Theoretical and Computational Investigations in Studying the Low-lying Excitations from Molecules to Materials as Efficient Energy Harvesters

Patra, Ramen (2026) Theoretical and Computational Investigations in Studying the Low-lying Excitations from Molecules to Materials as Efficient Energy Harvesters. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Ramen Patra (21RS061)) 21RS061.pdf - Submitted Version Restricted to Repository staff only Download (11MB)

Abstract

Low-dimensional strongly correlated organic π-conjugated molecules are promising candidates for potential applications in the field of optoelectronics and molecular electronics due to their exible electronic features, low production costs, and eco-friendliness. Understanding the ground state and the low-lying excited states of these molecules are crucial in determining their optoelectronic, charge transport, and thermoelectric properties. This thesis aims to understand the low-lying excited states and their role in predicting suitable organic light-emitting diodes, solar cells, spintronics and thermoelectric devices. The low-lying singlet and triplet excited states of smaller π-conjugated molecules are calculated using the numerically exact diagrammatic valence bond (DVB) method as well as the density matrix renormalization group (DMRG) method for larger molecules, within the framework of the Pariser-Parr-Pople (PPP) Hamiltonian. The results from these methods are compared with various quantum mechanical techniques, including TDDFT, EOM-CCSD, CASPT2, and DMRG-SCF and experimental data, to check their accuracy and reliability. Designing intermolecular singlet fission (x-SF) materials with conjugated molecules contributes immensely to the field of solar cells. Pyrene derivatives substituted with boron(B) and nitrogen(N) have been shown to meet the thermodynamic energy criteria for effective intermolecular singlet fission by adjusting the singlet-triplet energy gap. This adjustment enhances their potential use in organic electronics and photovoltaics. The in uences of appropriate structural modifications and electronic interactions on the stability of long-lived triplet excitons are also examined in s-indacene, pentalene and cyclobutadiene, which include both aromatic and anti-aromatic components alongside substituents. Although implementation of conventional DMRG technique in investigating fermionic model with periodic boundary condition (PBC) is still challenging due to demand of high computational facility, this symmetrized DMRG approach is utilized to calculate the low-lying correlated excited-states of radially π-conjugated cycloparaphenylene ([6]CPP) and [n]cyclacene molecules within model PPP Hamiltonian with adequate computational cost. This thesis also deals with the charge transport and thermoelectric characteristics of electrode-molecule-electrode systems based on radially π- conjugated molecules using the non-equilibrium Green's function (NEGF) method in combination with density functional theory (DFT). Molecular devices that feature cyclothiophenes, cyclophenylenethienylenes, and diphenylhexatriene (DPH) exhibit negative differential resistance, rectifying behavior, and dual spin-filtering effects by applying different bias voltages. The electronic transport processes are analyzed through transmission spectra, local density of states, transmission eigenstate and molecular projected selfconsistent Hamiltonian eigenstates, providing a thorough explanation of the aforementioned effect present in these devices. Moreover, the spindependent thermoelectric properties indicate favorable Seebeck coefficients and improved charge-spin conversion efficiency in uenced by chemical potential and temperature gradient. This suggests potential applications in spin caloritronic systems. Additionally, the effect of hydrostatic pressure on the structural, mechanical, electronic, and optical characteristics of DPH crystal is explored through DFT. The compression of the lattice due to pressure leads to a consistent decrease in the band gap and a redshift in dielectric constants, absorption coeffcients, and refractive indices, all occurring without any structural phase transition, even at high pressures. This suggests improved electronic conductivity and adjustable optoelectronic uses. In summary, these findings provide important insights for developing effective carbon-based molecular materials for applications in optoelectronics, thermoelectrics, spintronics, and advanced photovoltaic technologies.

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