Solar hot water and space heating systems constructed of commodity polymers have the potential to significantly reduce the initial cost of solar thermal systems. However, a polymer absorber must be prevented from exceeding its maximum service temperature during stagnation. Here we consider the addition of a thermotropic material to the surface of the absorber. The thermotropic layer provides passive overheat protection by switching from high transmittance during normal operation to high reflectance if the temperature of the absorber becomes too high. In this paper, a one dimensional model of a glazed, flat-plate collector with a polymer absorber and thermotropic material is used to determine the effects of the optical properties of a thermotropic material on optical efficiency and stagnation temperature of the collector. A key result is identification of the reflectance in the translucent state required to provide overheat protection for potential polymer absorber materials. For example, the reflectance of a thermotropic material in the translucent state should be greater than or equal to 51% for a polypropylene absorber which has a maximum service temperature of 115 °C.