Origin of Photocarriers in Phthalocyanines

The generation of photocarriers in copper and metal‐free phthalocyanines (CuPc and H₂Pc, respectively) is shown to be an extrinsic process, i.e., the result of absorbed oxygen impurities. ir Photoconduction found in CuPc is absent in H₂Pc. This suggests that the ir response may arise from the singlet–triplet absorption which is enhanced by the spin–orbit coupling of the Cu ion. The photocurrent is proportional to (light intensity)^s, where $s$ is the same constant for ir and visible (intrinsic) absorptions. This constancy supports the interpretation that both singlet and triplet excitons generate carriers via interaction with the same trapping centers.