▁on ▁am orph ous ▁sil icon ▁films ▁ ▁and ▁in ▁nan oc ry st all ine ▁sil icon ▁systems . ▁Ob ser ved ▁phot ov olt ages ▁were ▁found ▁to ▁reach ▁hundreds , ▁and ▁in ▁some ▁cases ▁even ▁thousands ▁of ▁vol ts . ▁The ▁films ▁in ▁which ▁this ▁effect ▁was ▁observed ▁were ▁generally ▁thin ▁sem icon duct ing ▁films ▁that ▁were ▁depos ited ▁by ▁vac u um ▁ev ap oration ▁onto ▁a ▁he ated ▁ins ulating ▁substr ate , ▁held ▁at ▁an ▁angle ▁with ▁respect ▁to ▁the ▁direction ▁of ▁the ▁incident ▁v apor . ▁However , ▁the ▁phot ov olt age ▁was ▁found ▁to ▁be ▁very ▁sensitive ▁to ▁the ▁conditions ▁and ▁procedure ▁at ▁which ▁the ▁samples ▁were ▁prepared . ▁This ▁made ▁it ▁difficult ▁to ▁get ▁reprodu cible ▁results ▁which ▁is ▁probably ▁the ▁reason ▁why ▁no ▁satisf actory ▁model ▁for ▁it ▁has ▁been ▁accepted ▁thus ▁far . ▁Several ▁models ▁were , ▁however , ▁suggested ▁to ▁account ▁for ▁the ▁extraordinary ▁phenomen on ▁and ▁they ▁are ▁briefly ▁out lined ▁below . ▁ ▁The ▁ob lique ▁de position ▁can ▁lead ▁to ▁several ▁structure ▁asym met ries ▁in ▁the ▁films . ▁Among ▁the ▁first ▁attempts ▁to ▁explain ▁the ▁A PE ▁were ▁few ▁that ▁treated ▁the ▁film ▁as ▁a ▁single ▁entity , ▁such ▁as ▁considering ▁the ▁variation ▁of ▁sample ▁thick ness ▁along ▁its ▁length ▁or ▁a ▁non - uniform ▁distribution ▁of ▁electron ▁tra ps . ▁However , ▁studies ▁that ▁followed ▁generally ▁supported ▁models ▁that ▁explain ▁the ▁effect ▁as ▁resulting ▁from ▁a ▁series ▁of ▁micro elements ▁contrib uting ▁add it