The solar cells that are on the market today are efficient, but expensive. Currently there is ongoing research to create solar cells that have a lower cost, in order to bring them into wide use. These solar cells are not made of the conventionally used doped silicon. Their chemistry is fundamentally different. The primary difference lies in the type of charge that is generated when sunlight hits the cell. In conventional solar cells, the excited electron and its corresponding hole are immediately separated and can easily travel away from each other. In the new solar cells the charge that is generated is known as an exciton, an electron/hole pair that are bound by Coulomb forces and cannot travel far without recombining (thus losing energy).
All solar cells have the function of taking the energy inherent in sunlight and converting it to electricity. The electrons and holes excited by sunlight travel from where they are generated to electrodes, generating an electric current. Because the electron and hole in an exciton cannot travel far without recombining, it is imperative that they only need to travel a few nanometers before separation. Excitons separate at the junction between the two materials that carry the holes and the electrons to their respective electrodes. As surface contact between the two materials increases, the number of excitons that can separate also increases. Another important issue is the relative energy levels of the materials used. Since electrons go to lower energy and holes to higher energy, it is important that the energy level of the electron carrier is below that of the hole carrier. Overall, excitonic solar cells work best when energetically compatible electron and hole carriers are intertwined on a nanometer length scale with high surface area between the two. Solid state devices are also preferred for their increased stability over time.
The vast majority of excitonic solar cells now developed have high surface area as well some mixing of the charge carriers on a nanometer length scale. However, these materials are not highly ordered, which leads to electrons becoming trapped as they travel, as well as preventing fully interpenetrating networks. To improve upon these systems a new technique is used, called Evaporation Induced Self-Assembly. Using this method a highly ordered mesoporous titania framework is created, which acts as the electron carrier. To begin, a sol-gel is synthesized using a titania precursor, a surfactant polymer (the template), ethanol (solvent), and hydrochloric acid. Next, the sol-gel is dip-coated onto a substrate (conducting glass or quartz). During evaporation of the film, titania forms a framework around micelles of surfactant. Following heat treatments cross-link the titania and remove the templating polymer. The titania that remains has a highly ordered mesoporous (5-10 nanometer diameter) structure that is amorphous on the atomic scale. The cubic structure of the titania means that all the pores are interconnected via small !'necks!( and that it is possible to find many paths from the top of the film all the way through to the substrate. This is crucial to the proper infiltration of the hole carrier (a charge conducting polymer), enabling it to come in contact with its electrode and also with the titania.
To facilitate charge transport, it is important to have a high level of crystallinity within the titania. This is obtained via calcining, which involves heating the titania to temperatures on the order of 400-600oC. However, larger crystallites destroy the pores, eliminating mesostructural order. It is necessary to experiment with different calcining methods that will optimize this balance, with the goal of finding a method that will have small crystallites and high order in the films. Also, one new heating technique has the possibility of changing the mesoscale order in such a way as to increase the ability of the hole-conducting polymer to more fully infiltrate the entire depth of the film. One way of determining the ease of electron transfer in the titania is by use of conductance measurements, specifically the four point probe technique that eliminates intrinsic resistance.
Changing the surface of the titania by way of bonding with a surface ligand has two potential benefits. One is that the energy levels of the titania and polymer become more closely aligned, leading to more efficient electron transfer. The other benefit is to encourage the diffusion of the polymer into the pores of the titania. A surface ligand can change the hydrophilic surface of the titania to one that is hydrophobic. The polymer will then be encouraged to diffuse. In order to study the affects of different surface ligands, a gas adsorption technique such as BET can be used to prove that the ligand has indeed bonded. Fluorescence can be used to compare different ligands. When electron transfer is achieved, fluorescence in the polymer will be quenched. This means that the excited electrons, which normally relax by fluorescing, now transfer to the titania. Therefore, the less fluorescence the better the electron transfer.
The hole carrier chosen is a polymer that conducts charge. The long chains of polymers enable them to efficiently conduct a charge along their lengths, unlike fullerenes, which tend to trap charges in the bucky ball. There is also a wide range of polymers which absorb in the visible range where the solar spectrum is the strongest. Polymers are also in the solid state. UV/Vis Absorbance is done to determine the energy levels of the polymer, including studying how they change once incorporated into the titania.
Another important aspect to consider is which electrodes to use. Typically the substrate is coated with the electrode to which the holes travel, while the top of the film is coated with the electrode to which the electrons travel. One side must be transparent to allow sunlight to enter. There are several substrates, such as glass coated with indium zinc oxide or indium tin oxide, that are promising for use with charge conducting polymers and titania. Their affect on the self-assembly of the titania network, as well as their energy levels, must be studied to determine their efficacy. The energy levels of the electrode on top of the film must be studied to ensure that the electrons will transfer from the titania to the electrode, while not allowing electrons to transfer back to the polymer. Gold and silver are two possibilities.
Over the next year, a new kind of excitonic solar cell will be developed that may improve the current technology in terms of cost and efficiency. Research must focus on understanding, then optimizing, energy transfer within the cell. The titania, surface ligands, polymers, and electrodes must all be studied in depth to optimize the system.