title: New insights into morphology of high performance BHJ photovoltaics revealed by high resolution AFM.
abstract: Direct imaging of the bulk heterojunction (BHJ) thin film morphology in polymer-based solar cells is essential to understand device function and optimize efficiency. The morphology of the BHJ active layer consists of bicontinuous domains of the donor and acceptor materials, having characteristic length scales of several tens of nanometers, that reduces charge recombination, enhances charge separation, and enables electron and hole transport to their respective electrodes. Direct imaging of the morphology from the molecular to macroscopic level, though, is lacking. Though transmission electron tomography provides a 3D, real-space image of the morphology, quantifying the structure is not possible. Here we used high-resolution atomic force microscopy (AFM) in the tapping and nanomechanical modes to investigate the BHJ active layer morphology that, when combined with Ar(+) etching, provided unique insights with unparalleled spatial resolution. PCBM was seen to form a network that interpenetrated into the fibrillar network of the hole-conducting polymer, both being imbedded in a mixture of the two components. The free surface was found to be enriched with polymer crystals having a "face-on" orientation and the morphology at the anode interface was markedly different.
authors.affiliation: WPI-Advanced Institute for Materials Research (WPI-AIMR), Tohoku University , 2-1-1 Katahira, Aoba, Sendai 980-8577, Japan.
authors.forename: Dong
authors.initials: D
authors.lastname: Wang
authors.forename: Feng
authors.initials: F
authors.lastname: Liu
authors.forename: Noritoshi
authors.initials: N
authors.lastname: Yagihashi
authors.forename: Masafumi
authors.initials: M
authors.lastname: Nakaya
authors.forename: Sunzida
authors.initials: S
authors.lastname: Ferdous
authors.forename: Xiaobin
authors.initials: X
authors.lastname: Liang
authors.forename: Atsushi
authors.initials: A
authors.lastname: Muramatsu
authors.forename: Ken
authors.initials: K
authors.lastname: Nakajima
authors.forename: Thomas P
authors.initials: TP
authors.lastname: Russell
keywords: Atomic Force Spectroscopy; Mechanical Properties; Morphology; Organic Photovoltaic; X-ray Scattering