Organic Heptamethine Salts for Photovoltaics and Detectors with Near‐Infrared Photoresponse up to 1600 nm

Abstract

1028 wileyonlinelibrary.com COMMUNICATION 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.Heptamethineindolium, 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 ] Solar cell devices with the structure indium tin oxide (ITO)/10 nm MoO 3 / t nm salt/40 nm C 60 /7.5 nm bathocuproine (BCP)/80 nm Ag were prepared using the four salts as a function of thickness ( Figure 2 a). Donor layers of each organic salt were spin-coated from N,N -dimethylformamide under nitrogen while other layers were thermally deposited in vacuum. The thickness for each salt was controlled by varying the solution concentration. For comparison purposes, the J -V and EQE for devices with similar salt thicknesses (12 ± 1 nm) are plotted in Figure 2 b,c and average performance metrics are shown in Table 1 . The fi ll factors (FF) for these devices, 0.3-0.5, are slightly lower than our previous demonstrations with larger bandgap organic salts due to decreased shunt resistances from the lower bandgap and series resistance from a potential interface barrier between the donor and MoO 3 . The exchange of BF 4 for TPFB nearly doubles the V OC from 0.13 to 0.33 V for cation 1 and 0.17 to 0.25 V for cation 2. This enhancement in

Polymer:Fullerene Bimolecular Crystals for Near‐Infrared Spectroscopic Photodetectors

“…Furthermore, metal–metal cavity photodetectors based on the P3HT:PCBM system as well as on the poly[3‐(5‐(4‐(2‐ethylhexyl)‐4H‐dithieno[3,2‐b:2′,3′‐d]pyrrol‐2‐yl)thiophen‐2‐yl)‐2,5‐bis(2‐hexyldecyl)‐6‐(thiophen‐2‐yl)‐2,5‐dihydropyrrolo[3,4‐c]pyrrole‐1,4‐dione]:1,1′,3,3′,8,8′,10,10′‐octaoxo‐2,2′,9,9′‐tetra(undecan‐6‐yl)‐1,1′,2,2′,3,3′,8,8′,9,9′,10,10′‐dodecahydro‐[5,5′‐bianthra[2,1,9‐def:6,5,10‐d′e′f′]diisoquinoline]‐13,13′‐dicarbonitrile (PDPPTDTPT:SdiCNPBI) blend are fabricated. For the latter, with a low E CT (≈ 0.7 eV), detection wavelengths are easily extended to a remarkable range of 1000–1700 nm (details in Sections SI‐3 and SI‐4, Supporting Information), which is among the longest detection wavelengths reported for organic photodetectors . However, due to the absence of intercalation, both blend systems result in photodetectors with low EQE (<10%), agreeing well with the results shown in Figure b.…”

“…By varying the combination of donor and acceptor materials, the CT absorption can be easily tuned up to 1700 nm, as we demonstrated with the PDPPTDTPT:SdiCNPBI system (details in Section SI‐4, Supporting Information). Tunability over a range of 700 nm demonstrates the potential of organic materials for wavelength‐selective photodetection far in the NIR . More importantly, our novel approach for extending the photodetection range is considerably simpler than other conventional methods that require the development of new absorbers with very low optical gaps, for which then a suitable acceptor is to be found.…”

Modulating cellular cytotoxicity and phototoxicity of fluorescent organic salts through counterion pairing

“…Reaction yield and purity of CyCoCB was similar to that of the other salts discussed here 30,31 . Immediately after irradiation, the media was replenished with fresh dye-laced media and allowed to incubate for another 24 hours.…”

Electronic Tuning of Mixed Quinoidal‐Aromatic Conjugated Polyelectrolytes: Direct Ionic Substitution on Polymer Main‐Chains

“…Thei AQM-based CPEs so generated show absorbances reaching into the NIR, displaying the efficacy of combining the donor-acceptor strategy with the BLA reduction strategy in the design of conjugated polymeric materials. [26,32,33]…”