Quantification of hot carrier thermalization in PbS colloidal quantum dots by power and temperature dependent photoluminescence spectroscopy

Abstract

PbS QDs are studied as attractive candidates to be applied as hot carrier solar cell absorbers.

Identification of Two Regimes of Carrier Thermalization in PbS Nanocrystal Assemblies

“…Our measurements suggest that previous hypotheses made to extract the carrier temperature within layers of PbS CQDs may need reconsideration; some past studies focused on the thermalization of free carriers at high energy, 18 while others relied on fitting the PL spectra with a sigmoid function that both encompasses the resonant absorption and the high-energy absorption tail. 16,17 It remains to be seen how these conclusions evolve under much higher pumping powers, when the Wien approximation of Planck's law is not valid anymore and optical nonlinearities must be taken into account, such as the fact that the absorption cross section of the emitters become strongly excitation-dependent. Likewise, it would be interesting to see how much thermalization remains for PbS CQDs in solution, because the sizable Stokes shifts that are also observed in such cases are also a consequence of interdot interactions.…”

“…As a corollary, the exciton temperature can be tuned from the lattice temperature to several more hundreds of kelvins by adjusting the pump power and the size of the ligands capping the semiconducting CQDs. Our measurements suggest that previous hypotheses made to extract the carrier temperature within layers of PbS CQDs may need reconsideration; some past studies focused on the thermalization of free carriers at high energy, while others relied on fitting the PL spectra with a sigmoid function that both encompasses the resonant absorption and the high-energy absorption tail. , It remains to be seen how these conclusions evolve under much higher pumping powers, when the Wien approximation of Planck’s law is not valid anymore and optical nonlinearities must be taken into account, such as the fact that the absorption cross section of the emitters become strongly excitation-dependent. Likewise, it would be interesting to see how much thermalization remains for PbS CQDs in solution, because the sizable Stokes shifts that are also observed in such cases are also a consequence of interdot interactions .…”

“…With the ongoing developments of solution-processed devices based on colloidal quantum dots (CQDs) − and nanoplatelets, − a few studies ,− have also investigated the possibility of thermalization within such systems. However, it appears that a clear identification of which carriers thermalize in these structures is still lacking; some studies assume that the thermalization concerns free carriers excited well above the first excitonic transition, while others consider that an equilibrium is reached for all electron–hole pairs, from the first excitonic resonance to all the other carriers excited at higher energy.…”

Enhancing the cooling potential of photoluminescent materials through evaluation of thermal and transmission loss mechanisms

“…Thermalization loss, as one of the important thermal loss mechanisms, refers to the non-radiative/thermal relaxation of excess energy from excited energy band to the bottom of conduction band (See Fig. 1 ) 9 , 16 . The deexcitation process from conduction band to valence band is different for fluorescent and phosphorescent materials.…”

Small Polarons and Surface Defects in Metal Oxide Photocatalysts Studied Using XUV Reflection–Absorption Spectroscopy

“…Semiconductor photocatalysis begins with the generation of charge carriers by light. Following rapid hot carrier thermalization, , the energy of the charge carriers are determined by the band alignment and occupancy in the semiconductor. In the excited state, there exists a competition between recombination and spatial charge separation, and these kinetics are strongly influenced by the presence of surface defects and midgap states in the material. − If the carriers avoid recombination long enough, the possibility for interfacial charge injection provides a pathway for photocatalytic surface chemistry. − The competition between charge generation, separation, recombination, and injection determines the efficiency of TMO photocatalysts. − Many of these processes occur on the ultrafast time scale.…”