Thick CdSe Research Articles

Back-surface electric field passivation of CdTe solar cells using sputter-deposited CdSe

CdTe back-surface passivation is one of the effective ways to enhance device performance. In recent years, the role of Se passivation in high performance CdSeTe thin film solar cell device has received increasing attention. However, current high-efficiency devices only have a considerable concentration of Se at the very front of the device, while most of the area on the back side of the absorber layer is not passivated by Se. This means that further efficiency improvements may be possible if more defects on the back surface of the absorber layer are passivated by Se, while maintaining the built-in field. Here, based on the magnetron sputtering process, chemical passivation and electric field passivation can be achieved when a small amount of CdSe is deposited on the CdTe back surface. When the CdSe thickness is ∼20 nm (Before annealing), the optimal cell current was increased from 23.8 mA/cm2 to 26 mA/cm2 and the efficiency was increased from 13.1% to 15.5%. External quantum efficiency and impedance spectroscopy measurements indicate that the performance improvement is due to the reduction of the interfacial recombination current on the back surface.

Size-dependent third-order optical nonlinearity in colloidal CdSe/CdS core/crown nanoplatelets with quasi-type Ⅱ band-structure

Colloidal semiconductor nanoplatelets, a kind of one-dimensional quantum confined materials, have been widely investigated due to their remarkable excitonic effect, in which excellent two-photon absorption property has been proved. In this work, 4.5 monolayers thick CdSe nanoplatelets laterally extended with CdS were synthesized and their crown size-dependent third-order nonlinear optical properties were measured through a picosecond Z-scan method. The detailed investigation combining the time-related photoluminescence spectroscopy reveals the effective passivation of the CdSe boundary with same area size of the core could improve the quantum yield up to 85% and third-order nonlinearity. In addition, the two-photon absorption cross-section can reach up to 6.8 × 106 GM due to the quasi-type Ⅱ band structure of these core/crown nanoplatelets.

Understanding the Interplay Between CdSe Thickness and Cu Doping Temperature in CdSe/CdTe Devices

CdSe thickness and Cu doping play significant roles in achieving high efficiency in CdTe solar cells. Using an evaporated CdSe/CdTe device stack to avoid vacuum breaks between deposition of layers, we investigated the role of the CdSe thickness on the device performance. When the CdSe thickness was greater than a critical value the device performance suffered primarily due to a reduction in the short-circuit current density (J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SC</sub> ). The critical thickness of CdSe was determined to be 120 nm. However, in some cases the CdSe thicknesses could be increased to larger values by increasing the Cu doping process temperature. The higher temperature process leads to an increase in J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SC</sub> and an overall improvement in device efficiency. Specifically, for a device with ∼270 nm of CdSe, the J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SC</sub> increased from 7.9 mAcm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−2</sup> with CuCl <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> processing temperature of 200°C to ∼29 mAcm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−2</sup> when the processing temperature was increased to 250°C.

Effect of PMMA polymer matrix on optical properties of CdSe nanoplatelets

Colloidal cadmium chalcogenide nanoplatelets (NPLs) possess unique properties such as ultrapure emission and giant-oscillator strength originating from their atomically-flat surfaces and one-dimensional carrier confinement. However, unlike quantum dots, the NPLs do not provide continuously tunable absorption and emission, which restricts the range of their possible applications. In this paper, we report a new approach for tuning opto-electronic properties of cadmium chalcogenide NPLs. We demonstrate that incorporation of CdSe NPLs into a poly(methyl methacrylate) (PMMA) matrix leads to a red-shift in all the low energy excitonic transitions and band-edge emission of the NPLs. The polymer matrix induced red-shift of the band-edge emission depends on thickness of the NPLs and ranges from 109 to 253 meV for 4.5 and 2.5 monolayer thick CdSe NPLs, respectively. In the case of CdSe/CdS core-shell NPLs, the polymer matrix induced red-shift of emission peak strongly depends on thickness of the shell and becomes negligible for thick shell CdSe/CdS core-shell NPLs. Possible explanations for the observed effect are presented. The demonstrated here simple approach allows one to tune optical and electronic properties of cadmium chalcogenide NPLs without altering their sizes and composition, which makes it interesting for different practical applications.

A comparative between CdS and CdSe quantum dots doped manganese for photovoltaic enhancement

In this study, we report the fabrication of the photovoltaic devices based on CdS/CdSe multilayer photoanodes, in which Mn2+ ions were doped into CdS or CdSe quantum dots with the doping concentrations in the range of 5% to 40%. The optical, photovoltaic and electrochemical properties of the quantum dots sensitized Solar cells were found to be optimized for 30% Mn2+ doping concentration with the thickness of CdSe:Mn2+ film at 3 cycles, and the efficiency performance of 3.9%.

Nonradiative recombination channel of dark excitons in colloidal CdSe nanoplatelets

Nonradiative recombination channel of spin-forbidden dark excitons in colloidal 4 ML thick CdSe nanoplatelets synthesized in argon and ambient atmosphere is revealed by theoretical analysis of time-resolved photoluminescence. The nonradiative recombination lifetime τnr ≈ 11 ns is found to be independent on nanoplatelets synthesis atmosphere. It is shown that the presence of the nonradiative recombination channel affects spin polarization of dark excitons and results in the time dependence of the circular polarization degree of photoluminescence in the external magnetic field.

Photoluminescence properties of epitaxial asymmetric triple CdSe quantum wells

The authors present the results of the photoluminescence characterization of three asymmetric triple quantum well (ATQW) systems. Each one contains three ultrathin CdSe quantum wells of 1, 2, and 3 monolayers (ML) thickness grown by atomic layer epitaxy between ZnSe separating barriers. Two systems have coupled QWs due to thin 5 nm thick ZnSe separating barriers, and the uncoupled system has 100 nm ZnSe separating barriers. Two of them (5 and 100 nm ZnSe barriers) were grown in the sequence 3–2–1, with the 3 ML QW closer to the substrate, while the third ATQW (5 nm ZnSe barrier) was grown in the reverse order. The photoluminescence spectrum of the uncoupled ATQW (3–2–1 sequence) presents three excitonic peaks, one peak for each QW, with the same emission energies of corresponding isolated single QWs. In the case of two coupled ATQW systems (3–2–1 and 1–2–3 growth sequence), the authors observed only one peak, corresponding to the lowest transition energy in the system formed by the three coupled QWs. The calculation of the wavefunctions shows no overlap for 100 nm ZnSe separating barriers, as expected for this thick barrier. For the systems with 5 nm ZnSe barriers, the calculations indicate that the overlap occurs mainly between the wavefunctions of the central 2 ML thick CdSe QW and the neighboring 1 ML and 3 ML QW wavefunctions; the overlap between the 1 and 3 ML QWs is small as a consequence of their 10 nm separation. The authors found that the effect of the coupling between the QWs is more easily identified by the reduction (or absence) of the intensity of the higher energy transitions of the multiple QW system than by the change of the ATQW energy levels with barrier thickness reduction.

Retorting Photocorrosion and Enhanced Charge Carrier Separation at CdSe Nanocapsules by Chemically Synthesized TiO2 Shell for Photocatalytic Hydrogen Fuel Generation

AbstractMetal chalcogenide‐based semiconductor nanostructures are promising candidates for photocatalytic or photoelectrocatalytic hydrogen generation. In order to protect CdSe from photocorrosion, a layer of TiO2 wrapped (shell) onto CdSe (core) nanocapsule via the post‐synthesis process. The morphology studies confirm that a thin crystalline TiO2 shell (3–8 nm) wrapped in all the three directions onto CdSe core and thickness of the shell can be controlled through modulating the titania precursor concentration. The feasibility of pristine CdSe nanocapsules and CdSe@TiO2 in transforming visible light to hydrogen conversion was tested through photocatalysis reaction. The CdSe@TiO2 nanocapsules generated a four‐fold high rate of hydrogen gas (21 mmol.h−1.g−1cat) than pristine CdSe. In order to understand the role of shell@core, we have studied the photoelectrochemical and impedance analysis. The CdSe@TiO2 nanocapsules showed higher photoelectric current generation and lower charge transfer resistance at electrode/electrolyte interfaces compared to pristine CdSe. These studies endorse that chemically synthesized crystalline TiO2 shell played a multifunctional role in (a) surface passivation from photocorrosion, (b) promoting photocharge carrier separation via tunneling process between CdSe and TiO2 interface. As a result, CdSe@TiO2 nanocapsules showed a high conversion efficiency of 12.9 % under visible light irradiation (328 mW.cm−2) and a TOF of 0.05018 s−1.

Atomically Thin Population of Colloidal CdSe Nanoplatelets: Growth of Rolled-up Nanosheets and Strong Circular Dichroism Induced by Ligand Exchange

Colloidal two-dimensional cadmium chalcogenides nanoplatelets have recently emerged as a class of semiconductor nanoparticles with the narrowest emission and absorption excitonic bands that are of interest for optical applications. Here, we have developed a synthesis protocol for 2.5-monolayer-(ML) thick CdSe nanosheets as a single population. We found that a two-step synthesis in the presence of water promoted the growth of atomically thin nanosheets with high structural and morphological perfection. Using a seeded-growth technique, we extended the lateral size of nanosheets up to 400 nm, which led to the formation of multiwall rolled-up nanostructures. Ligand exchange of native oleic acid, attached to Cd-rich (001) planes, with achiral thioglycolic acid and chiral N-acetylcysteine retains a scroll-like morphology of nanosheets, in contrast to a thicker 3.5 ML population. A reorientation from the [110] to [100] folding direction was found during the change from an achiral to a chiral ligand. In the case ...

Near‐Infrared‐Emitting Five‐Monolayer Thick Copper‐Doped CdSe Nanoplatelets

AbstractDoped nanocrystals are instrumental to the high‐performance luminescent solar concentrators (LSCs) and the color conversion devices. Recently, copper (Cu)‐doped three and four monolayer (ML) thick CdSe nanoplatelets (NPLs) have been shown superior to the existing Cu‐doped quantum dots (QDs) for their use in LSCs. However, additional improvement in the LSC performance can be achieved by further redshifting the emission into the near‐infrared (NIR) region of electromagnetic spectrum and increasing the absorbed portion of the solar irradiation. Cu‐doping into higher thicknesses of these atomically flat NPLs (e.g., ≥5 ML) can achieve these overarching goals. However, addition of the dopant ions during the nucleation stage disturbs this high‐temperature growth process and leads to multiple populations of NPLs and QDs. Here, by carefully controlling the precursor chemistry the successful doping of Cu in five ML thick NPLs by high‐temperature nucleation doping method is demonstrated. The optimized synthesis method shows nearly pure population of doped five ML thick NPLs, which possess ≈150 nm Stokes‐shifted NIR emission with high quantum yield of 65 ± 2%. Structural, elemental, and optical studies are conducted to confirm the successful doping and understand the detailed photophysics. Finally, these materials are tested experimentally and theoretically for their performance as promising LSC materials.

Light Emitting Spin Active Electronic States in Ultra-Thin Mn Doped CdSe Layered Nanosheets

The layered nanosheets exhibit a variety of physical and optical properties originating from amalgamation of intra- and inter- layer electronic interactions, which makes them promising materials for advanced devices with varsatile controlling channels. In particular, the dilute magnetic semiconductor multilayered nanosheets have promising optical, electrical and magnetic properties that have been less explored so far. Here, the spin permissible optical properties from solvothermally grown Mn doped CdSe (thickness ~2.26 nm) multilayered nanosheets are reported on. The presence of multi-phase magnetic orderings with a sharp ferromagnetic transition at temperature ~48 K pertinent to the stabilization and co-existence of Mn2+ and Mn3+ based local phases have been observed from the (Cd,Mn)Se layered nanosheets corroborating to the x-ray absorption near edge structure, electron paramagnetic resonance, Raman scattering and magnetic measurements. The optical absorption and photoluminescence (PL) studies at room temperature affirm wide array of optical properties in the visible regime corresponding to the band edge and intriguing dopant-phase mediated spin approved transitions. The circularly polarized magneto-PL and life time analysis exhibits the spin-polarized fast radiative transitions confirming the presence of spin-active electronic states.

Effect of CdSe thickness deposited by electrophoretic deposition for quantum-dot-sensitized solar cell

Abstract Cadmium selenide (CdSe) and titanium dioxide (TiO2) nanoparticles in chloroform were deposited on fluorine-doped tin oxide (FTO) conducting glass-substrate by the electrophoretic deposition (EPD) method at varying voltages and deposition times. Firstly, TiO2 nanoparticles were deposited on FTO conducting glass-substrate at 150 V for 30 s by EPD method. The TiO2 films were then heated at 450°C for 3 h. After that, CdSe was deposited on TiO2 layer at 100 V for 60 s, 90 s, and 120 s by EPD method. The CdSe/TiO2 films were characterized using UV-visible spectroscopy (UV-vis) and scanning electron microscope (SEM). A complete quantum-dots-sensitized solar cells (QDSSCs) were assembled using polysulfide electrolyte and Cu2S counter electrodes. Polysulfide electrolyte was prepared with mixture of sulphur, sodium sulphide, sodium hydroxide and methanol/deionized water. The power conversion efficiency of 2% was achieved using a CdSe/TiO2 (100 V, 120 s) electrode, under simulated AM 1.5, 100 mW cm-2 illumination.

Weak Exciton–Phonon Coupling in CdSe Nanoplatelets from Quantitative Resonance Raman Intensity Analysis

Resonance Raman spectra, cross sections, and depolarization ratios have been measured for 4.5 monolayer thick CdSe nanoplatelets dispersed in chloroform. Five excitation wavelengths between 514.5 and 476.5 nm were employed. The resonance Raman spectra are dominated by the longitudinal optical (LO) phonon near 201 cm–1 and its overtone, as in CdSe quantum dots. The absolute scattering intensity is much higher for excitation on resonance with the sharp, lowest-energy heavy-hole to conduction band transition than with higher energy transitions, decreasing by about a factor of 30 between 514.5 and 496.5 nm excitation. The LO phonon overtone is weak directly on resonance with the heavy-hole transition but much stronger at higher excitation energies, a result that is reproduced by simulations of the spectra using standard resonance Raman intensity theory. The absolute Raman cross sections imply a Huang–Rhys parameter for the LO phonon of about 0.08 on resonance with the lowest heavy-hole transition. This is a f...

The effect of thickness on the performance of CdSe:Cu2+—quantum dot-sensitized solar cells

The role of the thickness in the counter electrodes is one of the most important criteria for determining the photoelectric conversion efficiency. In this study, we report a novel strategy for optimizing the thickness of CdSe:Cu films in the quantum dot-sensitized solar cells by changing the successive ionic layer absorption and reaction cycles. The result shows that there was a sharp increase in photoelectric conversion efficiency from 1.8 to 4.23% with the SILAR cycles of CdSe:Cu films from 1 to 5 layers due to the effect of CdSe:Cu nanoparticles size on the optical and photovoltaic properties of the devices. In addition, the recombination resistances of the devices can be increased when the SILAR cycles of the counter electrodes changed from 1 layer to 5 layers. To study the above mentioned strategy, the dynamic processes were discussed in detail using the values of the dynamic resistances as: external and internal resistances (RD, Rd), series resistance (RS), shunt resistance (RSH) from one illuminatedI–V curve and resistance movement of charge at the Cu2S/electrolyte and the FTO/TiO2 interface (Rct1), resistance against the electron diffusion in the TiO2 and the charge recombination resistance at the TiO2/QDs/electrolyte interface (Rct2) from electrochemical impedance spectra.

Direct Synthesis of Six-Monolayer (1.9 nm) Thick Zinc-Blende CdSe Nanoplatelets Emitting at 585 nm

Direct Synthesis of Six-Monolayer (1.9 nm) Thick Zinc-Blende CdSe Nanoplatelets Emitting at 585 nm

Ligand exchange on CdSe nanoplatelets for the solar light sensitization of TiO2 and ZnO nanorod arrays

In quantum dot (QD) solar cells, the ex situ sensitization of wide band gap semiconductors (WBSCs) makes it possible to control the shape and the passivation of the nanosized sensitizer. Hence, ex situ techniques can be used to investigate how the band gap of the sensitizers affects the performance of quantum dot solar cells. The latter can be precisely controlled in 1D confined structures such as quasi-2D nanoplatelets (NPLs), the thickness of which is defined with an atomic precision. In this work, we tested and thoroughly characterized the attachment of 7, 9 and 11 monolayers thick CdSe NPLs (as well as QDs for the sake of comparison) to ZnO and to TiO2 nanorods. A crucial point of the ex situ techniques is the choice of bifunctional ligands that link the nanosized sensitizers to the WBSCs. Besides the well-known mercaptopropionic acid, we also studied two ‘atomic linkers’ (OH− and SH−) to minimize the distance between the sensitizer and the oxide. The as-prepared systems have been analyzed by UV/VIS absorption and Raman spectroscopy. Among them, SH− was found to be the most versatile linker that enabled the efficient attachment of all types of CdSe nanocrystals on ZnO and TiO2 nanorods.

Transient Absorption Spectroscopy of CdSe Nanoplatelets

Transient absorption (TA) spectra of 4.5 monolayer thick CdSe nanoplatelets are analyzed in terms of the heavy hole electronic structure. The nanoplatelets are treated with 4-methylbenzenethiol (MB...

Giant PbSe/CdSe/CdSe Quantum Dots: Crystal-Structure-Defined Ultrastable Near-Infrared Photoluminescence from Single Nanocrystals.

Toward a truly photostable PbSe quantum dot (QD), we apply the thick-shell or "giant" QD structural motif to this notoriously environmentally sensitive nanocrystal system. Namely, using a sequential application of two shell-growth techniques-partial-cation exchange and successive ionic layer adsorption and reaction (SILAR)-we are able to overcoat the PbSe QDs with sufficiently thick CdSe shells to impart new single-QD-level photostability, as evidenced by suppression of both photobleaching and blinking behavior. We further reveal that the crystal structure of the CdSe shell (cubic zinc-blende or hexagonal wurtzite) plays a key role in determining the photoluminescence properties of these giant QDs, with only cubic nanocrystals sufficiently bright and stable to be observed as single emitters. Moreover, we demonstrate that crystal structure and particle shape (cubic, spherical, or tetrapodal) and, thereby, emission properties can be synthetically tuned by either withholding or including the coordinating ligand, trioctylphosphine, in the SILAR component of the shell-growth process.

Strain Compensation in Single ZnSe/CdSe Quantum Wells: Analytical Model and Experimental Evidence.

The lattice mismatch between CdSe and ZnSe is known to limit the thickness of ZnSe/CdSe quantum wells on GaAs (001) substrates to about 2-3 monolayers. We demonstrate that this thickness can be enhanced significantly by using In0.12Ga0.88As pseudo substrates, which generate alternating tensile and compressive strains in the ZnSe/CdSe/ZnSe layers resulting in an efficient strain compensation. This method enables to design CdSe/ZnSe quantum wells with CdSe thicknesses ranging from 1 to 6 monolayers, covering the whole visible spectrum. The strain compensation effect is investigated by high resolution transmission electron microscopy and supported by molecular statics simulations. The model approach with the supporting experimental measurements is sufficiently general to be also applied to other highly mismatched material combinations for the design of advanced strained heterostructures.

Effect of lateral size and thickness on the electronic structure and optical properties of quasi two-dimensional CdSe and CdS nanoplatelets

The effect of lateral size and vertical thickness of CdSe and CdS nanoplatelets (NPLs) on their electronic structure and optical properties are investigated using an effective-mass envelope function theory based on the 8-band k ⋅ p model with valence force field considerations. Volumetrically larger NPLs have lower photon emission energy due to limited quantum confinement, but a greater transition matrix element (TME) due to larger electron-hole wavefunction overlap. The optical gain characteristics depend on several factors such as TME, Fermi factor, carrier density, NPL dimensions, material composition, and dephasing rate. There is a red shift in the peak position, more so with an increase in thickness than lateral size. For an increasing carrier density, the gain spectrum undergoes a slight blue shift due to band filling effect. For a fixed carrier density, the Fermi factor is higher for volumetrically larger NPLs and so is the difference between the quasi-Fermi level separation and the effective bandgap. The transparency injection carrier density (and thus input current density threshold) is dimension dependent and falls for volumetrically larger NPLs, as they can attain the requisite exciton count for transparency with a relatively lower density. Between CdSe and CdS, CdSe has lower emission energy due to smaller bandgap, but a higher TME due to lower effective mass. CdS, however, has a higher so hole contribution due to a lower spin-orbit splitting energy. Both CdSe and CdS NPLs are suitable candidates for short-wavelength LEDs and lasers in the visible spectrum, but CdSe is expected to exhibit better optical performance.