Trap states limited nanosecond response of organic solar cells

Measured transient current voltage characteristics of organic solar cells exhibit a tail in the decline characteristic which is proportional to t^-α. Common numerical drift-diffusion simulations neglecting dispersive charge carrier transport fail to describe the observed long tail of the current density decline up to micro seconds. Our approach is to account for the observed dispersive charge carrier transport by introducing an exponential distribution of trap states into our simulation. These trap states represent the tail of a bimodal distribution of energy states, which is commonly used to describe the distribution of transport sites of the highest occupied / lowest unoccupied molecular orbital (HOMO/LUMO) in organic materials. By doing so, we can reproduce the measured characteristics over four decades in time. Results are qualitatively and quantitatively in excellent accordance for different laser intensities, different applied biases and different device diameters at the same time.