Charge Transport without Recombination in Organic Solar Cells and Photodiodes

Decoupling charge generation and extraction is critical to understanding loss mechanisms in polymer:fullerene organic solar cells and photodiodes but has thus far proven to be a challenging task. Using steady-state and time-resolved light intensity dependent photocurrent (iPC) measurements in combination with transient photovoltage, we estimate the total charge inside a typical device during steady-state photoconduction, which is defined by the trapped, doping-induced, and mobile charge populations. Our results show that nongeminate recombination of any order can be avoided as long as this charge is much less than that capable of being stored on the electrodes-a criterion that is typically met in the linear iPC regime in donor:fullerene systems even with low, Unbalanced mobilities. Knowing the conditions under which nongeminate recombination is essentially absent is an important device and materials design consideration. Our work also demonstrates that the technique of iPC is not only useful to assess the charge extraction efficiency but can also be used to estimate the efficiency of free carrier generation in fully operational devices.