Abstract
AbstractTwo dipolar merocyanines consisting of the same π‐conjugated chromophore but different alkyl substituents adopt very different packing arrangements in their respective solid state with either H‐ or J‐type exciton coupling, leading to ultranarrow absorption bands at 477 and 750 nm, respectively, due to exchange narrowing. The social self‐sorting behavior of these push‐pull chromophores in their mixed thin films is evaluated and the impact on morphology as well as opto‐electronical properties is determined. The implementation of this well‐tuned two‐component material with tailored optical features allows to optimize planar heterojunction organic photodiodes with fullerene (C60) with either dual or single wavelength selectivity in the blue and NIR spectral range with ultranarrow bandwidths of only 11 nm (200 cm−1) and an external quantum efficiency of up to 18% at 754 nm under 0 V bias. The application of these photodiodes as low‐power consuming heart rate monitors is demonstrated by a reflectance‐mode photoplethysmography (PPG) sensor.
Highlights
Two dipolar merocyanines consisting of the same π-conjugated chromophore but different alkyl substituents adopt very different packing arrangements in their respective solid state with either H- or J-type exciton coupling, leading to ultranarrow absorption bands at 477 and 750 nm, respectively, due to exchange narrowing
The hole mobility of the mixed donors only varies between 1 × 10−5 and 10 × 10−5 cm2 V−1 s−1 for mixing ratios between 100:0 and 25:75, but significantly increases up to over 3 × 10−3 cm2 V−1 s−1 for pristine 1(Hex) layers (0:100). While these findings demonstrate decent charge transport in both neat and mixed layers, they do not completely explain the observed trend in EQEmax values of organic photodiodes (OPDs) devices as the lateral conductivity across 100 μm in an organic thin-film transistors (OTFTs) presumably cannot be directly compared to the vertical charge transport across an ≈ 10 nm thick donor layer in an OPD
By processing mixed thin films of the two donor materials 1(Pyrl) and 1(Hex), which individually exhibit prominent ultranarrow bandwidth H- and J-type bands in the solid state, respectively, it could be demonstrated that up to 40% of 1(Hex) molecules can be socially incorporated into the H-type coupled packing arrangement of 1(Pyrl)
Summary
Both push-pull dyes 1(Pyrl) and 1(Hex) are composed of the same 2-[4-(tert-butyl)thiazol-2(3H)-ylidene]malononitrile acceptor (A) and a 2-aminothiophene donor (D) unit and vary only in their alkyl substituents. For 1(Hex) the long and flexible hexyl groups induce a head-to-tail arrangement with predominant J-type coupling, as proven for similar chromophores, inducing a significantly bathochromically shifted absorption band to 750 nm with a FWHM of 19 nm (340 cm−1).[22,30] While a zig–zag arrangement, that is, non-ideal J-type packing arrangement with an additional weak H-band, was found for the related 1(Pr)[30] derivative, a sharper and more distinct J-band (and lack of an H-band) is observed for 1(Hex) Intrigued by these two distinct blue- and NIR-wavelength selective optical signatures of two molecularly such similar dyes, we were motivated to elucidate the impact of molecular interactions in mixed thin-films on optical properties and device performances. An enhanced intensity of the H-band is observed up to a ratio of 84% 1(Hex) and only for larger amounts of 1(Hex) the ratio of H- and J-type domains follows an ideal narcissistic behavior (i.e., phase separation) in which both dyes self-assemble into separated domains
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