Effect of Fabrication Parameters on Three-Dimensional Nanostructures of Bulk Heterojunctions Imaged by High-Resolution Scanning ToF-SIMS

Abstract

Solution processable fullerene and copolymer bulk heterojunctions are widely used as the active layers of solar cells. In this work, scanning time-of-flight secondary ion mass spectrometry (ToF-SIMS) is used to examine the distribution of [6,6]phenyl-C61-butyric acid methyl ester (PCBM) and regio-regular poly(3-hexylthiophene) (rrP3HT) that forms the bulk heterojunction. The planar phase separation of P3HT:PCBM is observed by ToF-SIMS imaging. The depth profile of the fragment distribution that reflects the molecular distribution is achieved by low energy Cs(+) ion sputtering. The depth profile clearly shows a vertical phase separation of P3HT:PCBM before annealing, and hence, the inverted device architecture is beneficial. After annealing, the phase segregation is suppressed, and the device efficiency is dramatically enhanced with a normal device structure. The 3D image is obtained by stacking the 2D ToF-SIMS images acquired at different sputtering times, and 50 nm features are clearly differentiated. The whole imaging process requires less than 2 h, making it both rapid and versatile.

Molecular dynamic‐secondary ion mass spectrometry (D‐SIMS) ionized by co‐sputtering with C60+ and Ar+

“…Co‐sputtering with a 10 kV C 60 + and a 0.2 kV Ar + beam is known to preserve the chemical structure of the remaining surface for various organic/polymeric materials . Using the co‐sputtering beam as the ionization source, molecular D‐SIMS spectra were also acquired (Fig.…”

“…The apparatus is based on a PHI 5000 VersaProbe (ULVAC‐PHI, Chigasaki, Japan) scanning X‐ray microprobe system operated at a base pressure of <1 × 10 –7 Pa, which was achieved by evacuation using ion getter pumps. The Ar + ion source was operated at 0.2 kV and 300 nA (measured on an Au target) using a floating voltage of 500 V. A voltage setting of 0.2 kV has been previously found to preserve the chemical structure of the remaining surface for various organic/polymeric materials while yielding a more stable sputtering rate . The ray path contained a 1° bend to filter fast Ar atoms.…”

“…Throughout the study of these cluster ion sources, buckminsterfullerene (C 60 + ) has received significant attention since it became a commercially available product in 2003 . Using surface analysis techniques and C 60 + ion sputtering, biological/medical and organic electronic materials have been widely studied.…”

“…This was feasible because the auxiliary Ar + beam could suppress the carbon deposition and/or remove graphitized materials through the removal of the top surface . To date, this co‐sputtering technique has only been used as a slicing tool that removes the outermost surface, while the newly exposed surface is analyzed using other means (namely, X‐ray photoelectron spectrometry and scanning probe microscopy). In previous studies, the sputtered materials were not analyzed, making it unclear how the auxiliary Ar + beam affected the mass distribution of secondary ions and whether the co‐sputtering technique could serve as an ionization source for the dynamic SIMS (D‐SIMS) analysis of organic materials.…”

3D In Situ ToF‐SIMS Imaging of Perovskite Films under Controlled Humidity Environmental Conditions

“…More recently, ToF‐SIMS equipped with in situ cluster ion sputtering, was used to study the depth‐dependent distribution of molecules in organic light‐emitting diodes (OLEDs) and their degradation mechanisms. In combination with the high lateral resolution of the scanning probe microscopy, cluster ion sputtering was also used to construct the 3D images of nanostructures at bulk heterojunctions . Recent studies have shown by using ToF‐SIMS that ion migration could be one of the degradation mechanisms …”