Hybrid Thin Film Encapsulation for Improving the Stability of PbS Quantum Dot Solar Cells.

The instability to moisture, heat, and ultraviolet (UV) light is the main problem in the application of quantum dot solar cells (QDSCs). Thin film encapsulation can effectively improve their operational stability. However, it is difficult to achieve multiple barrier effects with single layer of encapsulated film. Here, a hybrid thin-film encapsulation strategy is reported to encapsulate lead sulfide QDSCs, which can isolate moisture and partial thermal, and prevent the penetration of UV light, thus retarding the surface oxidation process of the quantum dots. After 60 h, the encapsulated device retains a normalized power conversion efficiency of 83.8% and 80.6% at 85% humidity and 75 °C, respectively, which is three and six times of the value obtained in unencapsulated devices. At continuous UV illumination, encapsulated device exhibits five times higher stability than the reference. This strategy provides the way for the overall improvement of the operating stability of lead sulfide QDSCs in harsh environments of high humidity, high temperature, and UV light.

Optimizing Energy Levels and Improving Film Compactness in PbS Quantum Dot Solar Cells by Silver Doping.

PbS quantum dot (QD) solar cells harvest near-infrared solar radiation. Their conventional hole transport layer has limited hole collection efficiency due to energy level mismatch and poor film quality. Here, how to resolve these two issues by using Ag-doped PbS QDs are demonstrated. On the one hand, Ag doping relieves the compressive stress during layer deposition and thus improves film compactness and homogeneity to suppress leakage currents. On the other hand, Ag doping increases hole concentration, which aligns energy levels and increases hole mobility to boost hole collection. Increased hole concentration also broadens the depletion region of the active layer, decreasing interface charge accumulation and promoting carrier extraction efficiency. A champion power conversion efficiency of 12.42% is achieved by optimizing the hole transport layer in PbS QD solar cells, compared to 9.38% for control devices. Doping can be combined with compressive strain relief to optimize carrier concentration and energy levels in QDs, and even introduce other novel phenomena such as improved film quality.

3D/2D Core/Shell Perovskite Nanocrystals for High-Performance Solar Cells.

All-inorganic lead halide perovskite nanocrystals (NCs) emerge as a rising star in photovoltaic fields on account of their excellent optoelectronic properties. However, it still remains challenging to further promote photovoltaic efficiency due to the susceptible surface and inevitable vacancies. Here, this work reports a 3D/2D core/shell perovskite heterojunction based on CsPbI3 NCs and its performance in solar cells. The guanidinium (GA+ ) rich 2D nanoshells can significantly passivate surface trap states and lower the capping ligand density, resulting in improved photoelectric properties and carrier transport and diminished nonradiative recombination centers via the hydrogen bonds from amino groups in GA+ ions. Consequently, an outstanding power conversion efficiency (PCE) of up to 15.53% is realized, substantially higher than the control device (13.77%). This work highlights the importance of surface chemistry and offers a feasible avenue to achieve high-performance perovskite NCs-based optoelectronic devices.

Elevating Surface-Enhanced Infrared Absorption with Quantum Mechanical Effects of Plasmonic Nanocavities.

Plasmonic nanocavities, with the ability to localize and concentrate light into nanometer-scale dimensions, have been widely used for ultrasensitive spectroscopy, biosensing, and photodetection. However, as the nanocavity gap approaches the subnanometer length scale, plasmonic enhancement, together with plasmonic enhanced optical processes, turns to quenching because of quantum mechanical effects. Here, instead of quenching, we show that quantum mechanical effects of plasmonic nanocavities can elevate surface-enhanced infrared absorption (SEIRA) of molecular moieties. The plasmonic nanocavities, nanojunctions of gold and cadmium oxide nanoparticles, support prominent mid-infrared plasmonic resonances and enable SEIRA of an alkanethiol monolayer (CH3(CH2)n-1SH, n = 3-16). With a subnanometer cavity gap (n < 6), plasmonic resonances turn to blue shift and the SEIRA signal starts a pronounced increase, benefiting from the quantum tunneling effect across the plasmonic nanocavities. Our findings demonstrate the new possibility of optimizing the field enhancement and SEIRA sensitivity of mid-infrared plasmonic nanocavities.

Breaking the Size Limitation of Directly-Synthesized PbS Quantum Dot Inks Toward Efficient Short-wavelength Infrared Optoelectronic Applications.

PbS quantum dots (QDs) are promising building blocks for solution-processed short-wavelength infrared (SWIR) devices. The recently developed direct synthesis of semi-conductive PbS QD inks has substantially simplified the preparation processing and reduced the material cost, while facing the challenge to synthesize large-size QDs with absorption covering the SWIR region. Herein, we for the first time realize a low-cost, scalable synthesis of SWIR PbS QD inks after an extensive investigation of the reaction kinetics. Finally, based on these PbS SWIR QD inks, the solar cell demonstrates a record-high power conversion efficiency (PCE) of 1.44 % through an 1100 nm cutoff silicon filter and the photodetector device shows a low dark current density of 2×10-6  A cm-2 at -0.8 V reverse bias with a high external quantum efficiency (EQE) of 70 % at ≈1300 nm. Our results realize the direct synthesis of low-cost and scalable SWIR QD inks and may accelerate the industrialization of consumer SWIR technologies.

Broadband Tunable Mid-infrared Plasmon Resonances in Cadmium Oxide Nanocrystals Induced by Size-Dependent Nonstoichiometry.

A central theme of nanocrystal (NC) research involves synthesis of dimension-controlled NCs and studyof size-dependent scaling laws governing their optical, electrical, magnetic, and thermodynamic properties. Here, we describe the synthesis of monodisperse CdO NCs that exhibit high quality-factor (up to 5.5) mid-infrared (MIR) localized surface plasmon resonances (LSPR) and elucidate the inverse scaling relationship between carrier concentration and NC size. The LSPR wavelength is readily tunable between 2.4 and ∼6.0 µm by controlling the size of CdO NCs. Structural and spectroscopic characterization provide strong evidence that free electrons primarily originate from self-doping due to NC surface-induced nonstoichiometry. The ability to probe and to control NC stoichiometry and intrinsic defects will pave the way toward predictive synthesis of doped NCs with desirable LSPR characteristics.

Ligand Mediated Transformation of Cesium Lead Bromide Perovskite Nanocrystals to Lead Depleted Cs4PbBr6 Nanocrystals.

Lead halide perovskite nanocrystals (NCs) have emerged as attractive nanomaterials owing to their excellent optical and optoelectronic properties. Their intrinsic instability and soft nature enable a post-synthetic controlled chemical transformation. We studied a ligand mediated transformation of presynthesized CsPbBr3 NCs to a new type of lead-halide depleted perovskite derivative nanocrystal, namely Cs4PbBr6. The transformation is initiated by amine addition, and the use of alkyl-thiol ligands greatly improves the size uniformity and chemical stability of the derived NCs. The thermodynamically driven transformation is governed by a two-step dissolution-recrystallization mechanism, which is monitored optically. Our results not only shed light on a decomposition pathway of CsPbBr3 NCs but also present a method to synthesize uniform colloidal Cs4PbBr6 NCs, which may actually be a common product of perovskite NCs degradation.

Flexible Broadband Graphene Photodetectors Enhanced by Plasmonic Cu3-x P Colloidal Nanocrystals.

The integration of graphene with colloidal quantum dots (QDs) that have tunable light absorption affords new opportunities for optoelectronic applications as such a hybrid system solves the problem of both quantity and mobility of photocarriers. In this work, a hybrid system comprising of monolayer graphene and self-doped colloidal copper phosphide (Cu3-x P) QDs is developed for efficient broadband photodetection. Unlike conventional PbS QDs that are toxic, Cu3-x P QDs are environmental friendly and have plasmonic resonant absorption in near-infrared (NIR) wavelength. The half-covered graphene with Cu3-x P nanocrystals (NCs) behaves as a self-driven p-n junction and shows durable photoresponse in NIR range. A comparison experiment reveals that the surface ligand attached to Cu3-x P NCs plays a key role in determining the charge transfer efficiency from Cu3-x P to graphene. The most efficient three-terminal photodetectors based on graphene-Cu3-x P exhibit broadband photoresponse from 400 to 1550 nm with an ultrahigh responsivity (1.59 × 105 A W-1 ) and high photoconductive gain (6.66 × 105 ) at visible wavelength (405 nm), and a good responsivity of 9.34 A W-1 at 1550 nm. The demonstration of flexible graphene-Cu3-x P photodetectors operated at NIR wavelengths may find potential applications in optical sensing, biological imaging, and wearable devices.

PbS/Cd3P2 quantum heterojunction colloidal quantum dot solar cells.

Here, we demonstrated the quantum heterojunction colloidal quantum dot (CQD) solar cells employing the PbS CQDs/Cd3P2 CQDs architecture in which both the p-type PbS and n-type Cd3P2 CQD layers are quantum-tunable and solution-processed light absorbers. We synthesized well-crystallized and nearly monodispersed tetragonal Cd3P2 CQDs and then engineered their energy band alignment with the p-type PbS by tuning the dot size and hence the bandgap to achieve efficient light absorbing and charge separation. We further optimized the device through the Ag-doping strategy of PbS CQDs that may leverage an expanded depletion region in the n-layer, which greatly enhances the photocurrent. The resulting devices showed an efficiency of 1.5%.

Bionic Scotopic Adaptation Transistors for Nighttime Low Illumination Imaging.

Human vision excels in perceiving nighttime low illumination due to biological feedforward adaptation. Replicating this ability in biomimetic vision using solid-state devices has been highly sought after. However, emulating scotopic adaptation, entailing a confluence of efficient photoexcitation and dynamic carrier modulation, presents formidable challenges. Here, we demonstrate a low-power and bionic scotopic adaptation transistor by coupling a light-absorption layer and an electron-trapping layer at the bottom of the semiconducting channel, enabling simultaneous achievement of efficient generation of free photocarriers and adaptive carrier accumulation within a single device. This innovation empowers our transistor to exhibit sensitivity-potentiated characteristics after adaptation, detecting scotopic-level illumination (0.001 lx) with exceptional photosensitivity up to 103 at low voltages below 2 V. Moreover, we have successfully replicated diverse scotopic vision functions, encompassing time-dependent visual threshold enhancement, light intensity-dependent adaptation index, imaging contrast enhancement for nighttime low illumination imaging, opening an opportunity for artificial night vision.

In situ synergistic halogen passivation of semiconducting PbS quantum dot inks for efficient photovoltaics.

Lead sulfide colloidal quantum dots (PbS CQDs) show great potential in next-generation photovoltaics. However, their high specific surface area and complex surface crystallography lead to a high surface trap density, which normally requires more than one type of capping ion or ligand to achieve effective surface passivation. In this study, we performed in situ mixed halogen passivation (MHP) during the direct synthesis of semiconducting PbS CQD inks by using different lead halogens. The different halogens can bind with the surface of the CQD throughout the nucleation/growth process, resulting in optimal surface configuration. As a result, the MHP CQD exhibited superior surface passivation compared to the conventionally iodine-capped CQDs. Finally, we achieved a substantial improvement in efficiency from 10.64% to 12.58% after the MHP treatment. Our work demonstrates the advantages of exploring efficient passivation in the directly synthesized CQD inks.

Charge Transport Layer Engineering toward Efficient and Stable Colloidal Quantum Dot Solar Cells.

Lead sulfide (PbS) colloidal quantum dot (CQD) solar cell, as a new type of solution-processed photovoltaic technology, have always attracted great interest. Early studies mainly focused on the surface passivation of CQDs and optimization of device structures. Recently, researchers further developed new charge transport layers and interfacial passivation strategies based on these foundations, which have significantly improved the device efficiency and stability. In this perspective, we summarize the important research progress in transport layer materials, structures, and interfacial passivation of CQD solar cells. We also discuss the remaining challenges and potential development directions for charge transport layers for high-performance and stable PbS CQD solar cells. We hope to draw attention to the enormous potential of charge transport layers in promoting CQD-based optoelectronics toward practical applications.

Merging Passivation in Synthesis Enabling the Lowest Open-Circuit Voltage Loss for PbS Quantum Dot Solar Cells.

The high open-circuit voltage (Voc ) loss arising from insufficient surface passivation is the main factor that limits the efficiency of current lead sulfide colloidal quantum dots (PbS CQDs) solar cell. Here, synergistic passivation is performed in the direct synthesis of conductive PbS CQD inks by introducing multifunctional ligands to well coordinate the complicated CQDs surface with the thermodynamically optimal configuration. The improved passivation effect is intactly delivered to the final photovoltaic device, leading to an order lower surface trap density and beneficial doping behavior compared to the control sample. The obtained CQD inks show the highest photoluminescence quantum yield (PLQY) of 24% for all photovoltaic PbS CQD inks, which is more than twice the reported average PLQY value of ≈10%. As a result, a high Voc of 0.71 V and power conversion efficiency (PCE) of 13.3% is achieved, which results in the lowest Voc loss (0.35 eV) for the reported PbS CQD solar cells with PCE >10%, comparable to that of perovskite solar cells. This work provides valuable insights into the future CQDs passivation strategies and also demonstrates the great potential for the direct-synthesis protocol of PbS CQDs.

Open-Shell Diradical-Sensitized Electron Transport Layer for High-Performance Colloidal Quantum Dot Solar Cells.

The zinc oxide (ZnO) nanoparticles (NPs) are well-documented as an excellent electron transport layer (ETL) in optoelectronic devices. However, the intrinsic surface flaw of the ZnO NPs can easily result in serious surface recombination of carriers. Exploring effective passivation methods of ZnO NPs is essential to maximize the device's performance. Herein, a hybrid strategy is explored for the first time to improve the quality of ZnO ETL by incorporating stable organic open-shell donor-acceptor type diradicaloids. The high electron-donating feature of the diradical molecules can efficiently passivate the deep-level trap states and improve the conductivity of ZnO NP film. The unique advantage of the radical strategy is that its passivation effectiveness is highly correlated with the electron-donating ability of radical molecules, which can be precisely controlled by the rational design of molecular chemical structures. The well-passivated ZnO ETL is applied in lead sulfide (PbS) colloidal quantum dot solar cells, delivering a power conversion efficiency of 13.54%. More importantly, as a proof-of-concept study, this work will inspire the exploration of general strategies using radical molecules to construct high-efficiency solution-processed optoelectronic devices.

Thickness-variation-insensitive near-infrared quantum dot LEDs.

In terms of tunable luminescence and high quantum efficiency, colloidal quantum dots (CQDs) are promising semiconductors for constructing near-infrared light-emitting diodes (NIR-LEDs). However, currently available NIR-LEDs are susceptible to variations in the emission layer thickness (EMLT), the highest external quantum efficiency (EQE) decreases to below 50% (relative to peak EQE) when the EMLT varies out of a narrow range of (±30 nm). This is due to the thickness-dependent carrier recombination rate and current density variation, resulting in batch-to-batch EQE fluctuations that limit LED reproducibility. Here we report efficient NIR-LEDs that exhibit EQE variations of less than 15% (relative to the champion EQE) over an EMLT range of 40-220 nm; the highest achievable EQE of ∼11.5% was obtained by encapsulating a 212 nm-thick CQD within a type-I inorganic shell to enhance the radiative recombination in the dots, resulting in a high photoluminescence quantum yield of 80%, and by post-treating the films with a bifunctional linking agent to improve and balance the hole and electron mobilities in the entire film (electron mobility: 8.23 × 10-3 cm2 V-1 s-1; hole mobility: 7.0 × 10-3 cm2 V-1 s-1). This work presents the first NIR-LEDs that exhibit EMLT-invariant EQE over an EMLT range of 40-220 nm, which represents the highest EQE among reported CQD NIR-LEDs with a QD thickness exceeding 100 nm.

Multifunctional Sensors Based on Doped Indium Oxide Nanocrystals.

There is an increasing need for multifunctional sensors that can detect radiation, biological activity, gas, etc. for efficient health monitoring, neurological medical devices, and human-machine interfaces in recent years. Herein, we demonstrated a multifunctional Sn-doped In2O3 nanocrystal (ITO NC) based device for ulyoutraviolet (UV)/infrared (IR) dual-band photodetection and light-activated efficient nitrogen dioxide (NO2) gas sensing at room temperature (RT). The effects of different surface ligands and annealing process of ITO NCs on their photodetection performance were investigated. The ITO NCs capped with 1,2-ethanedithiol (EDT) show a responsivity of 31.3/177.7 mA W-1 and normalized detectivity of ∼1 × 1010/109 cm Hz1/2 W-1 under UV/IR illumination at 375/2200 nm at RT. The potential of the ITO NCs sensors to monitor low concentrations of NO2 is activated by light illumination. The sensor has a higher response (4.2) to 1 ppm of NO2, shorter response/recovery time (156.8/554.2 s), and a lower detection limit (LOD) (219 ppb) under UV illumination compared within a dark environment. The LOD of the sensor is lower than the allowable exposure limit of NO2 specified in "Air Pollutant Limits" of the Occupational Safety and Health Administration (OSHA). Our work paves an alternative platform for the development of low-cost, integration-friendly multifunctional devices.

Homologous Bromides Treatment for Improving the Open-Circuit Voltage of Perovskite Solar Cells.

The power conversion efficiency (PCE) of solution-processed organic-inorganic mixed halide perovskite solar cells has achieved rapid improvement. However, it is imperative to minimize the voltage deficit (Woc  = Eg /q - Voc ) for their PCE to approach the theoretical limit. Herein, the strategy of depositing homologous bromide salts on the perovskite surface to achieve a surface and bulk passivation for the fabrication of solar cells with high open-circuit voltage is reported. Distinct from the conclusions given by previous works, that homologous bromides such as FABr only react with PbI2 to form a large-bandgap perovskite layer on top of the original perovskite, this work shows that the bromide also penetrates the perovskite film and passivates the perovskite in the bulk. This is confirmed by the small-bandgap enlargement observed by absorbance and photoluminescence, and the bromide element ratio increasing in the bulk by time-of-flight secondary-ion mass spectrometry and depth-resolved X-ray photoelectron spectroscopy. Furthermore, a clear suppression of non-radiative recombination is confirmed by a variety of characterization methods. This work provides a simple and universal way to reduce the Woc of single-junction perovskite solar cells and it will also shed light on developing other high-performance optoelectronic devices, including perovskite-based tandems and light-emitting diodes.

In Situ Growth of Strained Matrix on CsPbI3 Perovskite Quantum Dots for Balanced Conductivity and Stability.

Their nanoscale size endows perovskite quantum dots (QDs) with processing flexibility and high tunability of optoelectronic properties. The vast surface area also provides an opportunity for ligand engineering to offer QDs extra protection, which however, will impede charge transport in the QD array. Currently, the surface treatments that can balance both stability and conductivity of the perovskite QD array remain a huge challenge. Here, we report in situ growth of an atomic guanidinium lead iodide perovskite matrix on CsPbI3 QDs. In addition to the effect of trap passivation, the matrix can also provide substantial surface strain to improve the QD phase stability. Meanwhile, the ultrathin matrix allows efficient coupling and charge transport in the QD solids. As a result, the CsPbI3 QD solar cells can achieve both superior device stability and performance. We believe the development of a multifunctional surface matrix will become one of the future research focuses in perovskite QD-based devices.

In Situ Inorganic Ligand Replenishment Enables Bandgap Stability in Mixed-Halide Perovskite Quantum Dot Solids.

Instability in mixed-halide perovskites (MHPs) is a key issue limiting perovskite solar cells and light-emitting diodes (LEDs). One form of instability arises during the processing of MHP quantum dots using an antisolvent to precipitate and purify the dots forming surface traps that lead to decreased luminescence, compromised colloidal stability, and emission broadening. Here, the introduction of inorganic ligands in the antisolvents used in dot purification is reported in order to overcome this problem. MHPs that are colloidally stable for over 1 year at 25 °C and 40% humidity are demonstrated and films that are stable under 100 W cm-2 photoirradiation, 4× longer than the best previously reported MHPs, are reported. In LEDs, the materials enable an EQE of 24.4% (average 22.5 ± 1.3%) and narrow emission (full-width at half maximum of 30 nm). Sixfold-enhanced operating stability relative to the most stable prior red perovskite LEDs having external quantum efficiency >20% is reported.

Toward printable solar cells based on PbX colloidal quantum dot inks.

Lead chalcogenide (PbX, X = S, Se) colloidal quantum dots (CQDs) are promising solution-processed semiconductor materials for the construction of low-cost, large-area, and flexible solar cells. The properties of CQDs endow them with advantages in semi-conducting film deposition compared to other solution-processed photovoltaic materials, which is critical for the fabrication of efficient large-area solar cells towards industrialization. However, the development of large-area CQD solar cells is impeded by the conventional solid-state ligand exchange process, where the tedious processing with high expense is indispensable to facilitate charge transport of CQD films for photovoltaic applications. In the past several years, the rapid development of CQD inks has boosted the device performance and dramatically simplified the fabrication process. The CQD inks are compatible with most of the industrialized printing techniques, demonstrating potential in fabricating solar modules for commercialization. This article aims to review the recent advances in solar cells based on PbX CQD inks, including both lab-scale and large-area photovoltaic devices prepared from solution-phase ligand exchange (SPLE) as well as the recently invented "one-step" synthesis. We expect to draw attention to the enormous potential of CQD inks for developing high-efficiency and low-cost large-area photovoltaics.