Friday, April 20, 2012 3:30pm (334 JFB)
Title: Ultrafast Photophysics of pi-Conjugated Polymers for Organic Photovoltaic Applications"
In this work we used the pump and probe photomodulation (PM) spectroscopy technique to measure the transient excitation dynamics in various pi- conjugated polymers (PCPs) films and blends with appropriate molecular acceptors. Using two different ultrafast laser systems, we extended the PM spectrum to cover a broad spectral range from 0.25 – 2.5 eV in the time domain from 100 fs to 1 ns with 150 fs time resolution. We also used continuous wave (CW) photomodulation spectroscopy, photoluminescence, electro-absorption, doping-induced absorption, and x-ray diffraction to study the photoexcitations and doping induced states, as well as other optical properties of PCPs and polymer donor-fullerene acceptor blends. In addition to these measurements, we also fabricated organic photovoltaic (OPV) solar cell devices based on poly(3-hexyl-thiophene) [P3HT]/fullerene [PCBM] blend and estimated their power conversion efficiency (PCE) in relation to the optical studies. In (1.2:1) weight ratio of P3HT/PCBM blend that shows maximum donor (D) and acceptor (A) domain separation we found that although the intrachain excitons in the polymer domains decay within ~10 ps, no charge polarons are generated at their expense. Instead there is a built-up of charge-transfer (CT) excitons at the D-A interfaces, which may dissociate into separated ‘free’ polarons in the D and A domains at a later time. Although the CT excitons are photogenerated much faster in D-A blends with a smaller domain size (such as in P3HT with random order), their dissociation is less efficient because of larger binding energy. Our results elucidate the charge photogeneration mechanism in D-A blends, and unravel the important role of the binding energy in generating ‘free’ charge polarons.
We also studied the photophysics of a low band gap polymer, namely poly-thienophene-benzodithiophene (PTB7) film and its blend with acceptor [6,6] phenyl C71 butyric acid methyl ester [PC71BM]. In the CW PM spectrum of PTB7/PC71BM blend, clear signatures of polarons are observed. Whereas PA bands related to triplet excitons and trapped polarons are observed in the PM spectrum of pristine PTB7 film. The transient ultrafast PA of PTB7 is dominated by a singlet exciton band at ~0.95 eV. In the transient ultrafast PA spectrum of PTB7/PC71BM blend we found singlet exciton, charge transfer exciton and a polaron band that are generated simultaneously; this is different from the transient PM spectrum of P3HT/PCBM blend. We also found that the charge transfer exciton in PTB7/PC71BM dissociates faster than in P3HT/PCBM blends. This may be one of the reasons for getting higher PCE of ~7.4 % in the PTB7/PC71BM based solar cells compared to PCE ~4 % in P3HT/PCBM based solar cells.
 “Two-step charge photogeneration dynamics in polymer/fullerene blends for photovoltaic applications” Bill Pandit*, Sanjeev Singh*, Tek. P. Basel, Z. V. Vardeny, under review in Phys. Rev. B.* equal contributions.  “Photoexcitation dynamics in Poly [[4,8-bis[(2-ethylhexyl)oxy] benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl] (PTB7) and its fullerene blend” Bill Pandit, Tek. P. Basel, Ella Olejnik, Uyen Hyunh Z. V. Vardeny, in preparation for Phys. Rev. B.  “Ultrafast optical studies of ordered poly (3-thienylene-vinylene) films” E. Olejnik, B. Pandit, T. Basel, E. Lafalce, C.-X Sheng, C. Zhang, X. Jiang, Z. V. Vardeny, under review in Phys. Rev. B.  “Photoexcitation dynamics in isotope exchanged DOO-PPV” Ella Olejnik, Bill Pandit, Tek Basel Z. V. Vardeny, submitted to Appl. Phys. Lett.  “Photoexcitation dynamics in polythiophene/fullerene blends for photovoltaic applications” C.-X. Sheng, T. Basel, B. Pandit Z. V. Vardeny, Organic Electronics 13, 1031 (2012).  “Evidence for excimer photoexcitations in an ordered pi – conjugated polymer film” K. Aryanpour, C-X. Sheng, E. Olejnik, B. Pandit, D. Psiachos, S. Mazumdar, Z. V. Vardeny, Phys. Rev. B 83,155124 (2011).