PbS Quantum Dot Solar Cells Research Articles

Germylene Mediated Reductive C−C and C−N Coupling of an Isocyanide and its Device Application

AbstractWe have demonstrated a unique reductive coupling of 4‐iodophenyl isocyanide, facilitated by a perimidine‐based N‐heterocyclic germylene (NHGe), which yields a bis‐spirogerma compound featuring simultaneous C−C and C−N bond formation. This reaction, which leads to the oxidation of germanium from +2 to +4, represents a significant departure from previously documented isocyanide‐germylene interactions. The product exhibits extensive conjugation across its bicyclic C4Ge2N2 framework, conferring distinct photophysical properties, including prominent orange luminescence in both solution and solid states. The photophysical properties are supported by the TD‐DFT calculations confirming an n→π* transition. The potential application of this compound in optoelectronic devices, particularly as a hole transport layer in PbS quantum dot solar cells, is also explored, with promising preliminary results.

Germylene Mediated Reductive C-C and C-N Coupling of an Isocyanide and its Device Application.

We have demonstrated a unique reductive coupling of 4-iodophenyl isocyanide, facilitated by a perimidine-based N-heterocyclic germylene (NHGe), which yields a bis-spirogerma compound featuring simultaneous C-C and C-N bond formation. This reaction, which leads to the oxidation of germanium from +2 to +4, represents a significant departure from previously documented isocyanide-germylene interactions. The product exhibits extensive conjugation across its bicyclic C4Ge2N2 framework, conferring distinct photophysical properties, including prominent orange luminescence in both solution and solid states. The photophysical properties are supported by the TD-DFT calculations confirming an n→π* transition. The potential application of this compound in optoelectronic devices, particularly as a hole transport layer in PbS quantum dot solar cells, is also explored, with promising preliminary results.

Interface-resolved photovoltage generation dynamics and band structure evolution in a PbS quantum dot solar cell.

For directed development of solar cells using nanomaterials such as quantum dots, there is a need to understand the device function in detail. Understanding where photovoltage is generated in a device and where energy losses occur is a key aspect of this, and development of methods which can provide this information is needed. We have previously shown that time-resolved photoelectron spectroscopy of core levels can be used to follow the photovoltage dynamics at a specific interface of a lead sulfide quantum dot solar cell. Here, we use the method's selectivity and sample design to investigate the photovoltage generation in different parts of this solar cell and determine how the different layers (including the absorber layer thickness) contribute to charge separation. We show that all layers contribute to photovoltage generation and that a gold contact deposited on the quantum dots is necessary for full photovoltage generation and slow charge recombination. By combining the information obtained, we are able to experimentally follow the time evolution of the solar cell band structure during the charge separation process. Furthermore, we can identify which specific layers need to be optimized for an overall improvement of quantum dot cells. In the future, this methodology can be applied to other types of devices to provide insights into photovoltage generation mechanisms.

Enhanced carrier transfer performance on blade-coating PbS quantum dot solar cells by coordinated operation of iodine ligands

The significant breakthrough in the photovoltaic conversion efficiency of lead sulfide (PbS) quantum dot solar cells has primarily been achieved on small substrates using the spin-coating method. However, the blade-coating technique offers a scalable alternative for large-scale fabrication of absorption film, addressing the limitations of spin-coating for commercial applications. Given the inherent challenges of the blade-coating method, which often produces low-quality and defective films. Thus, an iodide ligand passivation strategy to realize the formation of high-quality films with reduced trap states was proposed. Typically, the iodide ligand is incorporated into the precursor solution with the aim of fine-tuning the nanoparticle’s functionality, facilitating the formation of systematically aligned and densely coupled film configurations during the blade coating procedure. The functional mechanisms of the aligned and compact-coupled configurations, as well as the effect of iodide introduction on defect mitigation, have been systematically investigated. The results reveal that iodide ligand introduction modulates the interparticle electrostatic potential and inter-dot space, facilitating the formation of a uniform structure with enhanced energy level alignment and reduced trap states. The highly aligned and compact structure improves interfacial integration with the ZnO electron transport layer, enabling efficient photon-electron extraction and transfer. As a result, PbS quantum dot solar cells fabricated using the blade coating technique have achieved an optimized efficiency of 4.68 %, offering significant potential for the development and production of directly-synthesized ink for quantum dot solar cells.

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 60h, 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.

Sn-Doped Zinc Oxide as an Electron Transporting Layer for Enhanced Performance in PbS Quantum Dot Solar Cells.

Colloidal PbS quantum dot solar cells (QDSCs) have been primarily demonstrated in n-i-p structures by incorporating a solution-processed ZnO electron transporting layer (ETL). Nevertheless, the inherent energy barrier for the electron extraction at the ZnO/PbS junction along with the defective nature significantly diminishes the performance of the PbS QDSCs. In this study, by employing Sn-doped ZnO (ZTO) ETL, we have tuned the conduction band offset at the junction from spike-type to cliff-type so that the electron extraction barrier can be eliminated and the overall photovoltaic parameters can be enhanced (open-circuit voltage of 0.7 V, fill factor over 70%, and efficiency of 11.3%) as compared with the counterpart with the undoped ZnO ETL. The X-ray photoelectron spectroscopy (XPS) analysis revealed a mitigation of oxygen vacancies in the ZTO ETL of our PbS QDSCs. Our work signifies the importance of Sn doping into the conventional ZnO ETL for the superior electron extraction in PbS QDSCs.

Effective Charge Collection of Electron Transport Layers for High-Performance Quantum Dot Infrared Solar Cells.

Infrared (IR) solar cells, capable of converting low-energy IR photons to electron-hole pairs, are promising optoelectronic devices by broadening the utilization range of the solar spectrum to the short-wavelength IR region. The emerging PbS colloidal quantum dot (QD) IR solar cells attract much attention due to their tunable band gaps in the IR region, potential multiple exciton generation, and facile solution processing. In PbS QD solar cells, ZnO is commonly utilized as an electron transport layer (ETL) to establish a depleted heterostructure with a QD photoactive layer. However, band gap shrinkage of large PbS QDs makes it necessary to tailor the behaviors of the ZnO ETL for efficient carrier extraction in the devices. Herein, the characteristics of ZnO ETL are efficiently and flexibly tailored to match the QD layer by handily adjusting the postannealing process of ZnO ETL. With a suitable temperature, the well-matched energy level alignment and suppressed trap states are simultaneously achieved in the ZnO ETL, effectively reducing the nonradiative recombination and accelerating the electron injection from the QD layer to ETL. As a consequence, a high-performance PbS QD photovoltaic device with power conversion efficiencies (PCEs) of 10.09% and 1.37% is obtained under AM 1.5 and 1100 nm filtered solar illumination, demonstrating a simple and effective approach for achieving high-performance IR photoelectric devices.

Stronger Coupling of Quantum Dots in Hole Transport Layer Through Intermediate Ligand Exchange to Enhance the Efficiency of PbS Quantum Dot Solar Cells.

Nowadays, the extensively used lead sulfide (PbS) quantum dot (QD) hole transport layer (HTL) relies on layer-by-layer method to replace long chain oleic acid (OA) ligands with short 1,2-ethanedithiol (EDT) ligands for preparation. However, the inevitable significant volume shrinkage caused by this traditional method will result in undesired cracks and disordered QD arrangement in the film, along with adverse increased defect density and inhomogeneous energy landscape. To solve the problem, a novel method for EDT passivated PbS QD (PbS-EDT) HTL preparation using small-sized benzoic acid (BA) as intermediate ligands is proposed in this work. BA is substituted for OA ligands in solution followed by ligand exchange with EDT layer by layer. With the new method, smoother PbS-EDT films with more ordered and closer QD packing are gained. It is demonstrated stronger coupling between QDs and reduced defects in the QD HTL owing to the intermediate BA ligand exchange. As a result, the suppressed nonradiative recombination and enhanced carrier mobility are achieved, contributing to ≈20% growth in short circuit current density (Jsc) and a 23.4% higher power conversion efficiency (PCE) of 13.2%. This work provides a general framework for layer-by-layer QD film manufacturing optimization.

Optimizing Surface Passivation of n‐Type Quantum Dots for Efficient PbS Quantum Dot Solar Cells

The n‐type quantum dot (QD) active layer is the core component of lead sulfide QD solar cells (PbS QDSCs). In the state‐of‐the‐art PbS QDSCs, the active layer is commonly obtained through liquid‐phase ligand exchange (LPLE). Due to the intricate chemical state of the ligand exchange solution providing halide ligand, therefore, the PbS‐OAQD solutions is used at concentrations of 20, 30, and 40 mg mL−1 for LPLE, aiming to investigate the reasons for different surface states post‐exchange and their impact on device performance. The results indicate that when the concentration of the PbS‐OA QD solution is 30 mg mL−1, the exchanged QDs exhibit complete removal of surface OA, a higher content of short‐chain ligand PbX2 (X = I, Br), Consequently, devices fabricated using PbS‐PbX2 QD obtained through the exchange of 30 mg mL−1 PbS‐OA QD solution achieve a higher power conversion efficiency (PCE) of 12.53%. This study presents a simple and effective strategy to enhance the performance of PbS QDSCs.

Reduced Surface Trap States of PbS Quantum Dots by Acetonitrile Treatment for Efficient SnO2-Based PbS Quantum Dot Solar Cells.

The solution-phase ligand-exchange strategy offers a simple pathway to prepare PbS quantum dots (QDs) and their corresponding solar cells. However, the production of high-quality PbS QDs with reduced surface trap state density for efficient PbS QD solar cells (QDSCs) still faces challenges. As the hydroxyl group (-OH) has been demonstrated to be the primary source of the surface trap states on PbS QDs in the general oleic acid method, here, we present an effective and facile strategy for reducing the surface -OH content of PbS QDs by using acetonitrile (ACN) as precipitant to wash the surface of QDs, which significantly decreases the trap state density and enables the preparation of superior PbS QDs. The resulting solar cell with an ITO/SnO2/n-PbS/p-PbS/Au structure obtained an improved photoelectric conversion efficiency (PCE) from 8.53 to 10.49% with an enhanced air storage stability, realizing a high PCE for SnO2-based PbS QDSCs.

Optimizing Energy Levels and Improving Film Compactness in PbS Quantum Dot Solar Cells by Silver Doping.

PbS quantum dot (QD) solar cells harvest near-infrared solar radiation. Their conventional hole transport layer has limited hole collection efficiency due to energy level mismatch and poor film quality. Here, how to resolve these two issues by using Ag-doped PbS QDs are demonstrated. On the one hand, Ag doping relieves the compressive stress during layer deposition and thus improves film compactness and homogeneity to suppress leakage currents. On the other hand, Ag doping increases hole concentration, which aligns energy levels and increases hole mobility to boost hole collection. Increased hole concentration also broadens the depletion region of the active layer, decreasing interface charge accumulation and promoting carrier extraction efficiency. A champion power conversion efficiency of 12.42% is achieved by optimizing the hole transport layer in PbS QD solar cells, compared to 9.38% for control devices. Doping can be combined with compressive strain relief to optimize carrier concentration and energy levels in QDs, and even introduce other novel phenomena such as improved film quality.

Pressurized Back-Junction Doping via Spray-Coating Silver Nanowires Top Electrodes for Efficient Charge Collection in Bifacial Colloidal PbS Quantum Dot Solar Cells.

Colloidal PbS quantum-dot solar cells (QDSCs) have long suffered from inefficient charge collection near the back-junction due to the lack of p-doping strategy, rendering their bifacial photovoltaic applications unsuccessful. Here, we report highly efficient photocarrier collection in bifacial colloidal PbS QDSCs by exploiting spray-coated silver nanowires (AgNWs) top electrodes. During our spray-coating process, pressurized Ag diffusion occurred toward the active layer, which induced effective p-doping and deep-level passivation. By manipulating the spray pressure, optimum AgNWs' stacking morphology enabling an appropriate level of Ag diffusion could be achieved, leading to Jsc over 30 mA/cm2 from the conventional n-i-p structure upon light illumination to the film side. The morphological and electrical behaviors of AgNWs according to the spray pressure are comprehensively explained in relation to the device performance. Finally, 50 bifacial cells were fabricated over 49 cm2 sized glass substrate, demonstrating the large-area processability and functionality of the spray-coated AgNWs with the effective back-junction engineering.

Design and optimization of the performance of PbS quantum dot based vertical photodetector

Abstract Quantum dots (QDS) are widely used as photoactive materials for various optoelectronic applications, such as light-emitting diodes, photodetectors, lasers, and solar cells, because of their unique and outstanding physical properties for fabricating devices via solution-processing techniques and for varying the sizes of QD to modulate the band gap. In this study, we constructed a numerical model of a vertical photodetector using lead sulfide (PbS) quantum dot layer undergoing ligand exchange treatment with tetrabutylammonium iodide (TBAI) as the light absorption layer to obtain an optimized high-performance photodetector using 1D-Solar Cell Capacitance Simulator (SCAPS) software. For the hole transport layer (HTL) and electron transport layer (ETL), a PbS quantum dot layer that employs an ethanedithiol (EDT) ligand exchange scheme and a zinc oxide (ZnO) layer were utilized, respectively, with gold (Au) and indium tin oxide (ITO) as the bottom and upper electrodes, respectively. After optimizing the thickness and doping concentration of the absorption layer, we obtained a maximum responsivity of 0.4675 A W−1 and detectivity of 2.44 * 10 13 Jones at a reverse bias of 0.5 volts after optimizing of thickness and doping concentration of absorption layer showing an improvement of 19% and 87.7% respectively compared to the initial structure whose parameters originated from studies of PbS quantum dot solar cells with the same structure and materials, although optimization of other transport layers do not provide as great performance enhancement as absorption layer. This study demonstrates that the structural parameters of solar cells are not always applicable to photodetectors, and that photodetectors have great potential for numerical optimization. Ethical Compliance: All procedures performed in this study involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki Declaration and its later amendments or comparable ethical standards.

Organic hole transport materials for high performance PbS quantum dot solar cells.

We developed a triazatruxene-based hole transport material (HTM), 3Ka-DBT-3Ka, aiming to enhance band alignment and augment charge generation and collection in devices, as an alternative for 1,2-ethanedithiol (EDT). The PbS CQD solar cells employing 3Ka-DBT-3Ka as the HTM achieve a peak efficiency of 11.4%, surpassing devices employing the conventional PbS-EDT HTM (8.9%).

Reducing the Open-Circuit Voltage Loss of PbS Quantum Dot Solar Cells via Hybrid Ligand Exchange Treatment.

The interface VOC loss between the active layer and the hole transport layer (HTL) of lead sulfide colloidal quantum dot (PbS-CQD) solar cells is a significant factor influencing the efficiency improvement of PbS colloidal quantum dot solar cells (PbS-CQDSCs). Currently, the most advanced solar cells adopt organic P-type HTLs (PbS-EDT) via solid-state ligand exchange with 1,2-ethanedithiol (EDT) on the CQD top active layer. However, EDT is unable to altogether remove the initial ligand oleic acid from the quantum dot surface, and its high reactivity leads to cracks in the HTL film caused by volume contractions, which inevitably results in significant VOC loss. These flaws prompted this research to develop a method involving hybrid organic ligand exchange using 3-mercaptopropionic acid (MPA) and 1,2-EDT (PbS-Hybrid) to overcome these drawbacks of VOC loss. The results indicated that the new exchange strategy improved the quality of the HTL film and benefited from the enhanced passivation of the quantum dot surface and better alignment of energy levels, and the average VOC of PbS-Hybrid devices is increased by approximately 25 mV compared to control devices. With the enhanced VOC, the average power conversion efficiency (PCE) of the devices is improved by 10%, with the highest PCE reaching 13.24%.

Performance optimization of efficient PbS quantum dots solar cells through numerical simulation

Colloidal quantum dots (CQDs) solar cells are less efficient because of the carrier recombination within the material. The electron and hole transport layers have high impact on the performance of CQDs based solar cells which makes its investigation a very important component of the development of the more efficient devices. In this work, we have tried performance optimization in tetrabutyl ammonium iodide capped lead sulfide (PbS) CQDs (PbS-TBAI) as absorber layers based solar cells by incorporating different hole transport layers (HTLs) to achieve better power conversion efficiency (PCE) in different device architectures by SCAPS—1D numerical simulation software. It was observed from the simulation that the ITO/TiO2/PbS-TBAI/HTL/Au device architecture shows higher power conversion efficiency as compared to the conventional experimentally realized device architecture of ITO/TiO2/PbS-TBAI/PbS-EDT/HTL/Au. The influence of interface defect density (IDD) at the interface TiO2/PbS-TBAI has also been studied where IDD is varied from 1 × 1013 cm−2 to 1 × 1018 cm−2 while keeping the rest of the device parameters intact. The result shows a noteworthy reduction in the PV performance of the device at higher IDD. This modelled device structure provides a new direction toward the experimental realization in high efficiency PbS QDs solar cells.

Lithium Salt‐Doped Organic Conjugated Hole Transport Layer for High‐Performance PbS Quantum Dot Solar Cells

The electrical properties of hole transport layers (HTL) are critical factors for the high performance of lead sulfide quantum dot solar cells (PbS CQDSCs). Herein, the impacts of doping the organic HTL PTB7‐Th with lithium bis(trifluoromethanesulfonimide) (LiTSFI) on the performance of quantum dot solar cells are explored and a notable enhancement in the fill factor is observed. A series of analytical characterizations of the doped HTLs are performed and it is found that doping with LiTFSI significantly deepens the Fermi level of the HTL, thereby improving the charge separation ability within the devices. Additionally, the conductivity of doped HTL is also improved. By optimizing the doping conditions, the doped device achieves a maximum power conversion efficiency of 12.68%. This doping strategy of organic HTL promotes the development of lead sulfide quantum dot solar cell techniques.

Metal Oxide Interlayers Enable Lower-Cost Electrodes in PbS QD Solar Cells

PbS quantum dots (QDs) are an emerging light-absorbing material for thin-film solar cells. However, state-of-the-art solar cell architectures rely on a gold electrode, which raises material costs to 3× more than the 2030 US DOE SunShot target module price. Here, we enable both aluminum and copper electrodes to replace gold through a NiOx interlayer. An additional indium tin oxide (ITO) buffer layer between the NiOx interlayer and electrode further enables longer shelf lifetimes. By combining these layers, we lower electrode material costs in PbS QD solar cells by a factor of 130000 without compromising power conversion efficiency or stability.

Merging Passivation in Synthesis Enabling the Lowest Open-Circuit Voltage Loss for PbS Quantum Dot Solar Cells.

The high open-circuit voltage (Voc ) loss arising from insufficient surface passivation is the main factor that limits the efficiency of current lead sulfide colloidal quantum dots (PbS CQDs) solar cell. Here, synergistic passivation is performed in the direct synthesis of conductive PbS CQD inks by introducing multifunctional ligands to well coordinate the complicated CQDs surface with the thermodynamically optimal configuration. The improved passivation effect is intactly delivered to the final photovoltaic device, leading to an order lower surface trap density and beneficial doping behavior compared to the control sample. The obtained CQD inks show the highest photoluminescence quantum yield (PLQY) of 24% for all photovoltaic PbS CQD inks, which is more than twice the reported average PLQY value of ≈10%. As a result, a high Voc of 0.71V and power conversion efficiency (PCE) of 13.3% is achieved, which results in the lowest Voc loss (0.35eV) for the reported PbS CQD solar cells with PCE >10%, comparable to that of perovskite solar cells. This work provides valuable insights into the future CQDs passivation strategies and also demonstrates the great potential for the direct-synthesis protocol of PbS CQDs.

Unraveling the Organic and Inorganic Passivation Mechanism of ZnO Nanowires for Construction of Efficient Bulk Heterojunction Quantum Dot Solar Cells.

Zinc oxide (ZnO) nanowire (NW) based lead sulfide (PbS) quantum dot solar cells (QDSCs), i.e., bulk heterojunction QDSCs, have been widely investigated because of the excellent photoelectronic properties of PbS QDs and ZnO NWs. To further improve the efficiency of this type of QDSCs, various passivation methods are applied to ZnO NWs to suppress interface recombination caused by trap defects. However, the comparison among passivation using organic, inorganic, and inorganic-organic hybrid materials with different properties has been less studied. In this work, the effect of passivation with inorganic Mg-doped ZnO (ZMO), organic 1,2-ethanedithiol (EDT) and both of them on ZnO NWs and PbS QDSCs are investigated. As a result, ZnO NWs purely passivated by organic material EDT show the best performance with fewer surface defects and better matched energy level with the PbS QD layer. A nearly 1.7 times larger power conversion efficiency (PCE) of 6.9% is achieved for the solar device using ZnO NW @EDT, compared with that (4.1%) of the untreated one. The work provides a promising way to impede interlayer charge recombination and facilitate carrier transport, thus enhancing the photovoltaic performance of the device.