Researchers from the University of Pennsylvania and Drexel University have demonstrated a new material that can both capture photons from visible light and get current to flow, paving the way for cheaper, more efficient solar PV cells.
Conventional solar panels are based around the interface of two materials: one which absorbs light and excites electrons, and one which causes them to flow in a consistent direction, producing an electric current. The interface which the excited electrons pass through is called the semiconductor p-n junction. Once an electron has crossed over, it cannot return the other way, thus creating the necessary flow.
However, some of the energy from photons is lost while electrons wait to make the jump through the junction. There’s even a name for the maximum theoretical efficiency of cells that use p-n junctions: the Shockley-Queisser limit. Multi-junction cells are able to overcome it, but this increases the complexity of the solar cell structure, which has a knock-on effect for production costs.
A small category of materials are able to send electrons off in a particular direction independently, without a junction; this is known as the ‘bulk’ photovoltaic effect rather than ‘interface’. The phenomenon has been known about since the 1970s but has previously only been shown to work with UV light. As most of the energy from the sun is in the visible and infrared spectrum, it hasn’t therefore been utilised for conventional solar cells.
A new material compound has been shown to generate the flow of electrons without a junction across a much wider spectrum of light. The compound created by the US researchers is a combination of a ‘parent’ material, potassium niobate, that lends it a bulk photovoltaic effect and a secondary one, barium nickel niobate, that lowers the threshold at which photons are absorbed, allowing it to capture more rays. The two materials are ground into fine powders, mixed and heated in an oven to create a ‘perovskite’ crystal that has the properties of both. The researchers fine-tuned the ratios involved until they hit upon the ideal combination.
“A solar cell based on the discovery could double power conversion efficiencies possible with conventional solar cells, theoretically”, says Professor Andrew Rappe at the University of Pennsylvania. It could also help to reduce the amount of materials used in a solar cell, and as perovskites are easier to process than silicon, make them more cost-effective too. The next step, Rappe says, is to create a full-scale solar cell that uses the modified perovskite, which should happen within two years.
The last 12 months have seen some promising breakthroughs in the dye sensitised solar cell (DSSC) field. Other types of engineered perovskite materials have been introduced as alternative light harvesters, replacing the layer of molecular sensitisers usually found in these cells. Efficiencies are already in the double-digit figures and are growing rapidly, according to Professor Michael Grätzel, a solar cell pioneer at the École Polytechnique Fédérale de Lausanne, Switzerland. – Sara Ver Bruggen
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