New materials could help boost the performance of perovskite solar cells (PSCs), say researchers at the Universities of Portsmouth, Bath, and Southampton. All three universities are in the United Kingdom.
The researchers wrote about their work in the journal Energy & Environmental Science (citation below). The authors were Nicola E. Courtier, James M. Cave, Jamie M. Foster, Alison B. Walker, and Giles Richardson.
A PSC is a type of solar cell that contains a perovskite-structured compound. The compound is most commonly a hybrid organic-inorganic lead or tin halide-based material. This material is the light-harvesting active layer. It is an emerging photovoltaic technology that has seen an impressive rise in power conversion efficiency exceeding 20%.
However, the perovskite material contains ion defects that can move about throughout the working day. The movement of these defects undermines the internal electric environment within the cell.
Function of perovskite material
In solar power, the perovskite material has two functions:
- It is responsible for absorbing light to create an electronic charge.
- It helps extract the charge into an external circuit before it is gone. We call this process ‘recombination.’
The terms solar energy and solar power refer to capturing the Sun’s energy (sunlight) and converting it into electricity.
Most detrimental recombination can occur in different parts of the inside of a solar cell. In some designs, it happens mainly within the perovskite. In other designs, it occurs at the edges of the perovskite where it comes into contact with the adjacent materials. We call these materials the transport layers.
Adjusting the transport layers’ properties
The researchers have now found a way to adjust the transport layers’ properties to encourage the perovskite’s ionic defects to move in such a way that they suppress recombination. This leads to significantly more efficient charge extraction. This subsequently increases the proportion of solar energy that strikes the surface of the cell that can ultimately be used.
Co-author, Dr. Jamie Foster, a Lecturer at the University of Portsmouth’s Department of Mathematics, 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.”
PSC performance depends on two things
The researchers showed that PSC performance depends strongly on the permittivity and effective doping density of the transport layers. Permittivity is the measure of any material’s ability to store an electric field.
Dr. Foster added:
“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.”
Transport layers with low permittivity and doping
The authors also suggest that if we use transport layers with low permittivity and doping to make PSCs, they are more stable. Specifically, more stable compared to those with high permittivity and doping.
This is because, within the perovskite layers, such cells show reduced ion vacancy accumulation. This has been associated with chemical degradation at the edges of the perovskite layer.
“How transport layer properties affect perovskite solar cell performance: insights from a coupled charge transport/ion migration model,” Nicola E. Courtier, James M. Cave, Jamie M. Foster, Alison B. Walker, and Giles Richardson. Energy & Environmental Science, published 21 December 2018. DOI: 10.1039/C8EE01576G.