Highly Stable and Printable Organic Solar Cells by Inhibiting Chemical Degradation of Photoactive Materials

Abstract

Organic solar cells (OSCs) has emerged as a next-generation energy source due to lightweight, color tunability, mechanical flexibility, and low-cost fabrication via easy/fast roll-to-roll solution processes. However, there are many challenging issues to be addressed for realizing OSCs: development of efficient photoactive materials, device engineering for high-performance devices, reliable printing techniques and long-term stability. Chapter 1 affords a general and fundamental overview related to the field of organic solar cells. First, the general features of conjugated semiconducting materials and photo-induced charge transport effect in bulk heterojunction (BHJ) model, which are blended with donor and acceptor materials, are described. Second, we discuss the importance of the morphology of BHJ film for high-performance OSCs to efficiently help charge transport. Third, we introduce the new perspective on the printing process of solution-based OSCs. Forth, the stability of OSCs is discussed. Finally, the state-of-the-art OSCs based on non-fullerene acceptors (NFAs) are introduced. For the commercialization of OSCs, the printability of photoactive solution is important because it is closely related to the mass production with low cost. In Chapter 2, we investigated that the formation of a photoactive layer in printing in ambient air. We found that the residue of the additive solvent (i.e., 1,8-diiodooctane), which is the commonly-used for the high efficient performance of OSCs, accelerate the chemical degradation of PCBM molecules. To overcome the instability of additive solvent, we introduced a novel additive solvent, which have the high stability by its resonance structure and spontaneously evaporate in the ambient air. From our strategy, we demonstrated the stable printed OSC device with 8.43 % power conversion efficiency (PCE) using doctor blade technology in the air. Moreover, the device lifetime significantly extended by more than 10 fold compared to that of 1,8-diiodooctane-used device without any encapsulation. From our results, we suggest the novel additive solvent to enable the printing process, indicating the high performance and the high air-stability. Another demanding requirement for the successful commercialization is the reproducibility of OSCs via printing process. However, it is a significantly challenging issue because the thickness of each layer of OSCs is optimized to be the nanometer-scale thickness. In addition, in a photoactive layer, which is composed of more than two organic compounds, the morphology as well as the thickness of films also strongly affects the device performance. Therefore, in order to secure the reliable performance of printed OSCs, the morphology dependence on its thickness should be clearly understood. In chapter 3, we discuss the morphology dependence on the thickness of the photoactive layer based on NFAs. In this chapter, for the first time, we proved that the morphology of the photoactive layer was severely dependent on its thickness and it is closely related to the surface energy mismatching between donor and acceptor materials. In order to solve this problem, we introduced the alkoxy groups on the side groups of NFA to closely reduce the surface energy difference. We successfully demonstrated the thickness-insensitive OSC devices though both systems of a spin coating and a doctor blade technique, thereby securing the desired morphology even at around 300 nm thickness of the photoactive layer without any additives. From our approach, we provide an important insight into the design of photoactive materials and morphology control for the printable OSCs using NFAs. To address OSCs as the ubiquitous power source, the high-performance of OSC devices have been demanded. The development of efficient NFAs provides the new perspective on the overall OSC field to overwhelm the properties such as absorption range, absorption coefficient and molecular stacking of fullerene-based accpetors. Among the NFAs, the derivatives of 3,9-Bis(2-methylene-(3-(1,1- dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)- dithieno[2,3-d:2’ ,3’ –d’ ]-s-indaceno[1,2-b:5,6-b’ ]dithiophene: ITIC) exhibit the unprecedented performance over 14% PCE. Despite of the outstanding PCE, the performance of OSCs-based on ITIC often reveal a rapid degradation under the illumination, which it is not clearly understood so far. In chapter 4, for the first time, we found that the photodegradation of OSCs is related with the photocatalytic activation of the transition metal oxide such as zinc oxide (ZnO) at the interface. To overcome this problem, we developed the new architecture by introducing the multiple passivation layers, which composed of the fullerene derivative and the polyelectrolyte on top of ZnO surface. The modified device structure dramatically extends the photostability more than 10 fold compared to that of ZnO single layer-used device under AM 1.5G irradiation using a Xenon lamp without UV filter. Moreover, we confirmed that our approach is generally applicable to various photoactive systems with ITIC-based NFAs. We believe that our findings provide a new perspective to improve the photostability of the NFA-based OSCs. In Chapter 5, we summarize an overall of the works and the conclusions drawn in this thesis.