Interface Engineering in SubPc/C60 based Organic Photovoltaics.

Abstract

Organic photovoltaic (OPV) devices have the potential to supply a significant portion of global electricity demand within the next 50 years. Though they’re capable of rapid, inexpensive processing, current research-grade devices suffer from low efficiency and lifetime. The relationship between processing, microscopic structure the interface, and device properties is becoming well-defined to the point that suggestions for molecular structure and morphology at the interface may enable commercial viability of OPVs.

Conventionally, bilayer OPV cells involve the deposition of the electron donor layer on top of a transparent anode, with the cathode deposited last. In this work, a comparison is made between conventional (SubPc/C60) and inverted (C60/SubPc) junctions. There is a significant trade-off between the open circuit voltage and short circuit photocurrent, attributed to the formation of a C60/ITO Schottky junction, and a change from exciton-quenching to exciton-blocking behavior of the SubPc:MoOX interface in inverted devices. The interfaces show significant impact deposition order can have on interfaces responsible for encouraging exciton diffusion to a heterojunction.

To probe the influence of molecular dipole on the open circuit voltage (VOC) of molecular heterojunction organic solar cells, axially fluorinated boron subphthalocyanine (SubPc-F) is synthesized and paired with fullerene as an acceptor. The energy levels and structure of the heteromolecular polaron pair are calculated, and a modified ideal organic diode model is presented, successfully reproducing the experimental SubPc-F device characteristics from the SubPc-Cl device fit. The reproducible difference in VOC is attributed to the permanent electric dipole on SubPc molecules effectively lowering the polaron pair binding energy, and thus influencing on polaron pair dynamics at the heterojunction. Importantly, this model has the ability to explain the low electric field dependence seen in some organic photovoltaics, as well as provide a more complete description polaron pair binding energy.

This work suggests a path forward for molecular design, with consideration for molecular orientation within the polaron pair, and incorporation of a permanent electric dipole capable of assisting polaron pair dissociation. The assistance of a dipole during dissociation could be a means of overcoming the tradeoff between high voltage and high current organic photovoltaics.