Solar panels are made up of solar cells which are diodes or silicon junctions or check valves that put out 0.5 to 0.6 volts each without load (open circuit). To charge a 12 volt battery, you put 10 solar cells in series to get roughly 20 volts which under load will drop down to closer to 15 volts.

Every solar cell is a diode and not only prevents the backwards flow of current it also prohibits forward flow of current without sunlight to excite the electrons and bridge the junctions.

As solar panels are strings diodes wired in series, if you partially block one diode by providing less light due to a shadow the performance of the entire panel suffers disproportionately. It’s actually bad for the panel to be exposed to shadows continously as when you are blocking flow of current due to shadows you are starting to wear down the silicon junctions in other cells eventually causing permanent damage to the panel.