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62 interaction between tetrahydrofuran solvent and zinc oxideFigure 5.1.: Spin-coating process. A drop of solution is placed on thesubstrate, which is then rotated at high speed in order tospread the fluid. Rotation is continued until the desiredthickness of the film is achieved.Some residual solvent molecules can bind to ZnO andpersist even after the syntesis process at the organic/inorganicinterface. Such contaminations of the ZnO/organicinterface can possibly affect the binding between the components,the interface morphology and the stability [53];furthermore they can generate dipoles (in case of polar solvent)that eventually affect the charge separation process.Among the solvents commonly used in combination withZnO there are xylene, dichlorobenzene, chlorobenzene, tetrahydrofuranand chloroform. In particular, tetrahydrofuran (THF)is commonly used in the production of hybrid ZnO-basedsolar cells due to its low freezing point and the ability tosolvate both polar and nonpolar compounds [105]. EachTHF molecule consists of one oxygen and four carbon atoms(each saturated by two hydrogens as in Figure 5.2) and itexists in different isoenergetic planar and non planar configurations(e.g. twisted or envelope) [106].Figure 5.2.: Molecule of THF in the planar configuration.At room temperature THF is liquid, with molecules weaklyinteracting through Coulombic and dispersive forces.