Improved modeling tools are needed for self-consistent simulation of photon and carrier transport in emerging photonic devices. Here we introduce the method of optical admittances to simplify the calculation of Green's functions and interference effects of energy transport in the recently introduced quantized fluctuational electrodynamics framework. Our approach enables a straightforward analytical method to calculate e.g. the local and nonlocal densities of states in photonic resonators. Furthermore, the resulting wave-optical treatment of emission enhancement and photon recycling can be coupled with drift-diffusion simulations using the so-called interference-exact radiative transfer equations to provide a full-device model of optical and electrical energy transport in arbitrary multilayer structures. We expect the presented framework to enable detailed new studies of emerging photonic devices based on e.g. thin-film technology.