PECS cells conversion efficiencies

It has been illustrated that polycrystalline materials can be operated in regenerative electrolytic solar cells yielding substantial fractions of the respectable energy conversion efficiency obtained by using single crystals. Pressure-sintered electrodes of CdSe subsequently doped with Cd vapor have presented solar conversion efficiencies approaching 3/4 of those exhibited by single-crystal CdSe electrodes in alkaline polysulfide PEC [84]. 

It is known that the photoelectrochemical cell (PEC), which is composed of a photoelectrode, a redox electrolyte, and a counter electrode, shows a solar light-to-current conversion efficiency of more than 10%. However, photoelectrodes such as n- and p-Si, n-and p-GaAs, n- and p-InP, and n-CdS frequently cause photocorrosion in the electrolyte solution under irradiation. This results in a poor cell stability therefore, many efforts have been made worldwide to develop a more stable PEC. 

Cd-chalcogenides (CdS, CdSe, CdTe) are among the most studied materials as photoelectrodes in a photoelectrochemical cell (PEC) (1,2 /3,4). Interest in such PEC s stems from the fact that, in aqueous polysulfide or polyselenide solutions, a drastic decrease in photocorrosion is observed, as compared to other aqueous solutions, while reasonable conversion efficiencies can be attained. 

Recently we showed that PEC s using alloys of Cd(Se,Te) as photoelectrodes can be prepared with stability characteristics much better than those obtained for CdTe based cells and with similar conversion efficiences (). Those efficiencies could be improved considerably by several chemical and photoelectrochemical surface treatments. 

Figure I. Results of an 8-month outdoors test of PEC containing a 0.8-cm2 thin film, painted CdSe photoelectrode (not photoetched), CoS counterelectrode, and 7M KOH, 2M S, 7M S, ImM Se solution. (OCV) open-circuit voltage (SCC) short-circuit current (EFF) solar conversion efficiency ( AMI.5). Between measurements the cell operated on maximum power (68 fi load). No appreciable change in fill-factor occurred during the test.

Jsc, and (2) the open-circuit photovoltage, which is denoted as Vbc- Jsc is generally related to the mobility of the redox couple in an electrolyte. Vqc is basically the energy difference between the energy level of the semiconductor electrode and the redox potential using a redox couple, as shown in Figure 15.2. A fill factor, which is denoted as FF, is reflected in the overall performance of a PEC cell. The conversion efficiency is the most important parameter in terms of the practicality of the PEC cells. This review focuses on Jsc in order to clarify the relationship between Jsc and the viscosity of the RTELs. 

Efficiency of PEC cells for solar-to-electrical energy conversion... 

In Japan, a photoelectrochemical system for outdoor operation, i.e., solar, has been constructed. Photoanodes were Ti plates with a I fim thick Ti02 surface layer with a total area of 0.17 m, cathodes were of Pt, and the electrolyte was KCl. The result was an output of 1.1 1 of hydrogen per day, the conversion efficiency was about 0.4 % which is considered relatively high compared with the small absorption wavelength range (3 % of the entire solar spectrum) by Ti02 [25]. In a more recent effort using a multi-band gap PEC cell structure, an efficiency of 12.4 % has been achieved [3]. 

An important limitation is that the three-electrode j-V measmement cannot be utilized to calculate a power conversion efficiency because the three-electrode scale represents only the half-cell voltage between the WE and RE (see Chapter Efficiency Definitions in the Field of PEC ). To emphasize why a half-cell efficiency is insufficient, Fig. 6.9a illustrates the case for two distinctly different hypothetical photoanodes. Photoanode A possesses a small Vonset, oer and large jfc OER while Photoanode B possesses a large Vonset,OER and small oer- Both photoanodes in this case would seemingly yield the same half-cell efficiency described by in Eq. (2.10) in Chapter Efficiency Definitions in the Field of PEC using the operating points Vop and jop that correspond to the point of maximum fill factor. However, further analysis in Fig. 6.9b reveals that a complete device which includes the addition of a photocathode would clearly distinguish Photoanode B as the superior component. 

---