Embedding Semiconductor Quantum Dots (QDs) into hybrid organic-inorganic solar cell holds promises for improving photovoltaic performances. Thanks to their strong coupling with electro-magnetic radiation field, QDs represent paradigmatic photon absorbers. Nevertheless, the quest for suitable charge separating hetero-interfaces is still an open challenge. Within this framework, the excited state interactions between QDs and fullerene derivatives are of great interest for ternary solar cells (polymer:QDs:fullerene). In this work, we investigated the exciton dynamics of core/shell CdSe/CdS QDs both in solution and in blends with fullerene derivative (PCBM). By means of transient optical techniques, we aimed to unveil the dynamics of the QDs-PCBM interaction. Indeed, the observed excited state depopulation of QDs in blends is compatible with an excited state interaction living on picosecond timescale. Through electron paramagnetic resonance, we delved into the nature of this interaction, identifying the presence of charge separated states. The concurrence of these observations suggest a fast electron transfer process, where QDs act as donors and PCBM molecules as acceptors, followed by effective charge separation. Therefore, our experimental results indicate the QDs-PCBM heterointerface as suitable exciton separating interface, paving the way for possible applications in photovoltaics.
Exciton interactions and dynamics are the most important factors determining the exceptional photophysical properties
of semiconductor quantum dots (QDs). In particular, best performances have been obtained for ingeniously engineered
core/shell QDs. We have studied two factors entering in the exciton decay dynamics with adverse effects for the
luminescence efficiency: exciton trapping at surface and interface traps, and non-radiative Auger recombination in QDs
carrying either net charges or multiple excitons. In this work, we present a detailed study into the optical absorption,
fluorescence dynamics and quantum yield, as well as ultrafast transient absorption properties of CdSe/CdS,
CdSe/Cd0.5Zn0.5S, and CdSe/ZnS QDs as a function of shell thickness. It turns out that de-trapping processes play a
pivotal role in determining steady state emission properties. By studying the excitation dependent photoluminescence
quantum yields (PLQY) in different CdSe/CdxZn1-xS (x = 0, 0.5, 1) QDs, we demonstrate the different role played by hot
and cold carrier trapping rates in determining fluorescence quantum yields. Finally, the use of global analysis allows us
untangling the complex ultrafast transient absorption signals. Smoothing of interface potential, together with effective
surface passivation, appear to be crucial factors in slowing down both Auger-based and exciton trapping recombination
processes.
The outstanding optical properties of Semiconductor Quantum Dots (QDs) have attracted much interest for over two
decades. The development of synthetic methods for the production of core-shell QDs has opened the way to attaining
almost ideal emitting properties. Their implementation in opto-electronic devices, such as light emitting diodes (LEDs)
and lasers, requires a full understanding of the fine details of their photophysics. The exciton dynamics of core and coreshell
QDs was extensively studied by means of pump and probe (P and P) and transient photoluminescence (TRPL)
spectroscopies. Nevertheless, the wealth of possible exciton and multi-exciton decay mechanisms, operating on
comparable time-scales, results in complex signals. In this work, the exciton dynamics of a complete CdSe/Cd1-xZnxS
series is investigated, with a focus on exciton trapping processes. Insights into the energy distribution of exciton traps are
unveiled by wavelength resolve QY measurements. Multicolor P and P measurements give a deeper insight into the
dynamics of exciton trapping and Auger recombinations. An inversion method is proposed as a powerful tool for
separating different contribution in complex P and P transients. The outcomes of this work clarify the role of core/shell
interfaces and surfaces in modulating the optical properties and suggest possible routes for their improvement.
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