▁and ▁going ▁from ▁A ▁to ▁B ▁to ▁C . ▁However , ▁it ▁will ▁almost ▁never ▁move ▁in ▁the ▁reverse ▁direction , ▁C ▁to ▁B ▁to ▁A , ▁because ▁the ▁transition ▁from ▁C ▁to ▁B ▁cannot ▁be ▁excited ▁by ▁phot ons , ▁but ▁instead ▁requires ▁an ▁impro bably ▁large ▁thermal ▁fl uct u ation . ▁Therefore , ▁there ▁is ▁a ▁net ▁right ward ▁phot oc urrent . ▁ ▁Because ▁the ▁electrons ▁under go ▁a ▁" shift " ▁each ▁time ▁they ▁abs orb ▁a ▁phot on ▁( on ▁average ), ▁this ▁phot oc urrent ▁is ▁sometimes ▁called ▁a ▁" shift ▁current ". ▁ ▁Dist ingu ishing ▁features ▁ ▁There ▁are ▁several ▁aspects ▁of ▁the ▁bulk ▁phot ov olta ic ▁effect ▁that ▁distinguish ▁it ▁from ▁other ▁kinds ▁of ▁effects : ▁In ▁the ▁power - gener ating ▁region ▁of ▁the ▁I - V ▁curve ▁( between ▁open - circ uit ▁and ▁short - circ uit ), ▁electrons ▁are ▁moving ▁in ▁the ▁opposite ▁direction ▁that ▁you ▁would ▁expect ▁from ▁the ▁dr ift - diff usion ▁equation , ▁i . e . ▁electrons ▁are ▁moving ▁towards ▁higher ▁ferm i ▁level ▁or ▁holes ▁are ▁moving ▁towards ▁lower ▁ferm i ▁level . ▁This ▁is ▁unusual : ▁For ▁example , ▁in ▁a ▁normal ▁sil icon ▁solar ▁cell , ▁electrons ▁move ▁in ▁the ▁direction ▁of ▁decre asing ▁electron - qu asi - fer mi ▁level , ▁and ▁holes ▁move ▁in ▁the ▁direction ▁of ▁increasing ▁hole - qu asi - fer mi - level , ▁consistent ▁with ▁the ▁dr ift - diff usion ▁equation