Tunable Near-Infrared Luminescence in Tin Halide Perovskite

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Tunable Near-Infrared Luminescence in Tin Halide Perovskite Devices May Ling Lai, Timothy Y.S. Tay, Aditya Sadhanala, Siân E. Dutton, Guangru Li, Richard H. Friend, and Zhi Kuang Tan J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b01047 • Publication Date (Web): 23 Jun 2016 Downloaded from http://pubs.acs.org on June 26, 2016

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The Journal of Physical Chemistry Letters

Tunable Near-Infrared Luminescence in Tin Halide Perovskite Devices May L. Lai1, Timothy Y. S. Tay1, Aditya Sadhanala1, Siân E. Dutton1, Guangru Li1, Richard H. Friend1* and Zhi-Kuang Tan1,2,3* 1 Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge, CB3 0HE, UK 2 Department of Chemistry, National University of Singapore, 3 Science Drive 3, S117543, Singapore 3 Solar Energy Research Institute of Singapore, National University of Singapore, 7 Engineering Drive 1, S117574, Singapore Corresponding Authors *E-mail: [email protected] *E-mail: [email protected]

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Infrared emitters are reasonably rare in solution-processed materials. Recently, research into hybrid organo-lead halide perovskite, originally popular in photovoltaics1-3, has gained traction in lightemitting diodes (LED) due to their low-cost solution processing and good performance.4-9 The leadbased electroluminescent materials show strong colorful emission in the visible region, but lack emissive variants further in the infrared. The concerns with the toxicity of lead may, additionally, limit their wide-scale applications. Here, we demonstrate tunable near-infrared electroluminescence from a lead-free organo-tin halide perovskite, using an ITO/PEDOT:PSS/CH3NH3Sn(Br1-xIx)3/F8/Ca/Ag device architecture. In our tin iodide (CH3NH3SnI3) LEDs, we achieved a 945 nm near-infrared emission with a radiance of 3.4 Wsr-1m-2 and a maximum external quantum efficiency of 0.72%, comparable with earlier lead-based devices. Increasing the bromide content in these tin perovskite devices widens the semiconductor bandgap and leads to shorter wavelength emissions, tunable down to 667 nm. These near-infrared LEDs could find useful applications in a range of optical communication, sensing and medical device applications.

TOC

Keywords: lead-free, light emitting diodes, bromide, iodide, infrared emission


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The achievement of high-efficiency solar cells using perovskite semiconductors has spurred significant research into these materials and has positioned them as strong contenders against other traditional semiconductors such as Si and GaAs. While the hybrid lead-halide perovskites were originally investigated as sensitisers in solid-state dye-sensitised solar cells,1-2, 10 it is becoming evident that these perovskites are capable of long-range charge transport11-14 and are therefore useful as a bulk semiconductor in planar heterojunction devices.15-17 The ease in material preparation (simple solution processing and low-temperature heating), coupled with the capacity to tune their electronic bandgaps,18 make these materials highly-desirable as semiconductors in optoelectronic devices. Earlier works have further shown perovskites to be capable of strong electroluminescence (EL) in a ‘charge-confined’ LED structure.4 This initial study demonstrates colour-tunable emission in the visible and the near-infrared, though their EL quantum efficiencies remain modest at 0.76%. Since then, considerable efforts were made to improve the performance of the perovskite LEDs (PeLED) via modifications to the device interfaces and active layers.5-8,

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For instance, applying a polyamine

interlayer improves perovskite film coverage,7 and blending perovskites into an insulating polymer matrix blocks off electrical shunting paths,19 both leading to significant enhancements in EL efficiencies. One concern with the lead halide perovskites, however, is that the material is toxic and an environmentally benign lead-free alternative would be preferred for consumer applications. Recent studies in perovskite solar cells have shown that tin can be used to replace lead in the perovskites, and these give reasonably good performance.20-21 However, tin halide perovskites are chemically less stable due to their tendency to oxidise from Sn2+ to Sn4+, which leads to p-type doping.22 The oxidation of tin remains a challenge in this work, though we expect this problem to be circumvented with better encapsulation technologies.


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Figure 1: (a) Unit cell of ABX3 perovskite crystal where A represents methylammonium (CH3NH3), B represents tin (Sn) and X represents bromide (Br) and iodide (I), (b) Normalized X-ray diffraction patterns of CH3NH3Sn(Br1-xIx)3 thin films. Insets show the (100) and (001) (upper) and the (200) and (002) (lower) diffraction patterns in more detail, (c) lattice spacing a (black) and c (green) of perovskite crystal and bandgap (red) with respect to the iodide content in the perovskite.

The perovskite structure is in the form of ABX3, where A and B are, in this instance, a monovalent methylammonium and a divalent tin cation, respectively, and X are halide anions. In the crystal structure shown in Figure 1a, an octahedral BX6 cage is located in the middle of the unit cell, and the corner-occupying A are 12-fold coordinated to X. However, this cubic structure in perovskite is typically distorted to give tetragonal and orthorhombic structures.


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In the CH3NH3Sn(Br1-xIx)3 perovskites, a pseudocubic tetragonal structure is observed across the entire bromide-iodide composition range. The lattice parameters, as determined from X-ray diffraction studies (Figure 1b), are observed to increase from a = 5.90353(5) Å, c = 5.90743(4) in CH3NH3SnBr3 to a = 6.23932(4)Å, c = 6.24087(11) in CH3NH3SnI3 (lattice parameters are plotted in Figure 1c). This is in good agreement with previous studies.20, 23 We note that the visible shoulders on the diffraction peaks is due to the presence of two closely located x-ray lines (Kα1 and Kα2) in the source. Further analysis of the x-ray diffraction measurements indicates that in addition to the change in scattering angles, the width of the peaks changes as a function of composition, with sharper peaks observed in the pure halide end members. This broadening can be partially explained by the increase in the degree of tetragonal distortion in the mixed-halide perovskites. However, strain from local variations in the crystal structure may also contribute to the observed broadening. The semiconductor bandgaps of the tin halide perovskites are estimated from the absorption onsets using the Tauc plots, as shown in Figure S1. Since the valence band of the perovskite has strong character of the halide p orbitals,24-25 increasing the bromide content (and decreasing iodide) causes an increase in the bandgap from 1.40 eV (in pure triiodide) to 2.15 eV (in pure tribromide).


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Figure 2: (a) Normalized absorbance spectra based on PDS measurements, (b) normalized photoluminescence (PL) spectra, (c) combined plot of Urbach energy and full-width at half maxima (FWHM) of PL and EL (electroluminescence) peaks, with respect to iodide content and (d) table of photoluminescence quantum efficiencies of perovskite samples.

The optical absorption spectra of the perovskite films were measured using photothermal deflection spectroscopy (PDS), as shown in Figure 2a. PDS measures the absorption via the heating of the sample due to non-radiative relaxation of the excited states. It is less affected by the optical effects such as light scattering and reflection that commonly affect optical transmission measurements. This enables the PDS to measure absorption with high sensitivity and dynamic range of 4-5 orders of magnitude. PDS data can be used to estimate an empirical energetic disorder parameter for the given semiconductor, known as Urbach energy ‘Eu’. It can be derived from the following expression: A = A0 exp((E-Eg)/Eu) where, A is the absorbance, A0 is a constant and Eg is the bandgap of the material.26-27


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In Figure 2b, we show the normalized photoluminescence (PL) spectra of the tin halide perovskite samples. We observe a monotonic red-shift in the PL emission peaks with increasing iodide content, which corresponds well with the shifts in the absorption onsets. The photoluminescence quantum efficiency of the perovskite thin films (summarized in Figure 2d) decreases generally with decreasing iodide content, from 5.3% in the 100% iodide sample to 0.1% in the 50% iodide sample. Samples with iodide content below 50% show negligible PL. These observations are consistent with our LED device studies, where we were unable to obtain electroluminescence in the devices with low iodide contents. By analysing the band-edge absorption of the samples (as in Figure 2a), we were able to quantify the energetic disorder in the perovskite semiconductors in the form of Urbach energy. A sharper (steeper) absorption band-edge gives smaller Urbach energy value. In Figure 2c, we plot the Urbach energy against the iodide content in the samples, and found the samples with higher iodide content (up to 90%) to possess the smallest Urbach energy, and therefore the lowest degree of energetic disorder. This is also reflected in the full width at half maximum (FWHM) of the PL and EL peaks, where the more disordered samples gave broader FWHM. We note that the lack of luminescence in the bromide-rich samples may be related to higher disorder within the perovskite crystals. This is reasonable considering that the disordered samples are likely to contain more traps and defect states that quench PL emission.


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Figure 3: (a) Device architecture of CH3NH3Sn(Br1-xIx)3 PeLED. (b) Energy levels of the layers in a PeLED device, displaying the conduction and valence band levels with respect to vacuum. The valence and conduction band levels of a tribromide perovskite (CH3NH3SnBr3) is shown here, and the values are obtained from Ref 21. The conduction bands lie deeper (more negative) for higher iodide content perovskites.

Using our range of tin halide perovskites, we fabricated light-emitting diodes in an indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS)/methylammonium tin halide CH3NH3Sn(Br1-xIx)3/poly(9,9’-dioctylfluorene) (F8)/calcium (Ca)/silver (Ag) (where 0

Tunable Near-Infrared Luminescence in Tin Halide Perovskite

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