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Diodes act as a one way valve for electricity so current only flows in one direction round a circuit. Like solar cells, a diode uses n and p type semiconductors but in a different arrangement to allow the current to flow as designed.
In solar energy systems they are found in multiple parts of the system, from protecting other components in a solar panel and voltage regulators, to altering the form of electricity before carrying on around the circuit in inverters, converters and rectifiers.