Figure 1a shows the absorbance spec

Figure 1a shows the absorbance spectra as well as chemical structures of the chosen active layer materials, i.e., the pristine polyindenoflurorene-8-triarylamine (PIF) and its (1:1) blends with the NFAs IDFBR and ITIC-4F. It can be observed that the contribution of PIF to the layer absorption is limited to the UV range of the spectrum, while the absorption range in the visible region is defined solely by the NFA. As observed in Figure 1b, this absorption governs the spectral responsivity of the OPDs under operation and thus, the extent of their color selectivity. At a bias voltage of 8 V, the spectral responsivity of the PIF:IDFBR devices reaches 72 mA W 1 and ranges from 450 to 600 nm. The device containing ITIC-4F achieves 229 mA W 1 and mainly absorbs above 600 up to 800 nm, reaching the NIR wavelength window. The respective responsivities are comparable or even outmatch previously demonstrated color-selective photodiodes based on organic or perovskite active layers demonstrating the potential of these nonconventional BHJ systems (see Table S1 in the Supporting Information).[6] A photograph of the inkjet-printed multicolor array with red and blue OPDs is shown in the inset of Figure 1b. The blue and red colors of the devices originate from the respective red and blue light absorption of PIF:ITIC-4F and PIF:IDFBR. Figure 1c depicts the viscosity of the pristine PIF ink formulation to which the NFAs were added at a 1:1 ratio. It can be observed that upon addition of the NFAs, the viscosity of the solution is minimally altered within less than 5% of the PIF value whereas the color change in the solutions is clearly visible. This feature allowed a simplified process development where the printing parameters are developed only for the pristine PIF ink and transferred without additional adjustment to the color-selective inks (see Figure S1 in the Supporting Information). This effective decoupling of the optical characteristics from the ink properties enables considerable reduction of processing complexity and enhanced spectral versatility as the ink-formulation only needs to be carried out once for various color-selective OPDs. Furthermore, the presented approach can be transferred to other transparent polymer donors and NFA combinations as shown in Figure S2 (Supporting Information). Generally, the functionality of BHJ systems does not only rely on strong and selective absorption of the active layer but also on an efficient exciton separation and charge extraction at the electrodes. Both are assisted by a favorable intermixed morphology.[29] We assume that the energetic alignment requires a HOMO energy offset (ΔEHOMO) between the PIF and the used NFAs to facilitate successful exciton separation and hole injection from the NFAs to PIF. The energy levels of the three materials are schematically displayed in Figure 2a. Previous studies report HOMO levels of 5.5, 5.7, and 5.8 eV for PIF,[30] ITIC-4F,[31] and IDFBR,[32] respectively, which would result in ΔEHOMO of 0.2 eV for PIF:ITIC-4F and 0.3 eV for PIF:IDFBR.