We have analyzed the decays of the fluorescence of colloidal CdSe quantum dots (QDs) suspensions during dilution and titration by the ligands. A ligand shell made of a combination of trioctylphosphine (TOP), oleylamine (OA), and stearic acid (SA) stabilizes the assynthesized QDs. The composition of the shell was analyzed and quantified using high resolution liquid state 1H nuclear magnetic resonance (NMR) spectroscopy. A quenching of the fluorescence of the QDs is observed upon removal of the ligands by diluting the stock solution of the QDs. The fluorescence is restored by the addition of TOP. We analyze the results by assuming a binomial distribution of quenchers among the QDs and predict a linear trend in the time-resolved fluorescence decays.We have used a nonparametric analysis to show that for our QDs, 3.0 ( 0.1 quenching sites per QD on average are revealed by the removal of TOP. We moreover show that the quenching rates of the quenching sites add up. The decay per quenching site can be compared with the decay at saturation of the dilution effect. This provides a value of 2.88 ( 0.02 for the number of quenchers per QD. We extract the quenching dynamics of one site. It appears to be a process with a distribution of rates that does not involve the ligands.
Low-bandgap
diketopyrrolopyrrole- and carbazole-based polymer bulk-heterojunction
solar cells exhibit much faster charge carrier recombination kinetics
than that encountered for less-recombining poly(3-hexylthiophene).
Solar cells comprising these polymers exhibit energy losses caused
by carrier recombination of approximately 100 mV, expressed as reduction
in open-circuit voltage, and consequently photovoltaic conversion
efficiency lowers in more than 20%. The analysis presented here unravels
the origin of that energy loss by connecting the limiting mechanism
governing recombination dynamics to the electronic coupling occurring
at the donor polymer and acceptor fullerene interfaces. Previous approaches
correlate carrier transport properties and recombination kinetics
by means of Langevin-like mechanisms. However, neither carrier mobility
nor polymer ionization energy helps understanding the variation of
the recombination coefficient among the studied polymers. In the framework
of the charge transfer Marcus theory, it is proposed that recombination
time scale is linked with charge transfer molecular mechanisms at
the polymer/fullerene interfaces. As expected for efficient organic
solar cells, small electronic coupling existing between donor polymers
and acceptor fullerene (Vif < 1 meV)
and large reorganization energy (λ ≈ 0.7 eV) are encountered.
Differences in the electronic coupling among polymer/fullerene blends
suffice to explain the slowest recombination exhibited by poly(3-hexylthiophene)-based
solar cells. Our approach reveals how to directly connect photovoltaic
parameters as open-circuit voltage to molecular properties of blended
materials.
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