## 2 Fundamental Principles  
The electronic and optical properties of organic semiconductors stem from their unique molecular structure, particularly their conjugated backbone. To understand these properties, it is instructive to compare conventional saturated polymers with conjugated systems. In saturated polymers like polyethylene (PE), carbon atoms form only \(\sigma\)-bonds, resulting in a large highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap typically approaching 7 eV [12]. This electronic structure makes PE an electrical insulator with minimal interaction with visible light. In contrast, conjugated polymers like polyacetylene (PA) feature alternating single and double bonds, where each carbon atom contributes one electron to the \(\pi\)-system. This conjugation creates delocalized \(\pi\)-electron clouds extending over multiple monomer units, significantly reducing the HOMO-LUMO gap to 1-1.5 eV [13], placing it in the semiconductor regime and enabling strong interactions with visible light. The \(\pi\)-conjugated system forms the basis for the semiconductor nature of these materials, creating an energy band structure analogous to inorganic semiconductors, though with important distinctions. The HOMO and LUMO in organic semiconductors correspond to the valence and conduction bands in inorganic materials. However, the localized nature of electronic states in organic semiconductors leads to fundamentally different light-matter interactions. When organic semiconductors absorb photons, they primarily generate excited states known as excitons, rather than the direct formation of free charge carriers typical in inorganic semiconductors. These excitons are electron-hole pairs bound by Coulombic interactions, with binding energies typically ranging from 0.3 to 1.0 eV [14; 15]. This high binding energy, resulting from the low dielectric constant of organic materials and strong electron-hole correlation, means that room temperature thermal energy (\(\approx\)0.026 eV) is insufficient for spontaneous exciton dissociation into free carriers. Excitons in organic semiconductors can be classified into several types based on their character [16; 17; 18; 19]. Frenkel excitons, localized on individual molecules or polymer chain segments, are the most common. These excitons can migrate through the material via energy transfer mechanisms, primarily Forster energy transfer for singlet excitons and Dexter energy transfer for triplet excitons, as illustrated in Figure 1. The exciton diffusion length, typically 5-20 nm in conjugated polymers, is a crucial parameter that influences device design and performance. The generation of free charge carriers from excitons requires additional energy or suitable interfaces. In photovoltaic applications, this is commonly achieved through the use of donor-acceptor heterojunctions, where the energy level offset between materials provides the driving force for exciton dissociation. The process involves exciton diffusion to the interface, followed by electron transfer to the acceptor material, resulting in charge separation. Understanding and controlling these processes is crucial for developing efficient photonic devices. In the context of light emission, the process operates in reverse. Injected electrons and holes form excitons, which can then recombine radiatively, producing photons. The efficiency of this process depends on various factors, including spin statistics, competing non-radiative decay pathways, and the molecular packing that influences both electronic and optical properties. The optical properties and phenomena in organic semiconductors arise from their unique electronic structure and can be understood through their energy band diagram. The absorption and emission processes are fundamentally linked to electronic transitions between the HOMO and LUMO levels, modified by vibrational states that create character  
Figure 1: Schematic view of the Forster energy transfer for singlet excitons and Dexter energy transfer for triplet excitons.