J. Phys. Chem. B 2000, 104, 2053-2059
2053
Nature of Photovoltaic Action in Dye-Sensitized Solar Cells David Cahen* and Gary Hodes* Department of Materials and Interfaces, Weizmann Institute of Science, RehoVot 76100, Israel
Michael Gra1 tzel‡ LPI, Institut Chimie Physique, EPFL, CH-1015 Lausanne, Switzerland
Jean Franc¸ ois Guillemoles§ Laboratoire d’Electrochimie (UMR 7575), F 75231 Paris Cedex, France
Ilan Riess⊥ Department of Physics, Technion, Haifa 32000, Israel ReceiVed: September 9, 1999; In Final Form: December 1, 1999
We explain the cause for the photocurrent and photovoltage in nanocrystalline, mesoporous dye-sensitized solar cells, in terms of the separation, recombination, and transport of electronic charge as well as in terms of electron energetics. On the basis of available experimental data, we confirm that the basic cause for the photovoltage is the change in the electron concentration in the nanocrystalline electron conductor that results from photoinduced charge injection from the dye. The maximum photovoltage is given by the difference in electron energies between the redox level and the bottom of the electron conductor’s conduction band, rather than by any difference in electrical potential in the cell, in the dark. Charge separation occurs because of the energetic and entropic driving forces that exist at the dye/electron conductor interface, with charge transport aided by such driving forces at the electron conductor/contact interface. The mesoporosity and nanocrystallinity of the semiconductor are important not only because of the large amount of dye that can be adsorbed on the system’s very large surface, but also for two additional reasons: (1) it allows the semiconductor small particles to become almost totally depleted upon immersion in the electrolyte (allowing for large photovoltages), and (2) the proximity of the electrolyte to all particles makes screening of injected electrons, and thus their transport, possible.
The use of dye molecules as light collectors in a solar cell (dye-sensitized solar cell; DSSC or DSS cell) has been most successful when the dye is adsorbed on a porous network of interconnected nanometer-sized crystallites of a wide band gap semiconductor.1 The best-studied example is that of Ru-bipyridyl dyes bound via carboxylate bonds to anatase (TiO2) crystallites. A generalized energy diagram for the cell is shown in Figure 1. From left to right, the cell components are platinized conducting SnO2, iodide/triiodide redox electrolyte (which will fill the pores of the anatase network), dye molecules, anatase, and conducting SnO2. In the figure, the vertical axis is the electron energy. Also shown are the processes thought to be involved in a working DSSC, which include injection of the electron into the TiO2, as well as regeneration of the dye via the iodide mediator in the redox electrolyte. The DSSC has been compared to both a classical (single crystal or polycrystalline) photoelectrochemical (PE)2 and to a p/n junction solar cell.3 While indeed it fits the general definition of a photovoltaic device (see below), we will argue that actually it functions in a manner quite different from PE or p/n junction cells. * Corresponding authors. E-mail:
[email protected] and
[email protected]. ‡ E-mail:
[email protected]. § E-mail:
[email protected]. ⊥ E-mail:
[email protected].
Figure 1. Schematic description of dye-sensitized solar cell, showing the principal processes involved. The vertical axis is the one-electron energy.12 The dotted line is the redox potential and the Fermi level of the electrodes. The relative positions of the energy levels are roughly to scale (compare also Figures 4 and 5, and text). hν: photon absorption; (1) electron injection; (2) recombination; (3) electron transport and collection at SnO2:F; (4) I- oxidation; (5) I3- reduction; (6) ion transport; SnO2 stands for SnO2:F.
Before proceeding, we list a number of apparent or real differences between the well-studied p/n (and PE) and the DSS cells (Table 1). Roughly following Table 1, we will consider the following issues: (A) What is/are the cause(s) for charge separation? (B) Why is recombination so low, notwithstanding the high surface and junction areas? (C) What is/are the cause(s) for and the limits of the photovoltage? (D) How are the separated charges transported through the cell?
10.1021/jp993187t CCC: $19.00 © 2000 American Chemical Society Published on Web 02/15/2000
2054 J. Phys. Chem. B, Vol. 104, No. 9, 2000
Cahen et al.
TABLE 1: Comparison between Mesoporous, Nanocrystalline, Dye-Sensitized Solar Cells and p/n Junction or Photoelectrochemical Cells
a
p/n or photoelectrochemical cell
DSSC, dye-sensitized solar cell
charge separation in space charge fielda grain boundaries; high recombination probability at junction should be avoided or minimizeda photovoltage from neutralization of internal built-in fielda both minority and majority carriers are present in the semiconductor
no significant space charge region grain surface maximization necessary; up to 103 times increased contact area existence (and importance) of an electrical field not established carriers of only one type are present in the semiconductor
Excluding nanocrystalline photoelectrochemical cells (as described, for example, in refs 4 and 5).
Figure 2. Schematic description of the charge distribution near the TiO2 surface (after ref 10), showing the electric field from the surface into the electrolyte, due to the cation adsorption on the TiO2 surface. The protons on the TiO2 come from the carboxylate or phosphate groups of the dye. The horizontal arrows show the direction of the electric field due to the Helmholtz potential, (shown as ∆φ(H) in Figure 5) in the dark. Under illumination this potential will decrease, as the electrons injected in the TiO2 will neutralize some of the positive charge at the surface.
While we will refer mainly to the cell, based on nanocrystalline, mesoporous anatase on conducting SnO2:F, with a Ru dye adsorbed on the anatase, immersed in an I-/I3- electrolyte, as described for example in ref 1, the model and discussion hold generally for DSSCs. A. What Is/Are the Cause(s) for Charge Separation? In p/n junction and classical PE cells, electronic contact between the components that form the photovoltaically active junction, and equilibrium between the electronic charge carriers in them, leads to space charge formation. Photogenerated charges are separated by the electric field in the space charge layer. In the DSSC, the individual particle size, lp, is too small to support a space charge,6-8 because lp , LD (the Debye length of the material) and, upon contact with the electrolyte, all of the particle is depleted. Thus, charge separation upon dye excitation must occur for other reasons. As pointed out earlier9 and discussed in detail by Zaban et al.,10,11 there does exist an electrical field at the electrolyte/ semiconductor interface, although not one due to space charge in the semiconductor. Figure 2, based on ref 10, illustrates its origin, viz., cation and dye adsorption on the surface. The dyes normally used are acids and release protons upon binding. These protons, together with other cations in the electrolyte, become part of the oxide surface region (for example, by converting terminal oxide into hydroxide groups). A dipole is formed across the Helmholtz layer between the negatively charged (iodide and dye) species and the cations. The (Galvani) electrical potential drop across the Helmholtz layer, ∆φ(H) (which we estimate at ∼0.3 eV; cf. also ref 9), will help to separate the charges and reduce recombination. This is shown schematically in Figure 5 as a drop in the local vacuum level. Normally, the dye’s lowest unoccupied molecular orbital (LUMO) is above (i.e., less negative than, on the solid state scale12) the semiconductor conduction band (CB) edge,13 and the ELUMO - ECB(anatase) difference presents an enthalpic driving force for electron injection.13-15 Similarly, as the dye’s highest occupied molecular orbital (HOMO) is below the iodide/triiodide
Eredox, this presents a driving force for hole injection into the electrolyte. This separates the charges and is the major mechanism for charge separation in the DSSC. Another reason for charge separation is an entropic one. The larger density of states in the anatase than in the dye (normalized per unit area) leads to a maximal possible driving force for charge separation of ≈0.1 eV. B. Why Is Recombination So Low? Recombination, i.e., the kinetics of the system, determines both the current and the open circuit voltage that are generated and produced in a photovoltaic cell. In a DSSC, injection of only one type of carrier from the dye into the semiconductor is energetically possible (electrons, in the case of the most common DSSCs). Because of the resulting virtual absence of holes, recombination will take place mostly between electrons injected into the semiconductor and holes on the triiodide, the hole carrier. This process is much more probable than recombination with the oxidized dye1, except near open circuit conditions.16,17 Recombination involving triiodide can occur only at the semiconductor/electrolyte interface (exclusion of the SnO2:F /electrolyte interface is discussed below) and then only if the electron can get within tunneling distance of the triiodide. This distance (
