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Objectives:

            The goals of the 3rd (1^st extended year) grant year (09-10) was to continue investigating  factors affecting exciton dissociation process in neat PbS quantum dots, and in hybrid PbS/PbSe (QDs) with conducting polymers. This year’s work focus on the characteristics of a long-lived gap state we have previously discovered.  Appropriate QD devices were fabricated and characterized.

 Status of effort:

            This year has seen our multi-tier efforts and results:

1) Investigation of gap state in PbS QDs

            Exciton states in lead selenide (PbSe) and lead sulfide (PbS) quantum dots have been studied extensively. However, relatively less attention has been paid to the states within the quantum dots bandgap. Our experimental results have revealed a single in-gap state which bears confinement dependence yet can not be explained by dark exciton theory, nor is it a trap state related to quantum dots surface defects as previously observed.   A detailed analysis of temperature dependence of photoluminescence, Stokes shift, absorption and photoinduced absorption reveals the unconventional G.S. is a new state of trapped exciton in QD film. With appropriate design engineering these trapped excitons might be harvested in solar cells and other optoelectronic devices. [1].

Fig. 1 Temperature dependences of PL (solid square, black) and the first excitonic absorption E₁ (open triangle, red) of a 4.2nm QD film. Black line is a linear fitting of PL data, and red line is fitting for E₁, at T > 50K. The inset is a plot of temperature dependence of PL intensity (solid diamond, black) [1].

2) Ligands effect on exciton dissociation process

            Following the ligand exchange effort of PbS QDs from last year, using a unique technology established in 2008 [2], we have measured photoinduced absorptions in hybrid film of ligand-exchanged PbS QDs with conducting polymer P3HT. Comparing with the mixture of pristine QDs, enhanced charge transfer was observed in P3HT blend with butylamine exchanged PbS QDs (Fig. 2), indicated by the disappearance of polaron peak (DP₂ ) from P3HT [3].

5). Training of graduate students on special optical characterizations

            In addition to Jason Lewis and Sheng Wu, who have jointed contributed to the findings of gap state characteristics [ref], a new graduate student (Evan Lafalce) has been trained on our spectroscopic gauge measurement employing photoinduced absorption spectroscopy the past year. He has acquired experimental skills such as vacuum technology, cryogenics and thin film technology, and organic chemistry. He has been involved with this project since he joined my group this year and has a paper on the way for submission [4].

Accomplishments/New findings:

            This year’s research resulted in 1 high-profile journal publications [1], and 1 paper manuscript under preparation [5].

      We believe the finding of the unconventional gap state is a novelty, since this state is a new state of trapped exciton in QD film. With appropriate design engineering these trapped excitons might be harvested in solar cells and other optoelectronic devices.

Interactions/Transitions:

1) Participation/presentations at meetings, conferences:

·       Jason Lewis, Jian Zhang and Xiaomei Jiang (2010). ‘Unconventional gap state in lead sulfide quantum dots; relevance to photovoltaic devices’, oral talk presented at the APS 2010 March Meeting, Portland, OR.

2) Collaboration/consultation:

            Our lab has continued our inter-institutional collaborations with Dr. Sun’s research group in Norfolk State University with prosperous result [6]. Our institutional collaboration with Dr. Schlaf at Electrical Engineering Department of USF has resulted in one submitted research grant to NSF. 

Technical impact:

            So far the results we have obtained with this grant are of paramount importance 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, the first external grant I have received. This grant has given me the initial support I needed in my career development, and it has helped me secure my current grants from New Energy Technology Inc and Florida High Technology Corridor Fund, through which my group has successfully made a prototype see-through SolarWindow, with much media reports [refs] and great public interest.  My PRF grant has helped the sustainability for my regular research activities, my participation in professional development workshops, and my attendance of scientific conferences. It serves as a milestone in my career development, as well as a great contribution to my institute’s educational programs.

References:

1. J. Lewis, S. Wu and X. Jiang, Unconventional gap state of trapped exciton in PbS quantum dots, accepted by Nanotechnology (2010).

2. J. Zhang and X. Jiang, Confinement-dependent below-gap state in PbS Quantum Dots Films probed by cw Photoinduced Absorption, Journal of Phys. Chem. B letters 112, 9557–9560 (2008).

3. Österbacka R, An CP, Jiang XM, Vardeny ZV (2000).  Two-dimentional electronic excitations in self-assembled conjugated polymer nanocrystals. Science 287, 839.

4. E. Lafalce, C. Zhang and X. Jiang, “Regioregularity dependent PTV”, manuscript, 2010.

5. E. Lafalce, X. Jiang. Enhanced charge transfer in PbS QD andP3HT with ligand exchange,   manuscript, 2010.

6. Cheng Zhang, Taina Matos, Rui Li, Eric Annih, and Sam-Shajing Sun; J.E. Lewis, J. Zhang and X. Jiang, Synthesis and Characterization of Fully Regioregular Head-to-Tail Poly(3-Dodedyl-2,5-Thienylenevinylene) for Opto-Electronic Applications, Polymer Chemistry, 1, 663-669, (2010).