Multilayer epitaxial growth of organic molecules for efficient small molecular organic solar cells

Abstract

Organic photovoltaic cells (OPVs) have been attracted in last decades due to their large area applicable, low-cost, non-toxicity and flexibility advantages. Unfortunately, OPVs have relatively low power conversion efficiency (ηp) compared to inorganic based photovoltaic devices. Moreover, the stability of the OPVs are too poor to use for the power generation. Thus, improvement of ηp and the stability are key issues for the manufacturing of the OPVs. In this thesis, the use of templating materials for improving the optoelectronic properties of the organic layers are reported and the mechanism of the templating effects is discussed. Firstly, we report that the use of molybdenum oxide (MoO3) and copper iodide (CuI) as a double interfacial layer improves the ηp and the photo-stability at the same time in zinc phthalocyanine (ZnPc) based photovoltaic cells. The absorption of the ZnPc film is enhanced 1.6 times by insertion of CuI as a template layer. Resultantly, the OPV with a CuI interfacial layer increases the efficiency significantly to 3.3%. However, the photo-stability is lowered even further. The time-of-flight secondary ion mass spectrometry (TOF-SIMS) analysis reveals that the diffusion of Cu atoms or ions is the origin of the photo-degradation of the device. Insertion of the MoO3 layer between the indium tin oxide (ITO) and CuI prevents the diffusion of Cu atoms or ions under UV illumination. To reveal the origin of the templating effects, the optoelectronic properties of lead-phthalocyanine (PbPc) and the performance of OPVs have been investigated by using three copper halogen compounds (CuCl, CuBr, CuI) possessing different lattice parameters as the templating layers. The crystallinity of the PbPc films was the highest on CuI followed by CuBr and CuCl, resulting in the broadening of Q-band absorption in the same order. The templating effects were able to be described by heteroepitaxial growth of organic molecules on the templating layers and the dimensionless potential calculated using a lattice model for the overlayer-substrate systems showed good correlation between the degree of epitaxy and the crystallinity of PbPc overlayers. Furthermore, the performance of OPVs was consistent with the prediction from the calculation results and the observation from the optical and structural analyses. Finally, the templating effect extends to multilayers to increase the crystallinity and to modify the orientation of the crystals of PbPc and C70 layers at the same time by adopting CuBr as a new templating layer on ITO. The formation of a monoclinic phase with a preferred orientation of (320) for PbPc and a fcc phase with a preferred orientation of (220) for C70 on the PbPc layer is revealed by X-ray diffraction (XRD) patterns. The multilayer epitaxy results in an increase of the exciton diffusion lengths from 5.6 to 8.8 nm for PbPc and from 6.9 to 13.8 nm for C70 to enhance the ηp of the planar heterojunction OPVs composed of PbPc and C70. The heteroepitaxy model also explains the multilayer epitaxy.