Temperature-Dependant, Absolute Quantum Measurements of Perovskites

Metal halide perovskites are a potential group of materials developed for a broad range of optoelectronic applications including light emitting diodes (LEDs), photovoltaics, optical sensing and lasers.1 They have received a great deal of attention because of their many desirable photophysical and synthetic properties, such as high tunability, solution processability, high charge carrier mobilities, and long charge carrier lifetimes.

An efficiency of 21% have been achieved by perovskite photovoltaic cells in a remarkably short time span. As a result, perovskite photovoltaic cells are starting to challenge the dominance of silicon.2

Perovskite nanocrystals have been proven to possess high PLQY and excellent wavelength tunability for light emission,3 while two dimensional perovskite structures have been demonstrated to be potential single component white light emitters.4 In this article, the FLS1000 Photoluminescence Spectrometer equipped with the Cryosphere accessory is used to investigate the photoluminescence quantum yield (PLQY) — one of the most significant photophysical parameters of perovskites for optoelectronic applications.

FLS1000 Photoluminescence Spectrometer with double excitation and emission monochromators.

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

A material’s PLQY is defined as the ratio of the number of photons emitted by that material (Nem), to the number of photons that were absorbed (Nabs), and is represented by the symbol η,

(1)

The PLQY is a measure of the competition between the non-radiative and radiative pathways in the material following photoexcitation, and hence the PLQY can also be expressed as,

(2)

where kr is the radiative rate and knr is the non-radiative recombination rate.

For effective operation of optoelectronic devices, the PLQY serves as a crucial material parameter. Since minimization of the non-radiative recombination rate will allow the production of devices with higher electroluminescence external quantum efficiency, emitter materials which are used in light emitting diodes must have a high PLQY. Therefore, PLQY measurements provide an effective technique to screen and optimize new potential emitters for LEDs, because materials with low PLQY will lead to inefficient LEDs and can be discarded.

PLQY is also a key parameter for creating efficient photovoltaic cells. All recombination inside the cell should be radiative to achieve the maximum open circuit voltage (VOC) of a photovoltaic cell. The theoretical maximum VOC of the commonly studied methylammonium lead iodide (MAPI) photovoltaic cell is 1.32 V under 1 sun illumination.5 However, presently, the VOC of MAPI cells continue to be less than 1.1 V, corresponding to a loss of 200 mV because of non-radiative recombination.5, 6 PLQY measurements can offer insight into these non-radiative losses and can help in material optimization.

An integrating sphere is the most accurate option for measuring the PLQY at room temperature. However, working optoelectronic devices are subjected to a broad range of temperatures that can depart considerably from the ambient temperature of a laboratory. In particular, photovoltaic cells are subjected to extremes in temperature, with ambient temperatures ranging between -20 °C and 40 °C. Furthermore, the working temperature inside the cell can significantly cross the ambient temperature owing to heating from solar irradiation, and in certain cases it has been shown to be as high as 70 °C.7 Hence, it is essential to understand the impact of temperature on the PLQY of optoelectronic devices.

In order to measure the temperature dependence of PLQY, Edinburgh Instruments has created a variable temperature integrating sphere called the Cryosphere, which can measure the PLQY of solid samples over 77 K to 500 K temperature range. With the use of an integrating sphere, the change in absorption coefficient as well as the change in PL intensity with temperature is explained. In this article, the FLS1000 with the Cryosphere accessory is used to measure the temperature dependence of the PLQY of the perovskite emitter CsPbBr3.

Sample holder (left) and integrating sphere (right) of the Cryosphere.

Figure2. Sample holder (left) and integrating sphere (right) of the Cryosphere.

Methods and Materials

The route described in Stoumpos et al. was employed to synthesize a single crystal of CsPbBr3.8 A Cryosphere which was fiber optically coupled to an FLS1000 Photoluminescence Spectrometer was used to measure the PLQY. The FLS1000 was fitted with a 450 W Xenon lamp, double monochromators, and a PMT-900 detector. The sample space was evacuated through a turbomolecular pump and temperature control provided by a constant flow of liquid nitrogen cryostat.

Results and Discussion

CsPbBr3 was first excited at 430 nm and the emission and scattering peaks were measured in the Cryosphere over 80-250 K temperature range. The scattering and emission peaks of a PTFE plug were also measured to serve as a reference. Figure 3 shows the change in CsPbBr3 emission with temperature. It can be observed that at 530 nm the PL has a maxima with a shoulder peak at 545 nm which is consistent with previous reports.8 As the temperature is decreased, the intensity of the PL increases, signifying a change in the PLQY of the material with temperature.

Variation of the photoluminescence emission spectrum of CsPbBr3 with temperature. The emission spectra were measured using the Cryosphere accessory for the FLS1000. Excitation source = 450 W Xenon Lamp, λex = 430 nm, Δλex = 7 nm, Δλem = 2 nm.

Figure 3. Variation of the photoluminescence emission spectrum of CsPbBr3 with temperature. The emission spectra were measured using the Cryosphere accessory for the FLS1000. Excitation source = 450 W Xenon Lamp, λex = 430 nm, Δλex = 7 nm, Δλem = 2 nm.

The quantum yield wizard of the FLS1000’s Fluoracle® operating software was used to calculate the PLQY of the sample at each temperature. The user chooses the desired emission and scattering spectra and sets the integration ranges, and Fluoracle subsequently combines over the spectra to establish the quantum yield. Figure 4 illustrates the interface of the quantum yield wizard for the measurement taken at 80 K. The scatter from the sample (blue), scatter from the reference (black), emission from sample (red), and background emission from reference (orange) were measured as four separate scans and integrated in the quantum yield wizard.

The emission and scattering peaks were recorded separately to enable the placement of an OD1.5 neutral density filter in the excitation light path when measuring the scattering peaks. It is possible to increase the monochromator bandwidths by placing a neutral density filter in the excitation light path, and this increase enhances the signal-to-noise of the emission peak measurements, without any saturation of the detector by the scattering peaks. Then, the transmission coefficient of the neutral density filter scales the intensities of the scattering peaks during post processing. This method is recommended when the PLQY of weakly emitting samples is being measured.

It is important to scale the background emission from the reference so that the higher intensity of scattered light in the sphere can be taken into consideration when the reference is present, which leads to a higher background emission signal. Fluoracle therefore has the option to automatically scale the background emission by the ratio of sample and reference scattering peak heights, which leads to the scaled reference background displayed in green, and enables accurate background subtraction during the PLQY calculation.

Scattering and emission peaks of the reference and the CsPbBr3 sample at 80 K, analysed using the Fluoracle quantum yield wizard. Excitation source = 450 W Xenon Lamp, λex = 430 nm, Δλex = 7 nm, Δλem = 2 nm. The scattering peaks were recorded with an OD1.5 neutral density filter in the excitation light path and then scaled by the transmission coefficient of the filter during post processing.

Figure 4. Scattering and emission peaks of the reference and the CsPbBr3 sample at 80 K, analysed using the Fluoracle quantum yield wizard. Excitation source = 450 W Xenon Lamp, λex = 430 nm, Δλex = 7 nm, Δλem = 2 nm. The scattering peaks were recorded with an OD1.5 neutral density filter in the excitation light path and then scaled by the transmission coefficient of the filter during post processing.

Fluoracle calculates the PLQY of the sample automatically by using,

(3)

where EA, SA, EB, SB are the integrated intensities of the emission and scattering peaks of the reference and sample, respectively. The ratio of the CsPbBr3 and reference scattering peaks were found to be more or less constant with temperature, indicating that the absorbance of the sample does not appreciably vary over the temperature range studied. The increase in photoluminescence intensity seen in Figure 3 when the sample is cooled must therefore be caused by an increase in PLQY.

Using the same integration gates as illustrated in Figure 4, the PLQY calculation was repeated for all temperatures to give the variation in PLQY with temperature shown in Figure 5. A nonlinear increase from 0.02% at 200 K to 0.43% at 80 K can be seen in the PLQY. This increase in PLQY is caused by the charge carriers becoming immobile as the temperature is decreased, and therefore increasingly unable to reach the non-radiative recombination centers (defects) that are present in the perovskite. Instead the carriers must combine radiatively which causes to an increase in PLQY with decreasing temperature.

Temperature dependence of the PLQY of CsPbBr3, measured using the Cryosphere accessory for the FLS1000. The PLQY was calculated using the quantum yield wizard in Fluoracle.

Figure 5. Temperature dependence of the PLQY of CsPbBr3, measured using the Cryosphere accessory for the FLS1000. The PLQY was calculated using the quantum yield wizard in Fluoracle.

Conclusion

By using the Cryosphere accessory for the FLS1000, the effect of temperature on the photoluminescence quantum yield of CsPbBr3 perovskite was examined. It was observed that the PLQY increased from 0.02% to 0.43% as the perovskite was cooled from 200 K to 80 K which corresponds to a suppression of non-radiative recombination mechanisms.

This article has shown that the Cryosphere is a powerful tool for investigating how temperature influences the loss mechanisms in optoelectronic devices.

References

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[2] M. A. Green, Y. Hishikawa, E. D. Dunlop, D. H. Levi, J. Hohl-Ebinger, and A. W. Y. Ho-Baillie, Solar Cell Efficiency Tables (Version 51), Prog. Photovolt. Res. Appl. 26, 3–12 (2018)

[3] X. Du, G. Wu, J. Cheng, H. Dang, K. Ma, Y. Zhang, Pe. Tana and S. Chen, High-Quality CsPbBr3 Perovskite Nanocrystals for Quantum Dot Light-Emitting Diodes, RSC Adv. 7 10391- 10396 (2017)

[4] M. D. Smith, and H. I. Karunadasa, White-Light Emission from Layered Halide Perovskites, Acc. Chem. Res. 51, 619-627 (2018)

[5] W. Tress, N. Marinova, O. Inganäs, M. K. Nazeeruddin, S. M. Zakeeruddin, M. Graetzel, Predicting the Open Circuit Voltage of CH3NH3PbI3 Perovskite Solar Cells Using Electroluminescence and Photovoltaic Quantum Efficiency Spectra: The role of Radiative and Non-Radiative Recombination, Adv. Energy Mater. 5, 1400812 (2015)

[6] Li. Gil-Escrig, G. Longo, A. Pertegás, C. Roldán-Carmona, A. Soriano, M. Sessolo and H. J. Bolink, Efficient Photovoltaic and Electroluminescent Perovskite Devices, Chem. Commun. 51, 569-571 (2015)

[7] E. Skoplaki, A. G. Boudouvis and J. A. Palyvos, A Simple Correlation for the Operating Temperature of Photovoltaic Module of Arbitrary Mounting, Sol. Energy Mater Sol. Cells 92, 1393-1402 (2008)

[8] C. C. Stoumpos, C. D. Malliakas, J. A. Peters, Z. Liu, M. Sebastian, J. Im, T. C. Chasapis, A. C. Wibowo, D. Y. Chung, A. J. Freeman, B. W. Wessels, and M. G. Kanatzidis, Crystal Growth of the Perovskite Semiconductor CsPbBr3: A New Material for High-Energy Radiation Detection, Cryst. Growth Des. 13, 2722−2727 (2013)

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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