This is a silly objection. That isn't how payback times are used.
Payback time is a quick indication of return on investment. You then compare that return on investment with the other options available to you, such as leaving the money in the bank.
If you included interest rates in payback time, you'd need to be constantly adjusting it as rates changed, and it would differ for different entities depending on their access to finance. Instead you keep it simple, and each entity has its own idea (based on circumstances and current interest rates) of what the effective payback time is of leaving the money in the bank (or not borrowing it, or investing it in other opportunities.) (For example, a start-up is likely to require a very short payback time - they're strapped for cash and are trying to get their Big New Idea to market where they hope it will make a fortune. Up-front money is then very expensive compared to down-the-road money. For them, it may make sense to lease a supercomputer even if buying it would have a two year payback time.)
What is missing from this analysis is depreciation of assets. After 6.4 years, money in the bank will have depreciated much less than the solar cells. Payback time is a rough guide - it tells you whether it is worth your while doing a more detailed analysis including finance cost, depreciation, tax implications etc.