This summer I finally had a chance to delve into semiconductors. These are widely used in all electronic devices, but I wanted to focus on something that not only brings innovation but has a social purpose too. Luckily, semiconductors are used in solar cells — they are responsible for sunlight absorption (band gap energy), the travel of the carriers within a solar cell (charge carrier mobility), and its ability to conduct heat without degrading (thermal conductivity) and much more. We chose perovskites due to their excellent optoelectronic properties but sadly, their thermal conductivity is ultralow, so we focused on lead-based halide perovskites due to their stability.

As we move towards more sustainable transportation, one idea is to integrate solar cells onto car roofs and bodies. However, most vehicles don’t have flat surfaces like Tesla Cyber Truck (Figure 1), which looks like it was designed by just using a ruler. So, for the rest of the vehicles, solar cells must bend to fit the car’s contours. 

Figure 1 — Tesla Cyber Track

Traditional materials like silicon are not up to this challenge — they are quite brittle, hard to grow on a flexible substrate, and lose efficiency quickly. This is where emerging flexible materials, such as perovskites, come into play. But this also introduces a new challenge — we need to understand how mechanical bending affects perovskites’ optoelectronics hence the ability of perovskite solar cells to convert solar energy into electrical power, and see if there is any chance we can tweak chemical composition to counteract this. 

I joined Dr. Stephanie Adeyemo (a researcher in the Electrical Engineering department), to learn about characterising perovskites. Then collaborated with Dr. Miloš Dubajić from Chemical Engineering and Biotechnology — we performed hyperspectral imaging on CsPbI₃ samples, and managed to agree on some extra thin films. I formed this link between the groups, as ironically perovskites are not grown in the EE department. However, none of this would be possible without Dr. Claire Barlow’s support and encouragement not only with sending out emails to former members of the Stranks group (reading whose thesis I found great fun), but also getting me actually do the research, then helping me with the poster and research report — all when things went very pear-shaped for me. Like perovskites, I needed a lot of ion concentration (sanity) adjustment, to counteract the effect of bending with a tiny radius. Unlike perovskites, Dr. Claire Barlow helped me, so, the pear-shaped things didn’t cause any halide ion segregation and lattice distortion, and I actually finished the work. To help perovskites, we look into strain engineering and introducing s-GO to deal with tightening of grain boundaries, and thermal expansion coefficients to prevent delamination and further lattice mismatch.

More findings? These are summarised on the poster, and research report! Indeed, it is possible to modify perovskite’s properties by changing its chemical composition — internal strain counteracts the external bending curvature. To investigate how this applies to a vehicle surface, I mapped a car roof’s curvature, adjusted the halide ions, and brought the properties back to optimal levels. No existing simulations attached perovskites to car rooftops or even looked into the impact of bending curvature on their optoelectronic properties, so I created my own from the data we gathered.

These insights allow us to design solar cells that can be attached to any vehicle surface and maintain their efficiency and performance. Also, the results of this research pave the way for developing next-generation Electrical Vehicle solar cell surfaces, wearable devices such as wearable sensors, and flexible perovskite displays; and somewhat satisfy my curiosity. Although, since some perovskites might degrade very quickly even without strain, somebody needs to look at their end-of-life applications and utilisation, to ensure that we don’t create more waste.

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