As Yoshino and co-workers discussed, the most room for improving polymeric photovoltaics lies in improving xien and the fill factor. They suggest the use of selective p- and n-doped donor–acceptor networks in which absorption and primary charge separations occur in undoped parts, and charge collection is by p–i and i–n fields. (The intrachain junctions created by by p-doping donor regions are p–i junctions. Likewise, i–n junctions are created by n-doping the acceptor regions.) Spectral sensitization by an excitonic antenna molecular component M, which has hnu absorption close to the charge transfer gap, can improve the xien factor.
Thus, there is a need to have high light-collection efficiency, and to have order
* at the molecular level if photogeneration and charge transport are to be efficient, and
* at the meso level if short distances between the site of irradiation and charge collection are to be realized (e.g., interpenetrating networks).
These requirements pose some exciting challenges and present some fascinating opportunities.
High light-collection efficiency. It is not surprising that there are few reports of attempts to improve the light-collection efficiency of conjugated polymers, given that the fundamental processes in polymer photovoltaic cells are still not well understood. In this regard, particular consideration needs to be given to the possibility that the most appropriate structure for exciton formation may not be the same as that for hole or electron transport. In addition, light absorption affects the polymer structure. For example, it has been reported that illumination of p-doped polythiophenes leads to its reduction or further undoping (19).
Two approaches to improving the high light-collection efficiency of polymeric photovoltaic devices could, therefore, be considered.
A separate excitonic or dye layer can be used in the device. This approach effectively mimics that taken in liquid-junction photovoltaic cells (22), in which a light-harvesting dye is attached to a thin film of nanocrystalline TiO2. The efficiency of these titania cells is fundamentally affected by how well the dye is bound to the semiconductor. This will also be important for dye-conjugated polymer devices.
Yoshino and co-workers (21) have constructed a solid-state photocell with octaethylporphyrin, the light-harvesting or excitonic layer, sandwiched between a phenylene vinylene donor layer and fullerene acceptor layer. Although the photo-current intensity is somewhat enhanced over the cell without porphyrin, a significant improvement is the wider spectrum obtained using conducting electroactive polymers (CEPs). There appear to be two different mechanisms of photo-induced illumination: from the donor side or from the acceptor side. This suggests that both donor-porphyrin and acceptor-porphyrin regions are contributing to charge generation.
The light-harvesting dye could be covalently attached to either the donor or acceptor material in the device. A wide variety of light-harvesting functionalities could be attached to the conjugated polymer. Thus, attaching conjugated substituents could enhance the light absorption of the polymer. For example, nitrostyryl side chains have been introduced into polythiophenes and shown to improve photoconductivity (23). We have investigated photocurrent generation with similar styryl polythiophenes having a range of donor and acceptor groups. Some variation in the photocurrent generation was observed for these polymers over a range of poised potentials, although this may have resulted from charge transport effects rather than enhanced exciton formation.
Alternatively, known dyes such as porphyrins and bi pyridyl metal complexes have been attached to conjugated polymer precursors (24, 25). There are few reports at this time, however, of the preparation of the functionalized conjugated polymers and their use in photovoltaic devices.
Order at the molecular level. The synthetic approach used to produce the conducting polymer of interest determines the degree of order in the resulting material. A number of synthetic routes for polythiophenes have been shown to result in stereoregular polymers. The best known of these are the McCulloch and the Rieke methods using alkylated thiophenes (26).
The starting materials are also important. Using oligomers can have a dramatic effect on the polymerization potential required. In fact, it is necessary to use at least monosubstituted thiophenes if the overoxidation of the polymer is to be avoided during the polymerization process. It is known that polymers grown at more extreme potentials are more susceptible to introduction of defects, and that lowering the potential by starting even with bithiophene results in higher yield and greater regularity in the polymer structure (27).
Others have shown that the particular electrochemical method used for polymerization is also important. For example, using pulsed potential methods produces more crystalline polymer materials. Substituents attached to the thiophene monomer can lower the oxidation potential (28) and also, as is evidenced in the Reike and McCulloch methods (26), play an important role in inducing order. In addition to the alkylated monomers often used for this purpose, other researchers including Ochiai et al. (29) have attached chiral groups to the thiophene backbone.
The choice of dopant incorporated into the CEP during synthesis is another important aspect in inducing order (30). This is most vividly illustrated with the polyanilines. Certain dopants, including CSA and DBSA, induce a structure that is more amenable to secondary doping, changing the polymer conformation from a tight coil to an expanded coil with a concomitant increase in conductivity. MacDiarmid and co-workers (31) introduced the concept of secondary doping and demonstrated the effect using m-cresol. More recently (32), we have shown that more “friendly” compounds such as thymol or carvacrol can generate the same effects.
Certain dopants have even been used to induce chirality in the polyaniline backbone. For example, using one enantiomer of camphor sulfonate is believed to induce helicity with a predominance of one enantiomer in polyaniline (33). Chirality can also be induced in sulfonated water-soluble polyanilines using acid–base pair interactions (34).
Creating a highly efficient interpenetrating polymer network containing light-harvesting molecules ensures efficient exciton generation, and the high interfacial nature of the polymer provides a means for separating and transporting the change. The chemical and physical properties of all polymeric components therefore must be such that they are compatible (they do not phase-segregate) to enable formation of these efficient interpenetrating networks.
