Reducing charge trapping in PbS colloidal quantum dot solids

Balazs, Daniel M.; Nugraha, Mohamad Insan; Bisri, Satria Zulkarnaen; Sytnyk, Mykhailo; Heiß, Wolfgang; Loi, Maria Antonietta

doi:10.1063/1.4869216

Cited by 72 publications

(118 citation statements)

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“…Thus, high FFs imply efficient charge extraction in spite of strong recombination, which suggests high charge carrier mobilities. Balazs et al 30 reported impressively high field effect transistor (FET) electron mobilities ~10 -2 cm²/Vs in colloidal quantum dot solids, but hole mobilities that were rather low, ~10 -4 cm²/Vs. Mobilities for CQDs with BDT ligand, measured in field effect transistors by Bisri et al, were significantly lower 31 , but these FET mobilities were shown to be strongly affected by traps at the interface between the CQD layer and the gate dielectric 31 and can be improved by 5 orders of magnitude using an ion gel gating 31 .…”

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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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Free carrier generation and recombination in PbS quantum dot solar cells

Kurpiers

Balazs

Paulke

et al. 2016

Applied Physics Letters

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“…However, this adsorption-desorption process appear to be reversible. [25] We calculated the value of the trap density under three different surrounding conditions, as seen in Table 1. The change of the trap density is caused by the gasadsorption and as expected, the lowest trap density is observed under vacuum.…”

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confidence: 99%

“…We speculate that electrons easily transfer from PbSe NRs to acceptor states created by molecules bound to the substrate and to the surface of the NRs, [28] which are desorbed upon annealing and vacuum treatments. [25] In order to better understand the transport mechanism in this NR assembly, the FETs were also characterized at lower temperatures. The temperature-dependent I D -V G transfer characteristics measured between 297 and 5 K are displayed in the form of 2D plots (as seen in Figure 4 and Figure S7 Information).…”

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confidence: 99%

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PbSe Nanorod Field‐Effect Transistors: Room‐ and Low‐Temperature Performance

Han

Balazs

Shulga

et al. 2018

Adv Elect Materials

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Bohr radius, quantum confinement effects emerge, the bandgap energy increases, and discrete energy levels appear in the density of states spectrum. From this point of view, PbSe is of particular interest for its large exciton Bohr radius (46 nm) and equal electron and hole effective masses, [7] which allows to achieve strong confinement even in relatively large particles. Moreover, PbSe has a narrow bandgap (0.28 eV in bulk), [8] with consequent optical activity in the near infrared spectral range [9] and displays a high dielectric constant (ε m = 23). [10] These versatile characteristics make PbSe QDs promising semiconductor building blocks for photovoltaic devices, [11] electronic circuits, [12] and mid-and near-infrared sensing. [13] Moreover, it has been demonstrated that the dimensionality of the nanostructure, in practice the aspect ratio, allows for tuning the physical properties. For example, quasi-1D PbSe nanorods (NRs) have been reported to exhibit more efficient electron transport, [14] enhanced multiple exciton generation, [15,16] reduced Auger recombination rates, [17] higher absorption coefficient, [18] and longer biexciton lifetime [19] relative to PbSe QDs. More recently, a maximum external quantum efficiency of 122% has been reported in PbSe NR solar cells. [15] Despite the prospects for high-performance PbSe NR solar cells, [15,20] there is still a lack of studies about their transport properties. This is especially surprising as due to their 1D confinement higher mobilities than in QDs can be expected. The fabrication of thinfilm FETs is a useful method for studying charge transport properties in solution-processed semiconductors, such as QDs.During synthesis colloidal QDs are capped with long-alkyl chain ligands, which provide solubility and stability in solution but also suppress charge carrier transport when the QDs are assembled in films. Ligand exchange with short molecules that can decrease the inter-QD spacing and passivate surface traps is essential for the fabrication of high performing devices. The choice of ligands largely depends on the desired QD properties. In early works, amines (especially hydrazine) were used for PbSe QD and nanowire-based transistors. [21,22] To cross-link PbSe QD films, a series of short-chain bifunctional molecules (e.g., 1,2-ethanedithiol, [23] 1,3-benzenedithiol, [24] 3-mercaptopropionic acid [25] etc.) were introduced in device fabrication. Most recently, the use of halide ions (Cl − , Br− ) have led to rapid improvements in PbS/PbSe QD solar cell power conversion efficiencies (up to 11.3%) and long-term stability in air. [1,26] The proposed

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“…9 These include doping via oxidation, [10][11][12][13][14][15] ligand control, 3,12,16,17 stoichiometry and defects, [18][19][20] and heterovalent impurities. [21][22][23] While n-type films have been fabricated through ligands such as hydrazine and halide salts, 17,24 p-type doping of thiol capped films has so far mostly been achieved via oxidation in ambient conditions.…”

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confidence: 99%