First principles study of CdSe quantum dots: Stability, surface unsaturations, and experimental validation

Yu, Min; Fernando, Gayanath; Li, R.; Papadimitrakopoulos, Fotios; Shi, Ning; Ramprasad, Rampi

doi:10.1063/1.2209195

Cited by 72 publications

(97 citation statements)

References 29 publications

(43 reference statements)

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“…[42] Reducing the density of trap states in the film boosts performance, since such states provide mid-gap recombination centers, limiting the open-circuit voltage and reducing carrier lifetimes through recombination. Removing and replacing capping ligands in situ, as in the solid-state exchange, carries an associated risk of degraded passivation: [43] as shown in Fig. 5(a), ligand exchange is efficient up to a certain fraction of surface coverage, at which point bound ligands sterically hinder the complete passivation of unterminated surface sites.…”

Section: Limitations Of Solid State Exchangementioning

confidence: 99%

Materials processing strategies for colloidal quantum dot solar cells: advances, present-day limitations, and pathways to improvement

et al. 2013

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Colloidal quantum dot photovoltaic devices have improved from initial, sub-1% solar power conversion efficiency to current record performance of over 7%. Rapid advances in materials processing and device physics have driven this impressive performance progress. The highestefficiency approaches rely on a fabrication process that starts with nanocrystals in solution, initially capped with long organic molecules. This solution is deposited and the resultant film is treated using a solution containing a second, shorter capping ligand, leading to a cross-linked, non-redispersible, and dense layer. This procedure is repeated, leading to the widely employed layer-by-layer solid-state ligand exchange. We will review the properties and features of this process, and will also discuss innovative pathways to creating even higher-performing films and photovoltaic devices.The most abundant and cleanest source of energy available on Earth, solar energy, is also the most underutilized. Its peak intensity is 1000 W/m 2 which, averaged over space and time, provides vastly more energy daily than the global population consumes. Solar energy thus offers, in principle, a compelling option to power a sustainable and increasingly electricitycentric future.[1] While a substantial photovoltaic industry is beginning to take shape worldwide, driven by early generation solar technology based on silicon and compound semiconductors such as cadmium telluride, the fact remains that, in order to compete with traditional energy sources, particularly with cheap and plentiful natural gas, solar photovoltaic systems must cost, fully installed, no more than $1 per watt-peak, which translates to a levelized cost of electricity of approximately $0.05/kWh over a system lifetime.[2] This is noticeably less than current first-and second-generation commercially available solar products, which are held back by higher materials and manufacturing costs; combined with installation costs influenced by the bulky, heavy, and rigid nature of the solar panels themselves.Third-generation photovoltaic systems, including organic, dye-sensitized, and colloidal quantum dot (CQD) solar cells, offer a path to low-weight, low-cost, and prospectively highefficiency solar energy capture and conversion. While thirdgeneration technologies currently function at lower efficiency than commercial first-generation modules, significant pathways exist to reach and surpass the conversion efficiencies required to be commercially feasible.CQD solar cells are of particular interest among the mentioned technologies due to the size tunability of these nanometer-scale semiconductor particles, pictured in Fig. 1(a). The optimal bandgap for a single-junction solar cell as reported by Shockley and Queisser [4] is 1.1 eV; while this offers a limited selection of bulk semiconductors, the quantum confinement effect allows the fabrication of quantum dots (QDs) with reasonable properties [5] using bulk small bandgap semiconductor compounds such as lead sulfide (PbS) and lead selenide (PbSe)...

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Section: Limitations Of Solid State Exchangementioning

confidence: 99%

Materials processing strategies for colloidal quantum dot solar cells: advances, present-day limitations, and pathways to improvement

et al. 2013

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“…The analogous construction of CdSe clusters from the bulk semiconductor has been used in previous theoretical studies 4,5,24 .…”

Section: Bare Clustersmentioning

confidence: 99%

Density Functional Study on the Morphology and Photoabsorption of CdSe Nanoclusters

et al. 2011

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The geometrical and electronic structures of a series of small CdSe quantum dots protected by various ligands have been studied by density functional theory. UV-vis spectra have been calculated by time-dependent density functional theory (TDDFT). The goal of this investigation is the rationalization of the basic properties of these systems, in particular, the nature of the exciton peaks. This study has been focused on the (CdSe)(x), x = 13, 19, 33, and 66, "magic-size" clusters that are characterized by high stability and large optical gaps. The geometries of the cluster are relaxed both in vacuum and in the presence of the surfactant ligands. To describe the interaction between the bare clusters and the surfactants, model types of ligands are introduced: fatty acids are modeled using formic and acetic acid and amines are modeled using ammonia and methyl amine. Present calculations demonstrate that the ligands play a crucial role in stabilizing the structure in a bulklike geometry and strongly affect the optical gap of the clusters, due to an optimal coordination of the surface atoms. For these "magic-size" clusters, the UV-vis spectrum is calculated at the TDDFT level. The calculated spectra are in good agreement with the experimental ones for clusters with the same dimension capped with the same type of Uganda. This suggests that our structures are realistic models of the actual quantum dots. 2

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“…12(b) but it is shifted with respect to the bulk band gap (shown by dashed lines) and more importantly, the wavefunctions of the states at the anticipated top of the VB (and bottom of the CB) are localized at the QD surface rather than extending through the volume of the QD. Since surface passivation is known to be important both from the perspective of enhancing measured optical properties 62,63 and calculated positions of energies in VB and CB, 50,64 we have added an extra atomic layer to the clusters which we investigated, making sure that there remains no dangling bond of the QD core. The passivation layer atoms are assumed to have the same hopping to the QD core atoms as the core atoms between each other and to keep the model simple, we assign a single on-site energy E pass to their s-and porbitals.…”

Section: Appendix B: Tight-binding Model For Simple Cubic Face Centementioning

confidence: 99%

Nodal ground states and orbital textures in semiconductor quantum dots

et al. 2014

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Conventional understanding implies that the ground state of a nonmagnetic quantum mechanical system should be nodeless. While this notion also provides a valuable guidance in understanding the ordering of energy levels in semiconductor nanostructures, there are reports that nodal ground states for holes are possible. However, the existence of such nodal states has been debated and even viewed merely as an artifact of a k·p model. Using complementary approaches of both k·p and tight-binding models, further supported by an effective Hamiltonian for a continuum model, we reveal that nodal ground states in quantum dots are not limited to a specific approach. Remarkably, the emergence of nodal hole states at the top of the valence band can be attributed to the formation of orbital vortex textures through competition between the hole kinetic energy and the coupling to the conduction band states. We suggest an experimental test for our predictions of the reversed energy ordering and the existence of nodal ground states. We discuss how our findings and the studies of orbital textures could be also relevant for other materials systems.

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First principles study of CdSe quantum dots: Stability, surface unsaturations, and experimental validation

Cited by 72 publications

References 29 publications

Materials processing strategies for colloidal quantum dot solar cells: advances, present-day limitations, and pathways to improvement

Materials processing strategies for colloidal quantum dot solar cells: advances, present-day limitations, and pathways to improvement

Density Functional Study on the Morphology and Photoabsorption of CdSe Nanoclusters

Nodal ground states and orbital textures in semiconductor quantum dots

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