Towards understanding the initial performance improvement of PbS quantum dot solar cells upon short-term air exposure

Stable Near-Infrared Photoluminescence of Hexagonal-Shaped PbS Nanoparticles with 1-Dodecanethiol Ligands

In this study, lead(II) sulphide (PbS) nanoparticles of varying particle sizes were synthesized using the hot injection method, employing 1-octadecene (ODE) as a coordinating ligand in conjunction with oleylamine (OAm). This synthesis approach was compared with the preparation of hexagonal-shaped nanoparticles through the ligand of 1-Dodecanethiol (DT), resulting in DT-capped PbS nanoparticles. The prepared nanoparticles were characterized using multiple techniques including photoluminescence (PL), transmission electron microscopy (TEM), X-ray diffraction (XRD), and Fourier-transform infrared spectroscopy (FT-IR). The condensation reaction of DT ligands led to various nanoparticles within the range of 34.87 nm to 35.87 nm across different synthesis temperatures (120 °C, 150 °C, 180 °C, 210 °C, and 240 °C). The PbS with DT ligands exhibited a highly crystalline and superhydrophilic structure. Interestingly, near-infrared (NIR)-PL analysis revealed peaks at 1100 nm, representing the lowest-energy excitonic absorption peak of PbS nanoparticles for both ligands. This suggests their potential utility in various applications, including IR photoreactors, as well as in the development of non-toxic nanoparticles for potential applications in in vivo bioimaging.

Understanding Colloidal Quantum Dot Device Characteristics with a Physical Model

Colloidal quantum dots (CQDs) are finding increasing applications in optoelectronic devices, such as photodetectors and solar cells, because of their high material quality, unique and attractive properties, and process flexibility without the constraints of lattice match and thermal budget. However, there is no adequate device model for colloidal quantum dot heterojunctions, and the popular Shockley-Quiesser diode model does not capture the underlying physics of CQD junctions. Here, we develop a compact, easy-to-use model for CQD devices rooted in physics. We show how quantum dot properties, QD ligand binding, and the heterointerface between quantum dots and the electron transport layer (ETL) affect device behaviors. We also show that the model can be simplified to a Shockley-like equation with analytical approximate expressions for reverse saturation current, ideality factor, and quantum efficiency. Our model agrees well with the experiment and can be used to describe and optimize CQD device performance.

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

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

Nanostructured PbS-Doped Inorganic Film Synthesized by Sol-Gel Route

IV-VI semiconductor quantum dots embedded into an inorganic matrix represent nanostructured composite materials with potential application in temperature sensor systems. This study explores the optical, structural, and morphological properties of a novel PbS quantum dots (QDs)-doped inorganic thin film belonging to the Al₂O₃-SiO₂-P₂O₅ system. The film was synthesized by the sol-gel method, spin coating technique, starting from a precursor solution deposited on a glass substrate in a multilayer process, followed by drying of each deposited layer. Crystalline PbS QDs embedded in the inorganic vitreous host matrix formed a nanocomposite material. Specific investigations such as X-ray diffraction (XRD), optical absorbance in the ultraviolet (UV)-visible (Vis)-near infrared (NIR) domain, NIR luminescence, Raman spectroscopy, scanning electron microscopy-energy dispersive X-ray (SEM-EDX), and atomic force microscopy (AFM) were used to obtain a comprehensive characterization of the deposited film. The dimensions of the PbS nanocrystallite phase were corroborated by XRD, SEM-EDX, and AFM results. The luminescence band from 1400 nm follows the luminescence peak of the precursor solution and that of the dopant solution. The emission of the PbS-doped film in the NIR domain is a premise for potential application in temperature sensing systems.

Oxygen-induced degradation in AgBiS2 nanocrystal solar cells

AgBiS₂ nanocrystal solar cells are among the most sustainable emerging photovoltaic technologies. Their environmentally-friendly composition and low energy consumption during fabrication make them particularly attractive for future applications. However, much remains unknown about the stability of these devices, in particular under operational conditions. In this study, we explore the effects of oxygen and light on the stability of AgBiS₂ nanocrystal solar cells and identify its dependence on the charge extraction layers. Normally, the rate of oxygen-induced degradation of nanocrystals is related to their ligands, which determine the access sites by steric hindrance. We demonstrate that the ligands, commonly used in AgBiS₂ solar cells, also play a crucial chemical role in the oxidation process. Specifically, we show that the tetramethylammonium iodide ligands enable their oxidation, leading to the formation of bismuth oxide and silver sulphide. Additionally, the rate of oxidation is impacted by the presence of water, often present at the surface of the ZnO electron extraction layer. Moreover, the degradation of the organic hole extraction layer also impacts the overall device stability and the materials' photophysics. The understanding of these degradation processes is necessary for the development of mitigation strategies for future generations of more stable AgBiS₂ nanocrystal solar cells.

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.

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

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

Functionalized rGO Interlayers Improve the Fill Factor and Current Density in PbS QDs-Based Solar Cells

Graphene-quantum dot nanocomposites attract significant attention for novel optoelectronic devices, such as ultrafast photodetectors and third-generation solar cells. Combining the remarkable optical properties of quantum dots (QDs) with the exceptional electrical properties of graphene derivatives opens a vast perspective for further growth in solar cell efficiency. Here, we applied (3-mercaptopropyl) trimethoxysilane functionalized reduced graphene oxide (f-rGO) to improve the QDs-based solar cell active layer. The different strategies of f-rGO embedding are explored. When f-rGO interlayers are inserted between PbS QD layers, the solar cells demonstrate a higher current density and a better fill factor. A combined study of the morphological and electrical parameters of the solar cells shows that the improved efficiency is associated with better layer homogeneity, lower trap-state densities, higher charge carrier concentrations, and the blocking of the minor charge carriers.