Since the invention of the dye-sensitized solar cell, great endeavors have continuously been applied to improve the performance. New dyes with different fascinating properties have been synthesized and tested. However, adsorption of dyes on the inorganic semiconductor substrates can always tune their properties due to either strong or weak interactions. The energy level alignment in the dye molecule/inorganic substrate heterojunction is a significant factor affecting the transfer of photoexcited electrons. Phthalocyanines are well-known dyes which have been widely used in photo-electronic devices and are good candidates in the solar cell application.
In this work, two different phthalocyanines (Pc); FePc and TiOPc adsorbed on single crystal rutile TiO₂(110) have been used as the test systems in ultra high vacuum conditions, using synchrotron based photoelectron spectroscopy (PES), near edge X-ray absorption fine structure (NEXAFS) and scanning tunneling microscopy (STM).
When adsorbed directly on the pristine unreconstructed (1x1) TiO₂(110) surface, both molecules display a strongly modified electronic structure in the first monolayer, whereas the ones in the second layer displays the "unperturbed" molecular electronic structure. Pre-adsorption of different pyridines on the TiO₂ surface reduces the strong interface interaction and the first layer Pc exhibits molecular electronic structure.
The different Pc morphologies and their different bond states lead to different energy level alignments at the interface on the bare substrate and on the modified surface. Hence, through this work, we show a possible route to improve the performance of solar cells by simply changing the surface properties through coadsorption of other organic molecules.