Quantification of Deep Traps in Nanocrystal Solids, Their Electronic Properties, and Their Influence on Device Behavior

Bozyigit, Deniz; Volk, Sebastian; Yarema, Olesya; Wood, Vanessa

doi:10.1021/nl402803h

Cited by 109 publications

(166 citation statements)

References 24 publications

(49 reference statements)

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“…Our results and other recent publications on the subject yielding similar values 36,37 indicate that while the mobility was comparable in the two material classes, the trap density differed by a factor of three. This motivated us to undertake a closer inspection of these trap densities and their connection to diffusion lengths and transport in CQD solids.…”

Section: Resultssupporting

confidence: 87%

Engineering colloidal quantum dot solids within and beyond the mobility-invariant regime

et al. 2014

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Colloidal quantum dots are attractive materials for efficient, low-cost and facile implementation of solution-processed optoelectronic devices. Despite impressive mobilities (1-30 cm 2 V À 1 s À 1 ) reported for new classes of quantum dot solids, it is-surprisingly-the much lower-mobility (10 À 3 -10 À 2 cm 2 V À 1 s À 1 ) solids that have produced the best photovoltaic performance. Here we show that it is not mobility, but instead the average spacing among recombination centres that governs the diffusion length of charges in today's quantum dot solids. In this regime, colloidal quantum dot films do not benefit from further improvements in charge carrier mobility. We develop a device model that accurately predicts the thickness dependence and diffusion length dependence of devices. Direct diffusion length measurements suggest the solid-state ligand exchange procedure as a potential origin of the detrimental recombination centres. We then present a novel avenue for in-solution passivation with tightly bound chlorothiols that retain passivation from solution to film, achieving an 8.5% power conversion efficiency.

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Section: Resultssupporting

confidence: 87%

Engineering colloidal quantum dot solids within and beyond the mobility-invariant regime

et al. 2014

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“…The observation of two distinct distributions may explain why some groups reported states close to the valence level and others associated them with the conduction level. [9][10][11][12][13][14]28] The surface chemistry of PbS CQDs is more complex than simple schemes such as the one in Figure 1a suggest. Even assynthesized CQDs do not only exhibit two differently polarized crystal facets ((001) and (111)), but are also covered with hydroxyl and oleate ions additional to oleic acid.…”

Section: Resultsmentioning

confidence: 99%

“…These classes of materials are level for EDT capped PbS was reported for electron transport experiments. [13,14] For PbS treated with tetrabutylammonium iodide (TBAI) a trap distribution with an activation energy of 0.34 eV was found via temperature-dependent impedance spectroscopy. [15] Using optical pump-probe spectroscopy, the groups of Asbury and Sargent observed a broad photoinduced absorption (PIA) around 0.3 eV with a long lifetime (still visible after 200 µs) for a range of ligands, including EDT and MPA, and attributed this observation to surface trap states.…”

Section: Introductionmentioning

confidence: 99%

Revealing Trap States in Lead Sulphide Colloidal Quantum Dots by Photoinduced Absorption Spectroscopy

Kahmann

Sytnyk

Schrenker

et al. 2017

Adv Elect Materials

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highly attractive for applications in the field of (opto-)electronics-mostly due to the feasible processing from solution and the tunability of their bandgap energy. Lead sulphide (PbS), in particular, attracts considerable interest due to its narrow bulk bandgap of 0.41 eV and the subsequent possibility to harness infrared photons in detectors and solar cells [1][2][3][4] or emit infrared light. [5][6][7] As-synthesized CQDs are commonly covered with long, electrically insulating surface ligands that need to be exchanged in order to promote electrical conduction. This exchange improves the charge carrier mobility, but can simultaneously affect the quantum dot (QD) density of states (DOS) and may introduce in-gap states (IGS). [8] In most cases, IGS are considered as trap states. The high surface to volume ratio renders the CQDs vulnerable to surface-related defects, which might be caused by incomplete surface passivation, surface oxidation, or interfacial states caused by the attachment of exchanged ligands. Such defect states were observed for PbS before by various techniques. Reported were especially a state 0.2 eV above the valence level of 1,2-ethanedithiol or 1,3-mercaptopropionic acid (EDT, MPA) capped CQDs via Kelvin probe force microscopy (KPFM) and scanning tunneling spectroscopy (STS), [9][10][11][12] Additionally, a quasimetallic midgap band ≈0.4 eV below the conduction

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“…The lifetime of a trapped carrier can be estimated using a thermal velocity ~10 6 cm sec -1 and average density of traps N ~10 14 cm -3 for PbS NCs as reported in literature. [30][31][32] It has been found that for ligand exchanged PbS NCs the electron capture coefficient ( ) is ~400 times greater than the hole capture coefficient, therefore electrons are more likely to be trapped. 33 Consequently the electron contribution to the photocurrent is small and the photoconductivity is predominantly due to holes.…”

Section: Pbs Nanocrystal Photodetectorsmentioning

confidence: 99%

“…The current focus of much research remains in deep understanding of trap associated charge transport behaviour in PbS NCs and their role in influencing the device performance. [30][31][32][33]45,46 As the gain is principally achieved via a prolonged carrier lifetime this limits the temporal response of the device that requires a fast recombination and subsequent collection of charge carriers. This sets up a fundamental trade-off between photoconductor gain and the bandwidth.…”

Section: Pbs Nanocrystal Photodetectorsmentioning

confidence: 99%

Lead sulphide nanocrystal photodetector technologies

2016

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2 Few other areas of study have uncovered the secrets of the universe like that of the interaction between light and matter, leading to many revolutionary scientific discoveries. The interaction of light, in particular with semiconducting materials, has enabled us to understand the behaviour of various fundamental phenomena and has laid the foundation of the optoelectronic systems that we rely on today. The majority of such systems necessitate the detection of light which is achieved via the use of photodetector devices. A photodetector converts incident photons into an electrical signal, which in the solid-state, is assembled together with an application-oriented readout integrated circuit (ROIC). Present-day commercially available photodetectors are typically made from gallium phosphide/silicon carbide (GaP)/(SiC), silicon (Si) and indium gallium arsenide/germanium (InGaAs)/(Ge) for detection in the ultraviolet (UV), visible and near-infrared (IR) regimes of the electromagnetic spectrum, respectively. For mid and far IR, lead sulphide (PbS), lead selenide (PbSe), indium antimonide (InSb), indium arsenide (InAs) and mercury cadmium telluride (HgCdTe) based photodetectors are commonly used. Photodetectors based on a photoemissive principle such as photomultiplier tubes (PMTs) are also widely used for ultrasensitive detection in the UV to near-IR spectral regime and are fabricated using alkali metals having a low workfunction. For applications demanding multispectral detection, 'two-colour' detectors are often manufactured with a bi-level structure, for example consisting of an IR transmitting Si photodiode mounted over an IR sensitive PbS photoconductor. Such a device structure has drawbacks that include added cost, increased complexity of device fabrication and associated issues in implementation. Furthermore, a significant majority of applications are based upon UV to near-IR light detection where the indirect bandgap of Si, the InGaAs epitaxial growth process, PMT bulkiness and high bias voltage requirement all present challenges and renders their use with flexible platforms impossible. As a result significant research effort continues to be expended on the development of a singlesystem multispectral photodetector to replace two or more detectors. In the last decade amongst all the candidate materials studied 1 PbS semiconductor nanocrystals (NCs), often referred as 'quantum dots', have emerged as the most promising material for the fabrication of 3 this type of photodetector. Such has been the progress in their development that reported PbS NC based photodetectors have already outperformed conventional state-of-the-art photodetectors in many aspects including low-cost room temperature device fabrication via solution processing, flexible substrate compatibility, broadband spectral sensitivity along with the figures of merit achieved, Fig 1a. In this review article, we present an overview of recent developments in PbS NC based photodetectors. We first provide a brief introduction to PbS NCs and their releva...

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Quantification of Deep Traps in Nanocrystal Solids, Their Electronic Properties, and Their Influence on Device Behavior

Cited by 109 publications

References 24 publications

Engineering colloidal quantum dot solids within and beyond the mobility-invariant regime

Engineering colloidal quantum dot solids within and beyond the mobility-invariant regime

Revealing Trap States in Lead Sulphide Colloidal Quantum Dots by Photoinduced Absorption Spectroscopy

Lead sulphide nanocrystal photodetector technologies

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