The origin of slow electron recombination processes in dye-sensitized solar cells with alumina barrier coatings

Abstract

We investigate the effect of a thin alumina coating of nanocrystalline TiO 2 films on recombination dynamics of dye-sensitized solar cells. Both coated and uncoated cells were measured by a combination of techniques: transient absorption spectroscopy, electrochemical impedance spectroscopy, and open-circuit voltage decay. It is found that the alumina barrier reduces the recombination of photoinjected electrons to both dye cations and the oxidized redox couple. It is proposed that this observed retardation can be attributed primarily to two effects: almost complete passivation of surface trap states in TiO 2 that are able to inject electrons to acceptor species, and slowing down by a factor of 3-4 the rate of interfacial charge transfer from conduction-band INTRODUCTIONNew concepts for photovoltaic energy conversion are increasingly focusing at the intersection between organic and inorganic materials. Such devices are based upon nanometerscale interpenetrating networks of electron and hole transporting materials 1-3 rather than employing the long diffusion length, high-purity crystals, as used in conventional silicon solar cells. Dye-sensitized solar cells (DSSCs) are so far the most efficient devices based on this concept. DSSCs base their ability to convert photon energy to electricity on fast conversion of photogenerated excitons to energetic charge carriers spatially separated in distinct nanoscale domains or phases. The energy associated with photoexcitation of the sensitizer dye is employed to inject electrons into the conduction band of nanocrystalline TiO 2 and the holes into the redox electrolyte, generally the I − / I 3 − redox couple in an organic solvent. Under illumination, the Fermi level of electrons in the TiO 2 is raised 4 relative to the chemical potential of the redox couple in the electrolyte, resulting in a large thermodynamic driving force (ca. 0.8 eV) for electron transfer from the TiO 2 to the redox couple. Slow kinetics for this charge recombination process are therefore essential to allow the charge carriers enough time to be collected into their respective contacts. 5 Recently, the concept of a "kinetic barrier" for interfacial charge recombination processes in DSSCs has been further demonstrated by the use of ultrathin ͑ഛ1 nm͒ conformal alumina ͑Al 2 O 3 ͒ layers deposited in situ onto the surface on the nanocrystalline metal oxide films, leading to a significant increase in DSSC energy conversion efficiencies, as reported by several groups. [6][7][8][9][10][11][12] Palomares et al. 11 investigated the electron recombination process at the interface between TiO 2 and the oxidized dye, and have shown slow kinetics for the electron transfer process when the TiO 2 nanocrystalline particles were coated with Al 2 O 3 . However, direct evidence for slow recombination kinetics between the electrons in the TiO 2 and the oxidized species in the electrolyte ͑I 3 − ͒ were not addressed. Due to the strong interest in this approach, 12,13 we decided to investigate further the origin of the effic...

ZnO−Al2O3 and ZnO−TiO2 Core−Shell Nanowire Dye-Sensitized Solar Cells

“…29, 38 Authors of one of the latter papers established that alumina shells retard the recombination dynamics of nanocrystalline TiO 2 by passivating surface recombination centers and slowing the rate of interfacial charge transfer. 39 However, we note that Al 2 O 3 coatings have been shown to improve the efficiency of only relatively inefficient devices (<5%) and that no core-shell cell has yet outperformed the best TiO 2 cells made by Grätzel et al In any case, these unpromising results discouraged further study of the ZnOAl 2 O 3 core-shell nanowire cells.…”

Physical electrochemistry of nanostructured devices

“…was shown that alumina coating improves considerably the electron lifetime by an almost complete passivation of surface trap states. 23,38 Absorption of molecules with different dipole moment modifies the energy level of a nanostructured semiconductor immersed in solution, as shown in Fig. 3 and discussed later.…”

Improved performance of porphyrin-based dye sensitised solar cells by phosphinic acid surface treatment

“…Therefore, both values are limited by the recombination processes 2,8,9 that occur when the photoinjected electron recombines with the oxidised dye molecule (R 1 ) or reacts with the oxidised form of the redox couple in the electrolyte (R 2 ). Process R 2 has been well studied [10][11][12][13][14] and has been deemed to have a greater impact on device performance than R 1 .…”