Advanced electrical characterization of organic–inorganic hybrid perovskite solar cells by impedance spectroscopy

Şahiner, Mehmet Cem
The solar cells employing organic–inorganic hybrid perovskites combine high power conversion efficiency with low-cost processability. Owing to the transfer of expertise acquired in dye-sensitized solar cell research, photovoltaic perovskite research expanded in an extraordinary pace. Efficiency values escalated rapidly mostly through refinement of fabrication methods and optimization of deposition conditions. The understanding of device physics, however, lacked such momentum because the focus was mainly on the competitive race for efficiency. Informative characterization techniques often left unattended and used mostly to connect the observed efficiency increase to modifications in fabrication conditions and methods. In this thesis, perovskite solar cells were studied by electrical impedance spectroscopic techniques, driven by the intention of contributing to the thorough understanding of operation principles. In this context, the impedance response of planar p–i–n heterojunctions employing perovskite light harvesters was modelled. A clear link between both high and low-frequency features of impedance spectra and underlying recombination process was revealed. For nickel oxide-based devices, recombination was shown to be coupled to the geometrical capacitance of device in the high frequency, and to the carrier accumulation on the interface in the low-frequency regime. Passivation of sputtered nickel oxide surface by an organic interlayer material was demonstrated. For the devices employing passivating interlayers, the low-frequency feature was inferred to be coupled to the ionic motion in perovskite absorber. Telltale signs of surface passivation were detected from impedance measurements through both the increase of recombination resistance and the disappearance of surface carrier accumulation.