Metal halide perovskites have arisen as a new family of semiconductors for room-temperature ionization radiation detectors due to their high stopping power, large and balanced electron-hole mobility-lifetime (mu-tau) product, and tunable bandgap. These materials are low-cost to make using solution process. The mu-tau of some perovskites are already close or superior to best CZT crystals, while the best resolution of gamma-ray spectrum does not catch up yet, most likely due to the ion migration and self-doping behavior in these materials. Here, we report a simple approach of additive-assisted inverse temperature crystallization (ITC) solution method to grow low-defect density single crystal. We also report our recent progress in developing flexible polycrystalline films for X-ray imaging with performance comparable to those of single crystals while they are much simpler to scale up to commercial relevant size.