Effect of Al-doped ZnO for Electron Transporting Layer in Planar Perovskite solar cells

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ScienceDirect Materials Today: Proceedings 17 (2019) 1259–1267

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MRS-Thailand 2017

Effect of Al-doped ZnO for Electron Transporting Layer in Planar Perovskite solar cells Chawalit Bhoomaneea,b, Pipat Ruankhama,b, Supab Choopuna,b, Aschariya Prathana,b, Duangmanee Wongratanaphisana,b,* a

Department of Physics and Materials Science, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand, Thailand Center of Excellence in Physics (ThEP center), Commission on Higher Education, Bangkok 10400, Thailand

b

Abstract Perovskite based solar cells have attracted the photovoltaic industry as storm due to their impressive efficiency, from under 4% in 2009 to over 20% in 2017. This work presents zinc oxide (ZnO) adopted as an electron transport material (ETM) layer in the planar perovskite solar cells due to its simple synthesis and excellent electrical properties. ZnO ETM in conventional perovskite solar cells shows the lower efficiency since ZnO will react with the organic cation (CH3NH3+) of perovskite. Extrinsically doping a small amount of Aluminium (Al) with ZnO (AZO) was used to improve the physicochemical properties of ZnO. The AZO ETM was prepared by spin coating technique with AZO sol-gel at 0, 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0 at% Al doping. Compared to the ZnO ETM based perovskite cell, the cell based on AZO ETM shows the highest Voc and Jsc. The perovskite cell with AZO ETM above 1.5 at% Al doping exhibits better and stable the performance cells because doing so can encourage the charge transporting and match the band energy with MAPbI3 compared to pure ZnO. © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of The First Materials Research Society of Thailand International Conference. Keywords: AZO; electron transporting layer; perovskite solar cells

1. Introduction Hybrid halide perovskites [1, 2] is a kind of compound between organic and inorganic with ABX3 formula where A is an ammonium cation (CH3NH3+) organic, B is a metal (Pb/Sn) and X is a halide anion (Cl, Br, or I [3]). The crystal structures of perovskite-type compounds are commonly used for solar cell [3] such as CH3NH3PbI3, CH3NH3PbCl3, CH3NH3PbBr3, CH3NH3GeCl3, and CH3NH3SnCl3. Especially, methylammonium lead iodide * Corresponding author. Tel.: +66 81575 9350; fax: +66 5335 7511. E-mail address: [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of The First Materials Research Society of Thailand International Conference.

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perovskite CH3NH3PbI3 or MAPbI3 is the first hybrid halide perovskite [2]. Perovskite based solar cells have attracted the photovoltaics industry by storm because their impressive efficiency has a rapid increasing in a few years, from under 4% efficiency in 2009 to over 20% efficiency in 2017 [2, 4]. ZnO is one material used for electron transport material (ETM) or electron transporting layer (ETL) as a hold blocking electrode [1]. ZnO films can be deposited by different methods [1] such as sputtering [5, 6], electrospraying [7], chemical vapor deposition (CVD), pulsed laser chemical spray, pulsed laser deposition (PLD), and sol-gel process. The sol-gel method [8] has a several advantage because this method easily controlled the composition, reduced cost product materials and run the process at low temperature. However, ZnO ETM has the poor chemical stability at the ZnO/perovskite interface [9] because the basic nature of ZnO may react with the organic cation (CH3NH3+) in perovskite [5]. Al-doped ZnO (AZO) is a suitable extraction layer which improved the effect of ZnO/perovskite interface [10] because AZO ETM has better band matching with perovskite and more resistance toward acid than ZnO films [5]. In this work, AZO was applied as an ETL in the conventional planar perovskite solar cells. The AZO ETM can significantly improve the perovskite cell efficiency by increase of the charge carrier density. 2. Experimental detail 2.1. Preparation of ZnO and AZO ETM films ZnO and Al-doped ZnO (AZO) thin films coated on an indium doped tin oxide (ITO) glass substrate were prepared by sol-gel method as shown in Figure 1. The ITO patterning was achieved using zinc powder and dilute solution of HCL [11]. Prior to modified sequential ETM deposition, the etched ITO substrates were subjected to strong cleanness under sonication with a 2% solution of alconox cleaning detergents, de-ionized water, acetone, alcohol and 2-proponal for 30 min each. After that, ITO substrates were dried with N2 gun and treated with ultraviolet ozone for 30 min. For ZnO precursor solution, 0.75 M precursor solution was prepared by dissolving zinc acetate dihydrate [ZnAc:(Zn(CH3COO)2.2H2O)] in 2-metheoxyethanol [2-MOE:C3H8O2] with equivalent molar ratio of ethanolamine [ETA:(C2H7NO)], stirred at 60oC for 3 hr and then kept at room temperature overnight to form clear homogenous. Next, the same conditions were applied for AZO precursor solution using aluminum chloride hexahydrate [(AlCl3.6H2O)] as doping agent with different Al/(Al+Zn) molar ratios of 0%, 0.5%, 1.0%, 1.5%, 2.0%, 2.5% and 3.0% solution. The cleaned ITO substrate was spin coated with a 0% precursor solution at 2000 rpm for 30 s, and annealed on a hotplate at 160oC for 30 min. The process was repeated 2 times. Next, the top AZO film at different doping ratio was coated on the ITO/ZnO substrates at the same speed and duration. The substrate was annealed again at 160oC for 30 min.

Figure 1 Schematic illustration of modified sequential ETM deposition.

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2.2. Preparation of MAPbI3 perovskite films The MAPbI3 perovskite films were prepared by two-step deposition method. Before MAPbI3 films preparation, the ETM surface was coated by Phenyl-C60-butyric acid methyl ester [(PCBM)] (10 mg/ml ) compact layer for blocking layer, dissolved in chlorobenzene [(CB)] and kept at 70oC overnight before spinning a 80 microliter portion of PCBM solution onto the ETM at 2000 rpm for 30 s. The first step, a 1 M solution of PbI2 in N,N-Dimethylformamide [(DMF)] was prepared and kept at 70oC for overnight. A PbI2 layer was deposited onto the surface of ZnO/AZO layer by spin-coating a 80 microliter portion of PbI2 solution at 3000 rpm for 30 sec inside a N2-filled glovebox. Then, the as-deposited film was annealed on a hotplate at 70oC for 10 min. The second step, a 200 microliter portion of a 10 mg/ml methylammonium iodide [(MAI)] in 2-proponal was dropped onto the PbI2 film and kept for 40 s before spinning the substrate at 2000 rpm for 30 s for MAPbI3 perovskite forming. Then, it was immediately dried at 100oC for 3 min. 2.3. Preparation of solar cells After the MAPbI3 perovskite films were deposited, a hold transport material (HTM) was coated with cobaltdoped poly (3-hexylthiophene-2,5-diyl) [Co-P3HT] by spin coating a 80 microliter portion of Co-P3HT solution at 1000 rpm for 30 s. The Co-P3HT [12] solution prepared in two steps; initially, 30 mg of P3HT was dissolved in 1mL of chlorobenzene [CB]. After sonication 0.5 hr, P3HT solution was stirred at room temperature for 2 hr. Finally, 16 microliter of acetonitrile, 8 microliter of 4-tert-butyl pyridine (tBP) and 8 microliter of Co(II)-TFSI were added to the P3HT solution. Finally, the Co-P3HT solution was kept at 40oC for 1 hr and then stirred at room temperature overnight. Finally, gold (Au) top electrode was thermally evaporated onto the Co-P3HT film of the perovskite solar cells. The schematic device structure fabricated in this work is shown in Figure 2.

Figure 2 Schematic diagram of perovskite solar cell structure.

2.4. Characterization techniques The surface morphology and cross section of the ETM and MAPbI3 perovskite films were observed with a field emission scanning electron microscope (FE-SEM). The crystallinity of the MAPbI3 perovskite films were investigated using the Bragg-Brentano geometry and X-ray diffractometry (Cu K-alpha; λ=0.154056 nm). The optical properties were obtained with a UV-Vis spectroscopy (UV-Vis). The photocurrent-voltage (J-V) curve characteristics of perovskite solar cells were measured using simulated solar light AM 1.5 at 100 mWcm-2 in ambient atmosphere. The active area for light irradiation was defined using a photo mask with area of 0.038 cm2. Charge dynamics of the perovskite solar cells were investigated using measurement of open-circuit voltage decay (OCVD). 3. Result and Discussion 3.1. Morphologies and crystallinity of the electron transport materials FE-SEM surface images of the ETM films by spin coating after annealing at 160oC are shown in Figure 3. The surface layer of the ZnO/AZO with 2 at% Al doping was smoother, denser and less crack than other surface layers as shown Figure 3(d). As seen in the figure, the crack of the surface layer was decrease with increasing Al doping.

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Figure 3 FE-SEM images of the surface layers of the ZnO ETM films with AZO at 0, 0.5, 1.5, and 2 at% Al.

3.2. Optical properties of the electron transport materials Moreover, the ZnO ETM films with at% Al doping show the average transmittance about 80% in visible region (400-700 nm) compared with ITO glass baseline as shown in Figure 4. Optical band gap energy (Eg) can be evaluated by the Beer-Lambert attenuation equation as follows [13]: 2 (1)

(α hν )

= C ( hν − Eg ) ,

where C is a constant, hυ is photon energy (eV), and α stands for absorption coefficient. The absorption coefficient is defined as follows:

1 d

(2)

1 T

α = ln( ), where T and d represent transmittance and thickness, respectively. 100

Transmittance (%)

80

60

0 at% Al 0.5 at% Al 1.0 at% Al 1.5 at% Al 2.0 at% Al 2.5 at% Al 3.0 at% Al

40

20

0 400

600

800

1000

Wavelength (nm) Figure 4 Optical properties of the ZnO electron transport layer films with AZO sol-gel at 0, 0.5, 1, 1.5, 2, 2.5, and 3 at% Al doping.

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The band gap energies of the ZnO electron transport layer films with AZO sol-gel at 0, 0.5, 1, 1.5, 2, 2.5, and 3 at% Al doping were shown in Table 1. The most band gap energy was approximately 3.32-3.34 eV. As a result, the band gap energy values do not closely correlate with at% Al doping since it has small amount of Al in ZnO structure. Table 1. The average transmittance (T), and optical band gap energy (Eg) of the ZnO electron transport layer films with AZO sol-gel at 0, 0.5, 1, 1.5, 2, 2.5, and 3 at% Al doping. ZnO/AZO with at% Al doping (%)

T (%)

Eg (eV)

0

85.65

3.33±0.01

0.5

80.98

3.33±0.01

1.0

81.32

3.34±0.01

1.5

83.27

3.33±0.01

2.0

79.10

3.34±0.01

2.5

73.81

3.32±0.01

3.0

80.15

3.33±0.01

3.3. Morphologies, crystallinity of the MAPbI3 perovskite materials and device characterization of perovskite solar cells The MAPbI3 perovskite film was prepared by spin coating. Figure 5 shows the surface of perovskite films on the ZnO electron transport layer films with AZO sol-gel at 0, 0.5, 1, 1.5, 2, 2.5, and 3 at% Al doping. All the films have large pinholes that cause direct contact between ETM and HTM layer.

Figure 5 FE-SEM images of the MAPbI3 perovskite films on the ZnO films with AZO sol-gel at 0, 0.5, 1, 1.5, 2, 2.5, and 3 at% Al doping.

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Figure 6 shows the FE-SEM cross section image of perovskite solar cell. The ZnO/AZO ETM layers coated onto the ITO glass substrate by spin-coating have the thickness approximately 200 nm. The MAPbI3 layers spun onto the surface of ETM layer have approximately 400 nm thick. The last layer, the thickness of the hole transport layer (HTM) with P3HT is about 120 nm. The electrode of perovskite solar cells using the Au by evaporation technique has a thickness of approximately 150 nm.

Figure 6 FE-SEM cross section (a) original and (b) graphic image of perovskite solar cell in this work.

The photovoltaic characteristics of different at% Al doping are shown in Figure 7 and the detailed photovoltaic parameters such as short-circuit photocurrent (Jsc), open-circuit voltage (Voc) and fill factor (FF) are summarized in Table 2. Initially, PCE of cell for 0.5 at% Al doping shows the slightly increase but when the at% Al doping increased then the PCE of cells increases significantly and stability trends at doping more than 1.5 at% Al doping. Moreover, the Jsc of cells increased which related the at% Al doping in ZnO significantly. Because ZnO ETM can react with the organic cation (CH3NH3+) of perovskite. Therefore, perovskite solar cells based on ZnO ETM have lower efficiency and stability. As well as AZO ETM was used to improve the physicochemical properties of ZnO due to AZO has higher conductivity, better band matching with MAPbI3 and more resistance toward acid than ZnO [5, 14].

(a)

2.0

0 at% Al 0.5 at% Al 1.0 at% Al 1.5 at% Al 2.0 at% Al 2.5 at% Al 3.0 at% Al

7.5

5.0

2.5

0.0 0.0

(b)

1.6

PCE (%)

Current density (mA/cm2)

10.0

1.2 0.8 0.4 0.0

0.2

0.4

Voltage (V)

0.6

0.8

0.0

0.5

1.0

1.5

2.0

2.5

3.0

at% Al doping (%)

Figure 7 Device characterization of perovskite solar cells. (a) J-V curves and (b) PCEs of the perovskite solar cells by ZnO electron transport layer films with AZO sol-gel at 0, 0.5, 1, 1.5, 2, 2.5, and 3 at% Al doping.

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Table 2. Photovoltaic parameters of the perovskite solar cells by ZnO electron transport layer films with AZO sol-gel at 0, 0.5, 1, 1.5, 2, 2.5, and 3 at% Al doping. The value in parenthesis indicates those of the best device. ZnO/AZO with at% Al doping (%)

Jsc (mA/cm2)

Voc (V)

FF

PCE (%)

0

1.02±0.15

0.61±0.03

0.40±0.03

0.26±0.03

0.5

1.70±0.51

0.68±0.10

0.23±0.12

0.27±0.06

1.0

4.55±0.35

0.62±0.03

0.27±0.05

0.74±0.06

1.5

6.59±0.37

0.66±0.02

0.35±0.02

1.50±0.13

2.0

7.71±1.25

0.67±0.03

0.34±0.02

1.74±0.18

2.5

8.46±0.64

0.64±0.01

0.34±0.02

1.83±0.24

3.0

8.59±0.43

0.64±0.02

0.33±0.01

1.81±0.16

The direct contact between ETM and HTM layer effects on decrease PCE of cells because it results in exciton recombination with low Voc [11, 15]. Open-circuit voltage decay (OCVD) technique was used to evaluate the charge lifetime for the further insight the charge recombination in perovskite solar cells. The decay of Voc after turning off the illumination is plotted as a function of time in Figure 8. The carrier lifetime (τ) can be calculated as follows [11, 16]; −1 (3)

τ =−

k BT  dVoc    , e  dt 

Lifetime (sec)

where kB is Boltzmann constant, e is the charge of the electron and T is the temperature. The carrier lifetimes of the cells from ZnO with AZO were significantly longer than of the ZnO without Al doping ETM.

0 at% Al doping 1.5 at% Al doping 3.0 at% Al doping 0.1

0.01 0.68

0.70

0.72

0.74

0.76

0.78

0.80

0.82

Volt (V) Figure 8 Carrier lifetime of the perovskite solar cells for ZnO electron transport layer films with AZO at 0, 1.5 and 3 at% Al doping.

XRD patterns of the MAPbI3 perovskite films on the ZnO electron transport layer films with AZO as shown in Figure 8 showed PbI2 formation at 2θ approximately 12.6o. However, it was found that the diffraction peaks decreased when the at% Al doping increased, while the diffraction peaks at 2θ approximately 14.5o increased. Especially, the diffraction peaks at 2θ approximately 14.5o indicated MAPbI3 perovskite information. The reaction of MAPbI3 from PbI2 can follow the following reaction [17]: (4) PbI +MAI → MAPbI . 2

3

The XRD results show the decrease of unconverted PbI2 amount. To qualitatively express the conversion of MAPbI3, the conversion (CMAPbI3) of MAPbI3 is defined as following [17, 18]

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CMAPbI3 ≡

I14.5o I12.6o + I14.5o

(5)

,

where I12.6o is the intensity of PbI2 peak at 12.6o and I14.5o is the intensity of MAPbI3 peak at 14.5o. The ratio of PbI2 and MAPbI3 peaks can explain the crystalline quality of perovskite layer due to the high amount of PbI2 remained in perovskite layer, which can affect to decrease the efficiency of cells. ∗ PbI2

♣ MAPbI3

(110) (001)

(220)

(202)

♣

∗

♦ ITO

ITO/ZnO/3 at%Al-ZnO/PCBM/MAPbI3

♣

♣

(222)(130) ♦

♣

(400) ♦

Intensity (a.u.)

ITO/ZnO/2.5 at%Al-ZnO/PCBM/MAPbI3

ITO/ZnO/2 at%Al-ZnO/PCBM/MAPbI3

ITO/ZnO/1.5 at%Al-ZnO/PCBM/MAPbI3

ITO/ZnO/1 at%Al-ZnO/PCBM/MAPbI3

ITO/ZnO/0.5 at%Al-ZnO/PCBM/MAPbI3

ITO/ZnO/PCBM/MAPbI3

10

15

20

25

30

35

Degree (2theta) Figure 9 The XRD spectra of perovskite layer on ITO/ ZnO electron transport layer films with AZO sol-gel at 0, 0.5, 1, 1.5, 2, 2.5, and 3 at% Al doping.

Figure 9 shows the conversion of MAPbI3 on the ZnO electron transport layer films with AZO sol-gel at 0, 0.5, 1, 1.5, 2, 2.5, and 3 at% Al doping. It suggested that the conversion values of MAPbI3 increased when the at% Al doping were increased. The XRD results show the (110) peak of MAPbI3 and the (001) peak of PbI2 which dominantly related with the conversion of MAPbI3. The conversion of MAPbI3 Gran size

0.8

74 72 70 68

0.7 66 0.6

0.5

CMAPbI3 ≡

0.0

0.5

64

I14.2° I12.7° + I14.2° 1.0

1.5

Gran size (nm)

The conversion of MAPbI3

0.9

62

2.0

2.5

3.0

60

at% Al doping (%) Figure 10 Crystalline conversion ratio into MAPbI3 and crystal grain size of MAPbI3.

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As shown in Figure 10, the result shows that the charge recombination in perovskite solar cells was suppressed more efficiently when AZO was used as ETM layer [5, 9, 14]. 4. Conclusion ZnO/AZO was adopted completely as an electron transport material (ETM) layer in the planar perovskite solar cells. Extrinsically doping a small amount of Al with ZnO (AZO) can improve the physicochemical properties of ZnO. Especially, the perovskite solar cells based on AZO ETM show the highest Voc and Jsc with better and stable performance. Therefore, the Al doping can encourage the charge transport and match the band energy with MAPbI3 better than undoping. It suggested that the AZO ETM can improve the reaction between the basic natures of ZnO with proton on CH3NH3+. Acknowledgements Chawalit Bhoomanee would like to acknowledge financial support from the Graduate School, Chiang Mai University and a 50th CMU Anniversary-Ph.D. Scholarship. References [1] H. AitDads, S. Bouzit, L. Nkhaili, A. Elkissani, A. Outzourhit, Sol. Energ. Mat. Sol. C. 148 (2016) 30-33. [2] N. Suhaili, M.F.M. Taib, M.K. Yaakob, O.H. Hassan, M.Z.A. Yahya, Mater. Today Proceeding. 4 (2017) 51545160. [3] T. Oku, in: L.A. Kosyachenko (Ed.) Solar Cells - New Approaches and Reviews, InTech, Rijeka, 2015, pp. Ch. 03. [4] Z. Xiao, Y. Yuan, Q. Wang, Y. Shao, Y. Bai, Y. Deng, Q. Dong, M. Hu, C. Bi, J. Huang, Mater. Sci. Eng. R. Rep. 101 (2016) 1-38. [5] Z.-L. Tseng, C.-H. Chiang, S. Chang, C.-G. Wu, Nano Energy. 28 (2016) 311-318. [6] C. Bhoomanee, S. Nilphai, S. Sutthana, P. Ruankham, S. Choopun, D. Wongratanaphisan, Integr. Ferroelectr. 165 (2015) 121-130. [7] K. Mahmood, B.S. Swain, H.S. Jung, Nanoscale. 6 (2014) 9127-9138. [8] A.A. Al-Ghamdi, O.A. Al-Hartomy, M. El Okr, A.M. Nawar, S. El-Gazzar, F. El-Tantawy, F. Yakuphanoglu, Spectrochim. Acta Part A Mol. Biomol. Spectrosc.131 (2014) 512-517. [9] X. Zhao, H. Shen, Y. Zhang, X. Li, X. Zhao, M. Tai, J. Li, J. Li, X. Li, H. Lin, ACS Appl. Mater. Interfaces. 8 (2016) 7826-7833. [10] X. Zhao, H. Shen, C. Zhou, S. Lin, X. Li, X. Zhao, X. Deng, J. Li, H. Lin, Thin Solid Films, 605 (2016) 208214. [11] P. Ruankham, D. Wongratanaphisan, A. Gardchareon, S. Phadungdhitidhada, S. Choopun, T. Sagawa, Appl. Surface Science. 410 (2017) 393-400. [12] J.W. Jung, J.-S. Park, I.K. Han, Y. Lee, C. Park, W. Kwon, M. Park, J. Mater. Chem. A. 5 (2017) 12158-12167. [13] J. Liu, W. Zhang, D. Song, Q. Ma, L. Zhang, H. Zhang, X. Ma, H. Song, Ceram. Int. 40 (2014) 12905-12915. [14] J. Yang, B.D. Siempelkamp, E. Mosconi, F. De Angelis, T.L. Kelly, Chem. Mater. 27 (2015) 4229-4236. [15] D.-Y. Son, K.-H. Bae, H.-S. Kim, N.-G. Park, J. Phys. Chem. C. 119 (2015) 10321-10328. [16] I. Hwang, M. Baek, K. Yong, ACS Appl. Mater. Interfaces. 7 (2015) 27863-27870. [17] T.-Y. Hsieh, T.-C. Wei, K.-L. Wu, M. Ikegami, T. Miyasaka, Chem. Commun. 51 (2015) 13294-13297. [18] A. Ariyarit, I. Takenaka, R. Yoshikawa, F. Gillot, S. Shiratori, RSC Adv. 6 (2016) 98052-98058.