What can wine tasting teach us about solar cell transparency?
If you are not specifically in the business of designing transparent materials, chances are you haven’t spent much time reflecting on the details of transparency. It seems simple. Transparent is see-through. A window is transparent — a wall is not. Water is transparent — milk is not. In our everyday lives, we rarely have reason to pay attention to any finer degrees of the in-between.
If you are into wine-tasting, however, you might have had reason to develop a more nuanced view of the subject. In the visual inspection of a wine, before you move on to scent and taste, there are typically three different aspects that are judged: clarity, colour, and intensity. Intensity is the aspect that directly relates to transparency — a wine of high intensity is nearly opaque, while a wine with low intensity has a transparent look.
This, however, does not mean that a wine of very low intensity resembles water — much like a high transparency value of a window pane doesn’t necessarily indicate that it gives you a great view of what is on the other side. Here is where the other two parameters come in: clarity and colour. Clarity is the opposite of cloudiness — if the wine is cloudy, you will only be able to see blurry shapes through it, while if clarity is high, you can make out the finer details. Colour is intuitive– a red wine of low intensity and high clarity allows you to see through it, but as through a red-coloured filter.
Just as when judging the visuals of a wine, classifying a transparent surface such as a window glass or a transparent solar cell is more complex than what can be represented by a single number. The transparency value that usually draws attention is the equivalent of what wine tasters call “intensity” — it is a measure of how much light that passes through, but it says nothing about other aspects implicit in the common understanding of the word “transparency”. If what we’re asking for is to what extent the surface will change our view of what is on the other side, we also need to account for colour and cloudiness (or “haze” as it is more often called when we speak about transparent surfaces).
In the case of transparent solar cells, the qualities of colour and haze becomes more important as we reach higher values of transparency. This is related to the somewhat counter-intuitive fact that even 50% transparency is generally perceived as “fully” transparent; normal windows, for example, are usually designed for a transparency around 50%. Increasing transparency of a technology much above 50% therefore has little value if we do not also address clarity and colour. A 60% transparent, but hazy and tinted surface, will still be perceived as less “see-through” than a clear and colourless, 50% transparent one.
So, how do different technologies hold up to these measures of transparency? Very few developers publish values beyond a transparency percentage, but it seems likely that achieving colourless solar cells would be a challenge for technologies that achieve transparency by absorbing light at the edges of the visible spectrum. The key to achieve a colourless surface is to intercept light evenly across the spectrum, so that no colour is “favoured” in the transmission: this could then only be achieved by staying completely out of the visible range, which severely limits the energy supply and makes indoor use infeasible.
Haze would be more of a problem for those technologies that achieve transparency by designing their cell as a mesh of thin lines. Such cells are see-through in the way a mosquito net is see-through, but they are unlikely to offer a clear view even if the transparency values are high.
The key to scoring high not just on one, but on all three measures, seems to be the absorption of small amounts of light evenly across the spectrum, while allowing the rest to pass through untouched and undisturbed. Our direct plasmonic solar cells are, as far as we can tell, the most promising avenue to reach there.