Who would win in a contest between a plant’s leaf and an artificial solar cell? My main task over the past week was to investigate how well RoboPlant’s mechanical solar cells (photovoltaics) will represent the photosynthetic machinery which plants use for the conversion of solar energy. In this article, we’ll be getting to grips with how photovoltaics work, and how plant cells intercept light energy so it can be converted into chemical form. This was no easy task – as a biologist I am far from home when I find myself wandering the realms of electrons, semiconductors, and even a cheeky bit of quantum physics. Saying that, even for a biologist, photosynthetic energy conversion is no walk in the park either. This article is not for the faint of heart, but if you do feel brave enough to venture into the twisted nether-realm at the boundary of physics and biology, then please, do read on…
Let’s begin by assessing the similarities between photovoltaic cells (PVCs) and plant photosynthesis. Both plants and PVCs utilise solar energy, which is delivered in the form of energy ‘packets’ called photons. Some photons are powerful enough to dislodge electrons – tiny subatomic particles which carry an electric charge – from their parent atoms, and it is these electrons which allow for the generation of electricity. A photoelectric material releases electrons in this way when exposed to light. PVCs typically use ‘semiconductor’ materials made of silicone and other atoms, while plants use a green pigment called chlorophyll. Both systems relying on photons is pretty much where the path splits, as these two photoelectric materials work in entirely different ways.
A photovoltaic cell typically consists of two layers made of semiconductor material. The top layer consists of an arrangement of silicone and phosphorus atoms, which contains a high concentration of free electrons (which aren’t ‘tied up’ in bonds between atoms). This causes the first layer to have a negative electric charge. The second, lower layer is (literally) the polar opposite, consisting of an arrangement of silicone and boron atoms. In this arrangement, there are very few free electrons but there are many ‘holes’ where electrons should be, causing the second layer to have a positive electric charge. The force of these polar opposites placed next to each other generates an electric field. When a free electron is hit by a photon of light with enough energy, it is dislodged from the lattice and drawn towards the positive layer. However, the electric field acts as a barrier, preventing free electrons from crossing over. You know that feeling when a pretty girl or handsome guy has caught your eye, but you’ve no idea how to break the ice? The electrons feel your pain. But if we connect an external circuit between the two layers using electrodes, the electrons can bypass the electric field barrier. Once on the other side, electrons can fill the holes in the second layer, but are dislodged again by more photons. The positive electrode then draws the electrons back across the barrier to the first layer, so they can go round the circuit again. The flow of electrons though the circuit gives us an electric current, which can be used to power an electronic device. The strength of the electric field determines the voltage. This is how the initial energy from the photons is converted into electricity, allowing a virtually infinite source of energy to be harnessed. Many photovoltaic cells are packed into the solar panels that you may notice out and about.
So that’s how solar cells do it, but how do the efforts of a living plant compare?
Solar energy conversion in plants is the first stage of photosynthesis. This is a highly complex process which takes place inside sub-cell structures called chloroplasts, within the microscopic cells that constitute a leaf. These contain the photoelectric pigment chlorophyll, which absorbs photons of light in the red and blue spectra, but reflects photons in the green spectrum (and now you know why plants are so green!). Like the semiconductors in PVCs, chlorophyll contains free electrons which are ‘knocked out’ when hit by photons. A free electron is then transported along a chain of different electron acceptor molecules, via another chlorophyll molecule, and eventually to an energy storage molecule, which is involved in turning carbon dioxide into stable sugars. Unlike in PVCs, the electrons do not go round in a continuous circuit, and therefore need to be replaced.
This is where photolysis (splitting of water using light) comes in. This provides a source of electrons and protons (larger positively charged subatomic particles) to drive photosynthetic mechanisms. Specialised chemicals enable the chemical bonds between hydrogen and oxygen to be broken when hit by photons. This releases two electrons which can bind to chlorophyll, and a hydrogen proton. The protons are used in the formation of energy storage molecules via a proton pump powered by the electron flow (which is essentially electricity). The oxygen is released as a waste product into the atmosphere. The need to replace the electrons is no issue for the plant, as long as there is a supply of water. The outcome of photosynthesis is the energy which originally came from the photons stored in the chemical bonds of sugar molecules. The overall process of photosynthetic energy conversion is far more complex than in photovoltaic cells!
All this takes place inside sub-cell structures called chloroplasts in the plant cells. Chloroplasts are particularly interesting from an evolutionary perspective, due to their striking similarities to photosynthetic microbes called cyanobacteria. Both have very similar internal photosynthetic machinery, and chloroplasts have their own DNA (separate to the ‘main’ DNA found in the cell’s nucleus), which is very similar to DNA found in cyanobacteria. This has led biologists to believe that chloroplasts were once free-living, microbes which were ‘captured’ by larger cells to do their bidding billions of years ago, and remain slaves to this day!
You don’t have to understand all of the energy conversion business to figure out that photovoltaics and photosynthesis are worlds apart. But how do they compare when it comes to efficiency? Which of the two gets more bang for its buck? It’s not the case of working out simple ratios, as ‘efficiency’ is a bit of a hazy term and there are many things to consider. For example, the energy converted by photovoltaics can be stored inside batteries or hydrogen fuel cells (via electrolysis), but batteries lose their charge over time, and hydrogen is unstable and difficult to store. Photosynthesis takes the bacon here as it stores energy inside chemical fuels, which can remain incredibly stable for millions of years. Crude oil is the embodiment of this – a fossil fuel that stores the solar energy that was initially captured by plants millions of years ago, and powers our global economy today! Photovoltaics, however, have the upper hand when it comes to exploiting the radiation spectrum, as they can convert energy from most visible light and UV light. Stacked photovoltaic cells, which have been designed to absorb photons at different wavelengths in the separate layers, can convert up to 40% of incoming solar energy to electricity. Plants, on the other hand, can only absorb light from 45% of the visible light spectrum. When factors such as reflection and metabolic requirements are taken into account, plants can only convert a measly 3-6% of solar energy to chemical fuels. However, that small fraction goes a very long way indeed! When we take it all the way down to the quantum level, we find that plants are far more efficient at converting photon energy (at the right wavelength) into free electrons. Quantum efficiency refers to the percentage of electric charge generated for one photon. In photovoltaic, cells, this is typically around 12-17%, while plants can convert a whopping 100% of photons (at the right wavelength) into free electrons.
So at the molecular level, photosynthesis is incredibly efficient, but how is this achieved? Recent research suggests that quantum mechanics may be behind this startling efficiency. It is now thought that as electrons pass from chlorophyll to different acceptor molecules, quantum signals are sent out, which detect the exact route the electron must take to the next acceptor. These signals take the form of ‘ripples’ which are sent out when the electron hits an acceptor, like the ripples you see when a pebble hits water. This not only allows the electrons to find their way along the chain incredibly quickly, but also, crucially, minimises the chance of them being lost along the way.
This is all getting rather complicated, so it may be best to leave quantum mechanics for the time being. As the famous physicist Richard Feynman once said – “if you think you understand quantum mechanics…you don’t understand quantum mechanics”. But the message we do need to take home is that the core processes of photosynthetic energy conversion are not only unimaginably complex, but also much more efficient than photovoltaics. Yet more amazing is the ability of plants to manufacture stable chemical fuels via this process. It’s for these reasons that scientists are busy scratching their bald heads looking for ways to do the same thing artificially, but to no avail yet. In RoboPlant, we’ll be using photovoltaic cells in solar panels to convert the energy from sunlight into electricity, and we’ll be using an electrolyser powered by this electricity to split water. They’re not as efficient, or as beautifully complex as the microscopic machinery which operates within plant cells, and we also have no way of turning carbon dioxide into sugars. Nevertheless, this is the best we can do for the time being!
Want to know more about semiconductors, electric fields and quantum mechanics? Well, sorry. Unfortunately, this is where I get off, but don’t let that stop you from continuing your adventure through the nether-realm by doing a little research of your own. The world could do with more brave warriors who are ready to embrace this sort of thing. If you feel up to it, the path ahead is unknown and the terrain unforgiving, but who knows what scientific riches you may find?