The ab-initio density matrix renormalization group's perspective for the fundamentals and analysis of singlet fission

Abstract

Singlet fission (SF), one of the most promising processes that can potentially improve solar cell efficiency, downconverts the higher energy photons and allows photovoltaic devices to surpass the Shockley-Queisser limit. This ultrafast process involves at least two chromophores or molecular fragments, with the initial exciton splitting into two triplets localized in different regions of space. These two triplets remain coupled in a multi-excitonic singlet state ¹(TT), making the overall process spin allowed. The fission of the correlated triplet pair leads to the generation of two electron-hole pairs per photon absorption and opens a path for SF devices to produce more energy under solar radiation. The inherent complexity of SF chromophores such as the strong correlation, involvement of dark electronic states and the existing similarities between SF and intersystem crossing (ISC) have posed severe experimental and theoretical challenges on the ability of SF to increase solar cell efficiencies in real applications. Besides, the dual excitonic nature of SF states has plagued the application of the popular time-dependent density functional theory to SF materials. Consequently, the relevant advancements in the SF research state two major limitations: (i) the mechanism of the SF process in organic molecules is poorly understood, and (ii) there are not many molecules fulfilling the SF criterion and undergoing the fission process.

Firstly, we proposed extensive wavefunction methodologies based on the Density Matrix Renormalization Group (DMRG) method and investigated the microscopic details of the fission process in pentacene dimer. The inclusion of a six-state vibronic coupling Hamiltonian derived from many electron ab-initio DMRG electronic and vibronic interactions enabled many interesting multistate and multimode pathways to form and separate the triplet pair. Our DMRG based two-particle spin descriptors are employed to quantify the local spin of SF basis states which distinguished the two localized triplets on distinct pentacene units along with the inter-dimer stacking and vibronic progression for the first time. These findings revealed that the coexistence of both lower-lying weak and high-lying strong charge-transfer (CT) states plays a crucial role in driving SF, as the triplet pair is vibronically bundled with these states via multimode interactions in the distinct vibronic region of compression and stretching, respectively. The indirect CT interactions are shown to delocalize the ¹(TT) state and reduce the binding energy of the triplet pair through the out-of-phase vibronic modulations, thus providing clear implications of T-T separations in pentacene dimer. Secondly, we performed a high-throughput virtual screening procedure to discover azaborine substituted organic SF materials by tuning the S-T gaps and other charge transport properties in the parent acene ring. To enhance the SF performance, we modified the structural packings and found the local maxima of SF rates in a six-dimensional space using Marcus theory. After extensive exploration and screening using a simplified frontier orbital model, we identified many potential dimer geometries for newly developed SF chromophores and reported their Davydoy splitting, binding energies and enhanced SF rates. These dimer structures can be utilized as new targets for crystal engineering and the possible chemical synthesis of covalently linked dimers. Our analysis highlighted a crucial point that the energetic criterion for monomers merely provides a general guideline, and it is molecular packing in the dimers that can dictate the SF kinetics and should be controlled together with the monomer-based excitonic properties for developing new SF materials.