Carrier multiplication in van der Waals layered transition metal dichalcogenides

© 2019, The Author(s).Carrier multiplication (CM) is a process in which high-energy free carriers relax by generation of additional electron-hole pairs rather than by heat dissipation. CM is promising disruptive improvements in photovoltaic energy conversion and light detection technologies. Current state-of-the-art nanomaterials including quantum dots and carbon nanotubes have demonstrated CM, but are not satisfactory owing to high-energy-loss and inherent difficulties with carrier extraction. Here, we report CM in van der Waals (vdW) MoTe2 and WSe2 films, and find characteristics, commencing close to the energy conservation limit and reaching up to 99% CM conversion efficiency with the standard model. This is demonstrated by ultrafast optical spectroscopy with independent approaches, photo-induced absorption, photo-induced bleach, and carrier population dynamics. Combined with a high lateral conductivity and an optimal bandgap below 1 eV, these superior CM characteristics identify vdW materials as an attractive candidate material for highly efficient and mechanically flexible solar cells in the future11sciescopu

Size-Dependent Exciton Formation Dynamics in Colloidal Silicon Quantum Dots

We report size-dependent exciton formation dynamics within colloidal silicon quantum dots (Si QDs) using time-resolved terahertz (THz) spectroscopy measurements. THz photoconductivity measurements are used to distinguish the initially created hot carriers from excitons that form at later times. At early pump/probe delays, the exciton formation dynamics are revealed by the temporal evolution of the THz transmission. We find an increase in the exciton formation time, from ∼500 to ∼900 fs, as the Si QD diameter is reduced from 7.3 to 3.4 nm and all sizes exhibit slower hot-carrier relaxation times compared to bulk Si. In addition, we determine the THz absorption cross section at early delay times is proportional to the carrier mobility while at later delays is proportional to the exciton polarizability, α_X. We extract a size-dependent α_X and find an ∼<i>r</i>⁴ dependence, consistent with previous reports for quantum-confined excitons in CdSe, InAs, and PbSe QDs. The observed slowing in exciton formation time for smaller Si QDs is attributed to decreased electron–phonon coupling due to increased quantum confinement. These results experimentally verify the modification of hot-carrier relaxation rates by quantum confinement in Si QDs, which likely plays a significant role in the high carrier multiplication efficiency observed in these nanomaterials

Ultrafast Electrical Measurements of Isolated Silicon Nanowires and Nanocrystals

We simultaneously determined the charge carrier mobility and picosecond to nanosecond carrier dynamics of isolated silicon nanowires (Si NWs) and nanocrystals (Si NCs) using time-resolved terahertz spectroscopy. We then compared these results to data measured on bulk c-Si as a function of excitation fluence. We find >1 ns carrier lifetimes in Si NWs that are dominated by surface recombination with surface recombination velocities (SRV) between ∼1100–1700 cm s^–1 depending on process conditions. The Si NCs have markedly different decay dynamics. Initially, free-carriers are produced, but relax within ∼1.5 ps to form bound excitons. Subsequently, the excitons decay with lifetimes >7 ns, similar to free carriers produced in bulk Si. The isolated Si NWs exhibit bulk-like mobilities that decrease with increasing excitation density, while the hot-carrier mobilities in the Si NCs are lower than bulk mobilities and could only be measured within the initial 1.5 ps decay. We discuss the implications of our measurements on the utilization of Si NWs and NCs in macroscopic optoelectronic applications

High-Performance CuInS₂ Quantum Dot Laminated Glass Luminescent Solar Concentrators for Windows

Building-integrated sunlight harvesting utilizing laminated glass luminescent solar concentrators (LSCs) is proposed. By incorporating high quantum yield (>90%), NIR-emitting CuInS₂/ZnS quantum dots into the polymer interlayer between two sheets of low-iron float glass, a record optical efficiency of 8.1% is demonstrated for a 10 cm × 10 cm device that transmits ∼44% visible light. After completing prototypes by attaching silicon solar cells along the perimeter of the device, the electrical power conversion efficiency was certified at 2.2% with a black background and at 2.9% using a reflective substrate. This “drop-in” LSC solution is particularly attractive because it fits within the existing glazing industry value chain with only modest changes to typical glazing products. Performance modeling predicts >1 GWh annual electricity production for a typical urban skyscraper in most major U.S. cities, enabling significant energy cost savings and potentially “net-zero” buildings

Control of PbSe Quantum Dot Surface Chemistry and Photophysics Using an Alkylselenide Ligand

We have synthesized alkylselenide reagents to replace the native oleate ligand on PbSe quantum dots (QDs) in order to investigate the effect of surface modification on their stoichiometry, photophysics, and air stability. The alkylselenide reagent removes all of the oleate on the QD surface and results in Se addition; however, complete Se enrichment does not occur, achieving a 53% decrease in the amount of excess Pb for 2 nm diameter QDs and a 23% decrease for 10 nm QDs. Our analysis suggests that the Se ligand preferentially binds to the {111} faces, which are more prevalent in smaller QDs. We find that attachment of the alkylselenide ligand to the QD surface enhances oxidative resistance, likely resulting from a more stable bond between surface Pb atoms and the alkylselenide ligand compared to Pb-oleate. However, binding of the alkylselenide ligand produces a separate nonradiative relaxation route that partially quenches PL, suggesting the formation of a dark hole-trap

Consensus statement: Standardized reporting of power-producing luminescent solar concentrator performance

Fair and meaningful device per- formance comparison among luminescent solar concentrator- photovoltaic (LSC-PV) reports cannot be realized without a gen- eral consensus on reporting stan- dards in LSC-PV research. There- fore, it is imperative to adopt standardized characterization protocols for these emerging types of PV devices that are consistent with other PV devices. This commentary highlights several common limitations in LSC literature and summarizes the best practices moving for- ward to harmonize with standard PV reporting, considering the greater nuances present with LSC-PV. Based on these prac- tices, a checklist of actionable items is provided to help stan- dardize the characterization/re- porting protocols and offer a set of baseline expectations for au- thors, reviewers, and editors. The general consensus combined with the checklist will ultimately guide LSC-PV research towards reliable and meaningful ad- vances

Consensus statement: Standardized reporting of power-producing luminescent solar concentrator performance

Fair and meaningful device performance comparison among luminescent solar concentrator-photovoltaic (LSC-PV) reports cannot be realized without a general consensus on reporting standards in LSC-PV research. Therefore, it is imperative to adopt standardized characterization protocols for these emerging types of PV devices that are consistent with other PV devices. This commentary highlights several common limitations in LSC literature and summarizes the best practices moving forward to harmonize with standard PV reporting, considering the greater nuances present with LSC-PV. Based on these practices, a checklist of actionable items is provided to help standardize the characterization/reporting protocols and offer a set of baseline expectations for authors, reviewers, and editors. The general consensus combined with the checklist will ultimately guide LSC-PV research towards reliable and meaningful advances