The influence of increasing the i-layer width of thick PIN a-Si:H diodes on the efficiency of X-ray detectors is analyzed. It is demonstrated that increasing the i-layer width to increase the interaction probability of the X-rays with the detector requires higher bias to achieve satisfactory collection efficiency. This higher bias produces, at the same time, an increase in the dark current that substantially decreases the overall detection efficiency. The use of complementary metal conversion layers is discussed and recommended and a Cr-a-Si:H PIN X-ray detector is fabricated and characterized. Experimental results are presented to validate expected behavior and recommended approach to increase the overall efficiency.