Molecular Control of Electrode Interfaces in Vertical and Lateral Organic Photovoltaics

Abstract

To improve the power conversion efficiencies of organic photovoltaics (OPVs), inserting an interfacial nanolayer between an organic photoactive layer and metal electrodes is one of the most effective and general ways. In this paper, we introduced effective and stable layers to anode and cathode interfaces and improved device performance in terms of both efficiency and lifetime. Additionally, we demonstrated the lateral OPVs with horizontally separated electrodes by controlling electrode interfaces. In Chapter 1, we employed a solution-processed ultrathin polymeric layer of poly(4-vinylphenol) (PHS), as an interfacial layer between the polymer:fullerene photoactive layer and the Al negative electrode for enhancing device performance in polymer bulk-heterojunction photovoltaic cells and investigated the roles of the interfacial nanolayer. The thin polymeric layer forms a dipole layer and causes the vacuum level of the adjacent negative electrode to shift upward, which resulted in increase of the built-in potential and facilitated carrier injection/extraction. As a result, open-circuit voltage, short-circuit current, fill factor, and power conversion efficiency of the device using a PHS nanolayer were improved. Secondly, we reported that insertion of a water-soluble ionic conjugated poly(phenylene) (ICP) nanolayer as a cathode interlayer in polymer solar cells resulted in improvement of both the device efficiency and stability. We discussed the role of the ICP with alkali metal counter ions at the electron extraction contact: An ionic dipole moment of the ICP induced the vacuum level of an adjacent cathode layer to shift upward and, consequently, increased built-in voltage and open-circuit voltage. As a result, PCE of the device using the ICP layer (5.0 %) significantly outperformed the device without an interlayer (3.0 %). In addition, the ICP layer greatly improved the device stability under continuous simulated solar irradiation. Therefore, the solution-processed PHS layer and ICP layer can replace the vacuum deposited metal fluoride and thus both are useful to realize low-cost, efficient and stable polymer solar cells. In Chapter 2, we focused on an anode interfacial layer. We synthesized water soluble conducting polymer including fluorocarbon polymer parts, PPFS-co-(PSS-g-PANI). The PPFS-co-(PSS-g-PANI) film formed nearly ohmic contact in a device due to high work function, and presented high hydrophobicity and smooth roughness. These characteristics caused higher Voc and FF, leading higher PCE (5.7 %) in OPVs, compared to that with conventional conducting polymer, PEODT:PSS (4.9 %) and water-soluble PSS-g-PANI (5.3 %). We also obtained remarkable increase in lifetime of the OPV introducing PPFS-co-(PSS-g-PANI) as a buffer layer. This multi-functional conducting polymer can be an attractive candidate for a buffer layer in OPVs. In Chapter 3, we designed and fabricated the lateral OPVs to overcome typical shortcomings of vertical OPVs such as limitation of photo-active layer thickness, difficulty of tandem OPVs by using solution processes and choice of different electrodes. We approached in two ways

e-beam lithography and organic nano-wire lithography. The lateral OPV using e-beam lithography exhibited power conversion efficiency (PCE) of 6.58 %. Then, we fabricated electrodes with lateral nano-gap by the simple and easy process, organic nano-wire lithography. We treated the mixed self-assembled monolayer (SAM), which was composed of 1H,1H,2H,2H-Perfluorodecanethiol and 1-Octadecanethiol, to ensure the difference of work function on electrodes. It showed the clear solar cell characteristics leading PCE of 0.8 %. These results offered excellent potential for the lateral OPVs by using the easy process.