But he only asked about maximum solar panel efficiency with cost no object that does not include either a charge controller or batteries - which is pretty funny for someone on a wet string internet connection.
A half decent solar panel comes out at around $800 /m^2 plus installation and unless you are getting some special "green" generation rate is hopelessly uneconomic. German market is distorted this way. Even off grid it is pretty marginal and there are cheaper ways of generating modest amounts of power. The only advantage I can see for PV is no moving parts and unattended operation.
The economics may be different when we can screen or inkjet print PV arrays from organic dyes at modest efficiency and plausible stability in bright sunlight. So far that remains a pipe dream
Regards, Martin Brown