Effects of Lattice Strain and Band Offset on Electron Transfer Rates in Type-II Nanorod Heterostructures

Type-II nanorod heterostructures (NRHs) exhibit efficient directional charge separation and provide the potential to control this flow of charges through changes in structure and composition. We use transient-absorption spectroscopy to investigate how the magnitude of band offset and lattice strain alters dynamics of photogenerated electrons in CdSe/CdTe type-II NRHs. In the absence of alloying and strain effects, electron transfer occurs in ∼300 fs. Reducing the conduction band offset by means of alloying leads to an even shorter charge-separation time (<200 fs), whereas curved NRHs with pronounced strain exhibit a longer charge-separation time of ∼700 fs

Effects of Lattice Strain and Band Offset on Electron Transfer Rates in Type-II Nanorod Heterostructures

Type-II nanorod heterostructures (NRHs) exhibit efficient directional charge separation and provide the potential to control this flow of charges through changes in structure and composition. We use transient-absorption spectroscopy to investigate how the magnitude of band offset and lattice strain alters dynamics of photogenerated electrons in CdSe/CdTe type-II NRHs. In the absence of alloying and strain effects, electron transfer occurs in ∼300 fs. Reducing the conduction band offset by means of alloying leads to an even shorter charge-separation time (<200 fs), whereas curved NRHs with pronounced strain exhibit a longer charge-separation time of ∼700 fs

Engineered CuInSe_<i>x</i>S_2–<i>x</i> Quantum Dots for Sensitized Solar Cells

Colloidal CuInSe_<i>x</i>S_2–<i>x</i> quantum dots (QDs) are an attractive less-toxic alternative to PbX and CdX (X = S, Se, and Te) QDs for solution-processed semiconductor devices. This relatively new class of QD materials is particularly suited to serving as an absorber in photovoltaics, owing to its high absorption coefficient and near-optimal and finely tunable band gap. Here, we engineer CuInSe_<i>x</i>S_2–<i>x</i> QD sensitizers for enhanced performance of QD-sensitized TiO₂ solar cells (QDSSCs). Our QD synthesis employs 1-dodecanethiol (DDT) as a low-cost solvent, which also serves as a ligand, and a sulfur precursor; addition of triakylphosphine selenide leads to incorporation of controlled amounts of selenium, reducing the band gap compared to that of pure CuInS₂ QDs. This enables significantly higher photocurrent in the near-infrared (IR) region of the solar spectrum without sacrificing photovoltage. In order to passivate QD surface recombination centers, we perform a surface–cation exchange with Cd prior to sensitization, which enhances chemical stability and leads to a further increase in photocurrent. We use the synthesized QDs to demonstrate proof-of-concept QDSSCs with up to 3.5% power conversion efficiency

Photocharging Artifacts in Measurements of Electron Transfer in Quantum-Dot-Sensitized Mesoporous Titania Films

Transient absorption and time-resolved photoluminescence measurements of high-performance mesoporous TiO₂ photoanodes sensitized with CuInSe_<i>x</i>S_2–<i>x</i> quantum dots reveal the importance of hole scavenging in the characterization of photoinduced electron transfer. The apparent characteristic time of this process strongly depends on the local environment of the quantum dot/TiO₂ junction due to accumulation of long-lived positive charges in the quantum dots. The presence of long-lived photoexcited holes introduces artifacts due to fast positive-trion Auger decay (60 ps time constant), which can dominate electron dynamics and thus mask true electron transfer. We show that the presence of a redox electrolyte is critical to the accurate characterization of charge transfer, since it enables fast extraction of holes and helps maintain charge neutrality of the quantum dots. Although electron transfer is observed to be relatively slow (19 ns time constant), a high electron extraction efficiency (>95%) can be achieved because in well-passivated CuInSe_<i>x</i>S_2–<i>x</i> quantum dots neutral excitons have significantly longer lifetimes of hundreds of nanoseconds

Quality Factor of Luminescent Solar Concentrators and Practical Concentration Limits Attainable with Semiconductor Quantum Dots

Luminescent solar concentrators (LSCs) can be utilized as both large-area collectors of solar radiation supplementing traditional photovoltaic cells as well as semitransparent “solar windows” that provide a desired degree of shading and simultaneously serve as power-generation units. An important characteristic of an LSC is a concentration factor (<i>C</i>) that can be thought of as a coefficient of effective enlargement (or contraction) of the area of a solar cell when it is coupled to the LSC. Here we use analytical and numerical Monte Carlo modeling in addition to experimental studies of quantum-dot-based LSCs to analyze the factors that influence optical concentration in practical devices. Our theoretical model indicates that the maximum value of <i>C</i> achievable with a given fluorophore is directly linked to the LSC quality factor (<i>Q</i>_LSC) defined as the ratio of absorption coefficients at the wavelengths of incident and reemitted light. In fact, we demonstrate that the ultimate concentration limit (<i>C</i>₀) realized in large-area devices scales linearly with the LSC quality factor and in the case of perfect emitters and devices without back reflectors is approximately equal to <i>Q</i>_LSC. To test the predictions of this model, we conduct experimental studies of LSCs based on visible-light emitting II–VI core/shell quantum dots with two distinct LSC quality factors. We also investigate devices based on near-infrared emitting CuInSe_<i>x</i>S_2–<i>x</i> quantum dots for which the large emission bandwidth allows us to assess the impact of varied <i>Q</i>_LSC on the concentration factor by simply varying the detection wavelength. In all cases, we find an excellent agreement between the model and the experimental observations, suggesting that the developed formalism can be utilized for express evaluation of prospective LSC performance based on the optical spectra of LSC fluorophores, which should facilitate future efforts on the development of high-performance devices based on quantum dots as well as other types of emitters

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

Simple yet Versatile Synthesis of CuInSe_<i>x</i>S_2–<i>x</i> Quantum Dots for Sunlight Harvesting

Common approaches to synthesizing alloyed CuInSe_<i>x</i>S_2–<i>x</i> quantum dots (QDs) employ high-cost, air-sensitive phosphine complexes as the selenium precursor. Such methods typically offer low chemical yields and only moderate emission efficiencies, particularly for selenium-rich compositions. Here we demonstrate that such hazardous and air-sensitive selenium precursors can be completely avoided by utilizing a combination of thiols and amines that is very effective at reducing and then complexing with elemental selenium to form a highly reactive selenium precursor at room temperature. The optical properties of the CuInSe_<i>x</i>S_2–<i>x</i> QDs synthesized by this new approach can be finely tuned for optimal sunlight harvesting through control of QD size and composition. In order to demonstrate the importance of such material tunability, we incorporate QDs into liquid-junction Grätzel solar cells and study correlations between varied QD size and composition and the resulting device performance. We also investigate charge transport in films of CuInSe_<i>x</i>S_2–<i>x</i> QDs by incorporating them into bottom-gate field effect transistors. Such films exhibit measurable <i>p</i>-type conductance even without exchange of the long native surface ligands, and the film’s conductance can be improved by more than 3 orders of magnitude by replacing native ligands with shorter ethanedithiol molecules. The results of this study indicate the significant promise of CuInSe_<i>x</i>S_2–<i>x</i> QDs synthesized by this method for applications in photovoltaics utilizing both sensitized and <i>p</i>–<i>n</i> junction architectures

Spectro-electrochemical Probing of Intrinsic and Extrinsic Processes in Exciton Recombination in I–III–VI₂ Nanocrystals

Ternary CuInS₂ nanocrystals (CIS NCs) are attracting attention as nontoxic alternatives to heavy metal–based chalcogenides for many technologically relevant applications. The photophysical processes underlying their emission mechanism are, however, still under debate. Here we address this problem by applying, for the first time, spectro-electrochemical methods to core-only CIS and core/shell CIS/ZnS NCs. The application of an electrochemical potential enables us to reversibly tune the NC Fermi energy and thereby control the occupancy of intragap defects involved in exciton decay. The results indicate that, in analogy to copper-doped II–VI NCs, emission occurs via radiative capture of a conduction-band electron by a hole localized on an intragap state likely associated with a Cu-related defect. We observe the increase in the emission efficiency under reductive electrochemical potential, which corresponds to raising the Fermi level, leading to progressive filling of intragap states with electrons. This indicates that the factor limiting the emission efficiency in these NCs is nonradiative electron trapping, while hole trapping is of lesser importance. This observation also suggests that the centers for radiative recombination are Cu²⁺ defects (preexisting and/or accumulated as a result of photoconversion of Cu¹⁺ ions) as these species contain a pre-existing hole without the need for capturing a valence-band hole generated by photoexcitation. Temperature-controlled photoluminescence experiments indicate that the intrinsic limit on the emission efficiency is imposed by multiphonon nonradiative recombination of a band-edge electron and a localized hole. This process affects both shelled and unshelled CIS NCs to a similar degree, and it can be suppressed by cooling samples to below 100 K. Finally, using experimentally measured decay rates, we formulate a model that describes the electrochemical modulation of the PL efficiency in terms of the availability of intragap electron traps as well as direct injection of electrons into the NC conduction band, which activates nonradiative Auger recombination, or electrochemical conversion of the Cu²⁺ states into the Cu¹⁺ species that are less emissive due to the need for their “activation” by the capture of photogenerated holes

Light Emission Mechanisms in CuInS₂ Quantum Dots Evaluated by Spectral Electrochemistry

Luminescent CuInS₂ (CIS) quantum dots (QDs) exhibit highly efficient intragap emission and long, hundreds-of-nanoseconds radiative lifetimes. These spectral properties, distinct from structurally similar II–VI QDs, can be explained by the involvement of intragap defect states containing a localized hole capable of coupling with a conduction band electron for a radiative transition. However, the absolute energies of the intragap and band-edge states, the structure of the emissive defect(s), and the role and origin of nonemissive decay channels still remain poorly understood. Here, we address these questions by applying methods of spectral electrochemistry. Cyclic voltammetry measurements reveal a well-defined intragap state whose redox potential is close to that of the Cu^<i>x</i> defect state (where <i>x</i> = 1+ or 2+). The energy offset of this state from the valence band accounts well for the apparent photoluminescence Stokes shift observed in optical spectra. These results provide direct evidence that Cu-related defects serve as emission centers responsible for strong intragap emission from CIS QDs. We then use <i>in situ</i> spectroelectrochemistry to reveal two distinct emission pathways based on the differing oxidation states of Cu defects, which can be controlled by altering QD stoichiometry (1+ for stoichiometric QDs and 2+ for Cu-deficient QDs)

Controlled Alloying of the Core–Shell Interface in CdSe/CdS Quantum Dots for Suppression of Auger Recombination

The influence of a CdSe_<i>x</i>S_1‑<i>x</i> interfacial alloyed layer on the photophysical properties of core/shell CdSe/CdS nanocrystal quantum dots (QDs) is investigated by comparing reference QDs with a sharp core/shell interface to alloyed structures with an intermediate CdSe_<i>x</i>S_1‑<i>x</i> layer at the core/shell interface. To fully realize the structural contrast, we have developed two novel synthetic approaches: a method for fast CdS-shell growth, which results in an abrupt core/shell boundary (no intentional or unintentional alloying), and a method for depositing a CdSe_<i>x</i>S_1‑<i>x</i> alloy layer of controlled composition onto the CdSe core prior to the growth of the CdS shell. Both types of QDs possess similar size-dependent single-exciton properties (photoluminescence energy, quantum yield, and decay lifetime). However the alloyed QDs show a significantly longer biexciton lifetime and up to a 3-fold increase in the biexciton emission efficiency compared to the reference samples. These results provide direct evidence that the structure of the QD interface has a significant effect on the rate of nonradiative Auger recombination, which dominates biexciton decay. We also observe that the energy gradient at the core–shell interface introduced by the alloyed layer accelerates hole trapping from the shell to the core states, which results in suppression of shell emission. This comparative study offers practical guidelines for controlling multicarrier Auger recombination without a significant effect on either spectral or dynamical properties of single excitons. The proposed strategy should be applicable to QDs of a variety of compositions (including, <i>e.g</i>., infrared-emitting QDs) and can benefit numerous applications from light emitting diodes and lasers to photodetectors and photovoltaics