Ultrafast Spectroelectrochemistry Reveals Photoinduced Carrier Dynamics in Positively Charged CdSe Nanocrystals

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 recombination via an 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 for in situ investigation of the charge carrier dynamics after optical excitation of the electrochemically charged QD assembly on TiO 2 . We observe that without bias (i.e., neutral QDs), subpicosecond hot carrier cooling is followed by multiple phases of the dynamics corresponding to electron injection and transfer to the TiO 2 . 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 TiO 2 , 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.

Excited States and Their Dynamics in CdSe Quantum Dots Studied by Two-Color 2D Spectroscopy

“… 6 Throughout the years, numerous studies have addressed a broad set of fundamental questions regarding excited states and their dynamics in QDs. 7 − 12 Several recent studies have addressed issues like high-intensity effects 13 − 16 and the influence of charging on excited-state dynamics 17 , 14 —all important from the point of view of possible optoelectronic applications of QDs.…”

“…Since then, the field has been expanding toward a broad combination of materials, sophisticated structures, and optoelectronic devices, such as light-emitting diodes and microspectrometers . Throughout the years, numerous studies have addressed a broad set of fundamental questions regarding excited states and their dynamics in QDs. − Several recent studies have addressed issues like high-intensity effects − and the influence of charging on excited-state dynamics , all important from the point of view of possible optoelectronic applications of QDs.…”

Ground- and Excited-State Properties of Charged Non-Stoichiometric Quantum Dots

“…These states are associated with a strongly localized positive charge in the presence of the lowest-energy optically weak exciton, which can be compared to the positive trion X + states detected in electrochemically positively charged CdSe QDs. 51 Because of the highly localized character of the electron−hole pair�with both an electron and a hole located on the surface selenium atoms�this charge-related state is optically inactive for Se-1, Se-2, and Se-0 (+) structures, Table 3.…”

In Situ Attenuated Total Reflectance Infrared Spectroelectrochemistry (ATR-IR-SEC) for the Characterization of Molecular Redox Processes on Surface-Proximal Doped Silicon ATR Crystal Working Electrodes

“…Spectroelectrochemistry (SEC) combines spectroscopy and electrochemistry, allowing spectroscopic changes to be directly correlated with applied potentials. It has been demonstrated with many different spectroscopies including UV–vis, IR, NMR, and Raman methods. − …”