Charge-extraction strategies for colloidal quantum dot photovoltaics

Passivation of 2D Cs2PbI2Cl2 Nanosheets for Efficient and Stable CsPbI3 Quantum Dot Solar Cells

Inorganic CsPbI3 perovskite quantum dots (PQDs) possess remarkable optical properties, making them highly promising for photovoltaic applications. However, the inadequate stability resulting from internal structural instability and the complex external surface chemical environment of CsPbI3 PQDs has hindered the development of CsPbI3 PQD solar cells (PQDSCs). In this work, the capping layer composed of inorganic two-dimensional (2D) Ruddlesden-Popper (RP) phase Cs2PbI2Cl2 nanosheets (NSs) is introduced, which may be effectively treated to improve the surface properties of the CsPbI3 PQD film. This modification serves to passivate defects by filling cesium and iodine vacancies while optimizing the energy band arrangement and preventing humidity intrusion, leading to the meliorative stability and photovoltaic performance. The optimized CsPbI3 PQDSCs achieve an enhanced power conversion efficiency (PCE) of 14.73%, with the superb stability of only a 16% efficiency loss after being exposed to ambient conditions (30 ± 5% RH) for 432 h.

Reducing the Open-Circuit Voltage Loss of PbS Quantum Dot Solar Cells via Hybrid Ligand Exchange Treatment

The interface VOC loss between the active layer and the hole transport layer (HTL) of lead sulfide colloidal quantum dot (PbS-CQD) solar cells is a significant factor influencing the efficiency improvement of PbS colloidal quantum dot solar cells (PbS-CQDSCs). Currently, the most advanced solar cells adopt organic P-type HTLs (PbS-EDT) via solid-state ligand exchange with 1,2-ethanedithiol (EDT) on the CQD top active layer. However, EDT is unable to altogether remove the initial ligand oleic acid from the quantum dot surface, and its high reactivity leads to cracks in the HTL film caused by volume contractions, which inevitably results in significant VOC loss. These flaws prompted this research to develop a method involving hybrid organic ligand exchange using 3-mercaptopropionic acid (MPA) and 1,2-EDT (PbS-Hybrid) to overcome these drawbacks of VOC loss. The results indicated that the new exchange strategy improved the quality of the HTL film and benefited from the enhanced passivation of the quantum dot surface and better alignment of energy levels, and the average VOC of PbS-Hybrid devices is increased by approximately 25 mV compared to control devices. With the enhanced VOC, the average power conversion efficiency (PCE) of the devices is improved by 10%, with the highest PCE reaching 13.24%.

Marcus Theory and Tunneling Method for the Electron Transfer Rate Analysis in Quantum Dot Sensitized Solar Cells in the Presence of Blocking Layer

In this research study, the effects of different parameters on the electron transfer rate from three quantum dots (QDs), CdSe, CdS, and CdTe, on three metal oxides (MOs), TiO2, SnO2, and SnO2, in quantum-dot-sensitized solar cells (QDSSCs) with porous structures in the presence of four types of blocking layers, ZnS, ZnO, TiO2, and Al2O3, are modeled and simulated using the Marcus theory and tunneling between two spheres for the first time. Here, the studied parameters include the change in the type and thickness of the blocking layer, the diameter of the QD, and the temperature effect. To model the effect of the blocking layer on the QD, the effective sphere method is used, and by applying it into the Marcus theory equation and the tunneling method, the electron transfer rate is calculated and analyzed. The obtained results in a wide range of temperatures of 250-400 °K demonstrate that, based on the composition of the MO-QD, the increase in the temperature could reduce or increase the electron transfer rate, and the change in the QD diameter could exacerbate the effects of the temperature. In addition, the results show which type and thickness of the blocking layer can achieve the highest electron transfer rate. In order to test the accuracy of the simulation method, we calculate the electron transfer rate in the presence of a blocking layer for a reported sample of a QDSSC manufacturing work, which was obtained with an error of ~3%. The results can be used to better interpret the experimental observations and to assist with the design and selection of the appropriate combination of MO-QD in the presence of a blocking layer effect.

Surface Stoichiometry Control of Colloidal Heterostructured Quantum Dots for High-Performance Photoelectrochemical Hydrogen Generation

Manipulating the separation and transfer behaviors of charges has long been pursued for promoting the photoelectrochemical (PEC) hydrogen generation based on II-VI quantum dot (QDs), but remains challenging due to the lack of effective strategies. Herein, a facile strategy is reported to regulate the recombination and transfer of interfacial charges through tuning the surface stoichiometry of heterostructured QDs. Using this method, it is demonstrated that the PEC cells based on CdSe-(Se_x S_{1- x} )₄ -(CdS)₂ core/shell QDs with a proper S_surface /Cd_surface ratio exhibits a remarkably improved photocurrent density (≈18.4 mA cm^-2 under one sun illumination), superior to the PEC cells based on QDs with Cd-rich or excessive S-rich surface. In-depth electrochemical and spectroscopic characterizations reveal the critical role (hole traps) of surface S atoms in suppressing the recombination of photogenerated charges, and further attribute the inferior performance of excessive S-rich QDs to the impeded charge transfer from QDs to TiO₂ and electrolyte. This work puts forward a simple surface engineering strategy for improving the performance of QDs PEC cells, providing an efficient method to guide the surface design of QDs for their applications in other optoelectronic devices.

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.

Supercapacitor and high k properties of CNT-PbS reinforced quinoxaline amine based polybenzoxazine composites

The new 2,3-diphenylquinoxalin-6-amine (dpqa) was designed and synthesized through an efficient and high yield condensation process. Data from FTIR and ¹H-NMR spectroscopy have been adopted to ascertain the molecular structure of benzoxazine compounds. Furthermore, the quinoxaline amine based benzoxazine (BA-dpqa) was synthesized using bisphenol-A and paraformaldehyde followed by combining different weight percentages (1, 5 and 10 wt%) of (3-glycidyloxypropyl)trimethoxysilane functionalized CNT-PbS with benzoxazine to obtain nanocomposites. The thermal and morphological properties of the quinoxaline amine based neat polybenzoxazine matrix poly(BA-dpqa) and CNT-PbS/poly(BA-dpqa) composites were analysed by XRD, TGA and SEM analysis. The values of the degradation temperature (T_d) obtained for neat poly(BA-dpqa) and 10 wt% CNT-PbS/poly(BA-dpqa) composites are 414 °C and 424 °C. Furthermore, the chair yield percentage was calculated as 33% and 35% respectively. The water contact angle of polybenzoxazine gradually increased from 89° to 127° proportional to the content of CNT-PbS. Among the composites, 10 wt% CNT-PbS reinforced poly(BA-dpqa) nanocomposites possess higher dielectric constant (k = 11.0) than other composites. The pseudocapacitor nature of the prepared electrodes is demonstrated by the good electrochemical performance according to the CV curve. Also, the prepared 10 wt% CNT-PbS/poly(BA-dpqa) (637 F g^-1 at 5 A g^-1 and 11.8 Ω) electrode shows better capacitance and lower charge transfer resistance values than 5 wt% CNT-PbS/poly(BA-dpqa) (613 F g^-1 at 5 A g^-1 and 13.2 Ω) and neat poly(BA-dpqa) (105 F g^-1 at 5 A g^-1 and 15.6 Ω) according to the charge/discharge curves and EIS spectra. 10 wt% CNT-PbS/poly(BA-dpqa) shows 99.2% cycling efficiency even at the 2000th cycle, which indicates the good electrochemical performance of the prepared electrode.

One-Step Ligand-Exchange Method to Produce Quantum Dot-DNA Conjugates for DNA-Directed Self-Assembly

To address the current challenges in making bright, stable, and small DNA-functionalized quantum dots (QDs), we have developed a one-step ligand-exchange method to produce QD-DNA conjugates from commonly available hydrophobic QDs. We show that by systematically adjusting the reaction conditions such as ligand-to-nanoparticle molar ratio, pH, and solvent composition, stable and highly photoluminescent water-soluble QD-DNA conjugates with relatively high ligand loadings can be produced. Moreover, by site specifically binding these QD-DNA conjugates to a DNA origami template, we demonstrate that these bioconjugates have sufficient colloidal stability for DNA-directed self-assembly. Fluorescence quenching by an adjacent gold nanoparticle (AuNP) was demonstrated. Such QD-AuNP dimers may serve as biosensors with improved sensitivity and reproducibility. Moreover, our simple method can facilitate the assembly of QDs into more complex superlattices and discrete clusters that may enable novel photophysical properties.

Elucidating the Gain Mechanism in PbS Colloidal Quantum Dot Visible-Near-Infrared Photodiodes

The responsivities of colloidal quantum dot (CQD) photodiodes are not satisfactory (∼0.3 A W^-1) due to the lack of gain. Here, visible-near-infrared PbS CQD photodiodes with a peak responsivity of ∼1 A W^-1 and external quantum efficiencies larger than 100% are demonstrated. The gain is realized by electron tunneling injection through the Schottky junction (PbS-EDT/Au) with barrier height reduced to 0.27 eV, originating from the capture of photogenerated holes at the negatively charged acceptor traps generated in the oxidized hole-transport layer PbS-EDT. The resulting device exhibits a peak detectivity of ∼8 × 10¹¹ jones at -1 V. Additionally, the response speed (400 μs) is not sacrificed by the trap states because of the dominated faster electron drift motion in the fully depleted device. Our results provide an accurate elucidation of the gain mechanism in CQD photodiodes and promise them great potential in weak light detection.

Nanocrystal Ordering Enhances Thermal Transport and Mechanics in Single-Domain Colloidal Nanocrystal Superlattices

Colloidal nanocrystal (NC) assemblies are promising for optoelectronic, photovoltaic, and thermoelectric applications. However, using these materials can be challenging in actual devices because they have a limited range of thermal conductivity and elastic modulus, which results in heat dissipation and mechanical robustness challenges. Here, we report thermal transport and mechanical measurements on single-domain colloidal PbS nanocrystal superlattices (NCSLs) that have long-range order as well as measurements on nanocrystal films (NCFs) that are comparatively disordered. Over an NC diameter range of 3.0-6.1 nm, we observe that NCSLs have thermal conductivities and Young's moduli that are up to ∼3 times higher than those of the corresponding NCFs. We also find that these properties are more sensitive to NC diameter in NCSLs relative to NCFs. Our measurements and computational modeling indicate that stronger ligand-ligand interactions due to enhanced ligand interdigitation and alignment in NCSLs account for the improved thermal transport and mechanical properties.

An Aggregation-Suppressed Polymer Blending Strategy Enables High-Performance Organic and Quantum Dot Hybrid Solar Cells

Solution-processing hybrid solar cells with organics and colloidal quantum dots (CQDs) have drawn substantial attention in the past decade. Nevertheless, hybrid solar cells based on the recently developed directly synthesized CQD inks are still unexplored. Herein, a facile polymer blending strategy is put forward to enable directly synthesized CQD/polymer hybrid solar cells with a champion efficiency of 13%, taking advantage of the conjugated polymer blends with finely optimized aggregation behaviors. The spectroscopic and electrical investigations on carrier transport and recombination indicate that polymer blends can endow fast carrier transport and less recombination over the single counterparts. Moreover, the blending strategy offers a "dilution effect" for top-notch photovoltaic polymers with excessively strong aggregation tendency, resulting in moderate feature domain size and surface roughness, which afford fast hole transport and therefore high photovoltaic performance. The effectiveness of this strategy is successfully validated using two pairs of photovoltaic polymers. Accordingly, the relationships between polymer morphology, carrier transport, and photovoltaic performance are established to advance the progress of CQD/polymer hybrid solar cells. Such progress stresses that the utilization of aggregation-suppressed polymer blends is a facile approach toward the fabrication of high-efficiency organic-inorganic hybrid solar cells.

Measuring the carrier diffusion length in quantum dot films using graphene as photocarrier density probe

The diffusion length of quantum dot (QD) films is a critical parameter to improve the performance of QD-based optoelectronic devices. The dot-to-dot hopping transport mechanism results in shorter diffusion lengths compared to bulk solids. Herein, we present an experimental method to measure the diffusion length in PbS QD films using single layer graphene as a charge collector to monitor the density of photogenerated carriers. By producing devices with different thicknesses, we can construct light absorption and photocarrier density profiles, allowing extracting light penetration depths and carrier diffusion lengths for electrons and holes. We realized devices with small (size: ∼2.5 nm) and large (size: ∼4.8 nm) QDs, and use λ = 532 nm and λ = 635 nm wavelength illumination. For small QDs, we obtain diffusion lengths of 180 nm for holes and 500 nm for electrons. For large QDs, we obtain diffusion lengths of 120 nm for holes and 150 nm for electrons. Our results show that films made of small QD films have longer diffusion lengths for holes and electrons. We also observe that wavelength illumination may have a small effect, with electrons showing a diffusion length of 500 and 420 nm under λ = 532 nm and λ = 635 nm illumination, respectively, which may be due to increased interactions between photocarriers for longer wavelengths with deeper penetration depths. Our results demonstrate an effective technique to calculate diffusion lengths of photogenerated electrons and holes and indicate that not only QD size but also wavelength illumination can play important roles in the diffusion and electrical transport of photocarriers in QD films.

Surface band bending and carrier dynamics in colloidal quantum dot solids

Band bending in colloidal quantum dot (CQD) solids has become important in driving charge carriers through devices. This is typically a result of band alignments at junctions in the device. Whether band bending is intrinsic to CQD solids, i.e. is band bending present at the surface-vacuum interface, has previously been unanswered. Here we use photoemission surface photovoltage measurements to show that depletion regions are present at the surface of n and p-type CQD solids with various ligand treatments (EDT, MPA, PbI₂, MAI/PbI₂). Using laser-pump photoemission-probe time-resolved measurements, we show that the timescale of carrier dynamics in the surface of CQD solids can vary over at least 6 orders of magnitude, with the fastest dynamics on the order of microseconds in PbS-MAI/PbI₂ solids and on the order of seconds for PbS-MPA and PbS-PbI₂. By investigating the surface chemistry of the solids, we find a correlation between the carrier dynamics timescales and the presence of oxygen contaminants, which we suggest are responsible for the slower dynamics due to deep trap formation.

Solution-processed two-dimensional materials for next-generation photovoltaics

In the ever-increasing energy demand scenario, the development of novel photovoltaic (PV) technologies is considered to be one of the key solutions to fulfil the energy request. In this context, graphene and related two-dimensional (2D) materials (GRMs), including nonlayered 2D materials and 2D perovskites, as well as their hybrid systems, are emerging as promising candidates to drive innovation in PV technologies. The mechanical, thermal, and optoelectronic properties of GRMs can be exploited in different active components of solar cells to design next-generation devices. These components include front (transparent) and back conductive electrodes, charge transporting layers, and interconnecting/recombination layers, as well as photoactive layers. The production and processing of GRMs in the liquid phase, coupled with the ability to "on-demand" tune their optoelectronic properties exploiting wet-chemical functionalization, enable their effective integration in advanced PV devices through scalable, reliable, and inexpensive printing/coating processes. Herein, we review the progresses in the use of solution-processed 2D materials in organic solar cells, dye-sensitized solar cells, perovskite solar cells, quantum dot solar cells, and organic-inorganic hybrid solar cells, as well as in tandem systems. We first provide a brief introduction on the properties of 2D materials and their production methods by solution-processing routes. Then, we discuss the functionality of 2D materials for electrodes, photoactive layer components/additives, charge transporting layers, and interconnecting layers through figures of merit, which allow the performance of solar cells to be determined and compared with the state-of-the-art values. We finally outline the roadmap for the further exploitation of solution-processed 2D materials to boost the performance of PV devices.

Enhancing Carrier Diffusion Length and Quantum Efficiency through Photoinduced Charge Transfer in Layered Graphene-Semiconducting Quantum Dot Devices

Hybrid devices consisting of graphene or transition metal dichalcogenides (TMDs) and semiconductor quantum dots (QDs) were widely studied for potential photodetector and photovoltaic applications, while for photodetector applications, high internal quantum efficiency (IQE) is required for photovoltaic applications and enhanced carrier diffusion length is also desirable. Here, we reported the electrical measurements on hybrid field-effect optoelectronic devices consisting of compact QD monolayer at controlled separations from single-layer graphene, and the structure is characterized by high IQE and large enhancement of minority carrier diffusion length. While the IQE ranges from 10.2% to 18.2% depending on QD-graphene separation, d_s, the carrier diffusion length, L_D, estimated from scanning photocurrent microscopy (SPCM) measurements, could be enhanced by a factor of 5-8 as compared to that of pristine graphene. IQE and L_D could be tuned by varying back gate voltage and controlling the extent of charge separation from the proximal QD layer due to photoexcitation. The obtained IQE values were remarkably high, considering that only a single QD layer was used, and the parameters could be further enhanced in such devices significantly by stacking multiple layers of QDs. Our results could have significant implications for utilizing these hybrid devices as photodetectors and active photovoltaic materials with high efficiency.

Environmental impacts of solar energy systems: A review

The annual increases in global energy consumption, along with its environmental issues and concerns, are playing significant roles in the massive sustainable and renewable global transmission of energy. Solar energy systems have been grabbing most attention among all the other renewable energy systems throughout the last decade. However, even renewable energies can have some adverse environmental repercussions; therefore, further attention and proper precautional procedures should be given. This paper discusses in detail the environmental impacts of several commercial and emerging solar energy systems at both small- and utility-scales. The study expands to some of the related advances, as well as some of the essential elements in their systems. The approach follows all the stages, starting with the designs, then throughout their manufacturing, materials, construction or installation phases, and over operation lifetime and decommissioning. Specific solutions for most systems such as waste minimization and recycling are discussed, alongside with some technically and ecologically favorable recommendations for mitigating the impacts.

Optimizing Surface Chemistry of PbS Colloidal Quantum Dot for Highly Efficient and Stable Solar Cells via Chemical Binding

The surface chemistry of colloidal quantum dots (CQD) play a crucial role in fabricating highly efficient and stable solar cells. However, as-synthesized PbS CQDs are significantly off-stoichiometric and contain inhomogeneously distributed S and Pb atoms at the surface, which results in undercharged Pb atoms, dangling bonds of S atoms and uncapped sites, thus causing surface trap states. Moreover, conventional ligand exchange processes cannot efficiently eliminate these undesired atom configurations and defect sites. Here, potassium triiodide (KI3) additives are combined with conventional PbX2 matrix ligands to simultaneously eliminate the undercharged Pb species and dangling S sites via reacting with molecular I2 generated from the reversible reaction KI3 ⇌ I2 + KI. Meanwhile, high surface coverage shells on PbS CQDs are built via PbX2 and KI ligands. The implementation of KI3 additives remarkably suppresses the surface trap states and enhances the device stability due to the surface chemistry optimization. The resultant solar cells achieve the best power convention efficiency of 12.1% and retain 94% of its initial efficiency under 20 h continuous operation in air, while the control devices with KI additive deliver an efficiency of 11.0% and retains 87% of their initial efficiency under the same conditions.

Highly efficient photon detection systems for noble liquid detectors based on perovskite quantum dots

Wavelength shifting photon detection systems (PDS) are the critical functioning components in noble liquid detectors used for high energy physics (HEP) experiments and dark matter search. The vacuum ultraviolet (VUV) scintillation light emitted by these Liquid argon (LAr) and liquid Xenon (LXe) detectors are shifted to higher wavelengths resulting in its efficient detection using the state-of-the-art photodetectors such as silicon photomultipliers (SiPM). The currently used organic wavelength shifting materials [such as 1,1,4,4 Tetraphenyl Butadiene (TPB)] have several disadvantages and are unreliable for longterm use. In this study, we demonstrate the application of the inorganic perovskite cesium lead bromide (CsPbBr₃) quantum dots (QDs) as highly efficient wavelength shifters. The absolute photoluminescence quantum yield of the PDS fabricated using these QDs exceeds 70%. CsPbBr₃-based PDS demonstrated an enhancement in the SiPM signal enhancement by up to 3 times when compared to a 3 µm-thick TPB-based PDS. The emission spectrum from the QDs was optimized to match the highest quantum efficiency region of the SiPMs. In addition, we have demonstrated the deposition of the QD-based wavelength shifting material on a large area PDS substrate using low capital cost and widely scalable solution-based techniques providing a pathway appropriate for meter-scale PDS fabrication and widespread use for other wavelength shifting applications.

Near full light absorption and full charge collection in 1-micron thick quantum dot photodetector using intercalated graphene monolayer electrodes

Quantum dots (QDs) offer several advantages in optoelectronics such as easy solution processing, strong light absorption and size tunable direct bandgap. However, their major limitation is their poor film mobility and short diffusion length (<250 nm). This has restricted the thickness of QD film to ∼200-300 nm due to the restriction that the diffusion length imposes on film thickness in order to keep efficient charge collection. Such thin films result in a significant decrease in quantum efficiency for λ > 700 nm in QDs photodetector and photovoltaic devices, causing a reduced photoresponsivity and a poor absorption towards the near-infrared part of the sunlight spectrum. Herein, we demonstrate 1 μm thick QDs photodetectors with intercalated graphene charge collectors that avoid the significant drop of quantum efficiency towards λ > 700 nm observed in most QD optoelectronic devices. The 1 μm thick intercalated QD films ensure strong light absorption while keeping efficient charge extraction with a quantum efficiency of 90%-70% from λ = 600 nm to 950 nm using intercalated graphene layers as charge collectors with interspacing distance of 100 nm. We demonstrate that the effect of graphene on light absorption is minimal. We achieve a time-modulation response of <1 s. We demonstrate that this technology can be implemented on flexible PET substrates, showing 70% of the original performance after 1000 times bending test. This system provides a novel approach towards high-performance photodetection and high conversion photovoltaic efficiency with quantum dots and on flexible substrates.

Photovoltaics

The conversion of solar energy into electricity via the photovoltaic (PV) effect has been rapidly developing in the last decades due to its potential for transition from fossil fuels to renewable energy based economies. In particular, the advances in PV technology and on the economy of scale permitted to reduce the cost of the energy produced with solar cells down to the energy cost of conventional fossil fuel. Thus, PV will play an important role to address the biggest challenges of our planet including global warming, climate change and air pollution. In this paper, we will introduce the photovoltaic technology recalling the working principle of the photovoltaic conversion and describing the different PV available on the market and under development. In the last section, we will focus more on the emerging technology of the halide perovskite, which is the research subject of the authors.

From coordination polymers to nanocrystals: general and facile synthesis of ultra-small metal oxide nanocrystals

Ultrasmall and well-dispersed ZnO QDs with special optical properties and polymerization activity were synthesised by using coordination polymers as templates.

Epitaxial growth and defect repair of heterostructured CuInSexS2-x/CdSeS/CdS quantum dots

Heterostructured quantum dots (hetero-QDs) have outstanding optical properties and chemical/photostability, which make them promising building blocks for use in various optoelectronic devices. Here, CuInSe_xS_2-x/CdSeS/CdS hetero-QDs were synthesized through a facile two-step method. Their particle size, three-dimensional (3D) shapes and the epitaxial relationship between the CuInSe_xS_2-x/CdSeS core and CdS shell were investigated by high-resolution transmission electron microscopy (HRTEM). Our investigation proves that the as-synthesized hetero-QDs have a regular tetrahedron 3D shape with four {111} crystal facets. The epitaxial relationship between the CuInSe_xS_2-x/CdSeS core and CdS shell is determined to be [110]_core//[110]_shell, {112}_core//{111}_shell. In situ HRTEM observations show that the screw dislocation inside the hetero-QDs can be efficiently repaired using e-beam irradiation. These results may help in designing hetero-QDs with high-quality interfaces and identifying the strategies for synthesizing defect-free hetero-QDs.

In situ interface engineering for probing the limit of quantum dot photovoltaic devices

Quantum dot (QD) photovoltaic devices are attractive for their low-cost synthesis, tunable band gap and potentially high power conversion efficiency (PCE). However, the experimentally achieved efficiency to date remains far from ideal. Here, we report an in-situ fabrication and investigation of single TiO₂-nanowire/CdSe-QD heterojunction solar cell (QDHSC) using a custom-designed photoelectric transmission electron microscope (TEM) holder. A mobile counter electrode is used to precisely tune the interface area for in situ photoelectrical measurements, which reveals a strong interface area dependent PCE. Theoretical simulations show that the simplified single nanowire solar cell structure can minimize the interface area and associated charge scattering to enable an efficient charge collection. Additionally, the optical antenna effect of nanowire-based QDHSCs can further enhance the absorption and boost the PCE. This study establishes a robust 'nanolab' platform in a TEM for in situ photoelectrical studies and provides valuable insight into the interfacial effects in nanoscale solar cells.

ZnO nanowires for solar cells: a comprehensive review

As an abundant and non-toxic wide band gap semiconductor with a high electron mobility, ZnO in the form of nanowires (NWs) has emerged as an important electron transporting material in a vast number of nanostructured solar cells. ZnO NWs are grown by low-cost chemical deposition techniques and their integration into solar cells presents, in principle, significant advantages including efficient optical absorption through light trapping phenomena and enhanced charge carrier separation and collection. However, they also raise some significant issues related to the control of the interface properties and to the technological integration. The present review is intended to report a detailed analysis of the state-of-the-art of all types of nanostructured solar cells integrating ZnO NWs, including extremely thin absorber solar cells, quantum dot solar cells, dye-sensitized solar cells, organic and hybrid solar cells, as well as halide perovskite-based solar cells.

Toward Efficient Preconcentrating Photocatalysis: 3D g-C3N4 Monolith with Isotype Heterojunctions Assembled from Hybrid 1D and 2D Nanoblocks

The macroscopic integration of the microscopic catalyst is one of the most promising strategies for photocatalytic technology in facing practical applications. However, in addition to the unsatisfactory photoactivated exciton separation, a new problem restricting the catalytic efficiency is the unmatched kinetics between the reactant diffusion and the photochemical reaction. Here, we report an isotype heterojunctional three-dimensional g-C₃N₄ monolith which is assembled from the hybrid building blocks of the nanowires and nanosheets. Benefiting from its hierarchically porous network and abundant heterojunctions, this catalytic system exhibits inherently promoted efficiency in light absorption and exciton separation, thus leading to a desirably improved photocatalytic performance. Furthermore, thanks to the structural and functional advantages of the constructed g-C₃N₄ monolith, a novel strategy of preconcentrating photocatalysis featuring the successive filtration, adsorption, and photocatalysis has been further developed, which could technically coordinate the kinetic differences and result in over-ten-time enhancement on the efficiency compared with the traditional photocatalytic system. Beyond providing new insights into the structural design and innovative application of the monolithic photocatalyst, this work may further open up novel technological revolutions in sewage treatment, air purification, microbial control, etc.

Non-Markovian Exciton-Phonon Interactions in Core-Shell Colloidal Quantum Dots at Femtosecond Timescales

We perform two-dimensional coherent spectroscopy on CdSe/CdZnS core-shell colloidal quantum dots at cryogenic temperatures. In the two-dimensional spectra, sidebands due to electronic coupling with CdSe lattice LO-phonon modes are observed to have evolutions deviating from the exponential dephasing expected from Markovian spectral diffusion, which is instantaneous and memoryless. Comparison to simulations provides evidence that LO-phonon coupling induces energy-gap fluctuations on the finite timescales of nuclear motion. The femtosecond resolution of our technique probes exciton dynamics directly on the timescales of phonon coupling in nanocrystals.

High efficiency perovskite quantum dot solar cells with charge separating heterostructure

Metal halide perovskite semiconductors possess outstanding characteristics for optoelectronic applications including but not limited to photovoltaics. Low-dimensional and nanostructured motifs impart added functionality which can be exploited further. Moreover, wider cation composition tunability and tunable surface ligand properties of colloidal quantum dot (QD) perovskites now enable unprecedented device architectures which differ from thin-film perovskites fabricated from solvated molecular precursors. Here, using layer-by-layer deposition of perovskite QDs, we demonstrate solar cells with abrupt compositional changes throughout the perovskite film. We utilize this ability to abruptly control composition to create an internal heterojunction that facilitates charge separation at the internal interface leading to improved photocarrier harvesting. We show how the photovoltaic performance depends upon the heterojunction position, as well as the composition of each component, and we describe an architecture that greatly improves the performance of perovskite QD photovoltaics.

Metal halide perovskites have wide tunability in both material and device structure. Here Zhao et al. fabricate heterojunctions of colloidal perovskite quantum dots with different composition using layer-by-layer deposition and demonstrate improved photovoltaic performance with enhanced photocarrier harvesting.

Cd-Cu-Fe-S quaternary nanocrystals exhibiting excellent optical/optoelectronic properties

Quaternary Cd-Cu-Fe-S nanocrystals (NCs) exhibiting a strong size tunable photoluminescence were synthesized for the first time by tuning the reaction temperature from 120 °C to 210 °C. The preparation procedure involved cadmium acetate, copper acetate, iron chloride, and sulfur powder dissolved in oleylamine as precursors. The wavelength of the emission can be tuned from 640 nm to nearly 1000 nm by only changing the size of the as-prepared NCs from 3.0 nm to 15 nm. Interestingly, these NCs possess a relatively high quantum yield of over 57% without coating any wide band-gap shell materials. The study on the optoelectronic properties of Cd-Cu-Fe-S NCs, where an order of photocurrent was enhanced under AM1.5 illumination, demonstrated their suitability as optically active components to fabricate optoelectronic devices.

Heterostructure of 1D Ta3 N5 Nanorod/BaTaO2 N Nanoparticle Fabricated by a One-Step Ammonia Thermal Route for Remarkably Promoted Solar Hydrogen Production

Heterostructures are widely fabricated for promotion of photogenerated charge separation and solar cell/fuel production. (Oxy)nitrides are extremely promising for solar energy conversion, but the fabrication of heterostructures based on nitrogen-containing semiconductors is still challenging. Here, a simple ammonia thermal synthesis of a heterostructure (denoted as Ta₃ N₅ /BTON) composed of 1D Ta₃ N₅ nanorods and BaTaO₂ N (BTON) nanoparticles (0D), which is demonstrated to result in a remarkable increase in photogenerated charge separation and solar hydrogen production from water, is introduced. As analyzed and discussed, the Ta₃ N₅ /BTON heterostructure is type II and tends to create intimate interfaces between the 1D nanorods and 0D nanoparticles. The 1D Ta₃ N₅ nanorods are demonstrated to transfer electrons along the rod orientation direction. Furthermore, the intimate interfaces of the heterostructure are believed to originate from the similar Ta-based octahedron units of Ta₃ N₅ and BTON. All of the above features are expected to integrally endow increased photoinduced charge separation and one order of magnitude higher solar overall water splitting activity with respect to counterpart systems. These results may open a new avenue to fabricate heterostructures on the basis of nitrogen-containing semiconductors that is extremely promising for solar energy conversion.

Improved Charge Extraction Beyond Diffusion Length by Layer-by-Layer Multistacking Intercalation of Graphene Layers inside Quantum Dots Films

Charge collection is critical in any photodetector or photovoltaic device. Novel materials such as quantum dots (QDs) have extraordinary light absorption properties, but their poor mobility and short diffusion length limit efficient charge collection using conventional top/bottom contacts. In this work, a novel architecture based on multiple intercalated chemical vapor deposition graphene monolayers distributed in an orderly manner inside a QD film is studied. The intercalated graphene layers ensure that at any point in the absorbing material, photocarriers will be efficiently collected and transported. The devices with intercalated graphene layers have superior quantum efficiency over single-bottom graphene/QD devices, overcoming the known restriction that the diffusion length imposes on film thickness. QD film with increased thickness shows efficient charge collection over the entire λ ≈ 500-1000 nm spectrum. This architecture could be applied to boost the performance of other low-cost materials with poor mobility, allowing efficient collection for films thicker than their diffusion length.

Sub-10 nm stable graphene quantum dots embedded in hexagonal boron nitride

Graphene quantum dots (GQDs), a zero-dimensional material system with distinct physical properties, have great potential in the applications of photonics, electronics, photovoltaics, and quantum information. In particular, GQDs are promising candidates for quantum computing. In principle, a sub-10 nm size is required for GQDs to present the intrinsic quantum properties. However, with such an extreme size, GQDs have predominant edges with lots of active dangling bonds and thus are not stable. Satisfying the demands of both quantum size and stability is therefore of great challenge in the design of GQDs. Herein we demonstrate the fabrication of sub-10 nm stable GQD arrays by embedding GQDs into large-bandgap hexagonal boron nitride (h-BN). With this method, the dangling bonds of GQDs were passivated by the surrounding h-BN lattice to ensure high stability, meanwhile maintaining their intrinsic quantum properties. The sub-10 nm nanopore array was first milled in h-BN using an advanced high-resolution helium ion microscope and then GQDs were directly grown in them through the chemical vapour deposition process. Stability analysis proved that the embedded GQDs show negligible property decay after baking at 100 °C in air for 100 days. The success in preparing sub-10 nm stable GQD arrays will promote the physical exploration and potential applications of this unique zero-dimensional in-plane quantum material.

Bandgap Engineering of Indium Phosphide-Based Core/Shell Heterostructures Through Shell Composition and Thickness

The large bulk bandgap (1.35 eV) and Bohr radius (~10 nm) of InP semiconductor nanocrystals provides bandgap tunability over a wide spectral range, providing superior color tuning compared to that of CdSe quantum dots. In this paper, the dependence of the bandgap, photoluminescence emission, and exciton radiative lifetime of core/shell quantum dot heterostructures has been investigated using colloidal InP core nanocrystals with multiple diameters (1.5, 2.5, and 3.7 nm). The shell thickness and composition dependence of the bandgap for type-I and type-II heterostructures was observed by coating the InP core with ZnS, ZnSe, CdS, or CdSe through one to ten iterations of a successive ion layer adsorption and reaction (SILAR)-based shell deposition. The empirical results are compared to bandgap energy predictions made with effective mass modeling. Photoluminescence emission colors have been successfully tuned throughout the visible and into the near infrared (NIR) wavelength ranges for type-I and type-II heterostructures, respectively. Based on sizing data from transmission electron microscopy (TEM), it is observed that at the same particle diameter, average radiative lifetimes can differ as much as 20-fold across different shell compositions due to the relative positions of valence and conduction bands. In this direct comparison of InP/ZnS, InP/ZnSe, InP/CdS, and InP/CdSe core/shell heterostructures, we clearly delineate the impact of core size, shell composition, and shell thickness on the resulting optical properties. Specifically, Zn-based shells yield type-I structures that are color tuned through core size, while the Cd-based shells yield type-II particles that emit in the NIR regardless of the starting core size if several layers of CdS(e) have been successfully deposited. Particles with thicker CdS(e) shells exhibit longer photoluminescence lifetimes, while little shell-thickness dependence is observed for the Zn-based shells. Taken together, these InP-based heterostructures demonstrate the extent to which we are able to precisely tailor the material properties of core/shell particles using core/shell dimensions and composition as variables.

Picosecond Charge Transfer and Long Carrier Diffusion Lengths in Colloidal Quantum Dot Solids

Quantum dots (QDs) are promising candidates for solution-processed thin-film optoelectronic devices. Both the diffusion length and the mobility of photoexcited charge carriers in QD solids are critical determinants of solar cell performance; yet various techniques offer diverse values of these key parameters even in notionally similar films. Here we report diffusion lengths and interdot charge transfer rates using a 3D donor/acceptor technique that directly monitors the rate at which photoexcitations reach small-bandgap dot inclusions having a known spacing within a larger-bandgap QD matrix. Instead of relying on photoluminescence (which can be weak in strongly coupled QD solids), we use ultrafast transient absorption spectroscopy, a method where sensitivity is undiminished by exciton dissociation. We measure record diffusion lengths of ∼300 nm in metal halide exchanged PbS QD solids that have led to power conversion efficiencies of 12%, and determine 8 ps interdot hopping of carriers following photoexcitation, among the fastest rates reported for PbS QD solids. We also find that QD solids composed of smaller QDs ( d = ∼3.2 nm) exhibit 5 times faster interdot charge transfer rates and 10 times lower trap state densities compared to larger ( d = ∼5.5 nm) QDs.

Butylamine-Catalyzed Synthesis of Nanocrystal Inks Enables Efficient Infrared CQD Solar Cells

The best-performing colloidal-quantum-dot (CQD) photovoltaic devices suffer from charge recombination within the quasi-neutral region near the back hole-extracting junction. Graded architectures, which provide a widened depletion region at the back junction of device, could overcome this challenge. However, since today's best materials are processed using solvents that lack orthogonality, these architectures have not yet been implemented using the best-performing CQD solids. Here, a new CQD ink that is stable in nonpolar solvents is developed via a neutral donor ligand that functions as a phase-transfer catalyst. This enables the realization of an efficient graded architecture that, with an engineered band-alignment at the back junction, improves the built-in field and charge extraction. As a result, optimized IR CQD solar cells (E_g ≈ 1.3 eV) exhibiting a power conversion efficiency (PCE) of 12.3% are reported. The strategy is applied to small-bandgap (1 eV) IR CQDs to augment the performance of perovskite and crystalline silicon (cSi) 4-terminal tandem solar cells. The devices show the highest PCE addition achieved using a solution-processed active layer: a value of +5% when illuminated through a 1.58 eV bandgap perovskite front filter, providing a pathway to exceed PCEs of 23% in 4T tandem configurations with IR CQD PVs.

Energy Level Alignment of Molybdenum Oxide on Colloidal Lead Sulfide (PbS) Thin Films for Optoelectronic Devices

Interfacial charge transport in optoelectronic devices is dependent on energetic alignment that occurs via a number of physical and chemical mechanisms. Herein, we directly connect device performance with measured thickness-dependent energy-level offsets and interfacial chemistry of 1,2-ethanedithiol-treated lead sulfide (PbS) quantum dots and molybdenum oxide. We show that interfacial energetic alignment results from partial charge transfer, quantified via the chemical ratios of Mo⁵⁺ relative to Mo⁶⁺. The combined effect mitigates leakage current in both the dark and the light, relative to a metal contact, with an overall improvement in open circuit voltage, fill factor, and short circuit current.

Understanding charge transfer and recombination by interface engineering for improving the efficiency of PbS quantum dot solar cells

In quantum dot heterojunction solar cells (QDHSCs), the QD active layer absorbs sunlight and then transfers the photogenerated electrons to an electron-transport layer (ETL). It is generally believed that the conduction band minimum (CBM) of the ETL should be lower than that of the QDs to enable efficient charge transfer from the QDs to the collection electrode (here, FTO) through the ETL. However, by employing Mg-doped ZnO (Zn_1-xMg_xO) as a model ETL in PbS QDHSCs, we found that an ETL with a lower CBM is not necessary to realize efficient charge transfer in QDHSCs. The existence of shallow defect states in the Zn_1-xMg_xO ETL can serve as additional charge-transfer pathways. In addition, the conduction band offset (CBO) between the ETL and the QD absorber has been, for the first time, revealed to significantly affect interfacial recombination in QDHSCs. We demonstrate that a spike in the band structure at the ETL/QD interface is useful for suppressing interfacial recombination and improving the open-circuit voltage. By varying the Mg doping level in ZnO, we were able to tune the CBM, defect distribution and carrier concentration in the ETL, which play key roles in charge transfer and recombination and therefore the device performance. PbS QDHSCs based on the optimized Zn_1-xMg_xO ETL exhibited a high power conversion efficiency of 10.6%. Our findings provide important guidance for enhancing the photovoltaic performance of QD-based solar cells.

Strain-Driven Stacking Faults in CdSe/CdS Core/Shell Nanorods

Colloidal semiconductor nanocrystals are commonly grown with a shell of a second semiconductor material to obtain desired physical properties, such as increased photoluminescence quantum yield. However, the growth of a lattice-mismatched shell results in strain within the nanocrystal, and this strain has the potential to produce crystalline defects. Here, we study CdSe/CdS core/shell nanorods as a model system to investigate the influence of core size and shape on the formation of stacking faults in the nanocrystal. Using a combination of high-angle annular dark-field scanning transmission electron microscopy and pair-distribution-function analysis of synchrotron X-ray scattering, we show that growth of the CdS shell on smaller, spherical CdSe cores results in relatively small strain and few stacking faults. By contrast, growth of the shell on larger, prolate spheroidal cores leads to significant strain in the CdS lattice, resulting in a high density of stacking faults.

Material challenges for solar cells in the twenty-first century: directions in emerging technologies

Photovoltaic generation has stepped up within the last decade from outsider status to one of the important contributors of the ongoing energy transition, with about 1.7% of world electricity provided by solar cells. Progress in materials and production processes has played an important part in this development. Yet, there are many challenges before photovoltaics could provide clean, abundant, and cheap energy. Here, we review this research direction, with a focus on the results obtained within a Japan-French cooperation program, NextPV, working on promising solar cell technologies. The cooperation was focused on efficient photovoltaic devices, such as multijunction, ultrathin, intermediate band, and hot-carrier solar cells, and on printable solar cell materials such as colloidal quantum dots.

Towards understanding the unusual photoluminescence intensity variation of ultrasmall colloidal PbS quantum dots with the formation of a thin CdS shell

In this study, we report anomalous size-dependent photoluminescence (PL) intensity variation of PbS quantum dots (QDs) with the formation of a thin CdS shell via a microwave-assisted cation exchange approach. Thin shell formation has been established as an effective strategy for increasing the PL of QDs. Nonetheless, herein we observed an unusual PL decrease in ultrasmall QDs upon shell formation. We attempted to understand this abnormal phenomenon from the perspective of trap density variation and the probability of electrons and holes reaching surface defects. To this end, the quantum yield (QY) and PL lifetime (on the ns-μs time scales) of pristine PbS QDs and PbS/CdS core/shell QDs were measured and the radiative and non-radiative recombination rates were derived and compared. Moreover, transient absorption (TA) analysis (on the fs-ns time scale) was performed to better understand exciton dynamics at early times that lead to and affect longer time dynamics and optical properties such as PL. These experimental results, in conjunction with theoretical calculations of electron and hole wave functions, provide a complete picture of the photophysics governing the core/shell system. A model was proposed to explain the size-dependent optical and dynamic properties observed.