Developing mechanically-robust materials for intrinsically-stretchable organic solar cells

Abstract

thus, mechanical functionality is a primary consideration in developing wearable devices. In particular, wearable OSCs fitted to arbitrary surfaces (e.g., textiles, skin, or curved objects) should maintain their optimal performance even when bent or stretched in any direction. The active layer is the most important component in stretchable OSC (SOSC) because it has the greatest impact on the photovoltaic performance and mechanical functionality of the entire device. Thus, to successfully develop SOSCs, active materials possessing intrinsically superior electrical and mechanical properties should be designed. In addition, the blend morphology of the active layer should be optimized considering the molecular interactions between the photoactive materials. In this study, an organic photoactive layer having high stretchability, strong cohesion, and high conductivity is developed and applied to SOSCs to secure high PCE and device stretchability close to commercialization. Chapter 1 covers the operating principle of organic solar cells, definition and classification of existing rigid/flexible/stretchable solar cells, organic thin film mechanical properties and measurement methods, molecular design key strategy and the SOSC device platform. Chapter 2 discusses developing a high molecular weight naphthalene diimide (NDI)-based conductive polymer acceptor and incorporating it into the photoactive layer as a third component. In this study, various polymer batches from low to ultra-high molecular weight were synthesized, which greatly affects mechanical properties and SOSC device performance, and added them into small molecular-based binary blend with a certain amount. We deeply investigated how molecular weight affects stretchability and efficiency of SOSCs. In particular, the ultra-high molecular weight increased tie molecule density and strong entanglements, resulting in superior stretchability and PCE compared to the control group with a low molecular weight. Chapter 3 presents a new strategy for designing effective interfacial stabilizers to enhance long-term stability and mechanical durability of conventional all-polymer solar cells (all-PSCs), using polymerized non-fullerene acceptors (PNFAs). The primary approach for molecular designs involves creating a new homopolymer by introducing a fused block containing one benzodithiophene (BDT) unit and two benzothiadiazole (BT) units. These stabilizers result in effective interfacial stabilization and thus suppress severe phase separation in blends due to their low interfacial energies with host polymer donor and acceptor materials. Importantly, ternary all-polymer blends containing the interfacial stabilizers demonstrate improved blend morphology with strengthened interfaces, resulting in better photovoltaic properties and thermal/mechanical stabilities. Resultingly, the stretchable PSCs (SPSCs) based on the ternary blend exhibit an excellent PCE over 13% and stretchability with a strain at PCE80% of 35%, which represent one of the highest values among SPSCs to the best of our knowledge. In Chapter 4, a new A-D-A type compatibilizer combining two fullerene derivatives and five thiophene rings as an electron donor intermediate unit is introduced into the photoactive layer to simultaneously enhance the thermal/mechanical stability of a fullerene-based blend films. When the compatibilizer was introduced as a third component, it was confirmed that the issue of aggregate formation of fullerene molecules and phase separation in blend caused by the thermal stress, as well as the resulting low mechanical cohesive energy of the thin film, could be effectively mitigated. The above-described research on the development of a photoactive layer thin film with high mechanical properties and conductivity is anticipated to serve as a future guideline for molecular design in the context of developing materials for wearable/stretchable solar cell devices.

Organic solar cells (OSCs) as an energy technology are attracting the attention of researchers attempting to respond to the rapidly growing demand for soft electronics. Compared to conventional silicon-based inorganic solar cells, OSCs have fascinating features such as transparency, light-weight, eco-friendliness, low-cost solution processability, and softness. In particular, the superior mechanical functionality of active layers based on the intrinsic softness of organic materials makes OSCs strong candidates for power sources of next-generation, self-powered wearable electronics. Recently, the power conversion efficiency (PCE) of lab-scale, single-junction OSCs has reached 18–19% and is growing very rapidly with considerable potential for industrial applications. While high PCEs have been achieved in rigid devices fabricated on glass substrates, they are not suitable for powering wearable or portable electronics. To be conformally attached to a human body, consisting of elastic skins and movable joints, materials must demonstrate high stretchability (tolerate up to ~50% strain)