About the transparent electrode of the organic photovoltaic cells

¹ Moltech Anjou, UMR 6200, Université de Nantes, 2 rue de la Houssinière, BP 92208, 44000 Nantes, France
² Institut Jean Rouxel (IMN), UMR 6502, Université de Nantes, 2 rue de la Houssinière, BP 92208, 44000 Nantes, France
³ LAMP, EA 3825, Faculté des Sciences et des Techniques, Université de Nantes, 2 rue de la Houssinière, BP 92208, 44000 Nantes, France
⁴ Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India

^a e-mail: jean-christian.bernede@univ-nantes.fr

Received: 23 March 2011
Revised: 29 August 2011
Accepted: 12 September 2011
Published online: 23 November 2011

Abstract

Electrodes and the nature of their contact with organic materials play a crucial role in the realization of efficient optoelectronic components. Whether the injection (organic light-emitting diodes – OLEDs) or collection (organic photovoltaic cells – OPV cells) of carriers, contacts must be as efficient as possible. To do this, it is customary to refer to electrode surface treatment and/or using a buffer layer all things to optimize the contact. Efficiency of organic photovoltaic cells based on organic electron donor/organic electron acceptor junctions can be strongly improved when the transparent conductive anode is coated with a buffer layer (ABL). We show that an ultra-thin gold (0.5 nm) or a thin molybdenum oxide (3–5 nm) can be used as efficient ABL. However, the effects of these ABL depend on the highest occupied molecular orbital (HOMO) of different electron donors of the OPV cells. The results indicate that, in the case of metal ABL, a good matching between the work function of the anode and the highest occupied molecular orbital of the donor material is the major factor limiting the hole transfer efficiency. Indeed, gold is efficient as ABL only when the HOMO of the organic donor is close to its work function Ф_Au. MoO₃ has a wider field of application as ABL than gold. The role of the oxide is not so clearly understood than that of Au, different models proposed to interpret the experimental results are discussed.