Effect of hole transport layer in planar inverted perovskite solar cells

Article abstract of DOI:10.1246/cl.150888

This study compares the photovoltaic performance of the inverted planar perovskite solar cells with and without poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) as the hole transport layer (HTL). The device ITO/CH₃NH₃PbI₃/PCBM/Al (PCBM: [6,6]-phenyl-C₆₁-butyric acid methyl ester) without PEDOT:PSS achieved a power conversion efficiency of 5.69%, which is much lower than that (ca. 10%) for the device with HTL. The effect of the HTL on the device performance was investigated by electronic impedance spectroscopy and transient photovoltage decay characterization. The results revealed that the low efficiency of HTL-free devices can be attributed to an inefficient electron-blocking capability. The absence of the HTL causes serious recombination at the ITO/perovskite interface, and thus results in low photovoltage and fill factor.

Full text of DOI:10.1246/cl.150888

Received: September 23, 2015 | Accepted: October 26, 2015 | Web Released: November 5, 2015
CL-150888
Effect of Hole Transport Layer in Planar Inverted Perovskite Solar Cells
Dan Li, Jin Cui, Hua Zhang, Hao Li, Mingkui Wang,* and Yan Shen
Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology,
Wuhan, Hubei 430074, P. R. China
(E-mail: mingkui.wang@mail.hust.edu.cn)
This study compares the photovoltaic performance of the
inverted planar perovskite solar cells with and without poly(3,4-
ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS)
as the hole transport layer (HTL). The device ITO/CH3NH3PbI3/
PCBM/Al (PCBM: [6,6]-phenyl-C61-butyric acid methyl ester)
without PEDOT:PSS achieved a power conversion efficiency of
5.69%, which is much lower than that (ca. 10%) for the device
with HTL. The effect of the HTL on the device performance was
investigated by electronic impedance spectroscopy and transient
photovoltage decay characterization. The results revealed that
the low efficiency of HTL-free devices can be attributed to an
inefficient electron-blocking capability. The absence of the HTL
causes serious recombination at the ITO/perovskite interface,
and thus results in low photovoltage and fill factor.
structured perovskite devices have several advantages that
attracted considerable attention, including simple device archi-
tecture and solution-processable fabrication process. This low-
temperature processability enables the manufacture of solar cells
on flexible substrates in large scale.23,24 In addition, the inverted
planar heterojunction structured perovskite devices show better
stability under ultraviolet light than the mesoporous structured
devices.25 Subsequently, extensive work has been carried out
on the planar heterojunction structured perovskite device, which
pushes the device PCEs higher and higher,16,26,27 including
progressive effort on optimizing the crystallization process and
morphology tuning of the perovskite films.6,28 A useful strategy
to improve perovskite solar cell performance is interfacial
engineering. For example, interfacial materials such as PDINO10
and PN4N26 have been adopted to modify the interface between
perovskite layer and charge-collecting electrodes. Various hole
transport layers (HTLs) possessing high work function were
adopted to selectively collect holes at the transparent electrode,
such as NiOx,29-32 CuSCN,33 V2O5,23 and conjugated polyelec-
trolytes,34 to enhance the device performance. Because of its
good film forming property, excellent transparency in the visible
and near infrared regions, and good conductivity,35 poly(3,4-
ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS)
has become the most preferred HTL in inverted planar hetero-
junction structured perovskite solar cells. PEDOT:PSS films are
extremely hygroscopic.36 Some papers have indicated that the
acidic nature of PEDOT:PSS can corrode the ITO substrate,
causing ions to diffuse into the PEDOT:PSS layer, and eventually
decreasing interfacial stability.37 Oxygen and moisture can be
easily absorbed by PEDOT:PSS, altering its structure and
reducing its conductivity.38
Here we show our finding on the inverted planar solar cell
based on CH3NH3PbI3 with and without a PEDOT:PSS as the
HTL. The effect of the HTL on the device performance was
studied by electronic impedance spectroscopy (IS) and transient
photovoltage decay (TPD) measurements. The CH3NH3PbI3-
based solar cell without the HTL achieved a PCE of 5.69%.
Compared with the complete devices showing a PCE of 10.01%,
the main difference is obviously reflected in the VOC and FF.
Impedance and transient experimental data reveal that those
decreases contributed to inefficient electron blocking capability
in the device without the HTL. Our results indicate that the
lack of the HTL layer could result in serious recombination at
the ITO/perovskite interface, and thus, obvious photovoltage
loss. These results may have important implications for the
future optimization of inverted planar perovskite solar cells.
Figure 1 shows the device configuration and the corre-
sponding energy levels for various components. The PEDOT
was considered to act as the HTL at the anode, and the [6,6]-
phenyl-C61-butyric acid methyl ester (PCBM) likewise as the
electron transport layer at the cathode. This structure is electri-
Perovskite solar cells have attracted considerable attention in
recent years due to their unique structures and excellent
performance. In 2009, Miyasaka et al. first introduced organo-
metallic halide perovskite as sensitizer for mesoporous solar
cells, obtaining a power conversion efficiency (PCE) of 3.8%.1
Since then, highly efficient perovskite devices have been
reported through extensive efforts toward altering device
architectures,2,3 morphology control,4-8 interfacial engineer-
ing,9-12 and so on. Presently, perovskite solar cells show higher
power conversion efficiency (PCE, the highest value being
20.1%) than polymer solar cells and dye-sensitized solar cells.
This can be attributed to the perovskite absorber layer with high
absorption coefficient in the ultraviolet-visible range,13 bipolar
transport characteristics,14 long carrier diffusion length,15 long
carrier lifetime,15 and excellent crystallinity as well as low defect
density.16-18 There are two main types of perovskite solar cells
under intensive investigation, i.e., mesoscopic structured and
planar heterojunction structured devices. Mesoscopic structured
devices initially employed n-type mesoporous metal oxide, for
example TiO2 layer, to extract and transport charge carriers.5,19-21
Lately, Snaith et al.2 replaced the n-type mesoporous TiO2 layer
with an insulating mesoporous Al2O3 layer as a scaffold and
reported devices with an exciting PCE of 10.9%. The existence
of an insulating mesoporous Al2O3 layer in the device config-
uration indicates that electrons can transport efficiently in the
perovskite itself. Later on, by reducing the scaffold thickness, the
authors began to explore the planar heterojunction configuration
of perovskite solar cells.2 Devices with impressive efficiencies of
above 15% were presented, in which deposition of a homoge-
neous pin-hole free perovskite layer and interfacial engineering
are critical parameters for further performance enhancement.11,22
In 2013, Guo et al.3 first introduced an inverted heterojunction
structured perovskite solar cell, achieving a PCE of 3.9%.
Even though this device showed lower efficiency compared to
the conventional perovskite solar cells, inverted heterojunction
Chem. Lett. 2016, 45, 89–91 | doi:10.1246/cl.150888
© 2016 The Chemical Society of Japan | 89

enhancement in FF and VOC. We suppose that the energy gap
(ca. 0.6 eV) between ITO and CH3NH3PbI3 is too large to form
an ohmic contact, resulting in inefficient hole injection and
serious back recombination at this interface. Insertion of the
PEDOT:PSS layer can increase the work function of ITO to
¹5.2 eV and thus form an ohmic contact for hole transport.
Electronic IS measurements were carried out to further
investigate the role of the HTL. Recently, IS has been used in the
characterization of perovskite solar cells.33,41,42 The IS measure-
ment was carried out at open circuit under about 0.1 sun
illumination. Figure S1 shows the Nyquist plots for the two
devices under illumination at a bias of ¹0.6 V in the frequency
range of 1 MHz to 20 mHz. The frequency response shows two
semicircles, representing two charge transfer processes. The
semicircle in the Nyquist plots in the high frequency regime can
be described with the charge transport and charge recombination
processes between CH3NH3PbI3 and the selective contact
interfaces. The second semicircle in the low frequency regime
is still under discussion, but has been attributed to ion migration
in CH3NH3PbI3 recently.43 We have fitted the IS data with an
equivalent circuit consisting of two series RC circuits, as shown
in the inset of Figure S1. There are several elements in the
equivalent circuit model that describe processes in the perovskite
solar cells: Rs is the series resistance, including the sheet
Figure 1. (a) Device architecture and (b) the corresponding energy
diagram.
Figure 2. (a) Current density-voltage (J-V) curves; (b) IPCE
spectra; and (c) dark current measurements of devices ITO/
PEDOT:PSS/perovskite/PCBM/Al (A) and ITO/perovskite/PCBM/
Al (B).
cally inverted with respect to the regular configuration that
characterizes most high-efficiency oxide-containing systems.
As depicted in Figure 1b, the valence and conduction band
energy levels of CH3NH3PbI3 are drawn with respect to the
work function of ITO and the LUMO of PCBM, respectively,
illustrating energetically favorable paths for hole and electron
travel to the collecting electrodes.
resistance of the ITO and the contact resistance of the cell. Rrec-1
,
C
rec-1, Rrec-2, and Crec-2 are the charge recombination resistance
and the corresponding capacitance of the charge recombination
process at the anode side (hole extraction) and cathode side
(electron extraction), respectively. For better fitting, all capacitor
elements were replaced by constant phase elements that, in any
case, keep the constant phase element (CPE) exponent p quite
close to the perfect capacitor value, p µ 1. As mentioned above,
the semicircle at high frequency relates to charge recombination
at the anode interface, which is crucial to device VOC. Figure 3a
Figure 2a shows the photocurrent-voltage curves of the
devices with (device A) and without (device B) PEDOT:PSS
¹2
measured under 100 mW cm AM 1.5 G simulated irradiation
at room temperature. The details of device fabrication and
characterization are described in Supporting Information.38
The corresponding photovoltaic parameters are summarized in
Table S1. Device B without the HTL exhibited a PCE of
5.69% with JSC of 15.67 mA cm¹2, FF of 0.61, and VOC of
0.59 V. Device A with the HTL showed a PCE of 10.01% with
VOC of 0.79 V, JSC of 17.55 mA cm¹2, and FF of 0.72. Obviously,
the existence of the HTL is essential for the high performance of
this type solar cell. Though both devices showed similar JSC
values, VOC and FF decreased significantly for the device without
the HTL. Figure 2b shows the incident photon to electron
conversion efficiency (IPCE) spectra of the two devices. Both
devices displayed broad IPCE curves in the wavelength region
of 300-800 nm. Device B depicts a lower IPCE value than
presents the interfacial charge recombination resistance Rrec-1.
The higher Rrec-1 for device A indicates that the interfacial
charge recombination process in the complete device is slower
than that in the incomplete one. The results are consistent
with the performance of device output photovoltage. A higher
recombination resistance Rrec-1 indicates lower recombination
mass flux (i.e., dark current).
TPD measurements were further performed to rationalize
the effect of the HTL on the device performance. This technique
is widely employed for the determination of charge lifetimes
(¸e) in sensitized solar cells.44,45 Figure S2 depicts the typical
voltage transient dynamics for all devices responding to the
light perturbation. We observed biphasic decays with double
device A, which is in good agreement with the result for JSC
.
We first carried out dark current characterization for
understanding the effect of the HTL on the photovoltaic
performance of the devices. Figure 2c illustrates the dark current
(JD) for both devices. The dark current characteristic shows
typical diode behavior. It shows a current leakage contribution at
low voltage and an exponential diffusion-dominated regime at
intermediate voltage as well as the space-charge limited drift
regime at high voltage.40 The dark current of device B is about
two orders of magnitude larger than that of device A. This result
suggests that the HTL can largely reduce the leakage current at
the ITO/CH3NH3PbI3 interface, and thus lead to a significant
Figure 3. The plots of (a) recombination resistance at the anode
side from impedance measurement and (b) electron lifetime from
transient decay measurement as a function of VOC for ITO/PEDOT/
perovskite/PCBM/Al (A) and ITO/perovskite/PCBM/Al (B).
90 | Chem. Lett. 2016, 45, 89–91 | doi:10.1246/cl.150888
© 2016 The Chemical Society of Japan

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tracted charge lifetime by fitting the photovoltage decay results
as a function of the open-circuit voltage for various devices. We
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inverted planar solar cell based on CH3NH3PbI3 was fabricated,
showing a PCE of 5.69%, with VOC of 0.59 V, JSC of
15.67 mA cm¹2, and FF of 0.61. Clearly, the HTL contributes
to enhancement of the solar cell performance, especially FF and
VOC. The experimental results reveal that the HTL in devices can
efficiently block electrons and suppress charge recombination at
the anode, which essentially influence the open circuit voltage
and fill factor.
Financial support from the Director Fund of the Wuhan
National Laboratory for Optoelectronics, the 973 Program of
China (Grants Nos. 2014CB643506 and 2013CB922104) is
gratefully acknowledged.
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Chem. Lett. 2016, 45, 89–91 | doi:10.1246/cl.150888
© 2016 The Chemical Society of Japan | 91