Charge Photogeneration in Donor鈥揂cceptor Conjugated Materials: Influence of Excess Excitation Energy and Chain Length

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• 作者：Raphael Tautz ; Enrico Da Como ; Christian Wiebeler ; Giancarlo Soavi ; Ines Dumsch ; Nils Fr枚hlich ; Giulia Grancini ; Sybille Allard ; Ullrich Scherf ; Giulio Cerullo ; Stefan Schumacher ; Jochen Feldmann
• 刊名：The Journal of the American Chemical Society
• 出版年：2013
• 出版时间：March 20, 2013
• 年：2013
• 卷：135
• 期：11
• 页码：4282-4290
• 全文大小：487K
• 年卷期：v.135,no.11(March 20, 2013)
• ISSN：1520-5126

文摘

We investigate the role of excess excitation energy on the nature of photoexcitations in donor鈥揳cceptor 蟺-conjugated materials. We compare the polymer poly(2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[1,2-b;3,4-b鈥瞉dithiophene)-4,7-benzo[2,1,3]thiadiazole) (PCPDTBT) and a short oligomer with identical constituents at different excitation wavelengths, from the near-infrared up to the ultraviolet spectral region. Ultrafast spectroscopic measurements clearly show an increased polaron pair yield for higher excess energies directly after photoexcitation when compared to the exciton population. This effect, already observable in the polymer, is even more pronounced for the shorter oligomer. Supported by quantum chemical simulations, we show that excitation in high-energy states generates electron and hole wave functions with reduced overlap, which likely act as precursors for the polaron pairs. Interestingly, in the oligomer we observe a lifetime of polaron pairs which is one order of magnitude longer. We suggest that this behavior results from the intermolecular nature of polaron pairs in oligomers. The study excludes the presence of carrier multiplication in these materials and highlights new aspects in the photophysics of donor鈥揳cceptor small molecules when compared to polymers. The former are identified as promising materials for efficient organic photovoltaics.