Strontium aluminate can be doped with europium to give a material with properties like you are expecting. This can be optimized to give maximum fluorescent yield as well, but keep in mind this will likely shift the wavelength.
The material must heat and cool in this process since absorption of light will not be 100% efficient and neither will the loss of light (some light may be emitted towards the material rather than outwards.). Most heat production should occur during the "charging" phase of the cycle though since the heat produced will be due to the transition from high energy levels to lower but still excited energy levels by vibrational loss of energy.
I am unsure what your application is, but here is a paper on the synthesis of these materials.