On the Design of Host–Guest Light‐Emitting Electrochemical Cells: Should the Guest be Physically Blended or Chemically Incorporated into the Host for Efficient Emission?

Abstract

It has recently been demonstrated that light‐emitting electrochemical cells (LECs) can be designed to deliver strong emission with high efficiency when the charge transport is effectuated by a majority host and the emission is executed by a minority guest. A relevant question is then: should the guest be physically blended with or chemically incorporated into the host? A systematic study is presented that establishes that for near‐infrared‐(NIR‐) emitting LECs based on poly(indacenodithieno[3,2‐b]thiophene) (PIDTT) as the host and 4,7‐bis(4,4‐bis(2‐ethylhexyl)‐4H‐silolo[3,2‐b:4,5‐b′]dithiophen‐2‐yl)benzo[c][1,2,5]‐thiadiazole (SBS) as the guest the chemical‐incorporation approach is preferable. The host‐to‐guest energy transfer in LEC devices is highly efficient at a low guest concentration of 0.5%, whereas guest aggregation and ion redistribution during device operation severly inhibits this transfer in the physical‐blend devices. The chemical‐incorporation approach also results in a redshifted emission with a somewhat lowered photoluminescence quantum yield, but the LEC performance is nevertheless very good. Specifically, an NIR‐LEC device comprising a guest‐dilute (0.5 molar%) PIDTT‐SBS copolymer delivers highly stabile operation at a high radiance of 263 µW cm−2 (peak wavelength = 725 nm) and with an external quantum efficiency of 0.214%, which is close to the theoretical limit for this particular emitter and device geometry.

Near‐Infrared Emission by Tuned Aggregation of a Porphyrin Compound in a Host–Guest Light‐Emitting Electrochemical Cell

“…For the electrolyte in the active material, we selected to employ the ionic liquid tetrahexylammonium tetrafluoroborate (THABF 4 , mass concentration = 5%), because of its broad electrochemical stability window is expected to inhibit non-desired electrolyte-induced side reactions. [42] The LEC devices were fabricated in an indium-tin-oxide (ITO)/poly (3,4- We find that all three device types feature a decreasing voltage and an increasing radiance during the initial operation at a constant current density of 75 mA cm −2 . This is the characteristic behavior of a well-behaved LEC device that features in situ conductivity-and injection-enhancing p-type and n-type doping at the two electrode interfaces, [43] and this observation thus yields further support to our previous conclusion that all three polymer hosts are functional LEC materials that can be both p-and n-type doped; see Figure 5 and related discussion.…”

“…For the electrolyte in the active material, we selected to employ the ionic liquid tetrahexylammonium tetrafluoroborate (THABF 4 , mass concentration = 5%), because of its broad electrochemical stability window is expected to inhibit non‐desired electrolyte‐induced side reactions. [ 42 ] The LEC devices were fabricated in an indium‐tin‐oxide (ITO)/poly(3,4‐ethylenediophene):poly(styrene‐sulfonate) (PEDOT:PSS)/host:guest:THABF 4 /Al configuration, and Figures a–c present the temporal evolution of such representative pristine devices with a) F8BT, b) PBDTSi‐BDD, and c) PBDTSi‐BDD‐Py as the host polymer.…”

Highly Stable Indacenodithieno[3,2-b]thiophene-Based Donor–Acceptor Copolymers for Hybrid Electrochromic and Energy Storage Applications

“…Spray-coated polymer films tend to exhibit a rougher morphology than their spin-coated counterparts, and the spray-coated films may accordingly facilitate better ingress of the electrolyte ions for improved charge transport and stabilization of the oxidized polymer. 60 , 64 , 77 , 78 This is observed as improved kinetics, particularly in the case of the spray-coated PIDTT–TBzT film. The same is true for PIDTT–TBT, which, however, did not show any obvious optical sign of charge trapping neither in the spin-coated nor spray-coated films.…”

“…Conjugated polymers incorporating extended thiophene-based structures, such as the rigid and planar indacenodithiophene (IDT) and indacenodithieno­[3,2- b ]­thiophene (IDTT) backbones, are known for their high absorption coefficients and excellent electrochemical stabilities and charge transport kinetics, which have made them attractive for use as photoactive materials in OPVs, − organic photodetectors (OPDs), , organic field-effect transistors (OFETs), , organic light-emitting diodes (OLEDs), and light-emitting electrochemical cells (LECs) , and recently in electrochromic devices , and hybrid electrochromic supercapacitors . However, significant improvements to the combined optical contrast and charge storage performance are needed for the realization of hybrid electrochromic and energy storage applications.…”

BODIPY‐Pt‐Porphyrins Polyads for Efficient Near‐Infrared Light‐Emitting Electrochemical Cells

“…[ 11,17 ] In this context, the chemical binding of host–guest units in a dyad‐type structure is desired over simple physical blending toward stable and efficient LECs. [ 11,15,29 ] For instance, we reported on intramolecular host–guest systems binding 1 BODIPY (BDP) unit to the periphery of a Zn‐porphyrin that yielded deep‐red LECs with one order of magnitude enhanced stabilities (>1000 h) keeping the low irradiance of the reference devices with the Zn‐porphyrin reference. [ 11 ] Edman and coworkers reported on the design and synthesis of a host–guest star‐shaped diketopyrrolopyrrole−Zn‐porphyrin, realizing NIR emission (≈900 nm) along with good irradiances of 36 μW cm −2 and stabilities of tens of minutes.…”