Abstract
Extra charges in semiconductor nanocrystals are of paramount importance for their electrically driven optoelectronic and photovoltaic applications. Optical excitations of such charged nanocrystals lead to rapid recombinationviaan Auger process, which can deteriorate the performance of the corresponding devices. While numerous articles report trion Auger processes in negatively charged nanocrystals, optical studies of well-controlled positive charging of nanocrystals and detailed studies of positive trions remain rare. In this work, we used electrochemistry to achieve positive charging of CdSe nanocrystals, so-called quantum dots (QDs), in a controlled way. Femtosecond transient absorption spectroscopy was applied forin situinvestigation of the charge carrier dynamics after optical excitation of the electrochemically charged QD assembly on TiO2. We observe that without bias (i.e., neutral QDs), sub-picosecond hot carrier cooling is followed by multiple phases of the dynamics corresponding to electron injection and transfer to the TiO2. Positive charging first leads to activation of the hole traps close to the valence band maximum, which opens a rapid recombination channel of the optical excitation. A further increase in the positive bias interrupts the electron injection to TiO2, and if nanocrystals are positively charged, it leads to Auger relaxation in a few hundred picosecond timescale. This study represents a step toward the understanding of the effect of positive charging on the performance of semiconductor nanocrystals under conditions which closely mimic their potential applications. (Less)
Highlights
At the nanoscale, material properties depend on their size owing to the quantum confinement
Measurement of cyclic voltammogram (CV) of the quantum dots (QDs) was done in a three-electrode electrochemical cell
The QD assembly onto TiO2-fluorine-doped tin oxide (FTO) serves as the working electrode, and a platinum plate is used as the counter electrode
Summary
Material properties depend on their size owing to the quantum confinement. When a QD absorbs a photon with energy larger than its band gap, an electron−hole pair is created, which is referred to as an exciton.[12] Biexciton is a pair of excitons, which can rapidly decay via a nonradiative Auger mechanism.[13] In this process, one exciton’s recombination energy is transferred to the electron or hole of the other exciton, bringing one of the charge carriers to a high energy level, which rapidly relaxes back to the band edge.[14] As a result of such a process, one exciton is lost. If a QD is charged (negatively or positively), light absorption leads to the formation of a socalled trion a collective three-body state consisting of an exciton combined with an additional charge: a hole or an electron.[13] In trions too, an Auger process takes place analogously to the biexcitons.[13] In optoelectronic applications, QDs are often highly excited or charged; the Auger recombination is an important mechanism, which can significantly affect the efficiency of the devices. For making the best-possible use of the nanomaterials, photophysics of charged QDs needs to be better understood
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