Open-Circuit Voltage Deficit, Radiative Sub-Bandgap States, and Prospects in Quantum Dot Solar Cells

Chuang, Chia-Hao Marcus; Maurano, Andrea; Brandt, Riley E.; Hwang, Gyu Weon; Jean, Joel; Buonassisi, Tonio; Bulović, Vladimir; Bawendi, Moungi G.

doi:10.1021/acs.nanolett.5b00513

Cited by 232 publications

(263 citation statements)

References 54 publications

(136 reference statements)

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“…9À11 Recently, Bawendi et al reported the presence of emissive IGS that form an "effective band" inside the band gap and ∼0.2 eV away from the band edge, which were believed to be at the origin of the high open-circuit voltage deficit in the QD solar cells. 12 Remarkably, this is in good agreement with our previous work, where we found a band of IGS ≈ 0.2 eV above the valence band of PbS QDs, which form percolation pathways assisting carrier transport. 13,14 In addition, Sargent et al and Klimov et al found similar IGS using transient absorption and scanning tunneling spectroscopy (STS) methods.…”

supporting

confidence: 93%

Molecular Oxygen Induced in-Gap States in PbS Quantum Dots

Zhang

Zherebetskyy

Bronstein³

et al. 2015

ACS Nano

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Artificial solids composed of semiconductor quantum dots (QDs) are being developed for large-area electronic and optoelectronic applications, but these materials often have defect-induced in-gap states (IGS) of unknown chemical origin.Here we performed scanning probe based spectroscopic analysis and density functional theory calculations to determine the nature of such states and their electronic structure. We found that IGS near the valence band occur frequently in the QDs except when treated with reducing agents. Calculations on various possible defects and chemical spectroscopy revealed that molecular oxygen is most likely at the origin of these IGS. We expect this impurity-induced deep IGS to be a common occurrence in ionic semiconductors, where the intrinsic vacancy defects either do not produce IGS or produce shallow states near band edges. Ionic QDs with surface passivation to block impurity adsorption are thus ideal for highefficiency optoelectronic device applications.

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supporting

confidence: 93%

Molecular Oxygen Induced in-Gap States in PbS Quantum Dots

Zhang

Zherebetskyy

Bronstein³

et al. 2015

ACS Nano

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“…Corroborating this claim, we observe an increased PL from energies lower than the bandgap for ligand exchanged CQDs ( Figure S6, Supporting Information), which, in accordance to several reports in the literature, we explain as trap-induced emission. [4,[19][20][21] Moreover, large CQDs, with their smaller surface to volume ratio and longer reaction time, are considered to exhibit a lower surface trap density, which we observe as a distinctly lower PIA signal for all employed ligands. This effect would not be explained by assuming intraband transitions to be responsible for the absorption bands.…”

Section: Resultsmentioning

confidence: 97%

“…[18] The corruption of a clean bandgap via ligand exchanging was furthermore related to the emergence of photoluminescence (PL) at energies below the direct bandgap transition. [4,[19][20][21] Indeed, these works underlined that these states lead to a significant reduction in open circuit voltage and thereby overall solar cell efficiency. [21,22] A clear understanding of these IGS and how to suppress them is thus crucial for further improvement of these materials for future applications.…”

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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“…To date, most works have focused on the transport in CQD solids [15][16][17] or the photophysics 18 , and only a few have investigated the working mechanisms of CQD solar cells in detail 14,19,20 . In this letter, we use timedelayed collection field (TDCF) 21 , bias-assisted charge extraction (BACE) 22 and steady state measurements to investigate free charge generation and recombination in PbS CQDs Schottky structure solar cells.…”

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confidence: 99%

Free carrier generation and recombination in PbS quantum dot solar cells

Kurpiers

Balazs

Paulke

et al. 2016

Applied Physics Letters

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Time Delayed Collection Field (TDCF) and Bias Assisted Charge Extraction (BACE) experiments are used to investigate the charge carrier dynamics in PbS colloidal quantum dot solar cells. We find that free charge carrier creation is slightly field dependent, thus providing an upper limit to the fill factor. BACE measurements reveal a rather high effective mobility of 2 × 10 cm²/Vs, meaning that charge extraction is efficient. On the other hand, a rather high steady state non-geminate recombination coefficient of 3 × 10 cm³/s is measured. We, therefore, propose rapid free charge recombination to constitute the main origin for the limited efficiency of PbS colloidal quantum dots cells.Colloidal quantum dots (CQDs) are a promising class of materials for optoelectronic applications due to their well-tunable optical and electronic properties. In the last years improvement in the quality of the CQDs, as a consequence of better and cleaner synthetic processes, has stimulated their use in a variety of applications from light emitting diodes 1 to solar cells 2,3 , field effect transistors 4-6 and photocatalytic devices 7,8 . The size-induced quantum confinement in the quantum dots can be tuned via precise tuning of the QDs dimensions. This quantum confinement can be partially maintained even in strongly coupled CQD solids 9 . Strong coupling is achieved through the ligand exchange process, by which the original, bulky organic ligands are replaced with shorter ones, enhancing the electronic coupling. Lead chalcogenide quantum dot solids exhibit a very broad absorption range, extending into the near infrared region 10 . This makes them particularly attractive for photovoltaic devices with high photocurrents 11 . From the early Schottky devices 9,11 , which show limited open circuit voltage (V oc ) and fill factors (FF), great improvement in the device structure and in the choice of ligands led to a record PbS CQD solar cell with efficiencies up to 10%. That record device showed a short circuit current of up to 22 mA/cm 2 , V oc of 0.63 V and FF of 0.7 12 . Still, the device performance seems to be limited by the V OC and the FF, purportedly due to recombination

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Open-Circuit Voltage Deficit, Radiative Sub-Bandgap States, and Prospects in Quantum Dot Solar Cells

Cited by 232 publications

References 54 publications

Molecular Oxygen Induced in-Gap States in PbS Quantum Dots

Molecular Oxygen Induced in-Gap States in PbS Quantum Dots

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

Free carrier generation and recombination in PbS quantum dot solar cells

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