The Hot Charge Transfer (CT) Exciton Mechanism in Organic Photovoltaics

Xiaoyang Zhu, University of Minnesota-Twin Cities, DMR 0804583

A charge transfer (CT) exciton can form across an interface, such as the donor/acceptor (D/A) interface in an organic photovoltaic cell. In the model systems of pentacene or tetracene/vacuum interfaces, we observe a series of CT excitons with binding energies ≤0.5 eV below the image band minimum, as shown by the pseudo-color presentation of two-photon photoemission spectra and solutions to the atomic-hydrogen-like Schrödinger equation in the figure (upper). The key issue at organic donor/acceptor interfaces is the difference in LUMO or HOMO levels. This difference represents an energy gain needed to overcome the exciton binding energy of the singlet Frenkel excitons (S0-Sn). A sufficiently large energy gain favors the formation of hot CTn (n >1) excitons that are energetically close to the charge separation (CS) state. These hot CT excitons are necessary intermediates for charge separation. Fig. 1

Building a new 2PPE spectrometer: applications to organic semiconductor thin films

Xiaoyang Zhu, University of Minnesota-Twin Cities, DMR 0804583

IPSSSTo more effectively probe the dynamics of charge separation at organic semiconductor interfaces, the PI upgraded the 2PPE spectrometer upon moving from Univ. Minn. to the Univ. of Texas at Austin last year. A key feature of the new instrument is an angle-resolved imaging electron detector, as illustrated by test date on Cu(111) for the dispersive surface state (SS) and image potential state (IPS) in the upper panel of Fig. 2.We are now successfully using this new spectrometer to map out the complex band structure and exciton dissociation dynamics at organic semiconductor interfaces. The model system under investigation is the CuPC/C60 donor/acceptor interface. As an illustration of the capability of this new instrument, the low panel of Fig. 2 shows the band structures of quantum well states of s and p symmetries in a monolayer C60 thin film. Fig. 2

Mentoring High School Students and Interaction with OPV Industry

Xiaoyang Zhu, University of Minnesota-Twin Cities, DMR 0804583

There has been substantial new development in the PI’s outreach activities since moving to the University of Texas at Austin on July 1st, 2009:• The PI participated in the Welch Foundation Summer Scholar

program and supervised a high school student, Daniel Nguyen, to carry out nanomaterial research in the PI’s lab in summer 2010. Daniel’s research has been submitted for publication In Journal of Highshool Research.

• In collaboration with Westlake Highschool in Austin, the PI became a member of the WHS's Independent Study Mentorship program and is currently supervising a student, Lucy Zha, in OPV research.

• The PI has started a very fruitful exchange program with Konarka Technologies, Inc. maker of organic solar cells. Konarka’s executive chairman, Mr. Howard Berke, and chief scientist, Prof. Alan Heeger (2000 Nobel Laureate in Chemistry), recently visited with the PI at Austin. The PI also visited Konarka’s headquarter in Lowell, MA. These exchanges involved extensive scientific discussions and focused how basic knowledge learned at the University lab can be used to guide the development of the critically important OPV technology.