Semiconductor quantum dots offer high photoluminescence quantum yields (PLQY), solution processability and highly tunable band gaps.
These qualities make quantum dots perfectly suited for photodiodes and solar cells, where they function as light absorbers. They are also ideal for optoelectronic devices, such as light emitting diodes and semiconductor lasers, where they serve as competent emitters.
Their light emission properties also make them a promising new type of fluorescent probe for biomedical fluorescence imaging, replacing traditional organic small molecule probes. Quantum confinement is another property of quantum dots that make them attractive, as this process provides researchers and manufacturers with exceptional control over their band gap.
The quantity of atoms in a bulk semiconductor is very large, and the overlap of this large number of atomic orbitals results in a continuum of tightly packed ‘molecular’ orbitals which make up the conduction and valence bands.
However, this changes when the semiconductor is reduced to a nanoscale size. At this point, the number of atomic orbitals overlapping decreases, and the conduction and valence bands are no longer continuous. Now, they are made up of discrete energy levels, and - more importantly - the width of the band gap between the conduction and valence bands increases. This is known as quantum confinement (Fig. 1).
Figure 1: The influence of particle size on the band gap and photoluminescence emission wavelength of quantum dots due to quantum confinement.
Nanoparticles that are small enough to have their band gap affected by quantum confinement are called quantum dots. Through careful and precise control of the size of the quantum dots throughout synthesis, it is possible to finely tune the photoluminescence emission and absorption wavelengths. This is extremely useful in optoelectronic applications.
In the past, quantum dots were dominated by chalcogenides, such as cadmium telluride and zinc selenide, but there has been growing interest in quantum dots based on halide perovskite semiconductors. Their role as cost-effective, high-efficiency absorbers in photovoltaic cells has already gained halide perovskites plenty of attention within the scientific community.
The solution processability, band gap tuneability and high PLQY that has made perovskite solar cells a great success also makes them favorable contenders for a new class of quantum dots. To improve the properties of perovskite quantum dots, additional research is now required. Photoluminescence and absorption spectroscopy are two leading techniques through which researchers can better characterize these promising materials.
This article details a complete photophysical characterization of two perovskite quantum dots using the highly adaptable FS5 Spectrofluorometer. Photoluminescence spectra, photoluminescence lifetime, absorption spectra and quantum yield were all explored as part of the study.
Figure 2: The FS5 Spectrofluorometer with TCSPC electronics and pulsed diode laser. The FS5 can be configured to measure the absorption spectra, emission spectra, lifetime and quantum yield of materials such as quantum dots.
Materials and Methods
PlasmaChem GmbH supplied the perovskite quantum dots. A solution of each quantum dot was prepared in cyclohexane. To prevent reabsorption errors during spectral and PLQY measurements, it was diluted to attain an absorbance of 0.1 OD at the band edge.
The solutions were inserted into 1 cm pathlength quartz cuvettes and measured using the FS5 Spectrofluorometer equipped with a PMT-900 detector and TCSPC lifetime electronics. Absorption spectra, photoluminescence emission spectra and photoluminescence decays were held with the SC-05 Cuvette Holder Module. The quantum dots were then measured using the SC-30 Integrating Sphere Module to determine the PLQY.
Results and Discussion
The FS5 Spectrofluorometer was then used to analyze the photophysics of two halide perovskite quantum dots, henceforth known as PQD-A and PQD-B. The FS5 is equipped with an absorption detector as standard, which allows the photoluminescence and absorption spectra to be measured without any need for additional instruments.
Fig. 3a shows the absorption and emission spectra of PQD-A. The emission is centered on 450 nm and, as is typical, has a narrow FWHM of only 14 nm. It is apparent that the emission takes place at the band edge of the quantum dot, as the emission peak is coincident with the abrupt drop in the absorbance that indicates the band edge.
Figure 3: Absorption and emission of perovskite quantum dots in cyclohexane. (a) Absorption and emission spectra of PQD A, (b) absorption and emission spectra of PQD-B and (c) chromacity coordinates of the PQD-A and PQD-B emission. The excitation source for the absorption and emission measurements was a 150 W Xenon Lamp. Absorption spectra parameters: Δλex = 2 nm. Emission spectra parameters: λex = 350 nm, Δλex = 1.0 nm, Δλem = 0.5 nm.
PQD-B initially seems to demonstrate similar absorption and emission spectra, with a narrow emission centered on 514 nm at the band edge of the quantum dot. However, differences become apparent when one compares the absorption behavior of PQD-A and PQD-B.
As is expected for a semiconductor, the absorbance in PQD-A drops drastically to zero at the band edge. Conversely, the absorbance of PQD-B does not drop fully to zero at the band edge, but instead demonstrates an extended and exponential decay in the absorbance following the band edge.
This gradual decay, which is known as an Urbach tail, stems from higher energetic disorder at the band edge as a result of defects and trapping sites 1-3. By examining the absorption spectra, we can thus determine that PDQ-B has a higher incidence of energetic disorder than PQD-A.
For display applications, it is more useful to define the emission in terms of its chromaticity coordinates, as opposed to the peak wavelength.
Using the wizard built into the Fluoracle® software of the FS5, it is possible to create a chromaticity plot from any emission spectrum, in either CIE 1931 or CIE 1976 color space. Fig. 3c displays the chromaticity coordinates of the two quantum dots, which were calculated in CIE 1931 color space.
In all perovskite quantum dot applications, a high PLQY is essential. While for quantum dot absorbers in solar cells, higher PLQYs will result in higher open circuit voltages and more efficient power conversion efficiencies.
It is therefore vital to ensure accurate measurement and optimization of the PLQY of recently developed perovskite quantum dots. An integrating sphere is the most reliable technique for measuring the PLQY of a sample.
The SC-30 Integrating Sphere Module for the FS5 was therefore used to measure the scattering and emission peaks of PQD-A, PQD B and the cyclohexane, as seen in Fig. 4.
Figure 4: The scattering and emission peaks of PQD-A and PQD-B dissolved in cyclohexane along with a reference of pure cyclohexane solvent. The scattering and emission integration ranges that were used to calculate the PLQY are shown in red. Excitation source = 150 W Xenon Lamp, λex = 350 nm, Δλex = 5 nm, Δλem = 0.3 nm.
The integrated quantum yield wizard of Fluoracle was then used to calculate the PLQY of the quantum dots. The desired scattering and emission integration ranges were defined in the wizard, after which Fluoracle integrates over the reference and sample spectra and calculates the quantum yield using:
where SSample and SRef represent the integrated intensities of the scattering peaks of the sample and reference, respectively, ESample represents the integrated intensity of the sample emission and ERefScaled represents the integrated intensity of the scaled reference emission.
The reference emission is scaled to account for higher intensity of scattered light in the sphere when the weakly absorbing reference is present. This greater scattering intensity results in a greater background emission signal in the sphere than when an absorbing sample is present.
Fluoracle offers users the choice to automatically scale the background emission by the ratio of sample and reference scattering peak heights. This results in the scaled reference background (seen in grey in Fig. 4) and offers greater accuracy of the background subtraction for the PLQY calculation.
While PQD-A demonstrated a poor PLQY of 3.3%, PQD-B demonstrated an outstanding PLQY of 56.0%. This high value for PQD-B highlights the great potential of these recently developed materials. In fact, values of over 90% have been reported for similar perovskite quantum dots in recent studies. This is comparable to traditional cadmium-based dots.4
The photoluminescence lifetime is the final photophysical parameter that is central to the characterization of perovskite quantum dots. This parameter helps to provide a greater understanding of the recombination processes inside the quantum dot. As illustrated in Fig. 5, the PL decays of PQD-A and PQD B were measured using Time-Correlated Single Photon Counting (TCSPC).
As has been noted in earlier reports4, the PL decays were found to be extremely complex and it is not possible to fit them with a single time constant. Likely causes of this complicated decay behavior include the multiple recombination processes that occur within the quantum dot, (such as radiative and non-radiative recombination channels), as well as the inevitable distribution of dots featuring slightly different lifetimes.
Using Fluoracle’s integrated reconvolution fitting routine, the PL decays were fit with a four component exponential decay model (Eq. 2). Reconvolution fitting accounts for the pulse shape of the excitation laser and the detector response to provide a more consistent fit.
Table 1: Fitting parameters of the PL decays in Figure 5
Table 1 shows the results of the fit for PQD-A and PQD-B. It is almost certain that the four components of the exponential fit do not correspond to four distinct physical processes, but can rather be applied to determine the average lifetime of the decay, 〈τ〉, which acts as a figure of merit for each quantum dot.
The average lifetime of the decays was determined using Eq. 3 and was found to be 16.1 ns for PQD-A, and 19.1 ns for PQD-B.
The FS5 Spectrofluorometer was used to analyze the photophysics of two perovskite quantum dots. Measurement of the absorption and emission spectra of the dots showed peak emission wavelengths at 450 nm and 514 nm, which corresponded with the band edge of the dots.
The PLQY was determined using the SC-30 Integrating Sphere Module, and the 450 nm emitting dot was determined to have a low PLQY of only 3%, while the 514 nm emitting dot offered a high PLQY of 56%.
Finally, TCSPC was used to analyze the PL lifetimes of the dots. This uncovered complicated recombination behavior with average lifetimes of 16.1ns and 19.1ns.
This article demonstrates how the FS5 Spectrofluorometer can deliver a comprehensive photophysical characterization of quantum dot emitters within one single, conveniently sized instrument.
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This information has been sourced, reviewed and adapted from materials provided by Edinburgh Instruments.
For more information on this source, please visit Edinburgh Instruments.