Ligand Exchange of Colloidal CdSe Nanocrystals with Stibanates Derived from Sb₂S₃ Dissolved in a Thiol-Amine Mixture

Molecular stibanates derived from the dissolution of bulk Sb₂S₃ in a binary ethylenediamine and mercaptoethanol solvent mixture have been studied as capping ligands for colloidal CdSe nanocrystals. A phase transfer ligand exchange strategy was utilized to effectively install the stibanate ligands onto the CdSe nanocrystals to form stable colloidal suspensions in polar solvents, such as formamide. This methodology was very effective in the removal of insulating native ligands on the as-prepared nanocrystals, with the resulting stibanate-capped CdSe nanocrystals giving low organic content thin films upon spin coating with improved interparticle coupling after heating to temperatures <300 °C. Photoelectrochemical measurements on stibinate-capped CdSe nanocrystal films showed that this novel ligand leads to a > 25-fold increase in photocurrent response relative to as-prepared CdSe nanocrystal films

Synthesis and Characterization of Wurtzite-Phase Copper Tin Selenide Nanocrystals

A new wurtzite phase of copper tin selenide (CTSe) was discovered, and the resulting nanocrystals were synthesized via a facile solution-phase method. The wurtzite CTSe nanocrystals were synthesized with dodecylamine and 1-dodecanethiol as coordinating solvents and di-<i>tert</i>-butyl diselenide (^<i>t</i>Bu₂Se₂) as the selenium source. Specific reaction control (i.e., a combination of 1-dodecanethiol with ^<i>t</i>Bu₂Se₂) was proven to be critical in order to obtain this new phase of CTSe, which was verified by powder X-ray diffraction and selected area electron diffraction. The wurtzite CTSe nanocrystals possess an optical and electrochemical band gap of 1.7 eV and display an electrochemical photoresponse indicative of a p-type semiconductor

Direct Spectroscopic Evidence of Ultrafast Electron Transfer from a Low Band Gap Polymer to CdSe Quantum Dots in Hybrid Photovoltaic Thin Films

Ultrafast transient absorption spectroscopy is used to study charge transfer dynamics in hybrid films composed of the low band gap polymer PCPDTBT and CdSe quantum dots capped with <i>tert</i>-butylthiol ligands. By selectively exciting the polymer, a spectral signature for electrons on the quantum dots appears on ultrafast time scales (≲ 65 fs), which indicates ultrafast electron transfer. From this time scale, the coupling between the polymer chains and the quantum dots is estimated to be <i>J</i> ≳ 17 meV. The reduced quantum dot acceptors exhibit an unambiguous spectral bleach signature, whose amplitude allows for the first direct calculation of the absolute electron transfer yield in a hybrid solar cell (82 ± 5%). We also show that a limitation of the hybrid system is rapid and measurable geminate recombination due to the small separation of the initial charge pair. The fast recombination is consistent with the internal quantum efficiency of the corresponding solar cell. We therefore have identified and quantified a main loss mechanism in this type of third generation solar cell

Tandem and Triple-Junction Polymer:Nanocrystal Hybrid Solar Cells Consisting of Identical Subcells

Tandem and triple-junction polymer:nanocrystal hybrid solar cells with identical subcells based on P3HT:CdSe nanocrystal bulk heterojunctions (BHJs) are reported for the first time showing 2-fold and 3-fold increases of open-circuit voltage (<i>V</i>_OC), respectively, relative to the single-junction cell. A combination of nanocrystalline ZnO and pH-neutral PEDOT:PSS is used as the interconnecting layer, and the thicknesses of subcells are optimized with the guidance of optical simulations. As a result, the average power conversion efficiency (PCE) exhibits a significant increase from 2.0% (<i>V</i>_OC = 0.57 V) in single-junction devices to 2.7% (champion 3.1%, <i>V</i>_OC = 1.28 V) in tandem devices and 2.3% (<i>V</i>_OC = 1.98 V) in triple-junction devices

Chalcogenol Ligand Toolbox for CdSe Nanocrystals and Their Influence on Exciton Relaxation Pathways

We have employed a simple modular approach to install small chalcogenol ligands on the surface of CdSe nanocrystals. This versatile modification strategy provides access to thiol, selenol, and tellurol ligand sets <i>via</i> the <i>in situ</i> reduction of R₂E₂ (R = ^<i>t</i>Bu, Bn, Ph; E = S, Se, Te) by diphenylphosphine (Ph₂PH). The ligand exchange chemistry was analyzed by solution NMR spectroscopy, which reveals that reduction of the R₂E₂ precursors by Ph₂PH directly yields active chalcogenol ligands that subsequently bind to the surface of the CdSe nanocrystals. Thermogravimetric analysis, FT-IR spectroscopy, and energy dispersive X-ray spectroscopy provide further evidence for chalcogenol addition to the CdSe surface with a concomitant reduction in overall organic content from the displacement of native ligands. Time-resolved and low temperature photoluminescence measurements showed that all of the phenylchalcogenol ligands rapidly quench the photoluminescence by hole localization onto the ligand. Selenol and tellurol ligands exhibit a larger driving force for hole transfer than thiol ligands and therefore quench the photoluminescence more efficiently. The hole transfer process could lead to engineering long-lived, partially separated excited states