Showing papers by "Arthur J. Nozik published in 2017"
TL;DR: It is shown that band edge positions of lead sulfide (PbS) colloidal semiconductor nanocrystals, specifically quantum dots (QDs), can be tuned over 2.0 eV through surface chemistry modification through simple, robust and scalable solution-phase ligand exchange methods.
Abstract: Band edge positions of semiconductors determine their functionality in many optoelectronic applications such as photovoltaics, photoelectrochemical cells and light emitting diodes. Here we show that band edge positions of lead sulfide (PbS) colloidal semiconductor nanocrystals, specifically quantum dots (QDs), can be tuned over 2.0 eV through surface chemistry modification. We achieved this remarkable control through the development of simple, robust and scalable solution-phase ligand exchange methods, which completely replace native ligands with functionalized cinnamate ligands, allowing for well-defined, highly tunable chemical systems. By combining experiments and ab initio simulations, we establish clear relationships between QD surface chemistry and the band edge positions of ligand/QD hybrid systems. We find that in addition to ligand dipole, inter-QD ligand shell inter-digitization contributes to the band edge shifts. We expect that our established relationships and principles can help guide future optimization of functional organic/inorganic hybrid nanostructures for diverse optoelectronic applications.
215 citations
TL;DR: It is found that varying the concentration of dopants exposed to the as-synthesized PbSe QDs controls the extent of exchange, and hypothesize that the majority of incorporated Ag remains at the QD surface and does not interact with the core electronic states of the QDs, which causes p-type doping behavior.
Abstract: Electronic impurity doping of bulk semiconductors is an essential component of semiconductor science and technology. Yet there are only a handful of studies demonstrating control of electronic impurities in semiconductor nanocrystals. Here, we studied electronic impurity doping of colloidal PbSe quantum dots (QDs) using a postsynthetic cation exchange reaction in which Pb is exchanged for Ag. We found that varying the concentration of dopants exposed to the as-synthesized PbSe QDs controls the extent of exchange. The electronic impurity doped QDs exhibit the fundamental spectroscopic signatures associated with injecting a free charge carrier into a QD under equilibrium conditions, including a bleach of the first exciton transition and the appearance of a quantum-confined, low-energy intraband absorption feature. Photoelectron spectroscopy confirms that Ag acts as a p-type dopant for PbSe QDs and infrared spectroscopy is consistent with k·p calculations of the size-dependent intraband transition energy. We...
57 citations
06 Nov 2017
TL;DR: In this paper, the authors showed that the theoretical power conversion efficiency of a multi-junction or tandem junction solar cell can reach 66% at 1-sun intensity for an infinite number of junctions in the stack.
Abstract: There are several approaches to increase the theoretical power conversion efficiency (PCE) of solar cells by using the high photon energies in the solar spectrum more efficiently or by recovering the energy of sub-bandgap photons. For the former, the multi-junction or tandem junction solar cell is the most successful approach which employs multiple p–n junctions of different bandgap (Eg) operating in tandem, where the Eg layers are designed to each absorb a different portion of the total incident sunlight. For an infinite number of junctions in the stack, the PCE reaches 66% at 1-sun intensity. Previous work demonstrates that high PCE is also possible in a single bandgap cell by utilizing the total excess kinetic energy of hot photogenerated carriers in several ways, such as where hot carriers are transported and collected at energy-selective contacts (max PCE = 66%), or by utilizing the excess energy of absorbed photons above the Eg to create additional electron-hole pairs and photocurrent (a process called Multiple Exciton Generation (MEG) in quantized semiconductors and impact ionization in bulk semiconductors.)
2 citations