*1.5.2 Charge separation*

*Nanostructures*

and PSCs have also been developed [14].

**1.5 Theoretical aspect**

*1.5.1 Light absorption*

presenting excellent market competitiveness and commercial prospect [6]. However, such DSSCs of a liquid-state electrolyte are with highly volatile solvents, which not only affect the long-term stability of the device but also limit its large-scale production. In 1998, Gratzel et al. unveiled a solid-state organic hole-transporting material, i.e., 2,2,7,7-tetrakis (N,N-di-p-methoxy-phenylamine)-9,9-spirobifluorene (spiro-OMeTAD), to replace the conventional liquid-state electrolyte for a solidstate DSSCs [7]. From 1998 to 2011, the power conversion efficiency of solid-state DSSCs increased steadily from 0.74% to 7.2% but still much lower than that obtained by liquid-state electrolyte-based DSSCs [8]. In 2009, Miyasaka and co-workers improved the power conversion efficiency of perovskite solar cell to 3.8% by replacing bromine (MAPbBr3) with iodine (MAPbI3) [9]. In 2011, Park et al. prepared MAPbI3 perovskite as quantum dots with a size of ca. 2–3 nm, resulting in an enhanced power conversion efficiency of 6.5% [10]. In the family of photovoltaic device, PSCs have demonstrated much stronger absorption than the standard N719 dye, but the limitation in perovskite-based solar cells was the rapid degradation of the device performance resulting in the dissolution of perovskites in liquidstate electrolytes. In 2012, Park's group works together with Gratzel's group and introduced perovskite-sensitized solar cell by using solid-state spiro-OMeTAD as a hole-transporting material by replacing liquid-state electrolytes [11], where the solid-state spiro-OMeTAD not only solved the problem of perovskite dissolution but also significantly improved the stability and power conversion efficiency (9.7%). Remarkable progress of perovskite solar cell has been made during 2013–2014 as the power conversion efficiency increased to a certified 16.2% and 20.1% [12, 13]. Sahil et al. prepared fully textured monolithic perovskite/silicon tandem solar cells with 25.2% certified power conversion efficiency. Besides the breakthrough in efficiency, novel designs of device architectures aiming for low-cost and highly stable DSSCs

After the invention of nanostructure-based photovoltaic solar cells, a lot of theoretical and experimental works has been carried out to explain its operation principle. The need for unique theoretical considerations of photovoltaic effect arises from the fundamental differences in the operation between DSSCs and PSCs

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

over traditional semiconductor *p-n and p-i-n* junction solar cells [15].

**66**

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 for driving efficiently.
