A solar cell is so much more than its efficiency

Jan Anton Koster jokingly calls himself the ‘device doctor’. Fellow researchers from all over the world can approach this professor at the University of Groningen if they want to know why their solar cell is not as good as they had hoped. Koster discovered that a simple measurement can reveal where energy is lost in a solar cell, and that a commonly used measurement is often performed incorrectly.

FSE Science Newsroom | Charlotte Vlek

Suppose you are creating a beautiful new solar cell, for example a perovskite solar cell. You can then take all kinds of measurements: how much power it generates at various light intensities, or when exposed to different colours from the spectrum. ‘I want to understand exactly how we should interpret these measurements,’ says Koster. ‘And how scientists can use these to improve their solar cell.’ 

Of course, the generated power tells you something about its efficiency, explains Koster. The efficiency of a solar cell is very strictly defined: it is a standard measurement taken under white light, at 25 °C, and at a set light intensity. ‘For many years, world records have been tracked for solar cells that are able to generate high power outputs under these specific conditions in the lab. By the way, Groningen is on that list too; it’s a record from a long time ago. But there is so much more that can be measured, and if you genuinely understand what you are measuring, it, in turn, will help you to develop new materials.’

A simple blue and red LED

Koster and his team discovered that you can quite easily take several measurements using various colours of light and that the combination of these measurements can reveal where energy is lost within the solar cell. A typical solar cell is made up of a middle layer, of perovskites for example, referred to as the ‘bulk’. Above and below are two layers that transport the generated charge, causing a current to flow. ‘We have shown that a comparison of the measurements for red and blue light shows where most energy is lost: in the bulk, between the bulk and the top layer, or between the bulk and the bottom layer.’

A simple blue and red LED was all that was needed, says Koster. A very simple experiment really. ‘All my colleagues should buy these LEDs! It would help them to make their solar cells even better.’ This is also Koster’s motivation for other techniques: by simulating on a computer exactly what happens in such a solar cell, he is developing targeted experiments that help us to understand solar cells much better. ‘A colleague of mine always says: we are developing a lot of solar cells, but we don’t think about them enough.’

Simple to measure, difficult to interpret

Koster gives an example: ‘There is an experiment that many researchers carry out but that is difficult to interpret. That is measuring impedance, the measure of motion in the material. To do this, you send a current through the solar cell, a direct current to be precise, and make it fluctuate slightly.’

The idea was that this measurement would tell us something about how long electrons continue to flow once they start moving. ‘That is true, but often the experiment was not carried out correctly: it was then measured at just the wrong voltage. We have shown that you then get the same result from your measurement, regardless of whether the solar cell is highly efficient or highly inefficient. So that tells us nothing.’

Impedance must be measured without applying a voltage and with exposure to light, states Koster. ‘And then you can indeed say something about how long the charged particles remain mobile. Using simulations, we now know exactly what the peak in such a measurement means. And we have also discovered why you could not derive any conclusions from that old experiment, carried out at the wrong voltage.’ 

A researcher with data

Fellow scientists quite frequently come to him with the request to interpret the measurements they have already taken themselves, says Koster. ‘But it’s more fun to use simulations to think about how these kinds of things work in general, rather than looking at every specific case.’

‘We once wanted to develop a “device doctor”, a kind of diagnostic tool that could be used to pinpoint where you could improve your solar cell,’ says Koster. ‘In the end, that became a flowchart, which is hanging here on the wall. But it is, of course, also a play on words. That “device doctor” is, in fact, me.’

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