Synthesis and characterization of tris-(5-amino-8-hydroxyquinoline) aluminum complexes and their use as anode buffer layers in inverted organic solar cells

Tris-(8-hydroxyquinoline)aluminum (Alq(3)) and its derivatives have been studied as light-emitting materials in organic light emitting diodes (OLEDs) and recently also as buffer layer materials in inverted organic solar cells based on the well-known bulk-heterojunction (BHJ) of poly(3-hexylthiophene) (P3HT) and C-61-butyric acid methyl ester (PCBM). Due to the positions of the highest occupied molecular orbital (HOMO) energy levels of P3HT and Alq(3), an extraction barrier for the photogenerated holes to escape the device is created. To reduce the height of the barrier, the position of the Al-complex HOMO level can be elevated by attaching different substituents on the 8-hydroxyquinoline ligand. In this study three new tris-(5-amino-8-hydroxyquinoline)aluminum complexes with electron donating amino substituents were synthesized, characterized and used as anode buffer layers in inverted organic solar cells. Results of the performed spectroscopic and electrochemical studies confirmed that 5-amino substitution of the hydroxyquinoline ligand is directly correlated with the position of HOMO levels in the complexes while lowest unoccupied molecular orbital (LUMO) levels remained unaffected. Although the complexes exhibit extremely low emission properties compared to the parent Alq(3), they performed nicely as charge transporting buffer layers between the photoactive layer and the gold anode in the organic solar cells.