Mapping the defects in back-contact perovskite solar cells
Back-contact perovskite solar cells could be set to improve further after researchers in Australia combined experiments and computer simulations to create a map revealing the strengths and limitations within devices.
Perovskite solar cells (PSCs) have emerged as an exciting next-generation renewable energy technology, thanks to their relatively low manufacturing cost, high defect tolerance and tuneable-optoelectronic properties.
Conventional PSCs employ a sandwich structure, with the perovskite absorbing layer placed between the extraction layers and the contact electrodes. The light has to pass through the substrates and the effective layers before reaching the perovskite layer.
Back-contact perovskite solar cells enjoy a higher theoretical efficiency, benefitting from the absence of parasitic absorption losses because the electrodes, as well as the transporting layers, are placed on the same side of the perovskite layer.
Moreover, this structure also allows for in-situ and in-operando characterisation of the perovskite layer. Over the years, device performance has increased from 6.54% for the first interdigitated (interlocked) back-contact PSCs to over 12%.
In back-contact PSCs, charge dynamics are impacted by the structure as well as the effectiveness of the transporting layers because they are arranged in an alternating array with a distance of a few micron meters, while the charge carrier diffusion length in perovskite is usually less than this.
To further improve the device performance and understand how the optoelectronic properties of perovskite changes over time and how they correlate to structure, it is beneficial to construct a map displaying such information.
Researchers from the ARC Centre of Excellence in Exciton Science, based at Monash University and Oxford University, took on the challenge by utilising a microscope with a camera and computer modeling to gain more understanding of how ideality factor changes spatially among different types of devices.
Contributing author Dr Boer Tan of Monash University said: "Ideality factor mapping can be used as routine analysis for us to identify defects in both device and electrode scale for photovoltaic devices.
"It is an intuitionistic method that allows us to gain a better understanding of the recombination processes in solar cells."
In this work, two different types of back-contact PSCs were studied, including one with compact titanium dioxide (TiO2) as the electron transporting layer. The other PSC had an additional mesoporous TiO2 layer.
The team found that the implementation of the mesoporous TiO2 layer greatly reduced recombination losses, allowing the devices to gain higher efficiency and photoluminescence intensity.
Accompanying computer simulations allowed the researchers to replicate and understand the results they had observed experimentally.
A key factor in determining the performance of the device is the uneven distribution of ions along the electron-hole transport layer interface.
Finding a way to prevent the ions from migrating will be crucial to advancing back-contact PSC technology.
The results of this work have been published in the journal Advanced Energy Materials and are available here.