13
Bull. Soc. Photogr. Imag. Japan. (2019) Vol. 29 No. 2: 13–19

Original Paper

Characterization of Hole Transport Layer in Perovskite Solar Cell via Its Performance and Electronic Structure
Yuka Yamano*, Tadaaki Tani**, Takayuki Uchida*

Abstract: hole transport layer (HTL) composed of spiro-OMeTAD as a hole transport material (HTM) in a perovskite solar cell (PSC) withA
CH3NH3PbI3 as a light-absorber was characterized by observing the performance of the cell with variation of the condition in HTL andcorrelating the result with the electronic structure of the layers, through which positive holes are transferred. While the time-dependent increase in the work function of the Au anode, the doping of a spiro-OMeTAD layer with Li-TFSI and tBP, and the light irradiation increased
the short-circuit current and the open-circuit voltage, they made the Fermi level of the HTL deeper and closer to the HOMO level of the
spiro-OMeTAD layer. Namely, the changes in the performances and the electronic structures are in accord with each other to indicate thatthe transport of positive holes in the spiro-OMeTAD layer was disturbed by positive hole traps in the layer, and could be improved by thechanges in the condition of the layer, which decreased the depth and density of positive hole traps. It is noted that the Fermi level of the
HTL with high performance should be much lower than that of the systems in equilibrium with ambient atmosphere, providing a problem
to be solved for the stabilization of PSC.

Key words:

Perovskite solar cell, Hole transport layer, Doping, Electronic structure, Photoelectron yield spectroscopy in air

1. Introduction
Solar cells with perovskite materials as light-absorbers1)
have rapidly become to attract keen interests of many researchers because of their significant performances such as
high power conversion efficiency (PCE), low cost, and easy
attitude for production2) -8). Perovskite solar cell (PSC) was
developed by replacing Ru-complexing dyes in dye-sensitized solar cell (DSC) by perovskite materials such as CH3NH3PbI3 1). Since perovskite materials tend to decompose
when they are in contact with solvents9), an electrolyte solution used for a hole-transporting layer (HTL) in DSC wasthen replaced by a solid-state hole-transporting material(HTM) such as 2,2',7,7'-tetrakis-(N,N-di-p-methoxyphenylamine) 9,9'-spirobifluorene (spiro-MeOTAD)10)-16) and CuSCN17) 18).
While CuSCN acting as a p-type semiconductor is available
for HTM by itself, it has been reported that spiro-MeOTAD
is electrically resistive and becomes to be available for HTL
on doping14)-16). However, studies on the interaction between
spiro-MeOTAD and dopants13) have not yet provided sufficient knowledge to make clear the mechanism of the doping.
One of the authors (TT) made an overview of PSC19) fromthe viewpoint of dye sensitization of silver halide (AgX) photographic systems with cyanine dyes and AgX grains as
sensitizers and substrates20), respectively, since the former
has been derived from the latter via photo-catalyst with

TiO2 as a substrate21), DSC with Ru-complexing dyes andTiO2 as sensitizers and substrate, respectively22). While theoverlapping of electronic clouds among cyanine dyes in
AgX photographic systems and Ru-complexing dyes in DSC
is small, the electronic energy levels of the lowest unoccupied molecular orbital (LUMO) and the highest occupied one(HOMO) of a sensitizing dye molecule with respect to the
conduction band minimum (CBM) and the valence band
maximum (VBM) of a substrate are essentially important forthe study of the light-induced electron transfer from the dyemolecule to the substrate. On the other hand, perovskite
materials in PSCs are substantially inorganic semiconductors from the viewpoint of their electronic structures5)19), it isnecessary to treat a perovskite material by making notice of
its Fermi level in addition to its CBM and VBM. However,such treatment has not yet been made well.
In this paper, we have studied the electronic structure
and behavior of an HTL in a PSC with CH3NH3PbI3 as a lightabsorber according to the above-stated consideration in order to enhance our understanding of the mechanisms of thedoping of spiro-MeOTAD layers and the transport propertyof positive holes in HTLs of PSC.

Received 1st May 2019; Accepted 4th October 2019.
*Graduate School of Engineering, Tokyo Polytech. Univ., 1583 Iiyama, Atsugi, Kanagawa 243-0297, Japan**Fellow, Soc. Photogr. Imaging Jpn., 2933-2 Yoshidajima, Kaisei-machi, Ashigarakami-gun, Kanagawa 258-0021, Japan

14

Bull. Soc. Photogr. Imag. Japan. Vol. 29 No. 2 (2019)

pared by depositing Au in vacuum on the surface of the
HTL.

2. Materials and Experiments
2.1 Preparation of PSC
The structure of the PSCs, which were prepared in thisstudy, was a mesoporous type as shown in Fig. 1. A compact
TiO2 layer and then mesoporous TiO2 layer were formed on
an FTO glass, which are composed of pyramid-shaped particles of transparent conductive metal oxide (TCO) on a
glass plate to enhance the scattering of incident light, andare used as a transparent electrode for solar cells (AGC Fabric Type-VU). A compact TiO2 layer was formed on a hot
plate at 400 ℃ by means of a spray thermal decomposition
method with a solution of 0.6 ml of titanium (Ⅳ) diisopropoxide bis (acetylacetonate) (Tokyo Chemical Industry Co., Ltd.)in 8 ml of ethanol. A mesoporous TiO2 layer with mean pore
diameter of 18 nm was formed by spin-coating a suspensionof 2g of TiO2 paste containing 17w% of TiO2 particles with
〜20 nm in size in water (JGC Catalysts & Chemicals Ltd.,
PST-18NR) in 7g of ethanol with 5000 rpm at 25 ℃ and annealing it at 500 ℃ for 20 min. Then, a layer of CH3NH3PbI3
was formed on the above-stated layers by the one step
method. Namely, a solution of CH3NH3I (Wako Pure Chemical Corp.) and PbI2 (Sigma-Aldrich) (mole ratio of 1:1) in DMF
dehydrated (Wako Pure Chemical Corp.) (45 wt%) was coated on the above-stated layers by a spin-coater with 2000 rpm
for 30 sec and heated on a hot plate at 70 ℃ for 30 min tovolatilize DMF. An HTL layer and an anode were thenformed on the above-stated perovskite layer. In this paper,spiro-OMeTAD and Au were used as HTL and anode, respectively. A layer of spiro-OMeTAD (LT-S922, Luminescence Technology Corp.) was formed by spin-coating its
chlorobenzene solution (12 wt%) with 3000 rpm for 30s. In
this study, Li-TFSI and tBP23) 24) were used as dopants for an
HTL layer of spiro-MeOTAD. The former was added to a
final concentration of 0.064M, while the latter was added toa final concentration of 0.193M. The chemical structures ofthese compounds are shown in Fig. 1. An anode was pre-

Au

Anode

spiro-OMeTAD doped with LiTFSI,tBP)+

HTL

CH3NH3PbI3-

mesoporous-TiO2compact-TiO2

ETL

TCO

FTO

hν

F3C

S
O

spiro-OMeTAD

CH3

O

O

N
Li

2.2 Analyses of HTL in PSC
The performances of the above-stated cells such as current-voltage (I-V) characteristics, open-circuit voltage (Voc),
short-circuit current (Jsc), fill factor (FF), and power conversion efficiency (PCE) were evaluated by means of a solar
simulator (HAL-320, Asahi Spectra), an apparatus for themeasurement of I-V characteristics (Peccell IV Analyzer 2.2
and Keithley 2400) with 1SUN (100 mW/cm²) and AM (AirMass) 1.5G. The light intensity was calibrated by the 1SUN
checker, which was attached to HAL-320. The performancesof the above-stated cells were traced for 20 days in room air
without any barrier film to get the information of their stability.
The work function, i.e., the energy difference between thehighest occupied level (Fermi level for Au, VBM for CH3NH3PbI3, and HOMO for spiro-OMeTAD) and the vacuum level were measured by means of photoelectron yield spectroscopy in air (PYSA; Riken Keiki AC-2) 25) and compared withthe performances of the cells in this study. Every PYSA
measurement was carried out in room air at room temperature in the range from 3.80 to 6.20 eV with energy step of0.05 eV by use of a light source (10 nW). Photoelectrons arecaptured by oxygen molecules in air to form superoxide
radicals (O2-*). Then, superoxide radicals are collected anddissociated for the measurement of the yield of photoelectrons in an open counter. The distribution of the measured
values for a sample with different experimental condition
was small. The standard deviation for the measurement of
the work function of an Au layer was ±0.02 eV. The work
function of a thin layer of Au used as an anode in the cell
was traced for 20 days in room air. The electronic structures of CH3NH3PbI3/HTM/Au, through which positive
holes were transported in the cell, were studied as follows.
The measurement of the work functions of CH3NH3PbI3,
HTM, and Au layers on glasses was made by means of
PYSA, and followed by the measurement of the work functions of CH3NH3PbI3/HTM and CH3NH3PbI3/HTM/Au. The
electronic structures of CH3NH3PbI3/HTM/Au was constructed with respect to the vacuum level on the basis of theresults of the above-stated measurements by taking into
account the Fermi levels of CH3NH3PbI3, which should beequalized to those of spiro-OMeTAD and Au to transfer
positive holes.

CF3

S

H3C

CH3

3. Results and Discussions

O
N

Dopants (LiTFSI, tBP)

Fig. 1 C ross sectional diagram showing the structure of a perovskite solar
cell studied (upper), and chemical structures of spiro-OMeTAD,
Li-TFSI, and tBP used for its hole transport layer (lower).

3.1 Change in condition of HTL
It is demonstrated in this study that the performance of
PSC was markedly influenced by the following three kindsof changes in the positive hole transport in the cells; thetime-dependent change in the work function of an Au anode,

40

0.5

Photoelectron yield /cps

0.5

the doping of a spiro-OMeTAD layer with Li-TFSI and tBP,
and the irradiation of the cells to light.
Fig. 2 shows the time-dependent change in the work function of a thin Au layer in room air, indicating that the work
function is increasing with time after its preparation. It is
known that the work function of pure Au in vacuum is 5.1
eV26), while that of a thin Au layer in room air becomes to beabout 4.8 eV owing to the pinning of its Fermi level to thatof an organic layer deposited from air on its surface27). According to a jellium model, the outermost surface region inside a metal is positively charged, and the same amount of
the negative charge with electrons is spilled out to severalÅ in vacuum, forming an electric double layer26). Then, thework function of a metal layer is decreased by the pushback effect, according to which the electric double layers
thus formed are depressed by a phenomenon where spilled
electrons are pushed back to the inside of the metal whenthe metal is in contact with an organic layer 26) 27). It is considered that the work function of a thin Au layer first decreased owing to the push-back effect when it was in contact with a thin organic layer coming from air, and thenincreased owing to the pinning of its Fermi level to that of
the organic layer coming from air27).
Fig. 3 shows the time-dependent change in the I-V characteristics of the cell, indicating that both Voc and Jsc increased during its storage for 4 days after its preparation.
The increase of Voc with time was in accord with the in-

(b)

day 0

30

1

3

15 4

(a)
2 20

6
8

20
10

0 4.4

4.8

5.2

Energy /eV

5.6

6.0

(b)

4.85
4.75
4.65

4.55
0

5

10

Time / day

10

day 0
day 4
day 9

8
6

4
2
0
0.0

0.2

0.4

0.6

0.8

Voltage/ V

4.95
Work function /eVcrease in the work function of a thin Au layer with time after its preparation (Fig. 2). While the big change in work
function taking place for such a short period as seen in Fig.
2 is quite probable and verified for metals including Au in air
27), it is not considered to be probable for organic materials
available for electronic devices. It is therefore consideredthat the time-dependent increase in the work function of the
Au anode in the cell brought about the time-dependent increase in Voc. While the work function of a thin Au layer
kept for 20 days after its preparation was larger than thatof the layer kept for 4 days, Voc of the cell kept for 20 daysin air after its preparation was smaller than that of the cell
kept for 4 days. It is considered that this discrepancy arosefrom the fact that the degradation of a CH3NH3PbI3 layer inthe cell was in progress as observed in the previous paper28)
since the cells were kept in air without any barrier film.
Then, the HTL composed of a thin spiro-OMeTAD layer
was doped with Li-TFSI and tBP23) 24) according to the procedure described in the previous section. Two cells with
undoped and doped spiro-OMeTAD layers as HTLs wereprepared and subjected to the measurements of their performances at different 16 positions in each cell. The results
are shown in Fig. 4. The averaged values and their coefficients of variation are listed in the tables in Fig. 4. The
fluctuation of the performances of the cells with an un-doped
spiro-OMeTAD layer was large, especially for Jsc, and could
be much improved by the doping.

0.78
Voltage/ V

(a)

15

Characterization of Hole Transport Layer in Perovskite Solar Cell via Its Performance and Electronic Structure

Current density/ mA cm-2

Yuka Yamano, Tadaaki Tani, Takayuki Uchida

15

20

Fig. 2 T ime-dependent change in the PYSA spectrum (a) and work function (b) of a thin Au layer, which was prepared in vacuum and thenkept in ambient atmosphere for the time period indicated on each
spectrum (a) and the abscissa (b).

0.76
0.74
0.72
0.70
0

3

6

9

Time /dayFig. 3 Time-dependent changes in J-V curve characteristics (a) andopen-circuit voltage (b) of a perovslite solar cell composed of FTO/
TiO2/ CH3NH3PbI3/spiro-OMeTAD/Au.

16

Bull. Soc. Photogr. Imag. Japan. Vol. 29 No. 2 (2019)

Fig. 5 shows the PYSA spectra of an un-doped spiro-OMeTAD layer, CH3NH3PbI3/un-doped spiro-OMeTAD layers,
and CH3NH3PbI3/doped spiro-OMeTAD layers. While thework function of spiro-OMeTAD was hardly influenced byits contact with MAPbI3, it was significantly increased by
the doping. It is considered from this result that the dopants
drew electrons from the spiro-OMeTAD layer to bringabout the increase in its work function and to decrease thephotoelectron yields from it. Fig. 6 shows the PYSA spectraof un-doped spiro-OMeTAD/Au layers, CH3NH3PbI3/undoped spiro-OMeTAD/Au layers, and CH3NH3PbI3/doped
spiro-OMeTAD/Au layers. The doping significantly decreased the photoelectron yield from Au layer in this system, and increased the work function of the Au layer. It isconsidered from this result that the dopants drew electrons
from the spiro-OMeTAD layer to lower its Fermi level andthat of the Au anode, decreasing the photoelectron yieldfrom the Au anode.
Fig. 7 shows the effect of irradiation on the PYSA spectraof the PSCs with undoped spiro-OMeTAD layer as an HTL.
As seen in this figure, the irradiation increased the work
function of the Au anode of the cell and decreased the photoelectron field from it. It is considered from the result inthis figure that positive holes created in the CH3NH3PbI3layer in the cell were transferred to the spiro-OMeTAD

layer and captured by traps to deepen the Fermi levels of
the spiro-OMeTAD layer and the Au anode of the cell, andthat the above-stated condition created by the light irradiation was kept for a relatively long time (more than 5 min).
3.2 Electronic structures of HTL
Fig. 8 (a) and (b) show the electronic structures of CH3NH3PbI3/undoped spiro-OMeTAD/Au and CH3NH3PbI3/

4.98
Undoped-spiro
MAPbI3/ Undoped spiro 5.00
spiro 5.34
MAPbI3/Fig. 5 P YSA spectra of an undoped spiro-OMeTAD layer, CH3NH3PbI3/
undoped spiro-OMeTAD layers, and CH3NH3PbI3/doped
spiro-OMeTAD layers. An inserted table shows their work functions.

(a)Doping of spiro (HTM)(b)

Fig. 4 I -V characteristics of perovskite solar cells with undoped (a) and doped (b) spiro-OMeTAD/Au layers as measured at 16
different points on each cell. Upper and lower tables show the average values and coefficients of variation (CV) of theperformances of the cells with I-V characteristics shown in (a) and (b), respectively.

Yuka Yamano, Tadaaki Tani, Takayuki Uchida

17

Characterization of Hole Transport Layer in Perovskite Solar Cell via Its Performance and Electronic Structure

0.58

Undoped-spiro/ Au
4.65
MAPbI3 /Undoped-spiro/ Au 4.52
MAPbI3 / Doped-spiro/Au 5.25

Electronic energy [eV]

-0

-0

-0.02

4.87
4.99
5.00

Fig. 7 P YSA spectra of a perovskite solar cell composed of FTO/TiO2/
CH3NH3PbI3/spiro-OMeTAD/Au before and after exposure tolight, and after 5 min since the exposure was ceased.

doped spiro-OMeTAD/Au layers, respectively, as constructed with respect to the vacuum level according to the process described in Section 2.2. In Fig. 8 (a), the ionization energies of CH3NH3PbI3 and spiro-OMeTAD were measured inthis study as 5.52 eV and 4.98 eV, respectively. The ionization energy of Au was reported as 5.1 eV26). The ionizationenergy of spiro-OMeTAD on CH3NH3PbI3 was measured as
5.00 eV, indicating the vacuum level shift as - 0.02 eV. The
ionization energy (i.e., work function) of Au on spiro-OMeTAD/CH3NH3PbI3 was measured as 4.52 eV, indicating thatthe vacuum level shift between spiro-OMeTAD and Au was
0.60 eV, and that the Fermi level of spiro-OMeTAD was by
0.48 eV higher than its HOMO level. The electronic structure in Fig.8 (b) was constructed similarly.
While the orientation of the single crystals of metals andorganic semiconductors influence their work functions30),
they are not taken into account for the construction of theelectronic structures in Fig. 8, since the samples were amorphous or polycrystalline with randomly oriented small crystallites. As seen in Fig. 8 (a), the Fermi level of the Au anode

-0.36

-3.95

-3.95

EF
-5

-4.52

Au

EF

-5

-5.00

-5.52

Fig. 6 P YSA spectra of undoped spiro-OMeTAD/Au layers, CH3NH3PbI3/undoped spiro-OMeTAD/Au layers, and CH3NH3PbI3/doped
spiro-OMeTAD/Au layers. An inserted table shows their work
functions.

-0.15

Undoped CH NH PbI
3
3
3
spiroOMeTAD

-5.25
Au

-5.34

-5.52

Doped
CH3NH3PbI3
spiroOMeTAD

Fig. 8 E lectronic structures of (a) CH3NH3PbI3/undoped spiro-OMeTAD/
Au layers and (b) CH3NH3PbI3/doped spiro-OMeTAD/Au layers,
where Ef indicates the position of Fermi level in a spiro-OMeTAD
layer.

in the layer with an undoped spiro-OMeTAD, which wasmeasured immediately after the preparation of the film, was-4.52 eV, being higher by 0.48 eV than the HOMO level of a
spiro-OMeTAD in the layer. This result indicates that positive hole traps with ≦0.48 eV in depth were present andshould hinder the transport of positive holes in an undoped
spiro-OMeTAD layer by trapping them. The Fermi level ofthe Au anode was close to that of Au layer immediately after its preparation, which then increased by 〜0.3 eV on its
storage in air for 4 days as seen in Fig. 2.
The comparison between Fig. 8 (a) and (b) reveals that the
dopants could lower the Fermi levels of, not only spiro-OMeTAD layer, but also Au anode and CH3NH3PbI3 layer bydrawing electrons from the spiro-OMeTAD layer. In the undoped spiro-OMeTAD layer, positive hole traps with ≦0.48
eV in depth were present and captured positive holes. On
the other hand, only traps with ≦0.09 eV in depth were
present and captured positive holes in the doped spiro-OMeTAD layer. This result could provide an explanation for theobserved result that the values of Jsc of the cell could besignificantly improved and their coefficient of variation
could be much decreased by the doping. In addition, it isconsidered that the big increase in the work function of the
Au anode by the dopants contributed to the increase of Voc
of the cell by the dopants.
As discussed above, the electronic structure of a
spiro-OMeTAD layer in a device could be evaluated and hasturned out to be useful for studying the behavior of positive
holes in the layer responsible for the performance of thedevice. The usefulness of the knowledge of the electronic
structure should be further enhanced when it would be supported by the measurements of the conductivity, mobility ofpositive holes, number and depth of positive hole traps in a
spiro-OMeTAD layer.

18

Bull. Soc. Photogr. Imag. Japan. Vol. 29 No. 2 (2019)

4. General Discussions
In order to characterize the performance of a PSC fromthe viewpoint of its electronic structure, the performance ofa PSC composed of FTO/TiO2/CH3NH3PbI3/spiro-OMeTAD/Au was studied with variation of factors relating to
the layers, through which positive holes were transferred.
Then, the results were correlated with the electronic structure of CH3NH3PbI3/spiro-OMeTAD/Au layers, which wereconstructed from the PYSA spectra of individual and combined layers of their components, as shown in Fig. 8.
The electronic structures of the CH3NH3PbI3/spiro-OMeTAD/Au layers were constructed under the ussumptions
that the Fermi levels were present in individual layers of its
components and brought about the electron transfer phenomena to equalize the Fermi levels among them. Since theelectronic structure of CH3NH3PbI3 is substantially an inorganic semiconductor, its Fermi level was measured and discussed in many papers19) 29). Although spiro-OMeTAD is anorganic semiconductor, the Fermi level of a spiro-OMeTAD
layer and its role in the electronic phenomena in the layer
were discussed in the literature13) 15). It is therefore considered that the equalization of Fermi level took place at theinterface between the spiro-OMeTAD/CH3NH3PbI3 layers
as well as the interface between the spiro-OMeTAD/Aulayers.
The Fermi level equalization should bring about the shiftof the vacuum levels of the layers involved26) 30). The shift
takes place at the interface between them withoutband-bending in the layers when the transferred electrons
are mostly located at the interface. On the other hand, theshift takes place between the bulks with band-bending in
the layers involved when the transferred electrons located,
not at their interface, but in the bulks of the layers involved.
Kahn and others did not observe any band-bending in a
CH3NH3PbI3 layer when the Fermi level equalization took
place on its contact with a TiO2 layer29). It is therefore considered that the Fermi level equalization between CH3NH3PbI3 and spiro-OMeTAD did not bring about any band-bending in the former. Seki and Ishii observed the vacuum level
shift at the interface between many organic semiconductors
and substrates when they were in contact with each other26).
In this paper, it is therefore considered that the Fermi level
equalization of a spiro-OMeTAD layer with a CH3NH3PbI3layer and an Au layer took place to cause the vacuum level
shifts at the interfaces, as shown in Fig. 8. Fig. 8 shows thevacuum level shifts at the interfaces between spiro-OMeTAD and CH3NH3PbI3 layers and those between spiro-OMeTAD and Au layers without and with Li-TFSI, which havebeen observed for the first time to our best knowledge, andreflected the electron transfer across the interfaces. The
observed vacuum level shift at the interface between undoped spiro-OMeTAD and CH3NH3PbI3 was not howeverlarge enough to indicate the presence of any substantial

electron transfer between them.
The short-circuit current of the PSCs prepared in thisstudy was increased by the doping of a spiro-OMeTAD layer with Li-TFSI and tBP. Being in accord with the electronic structures of the systems studied in this paper, the change
in the condition of HTL by the doping is considered to makethe Fermi level of the spiro-OMeTAD layer closer to its
HOMO level, and therefore to make the positive hole traps
shallower. This result indicates that positive hole traps in a
spiro-OMeTAD layer disturb the transport of positive holes
in it, and could be almost eliminated by the dopants employed in this study.
These results are in good accord with the observationthat the traps in a spiro-OMeTAD are filled by such a p-type
dopant as Li-TFSI and light-created positive holes to exhibit
ESR spectra attributable to long-lived positive holes in it31).
It is therefore considered that positive hole traps inspiro-OMeTAD layers are situated at the HOMO levels of
their molecules with wide distribution owing to their disordered condition.
As seen in Fig. 7, the change caused in a PSC by irradiation was kept for a relatively long time. A PSC usually exhibits severe hysteresis in I-V measurement and is being
studied in relation to some slow processes responsible forit32) 33). However, it seems that such slow processes are derived, not from the accumulation of positive hole traps in
HTL as analyzed in this study, but from ion migration or
re-orientation of dipoles of methyl ammonium cations in thebulk of CH3NH3PbI3, resulting in the appearance of hysteresis and slow recovery of light-induced changes as seen in
Fig. 7.
The Fermi level of thin metal layers including an Au layer however tends to be pinned to the Fermi level of an organic layer deposited from air (estimated to be about 4.8 eV
in the previous paper27) ), as observed in Fig. 2. The work
function of the Au anode on the doped spiro-OMeTAD layer
was 5.25 eV and larger than that of an organic layers deposited from air. It is probable that the Fermi level of the Au
anode in the cells tends to be pinned to that of an organic
layer deposited from air when the cell will be kept in air.
This phenomenon should bring about the electron transferfrom the organic layer to the Au anode, and then the electron transfer from the Au anode to the doped spiro-OMeTAD layer. It is concerned that this change with time deteriorate the positive hole transport in the doped
spiro-OMeTAD layer by increasing the depth and density ofpositive hole traps in it, indicating the necessity to keep thecell apart from air for the stabilization of its performance.

References
1)

Akihiro Kojima, Kenjiro Teshima, Yasuo Shirai, and Tsutomu

Yuka Yamano, Tadaaki Tani, Takayuki Uchida

Characterization of Hole Transport Layer in Perovskite Solar Cell via Its Performance and Electronic Structure

Miyasaka, “Organometal Halide Perovskites as Visible-Light
Sensitizers for Photovoltaic Cells”, J. Am. Chem. Soc., 131, 60506051 (2009).
2) Henry J. Snaith, “Perovskites: The Emergence of a New Era for
low-Cost, High-efficiency Solar Cells”, J. Phys. Chem. Lett., 4,3623-3630 (2013).
3) Hyun Suk Jung, Nam-Gyu Park, “Perovskite Solar Cells: From
Materials to Devices”, Small, 11, 10-25 (2015).
4) Tsutomu Miyasaka, “Perovskite Photovoltaics: Rare Functionsof Organo Lead Halide in Solar Cells and Optoelectronic Devices”, Chem. Lett., 44, 720-729 (2015).
5) Yoshihiko Kanemitsu, Yasuhiro Yamada, Takumi Yamada,“Free Carrier Radiative Recombination and Photon Recycling in
Lead Halide Perovskite Solar Cell Materials”, Bull. Chem. Soc.
Jpn., 90, 1129-1140 (2017).
6) Tsutomu Miyasaka, “Lead Halide Perovskites in Thin Film Photovoltaics: Background and Perspective”, Bull. Chem. Soc. Jpn.,
91, 1058-1068 (2018).
7) Jotaro Nakazaki, Hiroshi Segawa, “Evolution of organometal halide solar cells”, J. Photochem. Photobiol. C: Photochemistry Reviews, 35, 74-107 (2018).
8) Ajai Kumar Jena, Ashish Kulkami, Tsutomu Miyasaka, “Halide
Perovskite Photovoltaics: Background, Status, and Future Prospect”, Chem. Rev., 119, 3036-3103 (2019).
9) Jinli Yang, Braden D. Siempelkamp, Dianyi Liu, Timothy L. Kelly, “Investigation of CH3NH3PbI3 Degradation Rates and Mechanisms in Controlled Humidity Environments Using in Situ
Techniques”, ACS nano, 9, 1955-1963 (2015).
10) U. Bach, D. Luppo, P. Comte, J. E. Moser, F. Weissoertel, J. Salbeck, H.Spreitzer, & M. Graetzel, “Solid-state dye-sensitized
mesoporous TiO2 solar cells with high photon-to-electron conversion efficiencies”, Nature, 395, 583-585 (1998).
11) Hui-Seon Kim, Chang-Ryul Lee, Jeong-Hyeok Im, Ki-Beon Lee,
Thomas Moehl, Arianna Marchioro, Soo-Jin Moon, Robin Humphry-Baker, Jun-Ho Yum, Jacque E. Moser, Michael Grätzel,
Nam-Gyu Park, “Lead Iodide Perovskite Sensitized All-SolidState Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%”, Scientific Reports, 2, 591 (2012).
12) Michael M. Lee, Joël Teuscher, Tsutomu Miyasaka, Takurou N.
Murakami, Henry J. Snaith, “Efficient Hybrid Solar Cells Basedon Meso-Superstructured Organometal Halide Perovskites”,
Science, 338, 643-647 (2012).
13) Rebecka Schoelin, Martin H. Karlson, Susanna K. Erikson, Hans
Siegbahn, Erik M. J. Johansson, “Energy Level Shifts in
Spiro-OMeTAD Molecular Thin Films When Adding Li-TFSI”,
J. Phys. Chem. C 116, 26300-26305 (2012).
14) Ute B. Cappel, Torben Daeneke, Udo Bach, “Oxygen-Induced
Doping of Spiro-MeOTAD in Solid-State Dye-Sensitized Solar
Cells and Its Impact on Device Performance”, Nano Letters, 12,4925-4931 (2012).
15) Mi Xu, Yaoguang Rong, Zhiliang Ku, Anyi Mei, Xiong Li, Hongwei Han, “Improvement in Solid-State Dye Sensitized Solar
Cells by p-Type Doping with Lewis Acid SnCl4”, J. Phys. Chem.
C 117, 22492-22496 (2013).
16) Antonio Abate, Daniel R. Staff, Derek J. Hollman, Henry J.
Snaith, Alison B. Walker, “Influence of ionizing dopants on
charge transport in organic semiconductors”, Phys. Chem.
Chem. Phys., 16, 1132-1138 (2014).
17) Peng Qin, Soichiro Tanaka, Seigo Ito, Nicolas Tetreault, Kyohei
Manabe, Hitoshi Nishino, Mohammad Khaja Nazeeruden, Michael Graetzel, “Inorganic hole conductor-based lead halide perovskite solar cells with 12.4% conversion efficiency”, Nature
Commun., 2014, DOI:10.1038/ncomms4834.
18) Seigo Ito, Soichiro Tanaka, Henri Vahlman, Hitoshi Nishino, kyo-19

hei Manabe, Peter Lund, “Carbon-Double-Bond-free Printed Solar Cells from TiO2/CH3NH3PbI3/CuSCN/Au: Structural control and Photoaging Effects”, ChemPhysChem 15, 1194-1200(2014).
19) Tadaaki Tani, “Survey of Perovskite Solar Cells from Dye Sensitization in Silver Hialde Photographic Systems”, J. Soc. Photogr. Imaging Jpn., 81, 318-333 (2018).
20) Tadaaki Tani, “Photographic Science Advances in Nanoparticles, J-Aggregates, Dye Sensitization, and Organic Devices”,
Oxford University Press, Oxford, 2011.
21) Akira Fujishima and Kenichi Honda, “Electrochemical Photolysis of Water at a Semiconductor Electrode”, Nature, 238, 37-38(1972).
22) Brian OʼRegan and Michael Graetzel, “A low-cost, high-efficiency solar dell based on dye-sensitized colloidal TiO2 films”, Nature, 353, 737-740 (1991).
23) Shen Wang, Mahsa Sina, Pritesh Parikn, Taylor Uekert, Brian
Shahbazian, Arun Devaraj, Ying Shirley Meng, “Role of
4-tert-Butylpyridine as a Hole Transport Layer MorphologicalController in Perovskite Solar Cells”, Nano Lett., 16, 5594-5600(2016).
24) Naoki Ishida, Atsushi Wahamiya, Akinori Saeki, “Quantifying
Hole Transfer Yield from Perovskite to Polymer Layer: Statistical Correlation of Solar Cell Outputs with Kinetic and Energetic
Properties”, ACS Photonics, 3, 1678-1688 (2016).
25) Y. Nakajima, M. Hoshino, D. Yamashita, and M. Uda, “NearEdge Structure of Mg-, Co-, Cu-, and Zn-Tetraphenyulporphyrins Measured by PESA and Calculated with DV-Xα”, Advances in Quantum Chemistry, 42, 399-405 (2003).
26) Hisao Ishii and Kazuhiko Seki, “Energy Level Alignment at Organic-Metal Interfaces”, in “Conjugated Polymer and MolecularInterfaces Science and Technology for Photonic and Optoelectronic Applications”, William R. Salaneck, Kazuhiko Seki, Antoine Kahn, and Jean-Jacques Pireaux, eds., Marcel Dekker, Inc.,New York, Basel, 2001, pp.293-349.
27) Tadaaki Tani, Ryota Kan, Yuka Yamano, Takayuki Uchida,”Stabilization of Ag nanostructures by tuning their Fermi levels”, Jpn. J. Appl. Phys., 57, 055001 (2018).
28) Yuka Yamano, Ryota Kan, Tadaaki Tani, Takayuki Uchida,“Characteristics of the Hole Transport in Perovskite Solar Cells
and Their Aging Effects”, J. Soc. Photogr. Imaging Jpn., 80, 53-59(2017).
29) Philip Schulz, Eran Edri, Saar Kirmayer, Gary Hodes, David
Cahen, Artoine Kahn, “Interface energetics in organo-metal halide perovskite-based photovoltaic cells”, Energy & Environ.
Sci., 7, 1377-1381 (2014).
30) Tadaaki Tani (supervised by Hiroo Inokuchi), “Fundamentals
and Principles of Organic Semiconductors in Light of Inorganic
Semiconductors and Silver Halide Photography”, Maruzen Pub.
Co., Tokyo, 2014.
31) Miki Namatame, Masaki Yabusaki, Takehiro Watanabe, Yuhei Ogomi,
Shuzi Hayase, Kazuhiro Marumoto, “Direct observation of dramaticallyenhanced hole formation in a perovskite-solar-cell material spiro-OMeTAD by Li-TFSI doping”, Appl. Phys. Lett., 110, 123904 (2017).
32) Henry J. Snaith, Antonio Abate, James M. Ball, Giles E. Eperson, Tomas Leijtens, Nakita K. Noel, Samuel D. Stranks, Jacob Tse-Wei Wang,
Konrad Wojciechowski, Wei Zhang, “Anomalous Hysterisis in Perovskite Solar Cells”, J. Phys. Chem. Lett., 5, 1511-1515 (2014).
33) E. L. Unger, E. T. Hoke, C. D. Bailie, W. H. Nguyen, A, R. Bowring, T.
Heumueller, M. G. Christoforo, M. D. McGehee, “Hysteresis and transient behavior in current-voltage measurements of hybrid-perovskite
absorber solar cells”, Energy & Environmental Science, 7 (11), 36903696 (2014).