Anions for Near-Infrared Selective Organic Salt Photovoltaics

Christopher J. Traverse, Margaret Young, John Suddard-Bangsund, Tyler Patrick, Matthew Bates, Pei Chen, Brian Wingate, Sophia Y. Lunt, Annick Anctil, Richard R. Lunt
2017 Scientific Reports  
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
more » ... ndence 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. 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 (C 60 ) acceptor, and thus the open circuit voltage (V oc ) 1-3 . This allows for the fabrication of Cy-based devices that approach the excitonic voltage limit. Because the Cy + cation is primarily responsible for optical absorption, exchanging the anion does not significantly affect the spectral range or magnitude of the extinction coefficient (see Figure S1 ). Absorption is instead tuned via conjugation 4 of the cation molecule which can enable efficient NIR photoresponse at wavelengths of up to 1600nm 3 , ideal for applications in NIR-selective transparent photovoltaics (TPVs) 5,6 . While efficiencies have reached nearly 4% for this class of materials 7 , and properties such as the highest occupied molecular orbital (HOMO) and exciton diffusion have been linked to anion selection 2 , very little is known about the full range of tunability provided by anion pairing alone. In this work, we demonstrate alternative anions from two families, phenyl borates and carboranes, to determine the effects of the electron withdrawing groups and anion size on the performance and energy level alignment of devices utilizing the same Cy + cation. We select anions from the family of phenyl borates because they afford large-sized anions which have previously been correlated with high exciton diffusion lengths (EDLs) 2 . Salt-based devices with tetrakis(pentafluorphenyl)borate (TPFB) anions greatly out-performed equivalent devices with smaller anions due to a high EDL and larger interface gap. 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 also 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. Results and Discussion The phenyl borates investigated include tetraphenylborate (CyPhB), tetrakis(4-fluorophenyl)borate (CyFPhB), tetrakis(4-chlorophenyl)borate (CyClPhB), tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (
doi:10.1038/s41598-017-16539-3 pmid:29180694 pmcid:PMC5703893 fatcat:nqkxwaegxnespghcc4qvs4zuyq