Improvement of the efficiency of organic solar cells using the terthiophene-pyran-malononitrile (T3PM) as electron donor, through the use of a MoO3/CuI anode buffer layer

We focus on the contact between the anode and the electron donor (ED) in organic photovoltaic cells with planar heterojunction configuration. While the fullerene was the electron acceptor (EA), the terthiophene-pyran-malonitrile (T3PM) has been used as electron donor (ED). The HOMO and LUMO of T3PM have been estimated by electrochemical characterization to be -3.5 eV and -5.5 eV, which shows that it can be used as electron donor in a heterojunction with fullerene. The ITO contact plays a critical role in determining device efficiency, and the introduction of a thin anode buffer layer (ABL) between the ITO and the ED induces a significant improvement of the device efficiency. Different ABL have been used, the well-known Au and MoO3 and a more original, CuI, alone or coupled with MoO3. The thicknesses were 0.5 nm, 3.5 nm and 3 nm for Au, MoO3 and CuI respectively. It is shown that the ABL improve the cells performances. The best results are achieved with the couple MoO3/CuI through an increase of the open circuit voltage (Voc). Such improvement has been attributed to an increase of the shunt resistance. The influence of the ABLs can be explained partly by the fact that it raises the anode work function. Nevertheless, a study of the structures T3PM/ABL/ITO shows that the T3PM films grown on CuI are the most homogeneous which can explain the rise of the shunt resistance and therefore of Voc. (C) 2013 The Authors. Published by Elsevier Ltd. Open access under CC BY-NC-ND license. Selection and peer-review under responsibility of the European Material Research Society (E-MRS)