Photophysical pathways in efficient bilayer organic solar cells: The importance of interlayer energy transfer. (June 2021)

• Record Type: Journal Article
• Authors: Park, Song Yi; Chandrabose, Sreelakshmi; Price, Michael B.; Ryu, Hwa Sook; Lee, Tack Ho; Shin, Yun Seop; Wu, Ziang; Lee, Woojin; Chen, Kai; Dai, Shuixing; Zhu, Jingshuai; Xue, Peiyao; Zhan, Xiaowei; Woo, Han Young; Kim, Jin Young; Hodgkiss, Justin M.
• Is Part Of: Nano energy. Volume 84(2021)
• Publication Date: 2021-06
• Subjects: Organic solar cells -- Bilayer -- Bulk heterojunction -- Exciton diffusion -- Resonant energy transfer -- Non-fullerene acceptors -- Fused-ring electron acceptors; Nanoscience -- Periodicals; Nanotechnology -- Periodicals; Nanostructured materials -- Periodicals; Power resources -- Technological innovations -- Periodicals; Nanoscience; Nanostructured materials; Nanotechnology; Power resources -- Technological innovations; Periodicals; 621.042
• DOI: 10.1016/j.nanoen.2021.105924
• Languages: English
• ISSNs: 2211-2855

Abstract

Abstract: The development of organic photovoltaic (OPV) cells has long been guided by the idea that excitons – bound electron-hole pairs created by light absorption – diffuse only 5–10 nm. True for many materials, this constraint led to an inherently complex device architecture – the bulk heterojunction – that has obscured our understanding of device physics, and handicapped rational material design. Here, we investigate the photophysics of a series of planar bilayer heterojunction devices incorporating fused-ring electron acceptors with power conversion efficiencies up to 11%. Using ultrafast optical spectroscopy, we demonstrate the importance of long-range layer-to-layer energy transfer in planar structures, isolating this effect by including an insulating layer between the donor and acceptor layers to eliminate charge transfer effects. We show that the slab geometry facilitates substantially longer-range energy transfer than between isolated molecules or small domains. Along with high molecular packing densities, high absorption coefficients, and long exciton diffusion lengths, we show that these effects amount to exciton harvesting length scales that match the light absorption lengths and thereby enable efficient bilayer devices. Our quantitative analysis of bilayer structures also accounts for large domain sizes in bulk-heterojunction devices including fused-ring electron acceptors, and it quantifies the importance of strong resonant spectral overlap is for material selection and design for highly efficient OPVs. Graphical Abstract: ga1 Highlights: Bilayer organic solar cells can be as efficient as their bulk heterojunction counterparts. The photophysics of bilayer devices is fundamentally different to bulk heterojunctions. Long range interlayer energy transfer plays a key role in boosting bilayer device efficiency. Quantifying light absorption, exciton diffusion and energy transfer enables improved device design.