Determining the Photophysical Properties of White-Light Perovskites

Determining the Photophysical Properties of White-Light Perovskites

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Thanks to the tunability, low cost, solution processability, and high performance of hybrid, organic-inorganic halide perovskite semiconductors, they are highly promising candidates for use in optoelectronic devices. Due to the rapid increase in their cell efficiencies and their ability to provide high-efficiency, low-cost solar power, they have gained major attention as light harvesters in photovoltaic cells.1,2

Apart from their striking photovoltaic performance, it has been also found that halide perovskites exhibit exceptional light emitting properties and are being widely studied as a new category of solid-state light emitters.3-5

A promising application of perovskites is the development of solid-state white-light emitting devices. In commercially available solid-state white-light sources, white emission is produced by using a mix of multiple light emitting diodes and phosphor coatings.4 Such a multicomponent strategy is complicated as the different components in the emitter age at different rates and hence the emitter color will drift over time. Two-dimensional (2D) halide perovskites are promising materials for developing a single-component solid-state white-light source.4

This article describes the use of an FLS1000 Photoluminescence Spectrometer to characterize the properties of a white-light-emitting 2D perovskite with the help of steady-state and time-resolved photoluminescence spectroscopy.

FLS1000 Photoluminescence Spectrometer

Figure 1. FLS1000 Photoluminescence Spectrometer with double excitation and emission monochromators and TCSPC lifetime option.

Conventional halide perovskites are three-dimensional (3D) and have an ABX3 crystal structure, where X is the anion, and A and B are two different cations. The properties of perovskites can be tweaked over a broad range by modifying the choice of anions and cations that make up the structure, meaning perovskites are a highly versatile material. 2D or layered perovskites, with an ABX4 or A2BX4 crystal structure, have been gaining increasing attention. Here, the size of the A cation is made very large to lower the dimensionality of the structure.

α-(DMEN)PbBr4 is an example of a 2D perovskite, which is a promising white-light emitter. Figure 2 illustrates the structure of α-(DMEN)PbBr4.3 In α-(DMEN)PbBr4, the Pb-Br octahedra are separated into sheets by the large 2-(Dimethylamino)-ethylamine (DMEN) cations, thereby forming a quasi-2D system.

Crystal structure of α-(DMEN)PbBr4 and the organic cation 2-(Dimethylamino)ethylamine.

Figure 2. Crystal structure of α-(DMEN)PbBr4 and the organic cation 2-(Dimethylamino)ethylamine. Image adapted from Mao et al. and Smith et al.3,4

The transition from 3D to 2D has a huge impact on the photophysical properties of the perovskite. In  3D perovskites, the electrostatic attraction between photogenerated electron-hole pairs is screened by the polarizable 3D lattice, leading to free holes and electrons. On the other hand, in the case of 2D perovskites, their layered structure confines the photoexcited holes and electrons, leading to the formation of strongly bound electron-hole pairs known as excitons.

It is believed that the white-light emission in these systems is due to the self-trapping of these excitons.4 Photoluminescence spectroscopy is a powerful tool that can be used to gain clear insights into the photophysics of excitonic perovskites.

Methods and Materials

α-(DMEN)PbBr4 was produced by following the route described in Mao et al.3

An FLS1000 Photoluminescence Spectrometer equipped with a PMT-980 detector, EPL-405 pulsed diode laser, EPLED-255 pulsed light emitting diode, a 450 W Xenon lamp, and double monochromators was used to measure the photoluminescence emission (PL) spectra and decays.

The N-J03 front-face sample holder was used to position the perovskite sample.

Results and Discussion

Figure 3 illustrates the PL spectrum measured by exciting the α-(DMEN)PbBr4 sample at 280 nm. Typically, perovskite samples have low quantum yields, which could lead to difficulties in obtaining an accurate PL spectrum. The PL spectrum in Figure 3 was therefore measured using an FLS1000 with double monochromators on the excitation and the emission light paths.

It is essential to use double monochromators for highly scattering, low quantum yield samples such as perovskites since they allow the PL to be isolated from the Rayleigh scattered excitation light, thereby enabling an accurate spectrum to be measured. It is evident from Figure 3 that the emission from α-(DMEN)PbBr4 is exceptionally wide, extending from 400 to 930 nm, with a peak at 600 nm.

Photoluminescence emission spectrum of α-(DMEN)PbBr4 excited at 280 nm.

Figure 3. Photoluminescence emission spectrum of α-(DMEN)PbBr4 excited at 280 nm. Excitation source = 450 W Xenon Lamp, λex = 280 nm, Δλex = 10 nm, Δλem = 2 nm. The spectrum has been spectrally corrected to account for the monochromator and detector wavelength response.

A chromacity plot can be used to easily visualize the properties of the α-(DMEN)PbBr4 emission. The Fluoracle software provided in the FLS1000 includes a wizard to create a chromacity plot from any emission spectrum, in CIE 1931 or CIE 1976 color space.

Figure 4 shows the chromacity plot of the α-(DMEN)PbBr4 emission calculated in the CIE 1931 color space. The CIE 1931 coordinates of the emission of α-(DMEN)PbBr4 are (0.39, 0.40), which is close to the “pure white” point of (0.33, 0.33) and is hence a promising direction for the development of a solid-state white-light emitter.

Chromacity plot of α-(DMEN)PbBr4 emission in CIE 1931 color space

Figure 4. Chromacity plot of α-(DMEN)PbBr4 emission in CIE 1931 color space, calculated using Fluoracle from the emission spectrum in Figure 3. The white point (0.33, 0.33) is shown as a reference.

In order to gain better insights into the origin of the white-light emission in α-(DMEN)PbBr4, time-correlated single-photon counting (TCSPC) was employed to measure the PL decay.

Figure 5 shows the PL decays at λem = 600 nm when α-(DMEN)PbBr4 was excited at 255 and 405 nm. Since perovskites are inhomogeneous by nature, the PL lifetime does not consist of a single well-defined decay constant and the PL decays cannot be fit using only simple exponentials.7 Stretched exponential fitting is more suitable, since it accomodates the distribution of recombination rates that typically exist in perovskites.6-8

The FAST software package offered by Edinburgh Instruments can be used for analyzing complex decays. FAST includes a selection of advanced analysis options such as stretched exponential analysis, time-resolved anisotropy, and Förster kinetics.

PL decays of α-(DMEN)PbBr4 when excited at 255 and 405 nm and measured using TCSPC.

Figure 5. PL decays of α-(DMEN)PbBr4 when excited at 255 and 405 nm and measured using TCSPC. The PL decays were fit with two-component stretched exponentials and the weighted mean lifetime calculated. Excitation source = EPLED-255 Pulsed Light Emitting Diode and EPL-405 Pulsed Diode Laser, Rep Rate = 200 kHz, λem = 600 nm, Δλem = 15 nm.

A stretched exponential decay can be written as

where I(t) is the fluorescence intensity at time t, n is the number of components, Ii is the initial intensity, τi is the characteristic lifetime, and βi is the stretching exponent (0 < βi ≤ 1) of each component. When the value of β is reduced, the exponential is deformed such that the initial decay is rapid and the tail is longer. The PL decays in Figure 5 were fit with two-component stretched exponentials (stretched biexponentials) with the help of FAST.

Since there is no well-defined decay constant for stretched exponentials, their lifetime is better described by the mean relaxation time 〈τ〉 of the decay8,9

where (1/β) is the gamma function7,8

For multicomponent stretched exponentials, the mean relaxation time of each component is first calculated and the weighted mean relaxation time of all components is then calculated.

From Figure 5, it can be seen that the PL decay of α-(DMEN)PbBr4 is greatly influenced by the choice of excitation wavelength. The PL decay of the perovskite has a weighted mean relaxation time of 〈τ〉255 = 8 ns when excted at a wavelength of 255 nm. However, when excited at 405 nm, the weighted mean relaxation time is shorter, 〈τ〉405 = 1 ns, but the tail region is longer.

Since the PL lifetime is dependent on the excitation wavelength, it can be inferred that a number of excited states, with different lifetimes, may exist, which result in the white-light emission.

Conclusion

It can be difficult to accurately determine the photophysical properties of perovskite emitters such as α-(DMEN)PbBr4. Accurate emission spectra can be achieved using the FLS1000 Photoluminescence Spectrometer with double excitation and emission monochromators.

From the emission spectrum, it was evident that α-(DMEN)PbBr4 emits over a wide wavelength range, 400 nm to 930 nm, and is hence white-light emitting. The CIE 1931 chromacity coordinates calculated from the emission spectrum were (0.39, 0.40), close to the pure white coordinates of (0.33, 0.33). TCSPC was used to measure the PL lifetime, which was found to be dependent on the excitation wavelength.

FAST was used to fit the PL decays with stretched exponentials, with mean relaxation times of 8 ns and 1 ns when excited at 255 and 405 nm repectively.

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Acknowledgements

The authors would like to thank Ms L. Mao, Dr I. Hadar, and Professor M. G. Kanatzidis from Northwestern University for providing the α-(DMEN)PbBr4 sample used for this research.

References

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[7] D. W. de Quilettes, S. M. Vorpahl, S. D. Stranks, H. Nagaoka, G. E. Eperon, M. E. Ziffer, H. J. Snaith and D. S. Ginger, Solar cells. Impact of Microstructure on Local Carrier Lifetime in Perovskite Solar Cells, Science 348, 683–686 (2015)

[8] D. W. deQuilettes, S. Koch, S. Burke, R. K. Paranji, A. J. Shropshire, M. E. Ziffer, and D. S. Ginger, Photoluminescence Lifetimes Exceeding 8 μs and Quantum Yields Exceeding 30% in Hybrid Perovskite Thin Films by Ligand Passivation, ACS Energy Lett. 1, 438−444 (2016)

[9] C. P. Lindsey, and G. D. Patterson, Detailed Comparison of the Williams−Watts and Cole−Davidson Functions. J. Chem. Phys. 73, 3348−3357 (1980)

This information has been sourced, reviewed and adapted from materials provided by Edinburgh Instruments.

For more information on this source, please visit Edinburgh Instruments.

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