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

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.

Indeno[3,2-b]carbazole-Based Small Molecule Layer Enables Optimized Carrier Transport for PbS Quantum Dot NIR Photodetectors

Colloidal quantum dots (CQDs) have garnered considerable attention for photodetectors (PDs), attributable to exceptional photoelectric properties and ease solution-based processing. However, the prevalent use of 1,2-ethanedithiol (EDT) as a hole transport layer (HTL) has limitations, such as energy level discrepancies, requisite oxidation, and intricate multilayer assembly. Organic p-type materials, lauded for their superior attributes and synthetic versatility, are now stepping forward as viable substitutes for conventional EDT HTLs. In this work, we introduced an organic HTL derived from indolo[3,2-b]carbazole, named ZL004, leading to a marked improvement in carrier generation and collection, facilitated by the optimized band alignment and enhanced interfacial charge dynamics. The ZL004-based PDs exhibit a photoresponsivity of 0.45 A/W, a noise current of 1.8 × 10-11 A Hz-0.5, a specific detectivity of 4.6 × 109 Jones, and an expansive linear dynamic range of 107 dB─surpassing EDT-based devices across the board, demonstrating the extraordinary property of organic p-type materials for CQD-based PDs.

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.

High Detectivity of PbS Films Deposited on Quartz Substrates: The Role of Enhanced Photogenerated Carrier Separation

PbS films grown on quartz substrates by the chemical bath deposition method were annealed in an O2 atmosphere to investigate the role of oxygen in the sensitization process at different annealing temperatures. The average grain size of the PbS films gradually increased as the annealing temperature increased from 400 °C to 700 °C. At an annealing temperature of 650 °C, the photoresponsivity and detectivity reached 1.67 A W-1 and 1.22 × 1010 cm Hz1/2 W-1, respectively. The role of oxides in the sensitization process was analyzed in combination with X-ray diffraction and scanning electron microscopy results, and a three-dimensional network model of the sensitization mechanism of PbS films was proposed. During the annealing process, O functioned as a p-type impurity, forming p+-type PbS layers with high hole concentrations on the surface and between the PbS grains. As annealing proceeds, the p+-type PbS layers at the grain boundaries interconnect to form a three-dimensional network structure of hole transport channels, while the unoxidized p-type PbS layers act as electron transport channels. Under bias, photogenerated electron-hole pairs were efficiently separated by the formed p+-p charge separation junction, thereby reducing electron-hole recombination and facilitating a higher infrared response.

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.

Passivating Quantum Dot Carrier Transport Layer with Metal Salts

Quantum dots (QDs) have a wide range of applications in the field of optoelectronics. They have been playing multiple roles within the configuration of a device, by serving as the building blocks for both the active layer and the carrier transport layer. While the performance of various optoelectronic devices has been steadily improving via developments in passivating the QD active layer, the possible improvement via passivation of the QD-based carrier transport layer has been largely overlooked. Here, with lead sulfide QD photovoltaics as the platform of study, we demonstrate that the device performance can be significantly improved by passivating the QD hole transport layer (HTL) with zinc salt post-treatments. The power conversion efficiency is improved from 8.7% of the reference device to 10.2% and 9.5% for devices with zinc acetate (ZnAc)- and zinc iodide (ZnI₂)-treated HTLs, respectively. Transient absorption spectroscopy confirms that both treatments effectively reduce band-tail states and increase carrier lifetime of the HTLs. Further elemental analysis shows that ZnAc provides a higher amount of Zn²⁺ for passivation while maintaining the function of HTL by allowing essential p-doping oxidation. In contrast, the additional I^- passivation from ZnI₂ inhibits p-doping oxidation and limits the function of HTL. This work demonstrates the potential of improving device performance by passivating the QD-based HTLs, and the method developed is likely applicable to other optoelectronic devices.

Design Strategy of Quantum Dot Thin-Film Solar Cells

Quantum dots (QDs) are emerging photovoltaic materials that display exclusive characteristics that can be adjusted through modification of their size and surface chemistry. However, designing a QD-based optoelectronic device requires specialized approaches compared with designing conventional bulk-based solar cells. In this paper, design considerations for QD thin-film solar cells are introduced from two different viewpoints: optics and electrics. The confined energy level of QDs contributes to the adjustment of their band alignment, enabling their absorption characteristics to be adapted to a specific device purpose. However, the materials selected for this energy adjustment can increase the light loss induced by interface reflection. Thus, management of the light path is important for optical QD solar cell design, whereas surface modification is a crucial issue for the electrical design of QD solar cells. QD thin-film solar cell architectures are fabricated as a heterojunction today, and ligand exchange provides suitable doping states and enhanced carrier transfer for the junction. Lastly, the stability issues and methods on QD thin-film solar cells are surveyed. Through these strategies, a QD solar cell study can provide valuable insights for future-oriented solar cell technology.

Enhancing the Efficiency and Stability of PbS Quantum Dot Solar Cells through Engineering an Ultrathin NiO Nanocrystalline Interlayer

Significant progress in PbS quantum dot solar cells has been achieved through designing device architecture, engineering band alignment, and optimizing the surface chemistry of colloidal quantum dots (CQDs). However, developing a highly stable device while maintaining the desirable efficiency is still a challenging issue for these emerging solar cells. In this study, by introducing an ultrathin NiO nanocrystalline interlayer between Au electrodes and the hole-transport layer of the PbS-EDT, the resulting PbS CQD solar cell efficiency is improved from 9.3 to 10.4% because of the improved hole-extraction efficiency. More excitingly, the device stability is significantly enhanced owing to the passivation effect of the robust NiO nanocrystalline interlayer. The solar cells with the NiO nanocrystalline interlayer retain 95 and 97% of the initial efficiency when heated at 80 °C for 120 min and treated with oxygen plasma irradiation for 60 min, respectively. In contrast, the control devices without the NiO nanocrystalline interlayer retain only 75 and 63% of the initial efficiency under the same testing conditions.

Enhanced Passivation and Carrier Collection in Ink-Processed PbS Quantum Dot Solar Cells via a Supplementary Ligand Strategy

Solution-processed semiconductors have opened promising avenues for next-generation semiconductor and optoelectronic industries. Colloidal quantum dots (QDs) as one of the most typical materials are widely utilized for the design and development of light-emitting diodes, photodetectors, and solar cells. Recently, an emerging process of PbS QD ink has been employed to attain world record power conversion efficiency by surface passivation using a PbI₂ ligand to form PbI₂-PbS and the process optimization in the field of photovoltaics. However, the bonding and debonding of the ligands on the surface of PbS QDs are dynamic reversible processes in an ink environment. The uncoordinated Pb²⁺ defects induced by unbonded PbS QDs serve as the recombination sites. Thus, the present ink process leaves much room for the enhancement by surface passivation of PbS QDs. Herein, we devise an efficient strategy with a supplementary phenethylammonium iodide (PEAI) ligand for the formation of the PEAI-PbS interface in PbS QD ink-processed solar cells. This newly developed method can not only improve the passivation on the QD surface by iodine ions but also simultaneously enhance the carrier collection efficiency by a graded energy alignment between PbI₂-PbS and PEAI-PbS layers. The corresponding power conversion efficiency of the optimized device has significantly increased by approximately 20% more than the control device. As a result, such a robust and efficient method regarded as a strategic candidate can overcome the bottleneck of imperfect passivation caused by a large specific surface area and loose bonding ligands, eventually promoting the industrial application of QDs.

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.

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%.

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.

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.