Ligand-dependent exciton dynamics and photovoltaic properties of PbS quantum dot heterojunction solar cells (original) (raw)
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Performance of PbSe quantum dot based heterojunction solar cells: Dependence on ligand type
The Japan Society of Applied Physics, 2016
Univ. Electro-Commun., Kyushu Inst. Tech., Nanjing Tech. Univ., CREST JST Yaohong Zhang, Chao Ding, Shuzi Hayase,Yuhei Ogomi, Jin Chang, Taro Toyoda, Qing Shen, Email: shen@pc.uec.ac.jp Introduction Quantum dots (QDs) based solar cells have attracted more and more interests as a promising candidate for the next generation solar cells. Compared with conventional solar cells, QDs solar cells (QDSCs) are easier to prepare with low fabrication cost. Additionally, QDs present high extinction coefficients, tunable absorption spectra, and multiple exciton generation (MEG) effect. PbSe QDs have attracted attention due to their small bulk bandgap, high dielectric constant, and large exciton Bohr radius. However, the performance of PbSe based QDSCs tend to quickly degrade after the solar cells expose to air. Here, we found a modified method to synthesis air stable PbSe QDs and investigate the effect of ligand on the performance of PbSe QDSCs. Experimental Method Colloidal PbSe QDs were synthe...
Small, 2017
The power conversion efficiency of colloidal PbS quantum dot based solar cells is significantly hampered by lower than expected open circuit voltage (VOC). The VOC deficit is considerably higher in QD based solar cells compared to other types of existing solar cells due to in-gap trap induced bulk recombination of photogenerated carriers. Here, we report ligand exchange procedure based on a mixture of zinc iodide and 3-mercaptopropyonic acid to reduce the VOC deficit without compromising the high current density. This layer-by-layer solid state ligand exchange treatment enhanced the photovoltaic performance from 6.62% to 9.92% with a significant improvement in VOC from 0.58V to 0.66V. We further employed opto-electronic characterization, XPS and PL spectroscopy to understand the origin of VOC improvement. The mixed ligand treatment reduces the sub bandgap traps and significantly reduces the bulk recombination in the devices.
The Journal of Physical Chemistry C, 2016
We fabricated the long term air stable PbSe colloidal quantum dots (CQDs) based planar heterojunction solar cells (FTO/TiO 2 /PbSe/Au) with relatively larger active area (0.25 cm) using tetrabutylammonium iodide (TBAI, I-) as ligand in solid state ligand-exchange process. For the first time, we have achieved the whole preparation process of the device in the ambient atmosphere from PbSe CQDs collection to PbSe colloidal quantum dot solar cells (CQDSCs) fabrication, then storage and in their following measurements. Especially, TBAI-treated PbSe CQDSCs exhibited a power conversion efficiency (PCE) of 3.53% under AM 1.5 G in air, and also a remarkable long term stability (more than 90 days) of their storage in ambient atmosphere has been identified. By contrast, 1,2-ethanedithiol (EDT), 3-mercaptopropionic acid (MPA) and cetyltrimethylammonium bromide (CTAB, Br-) treated PbSe CQDSCs were further studied. The ligand-dependent exciton dissociation, recombination, energy level shift and air stability of PbSe CQDs treated with these different ligands were systematically investigated. It was noted that TBAI-treated PbSe CQDSCs exhibited suppressed recombination, faster charge transfer rate, and longer carrier lifetimes, which resulted in a higher power conversation efficiency (PCE) and long term air stability.
Ligands affect the crystal structure and photovoltaic performance of thin films of PbSe quantum dots
Journal of Materials Chemistry, 2011
We have prepared thin films of PbSe quantum dots (QDs) featuring three different ligands, oleic acid (OA), butylamine (BA), and 1,2-ethanedithiol (EDT), which have pronounced affects on the arrangement and photovoltaic performance of the PbSe QDs in the thin films. Transmission electron microscopy revealed that ligands that altered the inter-QD spacing induced significant changes in the packing of the PbSe QDs in localized regions of small areas (300 Â 300 nm) of the thin films: from a superlattice of OA-capped PbSe QDs to a chaotic pattern of EDT-capped PbSe QDs. Using a synchrotron X-ray reflectivity probe and data fitting, we determined that the roughness decreased and the average densities increased for large-area (1.5 Â 1.5 cm) PbSe QD thin films capped with BA and EDT, relative to those of the OA-capped PbSe QD film. In particular, the PbSe QDs' vertical packing density, which is critical for charge transport, increased substantially for the system incorporating EDT ligands. As a result, devices containing the EDT-treated PbSe QD film as the active layer displayed much improved power conversion efficiencies (PCEs) relative to those of corresponding devices featuring either the OA-or BA-capped PbSe QD films as active layers. Adopting a layer-by-layer technique, we fabricated a EDT-capped PbSe QD device that exhibited a PCE of 2.45%.
Recent Developments of Solar Cells from PbS Colloidal Quantum Dots
Applied Sciences
PbS (lead sulfide) colloidal quantum dots consist of crystallites with diameters in the nanometer range with organic molecules on their surfaces, partly with additional metal complexes as ligands. These surface molecules are responsible for solubility and prevent aggregation, but the interface between semiconductor quantum dots and ligands also influences the electronic structure. PbS quantum dots are especially interesting for optoelectronic applications and spectroscopic techniques, including photoluminescence, photodiodes and solar cells. Here we concentrate on the latter, giving an overview of the optical properties of solar cells prepared with PbS colloidal quantum dots, produced by different methods and combined with diverse other materials, to reach high efficiencies and fill factors.
ACS Energy Letters, 2017
To realize the full potential of colloidal quantum dot (CQD) based solar cells, it is important to address the issue of large open circuit voltage (VOC) deficit which is a major roadblock in reaching higher efficiencies. The origin of the VOC deficit in these solar cells lies primarily in the presence of sub-bandgap trap states of the QDs. Here, we present a synergistic engineering framework to passivate these sub-bandgap states in PbS QDs through chemical surface passivation and remote passivation exploiting ligand and architecture engineering. In particular we form bulk nano-heterojunctions (BNH) by mixing PbS QDs with ZnO nanocrystals in conjunction with mixed ligand treatments to passivate surface traps. We employ the mixed ligand system of zinc iodide and 3-mercatopropyonic acid (MPA) to leverage the benefits of both organic and inorganic ligands for surface passivation and improved charge transport. This mixed ligand treatment in BNH architectures leads to record low Voc deficit for PbS QDs of 0.4 V-0.55 V compared to previously reported 0.6-0.8 V for the range of 1.1-1.35 eV bandgap PbS QDs.
PbS QUANTUM DOT-BASED HETROJUNCTION SOLAR CELLS
2014
This study investigates the influence of nanoparticles (NPs) size on their optical properties, and the effect of combination of lead sulfide (PbS) quantum dots (QDs), with n-type and p-type NPs, on the photogenerated charge carriers transport across the heterojunction solar cell structure. PbS QDs, of a range of sizes, were synthesized using a co-precipitation process. In this study, p-type NPs, which are poly [3,4-ethylenedioxythiophene] –poly [styrenesulfonate] (PEDOT: PSS), copper oxide (CuO) and graphene oxide (GO); and n-type NPs which are zinc oxide (ZnO), titanium dioxide (TiO2), cadmium sulfide (CdS) and bismuth sulfide (Bi2S3), were synthesized and characterized by SEM and UV-visible spectrophotometers. The NPs with enhanced optical properties were utilized in heterojunction solar cell structures via spin coating, chemical bath deposition and SILAR cycle methods. The morphology and the theoretical band energy diagram for each cell were examined. The photovoltaic performance...
Solar Energy Materials and Solar Cells, 2017
Solution-processed colloidal quantum dots (CQDs) are promising candidates for large-scale, low-cost, and lightweight photovoltaic and electronic devices. Carrier transport dynamics has a substantial impact on device efficiency optimization. Coupled with photocarrier radiometry (PCR)a dynamic spectrally integrated frequency-domain photoluminescence (PL) modality, we report the derivation of a trap-state-mediated carrier hopping transport model for the extraction of multiple carrier transport parameters in PbS CQD thin films. These parameters, including effective carrier lifetime τ E , hopping diffusivity D h , trap-state-dependent carrier trapping rate R T , diffusion length L h , and carrier thermal emission rate e i , were obtained for CQD thin films with different dot size and capping ligands: tetrabutylammonium iodide (TBAI), 1,2-ethanedithiol (EDT), and methylammonium lead triiodide perovskite (MAPbI 3). Consistent with the framework of phonon-assisted carrier hopping mechanism, τ E , D h , and L h have demonstrated a monotonic dependence on temperature in the range from 100 K to 300 K. Perovskite-passivated PbS CQD thin films, especially those with larger dot sizes which are free of apparent defect induced PL emission and have higher τ E and D h at room temperature (ca. 0.51 μs and 1.80×10 −2 cm 2 /s, respectively) than their counterparts, demonstrate better photovoltaic material properties. Dot-size-dependent exciton binding energies (35.21-53.20 meV) were characterized using a dynamic PCR photo-thermal spectroscopy that also characterized the trap-state-mediated carrier hopping activation energies in the range from 100 meV to 280 meV. To test the reliability of the best-fitted results, computational fitting uniqueness was examined using a parametric theory.
Electrochimica Acta, 2016
Lead sulfide (PbS) quantum dot (QD) photovoltaics have reached impressive efficiencies of 12%, making them particularly promising for future applications. Like many other types of emerging photovoltaic devices, their environmental instability remains the Achilles heel of this technology. In this work, we demonstrate that the degradation processes in PbS QDs which are exposed to oxygenated environments are tightly related to the choice of ligands, rather than their intrinsic properties. In particular, we demonstrate that while 1,2-ethanedithiol (EDT) ligands result in significant oxidation of PbS, lead iodide/lead bromide (PbX 2) coated PbS QDs show no signs of oxidation or degradation. Consequently, since the former is ubiquitously used as a hole extraction layer in QD solar cells, it is predominantly responsible for the device performance evolution. The oxidation of EDT-PbS QDs results in a significantly reduced effective QD size, which triggers two competing processes: improved energetic alignment that enhances electron blocking, but reduced charge transport through the layer. At early times, the former process dominates, resulting in the commonly reported, but so far not fully explained initial increase in performance, while the latter governs the onset of degradation and deterioration of the photovoltaic performance. Our work highlights that the stability of PbS quantum dot solar cells can be significantly enhanced by an appropriate choice of ligands for all device components.
Comparing Halide Ligands in PbS Colloidal Quantum Dots for Field-Effect Transistors and Solar Cells
2018
Capping colloidal quantum dots (CQDs) with atomic ligands is a powerful approach to tune their properties and improve the charge carrier transport in CQD solids. Efficient passivation of the CQD surface, which can be achieved with halide ligands, is crucial for application in optoelectronic devices. Heavier halides, i.e., I– and Br–, have been thoroughly studied as capping ligands in the last years, but passivation with fluoride ions has not received sufficient consideration. In this work, effective coating of PbS CQDs with fluoride ligands is demonstrated and compared to the results obtained with other halides. The electron mobility in field-effect transistors of PbS CQDs treated with different halides shows an increase with the size of the atomic ligand (from 3.9 × 10–4 cm2/(V s) for fluoride-treated to 2.1 × 10–2 cm2/(V s) for iodide-treated), whereas the hole mobility remains unchanged in the range between 1 × 10–5 cm2/(V s) and 10–4cm2/(V s). This leads to a relatively more pro...