We consider a flat panel direct conversion x-ray image detector in which a photoconductor is used to directly convert absorbed x-ray photons to collectable electron hole pairs. The storage capacitor at each pixel stores a quantity of charge proportional to the incident radiation. The charge stored at each pixel is read out every at seconds. Based on a condition for a minimal dynamic constraint, we have examined the corresponding range of constraints on the material properties to establish approximate x-ray photoconductor selection criteria. We consider requirements for radiology and fluoroscopy in which Delta t = 1 and 1/30 s, respectively. When the electrical contacts are totally blocking, the dark current is limited by bulk thermal generation. Minimum band gap E-g requirements depend on the concentration of defects Nd around the Fermi level and their recombination cross section S-r. For single crystal photoconductors with a neutral-defect concentration of the order of similar to 10(14) cm(-3), we need E-g > 1.5 eV for fluoroscopy and E-g > 1.7 eV for radiology. For high purity single crystals, these band gap values can be lower, for example, E-g approximate to 1.42 eV (GaAs) requires N-d < 10(12) cm(-3). In the case of amorphous semiconductors, the dark current is due to the emission of carriers from a distribution of localized states in the energy gap. For a maximum integrated concentration of -10(-6) cm(-3) of localized states around the Fermi level, calculations suggest E-g >1.75 eV for fluoroscopy and E-g >1.9 eV for radiology; amorphous selenium readily satisfies these conditions. When the dark current is controlled to thermal emission from the metal electrode into the semiconductor over a potential barrier phi(B) (i.e., bulk thermal generation of carriers is negligible) then we find that the required Schottky barrier phi(B) can be expressed in terms of the band gap E-g. For Schottky barriers phi(B)similar to 0.6E(g), E-g >1.8 eV for fluoroscopy and >2.0 eV for radiology.