New research could make way for the design of new materials to help enhance the performance of perovskite solar cells (PSCs).
Perovskite solar cells are an up and coming photovoltaic technology that has seen an extraordinary rise in power conversion efficiency to above 20%.
However, PSC performance is impacted as the perovskite material contains ion defects that can move around during the course of a working day. As these defects travel, they influence the internal electric environment within the cell.
The perovskite material is responsible for absorbing light to form electronic charge, and also for helping to extract the charge into an external circuit before it is lost to a process known as “recombination”.
The majority of damaging recombination can take place in various locations within the solar cell. In certain designs, it occurs largely inside the perovskite, while in others it occurs at the edges of the perovskite where it contacts the neighboring materials referred to as transport layers.
Scientists from the Universities of Portsmouth, Southampton and Bath have currently developed a way to alter the properties of the transport layers to stimulate the ionic defects within the perovskite to travel in such a way that they suppress recombination and result in a more efficient charge extraction—boosting the proportion of the light energy falling on the surface of the cell that can in due course be used.
Dr Jamie Foster from the University of Portsmouth, who was involved in the research, said: “Careful cell design can manipulate the ionic defects to move to regions where they enhance the extraction of electronic charge, thereby increasing the useful power that a cell can deliver.”
The research, reported in Energy and Environmental Science, revealed that the performance of PSCs is intensely dependent on the permittivity (the measure of a material’s ability to store an electric field) and the effective doping density of the transport layers.
Understanding how and which transport layer properties affect cell performance is vital for informing the design of cell architectures in order to obtain the most power while minimizing degradation. We found that ion movement plays a significant role in the steady-state device performance, through the resulting accumulation of ionic charge and band bending in narrow layers adjacent to the interfaces between the perovskite and the transport layers. The distribution of the electric potential is key in determining the transient and steady-state behavior of a cell. Further to this, we suggest that the doping density and/or permittivities of each transport layer may be tuned to reduce losses due to interfacial recombination. Once this and the rate limiting charge carrier has been identified, our work provides a systematic tool to tune transport layer properties to enhance performance.
Dr Jamie Foster, Researcher, University of Portsmouth.
The scientists also propose that PSCs produced using transport layers with low permittivity and doping are more stable, than those with high permittivity and doping. This is because such cells display reduced ion vacancy accumulation inside the perovskite layers, which has been connected to chemical degradation at the edges of the perovskite layer.