Bimolecular recombination and complete photocurrent decay in metallophthalocyanine thin films

Abstract

Metallophthalocyanines (MPc) are a class of planar small molecule semiconductors that are of great interest due to their applications in organic solar cells. However, a thorough understanding of many of their charge transport properties, especially those related to trap states, is lacking. In previous works the increase of grain size in MPc thin films with higher substrate temperature during the thermal evaporation process was quantified. Here, we investigate the charge transport properties of phthalocyanine with varying grain sizes in five ways: 1) intensity-varied photocurrent measurements are used to study the order of recombination in zinc phthalocyanine (ZnPc) thin films; 2) the conduction dependence on oxygen doping in copper phthalocyanine (CuPc) and ZnPc is investigated through oxygen deprivation in a closed-cycle refrigerator; 3) the complete photocurrent decay in CuPc and ZnPc is explored; 4) the bandgap of CuPc and ZnPc is derived from wide temperature range (∼ 80K - 351K) resistance measurements; 5) thermally stimulated currents (TSC) in CuPc and ZnPc are induced using a modified Reber methodology. An explanation for oxygen doping in these type of thin films is presented and in addition to bimolecular recombination a second model is also applied to measurements of intensity-varied photocurrent. Our data suggest several interesting aspects of the charge transport properties in these organic semiconductors. Recombination may not be dominated by monomolecular processes but instead bimolecular. More amorphous (low deposition temperature) samples show a greater change in conductivity due to oxygen doping. The photocurrent of more crystalline samples decays more rapidly compared to amorphous samples, suggesting that deeper trap states are present in lower deposition temperature samples. The bandgap derived from temperature-varied resistance measurements is systematically low due to slow detrapping. Unlike TSC measurements in micrometer-thick organic films using current-perpendicular-to-plane geometry which show large peaks in current vs. temperature measurements, no such peaks were observed in our nanometer-thin current-in-plane geometry ZcPc and CuPc films