Photoelectrochemical cells. -
�� 2001 Macmillan Magazines Ltd insight review articles 338 NATURE | VOL 414 | 15 NOVEMBER 2001 | www.nature.com Echemicalthethehave ver since French scientist Edmond Becquerel1 discovered photoelectric effect, researchers and engineers been infatuated with the idea of converting light into electric power or fuels. Their common dream is to capture the energy that is freely available from sunlight and turn it into the valuable and strategically important asset that is electric power, or use it to generate fuels such as hydrogen. Photovoltaics takes advantage of the fact that photons falling on a semiconductor can create electron���hole pairs, and at a junction between two different materials, this effect can set up an electric potential difference across the interface. So far, the science of solar cells has been dominated by devices in which the junction is between inorganic solid-state materials, usually doped forms of crystalline or amorphous silicon, and profiting from the experience and material availability resulting from the semiconductor industry. Recently, we have seen more use of devices made from compound semiconductors ��� the III/V compounds for high-efficiency aerospace components and the copper���indium���sulphide/selenide materials for thin-film, low-cost terrestrial cells. But the dominance of the field by inorganic solid-state junction devices faces new challenges in the coming years. Increasingly, there is an awareness of the possible advantages of nanocrystalline and conducting polymer devices, for example, which are relatively cheap to fabricate (the expensive and energy-intensive high-temperature and high-vacuum processes needed for the traditional devices can be avoided), can be used on flexible substrates, and can be shaped or tinted to suit domestic devices or architectural or decorative applications. It is now even possible to depart completely from the classical solid-state junction device, by replacing the phase in contact with the semiconductor by an electrolyte (liquid, gel or organic solid), thereby forming a photoelectrochemical device. The development of these new types of solar cells is pro- moted by increasing public awareness that the Earth���s oil reserves could run out during this century. As the energy needs of the planet are likely to double within the next 50 years, the stage is set for a major energy shortage, unless renewable energy can cover the substantial deficit left by fossil fuels. Public concern has been heightened by the disastrous environmental pollution arising from all-too-frequent oil spills and the frightening climatic consequences of the green- house effect caused by fossil fuel combustion. Fortunately the supply of energy from the Sun to the Earth is gigantic: 321024 joules a year, or about 10,000 times more than the global population currently consumes. In other words, covering 0.1% of the Earth���s surface with solar cells with an efficiency of 10% would satisfy our present needs. But to tap into this huge energy reservoir remains an enormous challenge. Historical background Becquerel���s pioneering photoelectric experiments in 1839 were done with liquid not solid-state devices ��� a fact that is often ignored. His research, in which illumination of Photoelectrochemical cells Michael Gr��tzel Institute of Photonics and Interfaces, Swiss Federal Institute of Technology, CH-1015, Lausanne, Switzerland (e-mail firstname.lastname@example.org) Until now, photovoltaics ��� the conversion of sunlight to electrical power ��� has been dominated by solid- state junction devices, often made of silicon. But this dominance is now being challenged by the emergence of a new generation of photovoltaic cells, based, for example, on nanocrystalline materials and conducting polymer films. These offer the prospect of cheap fabrication together with other attractive features, such as flexibility. The phenomenal recent progress in fabricating and characterizing nanocrystalline materials has opened up whole new vistas of opportunity. Contrary to expectation, some of the new devices have strikingly high conversion efficiencies, which compete with those of conventional devices. Here I look into the historical background, and present status and development prospects for this new generation of photoelectrochemical cells. Semiconductor electrode Counter- electrode Semiconductor electrode Counter- electrode Valence band Valence band Ec Conduction band Ox Red h�� h+ e- e- e- Ev h+ H2 H2O O2 e - a b Conduction band e- - Ec h�� Ev Figure 1 Principle of operation of photoelectrochemical cells based on n-type semiconductors. a, Regenerative-type cell producing electric current from sunlight b, a cell that generates a chemical fuel, hydrogen, through the photo-cleavage of water.
�� 2001 Macmillan Magazines Ltd are unstable against photocorrosion. The width of the band gap is a measure of the chemical bond strength. Semiconductors stable under illumination, typically oxides of metals such as titanium or niobium, therefore have a wide band gap, an absorption edge towards the ultra- violet and consequently an insensitivity to the visible spectrum. The resolution of this dilemma came in the separation of the opti- cal absorption and charge-generating functions, using an electron transfer sensitizer absorbing in the visible to inject charge carriers across the semiconductor���electrolyte junction into a substrate with a wide band gap, and therefore stable. Figure 3 shows the operational principle of such a device. Nanocrystalline junctions and interpenetrating networks The need for dye-sensitized solar cells to absorb far more of the incident light was the driving force for the development of mesoscopic semiconductor materials16 ��� minutely structured materials with an enormous internal surface area ��� which have attracted great attention during recent years. Mesoporous oxide films are made up of arrays of tiny crystals measuring a few nanometres across. Oxides such as TiO2, ZnO, SnO2 and Nb2O5, or chalcogenides such as CdSe, are the preferred compounds. These are interconnected to allow electronic conduction to take place. Between the particles are mesoscopic pores filled with a semicon- ducting or a conducting medium, such as a p-type semiconductor, a polymer, a hole transmitter or an electrolyte. The net result is a junc- tion of extremely large contact area between two interpenetrating, individually continuous networks. Particularly intriguing is the ease with which charge carriers percolate across the mesoscopic particle network, making the huge internal surface area electronically addressable. Charge transport in such mesoporous insight review articles 340 NATURE | VOL 414 | 15 NOVEMBER 2001 | www.nature.com When a semiconductor is placed in contact with an electrolyte, electric current initially flows across the junction until electronic equilibrium is reached, where the Fermi energy of the electrons in the solid (EF) is equal to the redox potential of the electrolyte (Eredox), as shown in the figure. The transfer of electric charge produces a region on each side of the junction where the charge distribution differs from the bulk material, and this is known as the space-charge layer. On the electrolyte side, this corresponds to the familiar electrolytic double layer, that is, the compact (Helmholtz) layer followed by the diffuse (Gouy���Chapman) layer. On the semiconductor side of the junction the nature of the band bending depends on the position of the Fermi level in the solid. If the Fermi level of the electrode is equal to the flat band potential, there is no excess charge on either side of the junction and the bands are flat. If electrons accumulate at the semiconductor side one obtains an accumulation layer. If, however, they deplete from the solid into the solution, a depletion layer is formed, leaving behind a positive excess charge formed by immobile ionized donor states. Finally, electron depletion can go so far that their concentration at the interface falls below the intrinsic level. As a consequence, the semiconductor is p-type at the surface and n-type in the bulk, corresponding to an inversion layer. The illustration in the figure refers to n-type materials where electrons are the mobile charge carriers. For p-type semiconductors, analogous considerations apply. Positive holes are the mobile charge carriers and the immobile negatively charged states of the acceptor dopant form the excess space charge within the depletion layer. The flat band potential is a very useful quantity in photoelectrochemistry as it facilitates location of the energetic position of the valence and conduction band edge of a given semiconductor material. It is obtained by measuring the capacity of the semiconductor���electrolyte junction. The semiconductor is subjected to reverse bias ��� that is, a voltage is applied to increase the potential step across the junction ��� and the differential capacity is determined as a function of the applied potential, V. The space- charge capacity of the semiconductor (Csc) is in series with that of the Helmholtz layer (CH) present at the electrolyte side of the interface. In the depletion regime the condition CH���Csc applies, so the measured capacity is that of the space-charge layer. This depends on the applied bias voltage according to the Mott���Schottky equation: 1/(Csc)242 (DfscRT/F )/( o 1 N), where Dfsc4V1Vfb is the voltage drop in the space-charge layer, R is the gas constant, F the Faraday number, the dielectric constant of the semiconductor, o the permittivity of vacuum and 1N the ionized donor dopant concentration. A plot of the square of the reciprocal capacity against the applied voltage gives a straight line and this is extrapolated to 1/(Csc)240 to derive the flat band potential Vfb. Flat band potentials have been determined for a large number of materials49 and some representative examples are shown in Fig. 2. Apart from the type of semiconductor they depend on the nature and composition of the electrolyte. In aqueous solution the flat band potentials of most oxide semiconductors shifts by 0.059 V when the pH is changed by one unit. This is a consequence of the fact that protons are potential-determining ions for these solids. Box 1 The semiconductor���electrolyte interface Conduction band Conduction band E E Ef Ec Ev Ef Ec Ev Eredox Eredox Valence band Valence band Semiconductor Electrolyte + + + + + + + ��� ��� ��� ��� ��� ��� + Conduction band electrons Positive charge carriers Electrolyte anions + + + + + + + ��� ��� ��� ��� ��� ��� Conduction band Conduction band E E Ef Ec Ef Ec Ev Eredox Eredox Valence band Valence band + + + + + + + ��� ��� ��� ��� ��� ��� ��� ��� + + + + + + + ��� ��� ��� ��� ��� ��� a b c d Box 1 Figure Schematic showing the electronic energy levels at the interface between an n-type semiconductor and an electrolyte containing a redox couple. The four cases indicated are: a, flat band potential, where no space-charge layer exists in the semiconductor b, accumulation layer, where excess electrons have been injected into the solid producing a downward bending of the conduction and valence band towards the interface c, depletion layer, where electrons have moved from the semiconductor to the electrolyte, producing an upward bending of the bands and d, inversion layer where the electrons have been depleted below their intrinsic level, enhancing the upward band bending and rendering the semiconductor p-type at the surface.