How should you measure your excitonic solar cells?

The research field of photovoltaics is booming due to the recognised imperative to realise a long term solutions to clean and inexpensive power generation. With this rapid growth in activity, new concepts and new directions, it is becoming more and more important to be able to effectively compare technological advances made in different research groups, and between the different competing technologies. In this article it is illustrated how challenging it can be to fairly asses the efficiency of a solar cell. Specific examples of how much of an influence a range of measurement practices can have on the estimated short-circuit photocurrents and efficiencies are shown for a number of the most commonly researched excitonic solar cell concepts. In the worst case scenario, systematic overestimations by a factor of 5 can be made. For completeness, the influence of the light source, and the correct method to properly calibrate a solar simulator and estimate the spectral mismatch factor is also described. The error in calibrating the light source is actually likely to lead to a much smaller variance than the definition of the active area of the solar cell. The undeniable implication is that it is essential for excitonic solar cells to be correctly masked to define the active area, if the measured efficiency is to bear a true resemblance of the device performance. Adoption of standard protocols across the field will enable a much better comparison between literature values of efficiency and more rapid and well directed technological advancement will occur.