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Article

Photovoltaic Characteristics of CH3NH3PbI3 Perovskite Solar Cells Added with Ethylammonium Bromide and Formamidinium Iodide

by
Kousuke Nishi
,
Takeo Oku
*,
Taku Kishimoto
,
Naoki Ueoka
and
Atsushi Suzuki
Department of Materials Science, The University of Shiga Prefecture, 2500 Hassaka, Hikone, Shiga 522-8533, Japan
*
Author to whom correspondence should be addressed.
Coatings 2020, 10(4), 410; https://doi.org/10.3390/coatings10040410
Submission received: 1 April 2020 / Revised: 15 April 2020 / Accepted: 16 April 2020 / Published: 20 April 2020
(This article belongs to the Special Issue Advanced Thin Film Materials for Photovoltaic Applications)
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Abstract

:
Photovoltaic characteristics of solar cell devices in which ethylammonium (EA) and formamidinium (FA) were added to CH3NH3PbI3 perovskite photoactive layers were investigated. The thin films for the devices were deposited by an ordinary spin-coating technique in ambient air, and the X-ray diffraction analysis revealed changes of the lattice constants, crystallite sizes and crystal orientations. By adding FA and EA, surface defects of the perovskite layer decreased, and the photoelectric parameters were improved. In addition, the highly (100) crystal orientations and device stabilities were improved by the EA and FA addition.

1. Introduction

Organic-inorganic perovskite solar cells provide photoelectric conversion in wide wavelength ranges and exhibit excellent photovoltaic properties [1,2,3,4,5,6]. Since the film of CH3NH3PbI3 (MAPbI3) can be formed by a spin-coating method, there is an advantage that the production process is easy and low cost. In spite of these merits, there is a serious problem that the stability is extremely low. In order to solve this problem, research and development of devices with higher power conversion efficiency and stability using formamidinium (FA) [7,8,9,10,11,12,13], guanidinium [14,15] or alkali metal [16,17,18,19,20,21] doped perovskites for the methylammonium (MA) site have been conducted.
There also exists research and development of devices with ethylammonium (EA) added to perovskites [22,23,24,25,26]. EA has a larger ionic radius (2.74 Å) than that of MA (2.17 Å), and the addition of EA can be expected to improve stability from the viewpoint of calculations [25,27] and tolerance factor [1]. In addition, there is a report that the thermal stability and crystallinity are higher than those of MA, and the addition of EA to the perovskites showed a surface coating with fewer defects and improves the stability of the device [23,28]. However, it should be noted that excessive addition of EA leads to phase separation, a decrease in crystallinity, and precipitation of PbI2 as an impurity [29,30].
The purpose of this study is to examine the microstructures and photovoltaic characteristics of FA and EA co-added CH3NH3PbI3 perovskite solar cells. The stability of a MAPbI3 perovskite structure might be predicted by calculating the tolerance factor (t-factor) [31,32,33,34,35], which is given by t = r MA + r I 2 ( r Pb + r I ) , where r is an ionic radius [36]. When the t-factor is in the range of 0.81–1.1, perovskite structures could be formed [35]. If the t-factor is adjusted to 1.0, perovskite structures with cubic symmetry could be realized. The ionic radii of MA+, FA+, EA+, Pb2+, I−, Br−, and Cl− are 2.17, 2.53, 2.74, 1.19, 2.20, 1.96, and 1.81 Å, respectively [35,36]. By adding FA+ and EA+ with larger ionic radii than MA+, t-factor gets closer to 1, and the stability is expected to be improved. In addition, EA addition is expected to promote the crystal growth and improve the stability of the device [23,28], and there are few reports on simultaneous addition of FA and EA to the perovskite layer. The effects of the simultaneous addition to the perovskite compounds were analyzed by microstructural and photovoltaic characterization.

2. Materials and Methods

A cross-section and deposition process of the present perovskite solar cells is summarized and shown in Figure 1. A fluorine-doped tin oxide (FTO, Nippon Sheet Glass Company, Ltd., Tokyo, Japan) substrate was dipped and washed in an ultrasonic washing machine using acetone twice and methanol once, and cleaned with flowing N2. The 0.15 and 0.30 M precursor solutions of TiO2 were prepared from 0.055 and 0.11 mL titanium disopropoxide bis (acetyl acetonate) (Sigma Aldrich, Tokyo, Japan) and 1-btanol (1.0 mL, Nacalai Tesque, Kyoto, Japan). The solutions were cast on the transparent FTO, and spin-coated at 3000 rpm for 30 s and heat-treated at 125 °C for 5 min [37,38,39]. The processes with 0.30 M precursor solutions were repeated twice. In order to form a dense electron transport TiO2, the deposited samples were annealed at 550 °C for 30 min. The mesoporous TiO2 layer was deposited with TiO2 nanoparticles (P-25, Aerosil, Tokyo, Japan) and polyethylene glycol (Nacalai Tesque, Kyoto, Japan) in ultrapure water. The solution was blended with acethylacetone (20 μL, Fujifilm Wako Pure Chemical Corporation, Osaka, Japan) and triton-X-1001 (10 μL, Sigma Aldrich, Tokyo, Japan) for 30 min, and allowed to stand for 24 h to remove bubbles from the mixed solution. The prepared TiO2 mixed solution was spin-coated at 5000 rpm for 30 s and annealed at 550 °C for 30 min, and a mesoporous TiO2 layer was formed.
The perovskite precursor solutions were prepared as mixed solutions of methylamine hydroiodide CH3NH3I (MAI, 2.4 M, Tokyo Chemical Industry, Tokyo, Japan) and PbCl2 (0.8 M, Sigma Aldrich, Tokyo, Japan) in N,N-dimethylformamide (DMF) (0.5 mL, Sigma Aldrich, Tokyo, Japan) at 60 °C for 24 h. This is used as a standard cell, and the amount of MAI was reduced by adding formamidine hydroiodide CH(NH2)2I (FAI, Tokyo Chemical Industry, Tokyo, Japan), ethylamine hydrobromide CH3CH2NH3Br (EABr, Tokyo Chemical Industry, Tokyo, Japan), and ethylamine hydrochloride CH3CH2NH3Cl (EACl, Tokyo Chemical Industry, Tokyo, Japan). Detailed compositions of the perovskite compounds are listed in Table 1, together with the t-factors. The perovskite precursor solutions were spin-coated at 2000 rpm for 60 s and applied an air-blowing method during spin-coating [40,41]. The device was annealed at 150 °C for 20 min in the ambient air.
The hole-transport layer was deposited by spin-coating. A chlorobenzene solution (0.5 mL) of 2,2’,7,7’-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9’-spirobifluorene (spiro-OMeTAD, Fujifilm Wako Pure Chemical, Corporation, Osaka, Japan, 36.1 mg) was prepared by mixing it for 12 h. An acetonitrile solution (0.5 mL) of lithium bis (trifluoromethylsulfonyl) imide (Li-TFSI, Tokyo Chemical Industry, Tokyo, Japan) was also prepared by mixing it for 12 h. A mixture solution of the spiro-OMeTAD solution with 4-tertbutylpridine (14.4 μL, Sigma Aldrich, Tokyo, Japan) and Li-TFSI solution (8.8 μL) was prepared by mixing it at 70 °C for 30 min. The spiro-OMeTAD layer was deposited by spin-coating at 4000 rpm for 30 s. After that, gold (Au) thin film electrodes were deposited as electrodes by vacuum evaporation. As investigated in the previous works [42,43,44], layer thicknesses of the compact TiO2, mesoporous TiO2 + perovskite, spiro-OMeTAD, and Au layers were roughly estimated to be 40, 600, 50, and 200 nm, respectively.
The light-induced current density voltage (J–V) curves of the fabricated devices were obtained by using air mass 1.5 illuminator (San-ei Electric XES-301S, 100 mW·cm−2) and a current-voltage apparatus (B2901A, Keysight, Santa Rosa, CA, USA). In addition, the external quantum efficiencies of the devices were obtained (QE-R, Enli Technology, Kaohsiung, Taiwan). Optical microscopy (Eclipse E600, Nikon, Tokyo, Japan) and X-ray diffraction (D2 PHASER, Bruker, Billerica, MA, USA) measurements were performed to analyze the surface morphologies and nanoscopic structures.

3. Results and Discussion

J–V curves collected in the light condition for the fabricated perovskite solar cells are displayed in Figure 2. Table 2 shows summarized parameters of the fabricated solar cells. A conversion efficiency (η) of the standard cell is 6.72%. The JSC, VOC and η were improved from 19.2 mA·cm−2, 0.687 V and 6.72% to 21.5 mA·cm−2, 0.922 V and 14.25% by addition of FA 20% at the MA site. When EA 10% and FA 10% were added simultaneously, the JSC, VOC and η increased 19.9 mA cm−2, 0.946 V and 12.43%. Addition of EACl was also effective for the improvement of the device properties. Further addition of EA and FA would decrease the device performance.
Figure 3 is the J–V curves of the fabricated photovoltaic cells after 4 weeks in ambient air, and the estimated parameters are shown in Table 3. The conversion efficiency of the standard cell was lowered to 5.69%. Co-addition of small amount of EA and FA to MAPbI3 provided higher stability compared with the standard cells, as shown in Figure 4.
Optical microscopy images of the perovskites through spiro-OMeTAD are shown Figure 5. By adding EA and FA, surface defects of the perovskite layer decreased. Obtaining a perovskite layer with few defects enables efficient charge separation and charge extraction, which is thought to have led to improved device performance. In addition, defects in the perovskite layer are a cause of charge recombination, and it is considered that suppression of the defect has led to improvement in stability.
External quantum efficiency (EQE) spectra of the fabricated photovoltaic cells are shown in Figure 6. The band gap energies (Eg) were estimated from EQE spectra around 800 nm by linear fitting using band gap calculator software (Enli Technology, QE-R), and the measured band gap energies of the perovskite compounds increased from 1.54 to 1.57 eV by adding EA. The Eg value of the 20% EABr-added perovskite crystals was wider than that of the 20%FAI-added perovskite. The EQE values of the EABr-added device was lower between 350 and 750 nm than that of the FAI-added device, which led to a decrease of the JSC values.
X-ray diffraction (XRD) patterns of the fabricated cells added with EABr and FAI are shown in Figure 7a. Increases of (100) and (200) diffraction reflections are observed by adding FAI or EABr. In addition, only (100) and (200) peaks are observed, which indicates that the cells exhibited highly oriented (100) perovskite crystals by the air-blowing method [40].
Microstructural parameters of the present perovskite compounds are listed in Table 4. The lattice constants of the FAI-added perovskites were higher compared with the standard MAPbI3 material, whereas those of the EABr and FAI co-added perovskite decreased. Crystallite sizes were estimated from the (200) reflections, and they increased by the addition of FAI and EABr. The I100/I210 intensity ratios of (100) reflections (I100) to (210) reflections (I210) were measured from the XRD data in Figure 7a,b, and the results are shown in Table 4. If the CH3NH3PbI3 cubic perovskite particles are randomly oriented, then the I100/I210 value should be 2.08 [35]. For the standard cell prepared in the present study, the I100/I210 is 48, which means the (100) crystal surfaces of the cubic structures are strongly aligned in the solar cell. By the addition of FAI to the perovskite compounds, I100/I210 was increased to 1694, and the I100/I210 increased further to 1939 by adding EABr. This is 40 times higher than the I100/I210 of the standard perovskite device.
A schematic model showing molecular structures (MA, FA, and EA) and the lattice structure of the FAI and EABr added perovskites is shown in Figure 8a,b, respectively. The lattice constant a of 6.315 Å for a perovskite single crystal [35,45] is greater compared with the a of the perovskite compound in a cell configuration [46,47]. If the perovskite particles were synthesized and deposited on the mesoporous TiO2 layer, some of the CH3NH2 molecules might be desorbed. Then, MA vacancies could be formed, and the lattice constant (6.274 Å) of MAPbI3 is smaller than that of single crystal, as listed in Table 4. When FAI was added to the standard MAPbI3, the FA would occupy the defects and MA sites, and the lattice constant increased to 6.286 Å, as shown in Figure 8b and Table 4. As the size of Br− is fairly small compare with that of I−, a values of the EABr-added crystals decreased to 6.280 Å compared with FAI-added perovskite crystals, as indicated by arrows in Figure 8b. Combination of the present EA/FA with other molecules [15,48] and alkali metals [21,49] might also be effective for the stabilization of the perovskite compounds.

4. Conclusions

Solar cells using perovskite in the photoactive layer were produced by using spin-coating technique in ordinary air, and the influence on photovoltaic characteristics by adding EA and FA to the perovskite phase was investigated. From the results of J–V characteristics, the addition of EA and FA improved VOC and FF, leading to an improvement in photoelectric conversion efficiency. Devices with EA and FA added maintained photoelectric conversion efficiencies even after 4 weeks compared to that of the standard device. Optical microscope results showed surface improvement, and X-ray diffraction results showed FA and EA substitution at MA position of the perovskite. By substituting FA and EA, which have larger ionic radii than MA, the perovskite structures would have more stable cubic structures with higher stability.

Author Contributions

Conceptualization, K.N. and T.O.; Methodology, K.N., T.K., N.U., and A.S.; Formal Analysis, K.N. and T.O.; Investigation, K.N.; Data Curation, K.N. and T.O.; Writing-Original Draft Preparation, K.N. and T.O.; Writing-Review & Editing, T.O.; Visualization, K.N. and T.O.; Supervision, T.O.; Project Administration, T.O.; Funding Acquisition, T.O. All authors have read and agreed to the published version of the manuscript.

Funding

This research was partly funded by the Super Cluster Program of the Japan Science and Technology Agency (JST).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Cross-section of the cell and process conditions.
Figure 1. Cross-section of the cell and process conditions.
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Figure 2. J–V characteristics collected in light condition for the fabricated solar cells.
Figure 2. J–V characteristics collected in light condition for the fabricated solar cells.
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Figure 3. J–V characteristics collected in light condition for the fabricated solar cells after 4 weeks in ambient air without encapsulation.
Figure 3. J–V characteristics collected in light condition for the fabricated solar cells after 4 weeks in ambient air without encapsulation.
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Figure 4. Stability measurements of the fabricated perovskite devices.
Figure 4. Stability measurements of the fabricated perovskite devices.
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Figure 5. Optical microscopy images of cells with the compositions of (a) FAI 20%, (b) EABr 5% + FAI 20%, (c) EABr 10% + FAI 20%, and (d) EABr 20% + FAI 20%.
Figure 5. Optical microscopy images of cells with the compositions of (a) FAI 20%, (b) EABr 5% + FAI 20%, (c) EABr 10% + FAI 20%, and (d) EABr 20% + FAI 20%.
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Figure 6. External quantum efficiency spectra of the fabricated solar cells.
Figure 6. External quantum efficiency spectra of the fabricated solar cells.
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Figure 7. (a) X-ray diffraction (XRD) pattern of the present solar cells and (b) enlarged pattern of (a).
Figure 7. (a) X-ray diffraction (XRD) pattern of the present solar cells and (b) enlarged pattern of (a).
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Figure 8. Structures of (a) methylammonium (MA), formamidinium (FA), ethylammonium (EA) and (b) the present perovskite compounds.
Figure 8. Structures of (a) methylammonium (MA), formamidinium (FA), ethylammonium (EA) and (b) the present perovskite compounds.
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Table
Table 1. Compositions and calculated t-factors of the present perovskite compounds.
Table 1. Compositions and calculated t-factors of the present perovskite compounds.
Composition of PerovskiteEABr (%)FAI (%)t-Factor
MAPbI3000.912
MA0.9FA0.1PbI30100.919
MA0.8FA0.2PbI30200.927
MA0.5FA0.5PbI30500.949
MA0.8FA0.1EA0.1PbI2.7Br0.310100.933
MA0.75FA0.2EA0.05PbI2.85Br0.155200.933
MA0.7FA0.2EA0.1PbI2.7Br0.310200.940
MA0.6FA0.2EA0.2PbI2.4Br0.620200.954
MA0.75FA0.2EA0.05PbI2.85Cl0.155200.934
MA0.7FA0.2EA0.1PbI2.7Cl0.310200.941
Table
Table 2. Measured parameters of the cells fabricated in this study. JSC: short-circuit current density. VOC: open-circuit voltage. FF: fill factor. RS: series resistance. RSh: shunt resistance. η: conversion efficiency. ηave: averaged efficiency of three cells.
Table 2. Measured parameters of the cells fabricated in this study. JSC: short-circuit current density. VOC: open-circuit voltage. FF: fill factor. RS: series resistance. RSh: shunt resistance. η: conversion efficiency. ηave: averaged efficiency of three cells.
DeviceJSC
(mA·cm−2)		VOC
(V)		FFRS
(Ω·cm2)		RSh
(Ω·cm2)		η
(%)		ηave
(%)
Standard19.20.6870.5098.83376.726.35
+FAI 10%21.80.8160.5746.2166310.248.04
+FAI 20%21.50.9220.7193.4483914.2513.66
+FAI 50%15.70.9260.7124.713,54510.3610.31
EABr 10% + FAI 10%19.90.9460.6606.1466712.4312.23
EABr 5% + FAI 20%21.00.8340.6485.6495211.3310.63
EABr 10% + FAI 20%19.30.7890.5725.710158.478.70
EABr 20% + FAI 20%18.10.8510.5624.823408.688.27
EACl 5% + FAI 20%20.40.8790.6186.4187911.0610.63
EACl 10% + FAI 20%20.20.9330.6475.266,63712.2111.64
Table
Table 3. Measured photovoltaic parameters of the fabricated cells after 4 weeks.
Table 3. Measured photovoltaic parameters of the fabricated cells after 4 weeks.
DeviceJSC
(mA·cm−2)		VOC
(V)		FFRS
(Ω·cm2)		RSh
(Ω·cm2)		η
(%)		ηave
(%)
Standard19.00.6330.4748.92125.695.25
+FAI 10%17.30.9250.6158.751239.859.30
+FAI 20%20.70.9610.6754.6245513.4313.30
+FAI 50%14.80.9640.6846.075,9689.748.99
EABr 10% + FAI 10%17.30.9250.6158.751239.859.30
EABr 5% + FAI 20%18.60.9190.6995.219,97111.9311.41
EABr 10% + FAI 20%18.20.8190.5647.611298.396.86
EABr 20% + FAI 20%18.20.8700.5856.69469.268.77
EACl 5% + FAI 20%17.30.9000.6825.1409710.629.98
EACl 10% + FAI 20%17.00.9320.6645.8540710.549.49
Table
Table 4. Microstructural parameters for the perovskite crystals. Preferred crystal orientations were indicated with ratios of 100 diffraction intensities (I100) to 210 diffraction intensities (I210).
Table 4. Microstructural parameters for the perovskite crystals. Preferred crystal orientations were indicated with ratios of 100 diffraction intensities (I100) to 210 diffraction intensities (I210).
PerovskitesLattice Constant
a (Å)		Crystallite Size
D200 (Å)		Orientation
I100/I210
Standard6.274(1)47948
+FAI 20%6.286(1)6471694
EABr 5% + FAI 20%6.281(0)528460
EABr 10% + FAI 20%6.283(1)15061155
EABr 20% + FAI 20%6.280(2)8301939

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Nishi, K.; Oku, T.; Kishimoto, T.; Ueoka, N.; Suzuki, A. Photovoltaic Characteristics of CH3NH3PbI3 Perovskite Solar Cells Added with Ethylammonium Bromide and Formamidinium Iodide. Coatings 2020, 10, 410. https://doi.org/10.3390/coatings10040410

AMA Style

Nishi K, Oku T, Kishimoto T, Ueoka N, Suzuki A. Photovoltaic Characteristics of CH3NH3PbI3 Perovskite Solar Cells Added with Ethylammonium Bromide and Formamidinium Iodide. Coatings. 2020; 10(4):410. https://doi.org/10.3390/coatings10040410

Chicago/Turabian Style

Nishi, Kousuke, Takeo Oku, Taku Kishimoto, Naoki Ueoka, and Atsushi Suzuki. 2020. "Photovoltaic Characteristics of CH3NH3PbI3 Perovskite Solar Cells Added with Ethylammonium Bromide and Formamidinium Iodide" Coatings 10, no. 4: 410. https://doi.org/10.3390/coatings10040410

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