A radiation detector assembly including an organic photodetector that generate charge in response to an incident radiation, a thin film transistor array including a plurality of pixels. The plurality of pixels may produce electric signals corresponding to the charge generated by the organic photodetector. The radiation detector assembly also includes a spacer disposed on the thin film transistor array. The spacer surrounds one or more pixels and may confine the organic photodetector within the surrounded one or more pixels such that the surrounded one or more pixels are electrically isolated from a neighboring pixel.