Joint Mapping of Mobility and Trap Density in Colloidal Quantum Dot Solids

Abstract

Field-effect transistors have been widely used to study electronic transport and doping in colloidal quantum dot solids to great effect. However, the full power of these devices to elucidate the electronic structure of materials has yet to be harnessed. Here, we deploy nanodielectric field-effect transistors to map the energy landscape within the band gap of a colloidal quantum dot solid. We exploit the self-limiting nature of the potentiostatic anodization growth mode to produce the thinnest usable gate dielectric, subject to our voltage breakdown requirements defined by the Fermi sweep range of interest. Lead sulfide colloidal quantum dots are applied as the active region and are treated with varying solvents and ligands. In an analysis complementary to the mobility trends commonly extracted from field-effect transistor studies, we focus instead on the subthreshold regime and map out the density of trap states in these nanocrystal films. The findings point to the importance of comprehensively mapping the electronic band- and gap-structure within real quantum solids, and they suggest a new focus in investigating quantum dot solids with an aim toward improving optoelectronic device performance.

Colloidal Quantum Dot Solar Cells

“…Recently, Al/ Al 2 O 3 was employed as a gate dielectric to replace the commonly used Si/SiO 2 (Figure 12d). 145 The high permittivity of Al 2 O 3 allows the use of a very thin gate film and much lower gate voltage, which effectively reduces the effect of charges in the gate. On the basis of the new gate substrate, much more precise measurements were realized.…”

Photovoltaic light absorber with spatial energy band gradient using PbS quantum dot layers

“…The tunable optoelectronic property of CQDs in combination with their solution processability provides a powerful tool for implementing extraordinary photovoltaic device architectures based on spatial band engineering. A design of novel photovoltaic absorbers that uses spatial band engineering could realize enhanced charge carrier separation, the main motivation of this work, beyond previous pursuits of photovoltaic-quality quantum dot solids (QDSs) by modifying the surface passivation of individual CQDs [7][8][9][10].…”

Charge Transport Characterization of PbS Quantum Dot Solids for High Efficiency Solar Cells

“…We have found hysteretic I-V behaviour as shown in Fig. 2(a), which is physically related to the efficiency measurement problems recently issued for assessing emerging solar cells based on quantum dots [12] or perovskite materials [15]. The physical mechanism may be related to free carrier trapping or bound charge polarization, and in any case, it would be reasonable to assume a change in the electrostatic energy barrier to explain the hysteresis.…”

“…Initially multi-exciton generation phenomena in QD materials have driven CQD photovoltaics [5][6][7][8], which are now becoming one of the competitors for the next generation thin film solar cells with a lower cost per watt. Among the technology breakthroughs in CQD photovoltaics, the most fundamental and crucial achievement is in the CQD surface passivation in forming quantum dot solids (QDSs) [9][10][11][12]. The fundamental limitation of charge transport in QDS is summarized by QD to QD tunneling resistance and QD surface states charge trapping.…”