Time-Resolved Photoluminescence (TRPL)

Various types of set-up can be used for TRPL measurements, they can be broadly divided into three categories: spectrometers, confocal microscopes or combinations of both instruments.

For time-resolved experiments, the sample is excited by a pulsed laser, LED, or Xe-flash lamp. The emitted emission is detected by a detector with sensitivity in the UV/Vis or NIR spectral region.

Selection of the emission wavelength can be done via either a monochromator, long pass, band pass or variable filters. For lifetime measurements, either Time-Correlated Single Photon Counting (TCSPC) or Multi-Channel Scaling (MCS) electronics are used for data acquisition.

In summary, the essential components of a spectrometer set-up are:

• pulsed source (flash lamp, pulsed laser, LEDs)
• a way of separating fluorescence signal from excitation light (via monochromator or optical filters)
• single photon sensitive detector in the UV/Vis or NIR region
• for lifetime measurements: TCSPC or MCS unit to measure lifetimes from ps to ms

Systems that can be used for TRPL

• FluoTime 300 - High-end photoluminescence spectrometer >
• FluoTime 250 - Compact lifetime fluorometer >

Questions? Contact us >

Time-Resolved Emission Spectra (TRES) of gummy bears

Gummy bears are a popular type of gelatin-based candies from Germany, which were created by Hans Riegel, Sr. in 1922. Combing in all sizes and shapes, they are not only a popular treat (A) but are also nicely luminescent.

The luminescence properties a yellow and a red gummy bear were investigated with the help of the FluoMic microscope. Steady-state emission spectra of both were recorded upon excitation with a 440 nm pulsed diode laser (B). The two candies exhibit a similar emission band with a maximum at about 520 nm, with the red one having a broader tail towards the red spectral range. This is not surprising as their main ingredients are gelatin, sugars, starch, and will be differing only in flavoring and food coloring.

Recording TRES from the same candies yields more information (C and D): In both cases, a tri-exponential model fits the data very well. The two longer-lived components (blue and red curves) have a similar spectral shape with maxima at around 510 nm. The shortest-lived component (probably stemming from the food coloring) shows additional peaks at 585 and 610 nm for the red gummy bear.

Recording luminescence spectra from larger and irregularly shaped samples becomes very easy and fast using FluoMic. Plus, the spectroscopist get to enjoy the gummy bears afterwards.

Set-up:

• High-End photoluminescence sdpectrometer FluoTime 300 equipped with a compact upright widefield microscope FluoMic
• Excitation: 440 nm with a picosecond pulsed laser head from the LDH Series

Using TRPL to study charge carrier dynamics in semiconductors

Charge carrier dynamics in semiconductors are determined by the architecture and function of the respective device and directly relate to the nature and quality of wafer materials. Therefore characterizing these materials require precise and efficient measurement techniques to determine the free charge carrier lifetime. For particular classes of semiconductors, the characteristic charge carrier lifetime is highly dependent on the nature and dimensions of the materials and interfaces involved. Furthermore, surface effects, passivation, and energy transfer efficiency of sensitizers as well as the presence of dopants, impurities and defect sites can introduce significant variations in this parameter. Since the photoluminescence of semiconductors is a directly related to the charge carrier dynamics, TRPL using TCSPC is highly suited for their analysis and characterization, down to the sub-nanosecond time scale.

Set-up of example above:

• Compact upright photoluminescence microscope MicroTime 100
• Excitation: 635 nm with LDH-P-C-635B using a PDL 828
• Detection: PDM 1CTC-SPAD-Detector, detection wavelength > 664 nm

Sample courtesy of Andrea Knigge, Ferdinand-Braun-Institut, Berlin, Germany

Set-up of example below:

• Spectrometer FluoTime 300
• Excitation: with LDH Series lasers using a PDL 820
• Detection: with PMA detector
• Analysis: EasyTau

GaAs sample courtesy of Andrea Knigge, Ferdinand-Braun-Institut, Berlin, Germany
Si-nanodot sample courtesy of Maurizio Roczen, Helmholtz-Zentrum Berlin für Materialien und Energie, Germany

Determining electron-hole diffusion lengths in perovskite solar cells

A critical parameter in understanding the photophysics of semiconductor solar cells is the diffusion length of the photo-excited electrons and holes. TRPL quenching experiments are a valuable tool for determining these diffusion lengths. The example shows data obtained from mixed halide and triiodide organometal perovskite layers in presence of either an electron (blue) or hole (red) quenching layer, and a PMMA coating (black). The decay curves were recorded at 780 nm, corresponding to the peak emission of both materials. The measured decay dynamics can be fitted to a specific diffusion model, allowing deriving diffusion lengths. Here, the diffusion length of the electrons and holes in the mixed halide perovskite was 1 μm while the triiodide material featured a much shorter length of 100 nm, correlating well with performance of these materials as solar cells.

Set-up:

• High-End photoluminescence spectrometer FluoTime 300

Reference: S. D. Stranks et al., Science, 342 (2013), p.341

Label-free detection of native analyte fluorescence performed on a fluidic microchip

Time-resolved fluorescence microscopy in the deep UV can be employed in microfluidic environments to enable label-free detection and identification of various aromatic analytes in chip electrophoresis. The results show the electrophoretic separation of a mixture of small aromatics (188 µM Serotonin, 135 µM propranolol, 238 µM 3-phenoxy-1.2-propanediol and 196 µM tryptophan). Fluorescence decay curves are gathered on-the-fly and average lifetimes can be determined for different substances in the electropherogram to identify the aromatic compounds in the mixtures. Based on TCSPC, the background fluorescence can be discriminated which results in an improved signal-to-noise ratio. Additionally, microchip electrophoretic separations with fluorescence lifetime detection can be carried out for protein mixtures, which can be very useful for biopolymer analysis.

Set-up:

• MicroTime 200
• Excitation: 266 nm
• Detection: bandpass 350/50, DUG11x (285-365 nm), super bialkali PMT
• Analysis: SymPhoTime

Decay associated spectra of tryptophan

A solution of L-tryptophane in sline buffer was excited at 290 nm. A series of time-resolved emission spectra was collected in the wavelength range from 310 to 460 nm with a step size of 5 nm. Time-resolved emission spectroscopy (TRES) can identify different species of a mixture, even with strongly overlapping spectra, if their lifetimes are different. With modern TCSPC electronics and fast excitation sources one can get spectral as well as kinetic information within minutes.

Set-up:

• High-End photoluminescence spectrometer FluoTime 300
• Excitation at 290 nm using a PLS-290 and a PDL 820
• Analysis: EasyTau 2

Thermally-Activated Delayed Fluorescence (TADF) of acryflavine in PVA films

Acryflavine embedded in a PVA matrix exhibits both normal fluorescence with ns lifetime as well as thermally activated delayed fluorescence with lifetimes in ms time range. Upon excitation to its first excited singlet state S1, acryflavine undergoes efficient intersystem crossing (ISC) to the first excited triplet state T1. Since the energy gap between T1 and S1 states is small, there is a probability of thermally activated back ISC T1 → S1 followed by emission of TADF with lifetime of the T1 state, but at the same wavelength as normal fluorescence (S1 → S0 transition). When acryflavine is dissolved in water, the T1 state is efficiently quenched so that only the fast fluorescence can be observed.

With a single setup one can efficiently measure lifetimes in both ns as well as ms time ranges.

Set-up:

• High-End photoluminescence spectrometer FluoTime 300
• Excitation at 440 nm using a LDH-P-C-440 operated in pulsed as well as in burst mode using a PDL 820
• Analysis: EasyTau 2

White Paper:

Measuring steady-state and time-resolved photoluminescence from a positionable, micrometer-sized observation volume with the FluoMic

• Download White Paper