*1.5.1 Light absorption*

In photovoltaic solar cell, light absorption and charge transport occur in the same material, whereas in the DSSCs, photons get absorbed by dye molecules and charge transport is carried out in photoanode and electrolyte. The high efficiency of DSSCs is accomplished by coating the internal surfaces of porous metal oxidebased photoanode with special dye (N3, N719, and black dye) molecules which are tuned to absorb incoming photons of all wavelengths. The absorption of a photon by dye molecule takes place *via* an excitation between the electronic states of molecule. Similarly, in PSCs, the absorption spectrum demonstrates good lightharvesting capabilities over the visible to near-IR spectrum which is also stable during prolonged light exposure. For sensitized mesoporous metal oxide devices, it is inferred that after light absorption in the perovskite, electrons are transferred to the metal oxide-based electron transfer layer followed by the conducting substrate (i.e., fluorine-tin-oxide/indium-tin-oxide), and holes are transferred to the spiro-OMeTAD and then to the silver/gold contact electrode for driving the applied load. The enhancement in light absorption near the band edge can be carefully

**67**

**Figure 1.**

*Plausible electron recombination processes in the DSSCs.*

*Nanostructures in Dye-Sensitized and Perovskite Solar Cells*

increase the current followed by power conversion efficiency.

engineered through various nanostructures for better photon management to

As the next step of the conversion of light absorption into electrical current, a complete charge separation must be achieved. The charge separation in photovoltaic solar cell is induced by the electric field across the junction, while no such longrange electric fields are found in the DSSCs and PSCs. The charge separation in the DSSCs is basically an electron transfer process from dye molecule to photoanode and hole transport process from oxidized dye to electrolyte. The electron transfer mechanism depends on an electronic structure of dye molecules and energy level matching between the excited state of dye and conduction band of photoanode, i.e., metal oxide. The lowest unoccupied molecular orbital (LUMO) should be above the conduction band edge of photoanode, and the highest occupied molecular orbital (HOMO) should be below the chemical potential of redox pair of electrolyte, i.e., iodide/triiodide, which is supposed to be an energetic driving force for electron and hole separation. In addition, entropic factors play an important role for energetic charge separation. The large density of delocalized states in the metal oxide nanoparticles compared with dye molecules on the surface facilitates electron injection in its conduction band, which eventually increases the driving force in entropy (approximately 0.1 eV) for charge separation. In PSCs, the charge separation and transportation occur between metal oxide as an electron transfer layer and spiro-OMeTAD as a hole transfer layer interfacial surface. On exposing to the Sun's radiation, photo-excited electrons are injected from perovskite absorber layer into the conduction band of the metal oxide electron transfer layer, and the hole is transported to spiro-OMeTAD hole transfer layer followed by the charge collector

**Figure 1** presents the possible ways of recombination: (1) electron injection from dye-excited state to the conduction band of photoanode, (2) regeneration of dye cation by electron transfer from the redox couple, (3) charge recombination to the cation of dye, (4) recombination to the redox couple, and (5) excited-state decay to the ground state. The photo-injected electrons in the photoanode can have

*DOI: http://dx.doi.org/10.5772/intechopen.83803*

*1.5.2 Charge separation*

for driving efficiently.

*1.5.3 Recombination*

engineered through various nanostructures for better photon management to increase the current followed by power conversion efficiency.
