c itation . ▁ ▁General ▁properties ▁In ▁mole cular ▁cry st als ▁the ▁ener get ic ▁separation ▁between ▁the ▁top ▁of ▁the ▁val ence ▁band ▁and ▁the ▁bottom ▁condu ction ▁band , ▁i . e . ▁the ▁band ▁gap , ▁is ▁typically ▁ 2 . 5 – 4   e V , ▁while ▁in ▁in organ ic ▁sem icon duct ors ▁the ▁band ▁g aps ▁are ▁typically ▁ 1 – 2   e V . ▁This ▁implies ▁that ▁they ▁are , ▁in ▁fact , ▁ins ul ators ▁rather ▁than ▁sem icon duct ors ▁in ▁the ▁conventional ▁sense . ▁They ▁become ▁sem icon duct ing ▁only ▁when ▁charge ▁carri ers ▁are ▁either ▁inject ed ▁from ▁the ▁electro des ▁or ▁generated ▁by ▁intent ional ▁or ▁un intent ional ▁do ping . ▁Char ge ▁carri ers ▁can ▁also ▁be ▁generated ▁in ▁the ▁course ▁of ▁optical ▁exc itation . ▁It ▁is ▁important ▁to ▁realize , ▁however , ▁that ▁the ▁primary ▁optical ▁exc itations ▁are ▁neutral ▁exc it ons ▁with ▁a ▁C oul omb - binding ▁energy ▁of ▁typically ▁ 0 . 5 – 1 . 0   e V . ▁The ▁reason ▁is ▁that ▁in ▁organ ic ▁sem icon duct ors ▁their ▁die lect ric ▁constants ▁are ▁as ▁low ▁as ▁ 3 – 4 . ▁This ▁imp edes ▁efficient ▁phot og ener ation ▁of ▁charge ▁carri ers ▁in ▁neat ▁systems ▁in ▁the ▁bulk . ▁E fficient ▁phot og ener ation ▁can ▁only ▁occur ▁in ▁binary ▁systems ▁due ▁to ▁charge ▁transfer ▁between ▁don or ▁and ▁accept or ▁mo