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High Performance Flexible Perovskite Solar Cells on Ultra-Thin Glass: implications of the TCO Benjia Dou, Elisa M. Miller, Jeffrey A Christians, Erin M. Sanehira, Talysa Klein, Frank Barnes, Sean E. Shaheen, Sean Garner, Shuvaraj Ghosh, Arindam Mallick, Durga Basak, and Maikel F.A.M. van Hest J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b02128 • Publication Date (Web): 25 Sep 2017 Downloaded from http://pubs.acs.org on September 25, 2017

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The Journal of Physical Chemistry Letters

High Performance Flexible Perovskite Solar Cells on Ultra-Thin Glass: Implications of the TCO 1,2

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Benjia Dou , Elisa M. Miller , Jeffrey A. Christians , Erin M. Sanehira , Talysa R. Klein , Frank S. Barnes , Sean E. 2,4

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Shaheen , Sean Garner , Shuvaraj Ghosh , Arindam Mallick , Durga Basak , Maikel F.A.M. van Hest *

______________________________________________________________________ 1

National Renewable Energy Laboratory, Golden, CO 80401, United States  

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Department of Electrical, Computer, and Energy Engineering, University of Colorado Boulder, Boulder, CO 80309,

United States 3

Department of Electrical Engineering, University of Washington, Seattle, WA 98195, United States

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Renewable and Sustainable Energy Institute, University of Colorado Boulder, Boulder, CO 80309 United States

Corning Research and Development Corporation, Corning, NY, 14830, United States

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Solid State Physics Department, Indian association for the Cultivation of Science, Jadavpur, Kolkata-700032, India

*email correspondence to: [email protected]

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ACS Paragon Plus Environment


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Abstract For halide perovskite solar cells (PSC) to fulfil their vast potential for combining low-cost, high efficiency, and high throughput production they must be scaled using a truly transformative method, such as roll-to-roll processing. Bringing this reality closer to fruition, the present work demonstrates flexible perovskite solar cells with 18.1% power conversion efficiency on flexible Willow Glass substrates. We highlight the importance of the transparent conductive oxide (TCO) layers on device performance by studying various TCOs. While tin-doped indium oxide and indium zinc oxide based PSC devices demonstrate high photovoltaic performances, aluminum-doped zinc oxide (AZO) based devices underperformed in all device parameters. Analysis of x-ray photoemission spectroscopy data shows that the stoichiometry of the perovskite film surface changes dramatically when it is fabricated on AZO, demonstrating the importance of the substrate in perovskite film formation.

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Due to the need for high-efficiency, low-cost, and high-throughput solution processed solar cells, a significant research focus has been directed towards the family of halide perovskites, which share the general formula ABX3 (A = 1,2

monovalent cation, B = divalent metal, and X = halide). Specifically for the halide perovskites, A is methylammonium +

+

2+

(MA, CH3NH3 ), Cs , or formamidinium (FA, CH(NH2) ); B is Pb

2+

2+

-

-

-3

or Sn ; and X is Cl , Br , or I . Despite the

continuously decreasing cost and increasing production of Si and other commercial photovoltaic (PV) technologies, thus far only approximately 227 GW of PV have been installed as of 2015, far short desired PV deployment of 10 TW by 4

2030. Flexible perovskite solar cells (PSCs) offer an unique pathway toward terawatt-scale PV installation because of they can potentially be fabricated by incredibly high throughput roll-to-roll processing. Therefore, it is critical that high efficiency PSCs be developed on flexible substrates which are compatible with roll-to-roll processing. Beyond high throughput roll-to-roll processing, flexible PSCs will also enable more diverse applications for solar energy harvesting including, their facile integration into buildings, consumer electronics, automobiles, and various other portable applications. 5

Flexible PSCs have already been demonstrated on multiple flexible substrates , including polymer substrates 6

7

5

5

(such as polyethylene terephthalate (PET) and polyethylene naphthalate (PEN) ), titanium foil , stainless steel fiber , as 8

9

well as flexible glass (such as Corning® Willow Glass ). To date, most successful demonstrations of flexible PSCs are on 7

polymer substrates with efficiencies as high as 17.3% reported for a PET substrate . Despite this high efficiency, further performance improvement of flexible PSC based polymer substrates is potentially restricted by the low processing temperature limit of the polymer substrates. On the other hand, PSCs are known to be sensitive to water

10–12

, and

polymer substrates have relatively high water-vapor-transmission-rates (WVTR), e.g. WVTR of PET (thickness: 100 μm) -2

-1

13

is 1.1 g m day at 45 °C.

Flexible glass offers a promising alternative to polymer substrates as it can withstand

processing temperatures up to 700 °C and acts as an excellent water barrier (WVTR of Willow Glass with thickness of -6

-2

-1

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100 μm is less than 7 × 10 g m day at 45 °C), all while maintaining flexibility that allows for roll-to-roll processing and curved-surface integration. Previously, Tavakoli et al. demonstrated a PSC with 13.14% power conversion efficiency on flexible Willow Glass using the device stack Au/spiro-OMeTAD/MAPbI3/zinc oxide (ZnO)/ tin-doped indium oxide (ITO)/Willow 8

Glass. However, with advances in perovskite material properties and device engineering, multiple improvements can be made to further enhance the efficiency and stability of such Willow Glass-based flexible PSC. For example, 14

CH3NH3PbI3 is thermally unstable and highly sensitive to moisture,

10,12

and ZnO is known to accelerate the thermal

15

degradation of MAPbI3. (Ref. ) Moreover, Au is an expensive top contact choice for PSC and can be replaced by a more 11

affordable, and more stable, molybdenum oxide(MoOx)/Al contact.

Furthermore, while ITO has been shown to work well as a transparent conducting oxide (TCO) for PSCs, 16

investigation of TCOs beyond ITO would be very important as it can affect performance of solar cells . TCOs are electrically conductive and optically transparent metal oxide thin films, which are important components of optoelectronics such as solar cells and light emitting diodes. These films are typically made of impurity doped metal 16

oxides,

such as tin oxide (SnO2), ZnO, and indium oxide (In2O3). Introduction of the impurity atoms allows for 17

engineering of their transparency, conductivity, and surface roughness. Previous work developing and applying TCOs, specifically aluminum-doped zinc oxide (AZO)

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and indium zinc oxide (IZO),

demonstrated that careful selection of the TCO layer is critical.

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17,19,20

for other PV technologies has


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In this study, flexible Willow Glass based PSCs using three different TCOs (AZO, ITO and IZO) are investigated. By analyzing devices fabricated on these TCOs, this study highlights the importance of the TCO choice for PSCs beyond transmission/conductivity, demonstrating that the chemistry of the perovskite active layer can be dramatically influenced by the TCO on which it is grown. While ITO and IZO based PSC devices demonstrate high photovoltaic performances, AZO based devices underperformed in all device parameters, which is attributed to dramatic changes in the stoichiometry of the perovskite film.

Fig. 1. (a) Schematic representation of device architecture for the flexible perovskite solar cells. (b) A picture of the device not bent (top) and bent (bottom).

AZO, ITO, and IZO films are deposited onto the Willow Glass by RF sputtering of mixed metal oxide sources, as reported in literature

17,18,21

. On top of the TCO layers, a SnO2 electron transport layer (ETL) was spin-coated from an

aqueous SnO2 nanoparticle solution, to form a charge selective contact for the PSC, following the recent report by Jiang 22

et al.

2

The perovskite layer used for this study was a highly alloyed ABX3 system recently reported by Saliba et al., +

containing FA, MA, and Cs in the A-site, Pb

2+

-

-

in the B-site, and both I and Br in the X-site, with precursor solution

stoichiometry of Cs0.04MA0.16FA0.80Pb1.04I2.6Br0.48, abbreviated as FAMACs. These FAMACs films have been shown to be 2

more thermally stable than the more commonly studied MAPbI3. After the deposition of the perovskite layer, the devices were completed by spin-coating spiro-OMeTAD films as the hole transport layer (HTL), thermally evaporating 15 nm of molybdenum oxide (MoOx), and followed by evaporating 150 nm of aluminum (Al) as the top contact. The 2

device active area is 0.1 cm . Furthermore, to reduce reflection at the air/glass interface, a thin layer (70 nm) of MgF2 was deposited on the glass as an anti-reflection (AR) coating in the end. A schematic illustration of the Willow Glass/TCO/SnO2/FAMACs/Spiro-MeOTAD/MoOx/Al device stack is presented in Fig. 1a. As shown in Fig. 1b, the devices are very thin (99.5%) was from Lumtec. PbI2 (99.9985%), PbBr2 (99.999%), SnO2 (15% in H2O colloidal dispersion) and CsI were purchased from Alfa Aesar. All other chemicals and solvents were obtained from Sigma-Aldrich and used as received.

TCO deposition ITO deposited flexible Willow Glasses were obtained from The Center for Advanced Microelectronics Manufacturing at 21

the (Binghamton University). IZO is deposited by RF sputtering mixed metal oxide source containing In2O3 and ZnO2 17

(80 wt% In, 20 wt% Zn). Similarly, AZO (1 wt% Al, 99% Zn) films were deposited by sputtering a sputtering target of Al2O3 and ZnO which were mixed in proportion for several hours and pressed in the form of a pellet followed by a sintering process at 1000 °C for 24 hours.

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Perovskite film deposition 2

The FAMACs perovskite films were prepared by following the procedure described by Saliba et al. Briefly, the films were deposited by spin casting a precursor containing FAI (1 M), MABr (0.2 M), PbI2 (1.1 M), PbBr2(0.2 M) and CsI (0.05 M) in 4:1 v/v dimethyl formamide (DMF): dimethyl sulfoxide (DMSO), with spin-coating parameters as 1000 r.p.m. for 10 s and 6000 r. p. m. for 20 s. With approximately 5 s remaining in this spin-coating procedure, 0.1 mL of chlorobenzene was dripped onto the spinning substrate in a continuous stream. The films were then annealed on a hotplate for 1 hour at 100 °C. All precursor preparation, film deposition and annealing were performed in a N2 glovebox.

Solar Cell fabrication Flexible Willow Glass, with different TCOs, was cleaned by UV-ozone treatment for 15 min. SnO2 were deposited on the 22

cleaned substrates following the method reported by Jiang et al. SnO2 (15% in H2O colloidal dispersion) was diluted to 2.67% and spin cast onto the Willow Glass/TCO substrates with spinning procedure as 3,000 r.p.m. for 30 s. The as-spin cast films were annealed on a hotplate for 30 min at 150°C in air. The films were again cleaned by UV-ozone treatment

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for 15 min immediately before perovskite film deposition (described above). On top of the perovskite layer, spiroMeOTAD was deposited by spin casting (3000 r.p.m. for 30 s) a chlorobenzene solution containing 72 mg/mL spiroMeOTAD, 0.028 mL/mL 4-tert-butylpyridine, and 0.017 mL of a bis(trifluoromethanesulfonyl)imide lithium salt (LiTFSI) stock solution. The Li-TFSI stock solution consisted of 520mg of Li-TFSI dissolved in 1ml of acetonitrile. Following spiro-OMeTAD deposition, the films were left in a desiccator in air overnight. Finally, 15 nm of MoOx and 150 nm of Al were deposited on top of the spiro-MeOTAD by thermal evaporation to complete the devices.

Film Characterizations UV-visible absorption/transmission data were taken on a Shimadzu UV–vis–NIR 3600 spectrometer and PL data were taken with a Horiba Jobin Yvon fluoromax-4 spectrophotometers, both at room temperature. XRD spectrum were recorded with a Rigaku D-Max 2200 with Cu Kα radiation. XPS was performed using a Physical Electronics 5600 photoelectron spectrometer with monochromatic Al Kα excitation (1486.7 eV). The instrument was calibrated with metallic substrates that were cleaned with Ar ion bombardment. Peak areas are determined through a Gaussian/Lorentzian peak fitting algorithm. We typically assign a 5% error to the compositional analysis.

Device Characterizations J-V curves were taken in ambient environment with 100mA cm

-2

AM1.5G illumination. The solar simulator was

calibrated with an encapsulated Si cell certified by the NREL Cell and Module Characterization group. The PSCs were 2

masked with a metal aperture with size as 0.059 cm when performing the J-V scans, including both forward (-0.3 V to -1

1.2 V) and reverse (1.2 V to -0.3V) scans, which were done at 100mV s without prior voltage bias or light soaking. EQE spectra were recorded with a Newport Oriel IQE 200 in ambient environment.

Author contribution B. D. and M.v.H. conceived and designed the overall experiments. B.D. prepared and characterized the perovskite films and devices. E.M.M. performed XPS studies. B.D. and J.A.C. deposited MgF2. E.M.S. and B.D. performed EQE. T.R.K and M.v.H. prepared IZO films. F.S.B. and S.E.S. contributed results analysis. S.G. prepared the Willow Glasses. S.G., A.M. and D.B. provided the AZO films. B.D. wrote the first draft of manuscript with inputs from J.A.C., E.M.M. and M.v.H. All authors contributed to the discussion of the results and commented on the manuscript.

Acknowledgements This article was based upon work supported under the US–India Partnership to Advance Clean Energy-Research (PACER) for the Solar Energy Research Institute for India and the United States (SERIIUS), funded jointly by the U.S. Department of Energy (Office of Science, Office of Basic Energy Sciences, and Energy Efficiency and Renewable Energy, Solar Energy Technology Program, under Subcontract DE-AC36- 08GO28308 to the National Renewable Energy Laboratory, Golden, Colorado) and the Government of India, through the Department of Science and Technology under Subcontract IUSSTF/JCERDC- SERIIUS/2012 dated 22 November 2012. We thank Center for Advanced Microelectronic Manufacturing (CAMM) for coating ITO on Willow Glasses. The XPS data was supported by U.S. Department of Energy, Office of Science, Basic Energy Science, Division of Chemical Sciences, Geosciences and Biosciences, under subcontract DE-ACS-08GO28308. J.A.C. was supported by the Department of Energy (DOE) Office of Energy Efficiency and

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Renewable Energy (EERE) Postdoctoral Research Award under the EERE Solar Energy Technologies Office administered by the Oak Ridge Institute for Science and Education (ORISE) for the DOE under DOE contract number DE-SC00014664. E.M.S. was supported by a NASA Space Technology Research Fellowship.

Supporting Information Device performance; TCO transmittance; XRD data; XPS data; PL data.

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