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• Propose topology optimization formulation of thermophotonic problem for daytime passive radiative cooling. • Derive sensitivity of thermophotonic problem through adjoint method. • Propose density gradient based surface slope constraints for layer-wise design. • Demonstrate coupling effects between heat conduction, convection and radiation. Passive radiative cooling during the daytime requires efficient control of energy exchange in heat conduction, convection and radiation in the cooling system. In the paper, topology optimization formulations are proposed to design thermophotonic materials for passive radiative cooling. The first formulation solves pure photonic problem to achieve ideal absorption coefficients for radiation cooling. In the second formulation, we solve the coupled thermophotonic problem to maximize heat flux from the thermal load for efficient control of heat conduction, convection and radiation in passive radiative cooling. For better manufacturability, density gradient based boundary slope constraints are proposed to achieve layer-wise design. Sensitivity is derived through the adjoint method. While the objective function in the thermophotonic problem is independent of the density field, a derivation strategy is suggested to reduce the computational cost in sensitivity analysis. Numerical results demonstrate that passive radiative cooling can be achieved through topology optimization of thermophotonic materials. [ABSTRACT FROM AUTHOR]