Anna
Jancik Prochazkova
ab,
Felix
Mayr
a,
Katarina
Gugujonovic
a,
Bekele
Hailegnaw
a,
Jozef
Krajcovic
b,
Yolanda
Salinas
c,
Oliver
Brüggemann
c,
Niyazi Serdar
Sariciftci
a and
Markus C.
Scharber
*a
aInstitute of Physical Chemistry and Linz Institute of Organic Solar Cells (LIOS), Johannes Kepler University Linz, Altenbergerstrasse 69, 4040 Linz, Austria. E-mail: markus_clark.scharber@jku.at
bInstitute of Chemistry and Technology of Environmental Protection, Brno University of Technology, Purkynova 464/118, 61200 Brno, Czech Republic
cInstitute of Polymer Chemistry (ICP), Johannes Kepler University of Linz, Altenbergerstrasse 69, A-4040 Linz, Austria
First published on 25th July 2020
Photon cooling via anti-Stokes photoluminescence (ASPL) is a promising approach to realize all-solid-state cryo-refrigeration by photoexcitation. Photoluminescence quantum yields close to 100% and a strong coupling between phonons and excited states are required to achieve net cooling. We have studied the anti-Stokes photoluminescence of thin films of methylammonium lead bromide nanoparticles. We found that the anti-Stokes photoluminescence is thermally activated with an activation energy of ∼80 meV. At room temperature the ASPL up-conversion efficiency is ∼60% and it depends linearly on the excitation intensity. Our results suggest that upon further optimization of their optical properties, the investigated particles could be promising candidates for the demonstration of photon cooling in thin solid films.
Luminescence up-conversion is the process whereby a system absorbs low energy photons and emits higher energy photons. The most commonly studied luminescence up-conversion mechanisms result from multi-photon processes using rare earth dopants or triplet–triplet annihilation. Alternately, single photon up-conversion, also known as anti-Stokes photoluminescence (ASPL) can occur when the energy difference between the absorbed low energy photon and the emitted high energy photon is provided by phonons. Because this process does induce phonon annihilation, taking away the lattice thermal energy by optical radiation, this phenomenon can lead to “photoinduced cooling”. The possibility of using anti-Stokes photoluminescence to cool a fluorescent gas with radiation was first proposed by Pringsheim in 1929.9 In 1946, Landau developed the theoretical basis of the process by assigning entropy to light.10 He pointed out that the entropy of a radiation field is a function of both the solid angle of the propagating light and the frequency bandwidth. During ASPL cooling light with narrow spectral bandwidth and high directionality is converted into a broadband, isotropic luminescence, increasing entropy of the system even in the presence of local cooling. Experimentally, anti-Stokes cooling is difficult to achieve and successful cooling has first been reported in rare-earth doped glasses and a fluid solution of a laser dye.11,12 Anti-Stokes cooling in semiconductors and quantum dots remains challenging.
The requirements to achieve anti-Stokes PL cooling in quantum dot emitters have been formulated by Rakovich et al.:13
(a) Appropriate electronic structure with ground and excited states well separated from each other in energy, with the excited state split into two closely spaced levels
(b) Strong electron–phonon and hole–phonon interactions to ensure high rate of absorption of lattice phonons
(c) High photoluminescence quantum yield in order to minimize the dissipation of photon energy through sample heating due to non-radiative transitions
(d) Independence of the emission properties of the excitation wavelength
(e) Growth of the ASPL intensity with temperature.
A three-level model illustrating the basic processes for the generation of anti-Stokes photoluminescence is shown in Fig. 1(e). After photoexcitation (0 → 1) the higher lying state 2 is populated via the annihilation of a phonon. Radiative recombination to the ground state (2 → 0) leads to anti-Stokes photoluminescence.
In addition, net cooling often remains elusive due to large parasitic absorption in the band tail or due to impurities. The photoluminescence quantum efficiency required for cooling depends on the band gap of the emitter and needs to be >96% for an ideal system with an emission in the visible range.14 This explains why semiconductor structures, which can be cooled by light, are rare. Experiments on CdS nanobelts15 and different perovskite platelets16 suggest cooling due to anti-Stokes photoluminescence.
Many colloidal quantum dots based on perovskite semiconductor material appear to be ideal for photon cooling due to their outstanding optical properties. However, the nature of their lowest excited state, the anti-Stokes up conversion mechanism and the radiative and non-radiative recombination in these nanocrystals are still not fully understood. Self-trapped exciton,17 emission from trions,18 trap assisted recombination19 and the excited states splitting into dark and bright exciton13,20 have been suggested to explain the temperature dependence and the decay kinetics of the observed photoluminescence.
Here, we study the anti-Stokes photoluminescence of thin films of methylammonium lead bromide (CH3NH3PbBr3) colloidal nanoparticles with a size of about 7 nm. As the Bohr radius of the exciton is smaller compared to the particle size, the exciton is weakly confined.21 Temperature dependent Stokes and anti-Stokes photoluminescence and photoluminescence excitation spectra are recorded and the anti-Stokes quantum efficiency (ηASPL) is estimated as a function of the sample temperature. We find that ηASPL strongly depends on the temperature and decreases by a factor of 50 when cooled from room temperature to 100 K.
Fig. 2(a) and (b) show the photoluminescence of the nanoparticles when excited at 494 nm and 514 nm respectively. For comparison, the photoluminescence recorded with 405 nm excitation scaled to fit to the low energy part of the spectra is shown as well. Both spectra contain also an additional peak from the excitation. When excited at 494 nm, the shape and position of the recorded spectrum perfectly fits to the reference spectrum. When excited at 514 nm, a red shift is observed.
Fig. 2 Stokes and anti-Stokes PL spectra of CH3NH3PbBr3 nanoparticle thin film recorded at room temperature; excitation: (a) 494 nm, (b) 514 nm. |
The redshift of the anti-Stokes PL arises from the residual size distribution of the particle ensemble.14 Tuning the excitation wavelength to the low energy side of the nanoparticle absorption, the excitation of larger particles is preferred leading to a red spectral shift. This idea is supported by the fact that no redshift has been observed in the anti-Stokes PL spectrum of an individual perovskite nanoparticle.23
Fig. 3(a) shows the temperature dependence of the photoluminescence when the film is excited with 532 nm photons. In this experiment, a notch filter is placed in front of the monochromator to attenuate the intense laser radiation (18 mW). The anti-Stokes PL decreases significantly upon decreasing the temperature. Under the same experimental conditions, also the Stokes PL (405 nm, 0.06 mW) was recorded on the same spot of the thin film sample. In Fig. 3(b) the Stokes and anti-Stokes PL recorded at room temperature are compared.
Fig. 3 (a) Anti-Stokes PL recorded at different temperatures; (b) comparison between Stokes and anti-Stokes PL spectra recorded at room temperature. |
A strong redshift of the anti-Stokes emission is observed when excited with intense 532 nm laser light. An analysis of the Stokes and anti-stokes PL and the recorded excitation spectra allows the estimation of the anti-Stokes PL up-conversion efficiency given by equation:23
N ASPL and NPL correspond to the number of emitted photons when excited at 532 and 405 nm respectively. Nabs,405 and Nabs,532 correspond to the number of absorbed photons when excited at 405 and 532 nm respectively. In Fig. S1 in the ESI,† the normalized excitation spectra measured at different temperatures are shown. Several different spectra, recorded at different emission wavelengths are plotted on top of each other for each temperature. Nabs,532/Nabs,405 is estimated by extrapolation of the spectra to 532 nm and needs to be corrected for different laser intensities. Fig. S2† shows the temperature dependence of the ratio of absorbed photons at 405 nm and 532 nm normalized to the incoming photon flux. The red line represents a second order polynomial fit, which is used to extract data points at different temperatures between the measured data.
Fig. 4 shows the calculated anti-Stokes PL up-conversion efficiency as a function of the temperature. NASPL and NPL are obtained by integration of the measured PL spectra between 450 and 518 nm. Nabs,405/Nabs,532 is determined as described before. At room temperature, the up-conversion efficiency is high and drops quickly upon decreasing the temperature. Between room temperature and 200 K, the up-conversion efficiency shows Arrhenius type dependence with an activation energy of 80 meV.
In Fig. 5(a), the anti-Stokes PL spectra recorded at different laser intensities are plotted. Fig. 5(b) shows the log–log plot of the anti-Stokes PL peak intensity versus the light intensity. The line with a slope of 0.92 is the linear fit of data points.
Fig. 5 Light intensity dependence of the anti-Stokes PL. (a) ASPL spectra recorded at different light intensities; (b) log–log plot of the ASPL peak intensity versus light intensity. |
In this study we have demonstrated that an ensemble of CH3NH3PbBr3 nanoparticles efficiently up-convert low energy photons. The experimental results presented above suggest that CH3NH3PbBr3 nanocrystals are promising candidates for highly efficient anti-Stokes photoluminescence up-conversion. The anti-Stokes emission depends linearly on the excitation intensity suggesting that the underlying process is based on a one-photon mechanism. The observed temperature dependence suggests that the up-conversion process is thermally activated. Considering the moderate PLQY of the studied samples and the rather large energy difference (∼100 meV) between the barycenter of the photoluminescence spectrum (∼510 nm) and the photoexcitation (532 nm), the estimated ASPL conversion efficiency of ∼60% is very high. However, the current particle performance is not sufficient to achieve optical cooling. Preliminary photothermal deflection spectroscopy (PDS) studies show a strongly distorted absorbance spectrum of the studied film due to the photoluminescence of the nanoparticles but no evidence of optical cooling. To achieve cooling, the photoluminescence quantum yield of the CH3NH3PbBr3 nanoparticles needs to be >96% (ESI, Fig. S3†). This may be achievable by further improving the quantum dot synthesis and the film formation and by embedding the nanoparticles in an optically clear matrix for better out-coupling of the anti-Stokes photoluminescence.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0nr04545d |
This journal is © The Royal Society of Chemistry 2020 |