Journal Article Research Support, Non-U.S. Gov't Research Support, U.S. Gov't, Non-P.H.S. X‐ray detector absorption difference anode flexible substrate image array large‐area material identification metal halide perovskite mixed-valence multispectral X‐ray imaging mutual excitation quantum‐cutting stacked scintillators soft X‐ray sub‐cell matching model sulfur doping supercapacitors tandem solar cells thick organic films vanadium oxide