**2. Organic semiconductors**

The toleration of the ability to develop a long-term technology that is economically active for large-scale power generation based on environmentally safe

**413**

**Figure 2.**

*The energy levels in organic semiconductors [11].*

*Mechanism for Flexible Solar Cells*

ductor materials [10].

*DOI: http://dx.doi.org/10.5772/intechopen.93818*

materials with unlimited availability is caused by the organic materials. The organic semiconductors are less expensive materials than the inorganic semiconductors like Si; they have high optical absorption coefficients which offer the opportunity for the production of very thin solar cells. Also, thin flexible devices can be fabricated using high throughput and low temperature approaches that employ well established printing techniques in a roll-to-roll process. The electronic structure of all organic semiconductors is based on conjugated π-alternation between single and double carbon-carbon bonds. Single bonds are known as σ-bonds and are associated with localized electrons, and double bonds contain a σ-bond and a π-bond. The π-electrons are much more mobile than the σ-electrons; they can jump from site to site between carbon atoms thanks to the mutual overlap of π orbital's along the conjugation path, which causes the wave functions to delocalize over the conjugated backbone. The π-bands are either empty (called the Lowest Unoccupied Molecular Orbital (LUMO)) or filled with electrons (called the Highest Occupied Molecular Orbital (HOMO)). The band gap of these materials ranges from 1 to 4 eV. This π-electron system has all the essential electronic features of organic materials: light absorption and emission, charge generation and transport [9]. Also, molecular orbitals which form σ and π bonds represent the energy levels for organic semicon-

The denoted bonding molecular orbitals (σ and π) form the highest molecular orbital (highest energy levels) where the denoted anti-bonding molecular orbitals (σ\* and π\*) form the lowest molecular orbitals (lowest energy levels). These

molecular orbitals are similar to energy bands levels in inorganic materials. **Figure 2** shows the method of creating energy gap levels in organic semiconductor [11]. The anti-bonding π\* molecular orbitals (conduction band) joined the π bonding molecular orbitals (valance band) to create the Lowest Unoccupied Molecular Orbital (LUMO) and Highest Occupied Molecular Orbital (HOMO). The gap between the (LUMO) and (HOMO) is the energy gap where the conductivity in organic semiconductor depends on. Thus, from **Figure 2** it is clear that σ bonds are extremely filled with electrons where π bonds are empty. On the other hand, if the energy band gap becomes as small as possible the tolerance of electrons to move from the (HOMO) to (LUMO) increases. Some organic semiconductors have a very small band gap of <2 eV, which mean that it is good materials compared to some

inorganic semiconductors, which haves a large energy band gap [10].
