Thanks for the explanation of your plans with U3S and U3B. I think running the U3S on solar power on the ground is a great idea because you'll learn about PV the fun way. Then you'll be able to design power management systems for your U3B flights, ham shack, house, town, etc.
I've had good luck using electrically-conductive epoxy to contact PV cells that resist soldering. But it's expensive and fussy about cure temperature to provide good conductivity.
Isc is short-circuit current -- the current provided by the PV when the terminals are shorted. The PV current is lower out at the maximum power point where it matters. Enter "solar cell i-v curve" into a search engine for more.
I'll decide whether to use a blocking diode by measuring my array's shunt resistance in the dark at the battery voltage and slightly above it. If the current leakage loss seems substantial, then I'll evaluate whether it's worth the extra mass to add a tiny schottky or Ge diode and another PV cell to make up for the diode's voltage drop.
I don't trust ebay seller's PV cell performance claims considering the bogus battery capacity claims that dominate on ebay. I'll measure 'em myself. My initial estimates indicate that the 39x19mm cells will produce more current than I'll need, so one of my near-term challenges is to develop a cell-dicing technique that doesn't generate microcracks or shunt the pn junction at the edge.
I bought a tiny scale for $3.5 and have been weighing various components I may use.