Effect of Light Illumination on Mixed Halide Lead Perovskites

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Effect of Light Illumination on Mixed Halide Lead Perovskites: Reversible or Irreversible Transformation Weixin Huang, Seog joon Yoon, and Pitambar Sapkota ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00513 • Publication Date (Web): 29 May 2018 Downloaded from http://pubs.acs.org on May 29, 2018

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ACS Applied Energy Materials

Effect of Light Illumination on Mixed Halide Lead Perovskites: Reversible or Irreversible Transformation Weixin Huang1,2,*, #, Seog Joon Yoon1,2,*, #, Pitambar Sapkota1,3 1 2

Radiation Laboratory, University of Notre Dame, Notre Dame, IN 46556, USA

Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, IN 46556, USA 3

Department of Physics, University of Notre Dame, Notre Dame, IN 46556, USA

*Corresponding authors: [email protected], [email protected] #

Present Address: Seog Joon Yoon, Institute of Advanced Materials (INAM), Universitat Jaume I, 12006

Castelló, Spain ([email protected]) 1

ACS Paragon Plus Environment

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Abstract: One intriguing aspect of mixed halide lead perovskites (e.g., CH3NH3PbBrxI3–x) is halide ion segregation when subjected to visible illumination, but this aspect gives rise to the concerns regarding the influence of halide ion movement on the long−term stability and applications. Here, the impact of light illumination on mixed halide hybrid perovskite films was investigated by exposing such films to continuous wavelength laser (25 mW/cm2). It turns out that under illumination the segregated iodide−rich perovskite species further decompose and ultimately form metallic lead (Pb0). The chemical changes in decomposed films were found to cause an irreversible shift of absorption band edge and a significant change in film morphology, while no decomposition of MAPbI3 and MAPbBr3 films was observed after exposure to the same illumination condition. Our results unambiguously demonstrate the deleterious effect of phase segregation on material stability for mixed halide perovskites. KEYWORDS: mixed halide perovskites, phase segregation, transient absorption spectroscopy, in situ XPS, light illumination

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1. INTRODUCTION Organic−inorganic hybrid perovskites have received considerable attention in recent years because of low processing cost and their outstanding optoelectronic properties for solar energy application.1–5 The photovoltaic efficiencies of hybrid perovskite single−junction solar cells have shown an unprecedented advance from 3.5 % in 2009 to 22.7 % in 2017.6,7 The photovoltaic performance of devices is continually improved by compositional, structural, or interfacial engineering.8–11 With the successful application on single−junction solar cells, extensive efforts have been devoted to investigating high−bandgap perovskites as the top−cell materials for tandem solar cells.12,13 Researchers have also demonstrated the ability to tune the bandgap of perovskite materials.14–18 By manipulating the halide composition, the bandgap for mixed halide perovskites MAPbBrxI3−x (MA=CH3NH3+, x=0 to 3) can be tuned between 1.55 and 2.43 eV, thereby expanding the potential application as ideal materials for tandem cell applications.19 Besides the tunable bandgap, the mixed halide perovskites (MAPbBrxI3−x) exhibit interesting light−induced

halide ion migration properties.20–24 Under light

illumination, mixed

bromide−iodide perovskites undergo phase segregation into segregated bromide−rich and iodide−rich regions, while the light−induced halide segregation recovers under the dark conditions.20,23 Due to their sensitive spectral photo−response, these materials show potential applications in sensing, switching, memory and other photonics applications.25,26 Recent findings in halide segregation include effect of halide composition,27 segregated clusters forming at grain boundaries,25 and formation/recovery kinetics and charge transfer rate.21 While most of the work has been focused on the reversible process under short−term light illumination, there remain concerns about the influence of halide ion migration on the long−term stability and photovoltaic performance. Therefore, a detailed understanding of the light−induced chemical evolution of 3



perovskites is essential for furthering approaches to control the stability of mixed halide perovskites under illumination. Here, we reported a study of light−induced transformations in mixed halide thin films under 405 nm continuous−wave (CW) diode laser (25 mW/cm2) irradiation. It provides a detailed analysis of the changes manifest in the optical absorption, film morphology, excited state dynamics and surface chemistry when the mixed halide perovskite films are exposed to the illumination (0−5 hours). These results offer a comprehensive picture of the influence of illumination on the transformation of mixed halide perovskites.

2. EXPERIMENTAL SECTION We prepared CH3NH3PbI1.5Br1.5 perovskites from equimolar (0.3 M) methylammonium bromide (MABr, Dyesol), methylammonium iodide (MAI, Dyesol) and lead bromide (PbBr2, Alfa Aesar) and lead iodide (PbI2, Alfa Aesar) dissolved in 2 ml N,N-dimethylformamide (DMF) according to a procedure reported previously.16 The precursor solutions were injected through inorganic membrane filter with the pore size of 0.2 µm (Whatman). The FTO glass was cleaned by ultrasonication for 30 min in a detergent solution, acetone and ethanol. The FTO glass was plasma cleaned in air atmosphere for 5 min. The precursor solutions were spin-coated on the FTO glass substrate at 5500 rpm for 30s followed by annealing at 100 C for 5 minutes to form the halide mixed perovskite film. The spin coating and annealing were carried out in a dry nitrogen glove box. MAPbBr2.2I0.8, MAPbBr2I1, MAPbBr1.5I1.5, MAPbBr1I2, MAPbBr0.7I2.3 and PbI2 films were prepared by using the same method but with different ratios of precursors (Table S1 lists the detailed amounts of each precursor). All films were put in vacuum cell in N2 filled glove box to protect the perovskite films against to ambient air. Later, the N2 gas was pump out

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by a vacuum pump for 30 minutes so that the cell pressure would be lower than 1 x 10-2 mbar. All UV-vis and femtosecond transient absorption measurements were performed under vacuum. UV-Vis absorption spectra were carried out on Cary 50 Bio spectrophotometer (Varian) while the films were under the laser illumination. The laser beam was produced by a home-built optical system, in which a 405 nm continuous-wave (CW) 180 mW laser passed through different neutral density filters to reach an intensity of 25 mW/cm2. In the absorption measurement, a 420 nm long pass filter was located in front of detector to cut off laser irradiation. The film morphology was imaged using an FEI Magellan-400 field emission scanning electron microscope (FESEM). A Bruker D8 Advanced Davinci Powder X-Ray diffractometer (Cu Kα Xray beam) was used to measure the X-ray diffraction patterns with a step size of 0.025° and an acquisition time of 3 sec deg-1. The in situ characterization of surface chemistry of MAPbI1.5Br1.5 perovskite films was performed with a laboratory-based, ambient pressure X-ray photoelectron spectrometer with a monochromatic Al Kα X-ray source (1486.6 eV). During the experiments, the mixed halide films were kept in the analysis chamber (10-9 mbar) and were illuminated by a 405 nm CW laser with the intensity of 25 mW/cm2. The laser spot on the sample surface was an elliptical shape (0.15 x 0.4 cm) with area of ~ 0.188 cm2, and the X-ray spot on the sample surface is also an elliptical shape (0.13 x 0.36 cm) with area of ~ 0.146 cm2 (Scheme S1). The temperature was measured using a K-type thermocouple with a chromel–alumel junction placed between the sample holder and the sample. The ratios of I/Sn, Br/Sn, N/Sn and Pb0/Pbtotal were estimated from the following equation:

ேభ ேమ

ூ ௌ

= ூభ ௌమ , where N is the concentration of atoms, I is the peak intensity of మ భ

photoelectrons, and S is the atomic sensitivity factor provided by the instrument manufacturer.

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All spectra were calibrated to their corresponding C 1s which is 285.3 eV,28,29 and binding energy values for all fitted and assigned spectral peaks are stated with an accuracy of 0.1-0.2 eV. Detailed

femtosecond

time-resolved

transient absorption spectra were described

in

literatures.21,30 In brief, the time-resolved femtosecond transient absorption spectra were measured with

Helios

software

(Ultrafast Systems). 775 nm pulsed laser was generated

through Clark MXR CPA-2010 (130 fs FWHM, 1 µJ/pulse) with 1 kHz repetition rate. 95 % of the fundamental is doubled to 387 nm to use pump beam, and rest 5 % was used to generate white light probe beam passing through CaF2 crystal. The external CW laser (405 nm, with 25 mW/cm2) was irradiated to the entire perovskite film which includes the specific area for measurement.

3. RESULTS The mixed halide perovskite (MAPbBr1.5I1.5) thin films were deposited on FTO (Fluorine−doped tin oxide) substrates by spin−coating followed by annealing at 100 °C (See the Experimental section and Table S1 in Supporting Information for the details). Figure 1 shows the absorption spectra of the mixed halide film before, during and after illumination. The absorption spectrum of the pristine film exhibits a peak around 625 nm (spectrum a in Figure 1A), which corresponds to the bandgap of 1.89 eV for this mixed halide perovskite. Upon subjecting to CW laser illumination for 4 hours, the initial absorption at 625 nm quenches (spectrum b in Figure 1A). The difference between these two spectra is displayed in Figure 1B to highlight the changes in the absorption spectra of the mixed halide film under light illumination. It can be observed that the absorption signals increase at 530 and 725 nm. The small changes in the absorption towards higher and lower wavelengths correspond to the absorption from segregated iodide and bromide rich regions, respectively.21 In a previous study, we found that the changes in the absorption 6


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spectra are reversible under short term illumination condition at room temperature (

Effect of Light Illumination on Mixed Halide Lead Perovskites

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