Large scale modelling of photo-excitation processes in materials with application in organic photovoltaics

Resumen

This dissertation concerns fundamental research on the photo-excitation processes in organic optoelectronics, where theoretical chemistry and computational modelling are used in order to investigate large scale material properties and predict more efficient energy technologies. Chapter 1 opens the thesis with the fundamentals of electronic and optoelectronic processes in organic photovoltaics. It covers different device architectures from which donor:acceptor (D:A) bulk heterojunctions (BHJs) are highlighted. This Chapter also describes some of the current challenges in terms of power conversion efficiency. These are the charge transfer exciton binding energy, the presence of non-radiative decay channels and the device morphology. Throughout this thesis, the properties of electronically excited state are studied in great detail while the morphology is discussed briefly. Chapter 2 outlines the general objective of this dissertation, namely to study theoretically and computationally the microscopic processes that underly the photovoltaic mechanisms at large scale. The specific objectives are to improve the technical implementation of the discrete reaction field (DRF) method in the Amsterdam Density Functional (ADF) modelling suite, to study charge transfer and charge separation processes such as those in large D:A BHJs and to explore the potential energy surfaces (PESs) of representative optoelectronic materials. Chapter 3 describes the electronic structure methods, wave function and electron density based methods, used in this thesis. Single- and multi-determinant wave function methods are described first, followed by density functional theory (DFT). DRF, which is widely used in the modelling of polarizable environments, is also introduced. In Chapters 4 to 6, DFT is used to study the excited state properties of photovoltaic materials. In Chapter 7, in addition to DFT, complete active space self-consistent field (CASSCF) and complete active space second order perturbation theory (CASPT2) methods are used to determine non-radiative decay paths in optoelectronic materials. Chapter 4 presents technical work carried out at the company Software for Che- mistry & Materials (SCM), involving an extended implementation of default input parameters required to perform DFT/DRF calculations with ADF. This implementation enables the accurate description of electrostatic potentials and dipole-dipole interactions in large system. The DRF input script for ADF is provided as supplementary information. Chapter 5 reports a calibration of exchange-correlation functionals for charge transfer states. It shows the importance of including long-range exchange-correlation functionals when studying intra- and inter-molecular charge transfer states. The influence of the environment on the charge transfer is included via DRF. The computing timings are also included to show that DRF models the polarizable environment very well, without significant additional computing time. Such a calibration turns out to be very useful when computing the excited state properties of polymer:fullerene derivative BHJs, as discussed in Chapter 6. Chapter 6 provides a theoretical study of the charge transfer exciton binding energy in semiconductor materials for BHJs. It covers four oligomers (PEO-PPV, PTFB, PTB7 and PTB7-Th), two fullerene derivatives ([60]PCBM and [70]PCBM) and eight oligomer:fullerene derivative BHJs (all the possible combinations between the four oligomers and the two fullerene derivatives). The absorption properties of single oligomers and fullerene derivatives and the excited state properties of oligomers:fullerene derivative BHJs are studied. Time-dependent density functional theory (TD-DFT) and DRF in a quantum mechanics/molecular mechanics (QM/MM) framework are used in order to compute the exciton binding energy from charge transfer and charge separated states. The results suggest that donor-acceptor (D-A) type oligomers (PTFB, PTB7 and PTB7-Th) perform better than the highly polarizable donor type oligomer PEO-PPV when blended to [60]PCBM or [70]PCBM. In addition, predicted charge transfer exciton binding energies are lower when D-A-type oligomers are blended with [70]PCBM. Mutual polarization effects from QM and MM regions on the exciton binding energy play a crucial role in the charge separation process. Chapter 7 describes an ab initio quantum chemistry study of luminescence in \pi -conjugated compounds with applications to optoelectronics. The non-radiative decay mechanisms of the distyrylbenzene cyano-substituted (DCS) family are discussed. This family comprises 33 compounds and is classified into two groups, \alpha - and \beta -compounds, where \alpha and \beta refer to the position of the cyano groups with respect to the central ring on the DSC backbone. The emissive character largely depends on the position of the cyano substituents. \beta -compounds, according to the experimental fluorescence quantum yields and computed non-radiative decay rates, tend to be more emissive than \alpha -compounds, with a few exceptions. A simplified strategy to explore the PESs and predict differences in the non-radiative channels, activated by a conical intersection between ground and excited states, is presented. From this strategy two energy descriptors are defined. These are the energy difference between ground and excited states in the Franck-Condon region, namely the absorption energy, and the energy difference between ground and excited states in the pyramidalization region, which is the region immediately before reaching the conical intersection. A good correlation between the non-radiative decay rates and the two energy descriptors is found. Chapter 8 suggests two ways for continued studies of photo-excitation processes in organic photovoltaics, based on the implementation of DFT/DRF energy gradients for the ground and excited states as well as electronic coupling and decay rates from non-orthogonal configuration interaction calculations. Chapter 9 closes the thesis with general conclusions and an outlook for large scale modeling research on next generation organic optoelectronics.