A conventional CCD consists of a sandwich of semiconductor layers overlaid with a network of "gates", electrodes which serve to transfer charge from one pixel to another as the device is read out. At low energies (E<1>5 keV). due to photon loss through the front of the back-illuminated device.
Comparison of QE of front- and back-illuminated CCDs. The lack of gate structure at the exposed surface of the back-illuminated device, combined with its reduced physical thickness, results in improved QE for energies <3>5 keV), due to photon loss through the front of the back-illuminated device.
copied from http://cxc.harvard.edu/newsletters/news_05/node11.html
CCD detectors are not able to convert all of the photons that strike the surface into electrons for a variety of reasons. Quantum efficiency (QE), which describes the ability of the CCD to turn photons into a useful form of output, is basically the ratio of incoming photons to those photons actually detected by the CCD. The typical range of efficiency is from a few percent up to 90%. Efficiency will also vary with the frequency of light (color) observed. Front-illuminated thick chips are not as sensitive to blue light as they are to light with longer wavelengths.