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Objectives:

The goals of the second (last) grant year (08-09) was to continue investigating  exciton dissociation process in hybrid PbS/PbSe (QDs) with conducting polymers, as well as how chemical and thermal treatments of QDs affected their optoelectronic properties.  Appropriate QD devices were fabricated and characterized.

Status of effort:

The second (and last) grant year has seen our multi-tier efforts:

1) Ligands manipulations of PbS and PbSe QDs

Two different ligand manipulations were conducted:

• The first is ligand exchange with butylamine, which is a much shorter ligand than the original oleic acid (Fig. 1a). FTIR spectroscopy confirmed the success of ligand exchange (Fig. 1b).

• The second was ligand removal by Laser Pyrolysis. This part was accomplished through an institutional collaboration with Drs. Mukherjee and Witannachchi. More than three orders of magnitude increase of conductivity of the coupled PbSe QD film with ligands removed by CO₂ laser [2] (Fig. 2).

Fig.2. (a) Top view (upper) and cross sectional view (lower) of a PbSe quantum dots field effect transistor (FET) device on a patterned Si substrate showing a 2 μm channel. (b) comparison between the conductivity of the spin-coated film (square) and that of the laser-spay film (triangle). The inset shows the absorption of the two films, and high resolution transmission electron micrograph (TEM) image of a 9 nm ligand-free PbSe NC film [2].

2) Employing a spectroscopic measure for exciton dissociation measurements

Based on this unique technology established last year, we have measured photoinduced absorptions in hybrid film of ligand-exchanged PbS QDs with conducting polymer P3HT. Enhanced charge transfer was observed (Fig. 3), indicated by the change of intensity and position for IR-PA peak.  

 3). Synthesis and optical characterization of PbSe QDs

Our group started to synthesize PbSe QDs using a modified colloidal chemistry procedure through collaboration with Drs. Witannachchi and Mukherjee's group. As an example, Fig.4 shows an absorption spectrum of 9nm PbSe quantum dots synthesized by us.  The insert shows a high resolution transmission electron micrograph (HRTEM), demonstrating the uniform size distribution, which is confirmed by the narrow FWHM of the absorption peak at about 0.65 eV.

Fig. 4

4) Transport properties study in PbSe QDs

Transport properties such as mobility and conductivity measurements were carried out in either lateral (field effect transistor geometry) or sandwich structures. Methodologies involved Time of Flight and current-voltage characterization. As an example, Fig. 5 (next page) shows a typical conductivity measurement on a spin-coated PbSe quantum dot thin film (thickness < 10 nm) on a patterned p⁺-Si /SiO₂ substrate. The device has a channel length of 2 μm and a width of 1 mm. 

Fig. 5

5). Training of graduate students on special optical characterizations

In addition to Jason Lewis who was trained on how to fabricate thin film solar cells and necessary characterization methodologies last year [1], a new graduate student (Sheng Wu) has been trained on our spectroscopic gauge measurement employing photoinduced absorption spectroscopy the past year. He has been involved with this project since he joined my group last fall and has a paper on the way for submission [4].

Accomplishments/New findings:

This year's research resulted in two high-profile journal publications [1,2], and two paper manuscripts being on the way to submission [3,4]

Interactions/Transitions:

1) Participation/presentations at meetings, conferences:

·        MRS 2008 Fall Meeting (Boston, Dec. 1~5.). Oral talk ‘Optical and Transport Study of Nanocomposite Film of Polymer and PbS Quantum Dots', K1 338.

·        Invited talk at University of Florida (Gainesville, Feb 15, 2009). Title “Unconventional in-gap state in lead sulfide quantum dots probed by photoinduced absorption “

·        I have led USF team for a 2009 Global Venture Challenge (Knoxville, Mar 24-27, 2009) in Oakridge National Lab sponsored by DOE. USF team was selected as one of the fifteen semifinalists globally.  

·        Invited talk in ACS's Florida sectional conference (FAME 2009). Title “In-gap state in lead sulfide quantum dots and its relevance to photovoltaic devices”.

2) Collaboration/consultation:

Our lab has continued our inter-institutional collaborations with Dr. Sun's research group in Norfolk State University with prosperous result [3]. Our institutional collaboration with Dr. Schlaf at Electrical Engineering Department of USF has resulted in two submitted research grants to NSF.  

Technical impact:

The results we have obtained with this grant are important for the continuous research effort on possible multiple exciton dissociation in hybrid solar cells. Our findings about photoinduced charge transfer and the unique technique developed to probe it will help understand the essential process of exciton dissociation and give us a guideline to achieve high efficiency of the third generation solar cells, which has immediate impact on the cost-effective utilization of renewable energy.

Broader impact:

I have always been grateful for the ACS-PRF grant I received. Not only has it been the major supporting fund for my beginning years, it has been a tremendous help for my current grants from New Energy Technology Inc and Florida High Technology Corridor Fund, through which my group is working on developing transparent SolarWIndow.  My PRF It serves as a milestone in my career development.

References:

1) J. Lewis, J. Zhang and X. Jiang, “Fabrication of organic solar array for applications in microelectromechanical systems”, Journal of Renewable and Sustainable Energy, 1, 1, (2009).

2) G. Dedigamuwa, J. Lewis, J. Zhang, X. Jiang, P. Mukherjee and S. Witanachchi: A laser-assisted spray process for the growth of surfactant-free PbSe quantum dot films, accepted by Appl. Phys. Lett., (2009).

3) Cheng Zhang, Taina Matos, Rui Li, Eric Annih, and Sam-Shajing Sun; J. Zhang and X. Jiang, Optical and transport Characterizations of Fully Regioregular Head-to-Tail Poly(3-Dodedyl-2,5-Thienylenevinylene), submitted, (2009).

4) S. Wu, J. Lewis, J. Zhang & X. Jiang, Unconventional Gap State in PbS Quantum Dots, preprint, (2009).