CH3NH3PbI3 perovskite/fullerene planar-heterojunction hybrid solar cells

Article abstract of DOI:10.1002/adma.201301327

All-solid-state donor/acceptor planar-heterojunction (PHJ) hybrid solar cells are constructed and their excellent performance measured. The deposition of a thin C₆₀ fullerene or fullerene-derivative (acceptor) layer in vacuum on a CH₃NH₃PbI₃ perovskite (donor) layer creates a hybrid PHJ that displays the photovoltaic effect. Such heterojunctions are shown to be suitable for the development of newly structured, hybrid, efficient solar cells. Copyright

Full text of DOI:10.1002/adma.201301327

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CH3NH3PbI3 Perovskite/Fullerene Planar-Heterojunction
Hybrid Solar Cells
Jun-Yuan Jeng, Yi-Fang Chiang, Mu-Huan Lee, Shin-Rung Peng, Tzung-Fang Guo,*
Peter Chen,* and Ten-Chin Wen
Recent studies have reported organometal halide perovskite-
based mesoscopic solar cells with promising photovoltaic effi-
ciencies.[1–4] These devices employ a submicrometer, crystalline,
and well-oriented thin film of organo-lead iodide perovskite
as the light absorber (or as both light absorber and carrier con-
ductor) deposited on the surface of meso-structured titanium
dioxide (TiO2) or aluminum oxide (Al2O3) layers through a simple
spin-coating process.[3,4] Organo-lead iodide perovskite exhibits a
direct bandgap and a broad range of light absorption covering the
visible to near-infrared spectrum as well as a high extinction coef-
ficient.[2,4] All-solid-state mesoscopic solar cells with organo-lead
iodide perovskite as the light absorber exhibit high power conver-
sion efficiencies (PCEs) of great potential in real applications.[4]
Etgar et al. recently reported that spin-casting a methyl-
ammonium lead iodide (CH3NH3PbI3) solution on a 400 nm
This Communication presents a hybrid organic solar cell
that uses a glass/indium titanium oxide (ITO)/poly(3,4-ethyl-
enedioxythiophene) poly(styrene-sulfonate) (PEDOT:PSS) sub-
strate as the positive electrode, a PHJ of CH3NH3PbI3 perovs-
kite/fullerene (C60) structure as the active layer, a thin bathocu-
proine (BCP) film as an exciton- or hole-blocking layer (EBL or
HBL), and an aluminum (Al) negative electrode. The resulting
cell demonstrates very promising photovoltaic performance,
with an open-circuit voltage VOC = 0.55 V, a short-circuit current
JSC = 5.21 mA cm−2 , and a fill factor FF = 0.57, corresponding
to a PCE of 1.6% under standard 1 sun AM 1.5 simulated solar
irradiation. Additionally, applying [6,6]-phenyl C61-butyric acid
methyl ester (PCBM)[14] or indene-C60 bisadduct (ICBA)[15,16]
of higher lowest unoccupied molecular orbital (LUMO) level
instead of C60 as the acceptor elevates the magnitude of VOC
and PCE to 0.65 or 0.75 V and 2.4 or 2.1%, respectively. These
results verify the formation of a donor–acceptor interface at the
CH3NH3PbI3 perovskite/C60 PHJ and the modulation of photo-
voltaic performance by acceptors of varied LUMO levels.[17–19]
When the PHJ configuration is optimized, the hybrid cell can
deliver a 3.9% PCE with 10.32 mA cm−2 in JSC. This provides
a direction for developing newly designed, hybrid, efficient, all-
solid-state solar cells.
Several studies have reported that a thin layer of crystalline
CH3NH3PbI3 perovskite can be easily prepared by spin-casting
a γ-butyrolactone solution of equimolar CH3NH3I and PbI2
(precursor solution) on the substrate.[2,5] However, the crystal
growth may coarsen the film and markedly degrade device per-
formance or result in device failure. Accordingly, in this study,
we preheated a glass/ITO/PEDOT:PSS substrate at an elevated
temperature of 60 °C for 5 min, and cast the precursor solution
at a high spinning speed of 6000 rpm, to ensure solvent evap-
oration was fast and to inhibit the coarsening of the crystals.
The film becomes darker after annealing at 100 °C for 15 min.
Figure 1a shows its UV–vis spectrum. The film absorbs a wide
range of light from visible to the near-infrared, indicating the
formation of CH3NH3PbI3 perovskite on the substrate.[2] The
crystalline structure of CH3NH3PbI3 perovskite film on the
glass/PEDOT:PSS substrate was characterized by glancing
angle X-ray diffraction (GAXRD). The GAXRD patterns illus-
trated in Figure 1b agree well with the previously published
results.[1,2] The glass/PEDOT:PSS substrate shows no character-
istic peaks (lower curve in Figure 1b). The CH3NH3PbI3 per-
ovskite film prepared by the above procedures has a reflective
topography as viewed by the naked eye. The inset of Figure 1a
shows a photograph of the patterned glass/ITO/PEDOT:PSS
substrate both uncoated and coated with a thin layer of
CH3NH3PbI3 perovskite film for comparison.
thick TiO2 (anatase) nanosheet forms
a
mesoscopic
(CH3NH3PbI3)/TiO2 heterojunction.[5] CH3NH3PbI3 perovskite
successfully acts as both light harvester and hole conductor. No
additional hole transport layer is needed in their study. These
devices eliminate the difficulty of infiltrating a hole conductor,
such as 2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenyl-amine)-9,9′-
spirobifluorene (spiro-OMeTAD), in mesoporous TiO2 for the
fabrication of all-solid-state devices.[6,7] Generally speaking,
dual function (i.e., light absorber and carrier conductor) active
layers are widely used in fabricating polymer-, organic-, and
inorganic-based solar cells.[8–13] In other words, the function of
CH3NH3PbI3 perovskite in Etgar’s work is akin to that of the
“donor” material, in donor–acceptor polymer/organic planar-
and bulk-heterojunction solar cells. As a result, depositing or
growing a thin layer of “acceptor” material on the CH3NH3PbI3
perovskite film is expected to create a donor–acceptor con-
tact interface for charge separation, in which the hybrid
CH3NH3PbI3 perovskite/acceptor planar heterojunction (PHJ)
displays the photovoltaic effect under irradiation.
J.-Y. Jeng, Y.-F. Chiang, M.-H. Lee, Dr. S.-R. Peng,
Prof. T.-F. Guo, Prof. P. Chen
Department of Photonics
Advanced Optoelectronic Technology Center
National Cheng Kung University
Tainan, Taiwan 701, R.O.C.
E-mail: guotf@mail.ncku.edu.tw;
petercyc@mail.ncku.edu.tw
Prof. T.-C. Wen
Department of Chemical Engineering
National Cheng Kung University
Tainan, Taiwan 701, R.O.C.
DOI: 10.1002/adma.201301327
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Figure 2. a) Configuration of the hybrid solar cell in this study. b) Scheme
of the energy levels of each layer in the device.
Figure 1. a) UV–vis spectra of CH3NH3PbI3 perovskite films prepared
from γ-butyrolactone (6000 rpm) and dimethylformamide (DMF) (8000
rpm) solutions on quartz/PEDOT:PSS substrates. Inset: Photograph of
the patterned glass/ITO/PEDOT:PSS substrate uncoated (right) and
coated (left) with a thin layer of CH3NH3PbI3 perovskite film for com-
parison. b) GAXRD patterns of the glass/PEDOT:PSS/CH3NH3PbI3 and
the glass/PEDOT:PSS samples.
performance with VOC = 0.55 V, JSC = 5.21 mA cm−2 , and FF =
0.57, corresponding to a PCE of 1.6% under standard 1 sun
AM 1.5 simulated solar irradiation. The photovoltaic para-
meters of the devices are summarized in Table 1. As shown
in Figure 4, the incident photon to electron conversion effi-
ciency (IPCE) spectrum indicates that the device shows a
spectral response in the region from the visible to near-
infrared (300–800 nm) with a peak IPCE of 30% at approxi-
mately 500 nm. The IPCE spectrum of the device is coinci-
dent with the UV–vis spectrum of CH3NH3PbI3 perovskite
film as illustrated in Figure 1a. This observation suggests
that CH3NH3PbI3 perovskite acts as the light absorber and
is the primary contributor to the photocurrent. However, the
magnitudes of both VOC (0.55 V) and JSC (5.21 mA cm−2 ) for
the CH3NH3PbI3 perovskite/C60 PHJ solar cell are smaller
than those of the mesoscopic CH3NH3PbI3/TiO2 hetero-
junction solar cell (0.63 V, 16.1 mA cm−2 ) reported by Etgar
et al.[5] First, the CH3NH3PbI3 perovskite film used herein is
around 20–30 nm thick with an absorbance of around 0.3 near
500 nm, which is far from the optimum absorption of 1 sun
solar irradiation. The current stage of our study attempting
to increase the thickness of CH3NH3PbI3 perovskite film by
increasing the concentrations of the precursor solutions or
reducing the spinning speed during the spin-coating pro-
cess results in a very coarse topography of CH3NH3PbI3
perovskite film, which is not suitable for the fabrication of
Figure 2a illustrates the device configuration used in this
study and Figure 2b shows the energy level of each layer in
the device. In Figure 2b, the LUMO and the highest occupied
molecular orbital (HOMO) levels of CH3NH3PbI3 perovskite
are –3.9 and –5.4 eV, respectively, and those of C60 are –4.5 and
–6.2 eV.[5,20] Accordingly, when we deposit a thin layer of C60
in contact with CH3NH3PbI3 perovskite, C60 with a LUMO of
–4.5 eV could be an ideal “electron acceptor” for CH3NH3PbI3
perovskite to create a donor–acceptor PHJ system as illustrated
in Figure 2a,b. Under irradiation, excitons are generated by the
absorption of light in the CH3NH3PbI3 perovskite layer. The
oppositely charged holes and electrons in excitons are then sep-
arated at the donor–acceptor interface, as depicted in Figure 2b,
and are transported by CH3NH3PbI3 perovskite and C60, respec-
tively, resulting in the photovoltaic effect.
Figure 3a presents the current density–voltage (J–V) curves
of the hybrid organic solar cells (consisting of glass/ITO/
PEDOT:PSS as the positive electrode, a PHJ of CH3NH3PbI3
perovskite/C60 as the active layers, a thin BCP film as EBL,
and an Al negative electrode) under standard 1 sun AM 1.5
simulated solar irradiation. The device exhibits promising
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Figure 4. IPCE spectra of the perovskite/C60 (30 nm) (filled circles) and
perovskite/PCBM (25 nm) (filled squares) devices. Inset: The good diode
characteristics of the perovskite/PCBM (25 nm) device in the dark (empty
squares) and under irradiation (filled squares).
PHJ solar cells. Second, the application of mesoscopic TiO2
nanosheets or meso-superstructured metal oxide layers with
CH3NH3PbI3 perovskite (respectively reported by Etgar et al.[5]
and Lee et al.[4]) would still be favorable at the current stage
for photocurrent generation because of the markedly enlarged
contact interface of PHJ in hybrid solar cells. However, practi-
cally speaking, if a very thin film can produce sufficient light
harvesting efficiency, it is not necessary to use a mesoporous
structure, which would cause the pore-filling of solid material
into the mesoscopic junction to be a challenging issue. This is
reflected in Lee et al.’s paper (Figure S3, Supporting Informa-
tion), where the solar cells showed a wide distribution range
in terms of device performance.[4] Given the high extinction
coefficient of CH3NH3PbI3 perovskite,[2] the ultrathin “planar
junction” devices proposed here represent a new design for
excitonic photovoltaic devices without a mesoscopic structure.
We believe further improvements to film deposition and thick-
ness, along with surface morphological control, can contribute
to advances of this planar junction.
Figure 3. a ) J–V curves of CH3NH3PbI3 perovskite (γ-butyrolactone)/C60
or C60-derivative hybrid solar cells: Perovskite/C60 (30 nm) (filled circles),
perovskite/PCBM (25 nm) (filled squares), perovskite/ICBA (30 nm)
(filled diamonds) under standard 1 sun AM 1.5 simulated solar irradia-
tion, and perovskite/ICBA (30 nm) (empty diamonds) under 0.5 sun.
b) J–V curves of CH3NH3PbI3 perovskite (DMF)/C60 or C60-derivative
hybrid solar cells: Perovskite/C60 (30 nm) (circles), perovskite/PCBM
(25 nm) (squares), and perovskite/ICBA (30 nm) (diamonds) under
standard 1 sun AM 1.5 simulated solar irradiation.
T a b le 1 . The photovoltaic parameters of CH3NH3PbI3 perovskite/C60 or C60-derivative hybrid solar cells. Device configuration: glass/ITO/PEDOT:PSS/
CH3NH3PbI3 perovskite/C60 or C60 derivative/BCP/Al.
Device
VOC
[V]
JSC
[mA/cm2]
FF
PCE
[%]
RS
[Ω cm2]
RP
[MΩ]
Perovskite/C60 (30 nm)a)
0.55
0.65
0.75
0.62
0.55
0.60
0.58
5.21
5.89
0.57
0.63
0.51
0.62
0.61
0.63
0.58
1.6
2.4
2.1
2.5
3.0
3.9
3.4
4.49
4.50
8.31
2.01
19.39
17.21
Perovskite/PCBM (25 nm)a)
Perovskite/ICBA (30 nm)a)
5.44
Perovskite/ICBA (30 nm)b )
3.17
Perovskite(DMF)/C 60 (30 nm)a)
Perovskite(DMF)/PCBM (25 nm)a)
Perovskite(DMF)/ICBA (30 nm)a)
9.02
1.75
1.82
2.42
0.44
3.93
1.53
10.32
10.03
a)Under standard 1 sun AM 1.5 simulated solar irradiation; b)Under 0.5 sun AM 1.5 simulated solar irradiation.
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the acceptor with the CH3NH3PbI3 perovskite
film has VOC = 0.65 V, JSC = 5.89 mA cm−2 ,
and FF = 0.63, corresponding to a PCE of
2.4% under standard 1 sun AM 1.5 simulated
solar irradiation. The change of VOC from 0.55
to 0.65 V strengthens the formation of a PHJ
at the CH3NH3PbI3 perovskite/C60 or PCBM
interface to generate the photovoltaic effect,
because the energy level offset between the
HOMO level of the donor (CH3NH3PbI3 per-
ovskite) and the LUMO level of the acceptor
(C60 or PCBM) modulates the magnitude of
[17–19]
VOC
.
As shown in Figure 4, the IPCE
spectrum of the CH3NH3PbI3 perovskite/
PCBM-device with a peak IPCE of approxi-
mately 45% at 500 nm (higher than that of
the CH3NH3PbI3 perovskite/C60 device),
confirms the enhanced JSC and PCE. In addi-
tion, employing ICBA – which has a LUMO
level 0.17 eV higher than that of PCBM – as
the acceptor further increases the magni-
[21]
tude of VOC
.
The CH3NH3PbI3 perovskite/
ICBA PHJ device exhibits VOC = 0.75 V, JSC
=
5.44 mA cm−2 , and FF = 0.51, corresponding
to a PCE of 2.1% under standard 1 sun AM
1.5 simulated solar irradiation. Under 0.5 sun
irradiation, the device exhibits VOC = 0.62 V,
JSC = 3.17 mA cm−2 , and a superior FF = 0.62,
corresponding to a PCE of 2.5%. As a result,
the proposed CH3NH3PbI3 perovskite/C60
PHJ solar cells elucidate the effects of donor–
acceptor interfacial energy off-set and dem-
onstrate the feasibility of enhancing device
performance by manipulating hybrid donor–
acceptor materials with different energy
levels.
Figure 5a,c present high-resolution scan-
ning electron microscopy (HR-SEM) images
of the CH3NH3PbI3 perovskite films spin-cast
(6000 rpm) on the glass/ITO/PEDOT:PSS
substrate respectively without and with
preheating at 60 °C for 5 min. At the mag-
nification of 20 000× (see the scale bar in
Figure 5a,c), the CH3NH3PbI3 perovskite film
exhibits unique clustered-domain patterns
on the glass/ITO/PEDOT:PSS substrate and
the film prepared on the preheated substrate
(i.e., Figure 5c) has a denser morphology in
the clustered-domain region. As shown in
Figure 5c, the lengths of the domain regions
are approximately 1–3 μm. In Figure 5b,d, at a
magnification of 100 000× (see scale bar), the
Figure 5. HR-SEM images of CH3NH3PbI3 perovskite film spin-cast (6000 rpm) from
γ-butyrolactone solution on glass/ITO/PEDOT:PSS substrate: a,b) Without preheating of the
substrate for spin-coating (at two different magnifications); c,d) with preheating of the sub-
strate for spin-coating (at two different magnifications); e,f) CH3NH3PbI3 perovskite film pre-
pared from DMF solution with preheating of the substrate for spin-coating (at two different
magnifications). g,h) AFM height images of CH3NH3PbI3 perovskite film with preheating of
the substrate for spin-coating from γ-butyrolactone solution (6000 rpm) (g) and DMF solution
(8000 rpm) (h) on glass/ITO/PEDOT:PSS substrate.
Alternatively, using an acceptor material with a higher LUMO
level in a PHJ increases the magnitude of VOC and the overall
performance of the solar cells.[17–19] Figure 3a also presents the
J–V curves of hybrid organic solar cells with PCBM or ICBA as
the acceptor in the PHJ. Table 1 summarizes the photovoltaic
parameters of the devices. PCBM has a LUMO level of –3.9 eV,
while that of C60 is lower, at –4.5 eV.[20] The device with PCBM as
CH3NH3PbI3 perovskite crystal sizes in the clustered-domain
region are approximately 100–150 nm in diameter. Figure 5g
shows a height image obtained by atomic force microscopy
(AFM) indicating the clustered-domain and rough surface mor-
phology of the CH3NH3PbI3 perovskite film; the results agree
well with the images obtained by HR-SEM. It is worth noting
that the HR-SEM and AFM images of CH3NH3PbI3 perovskite
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films in Figure 5 are not the ideal film morphology, but the film
spin-cast on the preheated glass/ITO/PEDOT:PSS substrate
still has a smooth and reflective topography when viewed by
the naked eye (see the inset photograph of Figure 1a). The PHJ
device fabricated with the above-described film demonstrates
good diode characteristics, with 104–105 rectification ratio in the
dark (inset of Figure 4), and decent photovoltaic performance
(Figure 3a and Table 1). Therefore, we expect that improved
control of the CH3NH3PbI3 perovskite thin film morphologies
will result in better photovoltaic performance.
Figure 5e,f present the HR-SEM images of the film formed
by spin-casting (8000 rpm) the CH3NH3PbI3 perovskite N,N-
dimethylformamide (DMF) solution (equimolar CH3NH3I and
PbI2) on the preheated glass/ITO/PEDOT:PSS substrate (60 °C
for 5 min). The results show distinguishing features compared
to those in Figure 5a–d. In Figure 5e, the CH3NH3PbI3 per-
ovskite film exhibits densely interconnected crystalline strips
on the glass/ITO/PEDOT:PSS substrate at a magnification of
20 000×. As shown in Figure 5f at a magnification of 100 000×,
the crystal sizes are approximately 150–200 nm in diameter,
that is, larger than those in Figure 5d. The height AFM image
(Figure 5h) exhibits a surface morphology distinct from that
in Figure 5g. The CH3NH3PbI3 perovskite film has a higher
contact area in the PHJ and a suitable channel to transport
charge carriers configured as interconnected crystalline strips.
The device with the configuration glass/ITO/PEDOT:PSS/
CH3NH3PbI3 perovskite (from DMF solution, 8000 rpm on the
harvesting in the CH3NH3PbI3 perovskite films as discussed
above.
Some crevices remain in the crystal boundaries between
interconnected crystalline strips, which may reduce the mag-
nitude of the photovoltaic parameters. When DMF is used as
the solvent in preparing CH3NH3PbI3 perovskite film, changes
of atmosphere (solvent vapor pressure) in the glove box easily
modulate the thin film morphology, which causes variations
in the devices’ photovoltaic performance. Precise control over
the film formation conditions (e.g., solution concentration,
substrate preheating temperature and spinning rate, or sol-
vent atmosphere) would ensure CH3NH3PbI3 perovskite films
of suitable morphologies for device fabrication. Alternatively,
the synthesis and preparation of new perovskite-type materials
would be employed for the development of efficient perovskite-
based, hybrid PHJ solar cells.
In this Communication we have reported a device made
of CH3NH3PbI3 perovskite film spin-cast from DMF solution
with a decent PCE of 3.9%. Although the PCE of the hybrid
solar cells presented here is still lower than the highest PCE
reported by Etgar et al.[5] and Lee et al.,[4] further improvement
is expected through the proposed “planar junction” structure. It
is noted that Etgar et al.[5] reported PCE of 2.8% at 1 sun inten-
sity in a cell made from standard TiO2 nanoparticles rather
than mesoscopic TiO2 nanosheets (PCE of 5.5%). Lee et al.’s[4]
planar junction diode with a structure of fluorine-doped tin
oxide (FTO)/compact TiO2/CH3NH3PbI2Cl/spiro-OMeTAD/Ag
generates JSC of 7.13 mAcm−2 , VOC of 0.64 V, FF of 0.4, and
PCE of 1.8%. (The highest efficiencies of 7.6% and 10.9% are
achieved by cells made of, respectively, meso-superstructured
TiO2 and an Al2O3 layer that have a photoactive mesoscopic
heterojunction layer a few hundred nanometers thick.) Unlike
the above devices, which use standard TiO2 nanoparticles or
a compact TiO2 layer, in our study the photovoltaic effect is
generated through a CH3NH3PbI3 perovskite/fullerene inter-
face, which is an efficient PHJ with a thickness of a few tens
of nanometers. Our devices actually outperformed all other
reported “planar junction” architectures using CH3NH3PbI3 as
the absorber. We believe this progress represents a new concept
for the design of ultrathin excitonic photovoltaic devices. Our
approach significantly simplifies the processes for fabricating
all-solid-state, hybrid CH3NH3PbI3 perovskite-based solar
cells of decent performance without using mesoscopic TiO2
nanosheets or meso-superstructured metal oxide layers and an
additional hole transport layer. This might avoid the pore-filling
or reproducibility issues in mesoscopic junction devices. In the
future, CH3NH3PbI3 perovskite/C60 or C60 derivative PHJ solar
cells can be fabricated at relatively low temperatures on ITO-
patterned plastic substrates, offering great potential for applica-
tion in flexible, lightweight, and portable devices.
preheated substrate)/C60 (30 nm)/BCP (10 nm)/Al has VOC
=
0.55 V, JSC = 9.02 mA cm−2 , and FF = 0.61, corresponding to
a PCE of 3.0% under standard 1 sun AM 1.5 simulated solar
irradiation. The J–V curves are shown in Figure 3b and Table 1
lists the photovoltaic parameters of the hybrid solar cell. The
higher magnitude of JSC (9.02 mA cm−2 ) markedly increases
the magnitude of PCE, because of improved light harvesting
by means of improved coverage of the CH3NH3PbI3 perovs-
kite film on the glass/ITO/PEDOT:PSS substrate, as shown
in Figure 5e. Figure 1a also indicates a noticeable enhanced
absorbance in the UV–vis spectrum of the CH3NH3PbI3 per-
ovskite film prepared from its DMF solution. Accordingly, con-
trolling the qualities (morphology, roughness, crystalline size,
cluster domain, thickness, etc.) of CH3NH3PbI3 perovskite
films is found to improve the performance of hybrid solar cells.
The performance of the hybrid cells that include the
CH3NH3PbI3 perovskite (DMF) film is also improved by varying
the acceptor layer in the PHJ, using PCBM or ICBA instead
of C60. As shown in Figure 3b and Table 1, the CH3NH3PbI3
perovskite (DMF)/ICBA (30 nm) PHJ-solar cell exhibits VOC
=
0.58 V, JSC = 10.03 mA cm−2 , and FF = 0.58, corresponding to a
PCE of 3.4% under standard 1 sun AM 1.5 simulated solar irra-
diation. The CH3NH3PbI3 perovskite (DMF)/PCBM (25 nm)
PHJ device exhibits VOC = 0.60 V, JSC = 10.30 mA cm−2 , and FF =
0.63, corresponding to an optimized PCE of 3.9%. The increase
in the magnitude of VOC from 0.55 V (C60) to 0.58 V (ICBA)
and 0.60 V (PCBM) in the hybrid cells is not as obvious as that
seen in the CH3NH3PbI3 perovskite film (γ-butyrolactone solu-
tion) devices, though this part of the mechanism is still under
investigation. The significant increase in the magnitude of JSC
causing the markedly enhanced PCE can be attributed to the
appropriate morphologies, thicknesses, and improved light
In conclusion, we have successfully demonstrated the fabrica-
tion of all-solid-state, hybrid, donor–acceptor, CH3NH3PbI3 per-
ovskite/C60 PHJ solar cells with promising photovoltaic efficien-
cies. In the hybrid cell, the CH3NH3PbI3 perovskite serves a dual
function, as both the light absorber and hole conductor. Applying
acceptors with various LUMO levels modulates the magnitude
of VOC and PCE. These results verify the formation and func-
tion of a hybrid PHJ in the CH3NH3PbI3 perovskite/C60 inter-
face, displaying the photovoltaic effect under solar irradiation.
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Keithley 2400 sourcemeter, a chopper (SR540, Stanford Research
Systems Inc.), and a lock-in amplifier (SR510, Stanford Research
Systems Inc.). UV–vis spectra were taken using a U4100 spectrometer
(Hitachi). The crystallographic properties of CH3NH3PbI3 perovskite on
the glass/PEDOT:PSS substrate were characterized by GAXRD (Rigaku
D/MAX2500, 18 kW rotating anode X-ray Generator) at a scan rate of
4° min−1 . The HR-SEM images were obtained using a Zeiss AURIGA
39-50 field-emission scanning electron microscope. The height images
obtained by AFM were taken using a NanoScope IIIa instrument (Digital
Instruments Inc., Tonawanda, NY) in tapping mode.
The hybrid cell performs with a high VOC of 0.75 V, but its PCE
is limited by the magnitude of JSC, which can be improved by
increasing the thickness of the CH3NH3PbI3 perovskite active
layer. An ideal CH3NH3PbI3 perovskite film of the appropriate
thickness, morphology, and topography can be prepared by
adjusting the solvent, solution concentration, preheating temper-
ature, and atmosphere, or by synthesizing new perovskite-based
materials. Such results would further enhance photovoltaic per-
formance, and efforts in this direction are ongoing.
Acknowledgements
Experimental Section
The authors thank the National Science Council (NSC) of Taiwan
(NSC99-2113-M-006-008-MY3) for financially supporting this research.
Technical support from the NSC Instrument Center at NCKU for the
HR-SEM and GAXRD measurements is highly appreciated.
Materials and Sample Preparation: The ethylammonium lead iodide
(CH3NH3PbI3) perovskite was prepared as reported elsewhere.[1–5]
Methylamine (CH3NH2) (13.5 mL, 40 wt% in aqueous solution, Alfa Aesar)
and hydroiodic acid (HI) (15.0 mL, 57 wt% in water, Alfa Aesar) were stirred
at 0 °C under nitrogen atmosphere for 2 h. After the reaction, the solvent
of the solution was evaporated using a rotary evaporator. A white powder,
methyl ammonium iodide (CH3NH3I), was generated by the reaction. The
precipitate was washed with diethyl ether (Sigma–Aldrich) three times and
dried at 60 °C in a vacuum oven overnight. CH3NH3PbI3 was synthesized
by mixing CH3NH3I and lead iodide (PbI2) (Showa Chemical Industry Co.,
Ltd.) at 1:1 (0.8:2.3 g) equimolar ratio in γ-butyrolactone (Aldrich) (15.8 mL,
14.9 wt% solution) or anhydrous N,N-dimethylformamide (DMF) (Aldrich)
(17.2 wt% solution) at 60 °C, stirring for 12 h inside a nitrogen-filled glove
box with oxygen and moisture levels <1 ppm.
Fabrication of CH3NH3PbI3 Perovskite/Fullerence PHJ Hybrid Solar Cells:
The solar cells were fabricated in a standard arrangement by sandwiching
a PHJ of CH3NH3PbI3 perovskite/C60 structure between a precleaned and
prepatterned transparent glass/ITO (Ritek Corp., 15 Ω/sq.)/PEDOT:PSS
(Baytron P, Bayer AG, Germany) as the positive electrode, and a thin
BCP film as an EBL or HBL. The PEDOT:PSS layer is an interfacial buffer
to smooth the relatively rough ITO surface, which reduces the leakage
current and improves the stability of the device. The device was completed
by the thermal evaporation of Al as the negative electrode. Figure 2a
illustrates the device configuration and Figure 2b shows the energy levels
of the layers. Prior to the device fabrication, the ITO/glass substrates were
sequentially cleaned by ultrasonic treatment in detergent, deionized water,
acetone, and isopropyl alcohol. The glass/ITO/PEDOT:PSS substrates
were preheated at 60 °C for 5 min prior to spin-casting CH3NH3PbI3
γ-butyrolactone solution (14.9 wt%) at 6000 rpm for 30 s. The preheating
treatment of the substrate ensures the solvent evaporation is fast and
inhibits coarsening of the crystals. The film becomes darker after annealing
at 100 °C for 15 min. The thickness of the thin films was measured
using a step profiler (ET 4000M, Kosaka, Japan). A thin calibrated Al
layer was deposited on the surface of CH3NH3PbI3 perovskite film for a
more precise measurement of the thickness. The C60 (30 nm) (>99.5%,
Aldrich), PCBM)(25 nm) (>99.5%, Solenne, Netherlands), ICBA (30 nm)
(>99.5%, Solenne, Netherlands), BCP (10 nm) (Aldrich), and Al (100 nm)
were thermally deposited on the substrate inside a vacuum chamber
(10−6 Torr). The thickness was monitored by a quartz crystal monitor. The
active area of the device is 0.06 cm2. All the procedures were implemented
inside a nitrogen-filled glove box with oxygen and moisture levels <1 ppm
except for the casting of the PEDOT:PSS layer.
Received: March 25, 2013
Revised: April 22, 2013
Published online:
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Characterization: Device characteristics, J–V curves, were measured
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2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2013,
DOI: 10.1002/adma.201301327