semisweet vs bittersweet

semisweet is not a noun while bittersweet is a noun.

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

As adjectives, semisweet and bittersweet are synonyms defined as:

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

semisweet (noun)	bittersweet (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

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