Utilization of Eco-Friendly Materials for Printed Large-Area Organic Photovoltaics with High Efficiency and Stability

Abstract

Solution processed organic solar cells (OSCs) provide several advantages, such as low-cost, flexibility, and suitability for mass production through roll-to-roll processing based on various printing methods. The state-of-the-art OSCs have achieved commercially applicable power conversion efficiency (PCE) exceeding 18%, which exhibits similar level with other solar cells. However, in order to broaden and develop into the next generation OSCs more, further researches that can optimize each layer in OSCs are necessary to ensure the device performance and their practical application in the future. In this study, the extensive research findings include three following strategies using eco-friendly materials, which will detailedly discussed in chapter 2~4. (Chapter 1) It provides a general introduction and background to OSCs, printed electronics, and layer engineering in OSCs. (Chapter 2) Various studies in OSCs have been conducted on scalable coating methods that are compatible with large-area production of organic photovoltaic modules. However, it is still difficult to control the bulk heterojunction (BHJ) morphology of the active layer during large-scale fabrication of OSCs. This study reports a morphology-controllable strategy in OSCs using water treatment (WT) in the stirring process of the active solution, thus resulting in vortex agitation. The effects of WT and water injection volume are investigated based on three reference cells for the optimization of small- and large-area devices, and the physicochemical and optical properties of the films are compared with those without WT. The thin films with WT exhibit a smoother morphology than those without WT, indicating well-dispersed donor (D)–acceptor (A) phases. Therefore, enhanced efficiencies of the films are achieved via WT. Furthermore, large-area solar cell modules with a total effective area of 10 cm2 are fabricated, and they exhibit superior PCEs as high as 11.92% (H-NF-DIW10), indicating that the WT method is a simple and effective strategy to fabricate large-area organic photovoltaic modules. (Chapter 3) The introduction of molecular doping process is necessary to enhance the optic and electronic properties of organic semiconductors for facilitating charge transport. In particular, since the doping process has a positive influence on the charge transfer interaction between the host semiconductor and dopant, improved mobility has been efficiently achieved via these doping methods using p- or n-type dopants. Despite its advantages, doping technologies in OSCs, which are generally known to have better hole mobility, are emphasized to the development of n-type dopants used for balancing the electron and hole as increasing the electron mobility. In addition, since the BHJ microstructure in OSCs has randomly blended phases of the D and A, it is important to optimize charge extraction without loss by controlling the morphology. In this study, I report OSCs by p-type doping with formic acid (FA) into a BHJ photoactive layer comprised of PM6 and Y6. The resulting champion device yields a significantly improved power conversion efficiency from 14.3% to 15.3% with a high fill factor of 71.7%. It is found that the p-doped photoactive layer exhibits enhanced conductivity, improved carrier mobilities, suppressed charge recombination, and lowered leakage current. The p-type dopant, FA, simultaneously acts as a film morphology controller of the photoactive layer with enhanced phase separation to transport the charge efficiently. Thus, the doping process with FA can maintain the device performance in long-term stability tests (95.6% remaining of its initial PCE). This work demonstrates that controlling the charge transport and trap formation via directly introducing a small amount of FA dopant into photoactive layers is a promising strategy for further improvement of device efficiency and stability in OSCs. (Chapter 4) In OSCs, the role of the interlayer is important for efficient charge transport from the photoactive layer to electrodes. However, as a hole-transporting layer (HTL), molybdenum oxide (MoOx) and poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonate) (PEDOT:PSS) are still used for commercialization despite their drawbacks. The development of novel materials with suitable energy levels and solvent orthogonality is required to enhance hole extraction and optimize the film morphology. Herein, oxidized carbon soot (OCS) and OCS-functionalized octadecylamine (OCS-ODA) nanoparticles as HTL materials are synthesized. As a large number of oxygen functional groups are produced via Hummer’s method, the devices with the OCS-ODA exhibit high hole extraction ability. The ODA long alkyl chains functionalized by facile process also improve film morphology to minimize contact resistance and charge recombination. The small-area (SA), large-area, and flexible (F) OSCs with OCS-ODA show power conversion efficiencies of 15.04%, 14.57%, and 12.73%, respectively. In particular, OSCs with OCS-ODA are further demonstrated to possess storage stability in SA-OSCs (71% retention after 450 h) and mechanical stability in F-OSCs (78% retention after 1000 bendings).