Photovoltaic devices have become a promising alternative energy source in the last decades. They are expected to increasingly and significantly contribute to overall energy production over the coming years. The photovoltaic field, dominated mainly by inorganic solid-state junction cells, is now being challenged by the emergence of new devices based on nanocrystalline and conducting polymer films, which offer a very low-cost fabrication and attractive features such as transparency, flexibility, etc. that might facilitate the market entry. Among all of them, dye sensitized solar cells (DSSC) are devices that have shown to reach moderate efficiencies, thus being feasible competitors to conventional cells.
DSSC combine the optical absorption and charge-separation processes by the association of a sensitizer as light-absorbing material with a wide band-gap semiconductor (usually titanium dioxide). As early as the 1970s, it was found that titanium dioxide (TiO2) from photoelectrochemical cells could split water with a small bias voltage when exposed to light (Fujishima & Honda, 1972). However, due to the large band-gap for TiO2, which makes it transparent for visible light, the conversion efficiency was low when using the sun as illumination source. Dye sensitization of semiconductor electrodes dates to the 1960s (Gerischer & Tributsch, 1968). This pioneering research involved an absorption range extension of the system into the visible region, as well as the verification of the operating mechanism by injection of electrons from photoexcited dye molecules into the conduction band of the n-type semiconductor. Since only a monolayer of adsorbed dye molecules was photoactive, light absorption was low and limited when flat surfaces of the semiconductor electrode were employed. This inconvenience was solved by introducing polycrystalline TiO2 (anatase) films with a surface roughness factor of several hundreds (Desilvestro et al., 1985; Vlachopoulos et al., 1988). The amount of adsorbed dye was increased even further by using mesoporous electrodes, providing a huge active surface area thereby, and cells combining such electrodes and a redox electrolyte based on iodide/triiodide couple yielded 7% conversion efficiencies in 1991 (O’Regan & Gratzel, 1991). The current highest energy conversion efficiency is over 11% (Chiba et al., 2006), and further increase of the efficiency is possible by designing proper electrodes and sensitization dyes.
Figure 1 shows both a scheme and an energy level diagram of a liquid electrolyte dye sensitized solar cell. They usually consist of one electrode made of a layer of a few micrometers of titanium dioxide nanocrystals (average crystal size around 20 nm), that have been sintered together to allow electronic conduction to take place. A monolayer of a sensitizer dye, typically a ruthenium polypyridyl complex, is attached to the surface of the nanocrystalline electrode. This mesoporous film is deposited onto a conductive, transparent substrate, typically indium tin oxide (ITO) or fluorinated SnO2 (FTO), and soaked with a redox electrolyte, typically containing F/R – ion pairs. This electrolyte is also in contact with a colloidal platinum catalyst coated counter-electrode. Sunlight is harvested by the dye producing photo-excited electrons that are injected into the conduction band of the nanocrystalline semiconductor network, and then into the conducting substrate. At the same time the redox electrolyte reduces the oxidized dye and transports the electron acceptors species (I3-) to the counter-electrode, where the I3- is reduced back to I- and the electrical circuit is completed via electron migration through the external load.
Fig. 1. Cross section view of the design of a dye sensitized solar cell under illumination conditions (left), and energy levels of the different components of the cell that represent the energetics of operation of such devices (right).
In contrast to silicon devices, charge separation is primarily driven by the oxidation/reduction potentials of the different species at the TiO2/dye/electrolyte interface, being screened out any electric field gradient in the TiO2 electrode due to the high concentration of mobile ions employed in the liquid electrolyte (Zaban et al., 1997). Photoinduced charge separation takes place at the TiO2/electrolyte interface. Thus, electron injection requires the dye excited state to be more reducing than the TiO2 conduction band. In the same way, regeneration of the dye ground state by the redox couple requires the dye cation to be more oxidizing than the h/h – redox couple (Mori & Yanagida, 2006). The voltage output of the device is approximately given by the splitting between the TiO2 Fermi level and the chemical potential of the redox electrolyte, being the former related with the density of injected electrons and the density of charge traps in the band gap of TiO2. Under illumination conditions, the density of electrons injected into the semiconductor conduction band increases, raising the Fermi level towards the conduction-band edge and generating a photovoltage in the external circuit.
Charge transport processes within the cell are considered to be diffusive (Sodergren et al., 1994), (Cao et al., 1996), (Schwarzburg & Willig, 1999) and are driven by concentration gradients generated in the device, thus making electrons to go towards the working electrode and triiodide ions towards the counter electrode. During the diffusion process, photogenerated electrons can recombine with acceptors species, such us dye cations and triiodide ions. Another loss pathway includes decay of the dye excited state to ground (Huang et al., 1997), (Nelson et al., 2001). Kinetic competitions between the different forward and loss pathways are therefore critical to determine the quantum efficiencies of charge separation and collection, and so the conversion efficiency. A diagram showing the kinetics of a DSSC is presented in Figure 2. It should be noticed that not only energetics but also
kinetics must be taken into account, and they constitute the key issues to achieve high energy conversion devices.
The overall conversion efficiency (n) of the dye-sensitized solar cell is determined by the photocurrent density measured at short circuit (JSC), the open-circuit photovoltage (Voc), the fill factor (ff), and the power of the incident light (Pin). These values can be extracted from the photocurrent density-voltage characteristics (IV curves) under AM 1.5 full sunlight (Pin=100 mWcm-2). The relation between J and V is determined by varying the resistance of the outer circuit, being Jsc obtained when the resistance of the outer circuit is zero (thus voltage is zero) and Voc when the resistance is maximum (thus photocurrent is zero). The output power of the device equals the product of J and V, and the fill factor expresses the efficiency of the device compared to that of and ideal cell. Pmax is commonly reported as the output power of the commercial device and corresponds to the maximum value that can reach the output power. The performance of DSSC can be therefore estimated using the following equations:
A typical IV curve corresponding to a 7 pm thick dyed-TiO2 electrode measured under 1 sun illumination is displayed in Figure 3 (left). The solar radiation and the ruthenium dye absorptance spectra are shown in Figure 3 (right).
For a detailed description of DSSCs, we refer the reader to M. Graetzel (Graetzel, 2000) and M. Graetzel and J. Durrant (Graetzel & Durrant, 2008).
In the next section, we analyze two different approaches that contribute to the enhancement of DSSC efficiencies through the control of photon absorption into the cell. We put special emphasis to describe the integration of new materials known as porous one-dimensional photonic crystal due to their ease of integration and demonstrated promising performance.
Fig. 3. IV curve for a 7 gm thick TiO2 electrode, for which N719 as dye and a I-/I3- redox couple based liquid electrolyte have been employed (left), and mismatch between the dye absorption spectrum and that for the AM 1.5 solar spectrum (right).