Solar-to-hydrogen conversion efficiency

These early tests were not conducted with the most efficient solar cells available at that time. The record efficiency then was about 30% for a laboratory cell (see Fig. 4) and those cells were not easily obtainable. Today s record efficiency is 40.7%, and 35% efficient cells are commercially available.18 Therefore, 40% solar to hydrogen efficiency is expected in the near term assuming a heat boost of 40%, a multijunction solar cell efficiency of 35%, and an optical efficiency of 85%. A 40% multijunction solar cell would yield a solar to hydrogen conversion efficiency of almost 50%. Nevertheless, electrochemical theoretical results calculated by Licht, shown in Figure 10, are consistent with these predictions based on Solar Systems early experiments.15... 

Figure 7 Dependence of the solar to hydrogen conversion efficiency on current density in the cell p-InP(Ru)/3M-HCl/Pt under 84.7mWcm solar irradiance. The insert shows the current voltage characteristics of the platinum cathode and of the p-InP(Ru) photocathode under 84.7 mW cm sunlight...

To avoid confusion between IPCE and solar-to-hydrogen conversion efficiency (STH), we should note three critical differences. First, in contrast to IPCE (electrons out/photons in), STH describes efficiency in terms of power (power out/ power in). Secondly, whereas IPCE measurements can be conducted with any calibrated and monochromated illumination source (i.e. it need not be AM 1.5 G illumination that is monochromated as long as the number of impinging photons at each wavelength are counted), STH requires the use of broadband solar-simulated illumination. The reader is referred elsewhere [5] for a discussion of solar simulation using laboratory illumination sources. The integration of IPCE over the entire solar spectrum can provide an estimation of the maximum possible STH, if (and only if) no applied bias is used in the IPCE measurement. Lastly, conducting IPCE experiments with an applied bias is allowable whereas STH requires true zero-bias conditions. Of course, the authors of a publication must be explicit as to... 

If the incident light intensity is known, the energy conversion efficiency can be calculated from the photocurrent density. If one assumes that 100% of the photocurrent is used to actually split water (a reasonable assumption for a concentrated KOH or NaOH electrolyte), this also gives the solar-to-hydrogen conversion efficiency. The question is then at which applied bias potential one should take the photocurrent. Ideally, this should be at zero bias for a two-electrode measurement, and presumably also at zero bias vs. RHE for a three-electrode measurement. However, at these bias potentials the photocurrent is often very... 

Fig. 7.2 Illustration of multilayered III-V semiconductor device structure that has demonstrated high solar-to-hydrogen conversion efficiencies for limited durations...

Fig. 7.15 Maximum achievable photocurrent density levels and potential solar-to-hydrogen conversion efficiencies for single-junctions as a function of bandgap based on optical absorption limits. Highlighted are the high-bandgap materials that have demonstrated spontaneous PEC water splitting but at correspondingly low efficiencies...

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