nonradiative energy loss mechanisms is highly desirable. We note that nonradiative recombination processes can also occur, for instance, because of poor contacts at the electrodes and, in the case of nongeminate recombination, not only via singlet 1 CT states but also via triplet 3 CT states [30] (a topic of future studies in our group).Up to now, the theoretical investigations of the NR recombination process in organic solar cells have been conducted under the Born-Oppenheimer (BO) approximation. [31][32][33][34][35] Thus, the possible impact of nonadiabatic vibronic coupling due to the breakdown of the BO approximation (for instance, in particular, when the energy difference between the initial and final states is small) has been neglected. We note that the nonradiative recombination via nonadiabatic coupling (NAC) was initially investigated in inorganic semiconductors in the early 1950s; there, it was found to play an important role in reducing the number of photogenerated carriers, suppressing luminescence, and decreasing the carrier lifetimes. [36][37][38] In molecular systems, the NR transition between two excited states or between an excited state and the ground state (with the same spin multiplicity) due to nonadiabatic coupling, is referred to as internal conversion. [39] In the case of organic emitters, the theoretical studies of Shuai and co-workers have demonstrated that internal conversion significantly limits the fluorescence quantum yields. [40,41] Compared to the energies (usually in the range 2.0-4.0 eV) of the first excited states in organic emitters, the 1 CT 1 -state energies are generally much lower in organic solar cells, on the order of 0.5-1.7 eV. [22,23,29,42] Thus, the nonadiabatic coupling between the 1 CT 1 state and the ground state can be expected to be large and it becomes important to evaluate the role that nonadiabatic coupling can play in the NR recombination of 1 CT 1 states at donor-acceptor interfaces. We emphasize that the NR recombination rates of interfacial 1 CT 1 states are difficult to measure experimentally since the distribution of the 1 CT 1 states in transient experiments is far from equilibrium. [43,44] Here, we have chosen the pentacene-C 60 interface as a re presentative system to study the factors determining the NR recombination rates in the context of OPV applications, since a large number of data are available from earlier experimental and theoretical studies. [31,[45][46][47][48] As the recombination process is expected to depend on the local D-A interface geometry, we have considered both edge-on and face-on interfacial orientations of the pentacene molecules relative to C 60 . Also, we consider two different packing modes of the pentacene molecules: (i) a herringbone-type packing (referred to as [P:herringbone] hereafter) directly taken from the pentacene crystal structure; [49] and (ii) a co-facial-type packing (referred to as [P:co-facial] Organic photovoltaic (OPV) [1][2][3][4][5][6][7][8] devices have a great potential to become a low-cost technology fo...

We propose a new methodology for the first-principles description of the electronic properties relevant for charge transport in organic molecular crystals. This methodology, which is based on the combination of a nonempirical, optimally tuned range-separated hybrid functional with the polarizable continuum model, is applied to a series of eight representative molecular semiconductor crystals. We show that it provides ionization energies, electron affinities, and transport gaps in very good agreement with experimental values, as well as with the results of many-body perturbation theory within the GW approximation at a fraction of the computational costs. Hence, this approach represents an easily applicable and computationally efficient tool to estimate the gas-to-crystal phase shifts of the frontier-orbital quasiparticle energies in organic electronic materials.

Polarization energy corresponds to the stabilization of the cation or anion state of an atom or molecule when going from the gas phase to the solid state. The decrease in ionization energy and increase in electron affinity in the solid state are related to the (electronic and nuclear) polarization of the surrounding atoms and molecules in the presence of a charged entity. Here, through a combination of molecular mechanics and quantum mechanics calculations, we evaluate the polarization energies in two prototypical organic semiconductors, pentacene and 6,13-bis(2-(tri-isopropylsilyl)ethynyl)pentacene (TIPS-pentacene). Comparison of the results for the two systems reveals the critical role played by the molecular packing configurations in the determination of the polarization energies and provides physical insight into the experimental data reported by Lichtenberger and co-workers (J. Amer. Chem. Soc. 2010, 132, 580; J. Phys. Chem. C 2010, 114, 13838). Our results underline that the impact of packing configurations, well established in the case of the charge-transport properties, also extends to the polarization properties of π-conjugated materials.

Understanding the nature and magnitude of the electronic polarization due to the presence of a charge carrier in organic molecular solids is of fundamental importance in the description of charge-carrier transport. We present an approach to study these effects based on a polarizable force field that accounts for charge, dipole, quadrupole, and induced-dipole interactions. To demonstrate its general applicability, the method is applied to the oligoacene crystal series (naphthalene through pentacene) and perfluorinated derivatives of naphthalene and pentacene. Very good qualitative agreement with experimental results is achieved in terms of both the magnitude and asymmetry of the polarization as a function of the sign of the injected charge, with improved quantitative agreement versus previous theoretical assessments.

Organic solar cells hold promise of providing low‐cost, renewable power generation, with current devices providing up to 13% power conversion efficiency. The rational design of more performant systems requires an in‐depth understanding of the interactions between the electron donating and electron accepting materials within the active layers of these devices. Here, we explore works that give insight into the intermolecular interactions between electron donors and electron acceptors, and the impact of molecular orientations and environment on these interactions. We highlight, from a theoretical standpoint, the effects of intermolecular interactions on the stability of charge carriers at the donor/acceptor interface and in the bulk and how these interactions influence the nature of the charge transfer states as well as the charge separation and charge transport processes.

Non-covalent interactions determine in large part the thermodynamic aspects of molecular packing in organic crystals. Using a combination of symmetry-adapted perturbation theory (SAPT) and classical multipole electrostatics, we describe the interaction potential energy surfaces for dimers of the oligoacene family, from benzene to hexacene, including up to 5000 configurations for each system. An analysis of these surfaces and a thorough assessment of dimers extracted from the reported crystal structures underline that high-order interactions (i.e., three-body non-additive interactions) must be considered in order to rationalize the details of the crystal structures. A comparison of the SAPT electrostatic energy with the multipole interaction energy demonstrates the importance of the contribution of charge penetration, which is shown to account for up to 50% of the total interaction energy in dimers extracted from the experimental single crystals; in the case of the most stable co-facial model dimers, this contribution is even larger than the total interaction energy. Our results highlight the importance of taking account of charge penetration in studies of the larger oligoacenes.

The polarizable environment surrounding charge carriers in organic semiconductors impacts the efficiency of the charge transport process. Here, we consider two representative organic semiconductors, tetracene and rubrene, and evaluate their polarization energies in the bulk and at the organic-vacuum interface using a polarizable force field that accounts for induced-dipole and quadrupole interactions. Though both oligoacenes pack in a herringbone motif, the tetraphenyl substituents on the tetracene backbone of rubrene alter greatly the nature of the packing. The resulting change in relative orientations of neighboring molecules is found to reduce the bulk polarization energy of holes in rubrene by some 0.3 eV when compared to tetracene. The consideration of model organic-vacuum interfaces highlights the significant variation in the electrostatic environment for a charge carrier at a surface although the net change in polarization energy is small; interestingly, the environment of a charge even just one layer removed from the surface can be viewed already as representative of the bulk. Overall, it is found that in these herringbone-type layered crystals the polarization energy has a much stronger dependence on the intralayer packing density than interlayer packing density.

Switching bithiophene for thienothiophene reduces the number of rotational conformations, facilitating self-assembly with minimal effects on the electronic structure.