All-Solid Perovskite Solar Cells with HOCO-R-NH3+Iâ Anchor-Group

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All-solid perovskite solar cells with HOCO-R-NH3+I- anchorgroup inserted between porous titania and perovskite Yuhei Ogomi, Atsushi Morita, Syota Tsukamoto, Takahiro Saitho, Qing Shen, Taro Toyoda, Kenji Yoshino, Shyam S Pandey, Tingli Ma, and Shuzi Hayase J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp412627n • Publication Date (Web): 21 May 2014 Downloaded from http://pubs.acs.org on May 28, 2014

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The Journal of Physical Chemistry

All-solid Perovskite Solar Cells with HOCO-RNH3+I- Anchor-group Inserted between Porous Titania and Perovskite

Yuhei Ogomi, *,† Atsushi Morita,,† Shota Tsukamoto ,† Takahiro Saitho,† Qing Shen,*, ‡,§ Taro Toyoda, ‡,§, Kenji Yoshino, ¶,§, Shyam S. Pandey, † Tingli Ma, † and Shuzi Hayase, *,†,§

†

Graduate School of Life Science and Systems Engineering, Kyushu Institute of Technology,

2-4 Hibikino, Wakamatsu-ku, Kitakyushu, Fukuoka 808-0196, Japan ‡

Graduate School of Informatics and Engineering, University of Electro-Communications, 1-

5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan. ¶

Department of Electrical and Electronic Engineering, University of Miyazaki, 1-1, Gakuen

Kibanadai Nishi, Miyazaki, 889-2192, Japan §

CREST, Japan Science and Technology Agency (JST), 4-1-8 Honcho Kawaguchi, Saitama

332-0012, Japan

1 ACS Paragon Plus Environment


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Corresponding author Information. Yuhei Ogomi†, [email protected], +81-93-6950644, Qing Shen‡, [email protected], +81-424-43-5471, Shuzi Hayase†, [email protected], +81-93-695-0644

Abstract

HOCO-R-NH3+I monolayer working as an anchor for perovskite (CH3NH3PbI3 (abbreviation: PEROVI3)) was inserted between the surface of porous metal oxide (titania or alumina) and the PEROVI3. Power conversion efficiency (PCE) of PEROVI3 solar cells increased from 8 % to 10 % after the HOCO-R-NH3+I- monolayer was inserted. Moreover, PCE of 12% was achieved for cells fabricated at optimized conditions. This increase in the efficiency was explained by retardation of charge recombination, and better PEROVI3 crystal growth which improves PEROVI3 network on these porous metal oxides. It was proved that PEROVI3 crystal growth can be controlled by HOCO-R-NH3+I- on these porous metal oxides.

“Keywords”: surface passivation, dye sensitized, thin film solar cell, serf organization, trap distribution

Introduction All-solid state solar cells consisting of perovskite have recently attracted interest because of the high efficiency reaching 14-15%

1-5

. Perovskite solar cells consist of compact titania

layer, porous metal oxide layer, perovskite layer and p-type organic semi-conductor layer. Two mechanisms have been reported on the electron collection. N.G. Park and his coworkers 2 ACS Paragon Plus Environment

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have reported high efficiency (9.7%) perovskite solar cells (PEROV Ti solar cell) composed of TiO2/ perovskite (CH3NH3PbI3 abbreviated as PEROVI3) /(2,2',7,7'- Tetrakis[N,N-di (4methoxy phenyl) amino] - 9,9'- spirobifluorene) (SPIRO)2. The efficiency has further increased to 14.14 % (Certified efficiency) by using two-step perovskite fabrication process3. They imply that electrons are collected by porous titania layers. On the other hand, Snaith and his coworkers have reported porous titania-free perovskite solar cells (PEROV Al solar cells), in which they described that electrons are collected by the perovskite layer itself covering the porous alumina surface5,6.

Recently, perovskite solar cells with flat

heterojunction structure with 15.4 % efficiency were prepared by a co-evaporation process under vacuum7. In this cell, electrons are also collected by the perovskite layer. In our report, we took the former charge collection mechanism. We focused on the interface between porous titania and perovskite layers.

In conventional dye-sensitized solar cells (DSC),

organic dyes are bonded onto porous titania surface with anchoring groups such as carboxylic moieties1. Since swift electron injection is realized by bonding these dye molecules to titania surfaces, designs for these anchor groups were a key issue for developing high efficiency DSCs1.

However, trials on introducing anchoring group between titania surface and

perovskite layer have not been reported so far. This prompted us to design the interface structure between them. In this report, we insert HOCO-R-NH3+I- as an anchoring group between the porous titania and the perovskite layer. The relationship between the interface structure and the photovoltaic performance is discussed.



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EXPERIMENTS Cells were fabricated by the following process. F-doped SnO2 layered glass ((FTO glass), Nippon Sheet Glass Co. Ltd) was patterned by using Zn and 6N HCl aqueous solution. On this patterned FTO glass, titanium diisopropoxide bis(acetylacetonate) solution in ethanol were sprayed at 300 °C to prepare a compact TiO2 layer. The substrate was dipped in 40 mM solution of TiCl4 in water for 30 min followed by baking at 500 °C for 20 min. Compact layers for all substrates were prepared by the two step process. A porous TiO2 layer was fabricated by spin-coating TiO2 paste (PST-18NR, JGC Catalysts and Chemicals Ltd.) in ethanol (TiO2 paste: ethanol = 2:7 weight ratio), followed by heating the substrate at 550 °C for 30 min to give 150 nm thickness films. The spin-coating rate was adjusted to prepare the target film thickness in each experiment because room temperature varies every day. Representative spinning rate was 500 rpm for 5 sec., followed by 5000 rpm for 30 sec. The porous substrate was dipped in 5 mM solution of HOCO-R-NH3+I- in ethanol for 90 min for introducing anchor moieties (-NH3+I-) on the titania. 1 M solution of PbI2 in N,N’dimethylformamide (DMF) was spin-coated, followed by baking at 70 °C for 30 min.

After

that, the substrate coated with the porous layer was dipped in 10 mg/ml solution of CH3NH3+Iin 2-propanol for 30 sec. After rinsing, the substrate coated with porous titania and PEROVI3 was heated at 70 °C for 30 min, followed by spin-coating a mixture of 60 mM of tertbutylpyridine, 30 mM of lithium bis(trifluoromethylsyfonyl)imide salt, and 180 mM of SPIRO in chlorobenzene. Finally, Ag and Au electrodes were fabricated by a vacuum deposition method (ALS Tech E-299). Ag was evaporated at 0.5 Å/sec under 2.0 x 10-5 Pa, 4 ACS Paragon Plus Environment

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followed by Au evaporation at 0.2 Å/sec on a hole transport layer without exposed to air. Ag and Au thicknesses were monitored by quartz crystal microbalance. Photovoltaic performance of the cells was evaluated by using AM 1.5G 100mW/cm2 irradiance solar simulator (CEP2000, Bunkoukeiki Inc) and with 0.4 x 0.4 cm mask on the cells of size 0.5 x 0.5 cm. Solar cells consisting of porous alumina were fabricated by the spin coating in the same way as the fabrication of the porous titania. Commercially available alumina (Aldrich 702129: particle diameter: 50 nm) dispersed in 2-propanol, ethyl cellulose (Nisshin Kasei), and ethanol were mixed in the ratio of 1:1:2 (weight %), and the mixture was employed instead of the titania paste to give 150 nm thickness. Thermally stimulated current (TSC) was measured using a TS-FETT electron trapping measuring system (Rigaku) in the same way described in our previous report8. After a substrate was cooled to -180 °C, traps were filled with carriers by exposing ultraviolet lights to the sample. After the light exposure was stopped, the sample temperature was increased gradually. As the temperature of the sample increases, these carriers are released from these trap sites. These carries are detected as TSC. TSC at a higher temperature is due to the electrons released from deeper traps and that at a lower temperature is associated with electrons released from shallow traps. Trap depths and trap densities are obtained from the temperature and thermally stimulated current density at each temperature by Equations 1 and 2, respectively 9,10.

.



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Et = kTm x In (Tm4/β) ----------------------Equation 1

where, Et: Trap depth, k: Boltzman Cons., Tm: Temp(K), β: Programing rate (K/s).

N = (dI/dt) x (1/enP) --------------------------------------Equation 2

where, N: Trap density, dI/dt: Current/unit time, e: Elementary charge, n: Volume (thickness x gap), P: Porosity

Transient absorption (TA) measurements were carried out to characterize the charge separation and recombination dynamics by using the femtosecond TA setup (fs-TA) and nanosecond TA setup (ns-TA), respectively11-13. For measuring charge separation dynamic with the fs-TA, a titanium/sapphire laser (CPA-2010, Clark-MXR Inc.) with a wavelength of 775 nm, a repetition rate of 1 kHz, and a pulse width of 150 fs was employed. The light was separated into two parts. One part (775 nm light) was used as the probe pulse. The other part was used to pump an optical-parametric amplifier (OPA) (a TOAPS from Quantronix) to generate light pulses with a wavelength tunable from 290 nm to 3 µm. The latter was used as a pump light to excite the sample. In this study, 520 nm light was used for pumping. In the nsTA setup for measuring charge recombination dynamics, an optical-parametric oscillator (OPO) (Surelite II – 10FP) output excited by a Nd:YAG nanosecond pulse laser (Panther, Continuum, Electro-Optics Inc.) was used. 470 nm light with 5 ns pulse width and 0.5 Hz 6 ACS Paragon Plus Environment

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repetition was employed as the excitation. A fiber coupled CW semiconductor laser with a wavelength of 658 nm (measurement of charge recombination between electrons in TiO2 and holes in perovskite) or a wavelength of 1300 nm (measurement of charge recombination between electrons in TiO2 and holes in SPIRO) was used as the probe light. X ray diffraction pattern was measured by Rigaku model XRD apparatus.

Results and discussion Figure 1 shows the schematic diagram of the anode structure we fabricated. A compact titania layer, and a porous titania layer treated with HOCO-R-NH3+I- were fabricated on a transparent conductive oxide layer (F-doped SnO2 glass: FTO).

Three HOCO-R-NH3+I-

(alanine HI salt (AlaH+I- ), β-glycine HI salt (GlyH+I-), γ-amino butyric acid HI salt (GABAH+I-)), shown in Figure 2, were employed as the anchoring group. The adsorption of HOCO-R-NH3+I- onto titania surface was confirmed by infrared (IR) spectroscopy. 1781 nm peak assigned to COO stretching of HOCO-R-NH3+I- shifted to 1654 nm after HOCO-RNH3+I- were adsorbed on titania, demonstrating that the salt was adsorbed on the titania (see supporting information Figure S1). The IR peak shift similar to these results has been reported in the previous reports14,15. CH3NH3PbI3 (PEROVI3) has a perovskite structure as shown in Figure 1. PEROVI3 layer is conventionally prepared by two step process. To begin with, PbI2 layer was prepared on TiO2 layer by spin coating, followed by dipping CH3NH3+Isolution, during which, CH3NH3+I- is inserted to PbI2 layer to make CH3NH3+I- perovskite layer3,16.

In the same way, we expected that NH3+ moiety of the HOCO-R-NH3+I- grown 7 ACS Paragon Plus Environment


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from the surface of the porous titania surface is incorporated into the surface of PbI2 layers in order to anchor the perovskite material. The bonding length between the perovskite and titania surface was varied from one methylene of GlyH+I-(C:n=1) to three methylene of GABAH+I-(C:n=3) as shown in Figure 2.

PbI3CH3NH3+

HOCO-R-NH3+I-

Porous TiO2

Porous TiO2 Figure 1. Structure of titania anode where HOCO-R-NH3+ was inserted between perovskite and porous titania.


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H3N I

H3N I

H3N I

COOH HIHIsalt of glycine glycine (abbreviated as GlyH+I-)

COOH β-alanine HIHIsalt of β-alanine (3-aminopropanoic (abbreviated as AlaH+I-)acid) COOH HIHIsalt of γ-amino butyric acid GABA (abbreviated as GABAH+I-) (γ-aminobutyric acid )

Figure 2. Structure of HOCO-R-NH3+I- (Amino acid HI satls) Figure 3 (A, B, C, D) summarizes the photovoltaic performances (Efficiency, Jsc, Voc, FF) of perovskite solar cells (PEROV Ti solar cell) with and without HOCO-R-NH3+I--anchorgroups. Efficiencies of the PEROV Ti solar cell without HOCO-R-NH3+I- anchor-group were in the range from 6 to 8 %.

When GlyH+I- (C:n=1), AlaH+I- (C:n=2),or GABAH+I-

(C:n=3),was inserted between the perovskite (PEROVI3) and porous titania, the efficiency increased in the following order; GlyH+I-< AlaH+I- < GABAH+I-, leading to the conclusion that longer methylene group is effective for increasing the efficiency. Open circuit voltage (Voc)(Figure 3C), fill factor (FF)(Figure 3D), and short circuit current (Jsc)(Figure 3B) increased from around 0.8 V to 0.9 V, from around 0.6 to 0.7, from around 12 mA/cm2 to 18 mA/cm2, respectively, as the number of methylene increased from 1 to 3 (from Glycine to GABA). In addition, dispersion of efficiency data of the solar cells became smaller after the insertion of the GABAH+I-. We have already reported that cyanine dyes consisting of longer alkyl groups were adsorbed better on TiO2 surface than that with shorter alkyl chains because of self-organization of the alkyl groups17, 18. 9 ACS Paragon Plus Environment

without amino acid GAB C:n=3 Alanine C:n=2

Glycine C:n=1

Jsc [mA/cm2]

11.0 10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0.0

20.0 18.0 16.0 14.0 12.0 10.0 8.0 6.0 4.0 2.0 0.0

GABA C:n=3 Alanin C:n=2

without amino acid Glycine C:n=1

0.7 0.6

GABA C:n=3

Glycine without amino Alanine C:n=1 acid C:n=2

FF

1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0

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B

A

Voc [V]

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Efficiency [%]


0.5

GABA

0.4

C:n=3

0.3 0.2

without amino Alanine acid C:n=2 Glycine C:n=1

0.1 0.0

C

D

Figure 3. Photovoltaic performances of perovskite solar cell with HOCO-R-NH3+I- anchoring layer. A: Efficiency, B: Short circuit current, C: Open circuit voltage (Voc), C: Fill factor (FF). Glycine, Alanine and GABA stand for GlyH+I-, AlaH+I-, and GABAH+I- respectively. C:x stands for the number of carbons between -NH3I and –COOH groups in each aminoacid For example, GABA (C:n=3) means that -NH3I group is separated from –COOH by three CH2 units. In this case, it is also expected that HOCO-R-NH3+I- with longer alkyl chains would be adsorbed better on TiO2 layer because of the self-organization of alkyl groups. GlyH+I- has only one carbon between NH3+I- and COOH. Because of the random adsorption of the amino acids, uniform formation of PbI2 layer on GlyH+I- may be disturbed more seriously than that on bare TiO2. 10 ACS Paragon Plus Environment

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Figure 4 shows the photovoltaic performances for an optimized PEROV Ti solar cell with GABAH+I- anchor-group. The structure is as follows: compact titania layer: 20 nm, porous titania layer: 150 nm, Spiro 100 nm, Ag: 10nm, Au: 40nm. I-V measurement was carried out in the sweep direction from Jsc to Voc at 100 ms delay time. The efficiency, FF, Voc, Jsc, series resistance (Rs), and shunt resistance (Rsh) was improved to 12.0 %, 0.62, 1.00 V, 19.2 mA/cm2, 112.9 Ωcm2, 71320 Ωcm2, respectively.

20.0

Current density (mA/cm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


15.0 10.0 5.0 0.0 -5.0 -10.0 -0.5

-0.3

-0.1

0.1

0.3

0.5

0.7

0.9

Voltage (V) Figure 4. Photovoltaic performance of PEROV Ti solar cell after insertion of HOCO-R--NH3+ I-. I-V measurement was carried out from Jsc to Voc at 100 ms delay. Compact titania layer: 20nm, porous titania layer: 150 nm, Spiro: 100 nm, Ag: 10nm, Au: 40nm. Eff: 12.0%, FF: 0.62, Voc: 1.00 V, Jsc: 19.2 mA/cm2, Rs: 112.9 Ω, Rsh: 71320 Ω. The time of the sample



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treated with CH3NH3+I- was changed from 30 sec (described in experimental section) to 50 sec for the optimization. The effects of the inserted GABAH+I- group on charge recombination in PEROV Ti solar cells were investigated. The charge recombination dynamics between electrons in TiO2 and holes in PEROVI3, and those between electrons in TiO2 and holes in SPIRO were measured by using ns-TA (Transition absorption spectroscopy) with two different probe wavelengths of 658 nm and 1310 nm, respectively. First, the recombination of electrons in TiO2 and holes in PEROVI3 was measured with the probe light of 658 nm. The pump light of 470 nm was used to excite PEROVI3 only. Figure 5 shows the TA responses of PEROVI3 on TiO2 (PEROVI3/TiO2) without and with the GABAH+I- in a time scale of 100 µs. It is known that the lifetime of photoexcited carriers in PEROVI3 is the order of 10 ns as reported by Snaith19 and Gratzel20. We have also confirmed that the photoexcited carrier lifetime in PEROVI3 was about 11 ns from our fs-TA experiment as discussed later. In addition, we also observed that photoexcited electron injection from PEROVI3 to TiO2 occurs in a time scale of ns for both the samples with and without GABAH+I- anchor group inserted between TiO2 and PEROVI3 as discussed later. Since the time scale of µs is much longer than photoexcited carrier life time in PEROVI3 and the electron injection time from PEROVI3 to titania, the TA signal appeared in the time scale (longer than µs) in Figure 5 could correspond to the recombination process between electrons in TiO2 and holes in PEROVI3. As shown in Figure 5, for PEROVI3/TiO2 without GABAH+I-, no TA signal can be observed. It suggests that recombination time between 12 ACS Paragon Plus Environment

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electrons injected into TiO2 and holes in PEROVI3 was smaller than 1 µs. However, for PEROVI3/TiO2 with GABAH+I-, a TA signal can be observed clearly, which disappeared in around 10 µs. This result suggests that the recombination of the electron in TiO2 and holes in PEROVI3 could be suppressed greatly by inserting GABAH+I- anchor group between TiO2 and PEROVI3.

2 .0 x 1 0

Δ A

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-4

0 .0

w it h o u t G A B A H I w it h G A B A H I - 2 .0 x 1 0

-4

1 E- 6

1 E- 5

1 E- 4

T im e (s ) Figure 5. TA response of PEROVI3/TiO2 with (red) and without (black) inserting GABAH+Ianchor group between TiO2 and PEROVI3 measured with a pump light wavelength of 470 nm and a probe light wavelength of 658 nm.



2 .0 x 1 0

-3

(a) w it h o u t S p ir o

Δ A

1 .0 x 1 0

-3

0 .0

- 1 .0 x 1 0

-3

w it h o u t G A B A H I w it h G A B A H I - 2 .0 x 1 0

-3

1 E- 6

1 E- 5

1 E- 4

1 E- 3

T im e (s ) 2 .0 x 1 0

-3

(b ) w it h S p ir o 1 .0 x 1 0

Δ A

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-3

0 .0

- 1 .0 x 1 0

-3

- 2 .0 x 1 0

-3

1 E- 6

w it h o u t G A B A H I w it h G A B A H I f it t in g 1 E- 5

1 E- 4

1 E- 3

0 .0 1

T im e (s ) Figure 6. TA response of PEROVI3 on TiO2 (a) without and (b) with SPIRO used as hole transport layer (HTM), which were measured with a pump light wavelength of 470 nm and a probe light wavelength of 1310 nm. The black and red curves are the results of the samples without and with GABAH+I- anchor group inserted between TiO2 and PEROVI3. The blue


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solid line in (b) represents the fitting result with a one exponential function with a decay time of 470 µs To investigate the recombination of electrons in TiO2 and holes in SPIRO for PEROV Ti solar cells, we applied ns-TA to monitor the dynamics of the oxidation states of SPIRO (i.e., the holes injected into SPIRO) with a probe light wavelength of 1310 nm. The pump light of 470 nm was used to excite PEROVI3 only. Figure 6 shows the TA responses of PEROVI3/TiO2 (a) without and (b) with SPIRO, for both PEROV Ti solar cells without and with GABAH+I-. For the samples without SPIRO, no TA signal was observed for both samples without and with GABAH+I-. However, for the samples with SPIRO, TA signals was observed clearly for both of samples with and without GABAH+I-. These results indicate that the TA signal probed with 1310 nm light in the time scale longer than µs originated from the injected holes in SPIRO from PEROVI3. These signals were fitted very well with one exponential decay function with a time constant of about 470 µs as shown in Figure 6(b). We can know that recombinations between electrons in TiO2 and the holes in SPIRO were almost the same for PEROV Ti solar cells without and with GABAH+I-. Based on the above ns-TA experimental results, we concluded that GABAH+I- inserted at the interfaces mostly suppress the charge recombination at PEROVI3/TiO2 interface rather than the recombination at TiO2/ SPIRO interfaces. Three methylene groups should be more effective than two or one methylene groups for retarding the charge recombination. We have previously reported that the efficiency of PEROV Ti solar cells increased by passivating porous titania with Y2O3 because of the 15 ACS Paragon Plus Environment


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retardation of the charge recombination21,22. The effect of methylene spacers on the increase in the efficiency is explained by the surface passivation of porous titania and spatial separation between titania and perovskite. The result of transition absorption spectroscopy measurement clearly demonstrated that charge separation occurred at the interfaces between PEROVI3 and SPIRO (hole injection) as well as at the interfaces between PEROVI3 and TiO2 (electron injection). possibility on electron collection by the PEROVI3 still remains.

However, a

In order to make the

question clear, photovoltaic performances for the porous alumina based-perovskite solar cell (PEROV Al solar cell) was prepared and the anchor-insertion effect was evaluated. All data on I-V curves are shown in Figure 7. Jsc varied from 1 mA/cm2 to 9 mA/cm2 and Voc also varied from 0.75 to 0.88 V, depending on how to passivate the porous alumina surface. Efficiency, Jc, Voc, and FF were summarized in Figure 8. The absolute efficiency of PEROV Al solar cell was low, less than 1 % in our experimental conditions.


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12.0

Current density (mA/cm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Al2O3 / GABA/NH3CH3PbI3 Al2O3 / Ala/NH3CH3PbI3 Al2O3 / Gly/NH3CH3PbI3 Al2O3 /NH3CH3PbI3

10.0 8.0 6.0 4.0 2.0 0.0 0.0

0.2

0.4

0.6

0.8

1.0

Voltage (V)

Figure 7. Photovoltaic performance of PEROV Al solar cells after insertion of GABAH+II-V measurement was carried out in the direction from Jsc to Voc with 100 ms delay time. Compact titania layer: 20nm, porous alumina layer: 150 nm, Spiro 100 nm, Ag: 10nm, Au: 40nm.



12.0 GABA Alanine C:n=3 C:n=2

Efficiency [%]

4.0 3.0 2.0

without amino acid

Glycine C:n=1

Alanine C:n=2 GABA C:n=3

10.0

Jsc [mAcm-2]

5.0

1.0

8.0 6.0 4.0

without amino Glycine C:n=1 acid

2.0 0.0

0.0

B

A 0.7

0.8

0.4

0.6

GABA Alanine C:n=3

without Glycine amino C:n=2 C:n=1 acid

0.5

FF

0.6

Voc [V]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.4 without amino 0.3 acid

Glycine Alanine GABA C:n=2 C:n=3 C:n=1

0.2

0.2 0.1

0.0

0.0

C

D

Figure 8. Photovoltaic performance of PEROV Al solar cells after insertion of GABAH+IWhen HOCO-R-NH3+I- were inserted between the alumina surface and the perovskite, the efficiency increased in the following order: GlyH+I- (C:n=1) < AlaH+I- (C:n=2) < GABAH+I(C:n=3) as shown in Figure 8A. Amino acid having longer alkyl chains gave better results. The increase in the Jsc was mainly responsible for the efficiency improvement as shown in Figure 8B. Voc was also improved by these passivations as shown in Figure 8C. FF increased by the passivation with GlyH+I- and AlaH+I-, however, GABAH+I- gave lower FF value as shown in Figure 8D, probably because the high Jsc value compared with those of others. These results on perovskite solar cell based on porous alumina passivated with GABAH+Isuggests that the networks of the PEROVI3 layer reaching transparent conductive oxide layer 18 ACS Paragon Plus Environment

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grow well after the GABAH+I- passivation. In our experimental condition, perovskite layer may have defects in the networks which may retard efficient electron collection by the PEROVI3 to the transparent conductive oxide layer as shown in Figure 9 B.

When

GABAH+I- was inserted between the perovskite and the alumina surface, the PEROVI3 network may grow well as shown in Figure 9 A. The perovskite layer was fabricated by two steps. First of all, PbI2 crystal is fabricated on the porous alumina, followed by treated with CH3NH3+I-. To begin with, PbI2 network reaching transparent conductive oxide layer may grow well on the porous alumina layer after GABAH+I- passivation. This may be explained by the increase in miscibility between the HOCO-R-NH3+I- anchor-group and PbI2. As is discussed later, the fact that bigger PEROVI3 crystal forms on GABAH+I- layer also support the better electron transport by PEROVI3 layer networks after GABAH+I- passivation. In other words, the inserted HOCO-R-NH3+I- can control the crystal and network growth of PbI2 and the final perovskite material. Considering the results on porous alumina based perovskite solar cells, we are not able to exclude the possibility that PEROVI3 partially works as electron collection for porous titania based perovskite solar cells. In this solar cell, both of porous titania and perovskite layers may be responsible for the electron collection.



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Perovskite

Al2O3

Al2O3

Defect

Defect

FTO B

A

Figure 9. Perovskite structure on alumina with or without GABAH+IA : with GABAH+I-, B : without GABAH+ITo confirm electron injection from perovskite to titania as well as the effects of the inserted GABAH+I- anchor group on the electron injection process, photoexcited electron-hole pair relaxation dynamics were measured for PEROVI3 deposited on mesoporous Y2O3 (PEROVI3/Y2O3), Al2O3 (PEROVI3/Al2O3) and those deposited on mesoporous TiO2 (PEROVI3/TiO2) samples by using the fs-TA. Electrons in PEROVI3 could be injected into TiO2 but not be injected into Y2O3 and Al2O3, because the conduction band edge of PEROVI3 is higher than that of TiO2 but much lower than those of Y2O3 and Al2O3. These samples were 20 ACS Paragon Plus Environment

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excited at 470 nm (only PEROVI3 was excited at this wavelength), and these TA signals were probed at 775 nm which corresponded to the conduction band edge of PEROVI3.

N o r m aliz e d T A S ign al

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0 .0

- 0 .5

A l2O 3 Y 2O 3

- 1 .0 0

1000

2000

3000

T im e (p s ) Figure 10. Normalized TA responses of PEROVI3/Y2O3 and PEROVI3/Al2O3. The pump light was 470 nm and the probe light was 775 nm.

Figure 10 shows the normalized TA responses of PEROVI3/Y2O3 and PEROVI3/Al2O3 up to 3 ns without GABAH+I-. Bleaching signals were observed for all samples, showing that PEROVI3 was excited and the number of ground state PEROVI3 decreased. The decay of the bleaching signals corresponds to the recombination process of photo-generated carriers within PEROVI3, since there is no electron injection from the perovskite to Y2O3 and Al2O3 as mentioned above. It is worth noting that the normalized TA responses of the PEROVI3/Y2O3 21 ACS Paragon Plus Environment


and PEROVI3/Al2O3 overlapped with each other very well, indicating that photoexcited carriers in PEROVI3 showed same properties even if PEROVI3 was deposited on the different porous substrates in this study.


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0 .0

- 0 .5

w it h G A B A w it h o u t G A B A

- 1 .0 0

1000

T im e (p s )

2000

3000

Figure 11. Normalized TA responses of PEROVI3/Y2O3 without (black) and with (red) GABAH+I- inserted at the interface of PEROVI3 and Al2O3. The pump light wavelength was 470 nm and the probe light was 775 nm, respectively. The blue lines represent the theoretical fitting result of the TA responses with biexponential functions with the fitting parameters shown in Table 1.


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Figure 11 shows the normalized TA responses of PEROVI3/Al2O3 without and with GABAH+I-. It is interesting to note that the TA response decayed slower when GABAH+Iwas inserted between PEROVI3 and Al2O3 interface, indicating that the lifetimes of photoexcited carrier became longer due to GABAH+I- insertion. We found that there were two processes in the TA decay and the TA signal was able to be fitted very well to a biexponential function as follows:

Y = A1e − t / t1 + A2 e − t / t 2

(Equation 3)

where t1 and t2 are the time constants and A1 and A2 are the contributions of the two decay components. The fitted results are shown in Table 1. For the sample without GABAH+I-, the time constants t1 and t2 of the two charge recombination processes were 460±54 ps and 11.0±4.3 ns, and the relative numbers of carriers related to the two recombination processes were 50% (A1/(A1+A2)) and 50% (A2/(A1+A2)), respectively. The faster one can be considered to be a recombination of electrons and holes through trap states at PEROVI3/Al2O3 interfaces or traps having PEROVI3 crystal itself including the crystal grain boundaries. The slower one can be considered to be a direct recombination of electrons and holes, which is in good agreement with the radiative recombination lifetime in PEROVI3 as reported by Snaith and Gratzel groups19,20. As shown in Table 1, for the samples with GABAH+I- inserted at the interface of PEROVI3 and Al2O3, the time constants t1 and t2 were 461±68 ps and 11.7±3.5 ns, which were almost the same as those without GABAH+I-. However, the relative number of 23 ACS Paragon Plus Environment


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carriers related to the faster recombination decreased to 40% (A1/(A1+A2)), while that related to slower recombination increased up to 60% (A2/(A1+A2)), respectively. These results imply that the two recombination processes are not changed by inserting GABAH+I- at the interface, but the ratio of the fast process against the slow process was changed. Possible explanation is that the density of the interface trapping states between alumina and PEROVI3 decreased by inserting GABAH+I- at the interfaces. Another possible explanation is that PEROVI3 crystals grown on GABAH+I- layer on porous metal oxide are grown well. Figure 12 shows the PEROVI3 crystals grown on GABAH+I-/porous titania (A: with GABAH+I-) and bare porous titania (B: without GABAH+I-), where crystals on GABAH+I- were bigger than those grown on bare porous titania.

Figure 12. Perovskite crystals grown on porous titania A : PEROVI3/GABAH+I-/TiO2, B : PEROVI3/TiO2


(a)

0 .0

Wit h o u t G A B A H I

- 0 .5

T iO 2 A l2O 3 f it t in g

- 1 .0 0


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



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1000

T im e (p s )

2000

3000

Figure 13. Normalized Wit h G A B A H I TA responses of PEROV (b ) 0 .0

- 0 .5

T iO 2 A l2O 3 f it t in g

- 1 .0 0

1000

T im e (p s )

2000

3000

Figure 13. Normalized TA responses of PEROVI3/TiO2 and PEROVI3/Al2O3 without (a) and with (b) GABAH+I- inserted at the interface of PEROVI3 and the substrates. The pump light wavelength was 470 nm and the probe light was 775 nm, respectively. The blue lines



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represent the theoretical fitting result of the TA responses with biexponential functions with the fitting parameters shown in Table 1.

Figure 13(a) and (b) show the comparison between the normalized TA responses of PEROVI3/TiO2 and PEROVI3/Al2O3, without and with GABAH+I- inserted at the interfaces of PEROVI3 and these substrates. We found that the TA decay became faster for PEROVI3 on TiO2 substrates, especially when GABAH+I- was inserted. The TA responses of PEROVI3/TiO2 was able to be fitted very well with Equation 3 and the fitting results are shown in Table 1. The time constants t1 and t2 were 299±31 ps and 4.1±0.4 ns for PEROVI3/TiO2 without GABAH+I-, and were 344±57 ps and 2.3±0.3 ns for PEROVI3/TiO2 with GABAH+I-, respectively. Both of the photoexcited carrier in PEROVI3 on TiO2 decayed faster, compared to those on Al2O3, demonstrating that electron injection really occurred from PEROVI3 to TiO2. Then, we calculated the average lifetimes tave of the photoexcited carriers using the following equation 23:

2

t ave

A t + A2t2 = 11 A1t1 + A2t2

2

(Equation 4)

The results of tave for each sample are shown in Table 1. Very interestingly, tave decreased from 3.9 ns to 2.4 ns after inserting GABAH+I- for PEROVI3/TiO2. It means that electron 26 ACS Paragon Plus Environment

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injection from PEROVI3 to TiO2 became faster after GABAH+I- was inserted in the interface between TiO2 and perovskite. The electron injection rate constant kET can be calculated from Equation 5 as follows23:

k ET = 1 / tave( PEROVI3 / TiO2 ) − 1 / tave( PEROVI3 / Al 2 O3 )

Equation 5

The results of kET are shown in Table 1. kET was 0.16 x 109 s-1 for PEROVI3/TiO2 without GABAH+I-, and it increased to be 0.33 x 109 s-1 after GABAH+I- were inserted. Electron injection efficiency ηinj could be estimated by using the following Equation 6, which is used to calculate electron injection efficiency in dye sensitized solar cells (1).

η inj = (k ET ) /(k ET + 1 / t ave( PEROVI3 / Al O ) ) 2

Equation 6

3

The calculated results of ηinj are shown in Table 1. It is worth noting that ηinj increased greatly from 63% to 79% due to GABAH+I- insertion. Bigger crystal formation as shown in Figure 12 and better networks of perovskite grown on GABAH+I- expected from results on photovoltaic performance improvement on PEROVI3/GABAH+I-/Al2O3, may explain the fast electron injection from PEROVI3 to TiO2 for PEROVI3/ GABAH+I-/TiO2. Table 1. Carrier dynamics for perovskite solar cell. samples

t1 ( ps )a

t2 ( ns )a

A1a

A2a

tav ( ns kET(× )b 109 s-1)c

ηinjc



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PEROVI3/Al2O3

460±54

11.0±4.3 0.44

0.44

10.6

-

-

PEROVI3/TiO2

299±31

4.1±0.4

0.43

0.46

3.9

0.16

63 %

GABAH+I- inserted at the interfaces PEROVI3/ 461±68 GABAH+I-/Al2O3

11.7±3.5 0.34

0.53

11.5

-

-

PEROVI3/ GABAH+I-/TiO2

42.3±0.3 0.46

0.40

2.4

0.33

79 %

344±57

a: Fitting results of the experimental TA responses with the biexponential function of Equation 3. b: calculated average life time using Equation 4. c: Electron injection rate calculated using Equation 5 as well as electron injection efficiency using Equation 6. All measurements were carried out in N2 atmosphere using 470 nm laser pulse excitation. X ray diffraction pattern of PbI2 crystal structure formed on titania without GABAH+Ianchor-group was similar to those of commercially available PbI2 powder24 as shown in Figure 14. Peaks assigned to 001, 011, 102, 003, and 110 were observed. Only 001 and 003 peaks remained after the insertion of GABAH+I- anchor-group, leading to the conclusion that PbI2 crystals where 001 plane is emphasized, are grown on GABAH+I- mono-layer. This implies that GABAH+I- layer determines the direction of crystal growth of PbI2 which is the first step of preparation of perovskite layer. The direction of the crystal growth may be associated with the better carrier collection.


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ICCD 07-0235 PbI2 hexagonal

(100) (102)

(110)

(001)

(003) (111)

(100)

(103) (201)

A

Intensity Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


B

Glass/TiO2 5

10

15

20

25

30

35

40

45

50

2θ (deg.) (deg.) 2θ Figure 14. X ray diffraction pattern of PbI2 with or without GABAH+I- . A: With GABAH+I- , B: Without GABAH+I- .

Considering the fact that electrons are injected from the perovskite to the porous titania, the injected electrons diffuse, hopping trapping sites25-30.

Surface traps become charge

recombination centers and decrease solar cell performances. Therefore, surface trap density has to be lowered. We have already reported that Y2O3 passivation decreases the surface trap



density of titania21,22. The trap density distribution was measured by thermally stimulated current method (TSC)8 and the results are shown in Figure 15.

-4.0 -4.1 -4.2

D

C

B

-4.3

A Vacum level (eV)

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-4.4 -4.5 -4.6 -4.7 -4.8 -4.9 -5.0

Electron trap density (cm-3)

Figure 15. Trap distribution of porous TiO2 layer and perovskite layer A: Bare porous titania, B: Bare Titania with GABAH+I-, C: MeNH3+PbI3- coated on bare titania with GABAH+I-, D: MeNH3+PbI3-, Titania conduction band: -4.0 eV, MeNH3PbI3 conduction band: -3.93 eV.


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Conduction band level of titannia is -4.0 eV from vacuum level. Shallow traps were focused on because they seriously affect photovoltaic performances. The trap density at -4.2 eV from vacuum level (0.2 eV deeper than that of the TiO2 conduction band) was about 101617

/cm3. After the surface was passivated with GABAH+I-, the trap density decreased to

1015/cm3 at -4.2 eV. After the perovskite layer was fabricated on GABAH+I-/porous titania layer, the trap density distribution further decreased to 1014/cm3 at -4.2 eV. An interesting thing is that the trap density of the perovskite itself was extremely low (1011/cm3 at -4.2 eV) in the shallow trap area, compared to the bare porous titania layers. This may explain the unique carrier transport properties (long carrier diffusion length reaching 100 nm to 1000 nm) of the perovskite19, 20 and the high photovoltaic performance for the perovskite solar cell.

Conclusions It was proved that insertion of HOCO-R-NH3+I- anchor group improved photovoltaic performances for perovskite solar cells. A solar cell with 12 % efficiency was reported. The effect of GABAH+I- anchor group is summarized as follows. 1. Passivation of surface trap of porous titania layers 2. Better growth of perovskite networks on porous materials 3. Crystal growth of perovskite materials with uniform crystal plain 4. Large crystal growth on porous materials 5. Retardation of charge recombination by alkyl groups. 6. Promotion of electron injection into titania 31 ACS Paragon Plus Environment


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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.

Acknowledgements This research was supported by JST CREST in Japan. Supporting information Available. This information is available free of charge via the Internet at http://pubs.acs.org

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