Why is it difficult to compare efficiency between different IPV cells?

Efficiency depends on how the system is set up in terms of input (light) and output (electrical power). Outdoor solar cells have addressed this by using a common standard (AM1.5G) on the input side. By placing a solar cell under these defined lighting (input) conditions, the electrical power (output) can be measured in a defined manner. 

However, test conditions for indoor measurements often vary, complicating the input side. A standard for indoor conditions has only recently emerged: IEC TS 62607-7-2. The problem is that most efficiency measurements do not refer to this, or any specific standard, making comparisons difficult. At Epishine, we fully comply with IEC TS 62607-7-2, making our efficiency statements unambiguous. 

Read more about measuring standards for IPV here.