CHAPTER 8: New Science and New Technology in Semiconducting Polymers
Published:08 Sep 2015
Special Collection: 2015 ebook collection , 2011-2015 materials and nanoscience subject collectionSeries: Polymer Chemistry Series
L. Kaake, D. Moses, C. Luo, A. K. K. Kyaw, L. A. Perez, S. Patel, ... A. J. Heeger, in Polymer Photovoltaics: Materials, Physics, and Device Engineering, ed. F. Huang, H. Yip, and Y. Cao, The Royal Society of Chemistry, 2015, pp. 255-271.
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New Science: The field of bulk heterojunction (BHJ) solar cells was created as a result of the discovery of ultrafast charge transfer. The length scale for the wavefunction describing the probability amplitude for finding a photoexcitation at a particular point in space was estimated using position–momentum uncertainty as expressed by the uncertainty principle. The problem can also be considered semi-classically and a very similar estimate of the length scale of the photoexcitation wavefunction is obtained via the resolution limit of a microscope; (λ/2πn) where n is the index of refraction. Finally, the BHJ solar cell should be especially sensitive to the effective interaction volume of the photon because it is comprised of a densely packed collection of strong absorbers. The photoexcitation process, therefore, generates a delocalized coherent superposition of the eigenfunctions of the Schrodinger equation that describes the nanostructured blend, with an immediate probability amplitude for finding a photoexcitation near a BHJ boundary enabling charge transfer in the femtosecond regime over relatively long distances. New Technology: A general strategy is presented to self-assemble unidirectional alignment and efficient charge transport for semiconducting polymer films deposited on textured Si/SiO2 substrates. By employing sandwich casting in a tilted tunnel system, we utilize capillary action, generated by functionalized spacers, to self-assemble semiconducting polymers along uniaxial nano-grooves on the substrate. The strength of capillary action can be tailored by different surface treatments of the glass spacers. PTS functionalization yields highly oriented crystalline films with compact structure with µh = 25.4 cm2 V−1 s−1 and µh = 22.2 cm2 V−1 s−1 for PCDTPT and CDTBTZ, respectively. These values are limited by the S–D contact resistance, Rc. Using longer channels, Rc is significantly less than the channel resistance and µ = 36.3 cm2 V−1 s−1 was measured. Extrapolating to infinite channel length, the intrinsic mobility for PCDTPT is obtained at this degree of chain alignment and structural order: µ = 47 cm2 V−1 s−1. The mobility is strongly anisotropic with 13.6- and 17.6-fold higher values parallel to the direction of polymer alignment.