Abstract
Fulfilling the potential of colloidal semiconductor quantum dots (QDs) in electrically driven applications remains a challenge largely since operation of such devices involves charged QDs with drastically different photophysical properties compared to their well-studied neutral counterparts. In this work, the full picture of excited state dynamics in charged CdSe QDs at various time scales has been revealed via transient absorption spectroscopy combined with electrochemistry as a direct manipulation tool to control the negative charging of CdSe QDs. In trions, excited states of single charged QDs, the additional electron in the conduction band speeds up the hot electron cooling by enhanced electron-electron scattering followed by charge redistribution and polaron formation in a picosecond time scale. The trions are finally decayed by the Auger process in a 500 ps time scale. Double charging in QDs, on the other hand, decelerates the polaron formation process while accelerates the following Auger decay. Our work demonstrates the potential of photoelectrochemistry as a platform for ultrafast spectroscopy of charged species and paves the way for further studies to develop comprehensive knowledge of the photophysical processes in charged QDs more than the well-known Auger decay, facilitating their use in future optoelectronic applications. (Less)
Highlights
Quantum dots (QDs),[1] as semiconductor nanocrystals with a size smaller than Bohr’s radius, have been widely investigated both as model systems for fundamental research and for numerous applications.[2−6] The applications typically rely on separation and extraction of photogenerated electron−hole pairs for the efficient conversion of photons to free charge carriers.[7]
We combine electrochemistry with ultrafast transient absorption spectroscopy (TA) to monitor changes in the excited state dynamics of the QD under controlled charging.[12,20−23] By observing the changes in the steady state absorption during spectroelectrochemistry measurement, we have identified distinct potentials that correspond to the injection of one and two electrons to the QDs
By changing the potential of the working electrode linearly with time from one set initial potential to another preset chosen potential and reversing the scan linearly back to the initial potential while measuring the current that passes to the counter electrode, a cyclic voltammogram (CV) is obtained,[25] showing distinct bands at the potentials where charge carriers can enter the electronic states of the system.[14,24,26]
Summary
Quantum dots (QDs),[1] as semiconductor nanocrystals with a size smaller than Bohr’s radius, have been widely investigated both as model systems for fundamental research and for numerous applications.[2−6] The applications typically rely on separation and extraction of photogenerated electron−hole pairs for the efficient conversion of photons to free charge carriers.[7] This can be achieved through transfer of photogenerated charges to electron (hole) acceptors, which highly depends on the relative position of the band edges.[8] Charge dynamics in such systems have been extensively investigated by time-resolved spectroscopies.[7−9] Most of these studies are implemented on half-cell systems (i.e., only photoanodes or photocathodes) or under open-circuit conditions where electron/hole extraction through external circuits does not occur. To the biexciton, the trion can decay through Auger recombination.[15] The depopulation of trions through the Auger process competes with the extraction of charges and undermines the charge separation efficiency in the devices. We anticipate that our findings will pave the way to comprehensive knowledge and better understanding of the photophysical processes in charged QDs, leading to their efficient use in future nanotechnology applications
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