses of 8 ohm/square were purchased from Pilkington.
Methyl ammonium iodide Synthesis
Methylammonium iodide was synthesized by the previously reported procedure27 by
reacting 27.8 ml of methylamine (40 wt% in H2O) and 30 ml of hydroiodic acid (57 wt% in
water) at 0°C for 2 hours. Solid residue was obtained in a rotary evaporator by carefully
removing the solvents at 40 °C. The yellow crude product methyl ammonium iodide
(CH3NH3I) was washed with diethyl ether several times, and then finally recrystallized from
a mixed solvent of diethyl ether and ethanol. After filtration, the pure solid was collected and
dried at 70 °C in a vacuum oven overnight.
Subphthalocyanine Synthesis
Subphthalocyanine was synthesized with a modified procedure based on the report of
Zyskowski et al.28 Phthalonitrile (1.77 g, 13.8mmol) was dissolved with stirring in 1,2ꢀ
dichlorobenzene (70 mL) under argon atmosphere. To this solution BCl3 (9.26 mL of 1.0 M
solution (9.26mmol) in heptane) was added in a single portion. On gradual heating the
heptane was distilled off. When distillation was complete, the reactants were heated at reflux
for an additional 1.5 hours. Then the reaction mixture was cooled to room temperature and
the solvent was removed by rotary evaporation. The resulting crude product was purified on a
short pad of neutral alumina. The obtained goldenꢀbrown powder was then dried in the
vacuum oven yielding the compound of Figure 1: (1.13 g, 57%). 1H (400 MHz, CDCl3): δ =
7.96ꢀ7.98 (6H, m), 8.89ꢀ8.94 (6H, m); λmax(CHCl3)/nm 564.
Fabrication of Perovskite solar cells
FTOꢀcoated glass substrates were cut in pieces of dimensions 1 cm ×3 cm. One third
of the conductive layer was removed using zinc powder and hydrochloric acid. Then they
were washed with mild detergent, rinsed several times with distilled water and subsequently

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with ethanol in an ultrasonic bath, finally dried under air stream. This patterned and cleaned
FTO electrode was first treated in TiCl4 by dipping into a solution made of 0.04M TiCl4 in
H2O for 30 minutes, then copiously rinsing and finally calcining at 500˚C. A compact thin
layer of TiO2 was then deposited by aerosol spray pyrolysis using a solution of 0.2 M
Diisopropoxytitanium bis(acetylacetonate) in EtOH. After spraying, the samples were heated
for 1 hour at 500˚C. Subsequently, a mesoporous TiO2 layer composed of titania paste made
of P25 nanoparticles was spin coated at 4000 rpm for 30 seconds and then heated for 15
minutes at 500 ˚C. Finally, it was treated again in TiCl4 as above. Active perovskite layer was
deposited on the thus prepared titania film by modifying published procedures27. A precursor
solution was made by mixing 230 mg PbCl2 with 394 mg methyl ammonium iodide in 1 ml
of DMF. The atomic ratio Pb:Cl:I in the precursor solution was thus 1:2:3. The solution was
kept under stirring for about half an hour and then it was deposited by spin coating at 3000
rpm for 60 seconds under conditions of 15% relative humidity in a dry box. Then it was
heated at 80 ˚C for about 45 min, which made the sample’s color turn from yellow to black.
Holeꢀtransporting SubPc layer was then deposited by vacuum thermal evaporation. The last
step was the deposition of 80 nm thick gold electrodes also by thermal evaporation under
vacuum. These unit devices had an active size of 15 mm2 (10 mm x 1.5 mm) as defined by
the size of the gold electrodes. A schematic representation of cell components is shown also
in Figure 1.
CharacterizationꢀMethods
Illumination of the samples was made with a PECCELL PECꢀL01 Solar Simulator set
at 100 mW/cm2. J−V characteristic curves were recorded under ambient conditions with a
Keithley 2601 source meter that was controlled by Keithley computer software (LabTracer).
IPCE values were obtained with an Oriel IQE 200 system. UVꢀvis absorption spectra were
recorded using a Shimadzu model 2600 spectrophotometer equipped with an integration
sphere. XRD patters were obtained with a D8 Advance Bruker diffractometer and FESEM
images with a Zeiss SUPRA 35VP microscope. A T64000 micro Raman system in the triple
subtractive configuration was used in order to record Raman spectra down to ~10 cmꢀ1. The
excitation wavelength was 514.5 nm while the laser power on the sample was set at
particularly low values (10 ꢁW); spectra were also acquired using considerably higher laser
power values (0.8 mW). The focusing of the excitation beam was achieved by a 50x
microscope objective which offered spatial resolution of ~1 ꢁm. The backscattered photons

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were collected at ambient conditions and recorded by a 2DꢀCCD detector with a spectral
resolution of ~2.5 cm1.
RESULTS AND DISCUSSION
Materials characterization
The Pb:Cl:I ratio of the presently synthesized CH3NH3PbClxI3ꢀx perovskites has been
determined by EDX and reported in a previous publication29 and was 1:1.1:2.2. It indicates
that the quantity of iodine is twice as large as that of chlorine. Figure 2 shows the UVꢀvis
absorption spectra of SubPc on TiO2 films with and without perovskite. CH3NH3PbClxI3ꢀx
was deposited onto the mesoporus TiO2 surface by spin coating. After thermal annealing
SubPc was deposited by vacuum deposition on the surface of the perovskite. Alternatively,
SubPc was vacuum deposited directly on titania. Figure 2 also shows absorption spectrum of
CH3NH3PbClxI3ꢀx/TiO2 alone. The perovskite nanocrystals absorbed light over the whole
visible range. Subphthalocyanine absorbed light in wavelengths between 450ꢀ620 nm. When
the two materials came together, SubPc contribution markedly distinguished itself, indicating
a substantial contribution of SubPc to photon harvesting. Furthermore, Figure 2 shows the
absorption spectrum of the previously studied copper phthalocyanine (CuPc)21, for comparison. The
latter absorbs light in a broader range of the visible spectrum and it has a smaller band gap, i.e. 1.7 eV
vs 2.0 eV of SubPc. Figure 3 shows energy levels diagram of the respective components in the
solar cell device in comparison with the previously studied CuPc. The HOMO and LUMO
levels for SubPc are reported in literature to be ꢀ5.6 and ꢀ3.6, respectively30. These levels may
match the corresponding levels of CH3NH3PbClxI3ꢀx and titania, but they are surely less
favorable than those of CuPc. Nevertheless, the crossꢀsectional FESEM image of Figure 4
reveals the formation of wellꢀdefined layer structure with sharp interfaces. Therefore, the
chosen components guarantee successful solar cell assembly.
XRD diffractograms of successive organic layers on titania are presented in Figure 5.
Perovskite material gave a wellꢀdefined XRD pattern but no contribution was detected for
SubPc. It is possible that the deposited SubPc film is amorphous or too thin to give a
detectable XRD signal.
Solar cell performance
ꢀ
The current voltage (JꢀV) characteristics of the SubPc/CH3NH3PbClxI3ꢀx /TiO2 based
solar cell, measured under ambient conditions using simulated solar radiation are shown in

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Figure 6. Corresponding photovoltaic parameters are shown in Table 1 in comparison with
the previously published21 CuPc. On the cell assembly day, SubPc cell demonstrated a short
circuit current density (Jsc) of 14.0 mA/cm2, an open circuit voltage (Voc) of 0.60 V and fill
factor (FF) of 40% leading to a PCE 3.4%. However, when the cell was stored under ambient
conditions in the dark, it underwent an incubation period of 9 days during which, the
efficiency increased. Thus the final measured values were Jsc=21.3 mA/cm2, Voc=0.67 V and
fill factor of 46% leading to a PCE 6.6%. After the 9th day the cell slowly deteriorated. The
corresponding CuPc cells gave Jsc=19.2 mA/cm2, Voc=0.67 V and fill factor of 40% leading
to a PCE 5.0%. Voc of the SubPc and CuPc were the same so the enhancement of power
conversion efficiency was mainly due to the improvement of the values of Jsc and FF.
ꢀ
The action spectrum (IPCE%) of the SubPc/CH3NH3PbClxI3ꢀx /TiO2 based solar cell is
shown in Figure 2 together with the above discussed absorption spectra. Despite the
important contribution of SubPc in light harvesting, the action spectrum does not mark any
SubPc contribution. Therefore, SubPc role seems to be limited only to act as HTM. Its
superiority vs CuPc obviously indicates that it is a better hole transporter. This is only an
overall conclusion. More work is necessary to determine structural parameters that may be at
the origin of the differentiation between these two materials. All the above comparative data,
for example, the light absorption range, the value of the bandgap, the placement of the
HOMO and LUMO levels are in favor of CuPc. However, SubPc did give a better
performance. The only reasonable explanation is that this has to do with the molecular
configuration in each case and the packing of the material in the holeꢀtransporting layer.
SubPc is a cone shaped molecule and it is for sure less planar than CuPc21 or ZnPc22. The
presence of Cl counter ion may additionally prevent aggregation. It is its molecular
configuration what most probably makes the difference.
MicroꢀRaman Analysis of cell stability
The Raman spectra of PbCl2 and CH3NH3I precursors are given in Figure 7. Both
spectra were collected under inert atmosphere. The Raman spectrum of the CH3NH3PbClxI3ꢀx
perovskite at ambient conditions is also given in the same figure for comparison. It was
collected using 0.8 mW laser power, which is a considerably high value for photosensitive
materials. The peaks at 515, 397 and 144 cmꢀ1 are attributed to TiO2 (anatase) substrate and
the fact that they appear in the spectrum suggests that the laser power is adequate for the
photons to reach the substrate and the inelastically scattered light subsequently to reach the

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sample’s surface and be collected by the objective. The remaining peaks i.e. the ones at 97,
110, 163 and 215 cmꢀ1 are attributed to the perovskite layer and more particularly to PbꢀI and
PbꢀCl bonds in accordance with Ref.31. Due to the photosensitivity of the material, structural
modifications are expected when the material is illuminated by this value of laser power and
for this reason it has been reported31 that laser power in the order of 10 ꢁW should be used in
order to minimise photodegradation effects. The stability of the sample over time was
checked by the collection of a series of spectra at regular intervals using low laser power (10
ꢁW) on the sample (Figure 8). Despite the low signal to noise ratio, three peaks assigned to
the perovskite layer can be clearly resolved together with a weak anatase mode at ~145 cmꢀ1.
Using the intensity of the latter peak as a reference, the intensity of the perovskite bands may
be weighted as a function of time. Figure 8b depicts the intensity ratios of two of the
perovskite bands ~96 cmꢀ1 and ~109 cmꢀ1 with respect to the reference anatase band (~145
cmꢀ1) as a function of time up to approximately 2 days. The intensity of both bands decreased
with time, indicating degradation of the material at ambient conditions. More specifically, the
intensity of the 96 cmꢀ1 band decreased slightly faster than the respective intensity of the 109
cmꢀ1 band. The latter is not easily explained, however, it may be attributed to the different
stabilities of the Pb polyhedral species containing different amounts of Cl and I.
The data of the previous subꢀsection showed that the fully assembled cells underwent
an incubation period of 9 days and after that period the degradation could be detected. The
Raman spectra analysis showed that pristine perovskite rapidly degraded in a period of two
days. Vulnerability of organoꢀlead halide perovskites is mainly due to moisture31ꢀ33. It is then
concluded that the placement of SubPc on the top slows down the degradation process.
CONCLUSIONS
We have introduced a thermally evaporated, boron subphthalocyanine as hole
transporting material for organo lead halide perovskite solar cells. The devices were
optimized and gave satisfactory performance of power conversion efficiency 6.6%. These
results show the potential of sub phthalocyanine as hole transporting material for cost
effective solid state lead halide perovskite solar cells. Raman spectral analysis of titaniaꢀ
deposited pristine CH3NH3PbClxI3ꢀx layer indicated fast degradation under ambient
conditions. However, when the perovskite was covered with SubPc this degradation was
slowed down, indicating the importance of the necessity for protective measures to preserve
organometal halide perovskites from degradation.

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ACKNOWLEDGEMENTS
The present work was supported by the project 11SYN_7_298, implemented under
the Act "COOPERATION 2011 ꢀ Partnerships of Production and Research Institutions in
Focused Research and Technology" of the Operational Programmes “Competitiveness and
Entrepreneurship (EPAN II)” and “Regions in Transmission (NSFR 2007ꢀ13)”. The authors
are very thankful to Dr.V.Dracopoulos for his help with XRD and FESEM measurements.

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Table 1: CurrentꢀVoltage characteristics of photovoltaic measurements
HTM
SubPc
ꢀ//ꢀ
Day+
1
Voc (V)
0.60
0.61
0.61
0.62
0.63
0.64
0.64
0.67
0.67
0.66
0.66
0.67
Jsc (mA/cm2)
14.0
FF (%)
40
η (%)
3.4
3.7
4.1
4.3
4.9
5.4
5.7
6.3
6.6
5.3
5.0
4.6
2
15.1
40
3
16.7
40
ꢀ//ꢀ
4
16.9
41
ꢀ//ꢀ
5
18.9
41
ꢀ//ꢀ
6
19.1
42
ꢀ//ꢀ
7
20.2
44
ꢀ//ꢀ
8
20.5
46
ꢀ//ꢀ
ꢀ//ꢀ
9
21.3
46
10
11
12
20.1
40
ꢀ//ꢀ
19.6
39
ꢀ//ꢀ
18.4
37
ꢀ//ꢀ
ꢀ
CuPc
0.67
19.2
40
5.0
+Day since cell assembly, where day 1 is the day of cell assembly

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Figure 1. Molecular structure of subphthalocyanine (SubPc) and device architecture of the
perovskite solar cell

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Figure 2. UVꢀvis absorption spectra of (1) SubPc on mesoporous TiO2; (2)
CH3NH3PbClxI3ꢀx on mesoporous titania; (3) SubPc/CH3NH3PbClxI3ꢀx /TiO2; (4) Copper
phthalocyanine on mesoporous TiO2;
SubPc/CH3NH3PbClxI3ꢀx /TiO2 solar device.
and (5) IPCE% spectrum for the

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Figure 3. Energy level diagram of perovskite solar cells comprising subphthalocyanine30 in
comparison with copper phthalocyanine21.

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Figure 4. A crossꢀsectional image of FTO/ TiO2/CH3NH3PbClxI3ꢀx /SubPc/Au

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TiO2+Pkt+ Sub Pc
TiO2+ Sub Pc
TiO2+ Pkt
TiO2
10
20
30
40
50
60
70
80
90
2 theta (degree)
Figure 5. XRD diffractograms of various film combinations. The abbreviation Pkt stands for
perovskite.

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Figure 6. Currentꢀvoltage characteristics of the best FTO/TiO2/ CH3NH3PbClxI3ꢀx/SubPc/Au
cell: (1) 1st day and (2) 9th day

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144
110
97
163
216
CH3NH3PbI3-xClx
2964
397
515
992
117
52
3076
916
x 10
37
CH3NH3I
157
59
PbCl2
300
600
900
3000
Raman Shift [cm-1]
Figure 7. Raman spectra for the precursor samples (PbCl2 and CH3NH3I) under inert
atmosphere, as well as for the perovskite material CH3NH3PbClxI3ꢀx under ambient
conditions.

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[a]
[b]
145
I96/I145
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
CH3NH3PbClxI3ꢀx
I109/I145
96
109
213
0.8 mW
t=0.5h
1h
0.01 mW
t= 4h
7h
24h
31h
44h
LASER power:
0.01 mW
80
160
240
320
0
10
20
30
40
50
Raman Shift [cm-1]
Time [hours]
Figure 8. [a] Raman spectra of perovskite as a function of time using 0.8 mW and 0.01 mW
laser power on sample. [b] Intensity ratio of two of the perovskite bands (96 cmꢀ1 and 109 cmꢀ
1) with respect to the anatase band at 145 cmꢀ1, as a function of time for the case of low laser
power (0.01 mW).