Improvement in open circuit voltage of organic solar cells by inserting a thin phosphorescent iridium complex layer

The authors develop organic solar cells (OSCs) with a multicharge separation (MCS) interface by inserting $∼4nm$ thin layer of phosphorescent bis(2-(4-tertbutylphenyl) benzothiazolato-$N,C2,$⁠) iridium (acetylacetonate) $(t-bt)2Ir(acac)$ comparing with copper phthalocyanine (CuPc) between pentacene/$C60$ heterojunction. The result showed that open circuit voltage is remarkably enhanced without obviously changing external quantum efficiency $ηEQE$ and short-circuit current density. As a result, power conversion efficiency is improved from 1.53% for pentacene/$C60$ device to 1.83% with sandwich layer $(t-bt)2Ir(acac)$ and 1.98% with CuPc. Moreover, a suitable equivalent circuit model is proposed to intuitively reveal the inner photogeneration process of OSC with MCS interface.