Shining a Light on Solar - Some More Innovations

Alongside perovskite there have been a number of other solar cell developments.

Solar Tower

MIT researchers have experimented with different 3D solar cells, including developing solar towers. These 3D shapes can better harness electricity when the Sun is not directly overhead, as well as allowing for light to be reflected within the structure and so increasing the amount of light absorbed.

This has the advantage of generating electricity more uniformly throughout the day, the solar cells can be "flat-packed" for easier transport, and in higher latitudes (like the UK) they perform considerably better than traditional flat solar cells (see Figure 1). The increase in energy density (the amount of power generated divided by its area) was by a factor of 2-20, with an even greater increase in cloudy conditions. 

Fig. 1 Ratio of energy generated by an open cube compared to a flat panel for different seasons

Bi-triggered solar cells

These are flexible solar cells that, with an added layer of graphene, can generate electricity not just from light but also from rain. Light/water electricity conversion efficiency is still relatively low at 6.53%, and it requires further work to improve long-term stability. This is still a very new technology however, and so optimisation in the future is to be expected. Even if efficiency never reaches the levels of normal silicon solar cells, in places such as the UK, bi-triggered solar cells may well be more appropriate.

Organic Solar Cells

Similar to perovskite solar cells, organic solar cells offer a low-cost, thin and flexible solar cell solution. They present an opportunity for solar cells to be used almost like wallpaper, covering buildings and interiors. Organic solar cells have also experienced a rapid increase in efficiency; this has included structural changes to the solar cell design, improving both efficiency (to about 15% - this is the efficiency required for commercial viability) and stability without altering the actual material.

Thin film solar cell

What does it all mean?

The innovations discussed in this post and the perovskite one tackle some of the big criticisms that are often brought up for solar cells: the variable output, their unsuitability for higher latitudes, high costs and so on. As these advances are prepared for the market and rolled out into the world, renewable energy will begin replacing fossil fuels not only to reduce carbon emissions, but also because they are simply the better option. This article from Nature describes just how much support solar is receiving from Silicon Valley, the current global hub of blistering technological development. 

Ultimately, as the current energy mix is discussed and plans for a zero carbon future are made, I think it's right that we assume real and novel improvements from the renewable energy offering, as opposed to arguing that the world must adapt energy generation to the technology we have available to us today. There are almost certainly similar improvements across the renewables industry, including other ways of harness solar energy (like this).

Shining a Light on Solar - Perovskite

Most, if not all, of the innovations and new ideas I'll be looking at will likely fail to make a big impact on the global energy scene. It's also very easy to get swept up in all the glitzy hype that some of these products have. But all it takes are a few key successes to accelerate the solar and renewable markets up to the heady heights of fossil fuels.
Note: I' not suggesting all these new ideas will change the world overnight, it will take many decades still. As you read this, think of changes happening on the timescale of massive global transformation.

Perovskite solar cells use materials that share a particular crystal structure known as, you guessed it, the perovskite structure. Some perovskite materials act as semiconductors, and that is what makes them useful in solar cells. Current solar cells use silicon as the semiconductor - an expensive material of choice (and 2/3 of global supply comes from China). Perovskites on the other hand can be easily made in the lab. The advantages don't just extend to cost however. The perovskite solar cells are a thin film - lightweight and transparent - and perovskites can be easily tuned to different band gaps, to control the wavelength of light they absorb.

Olga Malinkiewicz, who founded Saul Technologies (the company from my earlier post), is looking to develop cheap, flexible cells that can charge mobile devices and cover roof tiles. If they're a success, they could easily see use in powering the IoT and all our gadgets. This would make it easier for consumers to introduce solar technology to their homes. The sooner consumers begin to use and see the benefits of solar, the sooner solar and renewable technologies in general will be normalised and adopted on larger scales - and I really think that's an important point.

But Olga isn't the only person working on perovskite. The materials are a source of interest for several companies and researchers who all see promise in their application; Oxford PV is one such example. They are creating tandem solar cells, with perovskite being combined with other materials like silicon in the short term. The long term goal is durable all-perovskite solar cells with complementary band gaps, keeping manufacturing costs and energy usage to a minimum. This could ultimately see perovskite solar cells being incorporated into buildings, with normal glass being replaced with solar cells, helping to create self-sufficient buildings and potentially buildings generating surpluses of energy to feed into the grid.

There are of course hurdles that must be overcome, some of which are detailed here. Perovskite solar cells are still not operating at the same efficiency as silicon solar cells, with a particular problem being size - perovskite solar cells are still typically very small and efficiency suffers when larger cells are produced. The crystals readily dissolve in water, questions still linger about their stability outside of lab conditions and some of the best perovskite solar cells so far have been made with lead, which is far from ideal.

What is encouraging however is the pace of development (see Figure 1). New records are being consistently achieved and hurdles overcome. In July 2016 perovskite solar cells over 1cm² reached a maximum efficiency of 20.5%. Several months later, a different team grew the largest perovskite solar cell crystal yet created at 16cm²  with an efficiency of 12.1%.

Fig. 1 Change in power efficiency conversion rates over time

Habisreutinger et al. (2014) have attempted to address the question of thermal stability with the introduction of carbon nanotubes. Further work by Kim et al. (2016) has improved the stability of perovskite solar cells in humid environments, where water ingress poses the risk of dissolving the perovskite (and potentially any lead in the cells). They've achieved this by replacing the existing hole transfer layer used in perovskite solar cells with a hydrophobic, additive-free (the previous additive sucked in moisture) material that still has high hole mobility.

Attempts are being made to remove lead from the solar cells. Two teams have found ways to replace the lead but the resultant efficiencies have been very low (5.7% and 6.4%). The hope is that lead-free perovskite cells will improve in efficiency at the same rate as leaded ones, which were about 6% efficient only three years ago.

Alongside the technical achievements, I think it's important to highlight the financial backing perovskite solar cells have received. As well as just research grants, the cells are actively being prepared for market. Oxford PV, with the aim of 'commercialising technology from Oxford University', has recently acquired a factory in order to begin developing market-ready manufacturing processes. Saule Technologies has also attracted considerable private investment. For perovskite solar cells to have attracted so much investment in such a short period of time is very promising for solar technology.


Revisit next week where I'll be looking further into the latest solar cell developments. I hope you all have a great Christmas!

Shining a Light on Solar - How do Solar Cells Work?

This post has a quick and easy to understand explanation of how solar cells work.
I've also written a slightly more technical explanation, which is still very accessible! The second explanation really offers some more context to understand the new developments in solar technology, and hopefully allows a greater appreciation for them.

Solar cells use two different types of silicon sandwiched next to each other. One is the more negative side, which has extra electrons in it. The other is the more positive side, which has holes for the electrons in it.
The two pieces are connected together with a wire.
When light hits the solar cell, electrons are released from the negative side and they can move through the wire to fill the holes in the positive side.
That wire now has electricity moving through it that we can use!
Solar cells can be made to work best with different wavelengths of light. Some are tuned for natural sunlight, others for artificial etc. The wavelength of light that the solar cells work best with is the band gap.

Now the more technical explanation...

Currently most solar cells use silicon, which has four outer electrons.
In Figure 1 you can see the structure of pure, N-type and P-type silicon.
In pure silicon, each atom is sharing its four electrons with the surrounding silicon atoms.
The silicon is then doped i.e. it has impurities added to it.
Silicon can have phosphorus atoms added, and these introduce an extra electron as phosphorous has five outer electrons - this is the negative side, N-type.
Or silicon can have boron added, and this creates a hole, as boron has only three outer electrons - this is the positive side, P-type.

Fig. 1 Pure and doped silicon

The P-type and N-type silicons are then sandwiched together. At this point, the free electrons in the N-type silicon jump over to the P-type silicon, resulting in negative ions. And the holes in the P-type silicon jump over to the N-type, resulting in positive ions.

This creates an area called the depleted region in the middle (see Figure 2 - the diagram helps a lot for this bit!). One side (the P-type silicon) of the depleted region is negative, as there is now an excess of negative ions. The other side (the N-type silicon) is positive, as there is an excess of positive ions. This creates an electric field.

Fig. 2 Solar cell showing the P-type and N-type layers, and the depleted region

If a photon of light hits the solar cell, an outer electron will be lost and a hole created - this is called an electron-hole pair. If this happens in the electric field, that electron is attracted to the positive side of the depleted region, and so ends up in the N-type silicon. The resultant hole moves to the negative side of the depleted region, and so ends up in the P-type silicon.

If you connect both sides with a wire, the electrons can now flow from the N-type silicon, through the wire and over to the P-type silicon. You now have electricity flowing through that wire!

Not every photon will result in an electron-hole pair being created. The photon must have sufficient energy - that energy is known as the 'band gap' (see Figure 3). Some solar cells can lose electrons very easily (low band gap), others require more energy (high band gap). But any excess energy that the photon has is lost as heat. So if the solar cell has a very low band gap, you can expect more heat to be created. If it has a high band gap, there won't be as many electron-hole pairs created, but less heat.

Fig. 3 Band gaps and efficiencies of different solar cells

These band gaps are important, and some of the developments in solar cells is trying to make use of more than just a single band gap.

Shining a Light on Solar - An Introduction

I thought I'd expand more on my previous two posts on perovskite solar cells, as well as bringing in some other recent developments in the solar world that I've found quite interesting. But before that, here's a quick introduction to solar energy.

Solar energy has some phenomenal potential. A United Nations assessment calculated the 'Annual Solar Energy Potential' to be a minimum of 1,575 exajoules (1 EJ = 10¹⁸ joules) and a maximum of 49,837 EJ. The IEA calculated the global annual energy generation from all sources to be approximately 570 EJ... that's a lot less than what solar could generate.

It's fair to say then that availability of solar energy isn't exactly the problem, if anything, it's quite encouraging. And yet, solar currently accounts for about 1% of total electricity generation (see Figure 1 for total energy comparison), even when compared to other renewable energy sources such as hydropower and wind.

Fig 1. 2014 fuel shares in world total primary energy supply

On the plus side, the growth in the solar sector has been incredible (see Figure 2), but it will need to grow a lot to reach the potential that many people believe it can. This ambition has seen developments in solar technology attracting increasing investment, and this, ultimately, is the driving force required to see solar energy proliferate.

Fig. 2 Annual growth rates of world renewables supply from 1990-2014

Hydropower currently constitutes ~85% of global renewable electricity and yet has serious limits on further development, and ethical and environmental issues. Solar power is thus likely to be key in increasing renewable energy's contribution to the global energy mix. In order to harvest this potential we require considerable advances in solar technology.

My next post will look at how solar cells work.

Perovskite - Saule Technologies

So I looked into perovskite solar cells some more and there seems to be some really exciting work going on. They present some interesting opportunities which link quite well to the discussion on the Internet of Things and changing energy demands. Solar could (and maybe must) have a big part to play in the energy mix of the future.

The video below is a promo for a company that's managed to attract venture capital alongside research funding.

Perovskite Solar Cells

Just came across this video on some research being done by Stanford and Oxford scientists.

They're looking at perovskite solar cells as an alternative to the current silicon ones - the type you see on top of people's homes. As is always the case with these things, the hype may be unwarranted, but it definitely sounds interesting.