▁across ▁the ▁sample ▁is ▁large , ▁potentially ▁much ▁larger ▁than ▁the ▁band g ap . ▁ ▁Second , ▁in ▁a ▁similar ▁manner , ▁certain ▁ferro elect ric ▁materials ▁can ▁develop ▁stri pes ▁consisting ▁of ▁parallel ▁ferro elect ric ▁domains , ▁where ▁each ▁domain ▁acts ▁like ▁a ▁phot ov olta ic ▁and ▁each ▁domain ▁wall ▁acts ▁like ▁a ▁contact ▁connecting ▁the ▁adjacent ▁phot ov olta ics ▁( or ▁vice ▁vers a ). ▁Again , ▁domains ▁add ▁in ▁series , ▁so ▁that ▁the ▁overall ▁open - circ uit ▁voltage ▁is ▁large . ▁ ▁Third , ▁a ▁perfect ▁single ▁cry stal ▁with ▁a ▁non - cent ros ym metric ▁structure ▁can ▁develop ▁a ▁giant ▁phot ov olt age . ▁This ▁is ▁specifically ▁called ▁the ▁bulk ▁phot ov olta ic ▁effect , ▁and ▁occurs ▁because ▁of ▁non - cent ros ym met ry . ▁Specifically , ▁the ▁electron ▁processes — photo - ex c itation , ▁scatter ing , ▁and ▁relax ation — oc cur ▁with ▁different ▁prob abilities ▁for ▁electron ▁motion ▁in ▁one ▁direction ▁versus ▁the ▁opposite ▁direction . ▁ ▁Series - sum ▁of ▁gra ins ▁in ▁a ▁pol yc ry stal ▁ ▁History ▁ ▁This ▁effect ▁was ▁discovered ▁by ▁St ark iew icz ▁et ▁al . ▁in ▁ 1 9 4 6 ▁on ▁P b S ▁films ▁and ▁was ▁later ▁observed ▁on ▁other ▁sem icon duct ing ▁pol yc ry st all ine ▁films ▁including ▁C d Te , ▁Sil icon , ▁German ium , ▁Z n Te ▁and ▁In P , ▁as ▁well ▁as