TiO2 Electron Collection Layer for Efficient Meso

Subscriber access provided by UB + Fachbibliothek Chemie | (FU-Bibliothekssystem)

Article

ZrO2/TiO2 electron collection layer for efficient meso-superstructured hybrid perovskite solar cells Mario Alejandro Mejía Escobar, Sandeep Pathak, Jiewei Liu, Henry J. Snaith, and Franklin Jaramillo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 26 Dec 2016 Downloaded from http://pubs.acs.org on December 26, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 24

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

ACS Applied Materials & Interfaces

ZrO2/TiO2 electron collection layer for efficient mesosuperstructured hybrid perovskite solar cells

Mario Alejandro Mejía Escobar a, Sandeep Pathak b, Jiewei Liu b, Henry J. Snaith b* and Franklin Jaramillo a* a

Centro de Investigación, Innovación y Desarrollo de Materiales – CIDEMAT, Universidad de Antioquia

UdeA, Calle 70 No 52-21, Medellin, 050010, Colombia. b

Clarendon Laboratory, Department of Physics, University of Oxford, Parks Road, Oxford, OX1 3PU,

United Kingdom

*Corresponding authors: [email protected] (MAM Escobar), [email protected] (F. Jaramillo), [email protected]

Abstract Since the first reports of efficient organic-inorganic perovskite solar cells in 2012, an explosion of research activity has emerged around the world, which has led to a rise in the power conversion efficiencies (PCEs) to over 20%. Despite the impressive efficiency, a key area of the device, which remains sub-optimal is the electron extraction layer and its interface with the photoactive perovskite. Here, we implement an electron collection “bi-layer” composed of a thin layer of zirconia coated with titania, siting upon the transparent conductive oxide fluorine doped tin oxide (FTO). With this double collection layer we have reached up to 17.9% power conversion efficiency, delivering a stabilized power output (SPO) of 17.0%, measured under simulated AM 1.5 sunlight of 100 mW cm-2 irradiance. Finally, we propose a

ACS Paragon Plus Environment


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

mechanism of the charge transfer processes within the fabricated architectures, in order to explain the obtained performance of the devices.

Keywords: Perovskite solar cell, electron collection layer, pinholes, anomalous hysteresis, Atomic force microscopy.

1.

Introduction

Over the last decade, deployed solar photovoltaics have started to become a significant fraction of the total global electricity production, representing over 1% of power production1. However, to succeed in producing electricity cheaper than burning fossil fuels, and have the environmental benefits, further reductions in cost and/or enhancements in efficiency are required to offset the inherent additional costs of electricity storage requirements. A new family of solar cell materials, which promises to combine both low cost with capacity for high efficiency, is based on metal halide perovskites CH3NH3PbX3 (X: I, Br, Cl or combination of them). These materials have been shown to have extraordinary properties such as high crystallinity and extinction coefficient2, 3, tunable band gap4, 5, long carrier lifetimes and combined with high charge carrier mobility6, long range hole and electron diffusion lengths 7, 8 resulting in unexpectedly high solar cell power conversion efficiencies9, 10. Devices based on the CH3NH3PbX3 perovskite have reached high efficiencies11-13, which markedly surpass other emerging technologies such as DSSCs, organic tandem and quantum-dots solar cells. Thanks to its ambipolar behavior, different device architectures can be fabricated with this material, with the perovskite behaving as an n-type, p-type or even as an intrinsic semiconductor material (i-type), and therefore adopt the roles of both light absorber and charge transporter in a simple thin-film planar


Page 2 of 24

Page 3 of 24

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


heterojunction configuration. In contrast to silicon PV and other commercial thin film technologies, all layers in the perovskite solar cell can be processed at low temperature (bellow 150 °C)14,

15

. Despite these excellent properties of the perovskites, in order to

obtain a highly efficient photovoltaic devices, the perovskite semiconductor needs to be contacted by a positively (p) and negatively (n) charged contact material to selectively extract electrons and holes from either side of the device. Different architectures have been proposed16, having in common an electron collection layer (or n-type layer) and holetransporting layer (HTL) (or p-type layer) material containing either mesoporous metal oxide scaffolds or planar junctions . The typical structure is in an “n-i-p” configuration, where the n-type collection layer is processed on the transparent conducting substrate. Typically, a compact layer of TiO2 (c-TiO2) is employed as the n-type collection material and

2,2′,7,7′-tetrakis-(N,N-di-pmethoxyphenylamine)9,9′-spirobifluorene

(Spiro-

OMeTAD) as the p-type layer17. In an “inverted-type” architecture, the p-type material poly(3,4-ethylenedioxythiophene)

(PEDOT:PPS)

is

processed

on

the

transparent

conducting substrate in a “p-i-n” configuration, with fullerene derivatives employed as the n-type material. The mentioned architectures have shown to promote remarkable power conversion efficiency, but it has been observed that the recombination losses at the perovskite p-type and perovskite n-type heterojunctions generally limit the efficiency of the cell. Recently, the use of multi-layer charge extraction layers have been shown to be advantageous. Such as Wojciechowski et al. employed the use of an organic electron accepting fullerene self-assembled monolayer (C60-SAM) on top of c-TiO2, which showed a significant improvement in the charge collection and hence efficiency values18, and Tao et al. have recently reported highly efficient cells employing a double layer of TiO2 and PC61BM19. On the other hand, numerous efforts have been undertaken to replace TiO2



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

using some other metal oxide semiconductors as buffer layer, such as ZnO or SnO2 for the n-type layer or replacing the p-type conductor with NiO20-22. It is therefore still very much an open question as to the ideal n and p-type contact materials for perovskite solar cells, should they be organic or inorganic, single or multi-layer in nature? So far, the electronically “cleanest” contact materials appear to be organic, however, for ultimate high thermal stability inorganic would be preferable. Here, we present a double n-type collection layer employing ZrO2 nanocrystals below a cTiO2 layer. Integrating this n-type collection layer into a “meso-superstructured” perovskite solar cell, we reach a high short-circuit current density (Jsc), fill factor (FF) and opencircuit voltage (Voc) in the fabricated devices, which delivers a stabilized power conversion efficiency of up to 17%.

2.

Materials and methods

Unless otherwise stated, all materials were purchased from Sigma-Aldrich or Alfa Aesar and used as received. Spiro-OMeTAD was purchased from Borun Chemicals. CH3NH3I was synthesized in-house according to a reported protocol3. Perovskite Solar Cell Preparation. Devices were fabricated on fluorine-doped tin oxide (FTO) coated glass (Pilkington, 7 Ω/square). Initially, FTO was removed from regions under the anode contact by etching the FTO with zinc powder and 2 M HCl. Substrates were then cleaned sequentially in commercial detergent (Hellma analytics, Hellmanex), acetone, isopropanol, and finally oxygen plasma. Then, zirconia layer (n-ZrO2) was fabricated by spin-coating a mildly acid solution of zirconyl chloride (100 µL, Sigma no. 464198) in anhydrous ethanol (3 mL, Sigma no. 459836) at 3500 rpm and annealed at 150 °C for 15 minutes and 500 ºC for 30


Page 4 of 24

Page 5 of 24

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


minutes subsequently. Using these conditions, the other electron-transporting layer (c-TiO2) was fabricated by spin-coating a mildly acidic solution of titanium isopropoxide in ethanol anhydrous (350 µL in 10.0 mL of ethanol with 0.016 M HCl). The control devices with cTiO2 and c-TiO2/n-ZrO2 were fabricated using the same conditions mentioned above. The mesoporous Al2O3 scaffold was deposited by spin coating a colloidal dispersion of < 50 nm Al2O3 nanoparticles at 20 wt% in isopropanol diluted with isopropanol at a volume ratio of 2:1 at 2500 rpm for 1 minute. After, the wet film was heat treated at 150 ºC for 10 minutes. The perovskite layer was then deposited by spin-coating a nonstoichiometric precursor solution of methylammonium iodide and lead chlorine (3:1 molar ratio; 0.88 M lead chloride and 2.64 M methylammonium iodide) in N,N-dimethylformamide (DMF) at 2000 rpm for 45 seconds. The films were then annealed at 100 ºC for 90 minutes and 120 ºC for 15 minutes. The hole-transporting layer was then deposited via spin-coating a 8.5 wt%

of

2,2′,7,7′-tetrakis-(N,N-di-pmethoxyphenylamine)9,9′-spirobifluorene

(Spiro-

OMeTAD) in chlorobenzene (CB), with additives of 4-tert-butylpyridine and lithium bis(trifluoromethanesulfonyl) imide. Spin coating was carried out at 2000 rpm for 60 seconds. Devices were then left overnight in a desiccator in order to dope the SpiroOMeTAD via oxidation23. Finally, 120 nm silver electrodes were thermally evaporated under vacuum (≈1.3x10-4 Pa) at a deposition rate around of ≈0.1 nm s−1 approximately, to complete the devices. Device Characterization. The current density-voltage (JV) curves were measured (2400 series sourcemeter, Keithley Instruments) under simulated AM1.5 sunlight at 100 mW cm-2 irradiance generated by an Abet Class AAB sun 2000 simulator, with the intensity calibrated with an NREL calibrated KG5 filtered Si reference cell. The mismatch factor was calculated to be



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

less than 1%. The solar cells were masked with a metal aperture to define the active area, typically 0.0625 cm2 (measured individually for each mask), and measured in a light-tight sample holder to minimize any edge effects and ensure that the reference cell and test cell are located during measurement in the same spot under the solar simulator. Optical Measurements. Transmittance and reflectance spectra were collected with a Varian Cary 300 UV-Vis spectrophotometer with an internally coupled integrating sphere. Film Characterization. A JEOL JSM-6490LV emission scanning electron microscope was used to acquire SEM images. Sample thicknesses were measured using a Veeco Dektak 150 surface profilometer. X-ray diffraction (XRD) spectra for n-ZrO2 were obtained from full devices with no evaporated electrodes, using a Panalytical X’Pert Pro X-ray diffractometer. The result was obtained using a wavelength of 1.54 Å (Cu-K-Alpha1), generator and tube voltage of 40 kV, and a scan step size of 0.0041778. AFM images were obtained using a MFP-3D Infinity AFM - Asylum Research. Here it was used an ORCA holder and TiIr Tip in contact mode. Additionally, all test were performed at 23 °C and humidity between 40 to 50% Rh.

3.

Results and discussion

In recent studies we have found that the compact TiO2 layer, can have a porosity of up to 4%24. Although this is low, it indicates that there could be many occurrences of pinholes where the perovskite could make direct contact to the Fluorine doped tin oxide (FTO). This could create charge recombination pathways, which would generally be disadvantageous


Page 6 of 24

Page 7 of 24

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


for performance and stabilized power output of the solar cells. In order to eliminate the shunting paths via pinholes and interfacial charge recombination in the solar cell, we investigated if incorporation of an additional thin layer of a wider band gap metal oxide layer (i.e. ZrO2) is beneficial. In the Figure S1 we show the x-ray diffraction (XRD) pattern of a thin layer of ZrO2 deposited upon a glass substrate. We assign the three characteristic peaks to the crystallographic planes associated to zirconium oxide (ZrO2) crystallized in a tetragonal phase25. From here on in, we will refer to this ZrO2 layer as n-ZrO2 (n for nanocrystalline). Then, we investigate two device architectures. First placing n-ZrO2 layer between the FTO and TiO2 (A2) and second, by placing the n-ZrO2 between the TiO2 and perovskite layer (A3). We also fabricated devices with TiO2 “compact layer” (c- TiO2) only (A1; control), which we illustrate in Figure 1. Normally, 50 nm of TiO2 is used to fabricate perovskite solar cells, here the c-TiO2 thickness is only 20 nm, since, as we show below, this corresponds to the optimum thickness in the bi-layer configuration.

Figure 1. Proposed architectures and diffractograms associated to their electron collection layers. Illustrations of the evaluated architectures in this study. Spiro-OMeTAD was used as the HTM. The active layer is composed by an alumina scaffold (280 nm) and infiltrated perovskite with a capping layer (190 nm),



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

detonated as m-PVKT. The thicknesses values were obtained using a profilometer and analyzing the crosssections from SEM images of the fabricated devices. Additionally, energetic levels showed here were taken of some reports published in the literature3, 6, 16, 26.

On the other hand, it was obtained the diffractograms for each proposed electron collection bi-layer. The obtained results are showed in Figure S2 from Supporting Information. In that figure it can be seem that there are no remarkable differences between them. Peaks related to the FTO layer are clearly identifiable. Probably, the main peaks associated to the titania (anatase phase) (2θ=25°) and tetragonal zirconia (2θ=30°) have been masked by the peaks belonging to the thicker FTO layer (340 nm) as compared with the c-TiO2 (20 nm) and n-ZrO2 (25 nm) thin layers. From these results

Figure 2. Optical properties and surfaces for the thin-layers configurations used in the proposed devices. a) Transmittance spectrum of the fabricated electron collection thin-layers. Spectrum for glass is associated to a substrate with 2 mm of thickness. b) Surface SEM images obtained for FTO, c) FTO/c-TiO2, d) FTO/nZrO2/c-TiO2 and e) FTO/c-TiO2/n-ZrO2 architecture, respectively. f) Characteristic cross section of the fabricated compact layers.


Page 8 of 24

Page 9 of 24

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


In order to maximize the light absorption within the perovskite film, the compact layer must be transparent in the visible to near IR regions. In Figure 2a we show transmittance (%) of the bare glass, FTO substrate, FTO/c-TiO2, FTO/c-TiO2/n-ZrO2 and FTO/n-ZrO2/cTiO2 substrate. It is evident that the bare FTO and TiO2 show 80% transmittance and insertion of ZrO2 layer does not absorb any additional incident light27. To a first order approximation, the different n-type layers appear to show negligible differences in their optical properties. Since the refractive indices of ZrO2 and TiO2 are similar (2.2 versus 2.5), this is expected. In Figure 2d and e we show the SEM images for FTO/n-ZrO2/c-TiO2 and FTO/c-TiO2/nZrO2 thin films. As mentioned earlier that the compact layers in our study has been maintained very thin (

TiO2 Electron Collection Layer for Efficient Meso

Recommend Documents