Heat does not become photons. Infrared photons do not stimulate a solar cell. They require visible range or UV photons (photons of much higher energy).
Radiant heat is conversion of thermal energy to photons by blackbody emission (radiance). It is only heat if the photons are absorbed somewhere else and result in conversion to thermal energy.
Blackbody emission is not the only way to produce light.
Heat is the movement of thermal energy, not the thermal energy itself.
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