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4.3 electronic and optical properties of the system 57acting oligomer (column H in Figure 4.6), indicates that thepresence of the ZnPc layer, hinders the P3HT/ZnO chargerecombination. The tendency of the electrons to drop intothe ZnO conduction band minimum while the holes remainin the P3HT can be further confirmed by ROKS calculations(column E in Figure 4.6).The previous analysis can be summarized as follows:• The e-h pairs can be generated both in the ZnPc moleculesand in the P3HT oligomers thanks to their comparablyhigh absorption coefficients.• Due to the existence of strongly mixed ZnO/ZnPclevels, the electrons reach easily the ZnO conductionband. The ZnPc HOMO is lowered (Figure 4.6, columnC) but its potential energy difference with theP3HT HOMO is raised (Figure 4.6, column D), favouringthe injection of the hole into the P3HT.• The injection is supported also by the close “face-toface”proximity of the organic moieties.• The presence of the ZnPc interlayer causes higherpotential energy of an excited electron in the P3HTLUMO (Figure 4.6, colum G) with respect to the P3HT/ZnOsystem (Figure 4.6, colum D). This results again in abetter injection of electrons into the ZnO conductionband through the ZnPc layer.• On the other hand, the hole transfer from the P3HTlayer to the ZnPc layer is not likely to occur due tothe lowering of the P3HT HOMO (Figure 4.6, columE).4.3.2 Charge densities and recombinationAs widely discussed in chapter 1, one of the major limitationsto the efficiency of hybrid interfaces is the recombinationbetween the charges. In the ternary system herestudied, the ZnPc layer act as an electronic spacer thathinders the e-h recombination. This assertion is shown inFigure 4.7, where the electrons and holes charge density(calculated by using the ROKS method) of the P3HT/ZnOand P3HT/ZnPc/ZnO systems are reported in panels C, D