Kinetic Monte Carlo (KMC) simulations are a powerful tool to describe microscopic processes in disordered materials such as charge carrier transport and recombination. However, describing the macroscopic performance of complete devices such as organic solar cells has proven exteremely challenging. In our latest paper published in Solar RRL, we introduce a KMC model that is able to describe the most important characteristic of an organic solar cell, its current-voltage curve under illumination.

This was made possible by precise experimental calibration of the KMC model. We demonstrate for an exemplary organic blend material that there are three critical properties that need to be calibrated:

  1. The morphology of the active layer and in particular the presence of aggregated and amorphous regions
  2. The height of the injection barriers at the contacts
  3. The lifetime of the interfacial charge transfer states, which are an intermediate state in the non-geminate recombination of photogenerated charge carriers
KMC Calibration Morphology
Aggregates in the active layer reduce both geminate and non-geminate recombination losses

All these three properties were parameterised in the KMC model and calibrated by independent experiments, namely transmission electron microscopy, charge extraction by linearly increasing voltage (CELIV) and transient absorption. Using the calibrated model, clear evidence is provided that the conclusions drawn from microscopic and transient KMC modeling are indeed relevant for real operating organic solar cell devices.

Reference:
S. Wilken, T. Upreti, A. Melianas, S. Dahlström, G. Persson, E. Olsson, R. Österbacka, M. Kemerink, Experimentally Calibrated Kinetic Monte Carlo Model Reproduces Organic Solar Cell Current–Voltage Curve, Sol. RRL 4, 2000029 (2020)