Direct Correlation of Excitonics with Efficiency in a Core-Shell Quantum Dot Solar Cell

CdSe Quantum Dots Bedecked on ZnO/TiO2/CuO Ternary Nanocomposite for Enhanced Photocatalytic and Photovoltaic Applications

Herein, we synthesized a CdSe quantum dots (QDs)-decorated ternary metal oxide nanocomposite of ZnO/TiO2/CuO through a simple hydrothermal method. The prepared nanocomposite exhibited monoclinic, hexagonal, and cubic phase structures in XRD (X-ray diffraction) analysis. UV-vis absorbance spectra showed the broad absorption spectrum. SEM (scanning electron microscopy) clearly showed the presence of nanoparticles and confirmed the elements through elemental mapping. TEM (transmission electron microscopy) confirmed the nanostructure of metal oxides decorated with QDs. The average particle size was 45 nm for metal oxides and 7 nm for QDs. XPS (X-ray photoelectron spectroscopy) also confirmed the surface elemental composition. The prepared nanocomposites were introduced as photoanodes in DSSCs (dye-sensitized solar cells) and as photocatalysts for industrial dye solution. Among these samples, CdSe@CuO/TiO2/ZnO showed an improved performance of PCE (photon conversion efficiency) of 3.68% in DSSC and 96% photocatalytic degradation efficiency. It showed a recycling efficiency of ∼92% after 4 cycles against methylene blue (MB) organic dye under visible light irradiation.

The cytotoxicity of core-shell or non-shell structure quantum dots and reflection on environmental friendly: A review

Quantum dots are widely applicated into bioindustry and research owing to its superior properties such as broad excitation spectra, narrow bandwidth emission spectra and high resistance to photo-bleaching. However, the toxicity of quantum dots should not be underestimated and aroused widespread concern. The surface properties and size of quantum dots are critical relevant properties on toxicity. Then, the core/shell structure becomes one common way to affect the activity of quantum dots such as enhance biocompatibility and stability. Except those toxicity it induced, the problem it brought into the environment such as the degradation of quantum dot similarly becomes a hot issue. This review initially took a brief scan of current research on the cytotoxicity of QDs and the mechanism behind that over the past five years. Mainly discussion concentrated on the diversity of structure on quantum dots whether played a key role on the cytotoxicty of quantum dots. It also discussed the role of different shells with metal or nonmetal cores and the influence on the environment.

Experimental and Theoretical Study into Interface Structure and Band Alignment of the Cu2Zn1-x Cd x SnS4 Heterointerface for Photovoltaic Applications

To improve the constraints of kesterite Cu₂ZnSnS₄ (CZTS) solar cell, such as undesirable band alignment at p-n interfaces, bandgap tuning, and fast carrier recombination, cadmium (Cd) is introduced into CZTS nanocrystals forming Cu₂Zn_1-x Cd _x SnS₄ through cost-effective solution-based method without postannealing or sulfurization treatments. A synergetic experimental-theoretical approach was employed to characterize and assess the optoelectronic properties of Cu₂Zn_1-x Cd _x SnS₄ materials. Tunable direct band gap energy ranging from 1.51 to 1.03 eV with high absorption coefficient was demonstrated for the Cu₂Zn_1-x Cd _x SnS₄ nanocrystals with changing Zn/Cd ratio. Such bandgap engineering in Cu₂Zn_1-x Cd _x SnS₄ helps in effective carrier separation at interface. Ultrafast spectroscopy reveals a longer lifetime and efficient separation of photoexcited charge carriers in Cu₂CdSnS₄ (CCTS) nanocrystals compared to that of CZTS. We found that there exists a type-II staggered band alignment at the CZTS (CCTS)/CdS interface, from cyclic voltammetric (CV) measurements, corroborated by first-principles density functional theory (DFT) calculations, predicting smaller conduction band offset (CBO) at the CCTS/CdS interface as compared to the CZTS/CdS interface. These results point toward efficient separation of photoexcited carriers across the p-n junction in the ultrafast time scale and highlight a route to improve device performances.

Strategies for extending charge separation in colloidal nanostructured quantum dot materials

Semiconductor colloidal metal chalcogenides (II-VI) in the form of quantum dots (QDs) and different heterostructures (core/shell, alloys, etc.) are of extensive interest in scientific research for both a fundamental understanding and technological applications because of their quantized size and different optical properties; however, due to their small size, the exciton (bound electron and hole) experiences a strong Coulombic attraction, which has a remarkable impact on the charge separation and photophysical properties of QDs. Thus, to achieve an efficient charge separation, numerous attempts have been made via the formation of different heterostructures, QD/molecular adsorbate (either organic or inorganic) assemblies, etc. These hybrid materials ameliorated the absorption of the incident light as well as charge separation. This article reviews the strategies for extending charge separation in these colloidal nanocrystals (NCs), which is one of the crucial steps to elevate the solar to electrical energy conversion efficiency in a quantum dot-sensitized solar cell (QDSC). The article summarizes the benefits of co-sensitization and experimental shreds of evidence for the multiple charge transfer processes involved in a QDSC. Studies have shown that in the co-sensitization process, prolonged charge separation occurs via the dual behavior of the molecular adsorbate, sensitization (electron injection) and capture of holes from photoexcited QDs. This perspective emphases band edge engineering and control of charge carrier dynamics in various core/shell structures. The impact of colloidal alloy NCs on charge separation and interesting photophysical properties was recapitulated via the steady-state and time-resolved photoluminescence (PL) and femtosecond transient absorption spectroscopic techniques. Finally, the prolonged lifetime and extent of charge separation for these hybrid NCs (or the composites) assisted in the development of a better light harvester as compared to the case of their pure counterparts.

Improving the Power-Conversion Efficiency through Alloying in Common Anion CdZnX (X=S, Se) Nanocrystal Sensitized Solar Cells

In this paper, we have investigated the possibility of utilizing CdZnS and CdZnSe alloy nanocrystals (NCs) as sensitizers in quantum-dot solar cells (QDSCs). The alloy NCs were synthesized by a high-temperature hot injection method and subsequently characterized through high photoluminescence quantum yield, along with larger size compared to binary NCs. Femtosecond transient absorption measurements revealed long-lived charge carriers in the alloy structure due to more structural rigidity and less defect states. Finally, the solar-cell efficiencies of the CdZnS (CdZnSe) NCs were found to be 3.05 % (3.69 %) as compared to 1.23 % (3.12 %) efficiencies for CdS (CdSe) NCs. Thus, common anion ternary NCs have been successfully utilized for solar-cell assembly and can be helpful for constructing tandem solar cells to harvest the high-energy portion of solar radiation.

Ultrafast Electron Injection from Photoexcited Perovskite CsPbI3 QDs into TiO2 Nanoparticles with Injection Efficiency near 99

Photoexcited electron injection dynamics from CsPbI₃ quantum dots (QDs) to wide gap metal oxides are studied by transient absorption spectroscopy. Experimental results show under a low excitation intensity that ∼99% of the photoexcited electrons in CsPbI₃ QDs can be injected into TiO₂ with a size-dependent rate ranging from 1.30 × 10¹⁰ to 2.10 × 10¹⁰ s^-1, which is also ∼2.5 times faster than that in the case of ZnO. A demonstration QD-sensitized solar cell based on a CsPbI₃/TiO₂ electrode is fabricated that delivers a power conversion efficiency of 5%.

Boosting the Efficiency of Quantum Dot-Sensitized Solar Cells through Formation of the Cation-Exchanged Hole Transporting Layer

In search of a viable way to enhance the power conversion efficiency (PCE) of quantum dot-sensitized solar cells, we have designed a method by introducing a hole transporting layer (HTL) of p-type CuS through partial cation exchange process in a postsynthetic ligand-assisted assembly of nanocrystals (NCs). High-quality CdSe and CdSSe gradient alloy NCs were synthesized through colloidal method, and the charge carrier dynamics was monitored through ultrafast transient absorption measurements. A notable increase in the short-circuit current concomitant with the increase in open-circuit voltage and the fill factor led to 45% increment in PCE for CdSe-based solar cells upon formation of the CuS HTL. Electrochemical impedance spectroscopy further revealed that the CuS layer formation increases recombination resistance at the TiO₂/NC/electrolyte interface, implying that interfacial recombination gets drastically reduced because of smooth hole transfer to the redox electrolyte. Utilizing the same approach for CdSSe alloy NCs, the highest PCE (4.03%) was obtained upon CuS layer formation compared to 3.26% PCE for the untreated one and 3.61% PCE with the conventional ZnS coating. Therefore, such strategies will help to overcome the kinetic barriers of hole transfer to electrolytes, which is one of the major obstacles of high-performance devices.