Chemical-potential locking by inter-junction radiative coupling and gap-dependent junction-order inversion in near-field multijunction thermoradiative devices

Thermoradiative (TR) diodes convert low-grade heat into electricity through their own net emission, and two routes have been proposed to raise their output: placing a receiver in the near field and stacking junctions with different band gaps. Multijunction TR analyses so far assume far-field exchange and junctions that do not exchange photons with each other, while near-field multijunction converter models resolve absorption in each subcell but not the emission of subcells at nonzero chemical potential. In a TR stack, however, all junctions share the emitter temperature and differ only in their photon chemical potential, so their mutual near-field exchange can couple their operating points. Modeling this requires layer-resolved emission from every absorbing layer, separation of interband from free-carrier absorption, wave-vector integration up to the inverse spacer thickness, and a two-dimensional voltage optimization. We develop such a model for a HgCdTe/InSb stack at 550 K facing a receiver at 300 K and validate it against published near-field, TR-device, and multijunction results. Inter-junction exchange locks the optimal chemical potentials together, to |μ_f − μ_b| = 10^−3 k_B T_h against 0.93 k_B T_h without it, and lowers the maximum power by factors of 14-55 for a 1-nm spacer. A ratio of inter-junction to junction-receiver radiative conductance predicts the decoupling spacer thickness, 19-69 nm for gaps of 10-100 nm. Wave-vector-resolved transmission maps show how the gap-facing junction screens the buried one, the preferred junction order inverts with gap, and the near field raises the external radiative efficiency of the gap-facing junction 18-fold. In the radiative limit a single junction of equal active thickness always delivers more power; with Auger and Shockley-Read-Hall processes, a thin decoupled stack with the narrow-gap junction in front delivers 51% more power and 74% higher efficiency at a 10-nm gap. In the radiative limit locked junctions lose less than 2% in series or parallel connection, whereas with Auger and SRH processes decoupled junctions lose up to 68% in parallel connection. These results establish inter-junction coupling as a controlling mechanism in near-field multijunction TR devices and give quantitative rules for spacer thickness, junction order, thickness, and electrical connection.