(O’Regan and Grätzel, 1991). In these cells, a dye is incorporated in a porous inorganic matrix such as TiO2, and a liquid electrolyte is used for positive charge transport. This type of cell has a potential to be low-cost. However, the efficiencies at present are quite low, and the stability of the cell in sunlight is unacceptable. Research is needed to improve performance in both respects.
Another area of intense research is that on the integration of organic and inorganic materials at the nanometer scale into hybrid solar cells. The current advancement in conductive polymers and the use of such polymers in electronic devices and displays provides the impetus for optimism. The nano-sized particles or rods of the suitable inorganic materials are embedded in the conductive organic polymer matrix. Once again, the research is in the early phase and the current efficiencies are quite low. However, the production of solar cells based either solely on conductive polymers or hybrids with inorganic materials has much potential to provide low-cost solar cells. It is hoped that one would be able to cast thin-film solar cells of such materials at a high speed, resulting in low cost.
Research is being done to create aqueous photoelectrochemical cells for direct conversion of solar energy to hydrogen (Grätzel, 2001).15 In this method, light is converted to electrical and chemical energy. A solid inorganic oxide electrode is used to absorb photons and provide oxygen and electrons. The electrons flow through an external circuit to a metal electrode, and hydrogen is liberated at this electrode. The candidate inorganic oxides are SrTiO3, KTaO3, TiO2, SnO2, and Fe2O3. If successful, such a method holds the promise of directly providing low-cost hydrogen from solar energy.
It seems that a photoelectrochemical device in which all of the functions of photon absorption and water splitting are combined in the same equipment may have better potential for hydrogen production at reasonable costs. However, a quick “back of the envelope” analysis shows that in order to compete with the hydrogen produced from fossil fuels, photoelectrochemical devices should recover hydrogen at an energy equivalent of $0.4 to $0.5/Wp. This cost challenge is similar to that for electricity production from solar cells.
Large-scale use of solar energy for a hydrogen economy will require research and development efforts on multiple fronts:
In the short term, there is a need to reduce the cost of thin-film solar cells. To do this will require the development of deposition techniques of thin films such as microcrystalline silicon and other materials that are robust and provide high throughput rates without sacrificing film efficiencies. In the short run, thin-film deposition methods can potentially gain from a fresh look at the overall process from the laboratory scale to the manufacturing scale. The research in this area is expensive. For such research, some additional centers in academia with industrial alliances could be beneficial. It will be necessary to collect interdisciplinary teams from different science and engineering disciplines for such studies.
In the midterm to long term, organic polymer-based solar cells hold promise for mass production at low cost. They have an appeal for being cast as thin films at very high speeds using known polymer film casting techniques. Currently, the efficiency of such a system is quite low (in the neighborhood of 3 to 4 percent or lower), and stability in sunlight is poor. However, due to the tremendous development in conducting polymers and other electronics-related applications, it is anticipated that research in such an area has a high potential for success. Similarly, the search for a stable dye material and better electrolyte material in dyesensitized cells (Grätzel cells) has a potential to lead to lower-cost solar cells. There is a need to increase the stable efficiency of such cells.
In the long run, the success of directly splitting water molecules by using photons is quite attractive. Research in this area can be very fruitful.
The current DOE target for photoelectrochemical hydrogen production in 2015 is $5/kg H2 at the plant gate.16 Even if this target is met, solar-energy-to-hydrogen is unlikely to be competitive. Therefore, a much more aggressive cost target for hydrogen production by photoelectrochemical methods is needed.
Since photoelectrochemical hydrogen production is in an embryonic stage, a parallel effort to reduce the cost of electricity production from PV modules must be made. A substantial reduction in PV module cost (lower than $0.5/Wp) coupled with similar reductions in electrolyzer costs (about $125/kW at reasonable high efficiency of about 70 percent on a lower heating value basis) can provide hydrogen at reasonable cost. The potential research opportunities listed in the preceding subsection for PV solar cells along with electrolyzers must be actively explored.
All of the current methods and the projected technologies for producing hydrogen from solar energy are much more expensive (greater than a factor of three) when compared with hydrogen production from coal or natural gas plants. This is due partly to the lower annual utilization factor of about 20 percent (as compared with, say, wind of 30 to 40 percent). This creates enormous pressure to reduce the cost