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

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 pi-system is sterically hindered, and bulk-heterojunctions (BHJs), where donor and acceptor molecules approach cofacially with the pi-system fully exposed. Results point to low geminate pair recombination due to higher donor-acceptor separation in crystalline bilayers, while maintaining ultrafast CT (similar to 10(9) s(-1)). Moreover, HOMO-LUMO coupling is an order of magnitude higher in cofacial orientation, leveraging CR in BHJs for which k(CR) similar to 10(6) s(-1) and k(CT) similar to 10(9) s(-1). This work provides a molecular perspective rationale for the high photoresponse reported for rubrene/TCNQ single-crystal bilayer interfaces.