How to measure solar cell efficiency correctly

The significance of new solar cell technologies tends to rest heavily on their measured efficiency. But compounding small mistakes in measuring that efficiency can lead to values up to five times higher than the true reading, says Henry Snaith from the University of Oxford, UK. 

Snaith has therefore set out a guide that illustrates the factors that should be taken into consideration when measuring efficiency, and outlines the potential sources of error. It is an attempt to restore confidence in literature claims and make them more easily comparable - both within fields and across different types of cells including dye-sensitised solar cells (DSSCs), organic photovoltaics and hybrid solar cells. The guidance includes how to mask cells to get an accurate measure of the test area; the type of lamps to use and how to calibrate them; and the importance of positioning the cell in exactly the same place as the calibration reference. 

’There’s an ongoing stream of papers in which it’s not entirely clear exactly how the measurements have been made,’ says Snaith. And worse than that, some papers claim values that appear to be grossly overinflated. That has an impact on genuine claims, Snaith explains. ’If, for example, someone claims their hybrid solar cell has an efficiency of 4% when it’s really more like 1%, that makes it problematic for someone else to write an exciting paper when they’ve genuinely improved something to 1.5%.’   

However, Snaith is quick to point out that his intention is not to point the finger of blame. ’The field has grown rapidly, so there are a lot of people coming in - without much device experience - who want to be able to make a solar cell and test it to see if their systems have made improvements,’ he adds. This influx brings new ideas and approaches, which is definitely to be encouraged. Unfortunately, there are some easy-to-make mistakes that can have drastic effects on measurements. ’There’s nothing particularly new or complex in the paper - the idea is to provide a clear protocol for how to get a value that accurately reflects the efficiency of the solar cell, and to point out the common pitfalls that can occur.’   

Nicolas T?treault, who develops DSSCs at the Swiss Federal Polytechnic School in Lausanne, agrees that having a single reference point for best practice will be very useful, especially one showing the possibility of such huge variations and illustrating how they relate to what’s going on in the cell. ’One of the benefits of showing these extremes is that it shows that the consequence of not doing it correctly can introduce errors that border on cheating!’   

T?treault adds that accurate measurements are even more important when trying to claim a new efficiency record. Snaith agrees, although in that case, he says, measurements should be independently certified by one of the national laboratories such as the US National Renewable Energy Laboratory.   

Phillip Broadwith