PbS Quantum Dot Solar Cells Research Articles

Tuning Areal Density and Surface Passivation of ZnO Nanowire Array Enable Efficient PbS QDs Solar Cells with Enhanced Current Density

AbstractColloidal PbS quantum dots (QDs) have provoked a revolution in the field of optoelectronic devices owing to their low‐cost fabrication processing and excellent physical properties. Recently, the fabrication of nanostructured PbS QD photovoltaic (PV) devices based on zinc oxide (ZnO) nanowire array appears as an effective strategy for improving the overall device performance. Despite its potentially strong impact on the device performance, the role of nanowire areal density on photon absorption and exciton dynamics has not yet been studied and still remains unexplored. Here, for the first time, the areal density of ZnO nanowires is tuned through controlling the precursor concentration and its impact on PbS QD PV performance is studied. It is found that the device with optimized ZnO nanowire areal density yields significantly increased power conversion efficiency (PCE) (10.1% vs 8.5% of control nanowire‐based device) due to improved antireflection effect and reduced surface recombination states. To further improve the photovoltaic performance, the ZnO nanowire surface is treated with hydrogen plasma. Transient photovoltage (TPV) measurement reveals that this passivation process noticeably reduces the nonradiative charge recombination yielding a champion device with a remarkable PCE of 10.8%.

Efficient and Stable PbS Quantum Dot Solar Cells by Triple-Cation Perovskite Passivation.

Solution-processed quantum dots (QDs) have a high potential for fabricating low-cost, flexible, and large-scale solar energy harvesting devices. It has recently been demonstrated that hybrid devices employing a single monovalent cation perovskite solution for PbS QD surface passivation exhibit enhanced photovoltaic performance when compared to standard ligand passivation. Herein, we demonstrate that the use of a triple cation Cs0.05(MA0.17FA0.83)0.95Pb(I0.9Br0.1)3 perovskite composition for surface passivation of the quantum dots results in highly efficient solar cells, which maintain 96% of their initial performance after 1200 h shelf storage. We confirm perovskite shell formation around the PbS nanocrystals by a range of spectroscopic techniques as well as high-resolution transmission electron microscopy. We find that the triple cation shell results in a favorable energetic alignment to the core of the dot, resulting in reduced recombination due to charge confinement without limiting transport in the active layer. Consequently, photovoltaic devices fabricated via a single-step film deposition reached a maximum AM1.5G power conversion efficiency of 11.3% surpassing most previous reports of PbS solar cells employing perovskite passivation.

Improved Reproducibility of PbS Colloidal Quantum Dots Solar Cells Using Atomic Layer-Deposited TiO2.

Thanks to their broadly tunable bandgap and strong absorption, colloidal lead chalcogenide quantum dots (QDs) are highly appealing as solution‐processable active layers for third‐generation solar cells. However, the modest reproducibility of this kind of solar cell is a pertinent issue, which inhibits the exploitation of this material class in optoelectronics. This issue is not necessarily imputable to the active layer but may originate from different constituents of the device structure. Herein, the deposition of TiO2 electron transport layer is focused on. Atomic layer deposition (ALD) greatly improves the reproducibility of PbS QD solar cells compared with the previously optimized sol–gel (SG) approach. Power conversion efficiency (PCE) of the solar cells using atomic layer–deposited TiO2 lies in the range between 5.5% and 7.2%, whereas solar cells with SG TiO2 have PCE ranging from 0.5% to 6.9% with a large portion of short‐circuited devices. Investigations of TiO2 layers by atomic force microscopy and scanning electron microscopy reveal that these films have very different surface morphologies. Whereas the TiO2 films prepared by SG synthesis and deposited by spin coating are very smooth, TiO2 films made by ALD repeat the surface texture of the fluorine‐doped tin oxide (FTO) substrate underneath.

Hole transport layer selection toward efficient colloidal PbS quantum dot solar cells.

The effect of energy level alignment between the hole transport layer (HTL) and active layer in PbS quantum dot (QD) solar cells was investigated. Here, a great variation in device performance was observed when employing different hole transporting materials. Devices using HTLs that could not block electrons only show poor device behaviors, while those employing wide band-gap hole transporting materials with shallow lowest unoccupied molecular orbital (LUMO) energies to block electrons exhibit reduced dark currents as well as enhanced device efficiencies. A power conversion efficiency of 4.4% was obtained by utilizing Poly-TPD as the HTL due to the optimized energy level alignment. These improvements were realized by preventing current leakage and consequent counter diode formation. The efficiency can be further improved to 4.9% by inserting EDT-treated PbS QD film (PbS-EDT) hole transporting materials with higher hole mobility as well as suitable energy levels that can increase the collection efficiency.

Air exposure oxidation and photooxidation of solution-phase treated PbS quantum dot thin films and solar cells

The air exposure oxidation mechanisms of PbS quantum dot (QD) thin films and solar cells are studied in the current paper. As a novel and advantageous method, QD thin films were prepared by the single-step deposition of colloidal QDs treated with different ligands of butylamine (BA), mercaptopropionic acid (MPA), tetrabutylammonium iodide (TBAI), methylammonium iodide (MAI) and methylammonium lead triiodide (MAPbI3, perovskite). Photoluminescence (PL) measurements evaluated the stability of different surface treated PbS QDs during the colloidal to thin film transformation, and over the air exposure times. Blue-shift and quenching the PL spectra suggested rapid oxidation of QD thin films at the first times of air exposure. However, the oxidation rates significantly reduced for the QD thin films treated with organic MPA and all the inorganic ligands. According to the X-ray photoelectron spectroscopy (XPS) studies, thin films oxidation accompanied with the formation of PbSO3 and PbSO4 compounds on the (100) facets of PbS QDs. Although MAPbI3 treatment led to the complete passivation of QDs in the air, perovskite shelling partially oxidized to PbO and PbCO3 compounds. Furthermore, the PL enhancement phenomenon observed at the first times of air exposure for the TBAI and MAPbI3-treated QD thin films, as a result of their strong surface passivation as well as the photoenhancement and photoelectrification mechanisms. Eventually, p-n and p-i-n structured solar cells were fabricated by the single-step deposition of solution-phase treated PbS QD inks. In this case, inorganic surface treatments not only increased the power conversion efficiency (PCE) of solar cells, but also led to a high stability of fabricated devices in the air environment (lower than 1% PCE loss after 500 h of storage in the air).

Efficient, Stable, and Low-Cost PbS Quantum Dot Solar Cells with Cr–Ag Electrodes

PbS quantum dots (QDs) are a promising nanostructured material for solar cells. However, limited works have been done to explore the active layer thickness, layer deposition techniques, stability improvement, and cost reduction for PbS QD solar cells. We address those issues of device fabrication herein and suggest their possible solutions. In our work, to get the maximum current density from a PbS QD solar cell, we estimated the optimized active layer thickness using Matlab simulation. After that, we fabricated a high-performance and low-cost QD photovoltaic (PV) device with the simulated optimized active layer thickness. We implemented this low-cost device using a 10 mg/mL PbS concentration. Here, spin coating and drop-cast layer deposition methods were used and compared. We found that the device prepared by the spin coating method was more efficient than that by the drop cast method. The spin-coated PbS QD solar cell provided 6.5% power conversion efficiency (PCE) for the AM1.5 light spectrum. Besides this, we observed that Cr (chromium) interfaced with the Ag (Cr–Ag) electrode can provide a highly air-stable electrode.

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.

Boosting the current density in inverted Schottky PbS quantum dot solar cells with conjugated electrolyte

Herein, we correlate the chemical structure of the electrolyte with the performance of inverted Schottky quantum dot (QD) solar cells (SCs) having a structure of FTO/electrolyte/p-type PbS QDs/MoOx/Au-Ag. QDSCs of polyethyleneimine (PEI) or poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-1,4-phenylene] (PFN) were fabricated for comparison. The open-circuit voltage (VOC) of QDSCs scaled with the workfunction of electrolyte – modified fluorine-doped tin oxide (FTO). Conjugated PFN electrolyte resulted in lower VOC but it boosted the current density (JSC) of QDSCs by lowering the interfacial potential barrier at FTO-PbS QDs contact.

Limiting factor of performance for solution-phase ligand-exchanged PbS quantum dot solar cell

Despite great increases in photovoltaic performance due to the insertion of ligand-exchanged PbS quantum dot layer via 1,2-ethanedithiol (EDT), detailed carrier transport characteristics of solar cells with solution-phase ligand-exchanged PbS quantum dot (QD) still remains unknown. In this paper, relaxation time, recombination resistance, chemical capacitance, carrier lifetime, carrier diffusion length, and defect density corresponding to the depletion width were measured through electrochemical impedance spectroscopy using the Schottky diode model to understand limiting factors for photovoltaic performance. The activation energy, which is estimated from the temperature-dependence of resistance, was reduced from 0.06 eV to 0.02 eV by inserting the PbS-EDT layer, leading to the improved photovoltaic performance. We also evaluated QD solar cells with varying thicknesses of PbS QD layers to investigate the impedance analysis and photovoltaic measurements. The depletion width as well as the carrier diffusion length was larger than the physical thickness of the layer, and this indicates that an increased short-circuit current density can be realized at the thick PbS QD layer due to the resulting large absorbance. Furthermore, the reduced trap density of the QD layer improves the open circuit voltage and fill factor. As a result, a high photoconversion efficiency of 9.41% was achieved.

Impact of Background Oxygen Pressure on the Pulsed-Laser Deposition of ZnO Nanolayers and on Their Corresponding Performance as Electron Acceptors in PbS Quantum-Dot Solar Cells

ZnO films are commonly employed as n-type semiconductors in heterojunctions with PbS colloidal quantum-dot (CQDs) films because of their outstanding optical transparency and electron acceptor properties, yet studies of the impact of ZnO film microstructure, composition, and defect qualities on the solar-cell performance are quite limited. Here we report on the fabrication of ZnO films via pulsed-laser deposition and use these films to investigate how different morphologies affect the PbS CQD solar-cell performance. By modification of the background gas O2 pressures during the ZnO deposition process, the device performance approaching a 7.8% energy conversion efficiency is achieved with an O2 pressure of 100 mTorr. Higher or lower O2 pressures led to significantly lower device efficiency. We employ various materials and device characterizations to highlight the differences in the physical properties introduced by the fabrication oxygen pressure. In particular, we have found that the differences in the type...

2D-MoS2 nanosheets as effective hole transport materials for colloidal PbS quantum dot solar cells.

Herein, we demonstrate for the first time matrix-free deposition of two dimensional (2D) MoS2 nanosheets as an efficient hole transport layer (HTL) for colloidal lead sulfide (PbS) quantum dot (QD) solar cells. We have developed all-solution-processed n–p–p+ architecture solar cells where ZnO nanoparticles were used as an n-type window layer, a PbS QD layer acted as a light absorbing p-type layer and 2D-MoS2 nanosheets acted as a p+-type hole transport layer. The MoS2 nanosheets allow better interface with the PbS QD layers. The incorporation of the MoS2 hole transport layer leads to superior fill factor, higher open circuit voltage and better performance in colloidal PbS QD solar cells. These results show that one layer of MoS2 nanosheets improves the power conversion efficiency of the device from 0.92% for a hole transport material free device to 2.48%. The present work reveals the development of 2D-MoS2 nanosheets as a new hole transport layer for the fabrication of cost-effective, durable and efficient colloidal PbS quantum dot solar cells.

Impact of Background Oxygen Pressure on the Pulsed-Laser Deposition of ZnO Nanolayers and on Their Corresponding Performance as Electron Acceptors in PbS Quantum-Dot Solar Cells

Impact of Background Oxygen Pressure on the Pulsed-Laser Deposition of ZnO Nanolayers and on Their Corresponding Performance as Electron Acceptors in PbS Quantum-Dot Solar Cells

Low-Temperature ZnO Thin Film and Its Application in PbS Quantum Dot Solar Cells

Zinc oxide (ZnO) has been widely deployed as electron conducting layer in emerging photovoltaics including quantum dot, perovskite and organic solar cells. Reducing the curing temperature of ZnO layer to below 200 oC is an essential requirement to reduce the cell fabrication cost enabled by large-scale processes such as ink-jet printing, spin coating or roll-roll printing. Herein, we present a novel water-based ZnO precursor stabilized with labile NH3, which allow us to spin coat crystalline ZnO thin films with temperatures below 200 oC. Thin film transistors (TFTs) and diode-type quantum dot solar cells (QD SCs) were fabricated using ZnO as electron conduction layer. In the QD SCs, a p-type 1,2-ethylenedithiol treated PbS QDs with a bandgap of 1.4 eV was spin-coated on top of ZnO layer by a layer-by-layer solid state ligand exchange process. Electron mobility of ZnO was about 0.1 cm2V-1s-1 as determined from TFT measurements. Power conversion efficiency of solar cells: FTO/ZnO/PbS/Au-Ag was 3.0% under AM1.5 irradiation conditions. The possibility of deposition of ZnO at low temperatures demonstrated herein is of important for solution processed electronic and optoelectronic devices. 
 Keywords
 ZnO, low-temperature, quantum dots, solar cells, TFTs
 References
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Enhancement of charge transport in quantum dots solar cells by N-butylamine-assisted sulfur-crosslinking of PbS quantum dots

A novel and facile strategy to realize selective inorganic ligand (S2−) exchange on Pb-rich surface of PbS colloidal quantum dots (QDs) has been demonstrated. This was achieved via xanthate ligand decomposition at room temperature without damaging the QDs surface. This proposed method offers an amicable solution for the limitation that inorganic-terminated colloidal QDs are restricted by the specific requirement to solvents with high dielectric constant. Furthermore, Introduction of S2− to form sulfur-crosslinking PbS QDs enables reaction force-induced QDs arrays and smooth surface morphology of the spin-coated QDs film as evidenced by atomic force microscopy. Passivation of QDs by bromide combined with sulfur led to stronger electronic coupling between adjacent QDs as compared to bromide-only passivated QDs counterparts. Bromide and sulfur hybrid-capped QDs exhibited remarkably enhanced carrier mobility from 1.66 × 10−4 cm2/V s to 5.0 × 10−1 cm2/V s as evidenced by the Hall- Effect measurement. The smooth QDs film morphology and higher charge transport contributed to the boost in the power conversion efficiency of QDs solar cells up 4.96% compared that using only CTAB passivated PbS QDs solar cells (3.04%). This controlled sulfurization approach paves a potential way for improved optoelectronic properties and devices based on QDs.

High-Efficiency PbS Quantum-Dot Solar Cells with Greatly Simplified Fabrication Processing via "Solvent-Curing".

PbS quantum-dot (QD) solar cells are promising candidates for low-cost solution-processed photovoltaics. However, the device fabrication usually requires ten more times film deposition and rinsing steps, which is not ideal for scalable manufacturing. Here, a greatly simplified deposition processing is demonstrated by replacing methanol with acetonitrile (ACN) as the rinsing solvent. It is discovered that ACN can effectively "cure" the film cracks generated from the volume loss during the solid-state ligand-exchange process, which enables the deposition of thick and dense films with much fewer deposition steps. Meanwhile, due to the aprotic nature of ACN, fewer trap states can be introduced during the rinsing process. As a result, with only three deposition steps for the active layer, a CPVT-certified 11.21% power conversion efficiency is obtained, which is the highest efficiency ever reported for PbS QD solar cells employing a solid-state ligand-exchange process. More importantly, the simple film-deposition processing provides an opportunity for the future application of QDs in low-cost printing of optoelectronic devices.

Influences of Morphology of Window Layer on the Characteristics of PbS Quantum Dot Solar Cells

Background: In QDs-based solar cell devices, the PbS QDs layer was mainly focused to optimize. The ZnO electron acceptor layer attracts less attention whereas it shows the key roles in extracting and transporting charge carriers in heterojunction. The utilization of 1-D ZnO structures has been demonstrated to be large interface areas and good carrier pathways for efficient carrier collection. However, the influences of the morphology of metal oxide nanostructures on the photovoltaic performance of QD-based solar cells have been few in-depth reports. Objective: In this work, ZnO NRs/PbS QD based solar cells were fabricated. The influences of the ZnO NRs array structures on characteristics of ZnO NRs/PbS QD based solar cells were investigated. Method: ZnO NRs/PbS QD based solar cells were fabricated via spin coating method. XRD, SEM, UV-VIS-NIR spectrophotometer, I-V and EQE measurement systems were utilized to investigate the fabricated samples. Results: We have found optimum combinations of the linked parameters of ZnO NRs, their length of (230 ± 5) nm and density of (1.50 ± 5)x1010 # of rods.cm-2, that exhibit maximum efficiency of ∼2.5% for the ZnO NR/PbS QDs based solar cell. Conclusion: The influences the ZnO NRs structures on the solar cell characteristics, including the absorption, external quantum efficiency, and current density-voltage curves, were investigated. There seems to be an optimum between NR length and their density for resulting in maximum efficiency. This could be due the interplay of solar flux absorption and junction area controlled by these two parameters of ZnO NR morphology.

Reduction of moisture sensitivity of PbS quantum dot solar cells by incorporation of reduced graphene oxide

PbS nanocrystals are an important narrow-gap material for solar cells and photodetectors. Nevertheless, their application may be limited because device performance can be affected by atmospheric conditions. Indeed, the presence of oxygen and/or water can degrade the active layers, possibly leading to device failure. Strategies to address this issue are therefore actively explored. Here we report a solution-processed PbS quantum dot solar cell, consisting of a PbS-silane functionalized reduced graphene oxide (PbS-rGO) layer on top of the PbS absorber film, which enhances device stability, especially when the solar cells are exposed to moisture. Power conversion efficiency (PCE) measurements demonstrate a slower degradation under continuous illumination for solar cells with PbS-rGO. When storing the samples under saturated water vapor, differences are even more remarkable: with PbS-rGO the solar cells essentially maintain their initial PCE, while the PCE of the PbS reference devices is reduced by 50% after 5 days. Scanning electron microscopy, energy dispersive X-ray and X-ray photoelectron spectroscopy reveal the damage to the PbS films and the formation of PbSOx crystals in the PbS reference devices. Such crystals are not observed in the PbS-rGO devices, further supporting the importance of the PbS-rGO barrier layer.

Efficiency enhancement for solution-processed PbS quantum dots solar cells by inserting graphene oxide as hole-transporting and interface modifying layer

Abstract Interface modifying between the active layer and anode solar cells is a key technique to improve the device performance. In this paper, the enhancement of power conversion efficiency (PCE) of ZnO/PbS heterojunction quantum dot (QD) solar cells Au/PbS-TBAI/ZnO/ITO was achieved by incorporating a graphene oxide (GO) layer between the PbS-TBAI active film and the Au anode. Our experimental data showed the GO interlayer was partially reduced to graphene after its post-annealing at 140 °C and it played the role of hole-transporting layer and the interface-modifying layer. In this way, the defects existed at the contact interface of PbS-TBAI/Au were reduced significantly after inserting and post-annealing GO interlayer, resulting to the enhancement of holes transport and collection efficiency, showing a PCE enhancement of 12.87% as compared to that of the control device Au/PbS-TBAI/ZnO/ITO. The influence of post-annealing treatment on the whole device, as the reduction method for GO, on the performance of the QD solar cells was also explored and discussed.

Achieving high-performance PbS quantum dot solar cells by improving hole extraction through Ag doping

PbS quantum dot solar cells are promising candidates for low-cost and highly efficient light harvesting devices owing to their solution processability and bandgap tunability. The p-type ethanedithiol (EDT) treated PbS quantum dot film plays an important role in PbS quantum dot solar cells with an n-i-p junction device structure. However, despite their sulphur-rich surface the EDT-treated PbS quantum dot film still have a relatively low carrier concentration. Higher carrier concentrations in this layer are desirable to extend depletion regions and improve hole extraction. Also imbalances in the charge mobility between the intrinsic layer and the p-type layer may lead to charge build-up at this interface. These obstacles limit further improvement of the device performance. Herein, we utilize EDT-treated Ag-doped PbS quantum dots as a p-type layer to fabricate PbS quantum dot photovoltaic cells. The carrier carrier concentration, mobility and band extrema as well as Fermi energy levels of Ag doped PbS quantum dot film can be tailored by tuning the Ag/Pb mole ratio from 0.0% to 2.0% during fabrication. The device performance has been significantly improved from 9.1% to 10.6% power conversion efficiency largely due to improvements in carrier concentration in the PbS-EDT layer through the incorporation of silver impurities.

Highly Efficient Inverted Structural Quantum Dot Solar Cells.

Highly efficient PbS colloidal quantum dot (QD) solar cells based on an inverted structure have been missing for a long time. The bottlenecks are the construction of an effective p-n heterojunction at the illumination side with smooth band alignment and the absence of serious interface carrier recombination. Here, solution-processed nickel oxide (NiO) as the p-type layer and lead sulfide (PbS) QDs with iodide ligand as the n-type layer are explored to build a p-n heterojunction at the illumination side. The large depletion region in the QD layer at the illumination side leads to high photocurrent. Interface carrier recombination at the interface is effectively prohibited by inserting a layer of slightly doped p-type QDs with 1,2-ethanedithiol as ligands, leading to improved voltage of the device. Based on this graded device structure design, the efficiency of inverted structural heterojunction PbS QD solar cells is improved to 9.7%, one time higher than the highest efficiency achieved before.