No matter how low the price of oil goes, evidence is growing that solar energy will keep giving fossil fuels a run for the money. That’s because solar cell efficiency keeps increasing, while the cost of materials keeps sinking, as illustrated by a new study just released by Stanford University. The research team was able to push up the efficiency of a low grade silicon solar cell by applying another relatively cheap material on top.
That would be the crystalline material perovskite, which is simple and inexpensive to synthesize. Just a couple of days ago we were remarking that a research team in South Korea has achieved a new perovskite solar cell efficiency that sets a new record, so let’s take a look and see what the Stanford team is up to.
Solar Cell Efficiency And The Perovskite Problem
For those of you new to the topic, we’ve been all over perovskite lately. Though materials in the perovskite class aren’t particularly durable (they dissolve in water, for one thing), researchers have been attracted by their potential in a number of clean tech applications, including EV batteries as well as solar cells.
One thing that attracts researchers is the ability of perovskite solar cells to absorb only the visible part of the light spectrum. That has its pluses and minuses, but the big plus is that a perovskite solar cell can guarantee you the biggest bang per photon of visible light (that’s in comparison to silicon solar cells, which harvest energy from infrared light as well as visible light).
We’re not ready to ditch graphene as the nanomaterial of the new millennium yet, but perovskite is catching up fast. According to Stanford, perovskite solar cells were introduced just a few years ago, in 2009. The first attempts ranged close to 4 percent efficiency, and since then it’s vaulted up to 20 percent.
The challenge is to translate that labwork into something that can withstand real world weather conditions.
Maker’s Dream: A Hand Made Perovskite Solar Cell
The research team, co-headed up by grad student Colin Bailie, decided to tackle the problem in a way that would help keep driving the cost of solar energy down. That’s why the new perovskite solar cell is based on a conventional silicon cell. Here’s the strategy as described by Bailie:
Our goal is to leverage the silicon factories that already exist around the world. With tandem solar cells, you don’t need a billion-dollar capital expenditure to build a new factory. Instead, you can start with a silicon module and add a layer of perovskite at relatively low cost.
A huge obstacle in the team’s way was figuring out how to enable some photons to pass through the perovskite layer, so they could get to the silicon base. The solution was to place a transparent electrode on top of the cell, apparently making this the first ever two-electrode perovskite solar cell.
That gave rise to a new problem, which was how to figure out a way to get apply the electrode onto the perovskite solar cell without damaging it.
The answer to that was to do it by hand, using a technique similar to a temporary tattoo. The team took a sheet of plastic embedded with silver nanowires, and “rubbed” it onto the perovskite cell with the help of a pressure tool.
The result: the new perovskite layer boosted the efficiency of a low grade solar cell from 11.4 percent to 17 percent. That’s pretty good, considering that the perovskite cell alone only had an efficiency of 12.7 percent.
So, now what? Well, there’s still that pesky little problem of degradation in water to solve. Perovskite doesn’t hold up really well when exposed directly to light, either. So although the team is looking forward to achieving a perovskite tandem solar cell efficiency of up to 30 percent within the next ten years or so, it could be a while before we see that popping up in your friendly neighborhood solar garden.
Note: If you’re wondering where the silicon is that the image above, the Stanford team also noodled around with solar cell efficiency using a cell made of copper indium diselenide (CIGS), so that’s what you’re seeing. The results for that are all right but not quite as impressive as the silicon version. Using the same 12.7 percent efficiency perovskite cell, they took a CIGS cell with a 17 percent efficiency and boosted the overall efficiency to 18.6 percent.
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