Layer-by-layer assembly of sintered CdSe(x)Te1-x nanocrystal solar cells

Gradient Type-II CdSe/CdSeTe/CdTe Core/Crown/Crown Heteronanoplatelets with Asymmetric Shape and Disproportional Excitonic Properties

Characterized by their strong 1D confinement and long-lifetime red-shifted emission spectra, colloidal nanoplatelets (NPLs) with type-II electronic structure provide an exciting ground to design complex heterostructures with remarkable properties. This work demonstrates the synthesis and optical characterization of CdSe/CdSeTe/CdTe core/crown/crown NPLs having a step-wise gradient electronic structure and disproportional wavefunction distribution, in which the excitonic properties of the electron and hole can be finely tuned through adjusting the geometry of the intermediate crown. The first crown with staggered configuration gives rise to a series of direct and indirect transition channels that activation/deactivation of each channel is possible through wavefunction engineering. Moreover, these NPLs allow for switching between active channels with temperature, where lattice contraction directly affects the electron-hole (e-h) overlap. Dominated by the indirect transition channels over direct transitions, the lifetime of the NPLs starts to increase at 9 K, indicative of low dark-bright exciton splitting energy. The charge transfer states from the two type-II interfaces promote a large number of indirect transitions, which effectively increase the absorption of low-energy photons critical for nonlinear properties. As a result, these NPLs demonstrate exceptionally high two-photon absorption cross-sections with the highest value of 12.9 × 10⁶ GM and superlinear behavior.

Efficient thin-film perovskite solar cells from a two-step sintering of nanocrystals

Creating semiconductor thin films from sintering of colloidal nanocrystals (NCs) represents a very important technology for high throughput and low cost thin-film photovoltaics. Here we report the creation of all-inorganic cesium lead bromide (CsPbBr₃) polycrystalline films with grain size exceeding 1 μm from the bottom up by sintering of CsPbBr₃ NCs terminated with short and low-boiling-point alky ligands that are ideal for use in sintered photovoltaics. The grain growth behavior during the sintering process was carefully investigated and correlated to the solar cell performance. To achieve precise control over the microstructural development we propose a facile two-step sintering process involving the grain growth via coarsening at a relative low temperature followed by densification at a high temperature. Compared with the one-step sintering, the two-step process yields a more uniform CsPbBr₃ bulk film with larger grain size, higher density and lower trap density. Consequently, the photovoltaic device based on the two-step sintering process demonstrates a significant enhancement of efficiency with reduced hysteresis that approaches the best reported CsPbBr₃ solar cells using a similar configuration. Our study specifies a rarely addressed perspective concerning the sintering mechanism of perovskite NCs and should contribute to the development of high-performance bulk perovskite devices based on the building blocks of perovskite NCs.

Colloidal Approaches to Zinc Oxide Nanocrystals

Zinc oxide is an extensively studied semiconductor with a wide band gap in the near-UV. Its many interesting properties have found use in optics, electronics, catalysis, sensing, as well as biomedicine and microbiology. In the nanoscale regime the functional properties of ZnO can be precisely tuned by manipulating its size, shape, chemical composition (doping), and surface states. In this review, we focus on the colloidal synthesis of ZnO nanocrystals (NCs) and provide a critical analysis of the synthetic methods currently available for preparing ZnO colloids. First, we outline key thermodynamic considerations for the nucleation and growth of colloidal nanoparticles, including an analysis of different reaction methodologies and of the role of dopant ions on nanoparticle formation. We then comprehensively review and discuss the literature on ZnO NC systems, including reactions in polar solvents that traditionally occur at low temperatures upon addition of a base, and high temperature reactions in organic, nonpolar solvents. A specific section is dedicated to doped NCs, highlighting both synthetic aspects and structure-property relationships. The versatility of these methods to achieve morphological and compositional control in ZnO is explicated. We then showcase some of the key applications of ZnO NCs, both as suspended colloids and as deposited coatings on supporting substrates. Finally, a critical analysis of the current state of the art for ZnO colloidal NCs is presented along with existing challenges and future directions for the field.

Impact of Annealing Temperature on the Morphological, Optical and Photoelectrochemical Properties of Cauliflower-like CdSe0.6Te0.4 Photoelectrodes; Enhanced Solar Cell Performance

We are reporting on the impact of air annealing temperatures on the physicochemical properties of electrochemically synthesized cadmium selenium telluride (CdSe0.6Te0.4) samples for their application in a photoelectrochemical (PEC) solar cell. The CdSe0.6Te0.4 samples were characterized with several sophisticated techniques to understand their characteristic properties. The XRD results presented the pure phase formation of the ternary CdSe0.6Te0.4 nanocompound with a hexagonal crystal structure, indicating that the annealing temperature influences the XRD peak intensity. The XPS study confirmed the existence of Cd, Se, and Te elements, indicating the formation of ternary CdSe0.6Te0.4 compounds. The FE-SEM results showed that the morphological engineering of the CdSe0.6Te0.4 samples can be achieved simply by changing the annealing temperatures from 300 to 400 °C with intervals of 50 °C. The efficiencies (ƞ) of the CdSe0.6Te0.4 photoelectrodes were found to be 2.0% for the non-annealed and 3.1, 3.6, and 2.5% for the annealed at 300, 350, and 400 °C, respectively. Most interestingly, the PEC cell analysis indicated that the annealing temperatures played an important role in boosting the performance of the photoelectrochemical properties of the solar cells.

Synthesis of Group II-VI Semiconductor Nanocrystals via Phosphine Free Method and Their Application in Solution Processed Photovoltaic Devices

Solution-processed CdTe semiconductor nanocrystals (NCs) have exhibited astonishing potential in fabricating low-cost, low materials consumption and highly efficient photovoltaic devices. However, most of the conventional CdTe NCs reported are synthesized through high temperature microemulsion method with high toxic trioctylphosphine tellurite (TOP-Te) or tributylphosphine tellurite (TBP-Te) as tellurium precursor. These hazardous substances used in the fabrication process of CdTe NCs are drawing them back from further application. Herein, we report a phosphine-free method for synthesizing group II-VI semiconductor NCs with alkyl amine and alkyl acid as ligands. Based on various characterizations like UV-vis absorption (UV), transmission electron microscope (TEM), and X-ray diffraction (XRD), among others, the properties of the as-synthesized CdS, CdSe, and CdTe NCs are determined. High-quality semiconductor NCs with easily controlled size and morphology could be fabricated through this phosphine-free method. To further investigate its potential to industrial application, NCs solar cells with device configuration of ITO/ZnO/CdSe/CdTe/Au and ITO/ZnO/CdS/CdTe/Au are fabricated based on NCs synthesized by this method. By optimizing the device fabrication conditions, the champion device exhibited power conversion efficiency (PCE) of 2.28%. This research paves the way for industrial production of low-cost and environmentally friendly NCs photovoltaic devices.

Efficient Nanocrystal Photovoltaics via Blade Coating Active Layer

CdTe semiconductor nanocrystal (NC) solar cells have attracted much attention in recent year due to their low-cost solution fabrication process. However, there are still few reports about the fabrication of large area NC solar cells under ambient conditions. Aiming to push CdTe NC solar cells one step forward to the industry, this study used a novel blade coating technique to fabricate CdTe NC solar cells with different areas (0.16, 0.3, 0.5 cm²) under ambient conditions. By optimizing the deposition parameters of the CdTe NC's active layer, the power conversion efficiency (PCE) of NC solar cells showed a large improvement. Compared to the conventional spin-coated device, a lower post-treatment temperature is required by blade coated NC solar cells. Under the optimal deposition conditions, the NC solar cells with 0.16, 0.3, and 0.5 cm² areas exhibited PCEs of 3.58, 2.82, and 1.93%, respectively. More importantly, the NC solar cells fabricated via the blading technique showed high stability where almost no efficiency degradation appeared after keeping the devices under ambient conditions for over 18 days. This is promising for low-cost, roll-by-roll, and large area industrial fabrication.

Metastable CdTe@HgTe Core@Shell Nanostructures Obtained by Partial Cation Exchange Evolve into Sintered CdTe Films Upon Annealing

Partial Hg²⁺ → Cd²⁺ cation exchange (CE) reactions were exploited to transform colloidal CdTe nanocrystals (NCs, 4-6 nm in size) into CdTe@HgTe core@shell nanostructures. This was achieved by working under a slow CE rate, which limited the exchange to the surface of the CdTe NCs. In such nanostructures, when annealed at mild temperatures (as low as 200 °C), the HgTe shell sublimated or melted and the NCs sintered together, with the concomitant desorption of their surface ligands. At the end of this process, the annealed samples consisted of ligand-free CdTe sintered films containing an amount of Hg²⁺ that was much lower than that of the starting CdTe@HgTe NCs. For example, the CdTe@HgTe NCs that initially contained 10% of Hg²⁺, after being annealed at 200 °C were transformed to CdTe sintered films containing only traces of Hg²⁺ (less than 1%). This procedure was then used to fabricate a proof-of-concept CdTe-based photodetector exhibiting a photoresponse of up to 0.5 A/W and a detectivity of ca. 9 × 10⁴ Jones under blue light illumination. Our strategy suggests that CE protocols might be exploited to lower the overall costs of production of CdTe thin films employed in photovoltaic technology, which are currently fabricated at high temperatures (above 350 °C), using post-process ligand-stripping steps.

Enhancement of photovoltaic efficiency in CdSe x Te1-x (where 0 ⩽ x ⩽ 1): insights from density functional theory

Recent advancements in CdTe photovoltaic efficiency have come from selenium grading, which reduces the band gap and significantly improves carrier lifetimes. In this work, density functional theory calculations were performed to understand the structural and electronic effects of Se alloying. Special quasirandom structures were used to simulate a random distribution of Se anions. Lattice parameters decrease linearly as Se concentration increases in line with Vegard's Law. The simulated band gap bowing shows strong agreement with experimental values. Selenium, by itself, does not introduce any defect states in the band gap and no significant changes to band structure around the [Formula: see text] point are found. Band offset values suggest a reduction of recombination across the CdSeTe/MgZnO interface at [Formula: see text], which corresponds with the Se concentration used experimentally. Band structure analysis of two cases [Formula: see text] and x  =  0.4375, shows a change from dominant Cd/Te contributions in the conduction band minimum to Cd/Se contributions as Se concentration is increased, hinting at a change in optical transition characteristics. Further calculations of optical absorption spectra suggest a reduced transition probability particularly at higher energies, which confirms experimental predictions that Se passivates the non-radiative recombination centres.

Building Solar Cells from Nanocrystal Inks

The use of solution-processed photovoltaics is a low cost, low material-consuming way to harvest abundant solar energy. Organic semiconductors based on perovskite or colloidal quantum dot photovoltaics have been well developed in recent years; however, stability is still an important issue for these photovoltaic devices. By combining solution processing, chemical treatment, and sintering technology, compact and efficient CdTe nanocrystal (NC) solar cells can be fabricated with high stability by optimizing the architecture of devices. Here, we review the progress on solution-processed CdTe NC-based photovoltaics. We focus particularly on NC materials and the design of devices that provide a good p–n junction quality, a graded bandgap for extending the spectrum response, and interface engineering to decrease carrier recombination. We summarize the progress in this field and give some insight into device processing, including element doping, new hole transport material application, and the design of new devices.

Finding and Fixing Traps in II-VI and III-V Colloidal Quantum Dots: The Importance of Z-Type Ligand Passivation

Energy levels in the band gap arising from surface states can dominate the optical and electronic properties of semiconductor nanocrystal quantum dots (QDs). Recent theoretical work has predicted that such trap states in II-VI and III-V QDs arise only from two-coordinated anions on the QD surface, offering the hypothesis that Lewis acid (Z-type) ligands should be able to completely passivate these anionic trap states. In this work, we provide experimental support for this hypothesis by demonstrating that Z-type ligation is the primary cause of PL QY increase when passivating undercoordinated CdTe QDs with various metal salts. Optimized treatments with InCl₃ or CdCl₂ afford a near-unity (>90%) photoluminescence quantum yield (PL QY), whereas other metal halogen or carboxylate salts provide a smaller increase in PL QY as a result of weaker binding or steric repulsion. The addition of non-Lewis acidic ligands (amines, alkylammonium chlorides) systematically gives a much smaller but non-negligible increase in the PL QY. We discuss possible reasons for this result, which points toward a more complex and dynamic QD surface. Finally we show that Z-type metal halide ligand treatments also lead to a strong increase in the PL QY of CdSe, CdS, and InP QDs and can increase the efficiency of sintered CdTe solar cells. These results show that surface anions are the dominant source of trap states in II-VI and III-V QDs and that passivation with Lewis acidic Z-type ligands is a general strategy to fix those traps. Our work also provides a method to tune the PL QY of QD samples from nearly zero up to near-unity values, without the need to grow epitaxial shells.

Finding and Fixing Traps in II-VI and III-V Colloidal Quantum Dots: The Importance of Z-Type Ligand Passivation

Energy levels in the band gap arising from surface states can dominate the optical and electronic properties of semiconductor nanocrystal quantum dots (QDs). Recent theoretical work has predicted that such trap states in II–VI and III–V QDs arise only from two-coordinated anions on the QD surface, offering the hypothesis that Lewis acid (Z-type) ligands should be able to completely passivate these anionic trap states. In this work, we provide experimental support for this hypothesis by demonstrating that Z-type ligation is the primary cause of PL QY increase when passivating undercoordinated CdTe QDs with various metal salts. Optimized treatments with InCl₃ or CdCl₂ afford a near-unity (>90%) photoluminescence quantum yield (PL QY), whereas other metal halogen or carboxylate salts provide a smaller increase in PL QY as a result of weaker binding or steric repulsion. The addition of non-Lewis acidic ligands (amines, alkylammonium chlorides) systematically gives a much smaller but non-negligible increase in the PL QY. We discuss possible reasons for this result, which points toward a more complex and dynamic QD surface. Finally we show that Z-type metal halide ligand treatments also lead to a strong increase in the PL QY of CdSe, CdS, and InP QDs and can increase the efficiency of sintered CdTe solar cells. These results show that surface anions are the dominant source of trap states in II–VI and III–V QDs and that passivation with Lewis acidic Z-type ligands is a general strategy to fix those traps. Our work also provides a method to tune the PL QY of QD samples from nearly zero up to near-unity values, without the need to grow epitaxial shells.

The Many "Facets" of Halide Ions in the Chemistry of Colloidal Inorganic Nanocrystals

Over the years, scientists have identified various synthetic "handles" while developing wet chemical protocols for achieving a high level of shape and compositional complexity in colloidal nanomaterials. Halide ions have emerged as one such handle which serve as important surface active species that regulate nanocrystal (NC) growth and concomitant physicochemical properties. Halide ions affect the NC growth kinetics through several means, including selective binding on crystal facets, complexation with the precursors, and oxidative etching. On the other hand, their presence on the surfaces of semiconducting NCs stimulates interesting changes in the intrinsic electronic structure and interparticle communication in the NC solids eventually assembled from them. Then again, halide ions also induce optoelectronic tunability in NCs where they form part of the core, through sheer composition variation. In this review, we describe these roles of halide ions in the growth of nanostructures and the physical changes introduced by them and thereafter demonstrate the commonality of these effects across different classes of nanomaterials.

Tunable Crystallization and Nucleation of Planar CH3NH3PbI3 through Solvent-Modified Interdiffusion

A smooth and compact light absorption perovskite layer is a highly desirable prerequisite for efficient planar perovskite solar cells. However, the rapid reaction between CH₃NH₃I methylammonium iodide (MAI) and PbI₂ often leads to an inconsistent CH₃NH₃PbI₃ crystal nucleation and growth rate along the film depth during the two-step sequential deposition process. Herein, a facile solvent additive strategy is reported to retard the crystallization kinetics of perovskite formation and accelerate the MAI diffusion across the PbI₂ layer. It was found that the ultrasmooth perovskite thin film with narrow crystallite size variation can be achieved by introducing favorable solvent additives into the MAI solution. The effects of dimethylformamide, dimethyl sulfoxide, γ-butyrolactone, chlorobenzene, and diethyl ether additives on the morphological properties and cross-sectional crystallite size distribution were investigated using atomic force microscopy, X-ray diffraction, and scanning electron microscopy. Furthermore, the light absorption and band structure of the as-prepared CH₃NH₃PbI₃ films were investigated and correlated with the photovoltaic performance of the equivalent solar cell devices. Details of perovskite nucleation and crystal growth processes are presented, which opens new avenues for the fabrication of more efficient planar solar cell devices with these ultrasmooth perovskite layers.

Role of alkyl chain length in diaminoalkane linked 2D Ruddlesden–Popper halide perovskites

A new series of 2D Ruddlesden–Popper halide perovskites with diaminoalkane as a bulky spacer with a compositional formula of (NH₃(CH₂)_xNH₃)(CH₃NH₃)₂Pb₃I₁₀.

Semiconductor Solid-Solution Nanostructures: Synthesis, Property Tailoring, and Applications

The innovation of band-gap engineering in advanced materials caused by the alloying of different semiconductors into solid-solution nanostructures provides numerous opportunities and advantages in optoelectronic property tailoring. The semiconductor solid-solution nanostructures have multifarious emission wavelength, adjustability of absorption edge, tunable electrical resistivity, and cutting-edge photoredox capability, and these advantages can be rationalized by the assorted synthesis strategies such as, binary, ternary, and quaternary solid-solutions. In addition, the abundance of elements in groups IIB, IIIA, VA, VIA, and VIIA provides sufficient room to tailor-make the semiconductor solid-solution nanostructures with the desired properties. Recent progress of semiconductor solid-solution nanostructures including synthesis strategies, structure and composition design, band-gap engineering related to the optical and electrical properties, and their applications in different fields is comprehensively reviewed. The classification, formation principle, synthesis routes, and the advantage of semiconductor solid-solution nanostructures are systematically reviewed. Moreover, the challenges faced in this area and the future prospects are discussed. By combining the information together, it is strongly anticipated that this Review may shed new light on understanding semiconductor solid-solution nanostructures while expected to have continuous breakthroughs in band-gap engineering and advanced optoelectronic nanodevices.

Rationally Controlled Synthesis of CdSexTe1-x Alloy Nanocrystals and Their Application in Efficient Graded Bandgap Solar Cells

CdSe_xTe_1−x semiconductor nanocrystals (NCs), being rod-shaped/irregular dot-shaped in morphology, have been fabricated via a simple hot-injection method. The NCs composition is well controlled through varying molar ratios of Se to Te precursors. Through changing the composition of the CdSe_xTe_1−x NCs, the spectral absorption of the NC thin film between 570–800 nm is proved to be tunable. It is shown that the bandgap of homogeneously alloyed CdSe_xTe_1−x active thin film is nonlinearly correlated with the different compositions, which is perceived as optical bowing. The solar cell devices based on CdSe_xTe_1−x NCs with the structure of ITO/ZnO/CdSe/CdSe_xTe_1−x/MoO_x/Au and the graded bandgap ITO/ZnO/CdSe(w/o)/CdSe_xTe_1−x/CdTe/MoO_x/Au are systematically evaluated. It was found that the performance of solar cells degrades almost linearly with the increase of alloy NC film thickness with respect to ITO/ZnO/CdSe/CdSe_0.2Te_0.8/MoO_x/Au. From another perspective, in terms of the graded bandgap structure of ITO/ZnO/CdSe/CdSe_xTe_1−x/CdTe/MoO_x/Au, the performance is improved in contrast with its single-junction analogues. The graded bandgap structure is proved to be efficient when absorbing spectrum and the solar cells fabricated under the structure of ITO/ZnO/CdSe_0.8Te_0.2/CdSe_0.2Te_0.8/CdTe/MoO_x/Au indicate power conversion efficiency (PCE) of 6.37%, a value among the highest for solution-processed inversely-structured CdSe_xTe_1−x NC solar cells. As the NC solar cells are solution-processed under environmental conditions, they are promising for fabricating solar cells at low cost, roll by roll and in large area.

High-Efficiency Aqueous-Processed Polymer/CdTe Nanocrystals Planar Heterojunction Solar Cells with Optimized Band Alignment and Reduced Interfacial Charge Recombination

Aqueous-processed nanocrystal solar cells have attracted increasing attention due to the advantage of its environmentally friendly nature, which provides a promising approach for large-scale production. The urgent affair is boosting the power conversion efficiency (PCE) for further commercial applications. The low PCE is mainly attributed to the imperfect device structure, which leads to abundant nonradiative recombination at the interfaces. In this work, an environmentally friendly and efficient method is developed to improve the performance of aqueous-processed CdTe nanocrystal solar cells. Polymer/CdTe planar heterojunction solar cells (PHSCs) with optimized band alignment are constructed, which results in reduced interfacial charge recombination, enhanced carrier collection efficiency and built-in field. Finally, a champion PCE of 5.9%, which is a record for aqueous-processed solar cells based on CdTe nanocrystals, is achieved after optimizing the photovoltaic device.

Tuning Band Structure of Cadmium Chalcogenide Nanoflake Arrays via Alloying for Efficient Photoelectrochemical Hydrogen Evolution

Owing to their high extinction coefficient and moderate band gap, cadmium chalcogenides are known as common semiconductors for photoelectric conversion. Nevertheless, no ideal cadmium chalcogenide with proper band structure is available yet for photoelectrochemical hydrogen evolution. In this work, we modified the band structure of CdTe via alloying with Se to achieve a ternary compound (CdSe_0.8Te_0.2) with n-type conduction, a narrower band gap, and a more negative band position compared to those of CdSe and CdTe. This novel material exhibits strong light absorption over a wider spectrum range and generates more vigorous electrons for hydrogen reduction. As a result, a photoelectrode based on nanoflake arrays of the new material could achieve a photocurrent density 2 times that of its CdSe counterpart, outperforming similar materials previously reported in the literature. Moreover, the quick transfer of holes achieved in the novel material was found to depress photocorrosion processes, which led to improved long-term working stability.

Hybrid tandem quantum dot/organic photovoltaic cells with complementary near infrared absorption

Monolithically integrated hybrid tandem solar cells that effectively combine solution-processed colloidal quantum dot (CQD) and organic bulk heterojunction subcells to achieve tandem performance that surpasses the individual subcell efficiencies have not been demonstrated to date. In this work, we demonstrate hybrid tandem cells with a low bandgap PbS CQD subcell harvesting the visible and near-infrared photons and a polymer:fullerene-poly (diketopyrrolopyrrole-terthiophene) (PDPP3T):[6,6]-phenyl-C₆₀-butyric acid methyl ester (PC₆₁BM)-top cell absorbing effectively the red and near-infrared photons of the solar spectrum in a complementary fashion. The two subcells are connected in series via an interconnecting layer (ICL) composed of a metal oxide layer, a conjugated polyelectrolyte, and an ultrathin layer of Au. The ultrathin layer of Au forms nano-islands in the ICL, reducing the series resistance, increasing the shunt resistance, and enhancing the device fill-factor. The hybrid tandems reach a power conversion efficiency (PCE) of 7.9%, significantly higher than the PCE of the corresponding individual single cells, representing one of the highest efficiencies reported to date for hybrid tandem solar cells based on CQD and polymer subcells.

CdTe Nanocrystal Hetero-Junction Solar Cells with High Open Circuit Voltage Based on Sb-doped TiO₂ Electron Acceptor Materials

We propose Sb-doped TiO₂ as electron acceptor material for depleted CdTe nanocrystal (NC) hetero-junction solar cells. Novel devices with the architecture of FTO/ZnO/Sb:TiO₂/CdTe/Au based on CdTe NC and TiO₂ precursor are fabricated by rational ambient solution process. By introducing TiO₂ with dopant concentration, we are able to tailor the optoelectronic properties of NC solar cells. Our novel devices demonstrate a very high open circuit voltage of 0.74 V, which is the highest V_oc reported for any CdTe NC based solar cells. The power conversion efficiency (PCE) of solar cells increases with the increase of Sb-doped content from 1% to 3%, then decreases almost linearly with further increase of Sb content due to the recombination effect. The champion device shows J_sc, V_oc, FF, and PCE of 14.65 mA/cm², 0.70 V, 34.44, and 3.53% respectively, which is prospective for solution processed NC solar cells with high V_oc.

Fabrication of Fully Solution Processed Inorganic Nanocrystal Photovoltaic Devices

We demonstrate a method for the preparation of fully solution processed inorganic solar cells from a spin and spray coating deposition of nanocrystal inks. For the photoactive absorber layer, colloidal CdTe and CdSe nanocrystals (3-5 nm) are synthesized using an inert hot injection technique and cleaned with precipitations to remove excess starting reagents. Similarly, gold nanocrystals (3-5 nm) are synthesized under ambient conditions and dissolved in organic solvents. In addition, precursor solutions for transparent conductive indium tin oxide (ITO) films are prepared from solutions of indium and tin salts paired with a reactive oxidizer. Layer-by-layer, these solutions are deposited onto a glass substrate following annealing (200-400 °C) to build the nanocrystal solar cell (glass/ITO/CdSe/CdTe/Au). Pre-annealing ligand exchange is required for CdSe and CdTe nanocrystals where films are dipped in NH4Cl:methanol to replace long-chain native ligands with small inorganic Cl(-) anions. NH4Cl(s) was found to act as a catalyst for the sintering reaction (as a non-toxic alternative to the conventional CdCl2(s) treatment) leading to grain growth (136±39 nm) during heating. The thickness and roughness of the prepared films are characterized with SEM and optical profilometry. FTIR is used to determine the degree of ligand exchange prior to sintering, and XRD is used to verify the crystallinity and phase of each material. UV/Vis spectra show high visible light transmission through the ITO layer and a red shift in the absorbance of the cadmium chalcogenide nanocrystals after thermal annealing. Current-voltage curves of completed devices are measured under simulated one sun illumination. Small differences in deposition techniques and reagents employed during ligand exchange have been shown to have a profound influence on the device properties. Here, we examine the effects of chemical (sintering and ligand exchange agents) and physical treatments (solution concentration, spray-pressure, annealing time and annealing temperature) on photovoltaic device performance.

Isoelectronically doped CdSe/Te nanoalloys as alternative solar cell materials: insight from computational analysis

CdSe/Te nanoalloy as a solar energy harvesting material.

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.

Aqueous-Processed Inorganic Thin-Film Solar Cells Based on CdSe(x)Te(1-x) Nanocrystals: The Impact of Composition on Photovoltaic Performance

Aqueous processed nanocrystal (NC) solar cells are attractive due to their environmental friendliness and cost effectiveness. Controlling the bandgap of absorbing layers is critical for achieving high efficiency for single and multijunction solar cells. Herein, we tune the bandgap of CdTe through the incorporation of Se via aqueous process. The photovoltaic performance of aqueous CdSexTe1-x NCs is systematically investigated, and the impacts of charge generation, transport, and injection on device performance for different compositions are deeply discussed. We discover that the performance degrades with the increasing Se content from CdTe to CdSe. This is mainly ascribed to the lower conduction band (CB) of CdSexTe1-x with higher Se content, which reduces the driving force for electron injection into TiO2. Finally, the performance is improved by mixing CdSexTe1-x NCs with conjugated polymer poly(p-phenylenevinylene) (PPV), and power conversion efficiency (PCE) of 3.35% is achieved based on ternary NCs. This work may provide some information to further optimize the aqueous-processed NC and hybrid solar cells.

Fully solution processed all inorganic nanocrystal solar cells

The effects of solution (sol) processed contacts of indium tin oxide (ITO-sol) and gold (Au-sol) on solar cells are tested. When combined with solution processed active layers of CdTe/CdSe, all-inorganic fully solution processed solar cells are produced on non-conductive glass substrates. Under AM 1.5G illumination, open circuit voltages (Voc), short circuit currents (Jsc) and efficiencies (η) of solar cells processed with evaporated Au and commercial ITO were found to be (Voc = 0.56 ± 0.04 V, Jsc = 17.2 ± 2.2 mA cm(-2), η = 3.8 ± 0.4%), with Au-sol replacement (Voc = 0.54 ± 0.03 V, Jsc = 12.6 ± 1.0 mA cm(-2), η = 2.0 ± 0.1%), with ITO-sol (Voc = 0.38 ± 0.04 V, Jsc = 12.3 ± 0.8 mA cm(-2), η = 1.3 ± 0.2%), and with each layer solution processed (Voc = 0.49 ± 0.01 V, Jsc = 10.0 ± 1.5 mA cm(-2), η = 1.5 ± 0.2%) with the champion fully-solution processed all-inorganic device showing η = 1.7%. Layers and devices were characterized with UV/Vis spectroscopy, optical profilometry, XRD, XPS and SEM. The results indicate that the reduced performance of the all-solution devices results primarily from increased roughness of the Au film and decreased conductivity of the ITO layer.

Tuning carrier mobilities and polarity of charge transport in films of CuInSe(x)S(2-x) quantum dots

CuInSe(x)S(2-x) quantum dot field-effect transistors show p-type, n-type, and ambipolar behaviors with carrier mobilities up to 0.03 cm(2) V(-1) s(-1). Although some design rules from studies of cadmium and lead containing quantum dots can be applied, remarkable differences are observed including a strong gating effect in as-synthesized nanocyrstals with long ligands.

Enhanced charge separation in ternary P3HT/PCBM/CuInS2 nanocrystals hybrid solar cells

Geminate recombination of bound polaron pairs at the donor/acceptor interface is one of the major loss mechanisms in organic bulk heterojunction solar cells. One way to overcome Coulomb attraction between opposite charge carriers and to achieve their full dissociation is the introduction of high dielectric permittivity materials such as nanoparticles of narrow band gap semiconductors. We selected CuInS2 nanocrystals of 7.4 nm size, which present intermediate energy levels with respect to poly(3-hexylthiophene) (P3HT) and Phenyl-C61-butyric acid methyl ester (PCBM). Efficient charge transfer from P3HT to nanocrystals takes place as evidenced by light-induced electron spin resonance. Charge transfer between nanocrystals and PCBM only occurs after replacing bulky dodecanethiol (DDT) surface ligands with shorter 1,2-ethylhexanethiol (EHT) ligands. Solar cells containing in the active layer a ternary blend of P3HT:PCBM:CuInS2-EHT nanocrystals in 1:1:0.5 mass ratio show strongly improved short circuit current density and a higher fill factor with respect to the P3HT:PCBM reference device. Complementary measurements of the absorption properties, external quantum efficiency and charge carrier mobility indicate that enhanced charge separation in the ternary blend is at the origin of the observed behavior. The same trend is observed for blends using the glassy polymer poly(triarylamine) (PTAA).

Efficient inorganic solar cells from aqueous nanocrystals: the impact of composition on carrier dynamics

Efficient inorganic thin-film solar cells are fabricated from aqueous CdTe nanocrystals and a power conversion efficiency of 5.73% is achieved. Annealing-induced variation of material composition and charge dynamics are investigated in detail.

Nanocrystal grain growth and device architectures for high-efficiency CdTe ink-based photovoltaics

We study the use of cadmium telluride (CdTe) nanocrystal colloids as a solution-processable "ink" for large-grain CdTe absorber layers in solar cells. The resulting grain structure and solar cell performance depend on the initial nanocrystal size, shape, and crystal structure. We find that inks of predominantly wurtzite tetrapod-shaped nanocrystals with arms ∼5.6 nm in diameter exhibit better device performance compared to inks composed of smaller tetrapods, irregular faceted nanocrystals, or spherical zincblende nanocrystals despite the fact that the final sintered film has a zincblende crystal structure. Five different working device architectures were investigated. The indium tin oxide (ITO)/CdTe/zinc oxide structure leads to our best performing device architecture (with efficiency >11%) compared to others including two structures with a cadmium sulfide (CdS) n-type layer typically used in high efficiency sublimation-grown CdTe solar cells. Moreover, devices without CdS have improved response at short wavelengths.

Enhancement of open-circuit voltage and the fill factor in CdTe nanocrystal solar cells by using interface materials

Interface states influence the operation of nanocrystal (NC) solar cell carrier transport, recombination and energetic mechanisms. In a typical CdTe NC solar cell with a normal structure of a ITO/p-CdTe NCs/n-acceptor (or without)/Al configuration, the contact between the ITO and CdTe is a non-ohm contact due to a different work function (for an ITO, the value is ~4.7 eV, while for CdTe NCs, the value is ~5.3 eV), which results in an energetic barrier at the ITO/CdTe interface and decreases the performance of the NC solar cells. This work investigates how interface materials (including Au, MoO(x) and C₆₀) affect the performance of NC solar cells. It is found that devices with interface materials have shown higher V(oc) than those without interface materials. For the case in which we used Au as an interface, we obtained a high open-circuit voltage of 0.65 V, coupled with a high fill factor (62%); this resulted in a higher energy conversion efficiency (ECE) of 5.3%, which showed a 30% increase in the ECE compared with those without the interlayer. The capacitance measurements indicate that the increased V(oc) in the case in which Au was used as the interface is likely due to good ohm contact between the Au's and the CdTe NCs' thin film, which decreases the energetic barrier at the ITO/CdTe interface.

Electrical transport and grain growth in solution-cast, chloride-terminated cadmium selenide nanocrystal thin films

We report the evolution of electrical transport and grain size during the sintering of thin films spin-cast from soluble phosphine and amine-bound, chloride-terminated cadmium selenide nanocrystals. Sintering of the nanocrystals occurs in three distinct stages as the annealing temperature is increased: (1) reversible desorption of the organic ligands (≤150 °C), (2) irreversible particle fusion (200-300 °C), and (3) ripening of the grains to >5 nm domains (>200 °C). Grain growth occurs at 200 °C in films with 8 atom % Cl(-), while films with 3 atom % Cl(-) resist growth until 300 °C. Fused nanocrystalline thin films (grain size = 4.5-5.5 nm) on thermally grown silicon dioxide gate dielectrics produce field-effect transistors with electron mobilities as high as 25 cm(2)/(Vs) and on/off ratios of 10(5) with less than 0.5 V hysteresis in threshold voltage without the addition of indium.

A route to phase controllable Cu2ZnSn(S(1-x)Se(x))4 nanocrystals with tunable energy bands

Cu₂ZnSn(S_1−xSe_x)₄ nanocrystals are an emerging family of functional materials with huge potential of industrial applications, however, it is an extremely challenging task to synthesize Cu₂ZnSn(S_1−xSe_x)₄ nanocrystals with both tunable energy band and phase purity. Here we show that a green and economic route could be designed for the synthesis of Cu₂ZnSn(S_1−xSe_x)₄ nanocrystals with bandgap tunable in the range of 1.5–1.12 eV. Consequently, conduction band edge shifted from −3.9 eV to −4.61 eV (relative to vacuum energy) is realized. The phase purity of Cu₂ZnSn(S_1−xSe_x)₄ nanocrystals is substantiated with in-depth combined optical and structural characterizations. Electrocatalytic and thermoelectric performances of Cu₂ZnSn(S_1−xSe_x)₄ nanocrystals verify their superior activity to replace noble metal Pt and materials containing heavy metals. This green and economic route will promote large-scale application of Cu₂ZnSn(S_1−xSe_x)₄ nanocrystals as solar cell materials, electrocatalysts, and thermoelectric materials.

Cl-capped CdSe nanocrystals via in situ generation of chloride anions

Halide ions cap and stabilize colloidal semiconductor nanocrystal (NC) surfaces allowing for NCs surface interactions that may improve the performance of NC thin film devices such as photo-detectors and/or solar cells. Current ways to introduce halide anions as ligands on surfaces of NCs produced by the hot injection method are based on post-synthetic treatments. In this work we explore the possibility to introduce Cl in the NC ligand shell in situ during the NCs synthesis. With this aim, the effect of 1,2-dichloroethane (DCE) in the synthesis of CdSe rod-like NCs produced under different Cd/Se precursor molar ratios has been studied. We report a double role of DCE depending on the Cd/Se precursor molar ratio (either under excess of cadmium or selenium precursor). According to mass spectrometry (ESI-TOF) and nuclear magnetic resonance ((1)H NMR), under excess of Se precursor (Se dissolved in trioctylphosphine, TOP) conditions at 265 °C ethane-1,2-diylbis(trioctylphosphonium)dichloride is released as a product of the reaction between DCE and TOP. According to XPS studies chlorine gets incorporated into the CdSe ligand shell, promoting re-shaping of rod-like NCs into pyramidal ones. In contrast, under excess Cd precursor (CdO) conditions, DCE reacts with the Cd complex releasing chlorine-containing non-active species which do not trigger NCs re-shaping. The amount of chlorine incorporated into the ligand shell can thus be controlled by properly tuning the Cd/Se precursor molar ratio.

High efficiency solution processed sintered CdTe nanocrystal solar cells: the role of interfaces

Solution processing of photovoltaic semiconducting layers offers the potential for drastic cost reduction through improved materials utilization and high device throughput. One compelling solution-based processing strategy utilizes semiconductor layers produced by sintering nanocrystals into large-grain semiconductors at relatively low temperatures. Using n-ZnO/p-CdTe as a model system, we fabricate sintered CdTe nanocrystal solar cells processed at 350 °C with power conversion efficiencies (PCE) as high as 12.3%. JSC of over 25 mA cm(-2) are achieved, which are comparable or higher than those achieved using traditional, close-space sublimated CdTe. We find that the VOC can be substantially increased by applying forward bias for short periods of time. Capacitance measurements as well as intensity- and temperature-dependent analysis indicate that the increased VOC is likely due to relaxation of an energetic barrier at the ITO/CdTe interface.

Controlling light absorption in charge-separating core/shell semiconductor nanocrystals

Semiconductor nanocrystals of different formulations have been extensively studied for use in thin-film photovoltaics. Materials used in such devices need to satisfy the stringent requirement of having large absorption cross sections. Hence, type-II semiconductor nanocrystals that are generally considered to be poor light absorbers have largely been ignored. In this article, we show that type-II semiconductor nanocrystals can be tailored to match the light-absorption abilities of other types of nanostructures as well as bulk semiconductors. We synthesize type-II ZnTe/CdS core/shell nanocrystals. This material is found to exhibit a tunable band gap as well as absorption cross sections that are comparable to CdTe. This result has significant implications for thin-film photovoltaics, where the use of type-II nanocrystals instead of pure semiconductors can improve charge separation while also providing a much needed handle to regulate device composition.

25th anniversary article: Colloidal quantum dot materials and devices: a quarter-century of advances

Colloidal quantum dot (CQD) optoelectronics offers a compelling combination of low-cost, large-area solution processing, and spectral tunability through the quantum size effect. Since early reports of size-tunable light emission from solution-synthesized CQDs over 25 years ago, tremendous progress has been made in synthesis and assembly, optical and electrical properties, materials processing, and optoelectronic applications of these materials. Here some of the major developments in this field are reviewed, touching on key milestones as well as future opportunities.

Photocurrent mapping of 3D CdSe/CdTe windowless solar cells

This paper details the use of scanning photocurrent microscopy to examine localized current collection efficiency of thin-film photovoltaic devices with in-plane patterning at a submicrometer length scale. The devices are based upon two interdigitated comb electrodes at the micrometer length scale prepatterned on a substrate, with CdSe electrodeposited on one electrode and CdTe deposited over the entire surface of the resulting structure by pulsed laser deposition. Photocurrent maps provide information on what limits the performance of the windowless CdSe/CdTe thin-film photovoltaic devices, revealing "dead zones" particularly above the electrodes contacting the CdTe which is interpreted as recombination over the back contact. Additionally, the impact of ammonium sulfide passivation is examined, which enables device efficiency to reach 4.3% under simulated air mass 1.5 illumination.

Solution-processed zinc phosphide (α-Zn3P2) colloidal semiconducting nanocrystals for thin film photovoltaic applications

Zinc phosphide (Zn3P2) is a promising earth-abundant material for thin film photovoltaic applications, due to strong optical absorption and near ideal band gap. In this work, crystalline zinc phosphide nanoparticles are synthesized using dimethylzinc and tri-n-octylphosphine as precursors. Transmission electron microscopy and X-ray diffraction data show that these nanoparticles have an average diameter of ∼8 nm and adopt the crystalline structure of tetragonal α-Zn3P2. The optical band gap is found to increase by 0.5 eV relative to bulk Zn3P2, while there is an asymmetric shift in the conduction and valence band levels. Utilizing layer-by-layer deposition of Zn3P2 nanoparticle films, heterojunction devices consisting of ITO/ZnO/Zn3P2/MoO3/Ag are fabricated and tested for photovoltaic performance. The devices are found to exhibit excellent rectification behavior (rectification ratio of 600) and strong photosensitivity (on/off ratio of ∼10(2)). X-ray photoelectron spectroscopy and ultraviolet photoemission spectroscopy analyses reveal the presence of a thin 1.5 nm phosphorus shell passivating the surface of the Zn3P2 nanoparticles. This shell is believed to form during the nanoparticle synthesis.

Near infrared absorption of CdSe(x)Te(1-x) alloyed quantum dot sensitized solar cells with more than 6% efficiency and high stability

CdSe0.45Te0.55 alloyed quantum dots (QDs) with excitonic absorption onset at 800 nm and particle size of 5.2 nm were prepared via a noninjection high-temperature pyrolysis route and used as a sensitizer in solar cells. A postsynthesis assembly approach with use of bifunctional linker molecule mercaptopropionic acid (MPA) capped water-soluble QDs, obtained via ex situ ligand exchange from the initial oil-dispersible QDs, was adopted for tethering QDs onto mesoporous TiO2 film. With the combination of high loading of the QD sensitizer and intrinsic superior optoelectronic properties (wide absorption range, high conduction band edge, high chemical stability, etc., relative to their constituents CdSe and CdTe) of the adopted CdSe0.45Te0.55 QD sensitizer, the resulting CdSexTe1-x alloyed QD-based solar cells exhibit a record conversion efficiency of 6.36% (Jsc = 19.35 mA/cm(2), Voc = 0.571 V, FF = 0.575) under full 1 sun illumination, which is remarkably better than that of the reference CdSe and CdTe QD based ones. Furthermore, the solar cells with Cu2S counter electrodes based on eletrodeposition of Cu on conductive glass show long-term (more than 500 h) stability.