FULL PAPER © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 wileyonlinelibrary.com order provide an opportunity to under- stand many fundamental physical prop- erties relevant to solar energy conversion. Additionally, organic crystals are prom- ising in their own right due to the efficient carrier and energy transport properties associated with their long-range order. In particular, crystalline and polycrys- talline films of pentacene (PEN) and its derivatives have high carrier mobility for charge transport (≈1-10 cm 2 /Vs hole mobility) [1] and significant photoconduc- tivity. [2,3] Moreover, PEN and many of its derivatives display a propensity for singlet fission (SF), [4,5] a phenomenon that results in greater than 100% internal quantum efficiency in organic photovoltaics. Numerous possible molecular functionali- zations may modify solid-state structural and optoelectronic properties. [6] Therefore, elucidating the structure-property relation is important for the design of new func- tional molecular materials. 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-PEN), [7] shown in Figure 1, has recently attracted much interest. The bulky groups functionalizing PEN enable solubility in organic solvents and allow for solution-processing of polycrystalline TIPS-PEN thin films with high carrier mobility and photo- conductivity. [2,3,7] While TIPS-PEN apparently retains optical properties similar to PEN, its enhanced carrier mobility [8] and Relating the Physical Structure and Optoelectronic Function of Crystalline TIPS-Pentacene Sahar Sharifzadeh,* Cathy Y. Wong, Hao Wu, Benjamin L. Cotts, Leeor Kronik, Naomi S. Ginsberg, and Jeffrey B. Neaton* Theory and experiment are combined to investigate the nature of low-energy excitons within ordered domains of 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-PEN) polycrystalline thin films. First-principles density functional theory and many-body perturbation theory calculations, along with polarization- dependent optical absorption spectro-microscopy on ordered domains, show multiple low-energy absorption peaks that are composed of excitonic states delocalized over several molecules. While the first absorption peak is com- posed of a single excitonic transition and retains the polarization-dependent behavior of the molecule, higher energy peaks are composed of multiple transitions with optical properties that can not be described by those of the molecule. The predicted structure-dependence of polarization-dependent absorption reveals the exact inter-grain orientation within the TIPS-PEN film. Additionally, the degree of exciton delocalization can be significantly tuned by modest changes in the solid-state structure and the spatial extent of the excitations along a given direction is correlated with the degree of electronic dispersion along the same direction. These findings pave the way for tailoring the singlet fission efficiency of organic crystals by solid-state structure. DOI: 10.1002/adfm.201403005 Prof. S. Sharifzadeh Molecular Foundry Lawrence Berkeley National Laboratory Berkeley, CA, 94720, USA E-mail: ssharifz@bu.edu; jbneaton@lbl.gov Prof. S. Sharifzadeh Department of Electrical and Computer Engineering Boston University Boston, MA, 02215, USA Prof. S. Sharifzadeh Materials Science and Engineering Division Boston University Boston, MA, 02215, USA Dr. C. Y. Wong, H. Wu, B. L. Cotts, Prof. N. S. Ginsberg Department of Chemistry University of California, Berkeley Berkeley, CA, 94720, US Prof. L. Kronik Department of Materials and Interfaces Weizmann Institute of Science Rehovoth, 76100, Israel Prof. N. S. Ginsberg Physical Biosciences Division Lawrence Berkeley National Laboratory Berkeley, CA, 94720, USA Prof. N. S. Ginsberg, Prof. J. B. Neaton Department of Physics University of California Berkeley Berkeley, CA, 94720, USA Prof. N. S. Ginsberg, Prof. J. B. Neaton Kavli Energy NanoSciences Institute at Berkeley Berkeley, CA, 94720, USA Prof. N. S. Ginsberg, Prof. J. B. Neaton Materials Sciences Division Lawrence Berkeley National Laboratory Berkeley, CA, 94720, CA 1. Introduction Organic semiconductors are a highly tunable class of inexpen- sive, easily processable, and optically active materials that are promising for next-generation photovoltaics and other opto- electronics applications. While many organic materials have varying degrees of disorder, crystalline films with long-range Adv. Funct. Mater. 2014, DOI: 10.1002/adfm.201403005 www.afm-journal.de www.MaterialsViews.com