Margaret Young, John Suddard-Bangsund, Tyler Patrick
Jul 1, 2016
Citations
1
Influential Citations
48
Citations
Journal
Advanced Optical Materials
Abstract
the ligand structures to affect aggregation, crystal structure, and intermolecular proximities. However, once molecules are designed and integrated into optoelectronic devices, their performance typically suffers from arbitrary energy level alignments, resulting in lower-than-ideal open-circuit voltages, low carrier mobilities and diffusion lengths, and limited absorbance past 1000 nm. In this work, we synthesize a new series of heptamethine salts [ 12 ] with the highest occupied molecular orbital (HOMO) levels that can be tuned by varying the anion electronegativity. [ 13 ] These organic salts are used in photovoltaic and photodetector cells to demonstrate photoresponse at deep NIR wavelengths and open-circuit voltages nearing their excitonic limit. Using optical modeling and open-circuit voltage tuning [ 14,15 ] we identify limiting factors for performance and strategies for performance enhancement. Heptamethine salts 1 (1-Butyl-2-(2-3-2-(1-butyl-1H-benzo[cd] indol-2-ylidene)-ethylidene-2-diphenylamino-cyclopent-1-enylvinyl)-benzo[cd]indolium, λ max = 996 nm) and 2 (1-Butyl-2(23-2-(1-butyl-1H-benzo[cd]indol-2-ylidene)-ethylidene-2-phenylcyclopent-1-enyl-vinyl)-benzo[cd]indolium, λ max = 1024 nm) coordinated with the counterions tetrafl uoroborate (BF 4 − ) and tetrakis(pentafl uorophenyl)borate (TPFB − ) are shown in Figure 1 a. We focus on these molecules for their absorption ranges that extend to 1400 and 1600 nm for cations 1 and 2, respectively, (Figure 1 b). Figure 1 c shows a summary of the m / z synthesis verifi cation for the cation and anion masses. In previous studies, weakly coordinating anions like TPFB have been shown to modulate the frontier energy levels of organic cations used as donors in photovoltaic confi gurations, thereby increasing the open circuit voltage ( V OC ) with little or no impact on the bandgap or absorption range. [ 13,16 ]