Solution processed electrodes for organic solar cells

Abstract

Organic photovoltaics (OPVs) have attracted great scientific interest due to their low weight, flexibility and possibility of roll-to-roll mass production, providing green energy with low cost. The next important research and development milestones of OPVs are considered to be the avoidance of high cost materials, such as the transparent conductor indium tin oxide (ITO), and the avoidance of high vacuum and energy intensive deposition techniques, such as thermal evaporator, which is commonly used for the deposition of the reflective metal top electrode. Through the use of solution-processed electrodes, OPVs can be fabricated in a roll to roll production line. The inkjet printing (IJP) deposition technique has the potential to be introduced in the roll to roll production of OPVs due to its ability to print two-dimensional patterns and save material with drop on demand technology. In this thesis (Chapter 3), the potential of replacing the rigid and expensive ITO with IJP copper grids was investigated. Copper (Cu) nanoparticle inks have drawn much attention since they are cheaper than silver nanoparticle inks. A printing and sintering optimization of a proposed Cu ink is presented, resulting in high quality and conductive Cu grid structures. However, challenges arise during the implementation of Cu grid in the fabrication of OPVs. Specifically, Cu oxidizes during the necessary annealing step of PEDOT:PSS at 140°C in air, resulting in reduced conductivity. A highly conductive PEDOT:PSS formulation was introduced in order to enhance the conductivity of the electrode and assist in current collection. 3.4% power conversion efficiency (PCE) was achieved whereas the reference ITO-based device exhibited 4.9% PCE. The two following chapters (Chapters 4 and 5) are focused on the top electrode of inverted OPVs. A lifetime study is presented, comparing a solution processed PEDOT:PSS hole transporting layer (HTL) with the evaporated MoO3 HTL under 65°C in dark. The results show that devices using PEDOT:PSS as HTL are significantly more stable over time under heat conditions, compared to the devices using evaporated MoO3 as HTL which are suffering from fast degradation. OPVs with PEDOT:PSS and MoO3 as bi-layer HTL in different configurations were fabricated in an approach to isolate the interfaces and the degradation parameters. The results show that P3HT:PCBM/MoO3 as well as MoO3/Ag interfaces, contribute to the fast degradation of inverted OPVs with MoO3/Ag as top electrode. In addition, the above aspect was further enhanced by applying reverse engineering method in inverted OPVs with MoO3/Ag as top electrode. Based on the lifetime results of Chapter 4, PEDOT:PSS HTL was used in combination with IJP silver (Ag) for the development of a fully solution processed evaporation-free top electrode (Chapter 5). In inverted OPVs, silver top electrode can be deposited with printable deposition techniques compatible with roll to roll production line. A fully solution processed electrode is presented in inverted OPVs consisting of a highly conductive PEDOT:PSS and IJP Ag. Thick PEDOT:PSS layer was used in order to address the diffusion of solvents contained in Ag ink, into the active layer. It is proposed that highly conductive PEDOT:PSS assists in current collection of the printed electrode. Finally, a mixture of commercially available Ag inks with different nanoparticle sizes, was used in order to retain high conductivity, to control the printability and to achieve intimate interfaces.