Directed Vertical Diffusion of Photovoltaic Active Layer Components into Porous ZnO‐Based Cathode Buffer Layers

• Published in: Small (Weinheim an der Bergstrasse, Germany) Germany), 2018-04, Vol.14 (15), p.e1704310-n/a
• Main Authors: Kang, Jia‐Jhen, Yang, Tsung‐Yu, Lan, Yi‐Kang, Wu, Wei‐Ru, Su, Chun‐Jen, Weng, Shih‐Chang, Yamada, Norifumi L., Su, An‐Chung, Jeng, U‐Ser
• Format: Article
• Language: English
• Subjects: Buffer layers; Cathodes; Conduction bands; Diffusion; Diffusion layers; Electron mobility; Fullerenes; Nanotechnology; neutron and anomalous X‐ray reflectivity; Photovoltaic cells; polymer solar cells; Polymers; Porosity; scattering contrast variation; Solar cells; vertical composition profile; Zinc oxide; ZnO‐based cathode buffer layer
• ISSN: 1613-6810; 1613-6829
• DOI: 10.1002/smll.201704310

Cathode buffer layers (CBLs) can effectively further the efficiency of polymer solar cells (PSCs), after optimization of the active layer. Hidden between the active layer and cathode of the inverted PSC device configuration is the critical yet often unattended vertical diffusion of the active layer components across CBL. Here, a novel methodology of contrast variation with neutron and anomalous X‐ray reflectivity to map the multicomponent depth compositions of inverted PSCs, covering from the active layer surface down to the bottom of the ZnO‐based CBL, is developed. Uniquely revealed for a high‐performance model PSC are the often overlooked porosity distributions of the ZnO‐based CBL and the differential diffusions of the polymer PTB7‐Th and fullerene derivative PC71BM of the active layer into the CBL. Interface modification of the ZnO‐based CBL with fullerene derivative PCBEOH for size‐selective nanochannels can selectively improve the diffusion of PC71BM more than that of the polymer. The deeper penetration of PC71BM establishes a gradient distribution of fullerene derivatives over the ZnO/PCBE‐OH CBL, resulting in markedly improved electron mobility and device efficiency of the inverted PSC. The result suggests a new CBL design concept of progressive matching of the conduction bands. A methodology of contrast variation with neutron and anomalous X‐ray reflectivity is developed to map the multicomponent depth compositions of an inverted polymer solar cell, from the active layer surface down to the bottom of the porous ZnO–C60 cathode buffer interlayer. Facilitated by size‐selective ZnO–C60 nanochannels, PC71BM diffusion from the active layer builds a gradient distribution throughout the interlayer.