. ▁Power ▁generation ▁is ▁possible ▁only ▁because ▁the ▁quasi - fer mi - level s ▁are ▁split . ▁A ▁bulk ▁phot ov olta ic , ▁by ▁contrast , ▁can ▁generate ▁power ▁without ▁any ▁splitting ▁of ▁quasi - fer mi - level s . ▁ ▁This ▁also ▁explains ▁why ▁large ▁open - circ uit ▁volt ages ▁tend ▁to ▁be ▁seen ▁only ▁in ▁cry st als ▁that ▁( in ▁the ▁dark ) ▁have ▁very ▁low ▁conduct ivity : ▁Any ▁electrons ▁that ▁can ▁freely ▁move ▁through ▁the ▁cry stal ▁( i . e ., ▁not ▁requiring ▁phot ons ▁to ▁move ) ▁will ▁follow ▁the ▁dr ift - diff usion ▁equation , ▁which ▁means ▁that ▁these ▁electrons ▁will ▁subtract ▁from ▁the ▁phot oc urrent ▁and ▁reduce ▁the ▁phot ov olta ic ▁effect . ▁ ▁Each ▁time ▁one ▁electron ▁absor bs ▁one ▁phot on ▁( in ▁the ▁power - gener ating ▁region ▁of ▁the ▁I - V ▁curve ), ▁the ▁resulting ▁electron ▁disp lacement ▁is , ▁on ▁average , ▁at ▁most ▁one ▁or ▁two ▁unit ▁cells ▁or ▁mean - free - paths ▁( this ▁disp lacement ▁is ▁sometimes ▁called ▁the ▁" an is ot ropy ▁distance "). ▁This ▁is ▁required ▁because ▁if ▁an ▁electron ▁is ▁excited ▁into ▁a ▁mobile , ▁del ocal ized ▁state , ▁and ▁then ▁it ▁sc at ters ▁a ▁few ▁times , ▁then ▁its ▁direction ▁is ▁now ▁random ized ▁and ▁it ▁will ▁naturally ▁start ▁following ▁the ▁dr ift - diff usion ▁equation . ▁However , ▁in ▁the ▁bulk ▁phot ov olta ic ▁effect , ▁the