The organic ligands passivating the surface of semiconductor quantum dots (QDs) and the solvents used strongly determine the photostability of QD solutions. Highly purified QD solutions in chloroform have been shown to photodegrade upon pulsed ultraviolet (UV) irradiation, irrespectively of the type of surface ligand. However, the photostability of QDs dissolved in n-octane, a more photochemically inert solvent, strongly depends on the ligands passivating their surface. In n-octane, hexadecylamine-coated QDs are completely stable and display no photochemical response to pulsed UV laser irradiation. In solutions of octanethiol-capped QDs, the photoluminescence intensity slightly decreases under irradiation. QDs coated with trioctylphosphine oxide exhibit a more complex pattern of photobleaching, which depends on the initial value of fluorescence quantum yield of QDs. This complex pattern may be accounted for by two competing processes: (1) ligand photodesorption accompanied by photobleaching due to specific alignment of the band levels of QDs and highest occupied molecular orbital of the ligand and (2) photoinduced decrease in the population of trapping states. Furthermore, practically no thermodynamic degradation of QD solutions has been observed for the micromolar QD concentration used in the study, in contrast to lower concentrations, thus confirming the photoinduced origin of the changes caused by UV irradiation. Obtained results show that the photostability of QDs may be strongly increased by careful selection of the ligands passivating their surface and the solvents used in the experiments.
Quantum dots (QDs) have numerous applications in optoelectronics due to their unique optical properties. Novel hybrid
organic light-emitting diodes (OLEDs) containing QDs as an active emissive layer are being extensively developed. The
performance of QD–OLED depends on the charge transport properties of the active layer and the degree of localization
of electrons and holes in QDs. Therefore, the type and the density of the ligands on the QD surface are very important.
We have fabricated OLEDs with a CdSe/ZnS QD active layer. These OLEDs contain hole and electron injection layers
consisting of poly(9-vinyl carbazole) and ZnO nanoparticles, respectively. The energy levels of these materials ensure
efficient injection of charge carriers into the QD emissive layer.
In order to enhance the charge transfer to the active QD layer and thereby increase the OLED efficiency, the QD surface
ligands (tri-n-octyl phosphine oxide, TOPO) were replaced with a series of aromatic amines and thiols. The substituents
were expected to enhance the charge carrier mobility in the QD layer. Surprisingly, the devices based on the original
TOPO-coated QDs were found to have the best performance, with a maximum brightness of 2400 Cd/m2 at 10 V. We
assume that this was due to a decrease in the charge localization within QDs when aromatic ligands are used. We
conclude that the surface ligands considerably affect the performance of QD–OLEDs, efficient charge localization in QD
cores being more important for good performance than a high charge transfer rate.
Changes of optical properties of the solutions of CdSe/ZnS quantum dots (QDs) covered with the trioctylphosphine oxide (TOPO) ligands under the pulsed ultraviolet (UV) laser irradiation are observed. The fluorescence quantum yield (QY) of QDs decreases by more than an order of magnitude when the radiation dose approaches 2 × 10-15 J per particle. This process is accompanied by a blue shift of both fluorescence and the first excitonic absorption peaks. The fluorescence quenching becomes less pronounced when the overall TOPO content in the solution is increased. When ТОРО ligands are replaced with n-hexadecylamine (HDA), QY and spectral properties are not changed at the same irradiation conditions. We assume that the above changes of the optical properties are associated with photooxidation of TOPO ligands by excited QD. Such process is less probable for the HDA ligand due to its different energy structure.
Quantum dots (QDs) with the highest possible photoluminescence quantum yields are necessary for modern nanotechnology applications to biosensing and optoelectronics. To date, core-shell QDs are the best. We suggest and demonstrate a novel approach to enhancement of charge-carrier confinement in the core of CdSe QDs by creating a ZnS/CdS/ZnS shell with staggered potential barrier. The CdS interlayer breaks the ZnS-shell structure continuity, which allows combining the benefits of a single ZnS-monolayer inner shell, creating the highest possible confinement potential, with a sufficient overall shell thickness and suitability for common surface modification techniques. This approach allows the preparation of CdSe-ZnS/CdS/ZnS QDs with photoluminescence quantum yields approaching 100% and small photoluminescence peak width.
Solar energy converters based on organic semiconductors are inexpensive, can be layered onto flexible surfaces, and
show great promise for photovoltaics. In bulk heterojunction polymer solar cells, charges are separated at the interface of
two materials, an electron donor and an electron acceptor. Typically, only the donor effectively absorbs light. Therefore,
the use of an acceptor with a wide absorption spectrum and high extinction coefficient and charge mobility should
increase the efficiency of bulk heterojunction polymer solar cells. Semiconductor nanocrystals (quantum dots and rods)
are good candidate acceptors for these solar cells. Recently, most progress in the development of bulk heterojunction
polymer solar cells was achieved using PCBM, a traditional fullerene acceptor, and two low band gap polymers, poly[N-
9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT) and poly
(PTB7). Therefore, the possibility of combining these polymers with semiconductor nanocrystals
deserves consideration.
Here, we present the first comparison of solar cells based on PCDTBT and PTB7 where CdSe quantum dots serve as
acceptors. We have found that PTB7-based cells are more efficient than PCDTBT-based ones. The efficiency also
strongly depends on the nanocrystal size. An increase in the QD diameter from 5 to 10 nm causes a more than fourfold
increase in the cell efficiency. This is determined by the relationship between the nanoparticle size and energy spectrum,
its pattern clearly demonstrating how the mutual positions of the donor and acceptor levels affect the solar cell
efficiency. These results will help to develop novel, improved nanohybrid components of solar cells based on organic
semiconductors and semiconductor nanocrystals.
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