Perovskite solar cells are on the threshold of outcompeting existing thin-film solar cells, and seem to symbolize an ideal solar cell: extremely efficient and economical—if there was not the problem of a weak long-term stability, which continues to be a challenge. Associated with this are unusual phenomena taking place in perovskite materials and devices, where extremely slow microscopic processes can provide them with a sort of “memory effect”.
For example, measuring the efficiency of a perovskite solar cell can rely on things like how long the device is illuminated before measurement or how the voltage was transmitted to it. Some years ago, this effect, called current-voltage hysteresis, resulted in disputes on how to accurately establish the efficiency of perovskites. Another instance of these unclear processes is a (partial) recovery of a formerly degraded solar cell during day-night cycling.
These types of effects are a worry when measuring the solar cells’ performance as a function of frequency, which is a standard measurement for characterizing these devices in depth (impedance spectroscopy). They result in large signals at low frequencies (Hz to mHz) and enormous capacitance values for the (mF/cm2), including peculiar, “unphysical” negative values that are still a mystery to the research community.
Currently, chemical engineers from the lab of Anders Hagfeldt at EPFL have cracked the mystery. Guided by Wolfgang Tress, a scientist in Hagfeldt’s lab, they learned that the large perovskite capacitances are not traditional capacitances in the sense of charge storage, but just look like as capacitances due to the cells’ sluggish response time.
The scientists demonstrated this by measurements in the time domain and with various voltage scan rates. They discover that the origin of the obvious capacitance is a slow alteration of the current passing the contact of the solar cells, which is controlled by a slow accumulation of mobile ionic charge. A gradually increasing current acts like a negative capacitance in the impedance spectra.
The work offers insight into the interaction between the photovoltaic effect in these devices and the ionic conductivity of perovskite materials. Acquiring such comprehensive understanding adds to the attempt to customize, stable perovskite solar cells.