WW Kolloquium: Prof. Dr. Carsten Deibel, From Ionic Defects in Halide Perovskites to Fill Factor Losses in Organic Solar Cells

Prof. Dr. Carsten Deibel
Institut für Physik, Technische Universität Chemnitz, Germany
From Ionic Defects in Halide Perovskites to Fill Factor Losses in Organic Solar Cells

Organic semiconductors and metal halide perovskite semiconductors offer different ways to complement the application areas of established photovoltaics technologies: organic solar cells that can be printed are interesting for building integration, perovskites with their tunable bandgaps are interesting in tandem solar cells with silicon or in their own right. Here I will focus on different fundamental aspects of these two material systems.
Metal halide perovskites have shown record power conversion efficiencies approaching silicon but have the special property to be mixed electronic–ionic conductors. The properties of charged point defects in semiconductors such as halide perovskites play a critical role in determining the efficiency and stability of solar cells. Using impedance spectroscopy and deep level transient spectroscopy, we observe ionic defects rather than the electronic defects created by them. Generally, the ionic defects fulfil the Meyer–Neldel rule, allowing a categorization of defects in halide perovskite compounds despite activation energies shifting for, e.g., even minute variations of the stoichiometry. By voltage-sweep rate dependent measurements, we can assign the current–voltage hysteresis to specific ionic defects.
In state-of-the-art organic solar cells, one of the major limiting factors of the performance is the impact of the low active layer conductivity on the transport resistance that, in turn, limits the fill factor. In literature, there are different approaches that describe the same effect in different ways. Central to them is that the superposition principle of the diode equation – the shifting down of the dark curve by the short circuit current to describe the illuminated curve – is not valid if the active layer conductivity is rather low. The resulting changes can be described by the figure of merit for transport resistance α, determined around open circuit conditions, or by a photoshunt that appears around short circuit conditions and increases with higher illumination intensity. A higher “photoshunt” corresponds to a lower active layer conductivity, which leads to a reduced fill factor. I want to discuss the different viewpoints and show that the photoshunt and the transport resistance are closely related. The transport resistance phenomenon is generally also relevant to perovskite solar cells, as low conductive transport layers will lead to fill factor losses as well.