All-solid Sn/Pb halide perovskite sensitized solar cells Yuhei Ogomi I, Atsushi Morital, Shota Tsukamotol Takahiro Saithol, Naotaka Fujikawal, Shen Qing2,4,

Taro Toyoda2,4, Kenji Yoshino3,4, Shyam S. Pandeyl, Tingli Mal, and Shuzi Hayase*I,4

I Graduate School of Life Science and Systems Engineering, Kyushu Institute of Technology, 2-4 Hibikino,

Wakamatsu-ku, Kitakyushu, Fukuoka 808-0196, Japan 2 Graduate School of Informatics and Engineering, University of Electro-Communications, 1-5-1

Chofugaoka, Chofu, Tokyo 182-8585, Japan. 3 Department of Electrical and Electronic Engineering, University of Miyazaki, 1-1, Gakuen Kibanadai

Nishi, Miyazaki, 889-2192, Japan 4CREST, Japan Science and Technology Agency (JST), 4-1-8 Honcho Kawaguchi, Saitama 332-0012,

Japan

Abstract - We have succeeded in harvesting energy in near infrared region (NIR) by using air stable Sn doped metal halide perovskite materials. The edge of the incident photon to current efficiency (IPCE) edge reached 1060 nm. 4.18 % efficiency is reported. The photovoltaic performance was compared with Pb halide perovskite solar cell with 14.4% efficiency, leading to the conclusion that the low FF and Voc are associated with low shunt resistance (Rsh). One of the methods to suppress the charge recombination will be reported, which includes passivation of porous titania surface states with aminoacid HI salts. The relationship among the surface states of titaniaor alumina, and crystallinity of the perovskites induced by surface passivation molecules, and photovoltaic performances are discussed.

Index Terms - perovskite, passivation, crystal grow, Sn, dye sensitized solar cell

I. INTRODUCTION

All-solid state solar cells consisting of perovskite have

recently attracted interest because of the high efficiency

reaching 12_16%.1.9 Perovskite solar cells consist of compact

titania layer, porous metal oxide layer, perovskite layer and p­

type organic semi-conductor layer. When porous titania layer

was used, electrons are collected by the porous titania layer.

N.G. Park and his coworkers have reported perovskite solar

cells with 9.7% efficiency, where, the cell is composed of

Ti02/ CH3NH3PbI3 /(2,2',7,7'- Tetrakis[N,N-di (4-methoxy

phenyl) amino] - 9,9'- spirobifluorene) (SPIRO)? The

efficiency has further increased to 14.14 % (Certified

efficiency) by using two-step perovskite fabrication process.4

When the perovskite layer was fabricated on porous alumina

layer, electrons are collected by the perovskite layer itself

covering the porous alumina surface. 5,6 Perovskite solar cells

with flat heterojunction structure prepared by a co-evaporation

process under vacuum have been reported and the efficiency

reached 15.4 %.9 However, light absorption spectrum edge for

Pb halide solar cells is limited to 800 nm. It has been reported

that Sn halide perovskites have the light absorption up to

1200nm.JO·J6 However, it has been reported that CH3NH3SnI3

is not stable for handling in air. We noticed that the Sn halide

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P3HT

PorousTi02 and CH3NH3Sn/PbI3

Fig. 1. Structure of Cocktail Sn/Pb perovskite solar cells

perovskite became stable in air when Pb halide perovskite was

added. This prompted us to evaluate cocktail SnlPb

perovskite solar cells.

II. PREP ARA TTON OF COCKTAIL PEROVSKITE SOLAR CELLS

Figure 1 shows the structure of cocktail SnlPb perovskite

solar cells which consists of FTO glass/ compact Ti02

layer/porous Ti02 layer containing CH3NH3SnxPb(l'

x)I3/P3HT/AgiAu. In this report, CH3NH3SnxPb(l.x)I3 is

P3HT in Chlorobenzene solution (spin coating)

I Ag(lOnm)/Au(40nm) (Thermal evaporation) I

In air '-------------'

Fig. 2. Procedure of cell preparation

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abbreviated as MASnPb(x/(1-x)).

The perovskite solar cells were prepared in the procedure

shown in Figure 2. The perovskite layer was prepared under

N2 atmosphere. SnI2, PbI2, and methylammonium iodide

(MA) were mixed and spincoated on the porous titania layer

(one pot process). P3HT polymers were employed as the hole

transporting layer instead of SPIRO. Photovoltaic

performances were evaluated in air without encapsulation.

III. PHOTOVOL T AlC PERFORMANCES

Figure 3 shows the stability of perovskite layers in air.

900nm absorption of MASn(1/0) decreased rapidly in air and

became zero in 60 min. However, MASnPb(0.5/0.5) survived

1.1 1.0

11 0.9 ..: 0.8 ." 0.7 � 0.6 � O.S 50.4 Z 0.3

0.2 0.1 0.0

A=900nm

o 10 20 30 40 SO 60

Time [min] Fig. 3. Stability of perovskite layers in air

in air and was able to handle in air.

Figure 4 shows absorption spectra for MASnPb(x/(l-x)).

The spectrum edge shifted from 800 nm to 1200 nm when x

was changed from 0 to 1, demonstrated that the edge (band

gap) is tunable by changing the x. MASnPb(0.5/0.5) have

1050 nm edge and was able to be handled in air. 2 e in XRD changed from 14.45 to 15.56 gradually when x increased from

o to 1, which shows that the repeating unit distance decreases

with the increase in x. This reflects that Pb ion (ionic radius:

1.3-1.5 A) is replaced partially with Sn ion (ionic radius: 0.93

A). XPS peaks for Sn 3d shifted from 492.33 eV to 492.91 eV

(higher binding energy) when x was changed from 0.3 to 1.0,

suggesting that Sn4+ content may increase with higher x valuel7.

Sn:Pb=O.7/0.3 Sn:Pb=O.5/0.5 Sn:Pb=O.9/0.1

_________ Sn:Pb=O.3/0.7 1.2 ,

I

1.0

0.$

0.6

0.'

0.2

Sn:Pb=O/l

-C1t .... '''H,Pbl, -C1£ .... '"H,SIro,Pb...J, - CI[r'14,s.o,Pbt�, - O£,'''H,s.o,'''-joI,

O[,'''H,s.oA1I, _C1r .... '"H,SaI,

0.0 '---�� -'-.......,,-----' 400 ,SOO (i(IO iOO 3(10 9(IC) 1000 1100 1200 UOO

Waveleoglh (nm)

Sn:Pb=l/O

fOCI 1000 llOO 1* IlOO \\"' ..... '-

Fig. 4 Absorption spectra of MASnPb(X/(l-X)

978-1-4799-4398-2/14/$31.00 ©2014 IEEE

ME � g .� = ..

"0

': .. ... ... = u

150

100

50

0

-50

-100

-150

. . . . Sn:Pb=O.7 /0.3 "

. Sn:Pb=0.5/0.5 .

' . Sn:Pb=0.9/0.1

Sn:Pb=0.3/:·

;·'·...

' •

• \ - ••••••

e_e_.. ••••• _-�

CH3NH3Pb'3 CH3NH3S''o,Pb07'3 CH3NH3Sno,Pbo,13 CH3NH3Sno.7Pb0.3'3 CH3NH3Sno9Pbo"3 CH3NH3Sn'3

Sn:Pb=O/l �. .... ;-..� .�� ...

.. �

.. ':

..

�

. ..

'

�

'. . . . . . . .

.. '.

I:: . e::;

-0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 Voltage (V)

Fig. 5. Dark current for MASnPb{x/{l-x)) solar cells

Figure 5 shows dark current for various MASnPb(x/(1-x).

Cocktail perovskite solar cells with higher x (1,0.9, and 0.7)

did not show any diode properties, which is consistent to the

speculation that Sn4+ content of cocktail perovskite with high x

value is higher and conductivity increased with an increase in

the Sn4+ content'6. It has been reported that Sn4+ is associated

with the conductivity. Diode performance was improved when

x is lower than X:0.5.

W U !!:

0.9

0.8

0.7

0.6

0.5 A

0.4

0.3

0.2

0.1

0.0 +----r--..----r--.---+---.-�__I 300 400 500 600 700 800 900 1000 1100

Wavelength [nm]

Fig. 6. Comparison of IPCE between

MASnPb(0.5/0.5)(A) and MASnPb(O/l)(B)

IPCE of MASnPb(0.5/0.5) covers from 300 nm to 1060 nm

where the wavelength region longer than 800 nm was not

photoactive for conventional MASnPb(OIl), demonstrating

that the cocktail perovskite solar cells are useful for

photoconversion in the area of NIR. Figure 7 shows the

photovoltaic performance and the details are summarized in

Table l. Jsc of both solar cells were almost the same and

about 20 mA/cm2

was observed. However, Voc and FF (0.42

V and 0.50) were lower than those of MASnPb(OIl) (0.98 V,

0.69). Series resistances (Rs) of both solar cells were almost

the same, around 6 ncm, however, large difference was

observed in shunt resistance (Rsh). The former (168.2 ncm)

was extremely low, compared with the latter (1873.6 Ocm).

This suggests that charge recombination frequently occur at

some interfaces, which have to be suppressed for further

0152

25.0 120.0 u

115.0 >- 10.0 .... ·iii :ii 5.0

" .... c 0.0 f ... <3 -5.0

-10.0 -0.2 0.0 0.2 0.4 0.6 Voltage M

0.8 1.0

Fig. 7. Comparison of IV curves between

MASnPb(O.5jO.5)(A) and MASnPb(Ojl)(B)

improvements.

HOMO of P3HT is at -4.67 from vacuum level which is

much lower than that of SPIRO (-5.22). The difference

between Ti02 conduction band (-4.0) and P3HT HOMO (-

4.67) is 0.67 V which corresponds to the expected maximum

Voc for MASnPb(0.5/0.5). The difference between the

expected 0.67 V and the experimentally obtained Voc (0.42 V)

is 0.25 V. The Voc loss is large compared with other solar

cells. For improving the Voc further, to employ hole

transporting materials with deep HOMO and to suppress

charge recombination are needed.

TABLE 1

COMPARION of IV CURVES BETWEEN MASNPB(0.5/0.5)(A) AND

MASNPB(O/l)(B)

Jse Voe Efficiency Rs Rsh FF

[mA/em') [V) [%) [Oem') [Oem')

A 20.04 0.42 0.50 4.18 6.00 168.2

B 21.15 0.98 0.69 14.28 6.21 1873.6

III. SUPPRESSION OF CHARGE RECOMBINATION

In conventional dye-sensitized solar cells, organic dyes are

bonded onto porous titania surface with anchoring groups such

as carboxylic moieties '. Since swift electron injection is

realized by bonding these dye molecules to titania surfaces, the

selections of these anchor groups and the substitution positions

is a key issue for developing high efficiency DSCs '. There

have been no discussions on how the perovskite is bonded to

the porous titania surface. In addition, crystal growth of

perovskite should be influenced by the surface condition of

porous titania. This prompted us to consider the interface

structure between them. We inserted amino acid HI salts as an

anchoring group between the porous titania or alumina and the

978-1-4799-4398-2/14/$31.00 ©2014 )EEE

perovskite layer. The relationship between the interface

structure and the photovoltaic performance was discussed.

Porous Ti02

Fig. 8. Structure of titania anode where aminoacid HI salts were inserted between perovskite and porous titania

Figure 8 shows the schematic diagram of the anode structure

we fabricated. Three amino acid HI salts (alanine HI salt

(AHI salt), �-glycine HI salt (GHI salt), y-amino butyric acid

(GABAHI salt)) (Figure 9) were employed as the anchoring

group. The adsorption of these amino acid IH salts onto titania

surface was confirmed by infrared (lR) spectroscopy.

CH3NH3PbI3 (PEROVI3) has a perovskite structure as shown

in Figure 1 and some of CH3NH3 + groups facing on titania

were replaced by NH3 + moiety of the amino acid HI salt grown

from the surface of the porous titania surface. The bonding

length between the perovskite and titania surface was varied

from one methylene of alanine HI (AHI salt) salt to three

methylene of glycine HI salt (GABAHI salt).

Figure 10 summarizes the photovoltaic performances of

perovskite sensitized solar cells (PEROV-S-DSC) with and

without amino acid HI salt-anchor-groups. When alanine HI

salt (AHI salts), glycine HI salt (GHI salts), or GABA HI salts

was inserted between the perovskite (PEROVI3) and porous

titania, the efficiency increased in the following order; GHI

salt<AHI salt < GABAHI salt, leading to the conclusion that

longer methylene group is effective for increasing the

efficiency.

Figure 11 shows the photovoltaic performances for Pb

perovskite solar cells

prepared on porous HJ�r""COOH alumina. In these HI glycine solar cells, electrons

H N�COOH J Hill-alanine

are collected through

perovskite layers

fabricated on porous

alumina layer

(insulator).

(3-aminopropanoic acid)

Pb perovskite solar

cell efficiency

increased as follows:

GHI salt<AHI salt <

GABAHI salt, which

was the same trend as

HIGABA

(y-aminobutyric acid )

Fig.9. Structure of amino acid HI

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• without • amino acid

• • * GABA t

Alanine •

• •

Glycine

11.0 20.0 10.0 I 18.0 9.0 without 16.0

� 8.0 amino acid � 14.0 GA8A * � 7.0 • t � 12.0 � 6.0 • � c: • oct 10.0 .. 5.0 • .§. 8.0 'u IS 4.0

Alanine • lil 6.0 3.0 • 2.0 4.0 Glycine 1.0 2.0

0.0 0.0 1.0 0.7

•

I a I • GABA *

• �

without • aminoaci

Alanine Glycine

0.9 • • I 0.6 0.8 • 0.7 Alanine without 0.5

Glycine amino acid 2: 0.6 0.4 u 0.5 ... ... g 0.4 0.3

0.3 0.2 0.2 0.1 0.1 0.0 0.0 L-______ _

Fig. 10. Photovoltaic performances of Pb perovskite sensitized solar

cells with amino acid HI salt anchoring layer

5.0 12.0 Alanin Q GABA

Alanin 10.0 0 4.0 0 GABA 0

*' Glycine 0 'N8.0 � �3.0 0 E 0 0 .:i 6.0 � c: 0 Glycine ..

E :g 2.0 without a 'i4.0 without a is minoacid 0 amino acid 0 0 0 1.0 a 2.0 a b �

'" a '" a

0.0 0.0 0.7 9 Q e 0.8 � GABA 0.6 8 a

� a

GABA without Alanine 0.5 9 Q 0.6 amino acid Glycine

without 0 2: ... 0.4 minoacid Alanine g 0.4 ... 0.3 Glycine

0.2 0.2 0.1 0.0 0.0

Fig. 1 1. Photovoltaic performances of Pb perovskite solar cells

fabricated on porous alumina. Aminoacid HI salts were inserted

between perovskite and porous alumina.

that of Pb perovskite solar cells fabricated on porous titania.

This suggests that perovskite crystal grows better on GABA

passivation surface than on bare porous alumina. Long alkyl

groups may assist better alignment of amino acid -NH3 +r groups from which perovskite layer grows.

The effect of GABA HI salt-anchor- group is summarized as

follows.

1. Passivation of surface trap of porous titania layers working

as charge carrier layer.

2. Increase in surface coverage area and uniform crystal

growth of perovskite materials on porous metal oxide layer.

3. Retardation of charge recombination by alkyl groups.

Electron injection from perovikite to titania actually occurs,

however, we are not able to exclude the possibility that

perovskite are also involved in the carrier diffusions, as far as

cells fabricated under our experimental conditions are

concerned.

978-1-4799-4398-2/14/$31.00 ©2014 IEEE

Perovskite

Defect

With GABA passivation Without GABA passivation

Fig. 12. Difference in perovskite layer fabricated on alumina

IV. CONCLUSION

Cocktail SnPb perovskite solar cells were proved to harvest

the light up to lO60 nm, maintaining the stability in air. The

band edge is tunable, which is effective for improving

efficiency of single cells and tandem cells. In order to

improve the efficiency, Rsh has to increase and suppress the

charge recombination. It was proves that surface passivation of

alumina and porous titania with aminoacid HI salts were

effective for the suppression of charge recombination as well

as crystal growth of perovskite layers.

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