High performance PbS Quantum Dot Sensitized Solar Cells exceeding 4% efficiency: the role of metal precursors in the electron injection and charge separation

Enhanced film quality of PbS QD solid by eliminating the oxide traps through an in situ surface etching and passivation

PbS QDs have attracted considerable interest in optoelectronics. However, their high susceptibility to oxidation results in the production of Pb oxides on PbS, which can induce sub-bandgap traps in PbS QDs that are detrimental to the performance of the resultant device. Here we report a facile strategy to enhance the film quality of PbS QD solids through an in situ surface etching and passivation route, carried out by immersing the PbS QD solid film in an I^-/I₂ solution at room temperature in ambient air. The process is simple and allows for the simultaneous removal of surface Pb oxides and the formation of a PbI₂ passivation layer on PbS QDs, leading to the elimination of traps in PbS QDs while preserving their optical properties and film morphology. As a result, charge recombination within the film is suppressed and charge carrier transport is enhanced. When used to fabricate a quantum dot sensitized solar cell, a large increase in cell performance was achieved.

Quantum Dot Sensitized Solar Cell: Photoanodes, Counter Electrodes, and Electrolytes

In this study, we provide the reader with an overview of quantum dot application in solar cells to replace dye molecules, where the quantum dots play a key role in photon absorption and excited charge generation in the device. The brief shows the types of quantum dot sensitized solar cells and presents the obtained results of them for each type of cell, and provides the advantages and disadvantages. Lastly, methods are proposed to improve the efficiency performance in the next researching.

Adsorption and Cation-Exchange Behavior of Zinc Sulfide on Mesoporous TiO2 Film and Its Applications to Solar Cells

Zinc sulfide (ZnS) was deposited onto the surface of mesoporous TiO₂ film by a typical successive ionic layer adsorption and reaction (SILAR) process. By inducing a spontaneous cation exchange between ZnS and a target cation (Pb²⁺, Cu²⁺, Ag⁺, or Bi³⁺) dissolved in a chemical bath when they are in contact, it was demonstrated successfully that white translucent ZnS on the substrate could be changed to new brown-colored metal chalcogenides and the amount of ZnS deposited originally by different conditions could be compared in a qualitative way with the degree of color change. By utilizing this simple but effective process, the evolution of a well-known ZnS passivation layer prepared from different chemical baths in quantum dot (QD)-sensitized solar cells could be tracked visually by checking the degree of color change of TiO₂/ZnS electrodes after the induced specific cation exchange. When applied to representative CdS QD-sensitized solar cells, it was revealed clearly how the different degrees and rates of ZnS deposition could affect the overall power conversion efficiency while finding an optimized passivation layer over TiO₂/CdS electrode. An acetate anion-coupled Zn²⁺ source was observed to give a much faster deposition of a ZnS passivation layer than a nitrate anion one because of its higher pH-induced more-favorable adsorption of Zn²⁺ on the surface of TiO₂. As another useful application of the ZnS-based cation exchange, as-deposited ZnS was used as a template for preparing a more complex metal chalcogenide onto a mesoporous TiO₂ film. The ZnS-derived Sb₂S₃-sensitized electrode showed a promising initial result of over 1.0% overall power conversion efficiency with a very thin ZrO₂ passivation layer between TiO₂ and Sb₂S₃.

Evidence of Skewness and Sub-Gaussian Character in Temperature-Dependent Distributions of One Million Electronic Excitation Energies in PbS Quantum Dots

Obtaining statistical distributions by sampling a large number of conformations is vital for an accurate description of temperature-dependent properties of chemical systems. However, constructing distributions with 10⁵-10⁶ samples is computationally challenging because of the prohibitively high computational cost of performing first-principles quantum mechanical calculations. In this work, we present a new technique called the effective stochastic potential configuration interaction singles (ESP-CIS) method to obtain excitation energies. The ESP-CIS method uses random matrix theory for the construction of an effective stochastic representation of the Fock operator and combines it with the CIS method. Excited-state energies of PbS quantum dots (0.75-1.75 nm) at temperatures of 10-400 K were calculated using the ESP-CIS method. Results from a total of 27 million excitation energy calculations revealed the distributions to be sub-Gaussian in nature with negative skewness, which progressively became red-shifted with increasing temperature. This study demonstrates the efficacy of the ESP-CIS method as a general-purpose method for efficient excited-state calculations.

Bandgap Tunable Ternary Cd x Sb2-y S3-δ Nanocrystals for Solar Cell Applications

We report the synthesis and photovoltaic performance of a new nonstoichiometric ternary metal sulfide alloyed semiconductor-Cd _x Sb_2-y S_3-δ nanocrystals prepared by the two-stage sequential ionic layer adsorption reaction technique. The synthesized Cd _x Sb_2-y S_3-δ nanocrystals retain the orthorhombic structure of the host Sb₂S₃ with Cd substituting a fraction (x = 0-0.15) of the cationic element Sb. The Cd _x Sb_2-y S_3-δ lattice expands relative to the host, Sb₂S_3, with its lattice constant a increasing linearly with Cd content x. Optical and external quantum efficiency (EQE) spectra revealed that the bandgap E _g of Cd _x Sb_2-y S_3-δ decreased from 1.99 to 1.69 eV (i.e., 625-737 nm) as x increased from 0 to 0.15. Liquid-junction Cd _x Sb_2-y S_3-δ quantum dot-sensitized solar cells were fabricated using the polyiodide electrolyte. The best cell yielded a power conversion efficiency (PCE) of 3.72% with the photovoltaic parameters of J _sc = 15.97 mA/cm², V _oc = 0.50 V, and FF = 46.6% under 1 sun. The PCE further increased to 4.86%, a respectable value for a new solar material, under a reduced light intensity of 10% sun. The PCE (4.86%) and J _sc (15.97 mA/cm²) are significantly larger than that (PCE = 1.8%, J _sc = 8.55 mA/cm²) of the Sb₂S₃ host. Electrochemical impedance spectroscopy showed that the ZnSe passivation coating increased the electron lifetime by three times. The EQE spectrum of Cd _x Sb_2-y S_3-δ has a maximal EQE of 82% at λ = 350 nm and covers the spectral range of 300-750 nm, which is significantly broader than that (300-625 nm) of the Sb₂S₃ host. The EQE-integrated current density yields a J _ph of 11.76 mA/cm². The tunable bandgap and a respectable PCE near 5% suggest that Cd _x Sb_2-y S_3-δ could be a potential candidate for a solar material.

Review of Core/Shell Quantum Dots Technology Integrated into Building’s Glazing

Skylights and windows are building openings that enhance human comfort and well-being in various ways. Recently, a massive drive is witnessed to replace traditional openings with building integrated photovoltaic (BIPV) systems to generate power in a bid to reduce buildings’ energy. The problem with most of the BIPV glazing lies in the obstruction of occupants’ vision of the outdoor view. In order to resolve this problem, new technology has emerged that utilizes quantum dots semiconductors (QDs) in glazing systems. QDs can absorb and re-emit the incoming radiation in the desired direction with the tunable spectrum, which renders them favorable for building integration. By redirecting the radiation towards edges of the glazing, they can be categorized as luminescent solar concentrators (QD-LSCs) that can help to generate electricity while maintaining transparency in the glazing. The aim of this paper is to review the different properties of core/shell quantum dots and their potential applications in buildings. Literature from various disciplines was reviewed to establish correlations between the optical and electrical properties of different types, sizes, thicknesses, and concentration ratios of QDs when used in transparent glazing. The current article will help building designers and system integrators assess the merits of integrating QDs on windows/skylights with regards to energy production and potential impact on admitted daylighting and visual comfort.

Hidden energy levels? Carrier transport ability of CdS/CdS1-xSex quantum dot solar cells impacted by Cd-Cd level formation

In quantum dot sensitized solar cells (QDSSC), a cascade energy level structure controlled by assembly of cadmium-chalcogenide quantum dots can remarkably improve the sunlight harvesting and charge carrier lifetime. Despite the advantages of using co-sensitizers, energy conversion efficiencies are still low. An increased understanding of the causes of the low photoconversion efficiency (PCE) will contribute to the development of a straightforward approach to improve solar cell performance by exploiting co-sensitization. Herein we discuss how an excess of cadmium causes structural disorder and defect levels impacting the PCE of QDSSC devices. Thus, outer CdS1-xSex/inner CdS QD-co-sensitized B,N,F-co-doped-TiO2 nanotubes (BNF-TNT) were prepared. Chalcogenides were deposited by the SILAR method on BNF-TNT, varying the load of CdS as the inner sensitizer, while for CdS1-xSex, five SILAR cycles were used (5-CdS1-xSex), controlling the nominal S/Se molar ratio of the ternary alloy. Cd defects named as Cd-Cd energy levels were observed during CdS sensitization. Although incorporation of outer CdS1-xSex provides a tunable band gap to achieve good band alignment for carrier separation, Cd-Cd energy levels in the sensitizers act as recombination centers, limiting the overall electron flow at the BNF-TNT/CdS/CdS1-xSex interface. A maximum PCE of 2.58% was reached under standard AM 1.5G solar illumination at 100 mW cm-2. Additional limitations of SILAR as a deposition strategy of QDs are also found to influence the PCE of QDSSC.

Ag8SnS6: a new IR solar absorber material with a near optimal bandgap

We report the synthesis and photovoltaic properties of a new ternary solar absorber – Ag₈SnS₆ nanocrystals prepared by successive ionic layer adsorption reaction (SILAR) technique. The synthesized Ag₈SnS₆ nanocrystals have a bandgap E_g of 1.24–1.41 eV as revealed from UV-Vis and external quantum efficiency (EQE) measurements. Its photovoltaic properties were characterized by assembling a liquid-junction Ag₈SnS₆ sensitized solar cell for the first time. The best cell yielded a J_sc of 9.29 mA cm⁻², a V_oc of 0.23 V, an FF of 31.3% and a power conversion efficiency (PCE) of 0.64% under 100% incident light illumination using polysulfide electrolyte and Au counter electrode. The efficiency improved to 1.43% at a reduced light intensity of 10% sun. When the polysulfide was replaced by a cobalt electrolyte with a lower redox level, the V_oc increased to 0.54 V and PCE increased to 2.29% under 0.1 sun, a respectable efficiency for a new solar material. The EQE spectrum covers the spectral range of 300–1000 nm with a maximum EQE of 77% at λ = 600 nm. The near optimal E_g and the respectable photovoltaic performance suggest that Ag₈SnS₆ nanocrystals have potential to be an efficient IR solar absorber.

We report the synthesis and photovoltaic properties of a new ternary solar absorber – Ag₈SnS₆ nanocrystals prepared by successive ionic layer adsorption reaction (SILAR) technique.

ZnS/SiO2 Passivation Layer for High-Performance of TiO2/CuInS2 Quantum Dot Sensitized Solar Cells

Suppressing the charge recombination at the interface of photoanode/electrolyte is the crucial way to improve the quantum dot sensitized solar cells (QDSSCs) performance. In this scenario, ZnS/SiO2 blocking layer was deposited on TiO2/CuInS2 QDs to inhibit the charge recombination at photoanode/electrolyte interface. As a result, the TiO2/CuInS2/ZnS/SiO2 based QDSSCs delivers a power conversion efficiency (η) value of 4.63%, which is much higher than the TiO2/CuInS2 (2.15%) and TiO2/CuInS2/ZnS (3.23%) based QDSSCs. Impedance spectroscopy and open circuit voltage decay analyses indicate that ZnS/SiO2 passivation layer on TiO2/CuInS2 suppress the charge recombination at the interface of photoanode/electrolyte and enhance the electron lifetime.

Inorganic Ligand Thiosulfate-Capped Quantum Dots for Efficient Quantum Dot Sensitized Solar Cells

The insulating nature of organic ligands containing long hydrocarbon tails brings forward serious limitations for presynthesized quantum dots (QDs) in photovoltaic applications. Replacing the initial organic hydrocarbon chain ligands with simple, cheap, and small inorganic ligands is regarded as an efficient strategy for improving the performance of the resulting photovoltaic devices. Herein, thiosulfate (S₂O₃^2-), and sulfide (S^2-) were employed as ligand-exchange reagents to get access to the inorganic ligand S₂O₃^2-- and S^2--capped CdSe QDs. The obtained inorganic ligand-capped QDs, together with the initial oleylamine-capped QDs, were used as light-absorbing materials in the construction of quantum dot sensitized solar cells (QDSCs). Photovoltaic results indicate that thiosulfate-capped QDs give excellent power conversion efficiency (PCE) of 6.11% under the illumination of full one sun, which is remarkably higher than those of sulfide- (3.36%) and OAm-capped QDs (0.84%) and is comparable to the state-of-the-art value based on mercaptocarboxylic acid capped QDs. Photoluminescence (PL) decay characterization demonstrates that thiosulfate-based QDSCs have a much-faster electron injection rate from QD to TiO₂ substrate in comparison with those of sulfide- and OAm-based QDSCs. Electrochemical impedance spectroscopy (EIS) results indicate that higher charge-recombination resistance between potoanode and eletrolyte interfaces were observed in the thiosulfate-based cells. To the best of our knowledge, this is the first application of thiosulfate-capped QDs in the fabrication of efficient QDSCs. This will lend a new perspective to boosting the performance of QDSCs furthermore.

Lead tin sulfide (Pb1-xSnxS) nanocrystals: A potential solar absorber material

We present a new ternary semiconductor absorber material - Pb_1-xSn_xS - for solar cells. Pb_1-xSn_xS nanocrystals (NCs) were synthesized using the successive ionic layer adsorption reaction (SILAR) process. Energy-dispersive X-ray spectroscopy revealed the Sn ratio for a sample prepared with five SILAR cycles to be x=0.55 (i.e. non-stoichiometric formula Pb_0.45Sn_0.55S). The optical spectra revealed that the energy gap E_g of the Pb_1-xSn_xS NCs decreased with an increasing number of SILAR cycles n, with E_g=1.67eV for the sample with n=5. Liquid-junction Pb_1-xSn_xS quantum dot-sensitized solar cells were fabricated using the polysulfide electrolyte. The best cell yielded a short-circuit current density J_sc of 10.1mA/cm², an open circuit voltage of 0.43V, a fill factor FF of 50% and an efficiency of 2.17% under 1 sun. The external quantum efficiency spectrum (EQE) covered a spectral range of 300-800nm with a maximum EQE of ∼67% at λ=650nm. At the reduced light of 0.1 sun, the efficiency increased to 3.31% (with a normalized J_sc=17.7mA/cm²) - a respectable efficiency for a new sensitizer. This work demonstrates that Pb_1-xSn_xS shows potential as a solar cell absorber.

Triumphing over Charge Transfer Limitations of PEDOT Nanofiber Reduction Catalyst by 1,2-Ethanedithiol Doping for Quantum Dot Solar Cells

Charge transfer between a conducting polymer-based counter electrode (CE) and a polysulfide (S^2-/S_n^2-) electrolyte mediator is a key limitation to improvements of solar energy conversion efficiency (ECE) in quantum-dot-sensitized solar cells (QDSCs). In this paper, 1,2-ethanedithiol (EDT) was doped into nanofibrous poly(3,4-ethylenedioxythiophene) (PEDOT NF) to overcome the charge transfer limitation between PEDOT NF and S^2-/S_n^2-. EDT not only helps to reduce the aggregation and thus enhance the linearization of the PEDOT chains but also changes the molecular conformation of the PEDOT chains from a benzoid to a quinoid structure. EDT-doped PEDOT NF-based CEs showed almost 3.7 times higher conductivity, better electrocatalytic activity, and improved compatibility with S^2-/S_n^2- in an aqueous electrolyte. As a result, the charge transfer resistance between the polymer-based CE and the S^2-/S_n^2- electrolyte was significantly reduced, resulting in over 3% ECE in QDSCs, more than double that of a bare PEDOT NF-based CE.

Mn doped CdS passivated CuInSe2 quantum dot sensitized solar cells with remarkably enhanced photovoltaic efficiency

This paper explores that novel architecture of CuInSe₂/Mn-CdS exhibits remarkable enhancement in photovoltaic performance of the QDSSCs, which presents an excellent power conversion efficiency of 3.96%.

Assembly of CdS nanoparticles on boron and fluoride co-doped TiO2 nanofilm for solar energy conversion applications

Boron and fluoride co-doped TiO₂ nanomaterial is successfully synthetized using a facile process, followed by chemical bath deposition in an organic solution to ensure high wettability and superior penetration ability of the B/F co-doped TiO₂ films.

Improving the Photocurrent in Quantum-Dot-Sensitized Solar Cells by Employing Alloy PbxCd1-xS Quantum Dots as Photosensitizers

Ternary alloy Pb_xCd_1−xS quantum dots (QDs) were explored as photosensitizers for quantum-dot-sensitized solar cells (QDSCs). Alloy Pb_xCd_1−xS QDs (Pb_0.54Cd_0.46S, Pb_0.31Cd_0.69S, and Pb_0.24Cd_0.76S) were found to substantially improve the photocurrent of the solar cells compared to the single CdS or PbS QDs. Moreover, it was found that the photocurrent increases and the photovoltage decreases when the ratio of Pb in Pb_xCd_1−xS is increased. Without surface protecting layer deposition, the highest short-circuit current density reaches 20 mA/cm² under simulated AM 1.5 illumination (100 mW/cm²). After an additional CdS coating layer was deposited onto the Pb_xCd_1−xS electrode, the photovoltaic performance further improved, with a photocurrent of 22.6 mA/cm² and an efficiency of 3.2%.

Enhanced Performance of PbS-quantum-dot-sensitized Solar Cells via Optimizing Precursor Solution and Electrolytes

This work reports a PbS-quantum-dot-sensitized solar cell (QDSC) with power conversion efficiency (PCE) of 4%. PbS quantum dots (QDs) were grown on mesoporous TiO₂ film using a successive ion layer absorption and reaction (SILAR) method. The growth of QDs was found to be profoundly affected by the concentration of the precursor solution. At low concentrations, the rate-limiting factor of the crystal growth was the adsorption of the precursor ions, and the surface growth of the crystal became the limiting factor in the high concentration solution. The optimal concentration of precursor solution with respect to the quantity and size of synthesized QDs was 0.06 M. To further increase the performance of QDSCs, the 30% deionized water of polysulfide electrolyte was replaced with methanol to improve the wettability and permeability of electrolytes in the TiO₂ film, which accelerated the redox couple diffusion in the electrolyte solution and improved charge transfer at the interfaces between photoanodes and electrolytes. The stability of PbS QDs in the electrolyte was also improved by methanol to reduce the charge recombination and prolong the electron lifetime. As a result, the PCE of QDSC was increased to 4.01%.

Improved charge separation and transport efficiency in panchromatic-sensitized solar cells with co-sensitization of PbS/CdS/ZnS quantum dots and dye molecules

Schematic energy diagram of carrier generation, transfer, and recombination in the TiO₂/PbS/CdS/ZnS/N719 film.

Improving photoelectrochemical performance of highly-ordered TiO2 nanotube arrays with cosensitization of PbS and CdS quantum dots

PbS and CdS quantum dots (QDs) were deposited on TiO₂ nanotube arrays (TNTAs) by a sonication-assisted successive ionic layer adsorption and reaction (S-SILAR) method.

Optimizing Photovoltaic Response by Tuning Light-Harvesting Nanocrystal Shape Synthesized Using a Quick Liquid-Gas Phase Reaction

The electron recombination lifetime in a sensitized semiconductor assembly is greatly influenced by the crystal structure and geometric form of the light-harvesting semiconductor nanocrystal. When such light harvesters with varying structural characteristics are configured in a photoanode, its interface with the electrolyte becomes equally important and directly influences the photovoltaic efficiency. We have systematically probed here the influence of nanocrystal crystallographic structure and shape on the electron recombination lifetime and its eventual influence on the light to electricity conversion efficiency of a liquid junction semiconductor sensitized solar cell. The light-harvesting cadmium sulfide (CdS) nanocrystals of distinctly different and controlled shapes are obtained using a novel and simple liquid-gas phase synthesis method performed at different temperatures involving very short reaction times. High-resolution synchrotron X-ray diffraction and spectroscopic studies respectively exhibit different crystallographic phase content and optical properties. When assembled on a mesoscopic TiO2 film by a linker molecule, they exhibit remarkable variation in electron recombination lifetime by 1 order of magnitude, as determined by ac-impedance spectroscopy. This also drastically affects the photovoltaic efficiency of the differently shaped nanocrystal sensitized solar cells.

Highly Efficient Copper-Indium-Selenide Quantum Dot Solar Cells: Suppression of Carrier Recombination by Controlled ZnS Overlayers

Copper-indium-selenide (CISe) quantum dots (QDs) are a promising alternative to the toxic cadmium- and lead-chalcogenide QDs generally used in photovoltaics due to their low toxicity, narrow band gap, and high absorption coefficient. Here, we demonstrate that the photovoltaic performance of CISe QD-sensitized solar cells (QDSCs) can be greatly enhanced simply by optimizing the thickness of ZnS overlayers on the QD-sensitized TiO2 electrodes. By roughly doubling the thickness of the overlayers compared to the conventional one, conversion efficiency is enhanced by about 40%. Impedance studies reveal that the thick ZnS overlayers do not affect the energetic characteristics of the photoanode, yet enhance the kinetic characteristics, leading to more efficient photovoltaic performance. In particular, both interfacial electron recombination with the electrolyte and nonradiative recombination associated with QDs are significantly reduced. As a result, our best cell yields a conversion efficiency of 8.10% under standard solar illumination, a record high for heavy metal-free QD solar cells to date.

CuS/CdS Quantum Dot Composite Sensitizer and Its Applications to Various TiO2 Mesoporous Film-Based Solar Cell Devices

A nanoscale composite sensitizer composed of CuS and CdS quantum dots (QDs) was prepared by a simple but effective layer-by-layer reaction between a metal cation (Cu(2+) or Cd(2+)) and a sulfide anion (S(2-)). The as-prepared composite CuS/CdS QD sensitizer displayed an enhanced photon-to-current conversion over the sensitizing range of the visible spectrum compared to the counterpart of the pure CdS sensitizer. At the optimized ratio of the deposited amounts of CuS and CdS, the best CuS/CdS-sensitized mesoporous TiO2 cell with a polysulfide electrolyte showed an overall power conversion efficiency of 3.60% with a short circuit current (Jsc) of 11.77 mA/cm(2), an open circuit voltage (Voc) of 0.65 V, and a fill factor (FF) of 0.47. From the transmission electron microscopy images, the initially deposited CuS seemed to take a nucleation site to accumulate more CdS in the later deposition. The kinetic studies by impedance and Voc decay measurements also revealed that the CuS/CdS and CdS QD sensitizers made a similar interface between TiO2 and the electrolyte, but the former had a larger resistance of charge transfer with a longer lifetime of excitons after light absorption than the latter. To enhance the sensitizing power further, a multilayer QD sensitizer of CuS/CdS/CdSe was prepared by successive ionic layer adsorption and reaction (SILAR). This led to the best performance of 4.32% overall power conversion efficiency. Finally, a hybrid sensitizing system of inorganic QD (CuS/CdS) and organic dye (coded MK-2) was tested with a [Co(bpy)3](2+/3+) redox mediator. The CuS/CdS/MK-2 dye-sensitized cell showed over 3.0% efficiency under the standard illumination condition (1 sun).

Modulating Exciton Dynamics in Composite Nanocrystals for Excitonic Solar Cells

Quantum dots (QDs) represent one of the most promising materials for third-generation solar cells due to their potential to boost the photoconversion efficiency beyond the Shockley-Queisser limit. Composite nanocrystals can challenge the current scenario by combining broad spectral response and tailored energy levels to favor charge extraction and reduce energy and charge recombination. We synthesized PbS/CdS QDs with different compositions at the surface of TiO2 nanoparticles assembled in a mesoporous film. The ultrafast photoinduced dynamics and the charge injection processes were investigated by pump-probe spectroscopy. We demonstrated good injection of photogenerated electrons from QDs to TiO2 in the PbS/CdS blend and used the QDs to fabricate solar cells. The fine-tuning of chemical composition and size of lead and cadmium chalcogenide QDs led to highly efficient PV devices (3% maximum photoconversion efficiency). This combined study paves the way to the full exploitation of QDs in next-generation photovoltaic (PV) devices.

Ag plasmonic nanostructures and a novel gel electrolyte in a high efficiency TiO2/CdS solar cell

A novel photoanode architecture with plasmonic silver (Ag) nanostructures embedded in titania (TiO2), which served as the wide band gap semiconducting support and CdS quantum dots (QDs), as light absorbers, is presented. Ag nanostructures were prepared by a polyol method and are comprised of clumps of nanorods, 15-35 nm wide, interspersed with globular nanoparticles and they were characterized by a face centered cubic lattice. Optimization of Ag nanostructures was achieved on the basis of a superior power conversion efficiency (PCE) obtained for the cell with a Ag/TiO2/CdS electrode encompassing a mixed morphology of Ag nano-rods and particles, relative to analogous cells with either Ag nanoparticles or Ag nanorods. Interfacial charge transfer kinetics was unraveled by fluorescence quenching and lifetime studies. Ag nanostructures improve the light harvesting ability of the TiO2/CdS photoanode via (a) plasmonic and scattering effects, which induce both near- and far-field enhancements which translate to higher photocurrent densities and (b) charging effects, whereby, photoexcited electron transfer from TiO2 to Ag is facilitated by Fermi level equilibration. Owing to the spectacular ability of Ag nanostructures to increase light absorption, a greatly increased PCE of 4.27% and a maximum external quantum efficiency of 55% (at 440 nm) was achieved for the cell based on Ag/TiO2/CdS, greater by 42 and 66%, respectively, compared to the TiO2/CdS based cell. In addition, the liquid S(2-) electrolyte was replaced by a S(2-) gel containing fumed silica, and the redox potential, conductivity and p-type conduction of the two were deduced to be comparable. Although the gel based cells showed diminished solar cell performances compared to their liquid counterparts, nonetheless, the Ag/TiO2/CdS electrode continued to outperform the TiO2/CdS electrode. Our studies demonstrate that Ag nanostructures effectively capture a significant chunk of the electromagnetic spectrum and aid QD solar cells in delivering high power conversion efficiencies.

Capping Ligand-Induced Self-Assembly for Quantum Dot Sensitized Solar Cells

Quantum dot-sensitized solar cells (QDSCs), having the advantages of low-cost assembling process, economically viable materials and intrinsic optoelectronic properties of QD sensitizers, are regarded as attractive candidates for the third-generation solar cells. In spite of the previous unsatisfied performance resulted from poor sensitization, an increasing power conversion efficiency has been experimentally confirmed with the development of effective deposition approaches in the last five years. In this Perspective article, we present an overview on versatile QD deposition methods, regarding mainly the effective loading of QDs and surface chemistry issues. Linker-assisted assembly, a most efficient sensitizer deposition approach to achieve fast, uniform and dense coverage of the sensitizers on mesoporous TiO2 film electrode, will be discussed with emphasis. Recent advances based on this deposition technique in achieving high efficiency are presented. Also, combined efforts regarding the overall improvement of the device have been discussed to provide more possible access to higher power conversion efficiencies of the QDSCs.

Band engineering in core/shell ZnTe/CdSe for photovoltage and efficiency enhancement in exciplex quantum dot sensitized solar cells

Even though previously reported CdTe/CdSe type-II core/shell QD sensitizers possess intrinsic superior optoelectronic properties (such as wide absorption range, fast charge separation, and slow charge recombination) in serving as light absorbers, the efficiency of the resultant solar cell is still limited by the relatively low photovoltage. To further enhance photovoltage and cell efficiency accordingly, ZnTe/CdSe type-II core/shell QDs with much larger conduction band (CB) offset in comparison with that of CdTe/CdSe (1.22 eV vs 0.27 eV) are adopted as sensitizers in the construction of quantum dot sensitized solar cells (QDSCs). The augment of band offset produces an increase of the charge accumulation across the QD/TiO2 interface under illumination and induces stronger dipole effects, therefore bringing forward an upward shift of the TiO2 CB edge after sensitization and resulting in enhancement of the photovoltage of the resultant cell devices. The variation of relative chemical capacitance, Cμ, between ZnTe/CdSe and reference CdTe/CdSe cells extracted from impedance spectroscopy (IS) characterization under dark and illumination conditions clearly demonstrates that, under light irradiation conditions, the sensitization of ZnTe/CdSe QDs upshifts the CB edge of TiO2 by the level of ∼ 50 mV related to that in the reference cell and results in the enhancement of V(oc) of the corresponding cell devices. In addition, charge extraction measurements have also confirmed the photovoltage enhancement in the ZnTe/CdSe cell related to reference CdTe/CdSe cell. Furthermore, transient grating (TG) measurements have revealed a faster electron injection rate for the ZnTe/CdSe-based QDSCs in comparison with the CdSe cells. The resultant ZnTe/CdSe QD-based QDSCs exhibit a champion power conversion efficiency of 7.17% and a certified efficiency of 6.82% under AM 1.5 G full one sun illumination, which is, as far as we know, one of the highest efficiencies for liquid-junction QDSCs.

Enhanced performance of PbS-sensitized solar cells via controlled successive ionic-layer adsorption and reaction

The performance of PbS-sensitized solar cells is significantly improved by controlling successive ionic layer adsorption and reaction.

Tunable growth of PbS quantum dot–ZnO heterostructure and mechanism analysis

Tunable growth of a PbS QDs on ZnO heterostructure by a room temperature SILAR method and mechanistic analysis.

Theoretical insight into a novel zinc di-corrole dye with excellent photoelectronic properties for solar cells

A new zinc di-corrole dye has been designed by substitution of Ga with Zn in a Ga di-corrole dye. Its optical and electronic properties were studied by extensive DFT calculations.

Fabrication and photoelectrochemical characteristics of CuInS2 and PbS quantum dot co-sensitized TiO2 nanorod photoelectrodes

A cascade structured PbS/CuInS₂/TiO₂ photoelectrode with co-sensitization effect obtains the energy conversion efficiency of 4.11% under one sun illumination.

Performance enhancement of quantum-dot-sensitized solar cells by potential-induced ionic layer adsorption and reaction

Successive ionic layer adsorption and reaction (SILAR) technique has been commonly adopted to fabricate quantum-dot-sensitized solar cells (QDSSCs) in the literature. However, pore blocking and poor distribution of quantum dots (QDs) in TiO2 matrices were always encountered. Herein, we report an efficient method, termed as potential-induced ionic layer adsorption and reaction (PILAR), for in situ synthesizing and assembling CdSe QDs into mesoporous TiO2 films. In the ion adsorption stage of this process, a negative bias was applied on the TiO2 film to induce the adsorption of precursor ions. The experimental results show that this bias greatly enhanced the ion adsorption, accumulating a large amount of cadmium ions on the film surface for the following reaction with selenide precursors. Furthermore, this bias also drove cations deep into the bottom region of a TiO2 film. These effects not only resulted in a higher deposited amount of CdSe, but also a more uniform distribution of the QDs along the TiO2 film. By using the PILAR process, as well as the SILAR process to replenish the incorporated CdSe, an energy conversion efficiency of 4.30% can be achieved by the CdSe-sensitized solar cell. This performance is much higher than that of a cell prepared by the traditional SILAR process.

Mn(II/III) complexes as promising redox mediators in quantum-dot-sensitized solar cells

The advancement of quantum dot sensitized solar cell (QDSSC) technology depends on optimizing directional charge transfer between light absorbing quantum dots, TiO2, and a redox mediator. The nature of the redox mediator plays a pivotal role in determining the photocurrent and photovoltage from the solar cell. Kinetically, reduction of oxidized quantum dots by the redox mediator should be rapid and faster than the back electron transfer between TiO2 and oxidized quantum dots to maintain photocurrent. Thermodynamically, the reduction potential of the redox mediator should be sufficiently positive to provide high photovoltages. To satisfy both criteria and enhance power conversion efficiencies, we introduced charge transfer spin-crossover Mn(II/III) complexes as promising redox mediator alternatives in QDSSCs. High photovoltages ∼ 1 V were achieved by a series of Mn poly(pyrazolyl)borates, with reduction potentials ∼ 0.51 V vs Ag/AgCl. Back electron transfer (recombination) rates were slower than Co(bpy)3, where bpy = 2,2'-bipyridine, evidenced by electron lifetimes up to 4 orders of magnitude longer. This is indicative of a large barrier to electron transport imposed by spin-crossover in these complexes. Low solubility prevented the redox mediators from sustaining high photocurrent due to mass transport limits. However, with high fill factors (∼ 0.6) and photovoltages, they demonstrate competitive efficiencies with Co(bpy)3 redox mediator at the same concentration. More positive reduction potentials and slower recombination rates compared to current redox mediators establish the viability of Mn poly(pyrazolyl)borates as promising redox mediators. By capitalizing on these characteristics, efficient Mn(II/III)-based QDSSCs can be achieved with more soluble Mn-complexes.