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Low-temperature Nb-doped SnO2 electron selective contact yields over 20% efficiency in planar perovskite solar cells Elham Halvani Anaraki, Ahmad Kermanpur, Matthew T. Mayer, Ludmilla Steier, Taha Ahmed, Silver Hamill Turren Cruz, Ji-Youn Seo, Jingshan Luo, Shaik M. Zakeeruddin, Wolfgang Tress, Tomas Edvinsson, Michael Grätzel, Anders Hagfeldt, and Juan-Pablo Correa-Baena ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b00055 • Publication Date (Web): 01 Mar 2018 Downloaded from http://pubs.acs.org on March 1, 2018

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ACS Energy Letters

Low-temperature Nb-doped SnO2 Electron Selective Contact Yields Over 20% Efficiency in Planar Perovskite Solar Cells Elham Halvani Anaraki,ab Ahmad Kermanpur,*b Matthew T Mayer,cd Ludmilla Steier,ce Taha Ahmed,f Silver-Hamill Turren-Cruz,a Jiyoun Seo,c Jingshan Luo,c Shaik Mohammad Zakeeruddin,ac Wolfgang Richard Tress,ac Tomas Edvinsson,f Michael Grätzel,c Anders Hagfeldt*a Juan-Pablo Correa-Baena,*ag a

Laboratory of Photomolecular Science, Institute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne, CH-1015-Lausanne, Switzerland b

Department of Materials Engineering, Isfahan University of Technology, Isfahan, 8415683111, Iran c

Laboratory of Photonics and Interfaces, Institute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne, CH-1015-Lausanne, Switzerland d

Young Investigator Group Electrochemical Conversion of CO2, Helmholtz Zentrum Berlin, Hahn-Meitner-Platz 1, 14109 Berlin, Germany

e

Department of Chemistry, Imperial College London, Kensington, London SW7 2AZ, UK

f

Department of Chemistry, Ångström Laboratory, Structural Chemistry, Uppsala University, 752 36 Uppsala, Sweden

g

Department of Mechanical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Ave, Cambridge, MA 02139, USA Corresponding Authors: AK [email protected]; AH [email protected]; JPCB [email protected]

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ABSTRACT Low temperature planar organic-inorganic lead halide perovskite solar cells have been at the center of attraction as power conversion efficiencies go beyond 20%. Here, we investigate Nbdoping of SnO2 deposited by a low-cost, upscalable chemical bath deposition (CBD) method. We study the effects of doping on compositional, structural, morphological, and device performance when these layers are employed as electron selective layers (ESLs) in planarstructured PSCs. We use doping concentrations of 0, 1, 5 and 10 mol.% Nb to Sn in solution. The ESLs were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), atomic force microscopy (AFM), and UV-vis spectroscopy. ESLs with an optimum 5 mol.% Nb-doping yielded, on average, an improvement of all the device photovoltaic parameters with a champion power conversion efficiency of 20.5% (20.1% stabilized). TABLE OF CONTENTS


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ACS Energy Letters

With the rapid development in power conversion efficiency (PCE), from 3.8%1 to 22.1%2, organic-inorganic lead halide perovskite solar cells (PSCs) have been at the center of attraction in the photovoltaic research community.3 In paving the way towards low cost highly-efficient stable PSCs with good reproducibility and hole blocking properties, many metal oxide (TiO2,4-7 ZnO,8,

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SnO210-12)-based ESLs have been developed. Developing low-temperature and easily-

scalable methods to prepare ESLs with controlled optoelectronic properties, morphology and modified interface (especially in contact with perovskite light harvester) is needed in order to achieve high efficiencies. SnO2 is a semiconducting material with high optical transparency, suitable energy level alignment with iodine-based lead-halide perovskites, high electron mobility at room temperature and low-temperature processability. These characteristics make SnO2 a promising ESL that has shown the potential to replace the most widely used ESL, TiO2, in PSCs.11-15 In 2016 PSCs with spin coated SnO2-based ESLs post-treated with a simple lowtemperature (< 180℃) CBD method led to highly stable planar devices with a PCE close to 21%.10 In the same year a certified PCE of 19.9±0.6% was reported for the low temperature (< 150℃) planar PCSs with ESLs spin coated from a SnO2 colloidal precursor16. In all cases, these highly-efficient PSCs have been limited by their FF, which are lower than the best performing devices based on mesoporous TiO22,

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(above 80% FF) or inverted devices.18 Given that the

remaining layers in the device architecture are, for the most part, the same (FTO, perovskite, Spiro-OMeTAD, gold), this deficiency is attributed to the ESL in this architecture. Lower FFs are typically associated to low conductivities of the contact layers, or of the photoabsorber itself. Doping metal oxides (TiO2,19-23 ZnO,24,

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SnO226-31) with other metals has been shown to

improve the electronic and interfacial properties of these ESLs enhancing electron extraction in PSCs. There are a few reports published on doped-SnO2 ESLs through replacing Sn4+ ions with


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suitable dopant elements such as Mg,28 Li,27 Y29 and Nb26 for applications in PSCs. Nb-doping was performed by spin coating SnO2 thin films as ESLs in planar PSCs, which yielded an increase of their PCE from 15.13% for the undoped to 17.57% for the doped case.26 Similarly, it has been shown that the PCE of PSCs with SnO2 ESLs prepared through a low temperature (Voc of the J-V curve measured in a backward scan of full devices under illumination). The improved hysteresis behavior in 5 mol.% Nb-doped PSCs could be attributed to the more balanced flux between electrons and holes, as electrons are extracted faster in the doped ESLs due to its improved conductivity.27

a. 1.20

b. 25

VOC / V

JSC / mA cm

-2

1.15

1.10

20

1.05

15

1.00 0

1

5

10

Nb:Sn / mol.%

c.

0

d.

1 5 Nb:Sn / mol.%

10

21 20

FF

PCE %

0.75

0.70

19 18 17 16

0.65

0

1

5

0

10

1

5

10

Nb:Sn / mol.%

Nb:Sn / mol.%

e.

f. 8 7 6 5 4 3 2 1 0

50 40 Rs/Ohm

Hysteresis / %

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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30 20 27.1±7.2

0

1

5

Nb:Sn / mol.%

10

29.2±6.2 21.3±3.3

10 0

1 5 Nb:Sn / mol.%

21.6±4.2

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Fig. 5 The full devices J-V statistics. The photovoltaic parameters: (a) VOC, (b) JSC, (c) FF, (d) PCE, (e) hysteresis (as the PCE difference between forward and backward scan) and (f) series resistance (approximated based on the backward J-V curve slope at 1.15-1.25 V voltage range)


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ACS Energy Letters

statistics of planar perovskite solar cells made based on: 0, 1, 5 and 10 mol.% Nb-doped SnO2 ESLs. Table. 2 The average J–V parameters of perovskite solar cells with (0, 1, 5 and 10 mol.% Nbdoped) SnO2 ESLs measured under standard AM1.5G illumination with a 0.16 cm2 active area. FF

PCE

hysteresis

VOC

JSC

[V]

[mA cm-2]

0

1.114±0.025

22.41±0.63

0.718±0.030

18.05±0.98

2.08±1.41

1

1.122±0.012

22.81±0.33

0.724±0.031

18.47±0.81

2.32±1.23

5

1.125±0.017

22.86±0.39

0.736±0.019

18.92±0.75

1.39±0.92

10

1.091±0.023

23.20±0.56

0.728±0.029

18.56±0.62

1.66±1.26

Nb:Sn (mol.%) in ESL

[%]

In this work we prepared Nb-doped SnO2 layers by a simple chemical bath deposition, for applications in planar PSCs. ESLs with an optimum Nb-doping (5 mol.%) resulted in the minimum average of series resistance, that resulted in the highest average of FF (74%) and suppressed J-V hysteresis behavior in their corresponding planar devices. The devices employing 5 mol.% doped ESLs showed an improvement in all J-V parameters with a higher reproducibility in comparison with those using undoped SnO2 ESLs. Higher concentrations (10 mol.%) of Nbdopant created poor surface coverage quality in contact with perovskite light absorber and showed a VOC and PCE drop in comparison with 5 mol.% doped one due to increased recombination centers in this defective interfacial contact. The optimum Nb-doping in solutionprocessed SnO2 ESLs was found to be promising for enhancing the electron extraction and reaching highly-efficient planar PSCs with the champion PCE of 20.5% (20.1% stabilized). The undoped counterpart showed a champion PCE of 19.7% (19% stabilized). Doping of SnO2 ESLs through this simple CBD method without a high-temperature step paves the way towards industrially scalable PSCs with improved efficiency.


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ASSOCIATED CONTENT Supporting Information. Details of all experimental procedures. Additional characterization, including: XRD, XPS, SEM, and AFM. AUTHOR INFORMATION Twitter: @jpcorreabaena, Website: www.jpcorreabaena.com ACKNOWLEDGEMENTS The authors are grateful to Dr. Mehdi Ranjbar for his invaluable comments on the manuscript. E. H. A. acknowledges support from Isfahan University of technology, the Ministry of Science, research and technology of Iran and Iran Nanotechnology Initiative Council. The authors thank Xavier Jeanbourquin for assistance with AFM measurements. References: (1) Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal halide perovskites as visible-light sensitizers for photovoltaic cells. J. Am. Chem. Soc. 2009, 131, 6050-6051. (2) Yang, W. S.; Park, B.-W.; Jung, E. H.; Jeon, N. J.; Kim, Y. C.; Lee, D. U.; Shin, S. S.; Seo, J.; Kim, E. K.; Noh, J. H.; Seok, S. I. Iodide management in formamidinium-lead-halide– based perovskite layers for efficient solar cells. Science 2017, 356, 1376-1379. (3) Correa-Baena, J.-P.; Saliba, M.; Buonassisi, T.; Grätzel, M.; Abate, A.; Tress, W.; Hagfeldt, A. Promises and challenges of perovskite solar cells. Science 2017, 358, 739-744. (4) Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; Snaith, H. J. Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites. Science 2012, 338, 643-647. (5) Yella, A.; Heiniger, L.-P.; Gao, P.; Nazeeruddin, M. K.; Grätzel, M. Nanocrystalline Rutile Electron Extraction Layer Enables Low-Temperature Solution Processed Perovskite Photovoltaics with 13.7% Efficiency. Nano Lett. 2014, 14, 2591-2596. (6) Wojciechowski, K.; Saliba, M.; Leijtens, T.; Abate, A.; Snaith, H. J. Sub-150 C processed meso-superstructured perovskite solar cells with enhanced efficiency. Energy Environ. Sci. 2014, 7, 1142-1147. (7) Chandiran, A. K.; Yella, A.; Mayer, M. T.; Gao, P.; Nazeeruddin, M. K.; Grätzel, M. Sub-Nanometer Conformal TiO2 Blocking Layer for High Efficiency Solid-State Perovskite Absorber Solar Cells. Adv. Mater. 2014, 26, 4309-4312.


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