Scientists at Imperial College London have conducted experiments to follow the direction in which electrons move in perovskite solar cells when they are generated with a short pulse of light. They found that the mobile charged defects are still present even in solar cells with very efficient contact materials, despite these cells showing no hysteresis. Hysteresis was only found when cells suffered the combined effects of both the defects and poor selectivity at the contacts.
perovskite films contain charged defects that tend to impair their performance. Slow movement of these defects is thought to be responsible for a process known as hysteresis, which leads to irregularities in the efficiency with which light is converted to electrical current. Light-generated electricity exits the solar cell in the form of electrons to be harnessed. This is done via ‘contacts’ that sandwich the light-absorbing film. Previously, scientists have managed to address hysteresis by using more ‘selective’ contact materials that ensure a one-way flow of electrons out of the solar cell.
In theory, changing these contact materials shouldn’t have an effect on the movement of the charged defects within the perovskite, so it has remained a mystery why this appeared to ‘fix’ the hysteresis problem.
The research team stated “Previously there was debate over whether the charged defects or the contact materials were responsible for hysteresis. A little bit like Agatha Christie’s Murder on the Orient Express, we’ve shown that they both ‘did it’". According to the team, improving the contacts is only part of the solution, and there is still a need to address the issue of charged defects moving inside the perovskite that may provide a chemical weak point which could lead to the eventual degradation of the perovskite film. This raises a potential concern over the solar cells’ long term stability.
The results of this study may help the research community to assess the extent of charged defect movement to help the future research needed to improve the stability and bring this technology to market.