**2.2 Morphology**

Morphology describes the form of the material employed in the construction of a photovoltaic device. It can make reference to the form in which the molecules are present in the substrate or on the physical form of the substrates surface.

Polymer can be present as unordered glass-like phases with the polymer chains in random orientations or as crystalline regions where the chains are ordered in

#### **Figure 3.**

*Schematic electron transport in polymers with conductivity islands within the same chain (A), between chains (B) and between islands (C).*

**Figure 4.**

*Gradual transition in heterojunction cells (left) and hard interphase in heterojunctions (right). Larger contact reduces the electrical resistance.*

parallel. This order or the lack of it will also be related to electronic properties of the substrate as polyconjugation is related to co-planar stretched structures that are lead more easily to crystalline regions while disorder requires generally more flexible structures. These are obtained when the π-system is interrupted due to larger torsion angles and the lack of overlap. Conjugation on the other hand is crucial for electron mobility and transport. Thus crystalline ordered regions will exhibit lower electrical resistance than the glass like zones and are the preferred form for high overall yields.

Morphology reflects to some extend the chemical properties of the molecules. Rigid annulated aromatic ring systems are more likely to form rigid rod-like structures than other open chain backbones. On the other hand, it can be controlled to a certain extend by the conditions used in the deposition of the layers. When deposed by sublimation slow deposition rates at relatively high temperatures favor that the polymer reaches the thermally most stable perconjugated form while fast deposition at relatively low temperatures fixes the higher disorder.

In homojunction devices various substrates have to be deposed at the same time forming a determined gradient. This adds several new parameters and complications to the manufacturing process as diffusion rates, inter-material interactions with the possibility of phase separation, differences in vapor pressure and different requirements for the formation of crystalline phases have to be controlled at the same time over the whole substrate surface. For these reasons results obtained are very difficult to reproduce.

While in homojunction cells charges can pass from one "layer" to the next along the whole gradient that exists between both, in heterojunction cells charges have to pass through the layer-layer-interface. This interface acts as an electrical resistance. Therefore increasing the contact surface with rough surfaces will help to increase the electrical yield. On the other hand, if grooves are too large in comparison with the layer thickness, there exists the risk of hole-formation and direct contact formation of layers that should be separated by the intermedium layer (see **Figure 4**). Surface morphology also depends on substrate and on deposition conditions. A review on morphology of thin film transistors that can be applied in large extend to solar cells can be found in [16].
