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Cite this: DOI: 10.1039/c4cp03573a Received 11th August 2014, Accepted 10th September 2014

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Temperature-dependent excitonic photoluminescence of hybrid organometal halide perovskite films Kewei Wu,a Ashok Bera,a Chun Ma,a Yuanmin Du,a Yang Yang,b Liang Lib and Tom Wu*a

DOI: 10.1039/c4cp03573a www.rsc.org/pccp

Organometal halide perovskites have recently attracted tremendous attention due to their potential for photovoltaic applications, and they are also considered as promising materials in light emitting and lasing devices. In this work, we investigated in detail the cryogenic steady state photoluminescence properties of a prototypical hybrid perovskite CH3NH3PbI3xClx. The evolution of the characteristics of two excitonic peaks coincides with the structural phase transition around 160 K. Our results further revealed an exciton binding energy of 62.3  8.9 meV and an optical phonon energy of 25.3  5.2 meV, along with an abnormal blue-shift of the band gap in the high-temperature tetragonal phase.

Organometal halide perovskite solar cells have rapidly risen to the forefront of competitive photovoltaic technologies, boasting high efficiency and cost-effective synthesis.1–8 For the recently reported highly efficient solar cells, mixed halide CH3NH3PbI3xClx was used as the active layer as a result of its improved physical properties compared to the widely used pure iodide counterpart.3,9 Their outstanding solar cell performance is driven by the large light absorption coefficients and long-range balanced electron– hole transport lengths in such hybrid perovskites.10,11 Furthermore, excellent coherent light emission properties were also reported from amplified spontaneous emission to optically pumped lasing in these solution-processed materials.12,13 A high quantum efficiency of 70% was demonstrated, which is promising for light-emitting device applications.12 By substituting iodine with chlorine and bromine, wavelength-tunable emissions from ultraviolet to near infrared were achieved.13 Importantly, the balanced ambipolar charge transport characteristics of the perovskite semiconductors support the potential of optoelectronic applications. Indeed, such hybrid materials with tailored composition and dimensionality were investigated in the 1990s in the context of light emitting diodes and field effect transistors.14,15

a

Materials Science and Engineering, King Abdullah University of Science and Technology (KAUST), Thuwal, 23955-6900, Saudi Arabia. E-mail: [email protected] b Advanced Imaging and Characterization Laboratory, King Abdullah University of Science and Technology (KAUST), Thuwal, 23955-6900, Saudi Arabia

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Compared to the intensive efforts on solar cell applications, the optical properties of hybrid halide perovskites have been less investigated. Depending on photo-excitation conditions, both bound excitons and free charges are generated in perovskite upon light illumination. An exciton binding energy in the range of 50 meV was estimated in the mixed-halide crystals by cryogenic absorption measurements.16,17 Furthermore, due to the low defect density and slow Auger recombination, amplified spontaneous emission was observed even in the presence of electron and hole quenchers.13 The outstanding properties of mixed halide perovskites make such solution-processed materials a competitive candidate for light-emitting and also other optoelectronic applications. Here we report on the stable-state photoluminescence (PL) properties of solution-processed CH3NH3PbI3xClx thin films in the temperature range of 77 K to 380 K. A transition around 160 K was revealed by examining the temperature dependence of the peak positions and widths, which is in accordance with the previously reported structural transformation. A binding energy EB of 62.3  8.9 meV was extracted by fitting to the temperature-dependent excitonic emission intensity. Furthermore, an unusual high-temperature blue shift of the band gap was revealed, underscoring the unique physical properties of such hybrid perovskite semiconductors. For the synthesis of hybrid perovskites, a mixture of PbCl2 and CH3NH3I (1 : 3 molar ratio) dissolved in dimethylformamide (DMF) was used as the precursor solution.3 Patterned fluorinedoped SnO2 (FTO) glass substrates and bare quartz substrates were ultrasonically cleaned by detergent, ethanol, acetone and deionized water. For optical measurements, only perovskite was spin-coated on the quartz substrates and heated at 115 1C for 60 minutes in the fume hood. The film thickness as determined by the cross-sectional scanning electron microscope is about 300 nm. For the fabrication of solar cells, an active area of 0.18 cm2 was used and all devices were fabricated in air. A compact TiO2 layer was spin-coated using 0.15 M titanium isopropoxide and baked at 510 1C for 30 minutes. Then a meso-porous TiO2 layer was deposited by spin-coating TiO2

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paste diluted in anhydrous ethanol. The layers were sintered at 540 1C for 30 minutes. The perovskite precursor solution was spin-coated on the TiO2-FTO substrates. Sprio-OMeTAD was coated onto the perovskite films as the hole transport layer. Cells were left in the dark in a desiccator overnight prior to sputtering 80 nm Ag electrodes. The X-ray diffraction patterns were recorded using a Bruker D8 Advance diffractometer with Cu Ka radiation. The absorption spectra were measured using an Agilent Cary 6000i UV-Visible-NIR spectrometer. The photoluminescence spectra were characterized using a Horiba JY LabRAM Aramis spectrometer with an Olympus 10 lens in a Linkam THMS600 stage. A 632.8 nm helium–neon laser from Melles Griot was used as the excitation source. The laser power density on the film surface was about 800 mW cm2. The solar cell performance was evaluated using a Newport Oriel Sol3A solar simulator under AM 1.5 sunlight at 100 mW cm2. IPCE spectra were recorded using a Newport IQE200 system. Fig. 1(a) presents the XRD pattern of the film deposited on quartz substrates. Characteristic pervoskite diffraction peaks at

Fig. 1 (a) XRD pattern of the CH3NH3PbI3xClx film on quartz. Inset: digital photograph of surface morphology taken under a 50 lens. (b) Typical solar cell performance. Inset: IPCE spectra of the solar cell.

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about 14.21, 28.51, 43.41, and 59.01 can be assigned to (110), (220), (330), (440) planes of crystalline CH3NH3PbI3xClx, respectively. No secondary phase (e.g. PbI2 at 12.31) was detected. The inset shows the surface morphology of the film on quartz observed with an optical 50 lens. The crystalline grains are quite uniform with a size of several micrometres. Fig. 1(b) shows the current density–voltage (J–V) curve of a typical solar cell measured in the dark and under AM1.5 conditions. The device exhibits a short-circuit photocurrent density (Jsc) of 18.6 mA cm2, open-circuit voltage (Voc) of 0.86 V, and fill factor (FF) of 0.59, yielding an efficiency (Eff) of 9.5%. The IPCE spectrum of the solar cell is shown in the inset of Fig. 1(b). These structural and performance data suggest that the CH3NH3PbI3xClx thin films have a high crystalline quality. As shown in Fig. 2(a), the room temperature PL peak of the CH3NH3PbI3xClx film is centered at 1.61 eV with a full width at half maximum (FWHM) of 91 meV. The onset of interband absorption can be identified as an edge positioned at around 1.60 eV, in agreement with the previously reported data on thin

Fig. 2 (a) Room-temperature PL and absorption spectra of the CH3NH3PbI3xClx film on quartz. Inset is the photograph taken during the measurements. (b) PL spectrum taken at 77 K along with Gaussian fittings.

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films of solution-processed mixed halide CH3NH3PbI3xClx.9,12 The emission line width is almost twice as broad as the spectral width of the absorption onset (55 meV, determined as the FWHM of the first derivative of the absorption edge), also consistent with the previous reports.18 It should be pointed out that substitution of iodine with chlorine atoms is expected to increase the band gap of perovskites.19 In our case, the observed PL peak position is still close to that of CH3NH3PbI3,20 consistent with the low chlorine content in the films. Different excitation positions were also checked as shown in the inset image taken using a 100 lens. Almost identical spectra were recorded at different spots with different excitation intensities (tuned by a neutral filter), suggesting reproducible and stable PL properties of the perovskite films. Fig. 2(b) presents the cryogenic PL spectrum taken at 77 K. Two excitonic peaks can be distinguished from the deconvolved Gaussian fitting (R2 = 0.999). One sharp peak is located at 1.66 eV (B748.8 nm) and another peak at 1.61 eV (B769.9 nm). Temperature-dependent PL measurements from 77 K to 380 K were carried out and the data are summarized in Fig. 3. Intensities

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are normalized to clarify the shift of emission peaks. In the spectra from 77 K to 160 K shown in Fig. 3(a), clear evolutions of peak position and intensity were revealed. As the temperature increases, the intensity of Peak 1 at 1.66 eV decreases and vanishes around 160 K. On the other hand, Peak 2 at 1.61 eV increases with temperature and dominates the spectra above 160 K shown in Fig. 3(b). The continuous spectral evolution indicates that both PL peaks should be attributed to free excitons (FE), not some kind of bound excitons (BE). At room temperature, the PL of the CH3NH3PbI3xClx film originates solely from Peak 2. Some interesting features of the temperature-dependent evolution of the two excitonic peaks were observed after the detailed Gaussian fittings were carried out on the PL spectra. As shown in Fig. 3(c), Peak 1 stays roughly at the same position, while the peak significantly broadens with temperature increasing from 77 K to 160 K. The broadening of Peak 1 intensifies at higher temperatures because of stronger exciton–phonon interaction. As shown in Fig. 3(d), upon increasing the temperature, the position of Peak 2 shows a red shift at temperatures higher than 100 K,

Fig. 3 (a) and (b) Temperature dependent PL spectra taken from 77 K to 380 K; for clarity, the intensities of the spectra are normalized. (c) and (d) Positions and widths of the two excitonic peaks, as determined by the deconvoluted Gaussian fittings, shown as a function of temperature.

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then a blue shift from 160 K to 380 K, forming a trough around 160 K. This anomalous behaviour of peak shift was also observed recently by other groups,13,17 and will be addressed later. A similar non-monotonous behaviour is also observed here in the temperature-dependent FWHM data. The temperature region around 160 K seems to be a transition point where Peak 1 disappears and the trends of peak position/ FMHW of Peak 2 change. In fact, this feature is consistent with the structural transition at around 162 K from the low-temperature orthorhombic phase to the high-temperature tetragonal phase in CH3NH3PbI3.21,22 Recently, cryogenic absorption measurements also revealed the occurrence of this phase transition in the temperature regimes of 150–170 K for CH3NH3PbI3 and 120– 140 K for hybrid CH3NH3PbI3xClx.17 It can be deduced that Peak 1 comes from the low-temperature orthorhombic phase, while Peak 2 from the high-temperature tetragonal phase. Furthermore, theoretical calculation revealed that the tetragonal phase has a smaller band gap than the orthorhombic phase,21 which is consistent with the PL results. We should also pointed out that Peak 2 still exists in the PL at 77 K, indicating significant phase coexistence in such solution-processed hybrid perovskite films. Our results underscore PL as a convenient tool to investigate the phase transition in organometal-halide perovskites. Since the tetragonal phase is stable above 160 K, important physical parameters of perovskites such as exciton binding energy, exciton–phonon interaction and band gap can be estimated from the PL data. The temperature-dependent FE emission intensity is plotted in Fig. 4(a), which can be fitted using,23,24 IðTÞ ¼

I0 1 þ AeEB =kB T

(1)

in which I0 is the intensity at 0 K, EB the binding energy, and kB the Boltzmann constant. From the fitting, a binding energy EB of 62.3  8.9 meV is extracted, which is close to the previously reported values of 55  20 meV estimated from the absorption data for the CH3NH3PbI3xClx film17 and B50 meV from the magneto-absorption data for the CH3NH3PbI3 film.16 Another recent microwave photo-conductance and PL study revealed a lower exciton binding energy of 32  5 meV in CH3NH3PbI3 samples spin coated on mesoporous Al2O3.17,24 Other values like 18–24 meV was also reported for CH3NH3PbI3.21 This comparison suggests that the exciton characteristics of hybrid perovskites are sensitive to the synthesis, composition and structure details. In general, substitution doping of Cl into CH3NH3PbI3 may hinder collective molecular motions, leading to larger exciton binding energy. However, we should note that the exciton binding energy was derived from the lowtemperature PL data, and the room-temperature exciton binding energy may be overestimated. In other words, 62.3  8.9 meV should be taken as the upper bound for EB although the Cl doping is expected to increase EB of halide perovskites. Exciton screening by collective orientational motion of the organic cations at room temperature can still lead to a large population of free carriers.21

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Fig. 4 Temperature dependent data of (a) integrated intensity, (b) width and (c) energy of Peak 2 above 160 K. The solid lines are fittings.

The temperature-dependent peak-width broadening of Peak 2 is plotted in Fig. 4(b) and fitted using the independent Boson model:25 GðTÞ ¼ G0 þ sT þ

Gop e

hoop =kB T 

1

(2)

in which the first term G0 is the inhomogeneous broadening contribution, s and Gop are the exciton-acoustic phonon interaction

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and the exciton-optical phonon contribution to the line width broadening, respectively. The first term dominates in eqn (2) at relatively low temperatures, and a nearly constant broadening was expected. As the temperature increases, acoustic and optical phonon contributions dominate and the line width increases almost linearly with temperature. Since Peak 2 dominates at temperatures above 160 K, it is a reasonable assumption that the contribution to width broadening from the acoustic phonons can be neglected. Accordingly, the measured FWHM data were fitted using eqn (2) with s = 0. It was found that G0 = 38.0  3.7 meV, Gop = 92.1  23.5 meV and more importantly, the optical phonon energy hoop = 25.3  5.2 meV. This result is consistent with a recent Raman scattering experiment on CH3NH3PbI3, which revealed optical phonons with an energy of 25–42 meV for the torsion mode of the organic cations, whereas the vibration of the inorganic cage and the libration of the organic cations are associated with much smaller energies.26 As shown in Fig. 4(c), the energy of Peak 2 changes almost linearly with the temperature. The continuous blue-shift for temperatures higher than 160 K can be described by a temperature coefficient a = qE/qT = 0.30 meV K1. This counterintuitive blue-shift was also observed in lead/copper chalcogenide semiconductors like PbS(Se/Te) and CuCl(Br/I).27 Traditional ¨ssler29 and Bose-Einstein30 models, empirical Varshni,28 Pa which are commonly used to describe the temperature dependence for conventional semiconductors, are no longer suitable for such materials. A key factor is presumably the interplay between the electron–phonon renormalization and the thermal expansion, which have opposite effects on the band gap energy. We should note here that at temperatures lower than 160 K, an opposite trend was observed for the temperature-dependent shift of Peak 2. The gradual change of energy gap and PL energy of about 50 meV at around 160 K could be a natural result of the structural phase transition.21 It was proposed recently that small inclusions of crystallites of the room-temperature tetragonal phase may coexist with the low-temperature orthorhombic phase.31 This phase coexistence scenario explains well the persistence appearance of Peak 2 throughout the whole temperature range of 77–380 K. On the other hand, the coexistence of both Peaks 1 and 2 could also be a result of transitions between different points on the Fermi surface. We should stress that the anomalous temperature dependent band gap of hybrid perovskites demands further structural and optical investigations. In summary, we reported on the temperature dependent PL properties of solution processed CH3NH3PbI3xClx films in the range of 77 K to 380 K. The revolution of two excitonic peaks confirms the structural transition from the orthorhombic phase to the tetragonal phase at the critical temperature of 160 K. An exciton binding energy of 62.3  8.9 meV and an optical phonon energy of 25.3  5.2 meV were estimated. We also found an abnormal blue shift in the band gap energy with increasing temperatures for the tetragonal phase. Our results provide useful insights into the optical properties of such important hybrid perovskites, promoting their future applications in optoelectronic devices.

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Acknowledgements This work is supported by King Abdullah University of Science and Technology (KAUST).

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Temperature-dependent excitonic photoluminescence of hybrid organometal halide perovskite films.

Organometal halide perovskites have recently attracted tremendous attention due to their potential for photovoltaic applications, and they are also co...
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