Quasi type-II PbSe/PbS quantum dots (QDs) are employed in a solid state high effi ciency QD/TiO 2 heterojunction solar cell. The QDs are deposited using layer-by-layer deposition on a half-micrometer-thick anatase TiO 2 nanosheet fi lm with (001) exposed facets. Theoretical calculations show that the carriers in PbSe/PbS quasi type-II QDs are delocalized over the entire core/shell structure, which results in better QD fi lm conductivity compared to PbSe QDs. Moreover, PbS shell permits better stability and facile electron injection from the QDs to the TiO 2 nanosheets. To complete the electrical circuit of the solar cell, a Au fi lm is evaporated as a back contact on top of the QDs. This PbSe/PbS QD/TiO 2 heterojunction solar cell produces a light to electric power conversion effi ciency ( η ) of 4% with short circuit photocurrent ( J sc ) of 17.3 mA/cm 2 . This report demonstrates highly effi cient core/shell near infrared QDs in a QD/TiO 2 heterojunction solar cell.
Controlling the synthesis of narrow band gap semiconductor nanocrystals (NCs) with a high-quality surface is of prime importance for scientific and technological interests. This Letter presents facile solution-phase syntheses of SnTe NCs and their corresponding core/shell heterostructures. Here, we synthesized monodisperse and highly crystalline SnTe NCs by employing an inexpensive, nontoxic precursor, SnCl2, the reactivity of which was enhanced by adding a reducing agent, 1,2-hexadecanediol. Moreover, we developed a synthesis procedure for the formation of SnTe-based core/shell NCs by combining the cation exchange and the Kirkendall effect. The cation exchange of Sn(2+) by Cd(2+) at the surface allowed primarily the formation of SnTe/CdTe core/shell NCs. Further continuation of the reaction promoted an intensive diffusion of the Cd(2+) ions, which via the Kirkendall effect led to the formation of the inverted CdTe/SnTe core/shell NCs.
Above band-edge photoexcitation of PbSe nanocrystals induces strong below band gap absorption as well as a multiphased buildup of bleaching in the 1Se1Sh transition. The amplitudes and kinetics of these features deviate from expectations based on biexciton shifts and state filling, which are the mechanisms usually evoked to explain them. To clarify these discrepancies, the same transitions are investigated here by double-pump-probe spectroscopy. Re-exciting in the below band gap induced absorption characteristic of hot excitons is shown to produce additional excitons with high probability. In addition, pump-probe experiments on a sample saturated with single relaxed excitons prove that the resulting 1Se1Sh bleach is not linear with the number of excitons per nanocrystal. This finding holds for two samples differing significantly in size, demonstrating its generality. Analysis of the results suggests that below band edge induced absorption in hot exciton states is due to excited-state absorption and not to shifted absorption of cold carriers and that 1Se1Sh bleach signals are not an accurate counter of sample excitons when their distribution includes multiexciton states.
The work focuses on the synthesis of smallsized PbSe/PbS core/shell colloidal quantum dots with the core diameter of 2−2.5 nm and the shell thickness of 0.5−1.0 nm. The PbSe/PbS core/shell CQDs are chemically stable under time-limited air exposure and have emission quantum efficiency of 60% at room temperature. The PbSe/PbS core/ shell CQDs have a tunable absorption edge around 1 μm, large exciton emission Stokes shift (∼150 meV), and small exchange interaction (∼1.5 meV). Theoretical calculations associate the mentioned parameters to the small-size regime as well as to a lift of band-edge degeneracy due to slight shape anisotropy. The specific parameters are of special interest in photovoltaic applications.
Small-sized colloidal quantum dots (QDs) consisting of IV−VI semiconductors with the PbSe/PbS core/shell structure were synthesized by a specially developed wet-chemistry method. Their electronic properties were determined by the comparison of theoretical calculations with continuous-wave and transient photoluminescence measurements conducted at various temperatures. The results revealed the formation of quantum dots characterized by a tunable band gap around 1 μm, lifetime exceeding 4.0 μs at room temperature, photoluminescence quantum yield >60%, and resistance to oxidation for a relatively long period of time. The properties of the QDs vary with the core/shell architecture, which is beneficial for their optoelectronic applications.
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