Photochemical investigation of porphyrins for use in Dye Sensitized Solar Cells

Abstract

As Dye Sensitized Solar Cells (DSSCs) have become a more prominent focus of the research being published on alternative energy sources it is important to consider basic principles that show promise for improving the energy conversion efficiency of these cells. By choosing and building a dye system carefully it is possible to promote a process called triplet-triplet annihilation (TTA) which involves the upconversion of two relatively low energy triplet excited molecules to produce one highly excited singlet molecule and a ground state molecule (2 T1 Sn + S0 [n>1]). This process will give DSSCs the ability to take advantage of the photon-rich near infrared portion of the solar spectrum making them inherently more efficient than systems which cannot undergo TTA. Two dye systems which show promise as TTA capable light harvesters are zinc tetraphenylporphyrin (ZnTPP) and tin(IV)-dichloro,tetraphenylporphyrin (SnCl2TPP). The photophysical properties of ZnTPP are studied in its standard form as well as in its ligated form. Ligation of the central atom with nitrogen containing bidentate ligands (i.e. pyrazine, 4,4- bipyridine etc…) can have a profound effect on both the properties of the monomer as well as the aggregate behavior of solvated dyes and their interaction with each other. The properties of SnCl2TPP are also examined and in particular methods for the formation of stacked molecular columns as manipulation of interactions between the individual dyes can significantly impact their ability to undergo TTA. Spectral data indicate that ligation of the ZnTPP leads to the formation of a short lived charge transfer state which greatly decreases the lifetime of the S2 state while having little effect on the S1 decay kinetics. It also shows that there is little or no formation of six-coordinate ZnTPP (i.e. doubly ligated) which makes the formation of stacked supramolecular structures very difficult. Data concerning the electrochemical synthesis and chemical synthesis of a SnCl2TPP based stack were inconclusive but show some promise that may give direction to future work.