Exciton Formation, Relaxation, and Decay in PCDTBT Natalie Banerji,��� Sarah Cowan,��� Mario Leclerc,��� Eric Vauthey,�� and Alan J. Heeger*,��� Center for Polymers and Organic Solids, UniVersity of California at Santa Barbara, Santa Barbara, California 93106-5090, United States, Department of Chemistry, UniVersite �� LaVal, G1K 7P4 Quebec City, Quebec, Canada, and Department of Physical Chemistry, UniVersity of GeneVa, 30 Quai Ernest-Ansermet, CH-1211 GeneVa 4, Switzerland Received June 16, 2010 E-mail: ajhe1@physics.ucsb.edu Abstract: The nature and time evolution of the primary excitations in the pristine conjugated polymer, PCDTBT, are investigated by femtosecond-resolved fluorescence up-conversion spectroscopy. The extensive study includes data from PCDTBT thin film and from PCDTBT in chlorobenzene solution, compares the fluorescence dynamics for several excitation and emission wavelengths, and is complemented by polarization-sensitive measurements. The results are consistent with the photogeneration of mobile electrons and holes by interband ��-��* transitions, which then self-localize within about 100 fs and evolve to a bound singlet exciton state in less than 1 ps. The excitons subsequently undergo successive migrations to lower energy localized states, which exist as a result of disorder. In parallel, there is also slow conformational relaxation of the polymer backbone. While the initial self-localization occurs faster than the time resolution of our experiment, the exciton formation, exciton migration, and conformational changes lead to a progressive relaxation of the inhomogeneously broadened emission spectrum with time constants ranging from about 500 fs to tens of picoseconds. The time scales found here for the relaxation processes in pristine PCDTBT are compared to the time scale ( 0.2 ps) previously reported for photoinduced charge transfer in phase- separated PCDTBT:fullerene blends (Phys. Rev. B 2010, 81, 125210). We point out that exciton formation and migration in PCDTBT occur at times much longer than the ultrafast photoinduced electron transfer time in PCDTBT:fullerene blends. This disparity in time scales is not consistent with the commonly proposed idea that photoinduced charge separation occurs after diffusion of the polymer exciton to a fullerene interface. We therefore discuss alternative mechanisms that are consistent with ultrafast charge separation before localization of the primary excitation to form a bound exciton. 1. Introduction In the past three decades, conjugated polymers have emerged as an important class of materials with unique properties for use in organic optoelectronics.1-5 ���Plastic��� solar cells represent one major application area, where the active layer is typically a blend of the semiconducting polymer with a fullerene derivative.6-11 Spontaneous phase separation leads to the formation of a nanoscale bulk heterojunction (BHJ) material.12 Ultrafast photoinduced charge separation between the polymer donor and the fullerene acceptor is the initial step in the generation of mobile charge carriers. The mobile charge carriers can then travel to the electrodes along phase-separated fullerene and polymer networks. To improve the performance of BHJ solar cells, synthesis has evolved toward materials with more complex molecular structures that are soluble and processable from solution, for example, functionalized poly(alkylthiophenes) and poly(phe- nylene vinylenes).1 Furthermore, the synthesis of alternating donor-acceptor copolymers yields small bandgap semiconduc- tors, which enable better harvesting of the solar spectrum. Also, the low HOMO energies in these donor-acceptor copolymers lead to better stability of the pristine material and increase the open circuit voltage (VOC) in BHJ solar cells.13-21 Power conversion efficiencies (PCE) up to 8% have been reported for solar cells comprising blends of such polymers.22 We focus on pristine PCDTBT, a copolymer within the poly(2,7-carbazole) family synthesized by Leclerc and co-workers the molecular ��� University of California at Santa Barbara. ��� Universite �� Laval. �� University of Geneva. (1) Heeger, A. J. Chem. Soc. 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structure is shown in the inset of Figure 1.23-25 For PCDTBT: PC70BM, photovoltaic cells with PCEs beyond 6%, internal quantum efficiencies (IQEs) approaching 100% have been demonstrated.26 We report here the ultrafast temporal evolution of the emission spectrum of pristine PCDTBT following light absorp- tion, as measured by femtosecond-resolved fluorescence up- conversion (FU). Data are presented for spin-cast polymer thin films and for PCDTBT in solution in chlorobenzene. For each, photoexcitations in the first and second absorption bands of the polymer are compared, and fluorescence polarization measure- ments are used to supplement the data. Time-resolved fluorescence measurements in the pristine semi- conducting polymer are of interest as an approach to study the nature and time evolution of the primary excitations. Because the photoinduced charge transfer in the PCDTBT:PC70BM blends occurs in less than 200 fs,27 we are particularly interested in the initial relaxation processes occurring directly after the absorption of light. There exist ultrafast spectroscopic studies of new genera- tion semiconducting polymers, such as PBTTT,28 PCPDTBT,29 or APFO3.30-34 However, detailed femtosecond investigation of the wavelength-dependent fluorescence dynamics, as described here, has not been reported for this class of materials. The nature and evolution of the primary excitations in more classic polymers, for example, PPV,35-46 or MEH-PPV,47-60 was explored by a variety of methods including FU. Still, despite (13) Liang, Y. Y. Wu, Y. Feng, D. Q. Tsai, S. T. Son, H. J. Li, G. Yu, L. P. J. Am. Chem. Soc. 2009, 131, 56���57. (14) Liang, Y. Y. Feng, D. Q. Wu, Y. Tsai, S. T. Li, G. Ray, C. Yu, L. P. J. Am. Chem. Soc. 2009, 131, 7792���7799. (15) Hou, J. H. Chen, H. Y. Zhang, S. Q. Chen, R. I. Yang, Y. Wu, Y. 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The dashed green lines identify the interband absorption edge from the absorption spectrum. The molecular structure of the polymer is shown in the inset. 17460 J. AM. CHEM. SOC. 9 VOL. 132, NO. 49, 2010 A R T I C L E S Banerji et al.