The efficiency of organic light-emitting devices (OLEDs) is often limited by roll-off, where efficiency decreases with increasing bias. In most OLEDs, roll-off primarily occurs due to exciton quenching, which is commonly assumed to be active only above device turn-on. Below turn-on, exciton and charge carrier densities are often presumed to be too small to cause quenching. Using lock-in detection of photoluminescence, we find that this assumption is not generally valid; luminescence can be quenched by >20% at biases below turn-on. We show that this low-bias quenching is due to hole accumulation induced by intrinsic polarization of the electron transport layer (ETL). Further, we demonstrate that selection of nonpolar ETLs or heating during deposition minimizes these losses, leading to efficiency enhancements of >15%. These results reveal design rules to optimize efficiency, clarify how ultrastable glasses improve OLED performance, and demonstrate the importance of quantifying exciton quenching at low bias.

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

and the inherent trade-off between the achieving a high V OC , by maximizing the interface gap, and effi ciently dissociating excitons, by maximizing donoracceptor lowest unoccupied molecular orbitals (LUMO) offset energies. With smaller bandgaps, it thus becomes more challenging to identify suitable acceptor pairings due to the heightened tolerance for optimal energy level alignment. For example, tin phthalocyanine (SnPc) and lead phthalocyanine (PbPc), which both have absorption cut-offs near 1000 nm, exhibit V OC 's of only 0.42 and 0.47 V, respectively, under 1 sun illumination, [ 5,6 ] nearly half of the realistic excitonic voltage limit. [ 7 ] Because of this voltage limitation and limited spectral coverage, current demonstrations of transparent organic photovoltaics (TPVs) with high transparency have been limited to 2%-4%. [ 8,9 ] To address this challenge and expand the catalog of effi cient selectively NIR harvesting molecules, we demonstrate a molecular design technique for sensitive energy level control to allow for the rapid realization of novel donor-acceptor pairings with both effi cient NIR response and high voltage near the excitonic limit. This demonstration can ultimately enhance the performance of panchromatic tandem cells, and single-and multijunction transparent photovoltaics.Polymethines, or cyanines (Cy), are a highly promising class of molecules that can satisfy the need for effi cient, NIRselective harvesting for transparent photovoltaics. They are a class of ionic organic salts that have gained research attention for applications in photodetectors, [ 10 ] all-optical switching, [ 11 ] transparent luminescent concentrators, [ 12 ] and photovoltaic applications due to their versatile chemistry and outstanding photophysical properties. [ 13,14 ] Notably, polymethines feature tunable absorption, high extinction coeffi cients, high solubility, and suppressible oscillator strength in the visible part of the solar spectrum. Recently, polymethines with absorption in the near-infrared have been employed in OPV devices with effi ciencies of 0.9%-2.8% for opaque devices and semitransparent devices. [ 15,16 ] Despite these promising achievements, the performance of polymethine OPVs is still constrained by V OC 's far below the A new series of organic salts with selective near-infrared (NIR) harvesting to 950 nm is reported, and anion selection and blending is demonstrated to allow for fi ne tuning of the open-circuit voltage. Extending photoresponse deeper into the NIR is a signifi cant challenge facing small molecule organic photovoltaics, and recent demonstrations have been limited by open-circuit voltages much lower than the theoretical and practical limits. This work presents molecular design strategies that enable facile tuning of energy level alignment and open-circuit voltages in organic salt-based photovoltaics. Anions are also shown to have a strong infl uence on exciton diffusion length. These insights provide a clear route toward achieving high effi ciency transparent and panchromatic pho...

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