Equilibrium or Non-Equilibrium - Implications for the Performance of Organic Solar Cells

With power conversion efficiencies approaching 20%, organic solar cells can no longer be considered the ugly duckling of photovoltaics. Successes notwithstanding, there is still a need for further improvement of organic solar cells, both regarding energy and current management in these devices. At present, there are different and mutually exclusive interpretation schemes for the associated losses of energy and charge, hampering the rational design of next generations of organic solar cells. One critical factor that affects voltage, current, and fill factor losses is whether or not photogenerated charges are effectively near or far away from thermodynamic equilibrium. While it is commonly agreed that both the vibronic and (disordered) energetic structure of organic semiconductors affect the solar cell characteristics, the degree to which deviations from near-equilibrium population of the associated energy level distributions matter for the photovoltaic performance is unclear: near-equilibrium as well as kinetic descriptions have provided seemingly convincing descriptions of a wide range of experiments. Here, the most important concepts in relation to experimental results are reviewed, open questions are addressed and implications for device performance and improvement are highlighted.