bittersweet vs semisweet

bittersweet is a noun but semisweet is not a noun.

Word	Noun	Adjective	Verb	Adverb
bittersweet	Yes	Yes	No	No
semisweet	No	Yes	No	No

As adjectives, bittersweet and semisweet are synonyms defined as:

bittersweet and semisweet: having a taste that is a mixture of bitterness and sweetness

bittersweet (noun)	semisweet (noun)
twining shrub of North America having yellow capsules enclosing scarlet seeds
poisonous perennial Old World vine having violet flowers and oval coral-red berries; widespread weed in North America

bittersweet (adjective)	semisweet (adjective)
tinged with sadness	having a taste that is a mixture of bitterness and sweetness
having a taste that is a mixture of bitterness and sweetness