dimensions, stacking thin-film devices in the vertical direction with a 1/1 film/airgap ratio can lead to a practical volumetric power density of more than 1

dimensions, stacking thin-film devices in the vertical direction with a 1/1 film/airgap ratio can lead to a practical volumetric power density of more than 1 kW m−3 (Supplemen- tary Fig. 24), potentially outperforming solar cells,