iet ies . ▁Otherwise ▁neutral ▁exc it ons ▁decay ▁radi atively ▁to ▁the ▁ground ▁state ▁– ▁thereby ▁em itting ▁phot ol um ines c ence ▁– ▁or ▁non - rad i atively . ▁The ▁optical ▁absor ption ▁edge ▁of ▁organ ic ▁sem icon duct ors ▁is ▁typically ▁ 1 . 7 – 3   e V , ▁equivalent ▁to ▁a ▁spectral ▁range ▁from ▁ 7 0 0 ▁to ▁ 4 0 0   nm ▁( which ▁corresponds ▁to ▁the ▁visible ▁spectrum ). ▁ ▁History ▁ ▁In ▁ 1 8 6 2 , ▁Henry ▁Let he by ▁obtained ▁a ▁partly ▁conduct ive ▁material ▁by ▁an od ic ▁ox id ation ▁of ▁an il ine ▁in ▁sul fur ic ▁acid . ▁The ▁material ▁was ▁probably ▁pol yan il ine . ▁In ▁the ▁ 1 9 5 0 s , ▁research ers ▁discovered ▁that ▁poly cyc lic ▁a rom atic ▁comp ounds ▁formed ▁semi - con duct ing ▁charge - trans fer ▁complex ▁sal ts ▁with ▁hal og ens . ▁In ▁particular , ▁high ▁conduct ivity ▁of ▁ 0 . 1 2   S / cm ▁was ▁reported ▁in ▁per yl ene – iod ine ▁complex ▁in ▁ 1 9 5 4 . ▁This ▁finding ▁indicated ▁that ▁organ ic ▁comp ounds ▁could ▁carry ▁current . ▁ ▁The ▁fact ▁that ▁organ ic ▁sem icon duct ors ▁are , ▁in ▁principle , ▁ins ul ators ▁but ▁become ▁sem icon duct ing ▁when ▁charge ▁carri ers ▁are ▁inject ed ▁from ▁the ▁electro de ( s ) ▁was ▁discovered ▁by