Photocurrent Generation in Bulk vs Bilayer Devices: Quantum Treatment of Model Rubrene/7,7,8,8-Tetracyanoquinodimethane Heterojunctions for Organic Solar Cells

Abstract

The primary kinetic processes leading to photocurrent generation in rubrene/7,7,8,8-tetracyanoquinodimethane (rubene/TCNQ) heterojunctions are investigated using a combination of quantum-chemical methods, Marcus nonadiabatic electron-transfer theory, and Onsager–Braun model for charge separation. Charge-transfer (CT), -recombination (CR), and -separation (CS) rates are obtained for heterodimers representative of two device models: single-crystal planar bilayer, in which crystal orientation is preserved and rubrene’s fused π-system is sterically hindered, and bulk-heterojunctions (BHJs), where donor and acceptor molecules approach cofacially with the π-system fully exposed. Results point to low geminate pair recombination due to higher donor–acceptor separation in crystalline bilayers, while maintaining ultrafast CT (∼109 s–1). Moreover, HOMO–LUMO coupling is an order of magnitude higher in cofacial orientation, leveraging CR in BHJs for which kCR ∼ 106 s–1 and kCT ∼ 109 s–1. This work provides a molecular...