• Journal of the Optical Society of Korea
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• v.19 no.3
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• pp.272-276
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• 2015

The PbS quantum dot is an emerging photovoltaic material, which may provide high efficiency breakthroughs. The most crucial element for the high efficiency solar cells's development is to understand charge transport characteristics of PbS quantum dot solids, which are also important in planning strategic research. We have investigated charge transport characteristics of PbS quantum dot solids thin films using space charge limited conduction analysis and assessed thickness dependent photovoltaic performances. The extracted carrier drift mobility was $low-10^{-2}cm^2/Vs$ with the estimated diffusion length about 50 nm. These and recently reported values were compared with those from a commercial photovoltaic material, and we present an essential element in further development of PbS quantum dot solids materials.

• Proceedings of the Korean Vacuum Society Conference
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• 2010.08a
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• pp.280-280
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• 2010

Semiconductor quantum dots are of great interest for both fundamental research and industrial applications due to their unique size dependant properties. The most promising application of colloidal semiconductor nanocrystals (quantum dots or QDs) is probably as emitters in biomedical labeling, LEDs, lasers etc. As compared to II-VI quantum dots, III-V have attracted greater interest owing to their less ionic lattice, larger exciton diameters and reduced toxicity. Among the III-V semiconductor quantum dots, Indium Phosphide (InP) is a popular material due to its bulk band gap of 1.35 (eV) which is responsible for the photoluminescence emission wavelength ranging from blue to near infrared with change in size of QDs. Nevertheless, in recent years, the exact type of collective properties that arise when semiconductor quantum dots (QDs) are assembled into two- or three-dimensional arrays has drawn much interest. The term "uantum dot solids" is used to indicate three-dimensional assemblies of semiconductor QDs. The optoelectronic properties of the quantum dot solids are known to depend on the electronic structure of the individual quantum dot building blocks and on their electronic interactions. This paper reports an efficient and rapid method to produce highly luminescent and monodisperse quantum dots solution and solid through fabrication of InP thin films. By varying the molar concentration of Indium to Ligand, QDs of different size were prepared. The absorption and emission behaviors were also studied. Similar measurements were also performed on InP quantum dot solid by fabricating InP thin films. The optical properties of the thin films are measured at different curing temperatures which show a blue shift with increase in temperature. The dielectric properties of the thin films were also investigated by Capacitance-voltage(C-V) measurements in a metal-insulator-semiconductor (MIS) device.

• Proceedings of the Korean Vacuum Society Conference
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• 2012.02a
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• pp.535-535
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• 2012

We present a new synthetic route and characterization for ultra small sized PbS quantum dots in extreme quantum confinement with 1.5 to 2.9 nm in diameter. We obtained a series of nanocrystals with first absorption wavelength ranging from 580 to 820 nm (2.1-1.5 eV). To get this result, PbS quantum dots size is finely controlled by adjusting the growth temperature in the range of $70-95^{\circ}C$. We demonstrate that photoluminescence (PL) shows a red shift with respect to the first absorption peak that increases with decreasing PbS quantum dots size and ranges from about 500 to 125 meV as the mean PbS quantum dots diameter increases from 1.5 to 2.9 nm. We further created the assembled PbS quantum dot solids and investigated the transport properties for energy applications.

• Proceedings of the Korean Vacuum Society Conference
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• 2012.02a
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• pp.541-541
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• 2012

InP quantum dots capped by myristic acid (InP-MA QDs) were synthesized by a typical hot injection method using MA as stablizing agent. The current density across the InP-MA QDs thin film which was fabricated by spin-coating method is about $10^{-4}A/cm^2$ at the electric field of 0.1 MV/cm from I-V measurement on a metal-insulator-metal (MIM) device. The low conductivity of the InP-MA QDs thin film is interpreted as due to the long interdistances among the dots governed by the MA molecules. Therefore, replacing the MA with thioacetic acid (TAA) by biphasic ligand exchange was conducted in order to obtain TAA capped InP QDs (InP-TAA). InP-TAA QDs were designed due to: 1) the TAA is very short molecule; 2) the thiolate groups on the surface of the InP-TAA QDs are expected to undergo condensation reaction upon thermal annealing which connects the QDs within the QD thin film through a very short linker -S-; and 3) TAA provides better passivation to the QDs both in the solution and thin film states which minimizing the effect of surface trapping states.

• Proceedings of the Korean Vacuum Society Conference
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• 2009.08a
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• pp.73-73
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• 2009

• Bulletin of the Korean Chemical Society
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• v.35 no.2
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• pp.457-465
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• 2014

Size-controlled lead sulfide (PbS) quantum dots were synthesized by the typical hot injection method using oleic acid (OA) as the stabilizing agent. Subsequently, the ligand exchange reaction between OA and thioacetic acid (TAA) was employed to obtain TAA-capped PbS quantum dots (PbS-TAA QDs). The condensation reaction of the TAA ligands on the surfaces of the QDs enhanced the conductivity of the PbS-TAA QDs thin films by about 2-4 orders of magnitude, as compared with that of the PbS-OA QDs thin films. The electron transport mechanism of the PbS-TAA QDs thin films was investigated by current-voltage (I-V) measurements at different temperatures in the range of 293 K-473 K. We found that the charge transport was due to sequential tunneling of charge carriers via the QDs, resulting in the thermally activated hopping process of Arrhenius behavior.

• Proceedings of the Korean Vacuum Society Conference
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• 2012.08a
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• pp.189-189
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• 2012

Lead sulfide (PbS) nanocrystal quantum dots (NQDs) are promising materials for various optoelectronic devices, especially solar cells, because of their tunability of the optical band-gap controlled by adjusting the diameter of NQDs. PbS is a IV-VI semiconductor enabling infrared-absorption and it can be synthesized using solution process methods. A wide choice of the diameter of PbS NQDs is also a benefit to achieve the quantum confinement regime due to its large Bohr exciton radius (20 nm). To exploit these desirable properties, many research groups have intensively studied to apply for the photovoltaic devices. There are several essential requirements to fabricate the efficient NQDs-based solar cell. First of all, highly confined PbS QDs should be synthesized resulting in a narrow peak with a small full width-half maximum value at the first exciton transition observed in UV-Vis absorbance and photoluminescence spectra. In other words, the size-uniformity of NQDs ought to secure under 5%. Second, PbS NQDs should be assembled carefully in order to enhance the electronic coupling between adjacent NQDs by controlling the inter-QDs distance. Finally, appropriate structure for the photovoltaic device is the key issue to extract the photo-generated carriers from light-absorbing layer in solar cell. In this step, workfunction and Fermi energy difference could be precisely considered for Schottky and hetero junction device, respectively. In this presentation, we introduce the strategy to obtain high performance solar cell fabricated using PbS NQDs below the size of the Bohr radius. The PbS NQDs with various diameters were synthesized using methods established by Hines with a few modifications. PbS NQDs solids were assembled using layer-by-layer spin-coating method. Subsequent ligand-exchange was carried out using 1,2-ethanedithiol (EDT) to reduce inter-NQDs distance. Finally, Schottky junction solar cells were fabricated on ITO-coated glass and 150 nm-thick Al was deposited on the top of PbS NQDs solids as a top electrode using thermal evaporation technique. To evaluate the solar cell performance, current-voltage (I-V) measurement were performed under AM 1.5G solar spectrum at 1 sun intensity. As a result, we could achieve the power conversion efficiency of 3.33% at Schottky junction solar cell. This result indicates that high performance solar cell is successfully fabricated by optimizing the all steps as mentioned above in this work.