Quantum-Dot-Based Solar Cells: Recent Advances, Strategies, and Challenges

Advances in Smart Photovoltaic Textiles

Energy harvesting textiles have emerged as a promising solution to sustainably power wearable electronics. Textile-based solar cells (SCs) interconnected with on-body electronics have emerged to meet such needs. These technologies are lightweight, flexible, and easy to transport while leveraging the abundant natural sunlight in an eco-friendly way. In this Review, we comprehensively explore the working mechanisms, diverse types, and advanced fabrication strategies of photovoltaic textiles. Furthermore, we provide a detailed analysis of the recent progress made in various types of photovoltaic textiles, emphasizing their electrochemical performance. The focal point of this review centers on smart photovoltaic textiles for wearable electronic applications. Finally, we offer insights and perspectives on potential solutions to overcome the existing limitations of textile-based photovoltaics to promote their industrial commercialization.

Performance optimization of efficient PbS quantum dots solar cells through numerical simulation

Colloidal quantum dots (CQDs) solar cells are less efficient because of the carrier recombination within the material. The electron and hole transport layers have high impact on the performance of CQDs based solar cells which makes its investigation a very important component of the development of the more efficient devices. In this work, we have tried performance optimization in tetrabutyl ammonium iodide capped lead sulfide (PbS) CQDs (PbS-TBAI) as absorber layers based solar cells by incorporating different hole transport layers (HTLs) to achieve better power conversion efficiency (PCE) in different device architectures by SCAPS-1D numerical simulation software. It was observed from the simulation that the ITO/TiO₂/PbS-TBAI/HTL/Au device architecture shows higher power conversion efficiency as compared to the conventional experimentally realized device architecture of ITO/TiO₂/PbS-TBAI/PbS-EDT/HTL/Au. The influence of interface defect density (IDD) at the interface TiO₂/PbS-TBAI has also been studied where IDD is varied from 1 × 10¹³ cm^-2 to 1 × 10¹⁸ cm^-2 while keeping the rest of the device parameters intact. The result shows a noteworthy reduction in the PV performance of the device at higher IDD. This modelled device structure provides a new direction toward the experimental realization in high efficiency PbS QDs solar cells.

Progress in the Development of Active-Matrix Quantum-Dot Light-Emitting Diodes Driven by Non-Si Thin-Film Transistors

This paper aims to discuss the key accomplishments and further prospects of active-matrix (AM) quantum-dot (QD) light-emitting diodes (QLEDs) display. We present an overview and state-of-the-art of QLEDs as a frontplane and non-Si-based thin-film transistors (TFTs) as a backplane to meet the requirements for the next-generation displays, such as flexibility, transparency, low power consumption, fast response, high efficiency, and operational reliability. After a brief introduction, we first review the research on non-Si-based TFTs using metal oxides, transition metal dichalcogenides, and semiconducting carbon nanotubes as the driving unit of display devices. Next, QLED technologies are analyzed in terms of the device structure, device engineering, and QD patterning technique to realize high-performance, full-color AM-QLEDs. Lastly, recent research on the monolithic integration of TFT-QLED is examined, which proposes a new perspective on the integrated device. We anticipate that this review will help the readership understand the fundamentals, current state, and issues on TFTs and QLEDs for future AM-QLED displays.

Study of Two-Dimensional Janus WXY (X≠Y= S, Se, and Te) Trilayer Homostructures for Photovoltaic Applications Using DFT Screening of Different Stacking Patterns

Based on the first-principles density functional theory, Janus WXY (X ≠ Y = S, Se, and Te) trilayer homostructures for different stacking patterns are studied in this work to analyze their appropriateness in fabricating photovoltaic (PV) devices. A total of fifteen trilayer homostructures are proposed, corresponding to the suitable five stacking patterns, such as AAA, AA'A, ABA, AB'A, and A'BA' for each Janus WXY (X ≠ Y = S, Se, and Te) material. Structural and energetic parameters for all the fifteen structures are evaluated and compared to find energetically stable structures, and dynamic stability is confirmed by phonon dispersion curves. All these configurations being homostructure, lattice mismatch is found to be very low (∼0.05%), unlike heterostructure, making them feasible for optoelectronics and PV applications. WSSe AAA, WSSe AA'A, and WSeTe AA'A are dynamically stable along with negative binding energy and show type-II band alignment, enabling effective spatial carrier separation of photogenerated carriers. The optical properties of dynamically stable WSSe AAA and WSSe AA'A structures are also calculated, and the absorption coefficients at the visible light region are found to be ∼3.5 × 10⁵ cm^-1, which is comparable to the perovskite material absorption coefficient. Moreover, we have compared the optical characteristics of dynamically stable WSSe AAA and WSSe AA'A structures with their monolayer structures to realize the significance of stacking trilayer structures. Electrical properties such as mobility and conductivity for dynamically stable WSSe AAA and WSSe AA'A structures are evaluated to suggest them as a probable efficient material in PV technology.

Electronic, Optical, and Thermoelectric Properties of Bulk and Monolayer Germanium Tellurides

Electronic, optical, and thermoelectric properties of germanium tellurides (GeTe) were investigated through a series of first-principles calculations of band structures, absorption coefficients, and thermoelectric transport coefficients. We consider bulk GeTe to consist of cubic and rhombohedral phases, while the two-dimensional (2D) GeTe monolayers can form as a 2D puckered or buckled honeycomb crystals. All of the GeTe variants in the bulk and monolayer shapes are excellent light absorbers in a wide frequency range: (1) bulk cubic GeTe in the near-infrared regime, (2) bulk rhombohedral GeTe and puckered monolayer GeTe in the visible-light regime, and (3) buckled monolayer GeTe in the ultraviolet regime. We also found specifically that the buckled monolayer GeTe exhibits remarkable thermoelectric performance compared to the other GeTe phases due to a combination of electronic band convergence, a moderately wide band gap, and unique 2D density of states from the quantum confinement effect.

Structural reconstruction and visible-light absorption versus internal electrostatic field in two-dimensional GaN-ZnO alloys

GaN-ZnO alloys are more promising semiconductors than their counterparts for optoelectronic applications due to the abrupt red shift in the visible-light range. Unfortunately, the strong internal electrostatic field (IEF) seriously hinders to further improve the optoelectronic performance due to the charge density of surface states. We point out a structural model to extremely improve the visible-light absorption by overcoming the bottleneck of the IEF in the two-dimensional (2D) nonisovalent alloys. The novel haeckelite (8|4) configuration with the nearly zero IEF shows much better optoelectronic performances than the conventional wurtzite configuration. Meanwhile, we explore the thickness-driven structural transitions from the planar hexagonal to the 8|4 and to the wurtzite configurations. The visible-light absorption efficiency quickly rises up from the bulk wurtzite to the bulk 8|4 to the 2D 8|4 and to the MoS2-based heterostructures with the different-layer 8|4 configurations. The heterointerfacial coupling is an effective way to further reduce the IEF and hence to significantly improve the visible-light absorptions by enlarging the population of band edge states in the 8|4 configuration. We suggest that the 8|4 configuration is more prospective for diverse optoelectronic applications in 2D GaN-ZnO alloys than in binary counterparts.

Metal-Organic Framework Materials for Perovskite Solar Cells

Metal-organic frameworks (MOFs) and MOF-derived materials have been used for several applications, such as hydrogen storage and separation, catalysis, and drug delivery, owing to them having a significantly large surface area and open pore structure. In recent years, MOFs have also been applied to thin-film solar cells, and attractive results have been obtained. In perovskite solar cells (PSCs), the MOF materials are used in the form of an additive for electron and hole transport layers, interlayer, and hybrid perovskite/MOF. MOFs have the potential to be used as a material for obtaining PSCs with high efficiency and stability. In this study, we briefly explain the synthesis of MOFs and the performance of organic and dye-sensitized solar cells with MOFs. Furthermore, we provide a detailed overview on the performance of the most recently reported PSCs using MOFs.

Photoexcited carrier dynamics in colloidal quantum dot solar cells: insights into individual quantum dots, quantum dot solid films and devices

The certified power conversion efficiency (PCE) record of colloidal quantum dot solar cells (QDSCs) has considerably improved from below 4% to 16.6% in the last few years. However, the record PCE value of QDSCs is still substantially lower than the theoretical efficiency. So far, there have been several reviews on recent and significant achievements in QDSCs, but reviews on photoexcited carrier dynamics in QDSCs are scarce. The photovoltaic performances of QDSCs are still limited by the photovoltage, photocurrent and fill factor that are mainly determined by the photoexcited carrier dynamics, including carrier (or exciton) generation, carrier extraction or transfer, and the carrier recombination process, in the devices. In this review, the photoexcited carrier dynamics in the whole QDSCs, originating from individual quantum dots (QDs) to the entire device as well as the characterization methods used for analyzing the photoexcited carrier dynamics are summarized and discussed. The recent research including photoexcited multiple exciton generation (MEG), hot electron extraction, and carrier transfer between adjacent QDs, as well as carrier injection and recombination at each interface of QDSCs are discussed in detail herein. The influence of photoexcited carrier dynamics on the physiochemical properties of QDs and photovoltaic performances of QDSC devices is also discussed.

The Frontiers of Nanomaterials (SnS, PbS and CuS) for Dye-Sensitized Solar Cell Applications: An Exciting New Infrared Material

To date, extensive studies have been done on solar cells on how to harness the unpleasant climatic condition for the binary benefits of renewable energy sources and potential energy solutions. Photovoltaic (PV) is considered as, not only as the future of humanity's source of green energy, but also as a reliable solution to the energy crisis due to its sustainability, abundance, easy fabrication, cost-friendly and environmentally hazard-free nature. PV is grouped into first, second and third-generation cells. Dye-sensitized solar cells (DSSCs), classified as third-generation PV, have gained more ground in recent times. This is linked to their transparency, high efficiency, shape, being cost-friendly and flexibility of colour. However, further improvement of DSSCs by quantum dot sensitized solar cells (QDSSCs) has increased their efficiency through the use of semiconducting materials, such as quantum dots (QDs), as sensitizers. This has paved way for the fabrication of semiconducting QDs to replace the ideal DSSCs with quantum dot sensitized solar cells (QDSSCs). Moreover, there are no absolute photosensitizers that can cover all the infrared spectrum, the infusion of QD metal sulphides with better absorption could serve as a breakthrough. Metal sulphides, such as PbS, SnS and CuS QDs could be used as photosensitizers due to their strong near infrared (NIR) absorption properties. A few great dependable and reproducible routes to synthesize better QD size have attained much ground in the past and of late. The injection of these QD materials, which display (NIR) absorption with localized surface plasmon resonances (SPR), due to self-doped p-type carriers and photocatalytic activity could enhance the performance of the solar cell. This review will be focused on QDs in solar cell applications, the recent advances in the synthesis method, their stability, and long term prospects of QDSSCs efficiency.

Nanostructured Perovskite Solar Cells

Over the past decade, lead halide perovskites have emerged as one of the leading photovoltaic materials due to their long carrier lifetimes, high absorption coefficients, high tolerance to defects, and facile processing methods. With a bandgap of ~1.6 eV, lead halide perovskite solar cells have achieved power conversion efficiencies in excess of 25%. Despite this, poor material stability along with lead contamination remains a significant barrier to commercialization. Recently, low-dimensional perovskites, where at least one of the structural dimensions is measured on the nanoscale, have demonstrated significantly higher stabilities, and although their power conversion efficiencies are slightly lower, these materials also open up the possibility of quantum-confinement effects such as carrier multiplication. Furthermore, both bulk perovskites and low-dimensional perovskites have been demonstrated to form hybrids with silicon nanocrystals, where numerous device architectures can be exploited to improve efficiency. In this review, we provide an overview of perovskite solar cells, and report the current progress in nanoscale perovskites, such as low-dimensional perovskites, perovskite quantum dots, and perovskite-nanocrystal hybrid solar cells.

High efficiency solar cells tailored using biomass-converted graded carbon quantum dots

The solar-to-electric conversion efficiency of mesoscopic solar cells is highly dependent on electron extraction under solar irradiation and determines the charge recombination processes within devices. Boosting charge transfer via building energy level alignment has been proved to be a promising method to enhance the output power of solar cells. In the current work, we present the successful fabrication of functional biomass-converted carbon quantum dots with graded energy levels by doping nitrogen or sulphur atoms from lotus root powder. When employed as light absorbers for mesoscopic solar cells, light harvesting and electron extraction can be markedly increased arising from the band alignment of graded carbon quantum dots, yielding maximal power conversion efficiencies of 0.158% and 0.208% for bi- and tri-cascaded photovoltaics, respectively. The primary results demonstrate that the employment of an energy-graded architecture is a promising strategy to optimize the device output. Following this line of thought, we further fabricate a co-sensitized device by integrating graded carbon quantum dots with N719 dyes to enhance the electron extraction capability. The final device yields an efficiency as high as 9.04%, showing the potential application of carbon quantum dots in high-performance solar cells.

High efficiency Si quantum dot heterojunction solar cells using a single SiOX:B layer

Si quantum dots (QDs) have been fabricated from SiO₂/SiO_x multilayer structures to create a homogeneous size. However, this structure achieved much lower efficiencies than would be expected in the Si QD photovoltaic field. This is because Si QD generation and photoexcited carrier transport is restricted by the adjacent SiO₂ layer. In this study, we applied a single SiO_x:B layer fabrication method to the Si QD heterojunction solar cells. The number of generated Si QDs and the photo-excited carrier lifetime was maximized when the oxygen partial pressure and boron doping concentration parameters were 2.7 × 10^-5 Torr and 2.27 × 10²¹ atoms cm^-3, respectively. As a result, in excess of 17% power conversion efficiency for the Si QD heterojunction solar cell was achieved using the single layer method.

Enhanced-performance of self-powered flexible quantum dot photodetectors by a double hole transport layer structure

The usefulness of self-powered quantum dot (QD) photodetectors is increased if they are fabricated on flexible substrates. However, the performance of such photodetectors is typically significantly worse than similar devices fabricated on glass substrates due to poor charge transport performance. Here, a novel flexible self-powered CdSexTe1-x QD photodetector with a double hole transport layer of PEDOT:PSS/P-TPD has been fabricated, which achieves a performance comparable to that of rigid devices. The energy level of the P-TPD layer matches well with that of the PEDOT:PSS and QD layers, which significantly enhances photodetection capability across a spectral region that spans the ultraviolet, visible and near infrared (UV-NIR). A low dark current density (1.03 × 10-6 mA cm-2) and a large specific detectivity of approximately 2.6 × 1012 Jones at a wavelength of 450 nm are demonstrated, significantly outperforming previously reported flexible QD-based detectors. This improvement in performance is attributed to both increased hole transport efficiency and the inhibition of electron transport from the QDs into the PEDOT:PSS layer. The photodetector also exhibits good sensitivity under weak illumination, producing a photocurrent of 196 × 10-6 mA cm-2 under an irradiance of 5 μW cm-2. Moreover, no significant performance degradation is observed after 150 bending cycles to an angle of 60 degrees.

Gradient-band-gap strategy for efficient solid-state PbS quantum-dot sensitized solar cells

To improve charge separation and enhance open-circuit voltage (Voc) in solid-state quantum-dot sensitized solar cells (QDSCs), gradient-band-gap PbS quantum-dots were first and easily constructed by two-step spin-coating the Pb(NO3)2 solution and the mixed solution of Na2S and 1,2-ethanedithiol via successive ionic layer absorption and reaction (SILAR). The fabricated solid-state gradient-band-gap PbS QDSCs exhibited a Voc of 0.70 V, a short-circuit photocurrent density (Jsc) of 9.65 mA·cm-2, a fill factor (FF) of 0.60, and a photoelectric conversion efficiency (PCE) of 4.08%, while the inverse gradient-band-gap PbS QDSCs showed a Voc of 0.59 V, a Jsc of 5.86 mA·cm-2, an FF of 0.49 and a PCE of 1.69%. By optimization, the best solid-state gradient-band-gap PbS QDSCs achieved a Voc of 0.65 V and a PCE of 6.29% under 1 sun, and a Voc of 0.60 V and a PCE of 7.21% under 0.5 sun. The Voc of 0.65 V was relatively high, and the PCE of 6.29% was the highest value among solid-state QDSCs constructed using SILAR.

Near-Infrared Active Lead Chalcogenide Quantum Dots: Preparation, Post-Synthesis Ligand Exchange, and Applications in Solar Cells

Quantum dots (QDs) of lead chalcogenides (e.g. PbS, PbSe, and PbTe) are attractive near-infrared (NIR) active materials that show great potential in a wide range of applications, such as, photovoltaics (PV), optoelectronics, sensors, and bio-electronics. The surface ligand plays an essential role in the production of QDs, post-synthesis modification, and their integration to practical applications. Therefore, it is critically important that the influence of surface ligands on the synthesis and properties of QDs is well understood for their applications in various devices. In this Review we elaborate the application of colloidal synthesis techniques for the preparation of lead chalcogenide based QDs. We specifically focus on the influence of surface ligands on the synthesis of QDs and their solution-phase ligand exchange. Given the importance of lead chalcogenide QDs as potential light harvesters, we also pay particular attention to the current progress of these QDs in photovoltaic applications.

Periodically-ordered one and two dimensional CdTe QD superstructures: a path forward in photovoltaics

By using the state-of-the-art theoretical method, we herein explore the potentiality of covalently linked periodically-ordered 1D chain, 2D hexagonal and square ordered superstructures of CdTe QDs in photovoltaics.

Heterojunction Area-Controlled Inorganic Nanocrystal Solar Cells Fabricated Using Supra-Quantum Dots

A supra-quantum dot (SQD) is a three-dimensional structure formed by the attachment of quantum dots. The SQDs have sizes of tens of nanometer and they maintain the characteristics of the individual quantum dots fairly well. Moreover, their sizes and elemental compositions can be tuned precisely. On the basis of their unique features, in this work, SQDs are used as constituents of the interpenetrating photoactive layers of inorganic nanocrystal p-n heterojunction solar cells to control the p-type and n-type domain sizes (i.e., p-n heterojunction areas) for optimizing the charge-carrier collection. SQD-containing p-n heterojunction solar cells exhibit improved charge transport and thereby higher power conversion efficiency (PCE) (3.03%) owing to their intermediate p-type and n-type domain sizes, which are between those of a bilayer nanorod p-n heterojunction solar cell (PCE: 1.21%) and an interpenetrating nanorod p-n heterojunction solar cell (PCE: 2.40%). This work demonstrates that the self-assembly of nanoscale materials can be utilized for tailoring the spatial distributions of charge carriers, which is beneficial for obtaining an enhanced device performance.

Quantum Dot Solar Cells: Small Beginnings Have Large Impacts

From a niche field over 30 years ago, quantum dots (QDs) have developed into viable materials for many commercial optoelectronic devices. We discuss the advancements in Pb-based QD solar cells (QDSCs) from a viewpoint of the pathways an excited state can take when relaxing back to the ground state. Systematically understanding the fundamental processes occurring in QDs has led to improvements in solar cell efficiency from ~3% to over 13% in 8 years. We compile data from ~200 articles reporting functioning QDSCs to give an overview of the current limitations in the technology. We find that the open circuit voltage limits the device efficiency and propose some strategies for overcoming this limitation.

Size-Dependent Band-Gap and Molar Absorption Coefficients of Colloidal CuInS2 Quantum Dots

The knowledge of the quantum dot (QD) concentration in a colloidal suspension and the quantitative understanding of the size-dependence of the band gap of QDs are of crucial importance from both applied and fundamental viewpoints. In this work, we investigate the size-dependence of the optical properties of nearly spherical wurtzite (wz) CuInS₂ (CIS) QDs in the 2.7 to 6.1 nm diameter range (polydispersity ≤10%). The QDs are synthesized by partial Cu⁺ for In³⁺ cation exchange in template Cu_{2- x}S nanocrystals, which yields CIS QDs with very small composition variations (In/Cu = 0.91 ± 0.11), regardless of their sizes. These well-defined QDs are used to investigate the size-dependence of the band gap of wz CIS QDs. A sizing curve is also constructed for chalcopyrite CIS QDs by collecting and reanalyzing literature data. We observe that both sizing curves follow primarily a 1/ d dependence. Moreover, the molar absorption coefficients and the absorption cross-section per CIS formula unit, both at 3.1 eV and at the band gap, are analyzed. The results demonstrate that the molar absorption coefficients of CIS QDs follow a power law at the first exciton transition energy (ε _E1 = 5208 d^2.45) and scale with the QD volume at 3.1 eV. This latter observation implies that the absorption cross-section per unit cell at 3.1 eV is size-independent and therefore can be estimated from bulk optical constants. These results also demonstrate that the molar absorption coefficients at 3.1 eV are more reliable for analytical purposes, since they are less sensitive to size and shape dispersion.

Suppressing the photobleaching and photoluminescence intermittency of single near-infrared CdSeTe/ZnS quantum dots with p-phenylenediamine

Intrinsic photobleaching and photoluminescence (PL) intermittency of single quantum dots (QDs), originating from photo-oxidation and photo-ionization respectively, are roadblocks for most single-dot applications. Here, we effectively suppress the photobleaching and the PL intermittency of single near-infrared emitting QDs with p-phenylenediamine (PPD). The PPD cannot only be used as a high-efficient reducing agent to remove reactive oxygen species around QDs to suppress the photo-oxidation, but can also bond with the surface defect sites of single QDs to reduce electron trap states to suppress the photo-ionization. It is shown that the survival time of single QDs, the on-state probability of PL intensity traces, and the total number of emitted photons are significantly increased for single QDs in PPD compared with that on glass coverslip.

Absorption and Emission of Light in Optoelectronic Nanomaterials: The Role of the Local Optical Environment

Tailoring the interaction of electromagnetic radiation with matter is central to the development of optoelectronic devices. This becomes particularly relevant for a new generation of devices offering the possibility of solution processing with competitive efficiencies as well as new functionalities. These devices, containing novel materials such as inorganic colloidal quantum dots or hybrid organic-inorganic lead halide perovskites, commonly demand thin (tens of nanometers) active layers in order to perform optimally and thus maximizing the way electromagnetic radiation interacts with these layers is essential. In this Perspective, we discuss the relevance of tailoring the optical environment of the active layer in an optoelectronic device and illustrate it with two real-world systems comprising photovoltaic cells and light emitting devices.

Nanoarchitectures in dye-sensitized solar cells: metal oxides, oxide perovskites and carbon-based materials

Dye-sensitized solar cells (DSSCs) have aroused great interest and been regarded as a potential renewable energy resource among the third-generation solar cell technologies to fulfill the 21^st century global energy demand. DSSCs have notable advantages such as low cost, easy fabrication process and being eco-friendly in nature. The progress of DSSCs over the last 20 years has been nearly constant due to some limitations, like poor long-term stability, narrow absorption spectrum, charge carrier transportation and collection losses and poor charge transfer mechanism for regeneration of dye molecules. The main challenge for the scientific community is to improve the performance of DSSCs by using different approaches, like finding new electrode materials with suitable nanoarchitectures, dyes in composition with promising semiconductors and metal quantum dot fluorescent dyes, and cost-effective hole transporting materials (HTMs). This review focuses on DSSC photo-physics, which includes charge separation, effective transportation, collection and recombination processes. Different nanostructured materials, including metal oxides, oxide perovskites and carbon-based composites, have been studied for photoanodes, and counter electrodes, which are crucial to achieve DSSC devices with higher efficiency and better stability.

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.

Quantum dot-sensitized solar cells

A comprehensive overview of the development of quantum dot-sensitized solar cells (QDSCs) is presented.

Charge recombination control for high efficiency CdS/CdSe quantum dot co-sensitized solar cells with multi-ZnS layers

ZnS as an inorganic passivation agent has been proven to be effective in suppressing charge recombination and enhancing power conversion efficiency (PCE) in quantum dot-sensitized solar cells (QDSCs).

Intriguing electronic and optical properties of two-dimensional Janus transition metal dichalcogenides

Two-dimensional Janus transition metal dichalcogenides with an asymmetric structure present intriguing electronic, transport, and optical properties, which make them ideally suitable for electronic and optoelectronic applications.

Facile, one-pot and scalable synthesis of highly emissive aqueous-based Ag,Ni:ZnCdS/ZnS core/shell quantum dots with high chemical and optical stability

We report here on a one-pot, mild and low cost aqueous-based synthetic route for the preparation of colloidally stable and highly luminescent dual-doped Ag,Ni:ZnCdS/ZnS core/shell quantum dots (QDs). The pure dopant emission of the Ni-doped core/shell QDs was found to be highly affected by the presence of a second dopant ion (Ag⁺). Results showed that the PL emission intensity increases while its peak position experiences an obvious blue shift with an increase in the content of Ag⁺ ions. Regarding the optical observations, we provide a simple scheme for absorption-recombination processes of the carriers through impurity centers. To obtain optimum conditions with a better emission characteristic, we also study the effect of different reaction parameters, such as refluxing temperature, the pH of the core and shell solution, molar ratio of the dopant ions (Ni:(Zn+Cd) and Ag:(Zn+Cd)), and concentration of the core and shell precursors. Nonetheless, the most effective parameter is the presence of the ZnS shell in a suitable amount to eliminate surface trap states and enhance their emission intensity. It can also improve the bio-compatibility of the prepared QDs by restricting the Cd²⁺ toxic ions inside the core of the QDs. The present suggested route also revealed the remarkable optical and chemical stability of the colloidal QDs which establishes them as a decent kind of nano-scale structure for light emitting applications, especially in biological technologies. The suggested process also has the potential to be scaled-up while maintaining the emission characteristics and structural quality necessary for industrial applications in optoelectronic devices.

Enhancement effect of defect fluorescence of ZnSe quantum dots on a heterojuction of ZnSe quantum dots and gold nanoparticles

We studied an enhancement effect of defect fluorescence of ZnSe quantum dots (QDs) on a heterojunction of ZnSe QDs and gold nanoparticles. The photoluminescence (PL) of Au/ZnSe heterojunction is excited by using a 150 nm diameter ultraviolet laser spot of a scanning near-field optical microscope. Owing to the charge transfer of photon-generated carriers from ZnSe QDs, the enhanced PL effect is observed, which results from the increase of the built-in electric field to hinder the electron transfer to gold nanoparticles and is trapped by the defect states of ZnSe QDs. The broadening of defect fluorescence spectra and the reduction of excitonic fluorescence in multi-heterojunction of ZnSe QDs and gold nanoparticles are also observed which is attributed to an increase of their contact areas. We believe that enhanced defect fluorescence method described in this paper have potential applications in forming uniform optoelectronic heterojunction in controlling and boosting fluorescent efficiency of weak PL devices.

Efficient Carrier Multiplication in Colloidal Silicon Nanorods

Auger recombination lifetimes, absorption cross sections, and the quantum yields of carrier multiplication (CM), or multiexciton generation (MEG), were determined for solvent-dispersed silicon (Si) nanorods using transient absorption spectroscopy (TAS). Nanorods with an average diameter of 7.5 nm and aspect ratios of 6.1, 19.3, and 33.2 were examined. Colloidal Si nanocrystals of similar diameters were also studied for comparison. The nanocrystals and nanorods were passivated with organic ligands by hydrosilylation to prevent surface oxidation and limit the effects of surface trapping of photoexcited carriers. All samples used in the study exhibited relatively efficient photoluminescence. The Auger lifetimes increased with nanorod length, and the nanorods exhibited higher CM quantum yield and efficiency than the nanocrystals with a similar band gap energy E_g. Beyond a critical length, the CM quantum yield decreases. Nanorods with the aspect ratio of 19.3 had the highest CM quantum yield of 1.6 ± 0.2 at 2.9E_g, which corresponded to a multiexciton yield that was twice as high as observed for the spherical nanocrystals.

Synthesis and Characterization of Ultrathin Silver Sulfide Nanoplatelets

We report the synthesis of ultrathin silver sulfide (Ag₂S) nanoplatelets (NPLs) synthesized via a one-pot method in ethylene glycol with 3-mercaptopropionic acid serving as both the sulfur precursor and the platelet ligand. The colloidally synthesized nanoplatelets are exceptionally thin, with a thickness of only 3.5 ± 0.2 Å and a 1S exciton Bohr diameter to confinement ratio of ∼12.6. The NPL growth is shown to be quantized by layer thickness using absorption and photoluminescence (PL) spectroscopy. Transmission electron microscopy, atomic force microscopy, and X-ray diffraction analyses of the NPLs show that they correspond to the (202) plane of the β-Ag₂S structure. The PL quantum yield of these NPLs is ∼30%, suggesting their potential use in biomedical imaging. Optoelectronic properties were evaluated via sensitized photocurrent spectroscopy with the resulting spectra closely matching the distinctive absorption spectral shape of the Ag₂S NPLs.

Enhanced Long-term and Thermal Stability of Polymer Solar Cells in Air at High Humidity with the Formation of Unusual Quantum Dot Networks

Due to the practical applications of polymer solar cells (PSCs), their stability recently has received increasing attention. Herein, a new strategy was developed to largely enhance the long-term and thermal stability of PSCs in air with a relatively high humidity of 50-60% without any encapsulation. In this strategy, semiconductor PbS/CdS core/shell quantum dots (QDs) were incorporated into the photoactive blend of poly(3-hexylthiophene) (P3HT) and phenyl-C₆₁-butyric acid methyl ester (PCBM). By replacing the initial ligands of oleic acid with halide ligands on the surface of PbS/CdS QDs via solution-phase ligand exchange, we were able to form unusual, continuous QD networks in the film of P3HT:PCBM, which effectively stabilized the photoactive layer. Air-processed PSCs based on the stabilized P3HT:PCBM film showed excellent long-term stability under high humidity, providing over 3% of power conversion efficiency (PCE) simultaneously. Around 91% of pristine PCE was retained after 30 days storage in high-humidity air without encapsulation. This constitutes a remarkable improvement compared to ∼53% retained PCE for the QD-free devices, which can be ascribed to the efficient suppression of both PCBM aggregation and oxidation of the thiophene ring in P3HT, thanks to the formation of robust QD networks. Furthermore, the presence of QD networks was able to enhance the stability of the P3HT:PCBM film against thermal stress/oxidation under high-humidity environment (50-60%) as well. The device kept 60% of pristine PCE after thermal treatment for 12 h at 85 °C in air, which is more than twice higher than that for the QD-free device. To the best of our knowledge, the work represents the first unambiguous demonstration of the formation of QD networks in the photoactive layer and of their important contribution to the stability of PSCs. This strategy is highly promising for other fullerene-based PSCs and opens a new avenue toward achieving PSCs with high PCE and excellent stability.

Fully-Inorganic Trihalide Perovskite Nanocrystals: A New Research Frontier of Optoelectronic Materials

All-inorganic trihalide perovskite nanocrystals (NCs) are emerging as a new class of superstar semiconductors with excellent optoelectronic properties and great potential for a broad range of applications in lighting, lasing, photon detection, and photovoltaics. This article provides an up-to-date review on the developments of fully-inorganic trihalide perovskite NCs by emphasizing their controllable solution fabrication strategies, structural phase transformation, tunable optoelectronic properties, stability, as well as their photovoltaic and optoelectronic applications. Among the properties to be surveyed, particular focus is on the size-, shape-, and composition-dependent photoluminescence properties. Finally, by identifying new challenges, suggestions are provided for further research and potential development of this area.

Improvement of Photovoltaic Performance of Colloidal Quantum Dot Solar Cells Using Organic Small Molecule as Hole-Selective Layer

A novel organic small molecule bis-triphenylamine with spiro(fluorene-9,9'-xanthene) as the conjugated system, named BTPA-4, is successfully synthesized and employed as the hole-selective layer (HSL) in colloidal quantum dots solar cells (CQDSCs). The introduction of BTPA-4 layer can significantly prolong effective carrier lifetime (τ_eff), increase charge recombination resistance (R_rec), and thus diminish the interfacial charge recombination at the PbS-QDs/Au electrode interface. The effect of BTPA-4 as HSL in the device performance is especially significant for the open-circuit voltage (V_oc) and power conversion efficiency (PCE), with a ∼ 10% and 15% enhancement respectively, comparing with those of device without the HSL. Furthermore, the PbS CQDSCs with BTPA-4 possessed a noticeably stable property for over 100 days of storage under ambient atmosphere.

Theoretically Manipulating Quantum Dots on Two-Dimensional TiO2 Monolayer for Effective Visible Light Absorption

Low solar energy harvesting and conversion efficiency has become a major problem in solar energy science and engineering owing to the difficulty in capturing solar energy across the wide solar spectrum, especially in the visible light range. Inspired by the extraordinary properties of materials arising from decreased dimensions, in this study, we explore a nanocontact system formed by a two-dimensional (2D) TiO₂ monolayer and II-VI semiconductor (CdX)₁₃ (X = S, Se, and Te) nanocages for engineering the visible light absorption. The nanocontact system, via either Ti-X or Cd-O bond coupling mechanism, forms an ideal type II band alignment, where the stronger donor-acceptor coupling in the Ti-X contact system more efficiently relaxes the coupled geometry and helps it to couple to more electrons, therefore leading to an enhancement of the absorption peaks in the visible frequency range. On changing the element X in (CdX)₁₃ from S to Se then Te, a red shift of the visible light absorption peaks accompanied by stimulating optical response of the whole nanocontact system was observed. Nanocontacting semiconductors comprising low-dimensional (CdX)₁₃ nanocage@TiO₂ monolayer systems, which promote charge separation and optical absorption in the visible range that arise from the effects of adsorbent nature, decreasing size, and efficient interfacial coupling mechanism, are therefore promising photovoltaic and photocatalytic materials.

Alloying Strategy in Cu-In-Ga-Se Quantum Dots for High Efficiency Quantum Dot Sensitized Solar Cells

I-III-VI₂ group "green" quantum dots (QDs) are attracting increasing attention in photoelectronic conversion applications. Herein, on the basis of the "simultaneous nucleation and growth" approach, Cu-In-Ga-Se (CIGSe) QDs with light harvesting range of about 1000 nm were synthesized and used as sensitizer to construct quantum dot sensitized solar cells (QDSCs). Inductively coupled plasma atomic emission spectrometry (ICP-AES), wild-angle X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) analyses demonstrate that the Ga element was alloyed in the Cu-In-Se (CISe) host. Ultraviolet photoelectron spectroscopy (UPS) and femtosecond (fs) resolution transient absorption (TA) measurement results indicate that the alloying strategy could optimize the electronic structure in the obtained CIGSe QD material, thus matching well with TiO₂ substrate and favoring the photogenerated electron extraction. Open circuit voltage decay (OCVD) and impedance spectroscopy (IS) tests indicate that the intrinsic recombination in CIGSe QDSCs was well suppressed relative to that in CISe QDSCs. As a result, CIGSe based QDSCs with use of titanium mesh supported mesoporous carbon counter electrode exhibited a champion efficiency of 11.49% (J_sc = 25.01 mA/cm², V_oc = 0.740 V, FF = 0.621) under the irradiation of full one sun in comparison with 9.46% for CISe QDSCs.

Performance of colloidal CdS sensitized solar cells with ZnO nanorods/nanoparticles

As an alternative photosensitizer in dye-sensitized solar cells, bovine serum albumin (BSA) (a nonhazardous protein) was used in the synthesis of colloidal CdS nanoparticles (NPs). This system has been employed to replace the commonly used N719 dye molecule. Various nanostructured forms of ZnO, namely, nanorod and nanoparticle-based photoanodes, have been sensitized with colloidal CdS NPs to evaluate their effective performance towards quantum dot sensitized solar cells (QDSSCs). A polysulphide (S _x^2-)-based electrolyte and Cu _x S counter electrode were used for cell fabrication and testing. An interesting improvement in the performance of the device by imposing nanorods as a scattering layer on a particle layer has been observed. As a consequence, a maximum conversion efficiency of 1.06% with an open-circuit voltage (V_OC) of 0.67 V was achieved for the ZnO nanorod/nanoparticle assembled structure. The introduction of ZnO nanorods over the nanoparticle led to a significant enhancement of the overall efficiency compared to the corresponding bare nanoparticles.

High performance of TiO2/CdS quantum dot sensitized solar cells with a Cu–ZnS passivation layer

A Cu–ZnS passivation layer effectively suppresses the charge recombination and increases the light harvesting in QDSSCs.

Optimizing photovoltaic performance in CuInS2 and CdS quantum dot-sensitized solar cells by using an agar-based gel polymer electrolyte

Quantum dot-sensitized solar cells (QDSSCs) offer new opportunities to address the clean energy challenge, being one of the top candidates for third generation photovoltaics.

Nitrogen-Doped Carbon Dots for "green" Quantum Dot Solar Cells

Considering the environment protection, "green" materials are increasingly explored for photovoltaics. Here, we developed a kind of quantum dots solar cell based on nitrogen-doped carbon dots. The nitrogen-doped carbon dots were prepared by direct pyrolysis of citric acid and ammonia. The nitrogen-doped carbon dots' excitonic absorption depends on the N-doping content in the carbon dots. The N-doping can be readily modified by the mass ratio of reactants. The constructed "green" nitrogen-doped carbon dots solar cell achieves the best power conversion efficiency of 0.79 % under AM 1.5 G one full sun illumination, which is the highest efficiency for carbon dot-based solar cells.

Highly Monodispersed PbS Quantum Dots for Outstanding Cascaded-Junction Solar Cells

High-performance cascaded-junction quantum dot solar cells (CJQDSCs) are fabricated from as-prepared highly monodispersed lead sulfide QDs. The cells have a high power conversion of 9.05% and a short-circuit current density of 32.51 mA cm^-2. A reliable and effective stratagem for fabricating high-quality lead sulfide quantum dots (QD) is explored through a "monomer" concentration-controlled experiment. Robust QDSC performances with different band gaps are demonstrated from the as-proposed synthesis and processing stratagems. Various potential CJQDSCs can be envisioned from the band edge evolution of the QDs as a function of size and ligands reported here.