David Paul McMahon

A transferable model for singlet-fission kinetics

Exciton fission is a process that occurs in certain organic materials whereby one singlet exciton splits into two independent triplets. In photovoltaic devices these two triplet excitons can each generate an electron, producing quantum yields per photon of >100% and potentially enabling single-junction power efficiencies above 40%. Here, we measure fission dynamics using ultrafast photoinduced absorption and present a first-principles expression that successfully reproduces the fission rate in materials with vastly different structures. Fission is non-adiabatic and Marcus-like in weakly interacting systems, becoming adiabatic and coupling-independent at larger interaction strengths. In neat films, we demonstrate fission yields near unity even when monomers are separated by >5 Å. For efficient solar cells, however, we show that fission must outcompete charge generation from the singlet exciton. This work lays the foundation for tailoring molecular properties like solubility and energy level alignment while maintaining the high fission yield required for photovoltaic applications.

Computational study of the structure and charge-transfer parameters in low-molecular-mass P3HT.

Using classical molecular dynamics simulations and quantum chemical calculations, the structure and charge-transfer parameters in crystalline poly(3-hexylthiophene) (P3HT) are investigated. The changes in polymer structure with temperature are studied and, by performing DFT calculations on configurations found from MD, the changes in the charge-transfer characteristics are investigated. The system is found to adopt a structure consistent with X-ray diffraction experiments on the so-called type-II polymorph of the poly(3-alkylthiophenes). Upon increasing temperature, a conformational change in the polymer side chains occurs, which is found to lead to increased disorder in the interring torsions, which modulates the charge transfer along the polymer backbone. The intrachain transfer integrals are found to decrease slightly with temperature, while their distribution broadens considerably due to increased thermal motion of the rings. The interchain transfer integrals are found to be appreciable for both nearest and next-nearest neighbor rings. This, taken with the fact that the positions of rings on neighboring chains are strongly correlated, has consequences for the development of more accurate phenomenological charge-transport models, such as variable range hopping models.

A realistic description of the charge carrier wave function in microcrystalline polymer semiconductors

The electronic structure of the charge carrier in one of the most commonly used semiconducting polymers (poly(3-hexylthiophene (P3HT)) is described using a combination of classical and quantum chemical methods. It is shown that the carriers are localized in correspondence with long-lived traps which are present also in the crystalline phase of the polymer. The existence of activated transport for very ordered polymer phases (regardless of the strength of the polaron formation energy) is explained, and the trapped states, postulated by many phenomenological models, are described for the first time with chemical detail. It is shown that computational chemistry methods can be used to fill the gap between phenomenological descriptions of charge transport in polymers and microscopic descriptions of the individual quantum dynamic processes.

Organic Semiconductors: Impact of Disorder at Different Timescales

The charge transport in organic materials, from molecular crystals to polymers, is determined by their degree of disorder. The dynamic disorder in ideal molecular crystals at room temperature and the static disorder in disordered polymers are just two limiting cases of the timescale of the fluctuations in the electronic Hamiltonian caused by nuclear motions. In fact, a very large number of important materials (e.g. liquid crystalline semiconductors) are actually in an intermediate regime where the disorder is neither purely static nor purely dynamic. This Minireview discusses the recent contribution of computational chemistry (molecular dynamics and quantum chemistry) to the characterization of these transport regimes and outlines the theoretical methods that can be used to relate the system characteristics to the measurable mobility.

Persistence time of charge carriers in defect states of molecular semiconductors.

Charge carriers in organic crystals are often trapped in point defects. The persistence time of the charge in these defect states is evaluated by computing the escape rate from this state using non-adiabatic rate theory. Two cases are considered (i) the hopping between separate identical defect states and (ii) the hopping between a defect state and the bulk (delocalized) states. We show that only the second process is likely to happen with realistic defect concentrations and highlight that the inclusion of an effective quantum mode of vibration is essential for accurate computation of the rate. The computed persistence time as a function of the trap energy indicates that trap states shallower than ∼0.3 eV cannot be effectively investigated with some slow spectroscopic techniques such as THz spectroscopy or EPR commonly used to study the nature of excess charge in semiconductors.

Prediction of Excited-State Energies and Singlet–Triplet Gaps of Charge-Transfer States Using a Restricted Open-Shell Kohn–Sham Approach

Organic molecules with charge-transfer (CT) excited states are widely used in industry and are especially attractive as candidates for fabrication of energy efficient OLEDs, as they can harvest energy from nonradiative triplets by means of thermally activated delayed fluorescence (TADF). It is therefore useful to have computational protocols for accurate estimation of their electronic spectra in order to screen candidate molecules for OLED applications. However, it is difficult to predict the photophysical properties of TADF molecules with LR-TDDFT, as semilocal LR-TDDFT is incapable of accurately modeling CT states. Herein, we study absorption energies, emission energies, zero-zero transition energies, and singlet-triplet gaps of TADF molecules using a restricted open-shell Kohn-Sham (ROKS) approach instead and discover that ROKS calculations with semilocal hybrid functionals are in good agreement with experiments-unlike TDDFT, which significantly underestimates energy gaps. We also propose a cheap computational protocol for studying excited states with large CT character that is found to give good agreement with experimental results without having to perform any excited-state geometry optimizations.

Electronic and Optical Properties at Organic/Organic Interfaces in Organic Solar Cells

In organic photovoltaic (OPV) devices the formation of free charges from a singlet excited state is the key step in converting light to electrical energy. However, questions still remain as to why the process is so fast and efficient in some OPV devices while not in others. Currently, it is not understood how the binding energy of the charge transfer state formed at an organic/organic interface, ~40 kT, is overcome in order to create free charge carriers. Given the difficulty of experimentally probing the electronic processes occurring at the organic/organic interface, it falls to theoretical and computational studies to provide essential insights into the processes occurring on the microscopic level. In this review we will cover the contributions made by theoretical studies to improve our understanding of the organic/organic interface. We will address the advantages and disadvantages of different theoretical approaches to studying the numerous interesting effects observed, such as shifts in the HOMO and LUMO levels due to the electrostatic environment, increased localization due to disorder, and the general impact of molecular orientation on different molecular properties. Further, we will discuss the currently proposed mechanisms of charge separation at the organic/organic interface and the implications that these mechanisms have on the choice of materials for use in OPV devices.

A predictive theory of charge separation in organic photovoltaics interfaces

The key process in organic photovoltaics cells is the separation of an exciton, close to the donor/acceptor interface into a free hole (in the donor) and a free electron (in the acceptor). In an efficient solar cell, the majority of absorbed photons generate such hole-electron pairs but it is not clear why such a charge separation process is so efficient in some blends (for example in the blend formed by poly(3- hexylthiophene) (P3HT) and a C60 derivative (PCBM)) and how can one design better OPV materials. The electronic and geometric structure of the prototypical polymer:fullerene interface (P3HT:PCBM) is investigated theoretically using a combination of classical and quantum simulation methods. It is shown that the electronic structure of P3HT in contact with PCBM is significantly altered compared to bulk P3HT. Due to the additional free volume of the interface, P3HT chains close to PCBM are more disordered and, consequently, they are characterized by an increased band gap. Excitons and holes are therefore repelled by the interface. This provides a possible explanation of the low recombination efficiency and supports the direct formation of “quasi-free” charge separated species at the interface. This idea is further explored here by using a more general system-independent model Hamiltonian. The long range exciton dissociation rate is computed as a function of the exciton distance from the interface and the average dissociation distance is evaluated by comparing this rate with the exciton migration rate with a kinetic model. The phenomenological model shows that also in a generic interface the direct formation if quasi-free charges is extremely likely.