Material Science

To describe the properties of organic semiconductors (OSCs), three different approaches are available, each with its own advantages and disadvantages. Small cluster calculations have achieved great success in modeling photo-induced relaxation processes in OSCs [1], but they fail miserably in describing absorption phenomena. Periodic boundary conditions typically describe absorption spectra very well [2], but they cannot simulate photo-induced processes. Finally, theoretical methods based on effective Hamiltonians provide detailed assignments of spectra [3], but they cannot predict new materials as many of the necessary parameters rely on available experimental data. We have developed a computational protocol based on the cluster approach that provides highly accurate (polarization-resolved) absorption spectra of thin films and crystals of organic semiconductors, such as Pentacene [4], Tetracene (Figure 1), and a squaraine compound. The key components of our protocol include the use of optimally tuned functionals [5] and the selection of clusters that reflect the essential symmetries of the crystal structure and allow for delocalized excitons. Comparisons between calculated and measured spectra (e.g., Figure 1) demonstrate that our protocol accurately accounts for both electronic and vibrational effects.

(a)	(b)
Figure 1: Comparison of the calculated and measured (a) full and (b) polarization-resolved absorption spectrum of Tetracene.

[1] The dimer-approach to characterize opto-electronic properties of and exciton trapping and diffusion in organic semiconductor aggregates and crystals. B. Engels, V. Engel, PCCP 2017, 19, 12604, https://doi.org/10.1039/C7CP01599B.

[2] Polarized absorbance and Davydov splitting in bulk and thin-film pentacene polymorphs. C. Cocchi, T. Breuer, G. Witte, C. Draxl PCCP 2018, 20, 29724, https://doi.org/10.1039/C8CP06384B.

[3] Charge-Transfer Excitations Steer the Davydov Splitting and Mediate Singlet Exciton Fission in Pentacene. D. Beljonne, H. Yamagata, J. L. Brédas, F. C. Spano, Y. Olivier, Phys. Rev. Lett. 2013, 110, 226402, https://doi.org/10.1103/PhysRevLett.110.226402.

[4] Accurate Polarization-Resolved Absorption Spectra of Organic Semiconductor Thin Films Using First-Principles Quantum-Chemical Methods: Pentacene as a Case Study. L. Craciunescu, S. Wirsing, S. Hammer, K. Broch, A. Dreuw, F. Fantuzzi, V. Sivanesan, P. Tegeder, B. Engels, J. Phys. Chem. Lett. 2022, 13(16), 3726–3731, https://doi.org/10.1021/acs.jpclett.2c00573.

[5] Reliable Prediction of Charge Transfer Excitations in Molecular Complexes Using Time-Dependent Density Functional Theory. T. Stein, L. Kronik, R. Baer, JACS 2009, 131(8), 2818–2820, https://doi.org/10.1021/ja8087482.