Voltage-Dependent Differential Conductance (dI/dV) Imaging of a Polymer:Fullerene Bulk-Heterojunction

TL;DR

This study uses scanning tunneling spectroscopy to image and analyze the electronic structure of P3HT:PCBM bulk-heterojunctions, revealing nano-domain structures and energy level bending at interfaces.

Contribution

It introduces a method to visualize nano-domains and interface regions in polymer:fullerene BHJs using voltage-dependent dI/dV imaging, providing detailed electronic insights.

Findings

Nano-domain structures observed in dI/dV images.

Energy level bending at the heterojunction interface.

Type-II energy level alignment identified.

Abstract

With scanning tunneling spectroscopy (STS), we probed differential conductance (dI/dV) images of P3HT:PCBM bulk-heterojunctions (BHJs). Since the materials are seen energetically in dI/dV images, the imaging process provides opportunities to view the nano-domains of the components in the BHJ. The images were recorded at different voltages to bring out the interface region between the two materials as well. The density of states (DOS) spectra of pristine materials provided location of energy levels of the polymer and the fullerene in forming energy diagram with a type-II alignment from the view-point of charge carriers. DOS spectra recorded in the P3HT:PCBM heterojunction in addition yielded energies that deviated from those of the components indicating bending of energy levels at the interface region.