um ination ▁direction ▁one ▁must ▁assume ▁that ▁there ▁exists ▁a ▁large ▁difference ▁in ▁recom bin ation ▁rates ▁at ▁opposite ▁faces ▁of ▁a ▁cry st all ite , ▁which ▁is ▁a ▁weak ness ▁of ▁this ▁model . ▁ ▁The ▁structure ▁transition ▁model ▁This ▁model ▁suggests ▁that ▁when ▁a ▁material ▁cry st all izes ▁both ▁in ▁cub ic ▁and ▁hex ag onal ▁structures , ▁an ▁asym metric ▁bar rier ▁can ▁be ▁formed ▁by ▁a ▁resid ual ▁di pole ▁layer ▁at ▁the ▁interface ▁between ▁the ▁two ▁structures . ▁A ▁potential ▁bar rier ▁is ▁formed ▁due ▁to ▁a ▁combination ▁of ▁the ▁band ▁gap ▁difference ▁and ▁the ▁electric ▁fields ▁produced ▁at ▁the ▁interface . ▁One ▁should ▁remember ▁that ▁this ▁model ▁can ▁be ▁invoked ▁to ▁explain ▁anom alous ▁P V ▁effect ▁only ▁in ▁those ▁materials ▁that ▁can ▁demonstrate ▁two ▁types ▁of ▁cry stal ▁structure . ▁ ▁The ▁p - n ▁j unction ▁model ▁ ▁It ▁was ▁suggested ▁by ▁St ark iew icz ▁ ▁that ▁the ▁anom alous ▁P V ▁is ▁developed ▁due ▁to ▁a ▁distribution ▁gradient ▁of ▁positive ▁and ▁negative ▁imp urity ▁ ions ▁through ▁the ▁micro c ry st all ites , ▁with ▁an ▁orientation ▁such ▁as ▁to ▁give ▁a ▁non - zero ▁total ▁phot ov olt age . ▁This ▁is ▁equivalent ▁to ▁an ▁array ▁of ▁p - n ▁j unction s . ▁However , ▁the ▁mechanism ▁by ▁which ▁such ▁p - n ▁j unction s ▁may ▁be ▁formed ▁was ▁not ▁explained . ▁ ▁The ▁surface ▁phot ov olt age ▁model ▁ ▁The ▁interface ▁between ▁cry st all