Exciton diffusion length analysis of mixed donor materials in organic solar cells by doping with phosphorescent iridium complex

In this paper, the photovoltaic characteristics of ITO/(t-bt)(2)Ir(acac):CuPc (mixed ratio R by weight)/C-60/BCP/Ag organic solar cells (OSCs) are analyzed in detail. The intrinsic properties of a (t-bt )(2)Ir(acac) : CuPc doped layer on device performance are discussed based on theoretical analysis of the experimental OSCs. By studying the photoluminescence densities of pure (t-bt)(2)Ir(acac) film and mixed films with R=0.75, 0.9, the key exciton diffusion lengths L-D were calculated to be 28.3 +/- 5.0 nm, 31.7 +/- 5.0 nm, and 33.0 +/- 2.0 nm, respectively. A new method is then proposed to calculate LD for films with R < 0.75. The analysis gives an exciton diffusion length of L-D = 17.4 +/- 2.5 nm for a mixed film with R=0.25, an improvement of 74% in comparison to a pure CuPc layer. Moreover, the exciton diffusion efficiency eta(ED) of an OSC with R=0.25 is obviously improved with the assistance of an increased exciton diffusion length. Finally, to reveal the influence of the dopant (t-bt)(2)Ir(acac) on charge carrier collection H(V), device energy loss is analyzed and discussed. (C) 2010 American Institute of Physics. [doi:10.1063/1.3514545]