Ni-Co-O hole transport materials: Gap state assisted hole extraction with superior electrical conductivity

Article abstract of DOI:10.1039/c9ta07331k

Organic-inorganic hybrid perovskite solar cells (PSCs) have been rapidly evolving as a promising candidate for next-generation photovoltaic technologies. P-type organics or polymers are generally used as hole transport materials (HTMs), which are expensive and non-sustainable for long-term applications. Herein, we demonstrate an inorganic Ni-Co-O hole conductor that displayed fast hole extraction and transport by the presence of gap states and relative high hole conductivity. Detailed structural inspection reveals that Co³⁺ and Ni²⁺ ions would reform into Co²⁺ and Ni³⁺ sites with the incorporation of Co, which contributes to greatly enhanced hole concentration. Inverted heterojunction devices based on NiCoO_x hole transport layers yielded a maximum power conversion efficiency (PCE) of 20.03%, with 16.9% improvement compared with those based on NiO_x layers (17.14%). This novel HTM with a facile synthesis process provides a new strategy for designing efficient carrier transport materials such that efficient charge collection and transport are achieved.

Full text of DOI:10.1039/c9ta07331k

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DOI: 10.1039/C9TA07331K.
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DOI: 10.1039/C9TA07331K
ARTICLE
Ni-Co-O Hole Transport Materials: Gap Sates Assisted Hole
Extraction with Superior Electrical Conductivity
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
a
c
d
Yu Hou,† Li Juan Tang,† Hong Wei Qiao, Zi Ren Zhou, Yu Lin Zhong, Li Rong Zheng, Meng Jiong
a
b
a
Chen, Shuang Yang,* Hua Gui Yang*
DOI: 10.1039/x0xx00000x
Organic-inorganic hybrid perovskite solar cells (PSCs) have been rapidly evolving as a promising candidate for the next-
generation photovoltaic technologies. P-type organics or polymers are generally used as hole transport materials (HTMs),
which are expensive and non-sustainable for long-term applications. Herein, we demonstrate an inorganic Ni-Co-O hole
conductor that displayed fast hole extraction and transport by the presence of gap states and relative high hole
conductivity. Detailed structural inspection reveals that the Co3+ and Ni2+ ions would reform into Co2+ and Ni3+ sites with
the incorporation of Co, which contributes to the greatly enhanced hole concentration. Inverted heterojunction devices
based on NiCoO
x
hole transport layer yielded a maximum power conversion efficiency (PCE) of 20.03%, with 16.9%
layer (17.14%). This novel HTM with facile synthesis process provides a
improvement compared with those based on NiO
x
new strategy for designing efficient carrier transport materials such that efficient charge collection and transport are
achieved.
To address these issues, several inorganic-based p-type
semiconductors have been explored as HTMs in the past few
Introduction
The superior photophysical properties of metal halide
perovskites (e.g. intense light absorption, long diffusion length
1
3-18
19-21
22
years, including metal oxides,
metal sulfides,
iodides
2
3
and thiocyanates.
Despite the electron blocking
characteristics of p-type HTMs, a significant fraction of carriers
may still recombine at HTM/perovskite interface or in the bulk
HTM layer before reaching the electrodes, thus limiting the
device fill factor (FF) and the open-circuit voltage (Voc).
Recently, time-resolved spectroscopic studies revealed that
the hole extraction kinetics in the state-of-the-art spiro-
OMeTAD layer can be further improved by incorporation
1
-6
and low exciton energy), are responsible for their highly
efficient solar cells with power conversion efficiency (PCE) over
7
2
5%. To assist in the charge extraction and conduction from
the perovskite thin film, additional charge transport layers are
required. To date, most of the best-performing PSC devices
with efficiencies over 20% relied on doped organic or
polymeric hole transport materials (HTMs), such as 2,2′,7,7′-
2
4
conductive carbon materials, which suggested that the
photoexcited charge carriers must to reduce chances of
recombination. Therefore, the development of inorganic HTMs
with excellent hole extraction and transport ability is critical to
achieve higher power conversion efficiency (PCE).
tetrakis(N,N-di-pmethoxyphenyl-amine)9,9′-spirobifluorene
spiro-OMeTAD)8
-10
and
poly-triarylamine
(PTAA).11,12
(
Unfortunately, these organic HTMs are costly and unstable for
large-scale and long-term applications. Moreover, dopants,
such
as
lithium-bis(trifluoromethane)sulfonimide,
are
Herein, a Ni-Co-O hole conductor has been developed as an
inexpensive and solution-processable inorganic HTM for PSCs
necessary for these HTMs to offer sufficient hole mobility.
(
Fig. 1). Ni-Co-O hole conductors are refered to NiO
and CoO with different Co and Ni ratios. We found that the
incorporation of Co species into NiO provided unique gap
X x
, NiCoO
X
a. Key Laboratory for Ultrafine Materials of Ministry of Education, School of
Materials Science and Engineering, East China University of Science and
Technology, 130 Meilong Road, Shanghai 200237, China. E-mail:
x
states above the valance bands (VB) region, which can
facilitate holes extraction from perovskite films. Furthermore,
the formation of Ni and Co antisites with varied charge state
could modify the electronic structure and improved the
electrical conductivity of the HTM layer by over one order of
magnitude. Owing to the large enhancement in the charge
extraction and conductivity, the inverted planar PSC devices
hgyang@ecust.edu.cn
b.
Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials,
Ministry of Education, School of Materials Science and Engineering, Shandong
University, Jinan, 250061, P. R. China. E-mail: syang@sdu.edu.cn
Centre for Clean Environment and Energy, Gold Coast Campus, Griffith University,
Queensland 4222, Australia
c.
d.
Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese
Academy of Sciences, Beijing, 100049, China
†These authors contributed equally to this paper
Electronic
Supplementary Information (ESI) available. See DOI: with NiCoO HTM layers exhibited a champion PCE of 20.03%,
x
1
0.1039/x0xx00000x
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Fig. 1 a) Device structure of perovskite solar cells. B) Energy-level diagrams of perovskite solar cells (relative to the vacuum level). The
dashed line shows the fermi level of the hole conducting materials deposited on FTO glass. The occurrence of gap states in NiCoO
x x
and CoO
could provide electronic pathways for hole transfer from perovskites.
are located at ~0.44 and ~0.59 eV below the Fermi level for NiCoO
which is significantly better (> 16%) compared with to controls and CoO , respectively. Assuming these gap states are normally
with NiO layer (17.14%). distributed, the energy levels of these states are centered at about -
.40 and -5.49 eV for NiCoO and CoO , both of which are well
aligned to the VB of MAPbI (Fig. 1b and Fig. S6b).
Steady-state and time-resolved photoluminescence (TRPL) decay
x
x
x
5
x
x
3
Results and discussion
The Ni-Co-O films in this work were prepared by a simple spin- measurements (Fig. S7) were then performed to reveal the charge
coating process of Ni and Co precursor, followed by thermal transfer behaviors of different HTM layers. With the presence of Co
annealing (see more details in the experimental section). The species in HTM layer, PL quenching behavior in the perovskite films
concentration of all metal ions in the precursor solution was found were dramatic in which the PL intensity of NiCoOx and CoOx
to be 0.214 M, as determined by the performances of the resultant samples were ~8.6% and ~3.4% relative to that of NiO , at excitation
x
devices (Fig. S1). Scanning electron microscope (SEM) images of the wavelength of 515 nm (Fig. S7a). As shown in Fig. S7b, the TRPL
NiO , NiCoO and CoO films revealed similar compacted and decay of perovskite films with different HTM layers showed a bi-
x x x
conforming surface morphologies (Fig. S2). In order to investigate exponential decay with a fast and a slow component and was fitted
the structure of HTM, we performed X-ray diffraction (XRD) to a biexponential equation: Y = A exp(−t/τ ) + A exp(−t/τ ).25 We
1
1
2
2
measurements. Only the diffraction peaks of SnO from fluorine- attributed the quenching of the fast decay process to the fast
2
doped tin oxide (FTO) can be observed in Fig. S3, which indicate charge transfer from perovskite into the hole conductor layers, and
that the as-prepared Ni-Co-O materials are amorphous. the slow decay to bimolecular radiative recombination. Evidently,
Furthermore, the identities of these films were confirmed by the HTM layers had a major influence on the fast decay process
Raman spectroscopy which showed the presence of CoO
as evident from their signature Raman bands (Fig. S4).
x
and NiO
x
which we attributed to the matching energy level alignment
between the gap states induced by Co species and the perovskite
layer, thus facilitating the charge transfer at the interface. The fast
Ultraviolet photoelectron spectroscopy (UPS) and ultraviolet-visible
UV-vis) spectroscopy measurements were carried out to
investigate the energy levels of the NiO , NiCoO and CoOx layers on
decay lifetime of NiCoO
and 2.7 ns, which is 4.8 and 8.0 times faster than the NiO
21.7 ns). Additionally, the NiO NiCoO and CoO
x
and CoO
x
samples were calculated to 4.5
sample
samples
(
x
x
x
(
x
,
x
x
FTO substrates. The optical band gaps of NiOx, NiCoOx and CoOx
were measured to be 3.66, 2.70 and 2.30 eV, respectively (Fig. S5).
As shown in Fig. S6a, the sample work functions were determined
from their secondary electron cut-offs, and their valence band
displayed similar slow decay lifetimes of 22.6, 23.6 and 27.3 ns,
respectively, suggesting that the HTM layers did not affect radiative
recombination velocity in the perovskite films.
maximums were determined from their the main on-set of phot X-ray photoelectron spectroscopy (XPS) was employed to probe the
electrons. The energy levels of HTMs and the perovskite layer (Fig. surface electronic structure of Ni-Co-O HTM samples. As shown in
S6b) shows that the Fermi levels of NiCoO
.38 eV) layers are higher compared to that of pristine NiO
eV) and are accompanied by lowering of their VB edges (with films obtained via solution methods. The peaks centered at binding
x
(-4.39 eV) and CoO
x
(- Fig. 2a, the Ni 2p3/2 peaks can be resolved into three distinct
4
x
(-4.07 components which are consistent with previous reports for NiOx
respect to vacuum level). Most intriguingly, apparent broadening of energy of ~853.2 eV and ~855.1 eV are attributed to Ni2+ and Ni3+
,
VB edges for the Co-containing samples can be observed, which respectively.26,27 It is noteworthy that the atomic ratio of Ni3+ was
suggests the existence of gap states. The edges of these gap states
2
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Fig. 2 a,b) The XPS spectra of (a) Ni 2p and (b) Co 2p for the samples of CoO
d) Co K-edge for the samples of NiCoO
XAFS oscillations for the samples of NiCoO
by half. The xy dimensions of the analyzed area of the films are 100 × 100 μm2.
x
and NiCoO
x
. XANES spectra in the range of (c) Ni K-edge and
(
x
and controlled samples. Corresponding Fourier transforms of (e) Ni K-edge XAFS and (f) Co K-edge
and controlled samples. Notes: the intensities of Ni foil and Co foil in (e) and (f) were reduced
x
x x
increased from ~63.7% of NiO film to ~78.7% of NiCoO film different types of bond distances (Ni-Ni or Ni-Co bonds). While in Co
determined by the integral area of XPS peaks. The XPS spectra of Co K-edge range, three FT peaks located at ~ 1.5 Å, ~ 2.5 Å and ~ 3.0 Å
2
p (Fig. 2b) show four fitted peaks at binding energies of ∼778.9, can be attributed to Co-O, Co-Co and Co-Ni, respectively (Fig. 2f).
3
+
∼
780.7, ∼794.2 and ∼795.9 eV, which correspond to Co 2p3/2
,
2+
This phenomenon suggests that the closest neighboring atoms of
Co atoms in NiCoO are similar with Co and CoO samples.
are estimated to be 0.50 and 0.61, Regarding the enhanced bulk valence states of Ni in NiCoO films
respectively. These observations suggest that Ni atoms may occupy are in agreement with the XPS analysis. Hence, we can reasonably
2
+
3+
2+
28
3+
Co 2p3/2, Co 2p1/2 and Co 2p1/2, respectively. The Co :Co
ratios for the NiCoO and CoO
x
3
O
4
x
x
x
x
3
+
3
3+
3+
x
Co sites and lead to higher Ni but lower Co ratios in NiCoO film. conclude that more Ni sites were formed at the expense of the
The O 1s high-resolution spectra (Fig. S8) show four distinct Co sites.
components of O1, O2, O3 and O4 ascribed to metal-oxygen bonds,
OH- groups, defect sites with low oxygen coordination, and
multiplicity of physi- and chemi-sorbed water, respectively.
3
+
Solar cells with the various sandwiched HTM layers were fabricated
with an inverted planar heterojunction configuration
FTO/HTM/MAPbI /phenyl-C61-butyric acid methyl ester (PCBM)/
2
6,29-32
[
3
To further investigate into the bulk electronic structures, X-ray bathocuproine (BCP)/Ag]. The UV-vis absorbance spectra of
absorption near edge structure (XANES) spectra were collected. The perovskite films deposited on the different HTM layers exhibited in
pre-edge of NiCoO
x
films in the Ni K-edge spectra shifted to higher slight light absorbance efficiency depression caused by
samples, revealing a incorporation of Co (Fig. S9). The J-V curves of the PSCs with
higher bulk valence states than Ni (Fig. 2c). Meanwhile, the bulk different Co contents were shown in Fig. 3a, and the photovoltaic
valence of Co species in NiCoO resembles that of Co and CoO parameters were summarized in Table S1. The NiO device had a
controlled samples (Fig. 2d). In addition, the Fourier-transformed short-circuit current density (Jsc) of 22.71 mA cm , an open-circuit
FT) spectra extended X-ray absorption fine structure (XAFS) spectra voltage (Voc) of 1.035 V and a fill factor (FF) of 0.730, yielding a
absorption energy compared to NiO and NiO
x
2
+
x
3
O
4
x
x
−2
(
of all the samples were collected and analyzed. Interatomic power conversion efficiency (PCE) of 17.14%. With the addition of
distances are shorter owing to the fact that FT spectra were not Co from 25% to 50%, the highest recorded Jsc, Voc, and FF of the
−2
phase-corrected. In Fig. 2e, the first (at ~ 1.5 Å) and second (at ~ 2.5 resulted devices improved from 21.95 to 22.27 mA cm , 1.045 to
Å) peak can be attributed to the single scattering paths of the Ni-O 1.080 V and 0.780 to 0.833, respectively, improving the PCE from
and the closest neighboring transition metals around absorbing 17.89% to 20.03%. Further increase of Co content resulted in
atoms Ni (Ni-Ni), respectively. It is noted that the third peak (at ~ poorer performance: PCE of devices with 75% Co content reduced
3
.0 Å) can only be observed in NiCoO
x
films, suggesting that the to 17.12%, whereas the pure CoO
x
devices produced a low
closest neighboring transition metals of Ni atom consisted of two efficiency of only 9.95%.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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x
precursors (Fig. 4a). Interestingly, CoO film was
DOI: 10.1039/C9TA07331K
found to exhibit low electrical conductivity of
-8
-1
~
3.8×10 S m . Mott−Schottky plots of the oxide
films are generated based on the capacitances
under AC condition (Fig. 4b). All three samples
show obvious P-type electrical characteristic. The
carrier density can be calculated according to the
3
5
following equation:
2
2
-1
N = (2/e
where e
constant, ε
0
εε
0
A ) [d(1/C )/dV]
(1)
0
is the electron charge, ε is the dielectric
is the permittivity of vacuum, N is the
0
carrier density, and C is the capacitance derived
from the electrochemical impedance. The carrier
1
8
-3
density of NiO
comparable to that of CoO
whereas, the carrier density of NiCoO
x
of 2.1×10 cm is lower
1
8
-3
x
films of 6.1×10 cm ,
x
1
8
-3
significantly larger (10.2×10 cm ) approaching
the carrier density of metallic materials (>10
1
9
-
3
cm ).
Fig. 3 Photovoltaic performance. a) J–V curves of PSCs based on NiCoO
different Co contents. b) J-V metrics for 15 independent devices based on different
HTM layers. c) J–V curve (reverse and forward scans) of the best NiCoO device
with Co content of 50%. All J–V measurements were operated under AM 1.5G
irradiation with a reverse scan rate of 0.15 V s−1 for small-area (0.0625 cm-2)
devices.
x
, films with
We continued to measure the devices under
operational conditions using electrochemical
impedance spectroscopy (EIS) to identify the
recombination behavior in devices and the typical
Nyquist plots of perovskite devices based on
x
different HTMs are depicted in Fig. 4c. The
Fig. 3b shows the J-V metrics for 15 independent devices. Similar
trend was observed for species Voc values are 1.038, 1.039, 1.068,
.943 and 0.974 V and the mean FF are 0.699, 0.768, 0.785, 0.769
and 0.452 with the Co content of 0%, 25%, 50%, 75% and 100%,
respectively. The results clearly show the synergistic improvement
x x
of HTM performance with the optimized 50% CoO and NiO
experiment data were fitted using the equivalent circuit shown in
the inset in Fig. 4c. The Nyquist plots typically show two distinct
arcs at low and high frequency for perovskite devices. R is an
s
additional contribution from series resistance. The high frequency
arc in the Nyquist plots is ascribed to the diffusion of holes through
0
36
content in which the role of Co can help to optimize the charge
transfer at the interface and enhance the FF and Voc as expected.
x
On the other hand, the NiO also plays an equal important role,
which may be related to charge transport and reduction of the
3
3,34
surface trap states.
The champion PCE of 20.03% was achieved by using NiCoO
x
layer
−2
with a Jsc of 22.27 mA cm , Voc of 1.080 V, and FF of 83.3 %
under forward and reverse scanning with negligible hysteresis, as
shown in Fig. 3c. The Jsc of the device was also confirmed by
external quantum efficiency (EQE) spectra, with an integrated
-2
photocurrent of 21.2 mA cm (Fig. S10). Furthermore, this NiCoO
x
device was held at a forward bias of 0.894 V to monitor the
stability of power output as a function of time (Fig. S11). The
photocurrent stabilized within seconds to a value of approximately
−2
2
1.23 mA cm that is similar to that obtained by the J-V tests. A
stabilized PCE of 18.99% was measured upon 100 s.
For photovoltaic devices, charge transport loss in bulk carrier
transport layers could be another key factor to evaluate a hole
transport layer and thus it is critical to examine the electrical
conductivity of the HTM films. All electrical conductivity test are
conducted based on the structure of FTO/HTM/Au under dark
Fig. 4 Charge carrier transport behavior. a) J-V curves of NiO
NiCoO and CoO films deposited on FTO glass. The thickness was
20 nm for all metal oxide films. Insets is the corresponding
electrical conductivity extracted from the J-V results. b) C-V
profiling of NiO , NiCoO and CoO films. c,d) Nyquist plots and
recombination resistance (Rrec) of devices based on NiO , NiCoO
x
,
x
x
1
condition. The conductivity of the NiO
increase over one order of magnitude from ~1.8×10 S m to
x
film was estimated to
x
x
x
-7
-1
x
x
-6
-1
~
2.5×10 S m by substituting 50% of Ni with Co species in the
x
and CoO HTM layers.
4
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1
2
3
4
5
6
7
8
9
A. Kojima, K. Teshima, Y. Shirai and T. Miyasaka,VJie.wAAmrti.clCe hOenlmine.
the HTM layer which is modeled by a charge transfer resistance (Rtr
and chemical capacitance (Ctr). The lower frequency arc is ascribed
to the recombination resistance (Rrec) and chemical capacitance (C
which are associated with the recombination of the carriers in the
whole device. Normally, the Rtr is almost indistinguishable due to
the simplified transmission line model.
)
Soc., 2009, 131, 6050-6051.
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The Rrec under different applied bias voltages extracted from
nyquist plots are listed in Fig. 4d. The Rrec of NiCoO
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x
based device is
x
x
concluded that that excess Co contents generate the trap states,
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Conflicts of interest
The authors declare no competing financial interest.
2
2
2
2 G. A. Sepalage, S. Meyer, A. Pascoe, A. D. Scully, F. Huang, U.
Bach and L. Spiccia, Adv. Funct. Mater., 2015, 25, 5650-5661.
Acknowledgements
This work was financially supported by National Natural 23 S. Ye, W. Sun, Y. Li, W. Yan, H. Peng, Z. Bian and C. Huang,
Nano Lett., 2015, 15, 3723-3728.
Science Foundation of China (51602103), National Natural
Science Funds for Distinguished Young Scholar (51725201),
Young Elite Scientists Sponsorship Program by CAST
2
4 R. Ihly, A. M. Dowgiallo, M. Yang, P. Schulz, N. J. Stanton, O.
G. Reid and J. L. Blackburn, Energy Environ. Sci., 2016, 9,
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439-1449.
(
2017QNRC001), Shanghai Pujiang Program (18PJD009),
Fundamental Research Funds for the Central Universities
222201718002), the Major Research plan of National Natural
2
5 Q. Han, S.-H. Bae, P. Sun, Y.-T. Hsieh, Y. Yang, Y. S. Rim, H.
Zhao, Q. Chen, W. Shi, G. Li and Y. Yang, Adv. Mater. 2016,
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Science Foundation of China (91534202) and Shanghai
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Table of Contents
An inorganic NiCoO
x
hole conductor was developed as HTM in PSCs. The champion
device yielded a PCE of 20.03%.