Overcoming the Ambient Manufacturability-Scalability-Performance Bottleneck in Colloidal Quantum Dot Photovoltaics

Synthesis and Processing Strategy for High-Bandgap PbS Quantum Dots: A Promising Candidate for Harvesting High-Energy Photons in Solar Cells

The wide tunability of the energy bandgap of colloidal lead sulfide (PbS) quantum dots (QDs) has uniquely positioned them for the development of single junction and tandem solar cells. While there have been substantial advancements in moderate and narrow bandgap PbS QDs-ideal for single junction solar cells and the bottom cell in tandem solar cells, respectively; progress has been limited in high-bandgap PbS QDs that are ideally suited for the formation of the top cell in tandem solar cells. The development of appropriate high bandgap PbS QDs would be a major advancement toward realizing efficient all-QD tandem solar cells utilizing different sizes of PbS QDs. Here, we report a comprehensive approach encompassing synthetic strategy, ligand engineering, and hole transport layer (HTL) modification to implement high-bandgap PbS QDs into solar cell devices. We achieved a greater degree of size homogeneity in high-bandgap PbS QDs through the use of a growth retarding agent and a partial passivation strategy. By adjusting the ligand polarity, we successfully grow HTL over the QD film to fabricate solar cells. With the aid of an interface modifying layer, we incorporated an organic HTL for the realization of high-performance solar cells. These solar cells exhibited an impressive open-circuit voltage of 0.824 V and a power conversion efficiency of 10.7%, marking a 360% improvement over previous results.

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.

Revealing oxygen effect on efficiency and stability of quantum dot photovoltaics

Colloidal quantum dot solar cells (CQDSCs) have received great attention in the development of scalable and stable photovoltaic devices. Despite the high power-conversion-efficiency (PCE) reported, stability investigations are still limited and the exact degradation mechanisms of CQDSCs remain unclear under different atmosphere conditions. In this study, the atmospheric influence on the ZnO electron transport layer material (ETL), halide-passivated lead sulfide CQDs (PbS-PbI2) photoactive layer material and 1,2-ethanedithiol-PbS CQDs (PbS-EDT) hole transport material on device stability in PbS CQDSCs is investigated. It was found that O2 had negligible influence on PbS-PbI2, but it did induce the increase in work function of ZnO ETL and PbS-EDT layers. Notably, the increase of the ZnO work function (WFZnO) induces the formation of interface barrier between ZnO and PbS-PbI2, leading to a deterioration in device efficiency. By further replacing ZnO ETL with SnO2, a multi-interface collaborative CQDSC was constructed to realize the PCE with high stability. This study identifies the efficiency evolution that is inherent in CQDSCs under different atmospheric conditions.

Improvement in Performance and Stability of PbS QD/IGZO Phototransistors Through the Introduction of Ga2O3 Film for Broadband Sensor Applications

The development of broadband photosensors has become crucial in various fields. Indium-gallium-zinc oxide (IGZO, In:Ga:Zn = 1:1:1) phototransistors with PbS quantum dots (QDs) have shown promising features for such sensors, such as reasonable mobility, low leakage current, good photosensitivity, and low-cost fabrication. However, the instability of PbS QD/IGZO phototransistors under an air atmosphere and prolonged storage remain serious concerns. In this article, two concepts to improve the reliability of PbS QD/IGZO phototransistors were implemented. P-type doping in the PbS QD layer through oxidation allows increasing the built-in potential between IGZO and PbS QDs, leading to enhancement in photoinduced electron-hole pair creation. Second, agglomeration and fusion of a PbS QDs layer were controlled via thermal annealing, which facilitated the transport of photocreated carriers. The p-type doping and interconnection of a PbS QD layer can be achieved by deposition and subsequent thermal annealing of gallium oxide (Ga2O3) on PbS QD/IGZO stacks. The resulting Ga2O3/PbS QD/IGZO phototransistors exhibited high-performance switching characteristics under dark conditions. Notably, they showed a remarkable photoresponsivity of 196.69 ± 4.05 A/W and a detectivity of (5.47 ± 1.4) × 1012 Jones even at a long-wavelength illumination of 1550 nm. While the unpassivated PbS/IGZO phototransistor suffered serious degradation in optical performance after 2 weeks of storage, the Ga2O3/PbS QD/IGZO phototransistor demonstrated enhanced stability, maintaining high performance for over 5 weeks. These findings suggest that Ga2O3/PbS QD/IGZO phototransistors offer a feasible approach for the fabrication of large-scale active matrix broadband photosensor arrays, potentially revolutionizing optical sensing in various cutting-edge applications.

Role of the ZnO electron transport layer in PbS colloidal quantum dot solar cell yield

The development of lead sulfide (PbS) colloidal quantum dot (CQD) solar cells has led to significant power conversion efficiency (PCE) improvements in recent years, with record efficiencies now over 15%. Many of the recent advances in improving PCE have focused on improving the interface between the PbS CQD active layer and the zinc oxide (ZnO) electron transport layer (ETL). Proper optimization of the ZnO ETL also increases yield, or the percentage of functioning devices per fabrication run. Simultaneous improvements in both PCE and yield will be critical as the field approaches commercialization. This review highlights recent advances in the synthesis of ZnO ETLs and discusses the impact and critical role of ZnO synthesis conditions on the PCE and yield of PbS CQD solar cells.

Improving the efficiency of quantum dot-sensitized solar cells by increasing the QD loading amount

In quantum dot-sensitized solar cells (QDSCs), optimized quantum dot (QD) loading mode and high QD loading amount are prerequisites for great device performance. Capping ligand-induced self-assembly (CLIS) mode represents the mainstream QD loading strategy in the fabrication of high-efficiency QDSCs. However, there remain limitations in CLIS that constrain further enhancement of QD loading levels. This review illustrates the development of various QD loading methods in QDSCs, with an emphasis on the outstanding merits and bottlenecks of CLIS. Subsequently, thermodynamic and kinetic factors dominating QD loading behaviors in CLIS are analyzed theoretically. Upon understanding driving forces, resistances, and energy effects in a QD assembly process, various novel strategies for improving the QD loading amount in CLIS are summarized, and the related functional mechanism is established. Finally, the article concludes and outlooks some remaining academic issues to be solved, so that higher QD loading amount and efficiencies of QDSCs can be anticipated in the future.

An ultra-sensitive colloidal quantum dot infrared photodiode exceeding 100 000% external quantum efficiency via photomultiplication

In this study, we present ultrasensitive infrared photodiodes based on PbS colloidal quantum dots (CQDs) using a double photomultiplication strategy that utilizes the accumulation of both electron and hole carriers. While electron accumulation was induced by ZnO trap states that were created by treatment in a humid atmosphere, hole accumulation was achieved using a long-chain ligand that increased the barrier to hole collection. Interestingly, we obtained the highest responsivity in photo-multiplicative devices with the long ligands, which contradicts the conventional belief that shorter ligands are more effective for optoelectronic devices. Using these two charge accumulation effects, we achieved an ultrasensitive detector with a responsivity above 7.84 × 102 A W-1 and an external quantum efficiency above 105% in the infrared region. We believe that the photomultiplication effect has great potential for surveillance systems, bioimaging, remote sensing, and quantum communication.

Interface Engineering to Drive High-Performance MXene/PbS Quantum Dot NIR Photodiode

The realization of a controllable transparent conducting system with selective light transparency is crucial for exploring many of the most intriguing effects in top-illuminated optoelectronic devices. However, the performance is limited by insufficient electrical conductivity, low work function, and vulnerable interface of traditional transparent conducting materials, such as tin-doped indium oxide. Here, it is reported that two-dimensional (2D) titanium carbide (Ti3 C2 Tx ) MXene film acts as an efficient transparent conducting electrode for the lead sulfide (PbS) colloidal quantum dots (CQDs) photodiode with controllable near infrared transmittance. The solution-processed interface engineering of MXene and PbS layers remarkably reduces the interface defects of MXene/PbS CQDs and the carrier concentration in the PbS layer. The stable Ti3 C2 Tx /PbS CQDs photodiodes give rise to a high specific detectivity of 5.51 × 1012  cm W-1  Hz1/2 , a large dynamic response range of 140 dB, and a large bandwidth of 0.76 MHz at 940 nm in the self-powered state, ranking among the most exceptional in terms of comprehensive performance among reported PbS CQDs photodiodes. In contrast with the traditional photodiode technologies, this efficient and stable approach opens a new horizon to construct widely used infrared photodiodes with CQDs and MXenes.

Colloidal PbS Quantum Dot Photodiode Imager with Suppressed Dark Current

Lead sulfide (PbS) colloidal quantum dots (CQDs) for photodetectors (PDs) have garnered great attention due to their potential use as low-cost, high-performance, and large-area infrared focal plane arrays. The prevailing device architecture employed for PbS CQD PDs is the p-i-n structure, where PbS CQD films treated with thiol molecules, such as 1,2-ethanedithiol (EDT), are widely used as p-type layers due to their favorable band alignment. However, PbS-EDT films face a critical challenge associated with low film quality, resulting in many defects that curtail the device performance. Herein, a controlled oxidization process is developed for better surface passivation of the PbS-EDT transport layer. The dark current density (Jd) of PbS CQD PDs based on optimized PbS-EDT layer shows a dramatic decrease by nearly 2 orders of magnitude. The increase of carrier lifetime and suppression of carrier recombination via controlled oxidation in PbS-EDT CQDs were confirmed by transient absorption spectra and electrochemical impedance spectra. The device based on the optimized PbS-EDT hole transport layer (HTL) exhibits a specific detectivity (D*) that is 3.4 times higher compared to the control device. Finally, the CQD PD employing oxidization PbS-EDT CQDs is integrated with a thin film transistor (TFT) readout circuit, which successfully accomplishes material discrimination imaging, material occlusion imaging, and smoke penetration imaging. The controlled oxidization strategy verifies the significance of surface management of CQD solids and is expected to help advance infrared optoelectronic applications based on CQDs.

Recent Progress in Colloidal Quantum Dot Thermoelectrics

Semiconducting colloidal quantum dots (CQDs) represent an emerging class of thermoelectric materials for use in a wide range of future applications. CQDs combine solution processability at low temperatures with the potential for upscalable manufacturing via printing techniques. Moreover, due to their low dimensionality, CQDs exhibit quantum confinement and a high density of grain boundaries, which can be independently exploited to tune the Seebeck coefficient and thermal conductivity, respectively. This unique combination of attractive attributes makes CQDs very promising for application in emerging thermoelectric generator (TEG) technologies operating near room temperature. Herein, recent progress in CQDs for application in emerging thin-film thermoelectrics is reviewed. First, the fundamental concepts of thermoelectricity in nanostructured materials are outlined, followed by an overview of the popular synthetic methods used to produce CQDs with controllable sizes and shapes. Recent strides in CQD-based thermoelectrics are then discussed with emphasis on their application in thin-film TEGs. Finally, the current challenges and future perspectives for further enhancing the performance of CQD-based thermoelectric materials for future applications are discussed.

Cation-Exchange Enables In Situ Preparation of PbSe Quantum Dot Ink for High Performance Solar Cells

Lead selenide (PbSe) colloidal quantum dots (CQDs) are promising candidates for optoelectronic applications. To date, PbSe CQDs capped by halide ligands exhibit improved stability and solar cells using these CQDs as active layers have reported a remarkable power conversion efficiency (PCE) up to 10%. However, PbSe CQDs are more prone to oxidation, requiring delicate control over their processability and compromising their applications. Herein, an efficient strategy that addresses this issue by an in situ cation-exchange process is reported. This is achieved by a two-phase ligand exchange process where PbI₂ serves as both a passivating ligand and cation-source inducing transformation of CdSe to PbSe. The defect density and carrier lifetime of PbSe CQD films are improved to 1.05 × 10¹⁶  cm^-3 and 12.2 ns, whereas the traditional PbSe CQD films possess 1.9 × 10¹⁶  cm^-3 defect density and 10.2 ns carrier lifetime. These improvements are translated into an enhancement of photovoltaic performance of PbSe solar cells, with a PCE of up to 11.6%, ≈10% higher than the previous record. Notably, the approach enables greatly improved stability and a two-month stability is successfully demonstrated. This strategy is expected to promote the fast development of PbSe CQD applications in low-cost and high-performance optoelectronic devices.

Recent progress and future prospects on halide perovskite nanocrystals for optoelectronics and beyond

As an emerging new class of semiconductor nanomaterials, halide perovskite (ABX3, X = Cl, Br, or I) nanocrystals (NCs) are attracting increasing attention owing to their great potential in optoelectronics and beyond. This field has experienced rapid breakthroughs over the past few years. In this comprehensive review, halide perovskite NCs that are either freestanding or embedded in a matrix (e.g., perovskites, metal-organic frameworks, glass) will be discussed. We will summarize recent progress on the synthesis and post-synthesis methods of halide perovskite NCs. Characterizations of halide perovskite NCs by using a variety of techniques will be present. Tremendous efforts to tailor the optical and electronic properties of halide perovskite NCs in terms of manipulating their size, surface, and component will be highlighted. Physical insights gained on the unique optical and charge-carrier transport properties will be provided. Importantly, the growing potential of halide perovskite NCs for advancing optoelectronic applications and beyond including light-emitting devices (LEDs), solar cells, scintillators and X-ray imaging, lasers, thin-film transistors (TFTs), artificial synapses, and light communication will be extensively discussed, along with prospecting their development in the future.

Colloidal Quantum Dot Solar Cells: Progressive Deposition Techniques and Future Prospects on Large-Area Fabrication

Colloidally grown nanosized semiconductors yield extremely high-quality optoelectronic materials. Many examples have pointed to near perfect photoluminescence quantum yields, allowing for technology-leading materials such as high purity color centers in display technology. Furthermore, because of high chemical yield, and improved understanding of the surfaces, these materials, particularly colloidal quantum dots (QDs) can also be ideal candidates for other optoelectronic applications. Given the urgent necessity toward carbon neutrality, electricity from solar photovoltaics will play a large role in the power generation sector. QDs are developed and shown dramatic improvements over the past 15 years as photoactive materials in photovoltaics with various innovative deposition properties which can lead to exceptionally low-cost and high-performance devices. Once the key issues related to charge transport in optically thick arrays are addressed, QD-based photovoltaic technology can become a better candidate for practical application. In this article, the authors show how the possibilities of different deposition techniques can bring QD-based solar cells to the industrial level and discuss the challenges for perovskite QD solar cells in particular, to achieve large-area fabrication for further advancing technology to solve pivotal energy and environmental issues.

Solution Annealing Induces Surface Chemical Reconstruction for High-Efficiency PbS Quantum Dot Solar Cells

Colloidal quantum dots (CQDs) have a large specific surface area and a complex surface structure. Their properties in diverse optoelectronic applications are largely determined by their surface chemistry. Therefore, it is essential to investigate the surface chemistry of CQDs for improving device performance. Herein, we realized an efficient surface chemistry optimization of lead sulfide (PbS) CQDs for photovoltaics by annealing the CQD solution with concentrated lead halide ligands after the conventional solution-phase ligand exchange. During the annealing process, the colloidal solution was used to transfer heat and create a secondary reaction environment, promoting the desorption of electrically insulating oleate ligands as well as the trap-related surface groups (Pb-hydroxyl and oxidized Pb species). This was accompanied by the binding of more conductive lead halide ligands on the CQD surface, eventually achieving a more complete ligand exchange. Furthermore, this strategy also minimized CQD polydispersity and decreased aggregation caused by conventional solution-phase ligand exchange, thereby contributing to yielding CQD films with twofold enhanced carrier mobility and twofold reduced trap-state density compared with those of the control. Based on these merits, the fabricated PbS CQD solar cells showed high efficiency of 11% under ambient conditions. Our strategy opens a novel and effective avenue to obtain high-efficiency CQD solar cells with diverse band gaps, providing meaningful guidance for controlling ligand reactivity and realizing subtly purified CQDs.

Guanidinium-Pseudohalide Perovskite Interfaces Enable Surface Reconstruction of Colloidal Quantum Dots for Efficient and Stable Photovoltaics

Complete surface passivation of colloidal quantum dots (CQDs) and their strong electronic coupling are key factors toward high-performance CQD-based photovoltaics (CQDPVs). Also, the CQD matrices must be protected from oxidative environments, such as ambient air and moisture, to guarantee air-stable operation of the CQDPVs. Herein, we devise a complementary and effective approach to reconstruct the oxidized CQD surface using guanidinium and pseudohalide. Unlike conventional halides, thiocyanate anions provide better surface passivation with effective replacement of surface oxygen species and additional filling of defective sites, whereas guanidinium cations promote the construction of epitaxial perovskite bridges within the CQD matrix and augment electronic coupling. Additionally, we replace a defective 1,2-ethanedithiol-treated CQD hole transport layer (HTL) with robust polymeric HTLs, based on a judicious consideration of the energy level alignment established at the CQD/HTL interface. These efforts collectively result in high-performance and stable CQDPVs with photocurrents over 30 mA cm^-2, ∼80% quantum efficiency at excitonic peaks and stable operation under humid and ambient conditions. Elucidation of carrier dynamics further reveals that interfacial recombination associated with band alignment governs both the CQDPV performance and stability.

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.

Solvent Engineering of Colloidal Quantum Dot Inks for Scalable Fabrication of Photovoltaics

Development of colloidal quantum dot (CQD) inks enables single-step spin-coating of compact CQD films of appropriate thickness, enabling the promising performance of CQD photovoltaics (CQDPVs). Today's highest-performing CQD inks rely on volatile n-butylamine (BTA), but it is incompatible with scalable deposition methods since a rapid solvent evaporation results in irregular film thickness with an uneven surface. Here, we present a hybrid solvent system, consisting of BTA and N,N-dimethylformamide, which has a favorable acidity for colloidal stability as well as an appropriate vapor pressure, enabling a stable CQD ink that can be used to fabricate homogeneous, large-area CQD films via spray-coating. CQDPVs fabricated with the CQD ink exhibit suppressed charge recombination as well as fast charge extraction compared with conventional CQD ink-based PVs, achieving an improved power conversion efficiency (PCE) of 12.22% in spin-coated devices and the highest ever reported PCE of 8.84% among spray-coated CQDPVs.

The effect of water on colloidal quantum dot solar cells

Almost all surfaces sensitive to the ambient environment are covered by water, whereas the impacts of water on surface-dominated colloidal quantum dot (CQD) semiconductor electronics have rarely been explored. Here, strongly hydrogen-bonded water on hydroxylated lead sulfide (PbS) CQD is identified. The water could pilot the thermally induced evolution of surface chemical environment, which significantly influences the nanostructures, carrier dynamics, and trap behaviors in CQD solar cells. The aggravation of surface hydroxylation and water adsorption triggers epitaxial CQD fusion during device fabrication under humid ambient, giving rise to the inter-band traps and deficiency in solar cells. To address this problem, meniscus-guided-coating technique is introduced to achieve dense-packed CQD solids and extrude ambient water, improving device performance and thermal stability. Our works not only elucidate the water involved PbS CQD surface chemistry, but may also achieve a comprehensive understanding of the impact of ambient water on CQD based electronics.

Open-Circuit Voltage Loss in Lead Chalcogenide Quantum Dot Solar Cells

Lead chalcogenide colloidal quantum dot solar cells (CQDSCs) have received considerable attention due to their broad and tunable absorption and high stability. Presently, lead chalcogenide CQDSC has achieved a power conversion efficiency of ≈14%. However, the state-of-the-art lead chalcogenide CQDSC still has an open-circuit voltage (V_oc ) loss of ≈0.45 V, which is significantly higher than those of c-Si and perovskite solar cells. Such high V_oc loss severely limits the performance improvement and commercialization of lead chalcogenide CQDSCs. In this review, the V_oc loss is first analyzed via detailed balance theory and the origin of V_oc loss from both solar absorber and interface is summarized. Subsequently, various strategies for improving the V_oc from the solar absorber, including the passivation strategies during the synthesis and ligand exchange are overviewed. The great impact of the ligand exchange process on CQD passivation is highlighted and the corresponding strategies to further reduce the V_oc loss are summarized. Finally, various strategies are discussed to reduce interface V_oc loss from charge transport layers. More importantly, the great potential of achieving performance breakthroughs via various organic hole transport layers is highlighted and the existing challenges toward commercialization are discussed.

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.

Doped Organic Hole Extraction Layers in Efficient PbS and AgBiS2 Quantum Dot Solar Cells

The efficiency of PbS quantum dot (QD) solar cells has significantly increased in recent years, strengthening their potential for industrial applications. The vast majority of state-of-the-art devices utilize 1,2-ethanedithiol (EDT)-coated PbS QD hole extraction layers, which lead to high initial performance, but result in poor device stability. While excellent performance has also been demonstrated with organic extraction layers, these devices include a molybdenum trioxide (MoO₃) layer, which is also known to decrease device stability. Herein, we demonstrate that organic layers based on a poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) polymer doped with C₆₀F₄₈ can serve as hole extraction layers for efficient EDT-free and MoO₃-free QD solar cells. Such layers are shown to offer high conductivity for facile hole transport to the anode, while effectively blocking electrons due to their low electron affinity. We show that our approach is versatile and is applicable also to AgBiS₂ QD solar cells.

Scalable PbS Quantum Dot Solar Cell Production by Blade Coating from Stable Inks

The recent development of phase transfer ligand exchange methods for PbS quantum dots (QD) has enhanced the performance of quantum dots solar cells and greatly simplified the complexity of film deposition. However, the dispersions of PbS QDs (inks) used for film fabrication often suffer from colloidal instability, which hinders large-scale solar cell production. In addition, the wasteful spin-coating method is still the main technique for the deposition of QD layer in solar cells. Here, we report a strategy for scalable solar cell fabrication from highly stable PbS QD inks. By dispersing PbS QDs capped with CH3NH3PbI3 in 2,6-difluoropyridine (DFP), we obtained inks that are colloidally stable for more than 3 months. Furthermore, we demonstrated that DFP yields stable dispersions even of large diameter PbS QDs, which are of great practical relevance owing to the extended coverage of the near-infrared region. The optimization of blade-coating deposition of DFP-based inks enabled the fabrication of PbS QD solar cells with power conversion efficiencies of up to 8.7%. It is important to underline that this performance is commensurate with the devices made by spin coating of inks with the same ligands. A good shelf life-time of these inks manifests itself in the comparatively high photovoltaic efficiency of 5.8% obtained with inks stored for more than 120 days.

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.

A method for studying pico to microsecond time-resolved core-level spectroscopy used to investigate electron dynamics in quantum dots

Time-resolved photoelectron spectroscopy can give insights into carrier dynamics and offers the possibility of element and site-specific information through the measurements of core levels. In this paper, we demonstrate that this method can access electrons dynamics in PbS quantum dots over a wide time window spanning from pico- to microseconds in a single experiment carried out at the synchrotron facility BESSY II. The method is sensitive to small changes in core level positions. Fast measurements at low pump fluences are enabled by the use of a pump laser at a lower repetition frequency than the repetition frequency of the X-ray pulses used to probe the core level electrons: Through the use of a time-resolved spectrometer, time-dependent analysis of data from all synchrotron pulses is possible. Furthermore, by picosecond control of the pump laser arrival at the sample relative to the X-ray pulses, a time-resolution limited only by the length of the X-ray pulses is achieved. Using this method, we studied the charge dynamics in thin film samples of PbS quantum dots on n-type MgZnO substrates through time-resolved measurements of the Pb 5d core level. We found a time-resolved core level shift, which we could assign to electron injection and charge accumulation at the MgZnO/PbS quantum dots interface. This assignment was confirmed through the measurement of PbS films with different thicknesses. Our results therefore give insight into the magnitude of the photovoltage generated specifically at the MgZnO/PbS interface and into the timescale of charge transport and electron injection, as well as into the timescale of charge recombination at this interface. It is a unique feature of our method that the timescale of both these processes can be accessed in a single experiment and investigated for a specific interface.

PbS Colloidal Quantum Dot Inks for Infrared Solar Cells

Infrared PbS colloidal quantum dot (CQD)-based materials receive significant attention because of its unique properties. The PbS CQD ink that originates from ligand exchange of CQDs is highly potential for efficient and stable infrared CQD solar cells (CQDSCs) using low-temperature solution-phase processing. In this review, we present a comprehensive overview of CQD inks for the development of efficient infrared solar cells, which can effectively harvest the photons from the infrared wavelength region of the solar spectrum, including the importance of infrared absorbers for solar cells, the unique properties of CQDs, ligand-exchange determined CQD inks, and related photovoltaic performance of CQDSCs. Finally, we present a brief conclusion, and the possible challenges and opportunities of the CQD inks are discussed in-depth to further develop highly efficient and stable infrared solar cells.

Hybrid Surface Passivation for Retrieving Charge Collection Efficiency of Colloidal Quantum Dot Photovoltaics

Efficient charge collection in photovoltaics is a key issue toward their high performance. Despite the promising performance of colloidal quantum dot (CQD)-based photovoltaics (CQDPVs), they suffer significant dissipation of photocurrent due to imperfect surface passivation of the CQD hole transport layer (HTL) by a single 1,2-ethaneditihol (EDT) ligand. To address the critical drawback of existing CQDPVs, we offer a hybrid passivation strategy, including both EDT and thiocyanate (SCN). The hybrid passivation leads to seamless surface passivation of CQDs, remarkably suppressing charge recombination. This strategy also augments the p-doping density of the CQD, resulting in a pronounced energy level bending at the active layer/HTL interface and facilitating efficient charge separation. Moreover, enhanced electronic coupling across the CQDs (originating from reduced inter-dot spacing) promotes rapid charge extraction. Consequently, the flawless charge collection by a hybrid-passivated HTL successfully retrieves the photocurrent, achieving an enhanced CQDPV power conversion efficiency of 12.70% compared with 11.49% for the control device.

Low-Cost, Air-Processed Quantum Dot Solar Cells via Diffusion-Controlled Synthesis

Despite significant advances in the development of high-efficiency and stable quantum dot (QD) solar cells (QDSCs), recent synthetic and fabrication routes still require improvements to render QDSCs commercially feasible. Here, we describe a low-cost, industrially viable fabrication method of QDSCs under an ambient atmosphere (humid air and room temperature) using stable, high-quality, and small-sized PbS QDs prepared with low-cost, greener precursors [i.e., thioacetamide (TAA)] compared to the widely used bis(trimethylsilyl)sulfide [(TMS)₂S], at low temperatures without requiring any stringent conditions. The low reaction temperature, medium reactivity of TAA, and diffusion-controlled particle growth adopted in this approach provide an opportunity to synthesize ultrasmall (emission peak ∼700 nm) to larger PbS QDs (emission peak ∼1050 nm). This also enables well-controlled large-scale (multigram) synthesis with a rough estimated production cost of PbS of 8.11 $ per gram (based on materials cost), which is the lowest among the available PbS QDs produced using wet chemistry routes. QDSCs fabricated using 3.25 nm PbS QDs (bandgap 1.29 eV) under ambient conditions yield a high circuit current density (J_sc) of 32.4 mA/cm² (one of the highest values of J_sc ever reported) with a power conversion efficiency of 7.8% under 1 sun simulated sunlight at AM 1.5 G (100 mW/cm²). These devices exhibit better photovoltaic performance compared to devices fabricated with more traditional PbS QDs synthesized with (TMS)₂S under an ambient atmosphere, confirming the quality of PbS QDs produced with our method. The diffusion-controlled TAA-based synthetic route developed herein is found to be very promising for synthesizing size-tunable PbS QDs for photovoltaic and other optoelectronic applications.

Carrier Mobility, Lifetime, and Diffusion Length in Optically Thin Quantum Dot Semiconductor Films

We propose a method to measure the fundamental parameters that govern diffusion transport in optically thin quantum dot semiconductor films and apply it to quantum dot materials with different ligands. Thin films are excited optically, and the profile of photogenerated carriers is modeled using diffusion-based transport equations and taking into account the optical cavity effects. Correlation with steady-state photoluminescence experiments on different stacks comprising a quenching layer allows the extraction of the carrier diffusion length accurately from the experimental data. In the time domain, the mapping of the transient PL data with the solutions of the time-dependent diffusion equation leads to accurate calculations of the photogenerated carrier mobility. These findings allow the estimation of the speed limitations for diffusion-based transport in QD absorbers.

A Chemically Orthogonal Hole Transport Layer for Efficient Colloidal Quantum Dot Solar Cells

Colloidal quantum dots (CQDs) are of interest in light of their solution-processing and bandgap tuning. Advances in the performance of CQD optoelectronic devices require fine control over the properties of each layer in the device materials stack. This is particularly challenging in the present best CQD solar cells, since these employ a p-type hole-transport layer (HTL) implemented using 1,2-ethanedithiol (EDT) ligand exchange on top of the CQD active layer. It is established that the high reactivity of EDT causes a severe chemical modification to the active layer that deteriorates charge extraction. By combining elemental mapping with the spatial charge collection efficiency in CQD solar cells, the key materials interface dominating the subpar performance of prior CQD PV devices is demonstrated. This motivates to develop a chemically orthogonal HTL that consists of malonic-acid-crosslinked CQDs. The new crosslinking strategy preserves the surface chemistry of the active layer beneath, and at the same time provides the needed efficient charge extraction. The new HTL enables a 1.4× increase in charge carrier diffusion length in the active layer; and as a result leads to an improvement in power conversion efficiency to 13.0% compared to EDT standard cells (12.2%).

Quantum-Dot Tandem Solar Cells Based on a Solution-Processed Nanoparticle Intermediate Layer

Tandem cells are one of the most effective ways of breaking the single junction Shockley-Queisser limit. Solution-processable phosphate-buffered saline (PbS) quantum dots are good candidates for producing multiple junction solar cells because of their size-tunable band gap. The intermediate recombination layer (RL) connecting the subcells in a tandem solar cell is crucial for device performance because it determines the charge recombination efficiency and electrical resistance. In this work, a solution-processed ultrathin NiO and Ag nanoparticle film serves as an intermediate layer to enhance the charge recombination efficiency in PbS QD dual-junction tandem solar cells. The champion devices with device architecture of indium tin oxide/S-ZnO/1.45 eV PbS-PbI₂/PbS-EDT/NiO/Ag NP/ZnO NP/1.22 eV PbS-PbI₂/PbS-EDT/Au deliver a 7.1% power conversion efficiency, which outperforms the optimized reference subcells. This result underscores the critical role of an appropriate nanocrystalline RL in producing high-performance solution-processed PbS QD tandem cells.

Cost-Effective and Semi-Transparent PbS Quantum Dot Solar Cells Using Copper Electrodes

PbS quantum dots (QDs) have gained significant attention as promising solution-based materials for third generation of photovoltaic (PV) devices, thanks to their size-tunable band gap, air stability, and low-cost solution processing. Gold (Au), despite its high cost, is the standard electrode in the conventional PbS QD PV architecture because of its perfect alignment with valence levels of PbS QDs. However, to comply with manufacturing requirements for scalable device processing, alternative cost-effective electrodes are urgently required. Here, we employed an interface engineering approach and deposited poly(3-hexylthiophene-2,5-diyl) as a hole transport layer on 1,2-ethanedithiol-capped PbS QDs in order to adjust the valence band of QDs with the work function of inexpensive copper (Cu) electrodes. In fact, this is the first report of a Au-free PbS QD PV system employing the conventional device structure. Our Cu-based device shows a maximum power conversion efficiency (PCE) of 8.7% which is comparable with that of the Au-based device (10.2%). Interestingly, the P3HT-based device shows improved stability with relatively 10% PCE loss after 230 h under continuous illumination. Moreover, using an ultrathin Cu electrode, a semitransparent PbS QD PV is fabricated with a remarkably high average visible transparency of 26% and a PCE of 7.4%.

Role of the electronically-active amorphous state in low-temperature processed In2O3 thin-film transistors

Metal oxide (MO) thin-film transistors (TFTs) are expected to enable low-cost flexible and printed electronics, given their excellent charge transport, low processing temperatures and solution processability. However, achieving adequate mobility when processed scalably at low temperatures compatible with plastic electronics is a challenge. Here, we explore process-structure-transport relationships in blade-coated indium oxide (In2O3) TFTs via both sol-gel and combustion chemistries. We find that the sol-gel chemistry enables n-type TFTs when annealed at 200 °C to 225 °C with noticeable electron mobility ((3.4 ± 1.3) cm2V-1s-1) yet minimal In2O3 crystallinity and surprisingly low levels of the metal-oxygen-metal (M-O-M) lattice content (≈46 %). Increased annealing temperatures result in the appearance of nanocrystalline domains and an increase in M-O-M content to ≈70 %, without any further increase in mobility. An actetylacetone combustion-assisted ink lowers the external thermal budget required for In2O3 crystallization but bypasses the electronically-active amorphous state and underperforms the sol-gel ink at low temperatures. Grain boundary formation and nanocrystalline inclusions in these films due to rapid combustion-assisted crystallization are suggested to be the likely origin behind the significantly compromised charge transport at low-temperatures. Overall, this study emphasizes the need to understand the complex interplay between local order (nanocrystallinity) and connectivity (grain boundary, amorphous phases) when optimizing low-temperature processed MO thin films.

Synergistic ligand exchange and UV curing of PbS quantum dots for effective surface passivation

Lead sulfide (PbS) quantum dots (QDs) are promising materials in solution-processed photovoltaic (PV) devices due to their tunable bandgap and low-cost processing. Replacing the long oleic acid ligands of the as-synthesized QDs with shorter ligands is a key step for making functional QD PVs with correctly tuned band energies and reduced non-radiative recombination centers. In this work, we study the effect of ultraviolet (UV) treatment of PbS QD layers on the QD surface states during ligand exchange. We demonstrate that this straightforward approach effectively reduces the surface trap states and passivates the surface of QDs. We find that UV treatment reduces the density of hydroxyl groups attached to the QD surface and improves the bonding of short ligands to the QD surface. Multiple analyses show the reduction of nonradiative recombination centers for the UV-treated sample. The power conversion efficiency (PCE) of our optimized PbS QD device reached 10.7% (vs. 9% for the control device) and was maintained above 10% after 230 h of constant illumination.

Colloidal-annealing of ZnO nanoparticles to passivate traps and improve charge extraction in colloidal quantum dot solar cells

The popularity of colloidal quantum dot (CQD) solar cells has increased owing to their tunable bandgap, multiple exciton generation, and low-cost solution processes. ZnO nanoparticle (NP) layers are generally employed as electron transport layers in CQD solar cells to efficiently extract the electrons. However, trap sites and the unfavorable band structure of the as-synthesized ZnO NPs have hindered their potential performance. Herein, we introduce a facile method of ZnO NP annealing in the colloidal state. Electrical, structural, and optical analyses demonstrated that the colloidal-annealing of ZnO NPs effectively passivated the defects and simultaneously shifted their band diagram; therefore, colloidal-annealing is a more favorable method as compared to conventional film-annealing. These CQD solar cells based on colloidal-annealed ZnO NPs exhibited efficient charge extraction, reduced recombination and achieved an enhanced power conversion efficiency (PCE) of 9.29%, whereas the CQD solar cells based on ZnO NPs without annealing had a PCE of 8.05%. Moreover, the CQD solar cells using colloidal-annealed ZnO NPs exhibited an improved air stability with 98% retention after 120 days, as compared to that of CQD solar cells using non-annealed ZnO NPs with 84% retention.

Improving Charge Collection from Colloidal Quantum Dot Photovoltaics by Single-Walled Carbon Nanotube Incorporation

Improving charge collection is one of the key issues for high-performance PbS colloidal quantum dot photovoltaics (CQDPVs) due to the considerable charge loss resulting from the low mobility and large defect densities of the 1,2-ethanedithiol-treated PbS quantum dot hole-transporting layer (HTL). To overcome these limitations, single-walled carbon nanotubes (SWNTs) and C₆₀-encapsulated SWNTs (C₆₀@SWNTs) are incorporated into the HTL in CQDPVs. SWNT-incorporated CQDPV demonstrates a significantly improved short-circuit current density (J_SC), and C₆₀@SWNT-incorporated CQDPV exhibits an even higher J_SC than that of pristine SWNT. Both result in improved power-conversion efficiencies. Hole-selective, photoinduced charge extraction with linearly increasing voltage measurements demonstrates that SWNT or C₆₀@SWNT incorporation improves hole-transporting behavior, rendering suppressed charge recombination and enhanced mobility of the HTL. The enhanced p-type characteristics and the improved hole diffusion lengths of SWNT- or C₆₀@SWNT-incorporated HTL bring improvement of the entire hole-transporting length and enable lossless hole collection, which results in the J_SC enhancement of the CQDPVs.

Oxygen Plasma-Induced p-Type Doping Improves Performance and Stability of PbS Quantum Dot Solar Cells

PbS quantum dots (QDs) have been extensively studied for photovoltaic applications, thanks to their facile and low-cost fabrication processing and interesting physical properties such as size dependent and tunable band gap. However, the performance of PbS QD-based solar cells is highly sensitive to the humidity level in the ambient air, which is a serious obstacle toward its practical applications. Although it has been previously revealed that oxygen doping of the hole transporting layer can mitigate the cause of this issue, the suggested methods to recover the device performance are time-consuming and relatively costly. Here, we report a low-power oxygen plasma treatment as a rapid and low-cost method to effectively recover the device performance and stability. Our optimization results show that a 10 min treatment is the best condition, resulting in an enhanced power conversion efficiency from 6.9% for the as-prepared device to 9% for the plasma treated one. Moreover, our modified device shows long-term shelf-life stability.

Thiol and Halometallate, Mutually Passivated Quantum Dot Ink for Photovoltaic Application

Tunable-band-gap colloidal QDs are a potential building block to harvest the wide-energy solar spectrum. The solution-phase surface passivation with lead halide-based halometallate ligands has remarkably simplified the processing of quantum dots (QDs) and enabled the proficient use of materials for the development of solar cells. It is, however, shown that the hallometalate ligand passivated QD ink allows the formation of thick crystalline shell layer, which limits the carrier transport of the QD solids. Organic thiols have long been used to develop QD solar cells using the solid-state ligand exchange approach. However, their use is limited in solution-phase passivation due to poor dispersity of thiol-treated QDs in common solvents. In this report, a joint passivation strategy using thiol and halometallate ligand is developed to prepare the QD ink. The mutually passivated QDs show a 50% reduction in shell thickness, reduced trap density, and improved monodispersity in their solid films. These improvements lead to a 4 times increase in carrier mobility and doubling of the diffusion length, which enable the carrier extraction from a much thicker absorbing layer. The photovoltaic devices show a high efficiency of 10.3% and reduced hysteresis effect. The improvement in surface passivation leads to reduced oxygen doping and improved ambient stability of the solar cells.

In Situ Back-Contact Passivation Improves Photovoltage and Fill Factor in Perovskite Solar Cells

Organic-inorganic hybrid perovskite solar cells (PSCs) have seen a rapid rise in power conversion efficiencies in recent years; however, they still suffer from interfacial recombination and charge extraction losses at interfaces between the perovskite absorber and the charge-transport layers. Here, in situ back-contact passivation (BCP) that reduces interfacial and extraction losses between the perovskite absorber and the hole transport layer (HTL) is reported. A thin layer of nondoped semiconducting polymer at the perovskite/HTL interface is introduced and it is shown that the use of the semiconductor polymer permits-in contrast with previously studied insulator-based passivants-the use of a relatively thick passivating layer. It is shown that a flat-band alignment between the perovskite and polymer passivation layers achieves a high photovoltage and fill factor: the resultant BCP enables a photovoltage of 1.15 V and a fill factor of 83% in 1.53 eV bandgap PSCs, leading to an efficiency of 21.6% in planar solar cells.

Comparing Halide Ligands in PbS Colloidal Quantum Dots for Field-Effect Transistors and Solar Cells

Capping colloidal quantum dots (CQDs) with atomic ligands is a powerful approach to tune their properties and improve the charge carrier transport in CQD solids. Efficient passivation of the CQD surface, which can be achieved with halide ligands, is crucial for application in optoelectronic devices. Heavier halides, i.e., I^- and Br^-, have been thoroughly studied as capping ligands in the last years, but passivation with fluoride ions has not received sufficient consideration. In this work, effective coating of PbS CQDs with fluoride ligands is demonstrated and compared to the results obtained with other halides. The electron mobility in field-effect transistors of PbS CQDs treated with different halides shows an increase with the size of the atomic ligand (from 3.9 × 10^-4 cm²/(V s) for fluoride-treated to 2.1 × 10^-2 cm²/(V s) for iodide-treated), whereas the hole mobility remains unchanged in the range between 1 × 10^-5 cm²/(V s) and 10^-4cm²/(V s). This leads to a relatively more pronounced p-type behavior of the fluoride- and chloride-treated films compared to the iodide-treated ones. Cl^-- and F^--capped PbS CQDs solids were then implemented as p-type layer in solar cells; these devices showed similar performance to those prepared with 1,2-ethanedithiol in the same function. The relatively stronger p-type character of the fluoride- and chloride-treated PbS CQD films broadens the utility of such materials in optoelectronic devices.

Surface and Interface Chemistry in Colloidal Quantum Dots for Solar Applications Studied by X-Ray Photoelectron Spectroscopy

Control of the surface and interface chemistry of colloidal quantum dots (CQDs) is critical to achieving a product with good air stability and high performing optoelectronic devices. Through various surface passivation treatments, vast improvements have been made in fields such as CQD photovoltaics; however devices have not currently reached commercial standards. We show how X-ray photoelectron spectroscopy (XPS) can provide a better understanding of exactly how surface treatments act on CQD surfaces, and the effect of surface composition on air stability and device performance.. We illustrate this with PbS-based CQDs, using XPS to measure oxidation processes, and to quantify the composition of the topmost surface layer after different surface treatments. We also demonstrate the use of synchrotron radiation-excited depth-profiling XPS, a powerful technique for determining the surface composition, chemistry and structure of CQDs. This review describes our recent progress in characterization of CQD surfaces using SR-excited depth profiling XPS and other photoemission techniques.