The electron transport layer (ETL) is somewhat easier to reduce and more difficult to oxidize than the hole transport layer (HTL). We want to find HTL chemicals that are easy to oxidize and form stable cation radicals, and ETL chemicals that are easier to reduce and form stable radical anions. This results in an energy offset between the two organic layers so that electrons injected into the ETL layer have a large energy barrier to surmount in order to move into the HTL. The holes injected into the HTL have a large barrier to move into ETL layer. As a consequence, at low applied fields the holes and electrons build up at or near the interface which is the site of recombination.