Solar cell hysteresis refers to the phenomenon where the performance parameters of a solar cell, such as power output, current, or voltage, depend on the direction and speed of the voltage or current sweep used during measurement. Essentially, the current-voltage (J-V) curve of the device looks different when measured by sweeping the voltage from open circuit to short circuit compared to sweeping it from short circuit to open circuit.
Understanding Hysteresis in Solar Cells
Hysteresis is particularly prevalent and problematic in certain types of next-generation solar cells, such as Perovskite Solar Cells (PSCs). Its presence significantly impacts how we measure and interpret the device's efficiency and stability.
Why Hysteresis Matters
- Influence on PV Properties: Hysteresis makes it difficult to determine the true performance of a solar cell, as the measured efficiency can vary depending on the scan direction and speed. This inconsistency hinders accurate characterization and comparison between devices.
- Impact on Device Stability: As highlighted in the provided reference, the presence of hysteresis in PSCs importantly influences device stability. The underlying mechanisms causing hysteresis can also contribute to degradation over time.
Causes of Solar Cell Hysteresis
According to the reference, the hysteric behavior in PSCs generally arises due to several factors:
- Ferroelectric Polarization: Some materials within the solar cell structure can exhibit ferroelectric properties, meaning they have spontaneous electric polarization that can be reversed by an external electric field. This polarization can build up and affect the internal electric fields and charge transport in the device.
- Charge Carrier Trapping/Detrapping: Charge carriers (electrons and holes) can get temporarily stuck in defects or traps within the material layers. The rate at which they are trapped and later released (detrapped) can be slow, leading to a delay in the device response as the voltage is swept.
- Ion Migration in Perovskite Materials: Perovskite materials, which are central to PSCs, can contain mobile ions. Under an applied voltage, these ions can move and accumulate at interfaces or grain boundaries. This ion migration creates internal electric fields that oppose or enhance the external field, changing the device's behavior depending on the direction and duration of the voltage sweep.
These combined effects mean that the internal state of the device lags behind the applied voltage, resulting in the different J-V curves observed during forward and reverse sweeps.
Mitigating Hysteresis
Researchers are actively working on strategies to reduce or eliminate hysteresis in solar cells, particularly PSCs. These include:
- Material engineering to reduce defects and ion mobility.
- Interface passivation to minimize trapping at boundaries between layers.
- Developing new device architectures and contact materials.
Addressing hysteresis is crucial for achieving reliable performance measurements and improving the long-term stability of these promising solar cell technologies.
