Mixed Lead Halide Passivation of Quantum Dots

Infrared‐absorbing colloidal quantum dots (IR CQDs) are materials of interest in tandem solar cells to augment perovskite and cSi photovoltaics (PV). Today's best IR CQD solar cells rely on the use of passivation strategies based on lead iodide; however, these fail to passivate the entire surface of IR CQDs. Lead chloride passivated CQDs show improved passivation, but worse charge transport. Lead bromide passivated CQDs have higher charge mobilities, but worse passivation. Here a mixed lead‐halide (MPbX) ligand exchange is introduced that enables thorough surface passivation without compromising transport. MPbX–PbS CQDs exhibit properties that exceed the best features of single lead‐halide PbS CQDs: they show improved passivation (43 ± 5 meV vs 44 ± 4 meV in Stokes shift) together with higher charge transport (4 × 10^‐2 ± 3 × 10^‐3 cm² V^‐1 s^‐1 vs 3 × 10^‐2 ± 3 × 10^‐3 cm² V^‐1 s^‐1 in mobility). This translates into PV devices having a record IR open‐circuit voltage (IR V_oc) of 0.46 ± 0.01 V while simultaneously having an external quantum efficiency of 81 ± 1%. They provide a 1.7× improvement in the power conversion efficiency of IR photons (>1.1 µm) relative to the single lead‐halide controls reported herein.     

Infrared Colloidal Quantum Dots for Photovoltaics: Fundamentals and Recent Progress

Colloidal quantum dots (CQDs) are solution‐processed semiconductors of interest in low‐cost photovoltaics. Tuning of the bandgap of CQD films via the quantum size effect enables customization of solar cells’ absorption profile to match the sun's broad visible‐ and infrared‐containing spectrum reaching the earth. Here we review recent progress in the realization of low‐cost, efficient solar cells based on CQDs. We focus in particular on CQD materials and approaches that provide both infrared and visible‐wavelength solar power conversion CQD photovoltaics now exceed 5% solar power conversion efficiency, achieved by the introduction of a new architecture, the depleted‐heterojunction CQD solar cell, that jointly maximizes current, voltage, and fill factor. CQD solar cells have also seen major progress in materials processing for stability, recently achieving extended operating lifetimes in an air ambient. We summarize progress both in device operation and also in gaining new insights into materials properties and processing – including new electrical contact materials and deposition techniques, as well as CQD synthesis, surface treatments, film‐forming technologies – that underpin these rapid advances.     

Slow‐Photon‐Effect‐Induced Photoelectrical‐Conversion Efficiency Enhancement for Carbon‐Quantum‐Dot‐Sensitized Inorganic CsPbBr3 Inverse Opal Perovskite Solar Cells

All‐inorganic cesium lead halide perovskite is suggested as a promising candidate for perovskite solar cells due to its prominent thermal stability and comparable light absorption ability. Designing textured perovskite films rather than using planar‐architectural perovskites can indeed optimize the optical and photoelectrical conversion performance of perovskite photovoltaics. Herein, for the first time, this study demonstrates a rational strategy for fabricating carbon quantum dot (CQD‐) sensitized all‐inorganic CsPbBr₃ perovskite inverse opal (IO) films via a template‐assisted, spin‐coating method. CsPbBr₃ IO introduces slow‐photon effect from tunable photonic band gaps, displaying novel optical response property visible to naked eyes, while CQD inlaid among the IO frameworks not only broadens the light absorption range but also improves the charge transfer process. Applied in the perovskite solar cells, compared with planar CsPbBr₃, slow‐photon effect of CsPbBr₃ IO greatly enhances the light utilization, while CQD effectively facilitates the electron–hole extraction and injection process, prolongs the carrier lifetime, jointly contributing to a double‐boosted power conversion efficiency (PCE) of 8.29% and an increased incident photon‐to‐electron conversion efficiency of up to 76.9%. The present strategy on CsPbBr₃ IO to enhance perovskite PCE can be extended to rationally design other novel optoelectronic devices.     

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%).     

Tuning Solute‐Redistribution Dynamics for Scalable Fabrication of Colloidal Quantum‐Dot Optoelectronics

Solution‐processed colloidal quantum dots (CQDs) are attractive materials for the realization of low‐cost and efficient optoelectronic devices. Although impressive CQD‐solar‐cell performance has been achieved, the fabrication of CQD films is still limited to laboratory‐scale small areas because of the complicated deposition of CQD inks. Large‐area, uniform deposition of lead sulfide (PbS) CQD inks is successfully realized for photovoltaic device applications by engineering the solute redistribution of CQD droplets. It is shown experimentally and theoretically that the solute‐redistribution dynamics of CQD droplets are highly dependent on the movement of the contact line and on the evaporation kinetics of the solvent. By lowering the friction constant of the contact line and increasing the evaporation rate of the droplets, a uniform deposition of CQD ink in length and width over large areas is realized. By utilizing a spray‐coating process, large‐area (up to 100 cm²) CQD films are fabricated with 3–7% thickness variation on various substrates including glass, indium tin oxide glass, and polyethylene terephthalate. Furthermore, scalable fabrication of CQD solar cells is demonstrated with 100 cm² CQD films which exhibits a notably high efficiency of 8.10%.     

Activated Electron‐Transport Layers for Infrared Quantum Dot Optoelectronics

Photovoltaic (PV) materials such as perovskites and silicon are generally unabsorptive at wavelengths longer than 1100 nm, leaving a significant portion of the IR solar spectrum unharvested. Small‐bandgap colloidal quantum dots (CQDs) are a promising platform to offer tandem complementary IR PV solutions. Today, the best performing CQD PVs use zinc oxide (ZnO) as an electron‐transport layer. However, these electrodes require ultraviolet (UV)‐light activation to overcome the low carrier density of ZnO, precluding the realization of CQD tandem photovoltaics. Here, a new sol–gel UV‐free electrode based on Al/Cl hybrid doping of ZnO (CAZO) is developed. Al heterovalent doping provides a strong n‐type character while Cl surface passivation leads to a more favorable band alignment for electron extraction. CAZO CQD IR solar cell devices exhibit, at wavelengths beyond the Si bandgap, an external quantum efficiency of 73%, leading to an additional 0.92% IR power conversion efficiency without UV activation. Conventional ZnO devices, on the other hand, add fewer than 0.01 power points at these operating conditions.     

Infrared Solution‐Processed Quantum Dot Solar Cells Reaching External Quantum Efficiency of 80% at 1.35 µm and Jsc in Excess of 34 mA cm−2

Developing low‐cost photovoltaic absorbers that can harvest the short‐wave infrared (SWIR) part of the solar spectrum, which remains unharnessed by current Si‐based and perovskite photovoltaic technologies, is a prerequisite for making high‐efficiency, low‐cost tandem solar cells. Here, infrared PbS colloidal quantum dot (CQD) solar cells employing a hybrid inorganic–organic ligand exchange process that results in an external quantum efficiency of 80% at 1.35 µm are reported, leading to a short‐circuit current density of 34 mA cm^?2 and a power conversion efficiency (PCE) up to 7.9%, which is a current record for SWIR CQD solar cells. When this cell is placed at the back of an MAPbI₃ perovskite film, it delivers an extra 3.3% PCE by harnessing light beyond 750 nm.     

High‐Efficiency Photovoltaic Devices using Trap‐Controlled Quantum‐Dot Ink prepared via Phase‐Transfer Exchange

Colloidal‐quantum‐dot (CQD) photovoltaic devices are promising candidates for low‐cost power sources owing to their low‐temperature solution processability and bandgap tunability. A power conversion efficiency (PCE) of >10% is achieved for these devices; however, there are several remaining obstacles to their commercialization, including their high energy loss due to surface trap states and the complexity of the multiple‐step CQD‐layer‐deposition process. Herein, high‐efficiency photovoltaic devices prepared with CQD‐ink using a phase‐transfer‐exchange (PTE) method are reported. Using CQD‐ink, the fabrication of active layers by single‐step coating and the suppression of surface trap states are achieved simultaneously. The CQD‐ink photovoltaic devices achieve much higher PCEs (10.15% with a certified PCE of 9.61%) than the control devices (7.85%) owing to improved charge drift and diffusion. Notably, the CQD‐ink devices show much lower energy loss than other reported high‐efficiency CQD devices. This result reveals that the PTE method is an effective strategy for controlling trap states in CQDs.     

Near‐Unity Quantum Yields from Chloride Treated CdTe Colloidal Quantum Dots

Colloidal quantum dots (CQDs) are promising materials for novel light sources and solar energy conversion. However, trap states associated with the CQD surface can produce non‐radiative charge recombination that significantly reduces device performance. Here a facile post‐synthetic treatment of CdTe CQDs is demonstrated that uses chloride ions to achieve near‐complete suppression of surface trapping, resulting in an increase of photoluminescence (PL) quantum yield (QY) from ca. 5% to up to 97.2 ± 2.5%. The effect of the treatment is characterised by absorption and PL spectroscopy, PL decay, scanning transmission electron microscopy, X‐ray diffraction and X‐ray photoelectron spectroscopy. This process also dramatically improves the air‐stability of the CQDs: before treatment the PL is largely quenched after 1 hour of air‐exposure, whilst the treated samples showed a PL QY of nearly 50% after more than 12 hours.     

Chloride Passivation of ZnO Electrodes Improves Charge Extraction in Colloidal Quantum Dot Photovoltaics

The tunable bandgap of colloidal quantum dots (CQDs) makes them an attractive material for photovoltaics (PV). The best present‐day CQD PV devices employ zinc oxide (ZnO) as an electron transport layer; however, it is found herein that ZnO's surface defect sites and unfavorable electrical band alignment prevent devices from realizing their full potential. Here, chloride (Cl)‐passivated ZnO generated from a solution of presynthesized ZnO nanoparticles treated using an organic‐solvent‐soluble Cl salt is reported. These new ZnO electrodes exhibit decreased surface trap densities and a favorable electronic band alignment, improving charge extraction from the CQD layer and achieving the best‐cell power conversion efficiency (PCE) of 11.6% and an average PCE of 11.4 ± 0.2%.     

Lead Selenide (PbSe) Colloidal Quantum Dot Solar Cells with >10% Efficiency

Low‐cost solution‐processed lead chalcogenide colloidal quantum dots (CQDs) have garnered great attention in photovoltaic (PV) applications. In particular, lead selenide (PbSe) CQDs are regarded as attractive active absorbers in solar cells due to their high multiple‐exciton generation and large exciton Bohr radius. However, their low air stability and occurrence of traps/defects during film formation restrict their further development. Air‐stable PbSe CQDs are first synthesized through a cation exchange technique, followed by a solution‐phase ligand exchange approach, and finally absorber films are prepared using a one‐step spin‐coating method. The best PV device fabricated using PbSe CQD inks exhibits a reproducible power conversion efficiency of 10.68%, 16% higher than the previous efficiency record (9.2%). Moreover, the device displays remarkably 40‐day storage and 8 h illuminating stability. This novel strategy could provide an alternative route toward the use of PbSe CQDs in low‐cost and high‐performance infrared optoelectronic devices, such as infrared photodetectors and multijunction solar cells.     

A Colloidal‐Quantum‐Dot‐Based Self‐Charging System via the Near‐Infrared Band

A novel self‐charging platform is proposed using colloidal‐quantum‐dot (CQD) photovoltaics (PVs) via the near‐infrared (NIR) band for low‐power electronics. Low‐bandgap CQDs can convert invisible NIR light sources to electrical energy more efficiently than wider spectra because of reduced thermalization loss. This energy‐conversion strategy via NIR photons ensures an enhanced photostability of the CQD devices. Furthermore, the NIR wireless charging system can be concealed using various colored and NIR‐transparent fabric or films, providing aesthetic freedom. Finally, an NIR‐driven wireless charging system is demonstrated for a wearable healthcare bracelet by integrating a CQD PVs receiver with a flexible lithium‐ion battery and entirely embedding them into a flexible strap, enabling permanent self‐charging without detachment.     

Enhanced open-circuit voltage in visible quantum dot photovoltaics by engineering of carrier-collecting electrodes

Colloidal quantum dots (CQDs) enable multijunction solar cells using a single material programmed using the quantum size effect. Here we report the systematic engineering of 1.6 eV PbS CQD solar cells, optimal as the front cell responsible for visible-wavelength harvesting in tandem photovoltaics. We rationally optimize each of the device's collecting electrodes-the heterointerface with electron-accepting TiO(2) and the deep-work-function hole-collecting MoO(3) for ohmic contact-for maximum efficiency. We report an open-circuit voltage of 0.70 V, the highest observed in a colloidal quantum dot solar cell operating at room temperature. We report an AM1.5 solar power conversion efficiency of 3.5%, the highest observed in >1.5 eV bandgap CQD PV device.     

Nanostructured Back Reflectors for Efficient Colloidal Quantum‐Dot Infrared Optoelectronics

Colloidal quantum dots (CQDs) can be used to extend the response of solar cells, enabling the utilization of solar power that lies to the red of the bandgap of c‐Si and perovskites. To achieve largely complete absorption of infrared (IR) photons in CQD solids requires thicknesses on the micrometer range; however, this exceeds the typical diffusion lengths (≈300 nm) of photoexcited charges in these materials. Nanostructured metal back electrodes that grant the cell efficient IR light trapping in thin active layers with no deterioration of the electrical properties are demonstrated. Specifically, a new hole‐transport layer (HTL) is developed and directly nanostructured. Firstly, a material set to replace conventional rigid HTLs in CQD devices is developed with a moldable HTL that combines the mechanical and chemical requisites for nanoimprint lithography with the optoelectronic properties necessary to retain efficient charge extraction through an optically thick layer. The new HTL is nanostructured in a 2D lattice and conformally coated with MoO₃/Ag. The photonic structure in the back electrode provides a record photoelectric conversion efficiency of 86%, beyond the Si bandgap, and a 22% higher IR power conversion efficiency compared to the best previous reports.     

Enhanced Open‐Circuit Voltage in Colloidal Quantum Dot Photovoltaics via Reactivity‐Controlled Solution‐Phase Ligand Exchange

The energy disorder that arises from colloidal quantum dot (CQD) polydispersity limits the open‐circuit voltage (V_OC) and efficiency of CQD photovoltaics. This energy broadening is significantly deteriorated today during CQD ligand exchange and film assembly. Here, a new solution‐phase ligand exchange that, via judicious incorporation of reactivity‐engineered additives, provides improved monodispersity in final CQD films is reported. It has been found that increasing the concentration of the less reactive species prevents CQD fusion and etching. As a result, CQD solar cells with a V_OC of 0.7 V (vs 0.61 V for the control) for CQD films with exciton peak at 1.28 eV and a power conversion efficiency of 10.9% (vs 10.1% for the control) is achieved.     

Quantum Dot-Siloxane Anchoring on Colloidal Quantum Dot Film for Flexible Photovoltaic Cell

Lead sulfide (PbS) colloidal quantum dots (CQDs) are promising materials for next-generation flexible solar cells because of near-infrared absorption, facile bandgap tunability, and superior air stability. However, CQD devices still lack enough flexibility to be applied to wearable devices owing to the poor mechanical properties of CQD films. In this study, a facile approach is proposed to improve the mechanical stability of CQDs solar cells without compromising the high power conversion efficiency (PCE) of the devices. (3-aminopropyl)triethoxysilane (APTS) is introduced on CQD films to strengthen the dot-to-dot bonding via QD-siloxane anchoring, and as a result, crack pattern analysis reveals that the treated devices become robust to mechanical stress. The device maintains 88% of the initial PCE under 12 000 cycles at a bending radius of 8.3 mm. In addition, APTS forms a dipole layer on CQD films, which improves the open circuit voltage (V_OC) of the device, achieving a PCE of 11.04%, one of the highest PCEs in flexible PbS CQD solar cells.     

A Facet‐Specific Quantum Dot Passivation Strategy for Colloid Management and Efficient Infrared Photovoltaics

Colloidal nanocrystals combine size‐ and facet‐dependent properties with solution processing. They offer thus a compelling suite of materials for technological applications. Their size‐ and facet‐tunable features are studied in synthesis; however, to exploit their features in optoelectronic devices, it will be essential to translate control over size and facets from the colloid all the way to the film. Larger‐diameter colloidal quantum dots (CQDs) offer the attractive possibility of harvesting infrared (IR) solar energy beyond absorption of silicon photovoltaics. These CQDs exhibit facets (nonpolar (100)) undisplayed in small‐diameter CQDs; and the materials chemistry of smaller nanocrystals fails consequently to translate to materials for the short‐wavelength IR regime. A new colloidal management strategy targeting the passivation of both (100) and (111) facets is demonstrated using distinct choices of cations and anions. The approach leads to narrow‐bandgap CQDs with impressive colloidal stability and photoluminescence quantum yield. Photophysical studies confirm a reduction both in Stokes shift (≈47 meV) and Urbach tail (≈29 meV). This approach provides a ≈50% increase in the power conversion efficiency of IR photovoltaics compared to controls, and a ≈70% external quantum efficiency at their excitonic peak.     

Hybrid passivated colloidal quantum dot solids

Colloidal quantum dot (CQD) films allow large-area solution processing and bandgap tuning through the quantum size effect. However, the high ratio of surface area to volume makes CQD films prone to high trap state densities if surfaces are imperfectly passivated, promoting recombination of charge carriers that is detrimental to device performance. Recent advances have replaced the long insulating ligands that enable colloidal stability following synthesis with shorter organic linkers or halide anions, leading to improved passivation and higher packing densities. Although this substitution has been performed using solid-state ligand exchange, a solution-based approach is preferable because it enables increased control over the balance of charges on the surface of the quantum dot, which is essential for eliminating midgap trap states. Furthermore, the solution-based approach leverages recent progress in metal:chalcogen chemistry in the liquid phase. Here, we quantify the density of midgap trap states in CQD solids and show that the performance of CQD-based photovoltaics is now limited by electron-hole recombination due to these states. Next, using density functional theory and optoelectronic device modelling, we show that to improve this performance it is essential to bind a suitable ligand to each potential trap site on the surface of the quantum dot. We then develop a robust hybrid passivation scheme that involves introducing halide anions during the end stages of the synthesis process, which can passivate trap sites that are inaccessible to much larger organic ligands. An organic crosslinking strategy is then used to form the film. Finally, we use our hybrid passivated CQD solid to fabricate a solar cell with a certified efficiency of 7.0%, which is a record for a CQD photovoltaic device.     

Fine Multi‐Phase Alignments in 2D Perovskite Solar Cells with Efficiency over 17% via Slow Post‐Annealing

Layered Ruddlesden–Popper (RP) phase (2D) halide perovskites have attracted tremendous attention due to the wide tunability on their optoelectronic properties and excellent robustness in photovoltaic devices. However, charge extraction/transport and ultimate power conversion efficiency (PCE) in 2D perovskite solar cells (PSCs) are still limited by the non‐eliminable quantum well effect. Here, a slow post‐annealing (SPA) process is proposed for BA₂MA₃Pb₄I₁₃ (n = 4) 2D PSCs by which a champion PCE of 17.26% is achieved with simultaneously enhanced open‐circuit voltage, short‐circuit current, and fill factor. Investigation with optical spectroscopy coupled with structural analyses indicates that enhanced crystal orientation and favorable alignment on the multiple perovskite phases (from the 2D phase near bottom to quasi‐3D phase near top regions) is obtained with SPA treatment, which promotes carrier transport/extraction and suppresses Shockley–Read–Hall charge recombination in the solar cell. As far as it is known, the reported PCE is so far the highest efficiency in RP phase 2D PSCs based on butylamine (BA) spacers (n = 4). The SPA‐processed devices exhibit a satisfactory stability with <4.5% degradation after 2000 h under N₂ environment without encapsulation. The demonstrated process strategy offers a promising route to push forward the performance in 2D PSCs toward realistic photovoltaic applications.     

0D–2D Quantum Dot: Metal Dichalcogenide Nanocomposite Photocatalyst Achieves Efficient Hydrogen Generation

Hydrogen generation via photocatalysis‐driven water splitting provides a convenient approach to turn solar energy into chemical fuel. The development of photocatalysis system that can effectively harvest visible light for hydrogen generation is an essential task in order to utilize this technology. Herein, a kind of cadmium free Zn–Ag–In–S (ZAIS) colloidal quantum dots (CQDs) that shows remarkably photocatalytic efficiency in the visible region is developed. More importantly, a nanocomposite based on the combination of 0D ZAIS CQDs and 2D MoS₂ nanosheet is developed. This can leverage the strong light harvesting capability of CQDs and catalytic performance of MoS₂ simultaneously. As a result, an excellent external quantum efficiency of 40.8% at 400 nm is achieved for CQD‐based hydrogen generation catalyst. This work presents a new platform for the development of high‐efficiency photocatalyst based on 0D–2D nanocomposite.