Bridging the Gap between Solar Cells and Batteries: Optical Design of Bifunctional Solar Batteries Based on 2D Carbon Nitrides

Abstract

While solar cell technology is booming, intermittent availability of sunlight motivates new vistas for multifunctional devices capable of energy capture and storage on the same material, i.e., direct or two‐electrode bifunctional solar batteries. Herein, simulations and experiments are utilized to take a closer look at efficiency limitations and design considerations, and guidelines are proposed to operate a solar battery comprised of the 2D carbon nitride potassium poly(heptazine imide), K‐PHI, as a bifunctional solar battery photoanode in conjunction with the separator poly(N‐vinylcarbazole) and cathode poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate. An optical design of this device is developed by proposing light absorption in a charge collection layer within the photoanode and calculating photocharging current and charging time as figures of merit. The much larger efficiency of operation via rear illumination for K‐PHI layer thicknesses >200 nm is highlighted and enhancement strategies without modifying the photoactive layer are proposed. Finally, adapted Ragone plots are introduced and it is shown how the solar batteries are capable of improving energy and charge output solely via illumination (for the design under 1 sun, the energy and charge output increase by 60% and 63%, respectively) without modifying the device.

Integrated Solar Batteries: Design and Device Concepts

“…However, the large oxidative power comes with the risk of easier degradation of the electrolyte. For readers interested in electrolyte decomposition in solar battery concepts, we recommend reviews discussing this effect in more detail. , In addition, the illumination pathway of very thick battery electrodes with intrinsic short carrier diffusion lengths presents conceptional difficulties: a thick layer is beneficial for capacity but detrimental for light absorption at the junction where the photocurrent is produced …”

“…An example is illumination area vs mass: Solar cell performance scales with area and generally requires thin films, while battery capacity scales with mass (Figure a). We have recently introduced a form factor ([g cm –1 ]) to account for the device’s geometry, which could be a viable strategy to compare different solar battery designs while also allowing normalization for both area and mass …”

“…We have recently introduced a form factor ([g cm −1 ]) to account for the device's geometry, which could be a viable strategy to compare different solar battery designs while also allowing normalization for both area and mass. 29 Examples of both IEC and VEC designs are shown in Figure 6. A straightforward example for IEC designs are threeelectrode two-device designs that we mentioned in the introduction: A solar cell is connected in parallel to the battery and thus provides a constant photocurrent charging the battery.…”

“…12,14 In addition, the illumination pathway of very thick battery electrodes with intrinsic short carrier diffusion lengths presents conceptional difficulties: a thick layer is beneficial for capacity but detrimental for light absorption at the junction where the photocurrent is produced. 29 ■ A BRIEF OVERVIEW OF CURRENT SOLAR BATTERY CONCEPTS As introduced above, the field of solar batteries is in its infancy and therefore rich yet heterogeneous in terms of concepts and approaches. Solar battery designs can be organized according to the type of charge storage mechanism: 12 Photogenerated charge carriers assist in ion (de)insertion or oxidize/reduce a redox mediator.…”