28-08-2020 | | By Liam Critchey
Modern-day solar cells are made of a number of materials, from inorganic materials that show superior efficiencies, to organic materials that enable solar cells to become flexible and printable. Both types have their own merits and specific areas where they are used, and one of the most promising materials for solar cells are perovskite materials, although they have not yet reached their potential for a number of reasons. But bio-inspired flexible perovskite solar cells offer a new innovation in the solar power space.
Perovskites are inorganic materials with a chemical structure of A2+B4+X32, where A and B are positively charged cations and X is a negatively charged anion that binds them together. Perovskite solar cells are known to be highly efficient, with over 25% power conversion efficiencies (PCEs) already being achieved by some perovskite solar cells. However, despite their efficiency, traditional perovskite solar cells are very expensive to produce as they require very expensive electrodes (such as gold) to function effectively, and cheaper electrode materials tend to cause the device to be unstable.
While they are inorganic in nature and traditionally rigid materials, researchers have started to create perovskite thin films out of perovskites, and this has led to the development of flexible perovskite solar cells. However, like their bulk counterparts, there are still a few issues that need to be ironed out before flexible perovskite solar cells will become widely adopted.
While they are not quite as efficient as bulkier perovskite solar cells, the flexible perovskite solar cells fabricated to date are still very efficient compared to other flexible solar power cells —which typically lack in efficiency in lieu of other benefits—and they have reached around 20% PCE thus far. Perovskite thin films have excellent photoelectric properties, are lightweight, can create solar cells of a lower cost than their bulk counterparts, and can be used in roll to roll processes. There are also a number of deposition methods that can be used to create perovskite thin films, and they offer an alternative to the many organic-based thin film solar cells which are being developed. However, like many solar cells that are undergoing extensive development, they do suffer from issues which are holding back their commercial development, but these issues are completely different to the challenges faced with bulk perovskite solar cells.
There are a number of different challenges that researchers are trying to solve with bio-inspired flexible perovskite solar cells. The first is that some of the fabrication methods struggle to translate from the smaller lab scale devices to the large area modules that are designed to be the end-product. This is true for both the spin coating and printing methods used and it can lead to an increased number of defects and a less uniform crystal growth (of the perovskite thin films themselves) over large device areas. Moreover, indium tin oxide (ITO) is often used in flexible perovskite solar cells as the electrode, and ITO also becomes fragile over the large surface areas in these solar cells.
New ways are being devised to try and combat these issues. However, anti-solvent treatments (which promote the crystallisation of materials) are not suitable with large-scale printing methods and the formation of perovskite-based solutes when printing tends to produce uneven results on large processing areas. This has made it very hard to control a number of the processing parameters beyond the lab scale, including the flatness, crystal nucleation, and growth of the perovskite films.
Some methods have produced promising results, such as adding polymers or surfactants to the printing mix to improve surface adhesion across large surface areas, to utilising printing assisted processes (preheating the substrate, air auxiliary and cover deposition methods), to developing alternative materials for the ITO layer. A lot of progress has been made in tackling the different challenges, however, the solutions to date only focus on one issue each. So, while one issue may be solved, the others still remain, meaning that they are still not suitable for large scale fabrication and processing. One of the main sticking points across all the solutions is the ITO electrode and its inflexibility, so researchers are now taking inspiration from nature to create modified ITO electrodes that are more flexible and more suitable for flexible perovskite solar cells.
Researchers from China have taken inspiration from our own spines and have now created a bionic-orientated crystal that mimics the vertebrae in our bodies. The biomimicry approach utilises the ITO electrode and perovskite junction layers as the ‘vertebrae’ and the cartilage in between is represented by a polymer interface layer—which is a polymer mixture composed of poly(3,4-ethylenedioxythiophene):poly(ethylene-co-vinyl acetate), i.e. PEDOT:EVA. The polymer interface layer is the key bit of research and was developed by creating a PEDOT:EVA ink via a mini-emulsion method, followed by deposition using vacuum evaporation methods.
The reason for adopting this biomimetic structure is that the vertebrae in the body are rigid and robust, but the cartilage around the vertebrae enables the rigid sections to flex into different orientations—something which many ITO electrodes within flexible perovskite solar cells struggle to achieve. The polymer network utilised enabled this biological structure to be mimicked, as well as providing a way of controlling the crystallisation and growth of the perovskite films—without inducing a lot of defects. It’s a process that is reminiscent of some of the bio crystallisation processes we see in the body.
One of the defining features of the polymer interface layer is that it not only allows the uniform deposition of the perovskite layer, it also has an excellent cohesion to both the ITO and perovskite layers that enables it act as a hole transport layer between the two layers. However, the key mechanistic feature of the polymer layer is that it absorbs and releases the mechanical stress generated in the rigid materials when the device is flexed (hence why complete cohesion is key).
As well as tackling the challenges associated with using rigid materials in a flexible solar cell, it is also a highly efficient solar cell. The efficiency does vary between the device size, with PCEs varying between 17.55% and 19.87% at effective active areas of 1.01 cm2 and 31.20 cm2 (the lower PCEs correlating to the higher surface area and vice versa). The solar modules were also tested as a solar power source in wearable electronic devices, showcasing that this approach not only tackles some of the key issues with flexible perovskite solar cells, but it can also be directly used to create solar modules that are suitable for use in real-world applications.