Interface layer design and analysis for small molecule organic solar cells

Recently, the fast technological development of organic solar cells (OSCs) proves that it has a promising future in the sustainable energy market. Interlayers play a significant role in OSCs, which can enhance device performance and stability. However, many of the fundamental mechanism and design principles of the interlayers have not been adequately studied. The theoretical chapters of this thesis deliver a summary of the state-of-the-art photovoltaic (PV) technologies and the fundamentals of organic photovoltaics (OPVs). The basic knowledge of interlayers within OSCs is introduced, too. The methodology chapter introduces the materials used in this thesis. Besides, fundamentals and technical parameters of the methodologies are summarised in this chapter, e.g. X-ray diffraction (XRD), grazing-incidence wide-angle X-ray scattering (GIWAXS), photoluminescence (PL), spectroscopic ellipsometry, external quantum efficiency (EQE), and internal quantum efficiency (IQE). In the experimental chapters, a new molecule (hexapropyltruxene) is introduced as an interlayer material in small molecule organic solar cells (SMOSCs), and its functions within OSCs are investigated. Firstly, hexapropyltruxene is used as an interlayer in the SubPc/C60 solar cell stack. The experimental results show that hexapropyltruxene works well as an exciton blocking layer (ExBL) between the MoOx and SubPc layers. By adding 3.8 nm of hexapropyltruxene between MoOx and SubPc layers, the power conversion efficiency (PCE) of the SubPc/C60 solar cells is enhanced from 2.6 % to 3.0 %, achieving a ca. 15 % improvement. Besides, it is found that hexapropyltruxene has a weak molecular template growth effect on the growth of SubPc thin film. Secondly, the application of hexapropyltruxene as an interlayer in the solar cell stack first demonstrated by interuniversity microelectronics centre (IMEC) is also investigated. The results reveal that hexapropyltruxene has a very weak exciton blocking effect between MoOx layer and alpha-6T donor layer. The structural characterization also suggests that hexapropyltruxene can induce the growth of different alpha-6T polymorphs. In the final experimental chapter, the polymorphism of alpha-6T induced by different substrates with hole contact layers (HCLs) is studied. The competing growth of low temperature (LT)-, high temperature (HT)-, and beta-phases of alpha-6T is observed in the thin film growth. The results of the experiments suggest that widely-used HCLs such as MoOx, PEDOT:PSS, and MeO-TPD have the template growth effect on alpha-6T molecules. In addition, the substrate temperature in the deposition process is significant in the competing growth of alpha-6T polymorphs. Corresponding solar cell performances show a correlation with the polymorph composition within alpha-6T layers and the performances of OSCs. To conclude, less "standing-up" but more "lying-down" molecules lead to better light absorption, which results in better solar cell performance. PEDOT:PSS is the most appropriate HCL for our alpha-6T/SubPc solar cell stack, and low substrate temperature (20 C) is more suitable than high temperature (120 C) in the growth of alpha-6T thin films for solar cell application.