Long-term stability remains a key challenge for perovskite solar cells, where very slow microscopic processes can create a memory effect.
For instance, measuring the efficiency of a perovskite solar cell can depend on things like how long the device is illuminated prior to measurement or how the voltage was applied. A few years ago, this effect, known as current-voltage hysteresis, led to disputes on how to accurately determine the efficiency of perovskites. Another example of these obscure processes is a (partial) recovery of a previously degraded solar cell during day-night cycling.
Such effects are a concern when measuring the solar cells' performance as a function of frequency, which is a typical measurement for characterizing these devices in more detail (impedance spectroscopy). They lead to large signals at low frequencies (Hz to mHz) and giant capacitance values for the (mF/cm2), including strange negative values.
The team at EPFL in Zurich and Sharif University of Technology in Iran have solved the mystery. Led by Wolfgang Tress, the team found that the large perovskite capacitances are not classical capacitances in the sense of charge storage, but just appear as capacitances because of the cells' slow response time.
The researchers show this by measurements in the time domain and with different voltage scan rates. They find that the origin of the apparent capacitance is a slow modification of the current passing the contact of the solar cells, which is regulated by a slow accumulation of mobile ionic charge. A slowly increasing current appears like a negative capacitance in the impedance spectra.
The work sheds light onto the interaction between the photovoltaic effect in these devices and the ionic conductivity of perovskite materials. Gaining such in-depth understanding contributes to the endeavor to tailored, stable perovskite solar cells.