Building a Flexible Plastic-Free Solar Cell Proof of Concept — a 2.5 Year Endeavour!
Over my Christmas break in December of 2020, I decided to take a few days to learn about nanotechnology. I remember initially being quite bored with the topic. This is because many of the applications I was coming across were biological (not exactly aligning with my interests in grade nine). However, as soon as I came across nanotech’s applications in solar energy, I was hooked.
Fast forward to mid-January, and I had begun to notice the problem of accessibility that solar cells face: although they are rapidly becoming more widespread, they are generally also only placed on roofs. I wanted to come up with an idea for a cell that could be placed on more surfaces utilizing nanomaterials, which is where a concept for a transparent, flexible solar panel arose. I began talking to industry experts about how I could make this happen, and by March had a general cell structure with layers I thought could work together.
Before I explain my idea, a brief explanation of how a solar cell works (specifically for a perovskite, which I will go over below) is as follows:
At the bottom is the transparent conducting oxide on glass acting as the conducting electrode. This is what allows the light to pass through the cell and to the active layer.
In the middle is the perovskite, which is a light-harvesting active layer in solar cells, shown to be both efficient and cheap. The active layer, when exposed to sunlight, will absorb photons to excite electrons from their ground state to a higher energy state. In other words, electron-hole pairs are produced, called excitons. These excitons form free carriers (free electrons and holes) to generate a current, due to an internal electric field that arises from differences in the energy levels of materials within the perovskite solar cell.
The electron transport material/layer (ETM or ETL) and hole transport material/layer (HTL) are designed to reduce what is called recombination. Recombination refers to the process where electrons and holes recombine and release the energy absorbed from photons as heat, rather than allowing them to contribute to the generation of an electric current. The ETL facilitates the efficient transport of electrons (negatively charged carriers) to the metal electrode, while the HTL helps transport holes (positively charged carriers) to the transparent conducting electrode. Essentially solar cells require that holes be collected at one contact and electrons at the other. This separation of charges allows for the flow of electric current and the generation of electrical power.
This image below was a 3D printed model of my idea (which slightly resembles a sandwich) in March 2021.
On the bottom and top were antireflective (AR) structures — the ones I wanted to use were called moth-eye structures, which drew on the idea of the nanostructured bumps found in actual moth eyes as they have limited reflectivity. Then the green layers were meant to represent graphene deposited on the white layers, willow glass, to act as the TCE. Willow glass is an ultra-thin flexible glass and graphene is a carbon allotrope being investigated to be used as transparent conducting electrode materials in solar cells. The gold layer was meant to represent graphene quantum dots, meant to further enhance the efficiency of the solar cell; the yellow layer was meant to act as the HTL and represented a material called Spiro-IA; and the brown layer in the middle was meant to represent a perovskite. Perovskites are light-harvesting active layers in solar cells, and have been shown to be both efficient and cheap. (If you notice, there is no ETL in my above idea; I do not think I had known this was necessary, or even existed, at the time.)
It’s interesting to look at now because of how far I have strayed from every layer that had been in my initial plan.
Ultimately, my above 3D printed model did not make complete sense. But really, this can only be figured out when trying to move from an idea to a prototype — because you firsthand have to work with these materials, rather than just research them. Very different things!
I continued to refine this idea over the following year. I got samples of willow glass shipped to my house (which I stored under my bed) and had conversations with graphene providers (I found out it would be quite difficult to use this). My research pivoted to silver nanowires, another promising transparent conducting electrode material. The graphene quantum dots sounded cool but I realized were quite unnecessary and added another layer of complexity. Spiro-IA was expensive and vulnerable to degradation. The perovskites, I stuck with and continued to research.
The summer of grade ten, I began working on my newer idea in Toronto Metropolitan University’s solar innovation lab run by Bryan Koivisto. I was also getting help from Judy Castillo. We didn’t work with the willow glass yet, instead wanting to first build out the layers on regular glass to make sure they worked.
Getting into a lab was my first time realizing how different my solar cell on paper vs in reality would be. I failed repeatedly at everything. A solid would not be soluble in the right liquid, or a substance would be light sensitive and tricky to work with, or a sample would break in the spin coater. The perovskites themselves required very specific conditions if they were not to degrade and had to be made in a glove box. The lab did not have this, so we bought a ‘blow-up glove box’ also called a ‘glove bag’ to make them. Although I had read about it in papers, witnessing firsthand how quickly the perovskites could degrade was alarming. They also contained lead thus it was important to handle them carefully.
The perovskites I made on FTO glass (traditional electrodes in solar energy) worked minimally, and the perovskites with the silver nanowire electrodes did not work at all.
The Following Year
My summer had ended on a bit of an underwhelming note. I had not realized how difficult everything would be, especially when individual layers had been made successfully in different papers I had read.
I worked in the lab over my Christmas break, for the first time experimenting with the willow glass. We had a problem: the willow glass could not be cut. It shattered every time we tried. We even attempted with a laser cutter, which did not work. It had become quite apparent that not even this would work.
The transparency and flexibility had both proven to be incredibly difficult to achieve, thus I talked to the professor and we came to the consensus that we would just focus on the flexibility. After all, there are more walls than windows. Also, due to the difficulties of perovskites that had become very apparent over the previous summer, we pivoted to an organic cell approach. Now, the structure looked a bit more like this:
The active layer is now a blend of what are called an electron-donor material and an electron-acceptor material. The electron donor material readily donates, or releases, electrons when it absorbs energy from photons and the electron acceptor accepts the electrons released by the electron donor material. These materials are mixed to create a network of donor and acceptor interfaces. When sunlight hits the active layer, it excites electrons in the donor material, creating excitons. The electron acceptor material then captures the excited electrons from the electron donor material, leaving the positively charged holes in the donor. In other words the electrons move to the acceptor material and holes remain in the donor material. The separated electrons are collected by the ETL and transported to the bottom electrode; the separated holes are collected by the HTL and transported to the top electrode. This charge separation and collection create an electric current which can be used to generate electrical power, similar to the process in a perovskite.
With this in mind, I still needed a new substrate to work with for my solar cells, because the willow glass was not working. Avoiding plastic was another goal. I began working with a paper material (which was very cheap) and successfully found a way to deposit nanowires onto it (after three or four other failed deposition attempts). This seemed promising.
I had a few weeks in the summer of 2023 to devote to my solar cell. Truthfully, this substrate was much easier to work with than I had expected. We found a way that we could deposit every layer, which was both quite inexpensive and effective. Now every single material being used was different to those in my sandwich idea, but most of the layers from the previous summer were still a part of this solar cell, except the ETL layer. We also had to optimize for low annealing temperatures (basically, the heat applied after a layer is deposited in order to make sure it adheres to the substrate).
We tested the solar cells on my final day before having to go back to Ottawa, and truly I thought there was a 1% chance they would work. They were my first solar cells on this new material that had never been experimented with before, and I had now had a lot of encounters with failure.
However, the cells picked up a signal, a significant milestone in my journey! They are so thin and light that they can be held by a flower.
Of course, this means more optimization, more research, etc — but presents exciting next steps!
Overall, this journey has taught me a lot about going from an idea to an actual prototype, and I am looking forward to seeing where the journey takes me next :)
Thank you so much for reading this! I’m a 17-year-old learning about where sustainability meets technology, and am the author of the “Chronicles of Illusions” duology. If you want to see more of my work, connect with me on LinkedIn, Twitter, or subscribe to my newsletter!