Orthogonal colloidal quantum dot inks enable efficient multilayer optoelectronic devices

Lee, Seungjin; Choi, Min; Sharma, Geetu; Biondi, Margherita; Chen, Bin; Baek, Se‐Woong; Najarian, Amin Morteza; Vafaie, Maral; Wicks, Joshua; Sagar, Laxmi Kishore; Hoogland, Sjoerd; Arquer, F. Pelayo Garcı́a de; Voznyy, Oleksandr; Sargent, Edward H.

doi:10.1038/s41467-020-18655-7

Cited by 65 publications

(64 citation statements)

References 40 publications

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“…We expect the solvent–ligand interaction to influence the fabrication of the HTL in state-of-art solar cells, where the exchange with 1,2-ethanedithiol (EDT) ligands, given the high reactivity of thiols, is fast and often leads to aggregation caused by the rapid volume contraction. − However, the solvent’s role in the SLE exchange kinetics and CQD aggregation is not fully explored and its effects, in the context of the solar cell device, are not yet fully understood.…”

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confidence: 93%

“…From AFM measurements (Figure S3 in the Supporting Information), we calculated the RMS surface roughness and found a decreased surface roughness in the case of PRN (1.6 ± 0.2 nm) compared to ACN (2.8 ± 0.4 nm). Additionally, from SEM images, we observe in the PRN sample a reduced number of cracks, caused by volume shrinkage following rapid exchange (Figure S4 in the Supporting Information). We attribute this to the slower exchange kinetics, which minimize the rate of volume contraction.…”

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confidence: 99%

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Control Over Ligand Exchange Reactivity in Hole Transport Layer Enables High-Efficiency Colloidal Quantum Dot Solar Cells

et al. 2021

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Colloidal quantum dot (CQD) solar cells are solutionprocessed photovoltaic devices that exhibit promise in harvesting the infrared solar spectrum. Solid-state ligand exchange is the method employed to fabricate the CQD hole transport layer (HTL) in these cells: insulating oleic acid ligands are substituted with short thiol ligands (1,2-ethanedithiol) to create conductive p-type CQD solids. Thiols' high reactivity with the CQD surface results in rapid exchange, giving rise to aggregates of dots and unpassivated sites on dots, each contributing to sub-bandgap trap states. Here we report a strategy to minimize trap states in the CQD HTL by controlling the solvent type in the exchange. By employing a less volatile solvent, we achieve a slower reaction, leading to increased order and a 2 times reduced trap density in CQD solids. These improvements enable a power conversion efficiency of 13.1 ± 0.1% in CQD solar cells compared to control devices showing 12.4 ± 0.1%.E merging solution-processed materials have seen intense investigation because they provide facile routes to highperforming optoelectronic devices: colloidal quantum dots (CQDs) 1−4 for their absorption tunable across the solar spectrum; two-dimensional nanomaterials 5−9 prized for their flexibility, transparency, and charge mobility; and organic materials 10 for engineered excitonics and metal halide perovskites for outstanding transport and excited state lifetime of free electron−hole pairs. 11,12 CQDs have seen particular study in light-emitting diodes, 13,14 bioimaging, 15 photodetectors, 16,17 and solar cells. 18−21 For solar cells, lead sulfide (PbS) has received attention for its strong absorption coefficient and large Bohr radius, all relevant to light-harvesting across the solar spectrum. Advances in CQD ligand exchange and device architecture have recently enabled power conversion efficiencies (PCE) of 13%. 18 PbS CQD solar cells can also complement both perovskites and c-Si in a four-terminal tandem configuration. 22,23 Recently, CQD solar cells showed potential in flexible devices owing to their low-temperature fabrication. 24−26 To achieve efficient charge transport and collection in CQD solids, it is imperative to replace bulky oleic acid (OA) native ligands capping the CQDs with short conductive ligands.

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Control Over Ligand Exchange Reactivity in Hole Transport Layer Enables High-Efficiency Colloidal Quantum Dot Solar Cells

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“…With n–i–p structure, the CQDSC with the MA‐PbS HTL can achieve a V oc of 0.64 V with a V oc loss of ≈0.45 V. Additionally, the group further introduced aromatic ligands to achieve the process‐orthogonal CQD inks, which can realize CQD film stacks. [ 202 ] The results revealed that the CQDs with aromatic ligands can achieve higher hole mobility and lower defect density, compared with those of EDT‐PbS HTLs. With n–i–p structure, the CQDSC with the CQD HTL of aromatic ligands can achieve a V oc of 0.52 V with a V oc loss of ≈0.39 V.…”

Section: Charge Transport Layermentioning

confidence: 99%

Open‐Circuit Voltage Loss in Lead Chalcogenide Quantum Dot Solar Cells

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et al. 2021

Advanced Materials

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absorption coefficients, which can be used to capture the far-infrared of solar radiation. [13,14] This outperforms silicon-based and other solution-based semiconductor materials. Additionally, lead chalcogenide CQDSCs have the potential to break through the Shockley-Quesser limit, via multiple exciton generation. [15][16][17][18] In the past decade, lead chalcogenide CQDSCs have attracted abundant attention and their power conversion efficiencies (PCEs) have increased from ≈3% to ≈14%. [19][20][21] Moreover, the facile solution-processability (synthesis and ligand exchange) allows the production of solar cells by spraying, scraping, and roll-to-roll process, enabling great commercial potential. [22][23][24][25] Recently, some reports discussed the commercial viability of lead chalcogenide CQDSCs. [26,27] The results demonstrated that the cost of flexible lead chalcogenide CQDSCs can be as low as ≈0.94 $ per W, indicating the strong competitiveness of these solution-processability solar cells. To reach this low production cost, the PCEs of the CQDSCs are assumed to be 19% and the devices are assumed to be manufactured via a roll-toroll process. [27] Thus, it is still a great challenge for the commercialization of lead chalcogenide CQDSCs, and the low PCE is still the main obstacle for its market competitiveness.We summarize the main developments of lead chalcogenide CQDSCs (see Figure 1). In the early stage, the Schottky structure was used and it achieved the highest PCE of ≈5.2% in 2013. [28] But after 2014, there exist few reports of lead chalcogenide CQDSCs with the Schottky structure, due to the mismatch between optical absorption and charge extraction. [29] Subsequently, the n-p structure and depleted bulk heterojunction (DBH) structure have greatly improved the PCEs of lead chalcogenide CQDSCs. [30,31] N-p structure uses p-n junction to improve carrier extraction efficiency and reduce recombination, thus increasing the PCE of lead chalcogenide CQDSCs from ≈3% to above 9% in 2015. [19,32] Additionally, the absorption coefficient of lead chalcogenide CQDSCs reaches ≈10 6 cm −3 , which means that it is essential to use ≈1 μm solar absorber to fully absorb solar radiation. The DBH structure uses a bulk electron transport layer (ETL) to address the challenge of insufficient carrier diffusion length for the n-p structure, thus increasing the absorber thickness and greatly improving the short-circuit current (J sc ). [33][34][35] Through continued efforts, the PCE of the DBH structure has reached ≈10.8%. [36] To date, more research 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 im...

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“…CQD inks—dispersions of CQDs capped with short conductive ligands—have recently been employed in high‐performing optoelectronic devices. [ 20,21 ] This is enabled by the combined effect of a high degree of surface passivation, [ 22 ] minimized energetic disorder, [ 20 ] and compatibility with scale‐up techniques, [ 23 ] as opposed to CQDs passivated by solid‐state ligand exchange. However, the reduced ligand length in CQD inks requires electrostatic colloidal stabilization, which complicates oriented assembly in CQD solids.…”

Section: Introductionmentioning

confidence: 99%

Facet‐Oriented Coupling Enables Fast and Sensitive Colloidal Quantum Dot Photodetectors

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Charge carrier transport in colloidal quantum dot (CQD) solids is strongly influenced by coupling among CQDs. The shape of as‐synthesized CQDs results in random orientational relationships among facets in CQD solids, and this limits the CQD coupling strength and the resultant performance of optoelectronic devices. Here, colloidal‐phase reconstruction of CQD surfaces, which improves facet alignment in CQD solids, is reported. This strategy enables control over CQD faceting and allows demonstration of enhanced coupling in CQD solids. The approach utilizes post‐synthetic resurfacing and unites surface passivation and colloidal stability with a propensity for dots to couple via (100):(100) facets, enabling increased hole mobility. Experimentally, the CQD solids exhibit a 10× increase in measured hole mobility compared to control CQD solids, and enable photodiodes (PDs) exhibiting 70% external quantum efficiency (vs 45% for control devices) and specific detectivity, D* > 1012 Jones, each at 1550 nm. The photodetectors feature a 7 ns response time for a 0.01 mm2 area—the fastest reported for solution‐processed short‐wavelength infrared PDs.

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Orthogonal colloidal quantum dot inks enable efficient multilayer optoelectronic devices

Cited by 65 publications

References 40 publications

Control Over Ligand Exchange Reactivity in Hole Transport Layer Enables High-Efficiency Colloidal Quantum Dot Solar Cells

Control Over Ligand Exchange Reactivity in Hole Transport Layer Enables High-Efficiency Colloidal Quantum Dot Solar Cells

Open‐Circuit Voltage Loss in Lead Chalcogenide Quantum Dot Solar Cells

Facet‐Oriented Coupling Enables Fast and Sensitive Colloidal Quantum Dot Photodetectors

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