Strategies to improve cyanine dye multi layer organic solar cells

We report the status and strategies to improve the performance of cyanine dye/C-60 bilayer heterojunction solar cells. Cyanines are characterized by convenience of synthesis and tunable light absorption. They have high light extinction coefficients, which offer the possibility to go around the constraints of small exciton diffusion length, and ultrathin films can be applied to fabricate multi layer organic solar cells. We optimized the cell performance by oxidative doping (with O-2/water/light, or NOBF4) of the cyanine layer. Doping increased the film conductivity and induced a steep rise in the short-circuit current and the open-circuit voltage. The external quantum efficiency approached 80% at maximum, meaning that the full cyanine layer of 30 nm thickness was active in charge generation. We argue that cyanine aggregates may be responsible for efficient charge transport over larger distances to the heterointerface. We also describe simple processes to match the energy to the low-lying hole-transporting cyanine level by using a thin organic salt layer or doped polyaniline as anode buffer layer. Thereby, hole injection was facilitated and good-performing devices (power conversion efficiency eta = 3%) with a high fill factor (FF similar to 60%) were fabricated. Cyanine dyes are accompanied by mobile counter ions. These ions can diffuse into adjacent layers and build up important space charge that alters the energy levels at heterojunctions. The control of the ionic space charge allows to control the flow of electronic charge carriers and to implement new functionalities unique to organic opto-electronic devices. Copyright (C) 2010 John Wiley & Sons, Ltd.