Dynamics of Interfacial Charge Transfer States and Carriers Separation in Dye-Sensitized Solar Cells: A Time-Resolved Terahertz Spectroscopy Study

Abstract

Electron injection from a photoexcited molecular sensitizer into a wide-bandgap semiconductor is the primary step toward charge separation in dye-sensitized solar cells (DSSCs). According to the current understanding of DSSCs functioning mechanism, charges are separated directly during this primary electron transfer process, yielding hot conduction band electrons in the semiconductor and positive holes localized on oxidized dye molecules at the surface. Comparing results of ultrafast transient absorption and time-resolved terahertz measurements, we show here that intermediate interfacial charge transfer states (CTSs) are rather formed upon ultrafast injection from photoexcited Ru(II)− bipyridyl dye-sensitizer molecules into mesoporous TiO 2 films. Formation and dissociation of these CTSs were found to strongly depend on their ionic environment and excess excitation energy. This finding establishes a new mechanism for charge separation in DSSCs. It also offers a rationale for the effect of electrolyte composition in liquid-based devices and of ion doping in solid-state solar cells under working conditions.

Charge migration and charge transfer in molecular systems

“…19). The latter is in line with the hypothesis of a charge-transfer exciton formed between the trapped electron and the cationic dye at the surface, which was inferred from THz studies by the Moser group; 259 and (c) the recovery kinetics of all traps exhibits a clear biexponential decay with time constants being longer for the injection case compared to the band gap excitation. This is due to the fact that the latter yields holes in the valence band that favour a faster electron-hole recombination.…”

Charge-transfer states and optical transitions at the pentacene-TiO2interface

“…Investigations on hybrid organic-inorganic systems [1][2][3][4][5][6][7][8] have opened up the possibility to combine the benefits of both constituents and devise new applications that outperform their either purely organic or inorganic counterparts. For example, the possibilities of tailoring the electronic band structure can be considerably extended compared to inorganic heterostructures, because the organic molecules can more flexibly adjust to the underlying lattice of the inorganic substrate [9,10].…”

“…The band structure of a hybrid system can be further tuned by modifying the properties of the organic material [1,10]. Such hybrid systems can also provide new types of semiconductor excitations, like the hybrid Frenkel-Wannier exciton [11] with a high oscillator strength combined with an enhancement of nonlinear optical response [2,12], or the hybrid charge-transfer exciton [5][6][7][8] where a bound electron-hole pair is formed with the electron and hole residing at different sides of the organic-inorganic interface of the system. These ideas have already been applied in photovoltaics [10], where dye-sensitized solar cells [3,[13][14][15][16][17] have been found to be promising in the search for efficient, low-cost, and environmentally friendly devices.…”

“…These ideas have already been applied in photovoltaics [10], where dye-sensitized solar cells [3,[13][14][15][16][17] have been found to be promising in the search for efficient, low-cost, and environmentally friendly devices. The operational principles of solar cells are fundamentally dependent on charge-transfer excitations at the interfaces between the two constituents [5][6][7][8]. Even though charge-transfer excitations have been intensively studied during the last decades [5][6][7][8][18][19][20][21][22], the charge-transfer nature of the excited states [5,8,23] is not well understood.…”

“…The operational principles of solar cells are fundamentally dependent on charge-transfer excitations at the interfaces between the two constituents [5][6][7][8]. Even though charge-transfer excitations have been intensively studied during the last decades [5][6][7][8][18][19][20][21][22], the charge-transfer nature of the excited states [5,8,23] is not well understood. In order to enhance the performance of photovoltaic devices, it is especially important to characterize the mechanisms involved in the formation [5,23] and dissociation [5,6] of such excitons [8].…”