ites ▁may ▁contain ▁tra ps ▁for ▁charge ▁carri ers . ▁This ▁may ▁lead ▁to ▁a ▁surface ▁charge ▁and ▁an ▁opposite ▁space ▁charge ▁region ▁in ▁the ▁cry st all ites , ▁in ▁case ▁that ▁the ▁cry st all ites ▁are ▁small ▁enough . ▁Under ▁ill um ination ▁of ▁the ▁incl ined ▁cry st all ites ▁electron - hole ▁pairs ▁are ▁generated ▁and ▁cause ▁a ▁compens ation ▁of ▁the ▁charge ▁in ▁the ▁surface ▁and ▁within ▁the ▁cry st all ites . ▁If ▁it ▁is ▁assumed ▁that ▁the ▁optical ▁absor ption ▁depth ▁is ▁much ▁less ▁than ▁the ▁space ▁charge ▁region ▁in ▁the ▁cry st all ites , ▁then , ▁because ▁of ▁their ▁incl ined ▁shape ▁more ▁light ▁is ▁absor bed ▁in ▁one ▁side ▁than ▁in ▁the ▁other . ▁Thus ▁a ▁difference ▁in ▁the ▁reduction ▁of ▁the ▁charge ▁is ▁created ▁between ▁the ▁two ▁sides . ▁This ▁way ▁a ▁phot ov olt age ▁parallel ▁to ▁the ▁surface ▁is ▁developed ▁in ▁each ▁cry st all ite . ▁ ▁Bul k ▁phot ov olta ic ▁effect ▁in ▁a ▁non - cent ros ym metric ▁single ▁cry stal ▁ ▁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 . ▁The ▁electron ▁processes ▁like ▁photo - ex c itation , ▁scatter ing , ▁and ▁relax ation ▁may ▁occur ▁with ▁different ▁prob abilities ▁for ▁electrons ▁moving