How Ions Effect the Interface between Perovskite and Other Materials

A lot of research recently has been focused on the Perovskite solar cell; the main focus being on improving efficiencies for applications such as energy harvesting. However less work has been on the role of ions inside the Perovskite materials. It has been shown that ion migration inside the Perovskite is linked to a reduction in photocurrent seen as J-V (current-voltage) hysteresis, which negatively impacts the solar cells ability for energy production. The proposed research will instead look at applications where ions can be used positively. Initially understanding the mechanisms involved with ions inside Perovskite will need to be investigated as it is believed there is currently gaps in the field. To investigate how ions effect the Perovskite, there will be a departure from the popular J-V scans in favour for a new technique that will investigate directly how ionic position effects the photo-current.

This new technique will give a tool to measure the effect of ions at the interface of the Perovskite. It is hoped that this techniques will give insight to band bending, energy barriers and recombination of electronic charge. It is also thought that this technique will allow control over ionic position and hence will give control of the current output of the solar cell. The novel approach which will be employed in this project is being called a "True J-V" measurement. The idea with this new approach is to get an exact representation of the current response from the solar cell with ionic position dependence. The basic idea of this method is to apply a voltage onto the solar cell so that the ions move to a new position. The current will be measured constantly; the voltage will be applied until the current reaches a stable value, indicating that all of the ions have moved and stabilized at this new position. Once a stable current has been reached a short spike of ("Probe") voltage will be applied the cell and the current produced by the probe voltage will be measured. The voltage will then return to its stabilizing value until the current has stabilized again then a new probe voltage will be applied. This process will repeat for a given set of probe voltages then the stabilization voltage will be changed and the process will begin again. From the current measurements taken at the peak of each probe voltage a J-V curve can be reconstructed is representative of the ionic positions. Instead of viewing two lines from the forward and reverse scan measurements (in the standard J-V scan), multiple, different J-V curves will be seen, each one representing a different stabilization voltage i.e ionic position.

Using this understanding and technique of ionic effects on the interface of Perovskite additional objectives are set to further explore the system:

1. Observe temperature dependency off a solar cells electrical properties using cryogenic techniques to freeze ionic positions. Use these findings along with True J-V measurement to calculate the Richardson Constant for Perovskite materials and the barrier heights at the Perovskite interface.
2. Photvoltage-Photocurrent decay measurements using light pulses to control recombination of charge carriers based on ionic position.
3. Design and fabricate new device architecture such as a lateral contact cell that allows further control of current flow based on ionic positions.
4. Develop devices that behave as memristors and transistors based on ionic positions.
This research will aim to develop understanding of how to increase the efficiency and stability of Perovskite solar cells through the understanding of how ions inside the Perovskite effect important electrical properties of the system. The research also aims to propose alternative computing hardware through the use of Perovskites as transistors and logic gates by controlling ionic position and new device architectures.