| Dye-sensitized solar cells are potentially inexpensive alternatives to traditional semiconductor solar cells. In order to optimize dyes for solar cells we systematically investigate the electronic structure and radiation effects for a variety of porphyrins and phthalocyanines and cytochrome c as a biological model system. All feature a central transition metal atom (or H2) surrounded by nitrogen atoms. NEXAFS spectroscopy of the N is and C is edges reveals the unoccupied molecular orbitals and the orientation of the molecules in thin films. The transition metal 2p edges reflect the oxidation state of the central metal atom, its spin state, and the ligand field of the surrounding N atoms. The goal of this work is to determine how the energy position of the HOMO, LUMO, and nearby levels can be tuned by tailoring the molecules and their deposition.;Practical findings: (1) Vacuum deposition provides oriented films with a predictable oxidation state. Fe and Mn containing phthalocyanines oxidize easily from +2 to +3 in air, however chlorinated porphyrins reduce from +3 to +2 during vacuum deposition at elevated temperatures. (2) Proteins are susceptible to radiation damage and are thus poor candidates for solar cells. However the conjugated nitrogen cage of phthalocyanines and porphyrins are stable. (3) The Fermi level EF shifts relative to the HOMO-LUMO gap in phthalocyanine films due to radiation-induced gap states. This effect has little influence on the optical absorption, but it changes the lineup between the energy levels of the absorbing dye and the acceptor/donor electrodes that collect the charge carriers in a solar cell. (4) The LUMO of metallic octaethyl porphyrins is largely independent of the metal atom, however the HOMO is tunable. (5) NEXAFS spectroscopy with DFT together form a more accurate account of the energy levels in phthalocyanines and porphyrins. (6) Above LUMO, Cu and Zn phthalocyanine and porphyrin states primarily have N 2p character. They also have significant metallic 4s states concentrated in a resonance ∼ 5 eV above LUMO. |
