Enhanced performance and stability of inverted organic solar cells by using novel zinc-benzothiazole complexes as anode buffer layers

Performance of inverted bulk-heterojunction solar cells with widely used tris(8-quinolinolate) aluminium(III) (Alq(3)) is compared with a series of novel zinc-benzothiazole (Znb(2)) derivatives as buffer layer. The devices including a Znb(2) thin layer between the poly(3-hexylthiophene) (P3HT):C(61)-butyric acid methyl ester (PCBM) blend and the Au anode show a significant improvement in the power conversion efficiency (eta), which is enhanced by 30% compared to Alq(3) cells. Moreover, by combining both Alq(3) and Znb(2) in the device as a double buffer layer prior to the metal electrode deposition, the efficiency improves by 40%. The reasons for the enhanced performance of Znb(2) cells are attributed to the efficient charge transport and electron/exciton blocking properties. Furthermore it is expected that the deposition of Znb(2) modifies the Au work function to facilitate the hole transport and collection at the anode, and encapsulate the P3HT/PCBM blend during the electrode deposition. The presented photovoltaic cells also show high stability in ambient air conditions over a period of 245 days, which evidences the need of Znb(2) buffer layers for long-term device durability.