Polymer solar cells have been shown to degrade under X-rays. Here, in situ polymer photovoltaic performance and recombination lifetimes are measured and it is found that charge accumulation is the primary reason for degradation of solar cells. This is affected by the mixing ratio of donor and acceptor in the bulk heterojunction. Both a quantitative understanding and the physical model of the degradation mechanism are presented. Understanding of the degradation mechanism is extended in polymer donor–acceptor bulk heterojunction systems to propose a material combination for making radiation hard diodes that can find important application in fields ranging from memory arrays to organic X-ray detectors for medical imaging.