Abstract
Organic molecular salts are an emerging and highly tunable class of materials for organic and transparent photovoltaics. In this work, we demonstrate novel phenyl borate and carborane-based anions paired with a near-infrared (NIR)-selective heptamethine cation. We further explore the effects of anion structures and functional groups on both device performance and physical properties. Changing the functional groups on the anion significantly alters the open circuit voltage and yields a clear dependence on electron withdrawing groups. Anion exchange is also shown to selectively alter the solubility and film surface energy of the resulting molecular salt, enabling the potential fabrication of solution-deposited cascade or multi-junction devices from orthogonal solvents. This study further expands the catalog and properties of organic salts for inexpensive, and stable NIR-selective molecular salt photovoltaics.
Highlights
Anion exchange with near-infrared (NIR)-selective cyanine (Cy+) heptamethine cations enables facile tuning of the frontier orbital energies, the interface gap between the salt donor and fullerene (C60) acceptor, and the open circuit voltage (Voc)[1,2,3]
Photovoltaic devices were fabricated in the following architecture (Fig. 1c): indium tin oxide (ITO) (120 nm) / MoO3 (10 nm) / CyX (y nm) / C60 (40 nm) / bathocuproine (BCP) (7.5 nm) / Ag (80 nm), where X is the anion paired with Cy+ and y is the donor layer thickness
Cy layer thicknesses were controlled by varying the solution concentrations between 2–12 mg/mL to determine the effects of energy band bending on the interface gap
Summary
Anion exchange with near-infrared (NIR)-selective cyanine (Cy+) heptamethine cations enables facile tuning of the frontier orbital energies, the interface gap (the difference in the donor highest occupied molecular orbital and the acceptor lowest unoccupied molecular orbital) between the salt donor and fullerene (C60) acceptor, and the open circuit voltage (Voc)[1,2,3] This allows for the fabrication of Cy-based devices that approach the excitonic voltage limit. Carboranes are highly stable carbon-boron molecular clusters that have emerged recently for use as superacids[8,9] and have been shown to serve as extremely low coordinating anions for use in organic salts, with demonstrated applications in fabricating molecular wires[10] and electrochemical capacitors[11] They have been employed in manipulating the band gap of emission layers to tune the emission color of phosphorescent organic light emitting diodes[3,12]. Because of their low coordination, these anions may yield a path toward exceptionally stable organic salt PVs
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