Fig. 5. (a) Variation of maximum STH efficiency (%) with OEP/HEP compartment redox mediator potential (V vs. SHE) in tandem configuration with HEP (red) or OEP (blue) on top. The dashed and slide lines indicate the ideal case (no overpotentials) and non-ideal case (with overpotentials), respectively. (b) STH efficiency (%) as a function of the HEP's (x-axis) and OEP's (y-axis) bandgap energy in top-bottom configuration (in terms of optical arrangement) with HEP on top at redox mediator potential of OER compartment of 1.0 V vs. SHE (redox mediator of HER is insensitive).