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Fabrication parameters of low-temperature ZnO-based hole-transport-free perovskite solar cells
Accepted Manuscript Title: Fabrication parameters of low-temperature ZnO-based hole-transport-free perovskite solar cells Authors: Mohammad Hatamvand, Seyed Abbas Mirjalili, Maryam Sharzehee, Abbas Behjat, Mostafa Jabbari, Mikael Skrifvars PII: DOI: Reference:
S0030-4026(17)30253-X http://dx.doi.org/doi:10.1016/j.ijleo.2017.02.101 IJLEO 58917
To appear in: Received date: Accepted date:
7-12-2016 28-2-2017
Please cite this article as: Mohammad Hatamvand, Seyed Abbas Mirjalili, Maryam Sharzehee, Abbas Behjat, Mostafa Jabbari, Mikael Skrifvars, Fabrication parameters of low-temperature ZnO-based hole-transport-free perovskite solar cells, Optik - International Journal for Light and Electron Optics http://dx.doi.org/10.1016/j.ijleo.2017.02.101 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Fabrication Parameters of Low-temperature ZnO-based Hole-transport-free Perovskite Solar Cells Mohammad Hatamvanda,b,d, Seyed Abbas Mirjalilia*, Maryam Sharzeheea, Abbas Behjatb,c, Mostafa Jabbarid, Mikael Skrifvarsd a
Department of Textile Engineering, Yazd University, Yazd, Iran
b
Photonics Research Group, Engineering Research Center, Yazd University, Yazd, Iran
c
Atomic and Molecular Division, Faculty of Physics, Yazd University, Yazd, Iran
d
Swedish Centre for Resource Recovery (SCRR), University of Borås, 50190 Borås,
Sweden *Corresponding author e-mail: amirjalili@ yazd.ac.ir
Abstract Perovskite solar cells (PSCs) are a new generation solar cells. Low-Temperature techniques are used for fabrication PSCs on a flexible substrate that has a low thermal tolerance. In this paper, low-temperature PSCs with ZnO nanoparticles were prepared as electron transport material (ETM) without hole transport material (HTM). Effects of some of the fabrication parameters of low-temperature ZnO based PSCs without HTM, on their principal characteristics and performance, were investigated. Parameters such as the concentration of ZnO nanoparticles (NPs) dispersion, spin coating speed of ZnO NPs, and concentration of CH3NH3I on characteristics and performance of fabricated low-temperature PSCs were studied. The study shows that by changing these parameters, the performance of the fabricated PSCs changes considerably. Keywords: Perovskite solar cells, Low-temperature solar cells, Flexible solar cell, ZnO-based perovskite solar cells, ZnO nanoparticles 1. Introduction Solar energy is abundant, renewable, unlimited, clean, eco-friendly, sustainable and available. Due to these reasons, solar energy is an excellent alternative for substitution of fossil fuels which are limited, air pollutant and costly. Therefore, the development of solar cells as a replacement of fossil fuels to supply energy in the future is essential. Perovskite solar cells (PSCs) are a new generation of solar cells that are developing extremely rapidly due to their excellent photovoltaic properties such as having a high absorption coefficient, tunable band gaps, long electron–hole diffusion lengths, and a high charge carrier mobility, as well as facile fabrication processes, high efficiencies and low cost [1] compared to other sorts of solar cells [2–4]. PSCs are made out of organic-inorganic materials of methylammonium lead halides (CH3NH3PbX3, where X = I, Br, or Cl) by different techniques and structures such as vacuum evaporation and solution schemes, and with planar and mesoporous heterojunction structures [5]. Typically metal oxides such as TiO2, Al2O3, and ZnO are used as an electron transport material (ETM) in the structure of PSCs. In some cases, solar cells have to be flexible. Hence, researchers have tried to fabricate flexible PSCs recently [6,7]. Flexible solar cells are fabricated on flexible polymeric substrates such as
polypropylene (PP) and polyethylene terephthalate (PET) which have low melting points and degrades in relatively lower temperatures (compared to inorganic materials). However, fabrication of flexible solar cells requires low-temperature manufacturing methods, which is the need of the flexible substrates. In general, metal oxides need to be heated to a high temperature (>400°) for annealing to enhance the crystallinity of the ETM structure which leads to a better transportation of electrons and a higher efficiency [8–11]. ZnO is a compound that does not need a high temperature to show a better performance; therefore, it could function as ETM instead of other metal oxides which are processed at high temperatures. In a low-temperature PSCs, ZnO have been coated on both rigid and flexible substrates, by different techniques such as electrodeposition [12] and spin coating [13] in the forms of nanolayer [14,15], nanowalls [16], nanosheets [17] and nanorods [18] as compact layer, as well as ETM and hole blocking layer. In this study, we designed a low-temperature photovoltaic nanostructure to apply on a flexible substrate for fabrication of a flexible PSC. The first step for fabrication of a ZnO-based flexible PSC is to obtain a repeatable and sustainable photovoltaic nanostructure on a glass substrate. After that, this nanostructure will be applied on a flexible substrate. The focus of this paper is to study the fabrication parameters for nanostructured, lowtemperature ZnO-based PSCs without a hole transport material (HTM), and to study the effects of these parameters on the performance and principal characteristics for the fabricated PSCs. Therefore, we investigated mainly three parameters; the concentration of ZnO nanoparticles (NPs) in the dispersion, the spin coating speed (rpm) of ZnO and the concentration of CH3NH3I (MAI). Their effects on fabricated low-temperature ZnO-based PSCs samples were also investigated. 2. Materials and Methods 2.1. Fabrication of Low-temperature Solar cell Fabrication of Low-temperature PSCs was done according to the previous reports [14]. At first, to cut the two electrodes, a specific zone of fluorine-doped tin oxide (FTO) coated glass (with the sheet resistance of 14 Ω/sq) was etched by Zn powder and HCl (2 Molar). Then the etched glass was cleaned by sequential washing in an ultrasonic bath by deionized water, acetone, and ethanol, respectively for 20 minutes for each step and followed by drying in an air flow. ZnO NPs with the size of 10-30 nm were obtained from US Research Nanomaterials, Inc. A ZnO compact layer including ZnO NPs at different solvent concentrations (dispersed in 70 ml butanol, 5 ml methanol, 5ml chloroform) was spin coated on FTO surface at various speeds (rpm) for 35 seconds. After sintering at 150⁰C for 20 minutes, a PbI2 solution (dissolved in N,Ndimethylformamide at a concentration of 460 mg/mL) was spin coated on the ZnO nanolayer at 3000 rpm for 20 seconds. Then it was dried at 70 °C on a hot plate for 20 minutes. To form perovskite grains as light absorbers, the substrate was dipped in a solution of CH3NH3I (MAI) which had dissolved in isopropanol at different concentration for 2 minutes. The yellow color of deposited PbI2 was changing to dark brown-black gradually. As the structure was without HTM, the final stage was the coating of gold with a thickness of 170 nm as top contact electrode by thermal evaporation technique under the conditions of 3×10-5 bar vacuum pressure by coating rate of 0.1 nm/s. The coating was carried out by VAS BUC model:78535 thermal evaporator system. The fabrication process is depicted in Figure 1. 2.2. Design of experiments
For investigation of the effect of ZnO NPs dispersion concentration on solar cell performance, we varied the concentration of ZnO nanoparticles in the solvent at six levels; 10 mg/ml, 20 mg/ml, 30 mg/ml, 40 mg/ml, 60 mg/ml and 80 mg/ml. Samples then were fabricated by 2500 spin coating speed (rpm) of ZnO in 35 seconds and dip coating of them in (MAI) (dissolved in isopropanol at a concentration of 10 mg/mL) for 2 minutes. To evaluate the effect of ZnO spin coating speed (rpm) on the solar cell characteristics, four speeds of 2000, 3000, 4000 and 5000 rpm were used. The ZnO concentration was 40 mg/ml and the samples were dip coated on MAI dissolved in isopropanol at a concentration of 10 mg/ml for 2 minutes. For assessment of the effect MAI concentration in isopropanol, we fabricated samples by three concentrations of 8 mg/ml, 10mg/ml and 12 mg/ml for 2 minutes. The ZnO concentration was 40 mg/ml and the spin coating speed for ZnO was 5000 rpm in 35 seconds. All samples were fabricated by 3000 rpm spin coating of PbI2 solution (dissolved in N,Ndimethylformamide at a concentration of 460 mg/mL) in 20 seconds and with a coating of gold as a top electrode with the thickness of 170 nm. 2.3. Characterization Solar light simulator (300 W, Xenon lamp) calibrated by a reference Si solar cell under illumination of AM 1.5 (100 mW/cm2). A Keithley 2400 digital source meter were used for measurement of current density-voltage(J-V) characteristics of the PSCs. Scanning electronic microscope (Vega3 TESCAN SEM system) was used to study of surface morphology and crosssectional view of PSC nanostructures. The UV spectra graphs were measured via UV–vis spectrophotometry (Ocean Optics HR 4000). 3. Results and discussion Figure 1 shows a schematic description of the fabrication process for low-temperature PSCs with ZnO ETM without HTM. The configuration of the fabricated solar cell is FTO/compact layer of ZnO NPs (blocking layer)/CH3NH3PbI3/Au electrode. After etching the FTO-coated glass, the compact layer of ZnO NPs was spin coated. After that, the perovskite grains in two steps, as mentioned earlier, were formed. The final layer was a gold nanolayer as a top electrode. A cross-sectional SEM images of the mentioned structure with the sizes of different nanolayers are shown in Figure 2. Figure 3 depicts the arrangement of energy levels for separation the couple of electron–hole (exciton) and their transferring to the electrodes. Figure 4 shows the effect of ZnO NPs concentration in the solvent on the current density-voltage (J-V) property of the fabricated solar cell. It is clear that the curve for ZnO NPs concentration of 40 mg/ml poses the best performance of the fabricated solar cell. There is no obvious trend in the results, e.g., the performance for the 10 mg/ml sample is the weakest while the 80 mg/ml is weaker than 20 mg/ml. As Table 1 illustrates, the power conversion efficiency (PCE) enhancement of the fabricated PSCs in the concentration of 40 mg/ml is significant. By increasing the concentration of ZnO nanoparticles, the amount of ZnO nanoparticles increases and results in a thicker compact layer of ZnO as an ETM. It seems that a better transfer of electrons and less recombination lead to an elevation of performance until 40 mg/ml concentration. This result is in agreement with other researchers’ reports [19–21]. Increasing the concentration of ZnO NPs in the solvent leads to both unstable dispersion and agglomeration as well as poor transfer of electrons where results in a reduction in cell performance. As Hadouchi et. al. [20] discussed, ZnO acts a role as a hole blocking layer in planar perovskite. Moreover,
ZnO has been used to prepare perovskite absorber-based photovoltaic devices under low temperatures (Kun Mu et. al. [19]). Table 1 shows the performance of the prepared solar cells. As it can be seen, the best performance is for the 40 mg/ml concentration. Although this performance is lower than 7% (reported by Kun Mu el. al. [19] which used a HTM), our solar cell does not contain any HTM. Hence, the solar cell prepared via the method showed in this paper is low-cost, has a facile preparation process (due to having low-temperature sintering). Figure 5 depicts the speed of spin coating ZnO NPs has an effect on Low-temperature PSCs. By increasing the rpm, the surface of ZnO NPs compact layer was more uniform and smooth and resulted in a better arrangement of NPs which led to a better electron transport and a higher performance of PSC. The values for the effect of spin coating speed on the solar cell characteristics are tabulated in Table 2. The enhancement of 266 % for PCE in 5000 rpm is considerable. To investigate the effect of MAI concentration in isopropanol, three concentrations were chosen: 8 mg/ml, 10 mg/ml and 12 mg/ml. Results showed that increasing the concentration of MAI lead to the growth of the smaller size perovskite grains which gave a larger contact surface with the bottom layer (i. e., ZnO), should result in more electron transport and better performance of the solar cell. Figure 6 shows the J-V curves for different concentrations of MAI. Table 3 depicts the differences between the characteristics of fabricated solar cells in three concentrations of MAI. By increasing the concentration of MAI, PCE enhancement reached to 238%. The SEM micrographs showing growth of perovskite grains in various concentration of MAI are presented in Figure 7. By increasing the MAI concentration from 8 mg/ml to 12 mg/ml, the average size of the perovskite grains is decreased by 80%. Figure 8 shows the corresponding absorbance spectra of fabricated solar cells with different sizes of perovskite grains resulting from various concentrations of MAI. The concentration of 12 mg/ml has greater absorbance compared to the absorbance of the other concentrations. It can be concluded that the smaller perovskite grains can absorb more sunlight due to larger contact surfaces with sunlight. These results are in agreement with other researcher’s findings [21].
4. Conclusions Low-temperature HTM-free PSCs were fabricated successfully. It was shown that the concentration of ZnO NPs in the solvent is an effective fabrication parameter on principal characteristics of Low-temperature PSCs. It was also demonstrated that it is necessary to achieve a stable dispersion to prepare a uniform coating of the compact layer as an ETM layer for better transfer of electrons and preventing the recombination of electrons and holes. By increasing the concentration of ZnO NPs to 40 mg/ml, a thicker ZnO compact layer was obtained containing a higher amount of ZnO NPs, resulting in an improvement in cell performance. Increasing the concentration of ZnO NPs to more than 40 mg/ml led to an unstable dispersion and NPs agglomeration, which resulted in the decline of PSCs performance. The speed of ZnO spin coating for obtaining a smooth compact layer on FTO was effective on PSCs performance. Increasing spin coating speed of ZnO NPs led to a uniform and smooth compact layer with a more orderly arrangement of ZnO NPs and suggested a better transfer of electron and less recombination. The concentration of MAI in isopropanol for growing the perovskite grains was effective on Low-temperature PSCs performance. Higher concentration of the MAI led to growing perovskite grains with smaller size and more contact surface with sunlight that resulted in more absorbance and better performance of Low-temperature PSCs. This structure
has the potential to be used on flexible substrates to prepare cost-effective perovskite flexible solar cells. Acknowledgments We would like to thank Photonic Research Group (YPRG), Yazd University and Isatis Nanostructure-based Optical Devices Co. for their laboratory and technical supports. Also, this research was supported financially by Iran Nanotechnology Initiative Council.
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Figure and tables
Figure 1. Schematic description of the fabrication process for a low-temperature solar cell using ZnO as ETM without HTM.
Figure 2. Cross section SEM picture of nano structure Low-temperature PSC with ZnO compact layer as ETM.
Figure 3. Schematic energy levels description of Low-temperature PSCs with ZnO as ETM and without HTM.
Figur 4. J–V characteristics of the low-temperature PSCs for different concentration of ZnO NPs dispersion in a solvent for spin coating ZnO NPs as a compact layer as ETM.
Figur 5. J–V characteristics of Low-temperature PSC for different spin coating speed (rpm) of ZnO as compact layer to electron transport
Figur 6. J–V characteristics of Low-temperature PSCs for different concentration of MAI in isopropanol solvent
Figure 7. SEM images of perovskite grains for different concentration of MAI in isopropanol solvent (a) concentration of 8 mg/ml with the average size of 170 nm (b) concentration of 10 mg/ml with the average size of 90 nm and (c) concentration of 12 mg/ml with the average size of 35 nm
Figure 8. Absorbance spectrum of Low-temperature PSCs for different concentration of MAI in IsoPropanol solvent
Table 1. Measured characterizations of fabricated Low-temperature PSCs in different concentration of ZnO nano particles dispersion in solvent Concentration JSC VOC (V) 2 (mg/ml) (mA/cm )
FF
PCE (%)
Enhancement (%)
10
0.39
0.34
0.43
0.06
-
20
0.52
0.37
0.38
0.07
17
30
0.94
0.44
0.49
0.21
250
40
1.41
0.42
0.44
0.26
333
60
0.63
0.36
0.39
0.09
50
80
0.48
0.33
0.39
0.06
-
Table 2. Measured characterizations of fabricated Low-temperature perovskite solar cells at different spin coating rpm of ZnO as compact layer to electron transport Spin coating (rpm)
JSC (mA/cm2)
VOC (V)
FF
PCE (%)
Enhancement (%)
2000
0.43
0.23
0.33
0.03
-
3000
0.59
0.23
0.35
0.05
66
4000
0.77
0.28
0.31
0.07
133
5000
1.18
0.25
0.40
0.11
266
Table 3. Measured characterizations of fabricated low-temperature PSCs at various concentration of MAI in isopropanol solvent for growth the perovskite grains Concentration of MAI (mg/ml)
JSC (mA/cm2)
VOC (V)
FF
PCE (%)
Enhancement (%)
8
0.35
0.14
0.28
0.013
-
10
0.55
0.14
0.27
0.020
54
12
0.62
0.20
0.36
0.044
238
S0030-4026(17)30253-X http://dx.doi.org/doi:10.1016/j.ijleo.2017.02.101 IJLEO 58917
To appear in: Received date: Accepted date:
7-12-2016 28-2-2017
Please cite this article as: Mohammad Hatamvand, Seyed Abbas Mirjalili, Maryam Sharzehee, Abbas Behjat, Mostafa Jabbari, Mikael Skrifvars, Fabrication parameters of low-temperature ZnO-based hole-transport-free perovskite solar cells, Optik - International Journal for Light and Electron Optics http://dx.doi.org/10.1016/j.ijleo.2017.02.101 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Fabrication Parameters of Low-temperature ZnO-based Hole-transport-free Perovskite Solar Cells Mohammad Hatamvanda,b,d, Seyed Abbas Mirjalilia*, Maryam Sharzeheea, Abbas Behjatb,c, Mostafa Jabbarid, Mikael Skrifvarsd a
Department of Textile Engineering, Yazd University, Yazd, Iran
b
Photonics Research Group, Engineering Research Center, Yazd University, Yazd, Iran
c
Atomic and Molecular Division, Faculty of Physics, Yazd University, Yazd, Iran
d
Swedish Centre for Resource Recovery (SCRR), University of Borås, 50190 Borås,
Sweden *Corresponding author e-mail: amirjalili@ yazd.ac.ir
Abstract Perovskite solar cells (PSCs) are a new generation solar cells. Low-Temperature techniques are used for fabrication PSCs on a flexible substrate that has a low thermal tolerance. In this paper, low-temperature PSCs with ZnO nanoparticles were prepared as electron transport material (ETM) without hole transport material (HTM). Effects of some of the fabrication parameters of low-temperature ZnO based PSCs without HTM, on their principal characteristics and performance, were investigated. Parameters such as the concentration of ZnO nanoparticles (NPs) dispersion, spin coating speed of ZnO NPs, and concentration of CH3NH3I on characteristics and performance of fabricated low-temperature PSCs were studied. The study shows that by changing these parameters, the performance of the fabricated PSCs changes considerably. Keywords: Perovskite solar cells, Low-temperature solar cells, Flexible solar cell, ZnO-based perovskite solar cells, ZnO nanoparticles 1. Introduction Solar energy is abundant, renewable, unlimited, clean, eco-friendly, sustainable and available. Due to these reasons, solar energy is an excellent alternative for substitution of fossil fuels which are limited, air pollutant and costly. Therefore, the development of solar cells as a replacement of fossil fuels to supply energy in the future is essential. Perovskite solar cells (PSCs) are a new generation of solar cells that are developing extremely rapidly due to their excellent photovoltaic properties such as having a high absorption coefficient, tunable band gaps, long electron–hole diffusion lengths, and a high charge carrier mobility, as well as facile fabrication processes, high efficiencies and low cost [1] compared to other sorts of solar cells [2–4]. PSCs are made out of organic-inorganic materials of methylammonium lead halides (CH3NH3PbX3, where X = I, Br, or Cl) by different techniques and structures such as vacuum evaporation and solution schemes, and with planar and mesoporous heterojunction structures [5]. Typically metal oxides such as TiO2, Al2O3, and ZnO are used as an electron transport material (ETM) in the structure of PSCs. In some cases, solar cells have to be flexible. Hence, researchers have tried to fabricate flexible PSCs recently [6,7]. Flexible solar cells are fabricated on flexible polymeric substrates such as
polypropylene (PP) and polyethylene terephthalate (PET) which have low melting points and degrades in relatively lower temperatures (compared to inorganic materials). However, fabrication of flexible solar cells requires low-temperature manufacturing methods, which is the need of the flexible substrates. In general, metal oxides need to be heated to a high temperature (>400°) for annealing to enhance the crystallinity of the ETM structure which leads to a better transportation of electrons and a higher efficiency [8–11]. ZnO is a compound that does not need a high temperature to show a better performance; therefore, it could function as ETM instead of other metal oxides which are processed at high temperatures. In a low-temperature PSCs, ZnO have been coated on both rigid and flexible substrates, by different techniques such as electrodeposition [12] and spin coating [13] in the forms of nanolayer [14,15], nanowalls [16], nanosheets [17] and nanorods [18] as compact layer, as well as ETM and hole blocking layer. In this study, we designed a low-temperature photovoltaic nanostructure to apply on a flexible substrate for fabrication of a flexible PSC. The first step for fabrication of a ZnO-based flexible PSC is to obtain a repeatable and sustainable photovoltaic nanostructure on a glass substrate. After that, this nanostructure will be applied on a flexible substrate. The focus of this paper is to study the fabrication parameters for nanostructured, lowtemperature ZnO-based PSCs without a hole transport material (HTM), and to study the effects of these parameters on the performance and principal characteristics for the fabricated PSCs. Therefore, we investigated mainly three parameters; the concentration of ZnO nanoparticles (NPs) in the dispersion, the spin coating speed (rpm) of ZnO and the concentration of CH3NH3I (MAI). Their effects on fabricated low-temperature ZnO-based PSCs samples were also investigated. 2. Materials and Methods 2.1. Fabrication of Low-temperature Solar cell Fabrication of Low-temperature PSCs was done according to the previous reports [14]. At first, to cut the two electrodes, a specific zone of fluorine-doped tin oxide (FTO) coated glass (with the sheet resistance of 14 Ω/sq) was etched by Zn powder and HCl (2 Molar). Then the etched glass was cleaned by sequential washing in an ultrasonic bath by deionized water, acetone, and ethanol, respectively for 20 minutes for each step and followed by drying in an air flow. ZnO NPs with the size of 10-30 nm were obtained from US Research Nanomaterials, Inc. A ZnO compact layer including ZnO NPs at different solvent concentrations (dispersed in 70 ml butanol, 5 ml methanol, 5ml chloroform) was spin coated on FTO surface at various speeds (rpm) for 35 seconds. After sintering at 150⁰C for 20 minutes, a PbI2 solution (dissolved in N,Ndimethylformamide at a concentration of 460 mg/mL) was spin coated on the ZnO nanolayer at 3000 rpm for 20 seconds. Then it was dried at 70 °C on a hot plate for 20 minutes. To form perovskite grains as light absorbers, the substrate was dipped in a solution of CH3NH3I (MAI) which had dissolved in isopropanol at different concentration for 2 minutes. The yellow color of deposited PbI2 was changing to dark brown-black gradually. As the structure was without HTM, the final stage was the coating of gold with a thickness of 170 nm as top contact electrode by thermal evaporation technique under the conditions of 3×10-5 bar vacuum pressure by coating rate of 0.1 nm/s. The coating was carried out by VAS BUC model:78535 thermal evaporator system. The fabrication process is depicted in Figure 1. 2.2. Design of experiments
For investigation of the effect of ZnO NPs dispersion concentration on solar cell performance, we varied the concentration of ZnO nanoparticles in the solvent at six levels; 10 mg/ml, 20 mg/ml, 30 mg/ml, 40 mg/ml, 60 mg/ml and 80 mg/ml. Samples then were fabricated by 2500 spin coating speed (rpm) of ZnO in 35 seconds and dip coating of them in (MAI) (dissolved in isopropanol at a concentration of 10 mg/mL) for 2 minutes. To evaluate the effect of ZnO spin coating speed (rpm) on the solar cell characteristics, four speeds of 2000, 3000, 4000 and 5000 rpm were used. The ZnO concentration was 40 mg/ml and the samples were dip coated on MAI dissolved in isopropanol at a concentration of 10 mg/ml for 2 minutes. For assessment of the effect MAI concentration in isopropanol, we fabricated samples by three concentrations of 8 mg/ml, 10mg/ml and 12 mg/ml for 2 minutes. The ZnO concentration was 40 mg/ml and the spin coating speed for ZnO was 5000 rpm in 35 seconds. All samples were fabricated by 3000 rpm spin coating of PbI2 solution (dissolved in N,Ndimethylformamide at a concentration of 460 mg/mL) in 20 seconds and with a coating of gold as a top electrode with the thickness of 170 nm. 2.3. Characterization Solar light simulator (300 W, Xenon lamp) calibrated by a reference Si solar cell under illumination of AM 1.5 (100 mW/cm2). A Keithley 2400 digital source meter were used for measurement of current density-voltage(J-V) characteristics of the PSCs. Scanning electronic microscope (Vega3 TESCAN SEM system) was used to study of surface morphology and crosssectional view of PSC nanostructures. The UV spectra graphs were measured via UV–vis spectrophotometry (Ocean Optics HR 4000). 3. Results and discussion Figure 1 shows a schematic description of the fabrication process for low-temperature PSCs with ZnO ETM without HTM. The configuration of the fabricated solar cell is FTO/compact layer of ZnO NPs (blocking layer)/CH3NH3PbI3/Au electrode. After etching the FTO-coated glass, the compact layer of ZnO NPs was spin coated. After that, the perovskite grains in two steps, as mentioned earlier, were formed. The final layer was a gold nanolayer as a top electrode. A cross-sectional SEM images of the mentioned structure with the sizes of different nanolayers are shown in Figure 2. Figure 3 depicts the arrangement of energy levels for separation the couple of electron–hole (exciton) and their transferring to the electrodes. Figure 4 shows the effect of ZnO NPs concentration in the solvent on the current density-voltage (J-V) property of the fabricated solar cell. It is clear that the curve for ZnO NPs concentration of 40 mg/ml poses the best performance of the fabricated solar cell. There is no obvious trend in the results, e.g., the performance for the 10 mg/ml sample is the weakest while the 80 mg/ml is weaker than 20 mg/ml. As Table 1 illustrates, the power conversion efficiency (PCE) enhancement of the fabricated PSCs in the concentration of 40 mg/ml is significant. By increasing the concentration of ZnO nanoparticles, the amount of ZnO nanoparticles increases and results in a thicker compact layer of ZnO as an ETM. It seems that a better transfer of electrons and less recombination lead to an elevation of performance until 40 mg/ml concentration. This result is in agreement with other researchers’ reports [19–21]. Increasing the concentration of ZnO NPs in the solvent leads to both unstable dispersion and agglomeration as well as poor transfer of electrons where results in a reduction in cell performance. As Hadouchi et. al. [20] discussed, ZnO acts a role as a hole blocking layer in planar perovskite. Moreover,
ZnO has been used to prepare perovskite absorber-based photovoltaic devices under low temperatures (Kun Mu et. al. [19]). Table 1 shows the performance of the prepared solar cells. As it can be seen, the best performance is for the 40 mg/ml concentration. Although this performance is lower than 7% (reported by Kun Mu el. al. [19] which used a HTM), our solar cell does not contain any HTM. Hence, the solar cell prepared via the method showed in this paper is low-cost, has a facile preparation process (due to having low-temperature sintering). Figure 5 depicts the speed of spin coating ZnO NPs has an effect on Low-temperature PSCs. By increasing the rpm, the surface of ZnO NPs compact layer was more uniform and smooth and resulted in a better arrangement of NPs which led to a better electron transport and a higher performance of PSC. The values for the effect of spin coating speed on the solar cell characteristics are tabulated in Table 2. The enhancement of 266 % for PCE in 5000 rpm is considerable. To investigate the effect of MAI concentration in isopropanol, three concentrations were chosen: 8 mg/ml, 10 mg/ml and 12 mg/ml. Results showed that increasing the concentration of MAI lead to the growth of the smaller size perovskite grains which gave a larger contact surface with the bottom layer (i. e., ZnO), should result in more electron transport and better performance of the solar cell. Figure 6 shows the J-V curves for different concentrations of MAI. Table 3 depicts the differences between the characteristics of fabricated solar cells in three concentrations of MAI. By increasing the concentration of MAI, PCE enhancement reached to 238%. The SEM micrographs showing growth of perovskite grains in various concentration of MAI are presented in Figure 7. By increasing the MAI concentration from 8 mg/ml to 12 mg/ml, the average size of the perovskite grains is decreased by 80%. Figure 8 shows the corresponding absorbance spectra of fabricated solar cells with different sizes of perovskite grains resulting from various concentrations of MAI. The concentration of 12 mg/ml has greater absorbance compared to the absorbance of the other concentrations. It can be concluded that the smaller perovskite grains can absorb more sunlight due to larger contact surfaces with sunlight. These results are in agreement with other researcher’s findings [21].
4. Conclusions Low-temperature HTM-free PSCs were fabricated successfully. It was shown that the concentration of ZnO NPs in the solvent is an effective fabrication parameter on principal characteristics of Low-temperature PSCs. It was also demonstrated that it is necessary to achieve a stable dispersion to prepare a uniform coating of the compact layer as an ETM layer for better transfer of electrons and preventing the recombination of electrons and holes. By increasing the concentration of ZnO NPs to 40 mg/ml, a thicker ZnO compact layer was obtained containing a higher amount of ZnO NPs, resulting in an improvement in cell performance. Increasing the concentration of ZnO NPs to more than 40 mg/ml led to an unstable dispersion and NPs agglomeration, which resulted in the decline of PSCs performance. The speed of ZnO spin coating for obtaining a smooth compact layer on FTO was effective on PSCs performance. Increasing spin coating speed of ZnO NPs led to a uniform and smooth compact layer with a more orderly arrangement of ZnO NPs and suggested a better transfer of electron and less recombination. The concentration of MAI in isopropanol for growing the perovskite grains was effective on Low-temperature PSCs performance. Higher concentration of the MAI led to growing perovskite grains with smaller size and more contact surface with sunlight that resulted in more absorbance and better performance of Low-temperature PSCs. This structure
has the potential to be used on flexible substrates to prepare cost-effective perovskite flexible solar cells. Acknowledgments We would like to thank Photonic Research Group (YPRG), Yazd University and Isatis Nanostructure-based Optical Devices Co. for their laboratory and technical supports. Also, this research was supported financially by Iran Nanotechnology Initiative Council.
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Figure and tables
Figure 1. Schematic description of the fabrication process for a low-temperature solar cell using ZnO as ETM without HTM.
Figure 2. Cross section SEM picture of nano structure Low-temperature PSC with ZnO compact layer as ETM.
Figure 3. Schematic energy levels description of Low-temperature PSCs with ZnO as ETM and without HTM.
Figur 4. J–V characteristics of the low-temperature PSCs for different concentration of ZnO NPs dispersion in a solvent for spin coating ZnO NPs as a compact layer as ETM.
Figur 5. J–V characteristics of Low-temperature PSC for different spin coating speed (rpm) of ZnO as compact layer to electron transport
Figur 6. J–V characteristics of Low-temperature PSCs for different concentration of MAI in isopropanol solvent
Figure 7. SEM images of perovskite grains for different concentration of MAI in isopropanol solvent (a) concentration of 8 mg/ml with the average size of 170 nm (b) concentration of 10 mg/ml with the average size of 90 nm and (c) concentration of 12 mg/ml with the average size of 35 nm
Figure 8. Absorbance spectrum of Low-temperature PSCs for different concentration of MAI in IsoPropanol solvent
Table 1. Measured characterizations of fabricated Low-temperature PSCs in different concentration of ZnO nano particles dispersion in solvent Concentration JSC VOC (V) 2 (mg/ml) (mA/cm )
FF
PCE (%)
Enhancement (%)
10
0.39
0.34
0.43
0.06
-
20
0.52
0.37
0.38
0.07
17
30
0.94
0.44
0.49
0.21
250
40
1.41
0.42
0.44
0.26
333
60
0.63
0.36
0.39
0.09
50
80
0.48
0.33
0.39
0.06
-
Table 2. Measured characterizations of fabricated Low-temperature perovskite solar cells at different spin coating rpm of ZnO as compact layer to electron transport Spin coating (rpm)
JSC (mA/cm2)
VOC (V)
FF
PCE (%)
Enhancement (%)
2000
0.43
0.23
0.33
0.03
-
3000
0.59
0.23
0.35
0.05
66
4000
0.77
0.28
0.31
0.07
133
5000
1.18
0.25
0.40
0.11
266
Table 3. Measured characterizations of fabricated low-temperature PSCs at various concentration of MAI in isopropanol solvent for growth the perovskite grains Concentration of MAI (mg/ml)
JSC (mA/cm2)
VOC (V)
FF
PCE (%)
Enhancement (%)
8
0.35
0.14
0.28
0.013
-
10
0.55
0.14
0.27
0.020
54
12
0.62
0.20
0.36
0.044
238