arg he eb . com ▁ ▁Islam ic ▁Encyclopedia ; ▁Read ▁Online ▁ ▁Category : En cyc loped ias ▁of ▁Islam ▁Category : 2 0 1 0 ▁non - f iction ▁books <0x0A> </s> ▁The ▁anom alous ▁phot ov olta ic ▁effect ▁( AP E ), ▁also ▁called ▁the ▁bulk ▁phot ov olta ic ▁effect ▁in ▁certain ▁cases , ▁is ▁a ▁type ▁of ▁a ▁phot ov olta ic ▁effect ▁which ▁occurs ▁in ▁certain ▁sem icon duct ors ▁and ▁ins ul ators . ▁The ▁" an om alous " ▁refers ▁to ▁those ▁cases ▁where ▁the ▁phot ov olt age ▁( i . e ., ▁the ▁open - circ uit ▁voltage ▁caused ▁by ▁the ▁light ) ▁is ▁larger ▁than ▁the ▁band ▁gap ▁of ▁the ▁corresponding ▁sem icon duct or . ▁In ▁some ▁cases , ▁the ▁voltage ▁may ▁reach ▁thousands ▁of ▁vol ts . ▁ ▁Although ▁the ▁voltage ▁is ▁unus ually ▁high , ▁the ▁short - circ uit ▁current ▁is ▁unus ually ▁low . ▁Over all , ▁materials ▁that ▁exhib it ▁the ▁anom alous ▁phot ov olta ic ▁effect ▁have ▁very ▁low ▁power ▁generation ▁effic ien cies , ▁and ▁are ▁never ▁used ▁in ▁practical ▁power - gener ation ▁systems . ▁ ▁There ▁are ▁several ▁situations ▁in ▁which ▁A PE ▁can ▁arise . ▁ ▁First , ▁in ▁pol yc ry st all ine ▁materials , ▁each ▁micro scop ic ▁gra in ▁can ▁act ▁as ▁a ▁phot ov olta ic . ▁Then ▁the ▁gra ins ▁add ▁in ▁series , ▁so ▁that ▁the ▁overall ▁open - circ uit ▁voltage