Light Intensity Modulated Photo-Electrochemical Methods As a Tool in Investigation of Perovskites for Photovoltaics
Authors: Vanysek Petr , Novák Vitězslav 
Published: May 2020

Electrical Impedance Spectroscopy, abbreviated is EIS and often thus explained as electrochemical impedance spectroscopy, found its place in material studies as a routine experiment. ISE proliferation became possible through advances in electronics. Current-voltage characteristics were standard features of potentiostats. Impedance techniques require additional components embedded into a potentiostat, namely signal generator and signal analyzer. Relatively simple devices are available, although dedicated instrumentation with little noise, broad frequency control and perhaps sample temperature control or automated sample handling are particularly desired and command premium in cost.

Along with impedance over frequency data collection comes data interpretation, which is done with the assistance of software. Entirely automated data interpretation is also possible, though its utility in inexperienced hands is debatable. EIS as an electrical technique uses as the perturbing signal alternating electrical signal, either potential of current. In general, impedance studies are intended to study a transfer function and the input and output do not have to be only voltage-current or current-voltage as it would be in EIS. In mechanical engineering are for example useful studies focused on mechanical perturbation.

In the field of material studies on system responding to illumination is finding its way impedance study in which the perturbing signal is light of certain wavelength (or white light) with variable intensity.

Here we will look at the link between EIS and electrical response spectroscopy induced by fluctuating light. This is sometimes for short dubbed as optical spectroscopy although it does make a use of the of instruments for optical spectroscopy (such as UV-VIS, IR etc.) as they are known from analytical chemistry literature. Rather, an electric signal in a photoactive material is generated by intensity modulated light at different frequencies (similar range to electrical impedance spectroscopy) and a response (the magnitude and the phase shift) is analyzed. In this respect the magnitude and phase shift correspond to the magnitude and a vector of resistance on the polar plot of electrical impedance measurement. However, the data obtained serve a different purpose. For example, in a photovoltaic shorted system, the time constant associated with the response corresponds to mobility of the photogenerated charge in the material. For an open-circuit potential of such a photovoltaic system the measured time constant response corresponds to the rate of recombination of the generated hole-electron species.

As the time constants are important in development of a photovoltaic system for harvesting energy from the Sun, such studies are complementary to obtaining such practical data as the fill factor or maximum power point. We will demonstrate some of these results on perovskite-type materials designed by our collaborative partners.

Such materials and related constructions of particular solar cells could be possible replacements for the current silicon-based photovoltaic cells. Solar cells based on the general perovskite structure (ABX3) attracted in recent years much attention. These materials, due to the possible variability of both cations and anions in their composition, offer almost endless possibilities for synthetic preparation. For the active layer of a solar cell is often used the CH3NH3PbI3-2Cl2 perovskite structure, i.e., a structure containing an organic component. We have built and then tested such photovoltaic cells by static and dynamic methods using a unique photoelectric research instrument using the method CIMPS (Controlled Intensity Modulated Photocurrent Spectroscopy).