Atomic layer deposition of zinc oxide onto and into P3HT for hybrid photovoltaics†
Hybrid organic–inorganic bulk heterojunction (BHJ) photovoltaic devices continue to be a promising alternative for present semiconductor solar cell technology. However, to become competitive hybrid devices must improve their currently low efficiencies. A major challenge to overcome is the control over the hybrid morphology, and more specifically, directing interpenetrated nano-scale phase separated continuous networks through the active layer. Here we demonstrate that atomic layer deposition (ALD) can be used to deposit hybrid BHJ photovoltaic films with exceptional control over film composition and morphology. The BHJ is prepared by exposing a pre-formed conjugated polymer film to an ALD alternating sequence of a metal oxide precursor and water. In this study ZnO was grown from cycles of diethyl zinc (DEZ) and water inside pre-formed P3HT films. We find that DEZ diffuses into the amorphous regions of the P3HT film, followed by oxidation by water to form ZnO crystalline particles (5–10 nm). Importantly, the inorganic crystalline phase is formed within the polymer amorphous regions while the ordered polymer domains are maintained. Investigation of the growth mechanism and control over the number of ALD cycles allowed us to direct a BHJ morphology with: (I) a continuous ZnO network through the P3HT film; (II) a descending concentration gradient of ZnO from the top surface down to the substrate; and (III) a dense ZnO electron transporting layer on the polymer film surface. The successful morphology-control is manifested in efficient photocurrent generation, evident from time resolved microwave-photoconductivity measurements and device performances that are similar to those reported for intensely optimized conjugated polymer/metal oxide solar cells.