Abstract

The charge recombination and exciton dissociation are generally recognized as the basic electronic processes limiting the efficiency of photovoltaic devices. The aim of this thesis is to understand the electronic processes limiting the operation of efficient solar cells with particular emphasis on the role of electronic states endowed with magnetic dipole moment. The research work is divided into two parts. The first part deals with dye-sensitized solar cells (DSSCs) with sensitizers based on organic molecules (squaraine) and ruthenium complexes, while the second one regards organic solar cells (OSC) of a single-layer (squaraine) and bulk-heterojunction (squaraine:fullerene) architecture. A detailed mechanism of photocurrent generation in investigated solar cells systems examined by magnetic field effect (MFE) technique is proposed. The observed MFE in DSSCs is attributed to magnetic-field-induced spin-mixing of singlet and triplet electron-hole (e-h) pairs according to the Δg mechanism with spin dephasing due to different Lande g factors of the electron and hole entities. In OSCs under a weak external magnetic field, the change in photocurrent is due to electron and hole (e-h) pairs that experience a modulating hyperfine interaction associated with nuclear (mainly proton) magnetic moments, while in strong magnetic fields the photocurrent is affected by the Δg mechanism.

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