The efficiency measurement of photoelectrodes plays an important role in determining the light-sensitive ability of the photoelectrodes for future applications. Several types of efficiency are often reported in articles, such as incident photon-to-cuirent conversion efficiency (IPCE) or external quantum efficiency (EQE), absorbed photon-to-current conversion efficiency (APCE) or internal quantum efficiency (IQE), solar to hydrogen (STH) conversion efficiency, applied bias photon-to-current conversion efficiency (ABPE). INCIDENT PHOTON-TO-CURRENT EFFICIENCY (IPCE)

It is defined as the amount of photocurrent generated per incident photon flux or in other words, it depicts the number of electrons generated with the number of photons incident on it. IPCE (also known as, EQE) of a photoelectrode depends on the optical absorption, width of the space charge layer, and on the minority carrier diffusion length (Xiao et al., 2015). It can be measured using a two or three-electrode configuration. For instance, if two-electrode set up is used for measurements then the photocurrent density has dependence over the type as w’ell as the distance of the CEs. Thus, it can be erroneous to claim the appropriateness of the observed photocurrent density values. Thus to tackle with this a third electrode (REs) is used while carrying out the photoelectrochemical measurements. Thus, it stays independent of the type of CE, the distance between the electrodes, while keeping the photocurrent values unaffected. Hence, it characterizes the ability of the photoelectrodes to generate electrons/holes by incidenting photons of a particular wavelength and can be presented by the expression: ABSORBED PHOTON-TO-CURRENT EFFICIENCY (APCE)

While calculating IPCE the optical losses are not taken into account which ultimately makes the efficiency less apparent. To make the estimations accurate regarding the photoelectrochemical performance of the photoelectrodes the APCE or the IQE is estimated. It may be defined as the number of photogenerated charge earners involved in the generation of photocurrent per absorbed photon and formulated mathematically by the equation:

where, A, R, T represents absorption, reflection, and transmission respectively. SOLAR TO HYDROGEN CONVERSION (STH) EFFICIENCY

It is defined as the ratio of chemical energy produced to that of input solar energy under illumination and zero bias condition. Thus, STH efficiency is regarded as the water-splitting ability of a photoelectrode. The chemical energy stored is equal to the product of rate of hydrogen production and change in Gibbs free energy (AG) per mol of H, (237 kJ/mol), while the solar energy input is the product of incident power density (P, 100 mW/cm2, AM l.5G) with the sample area under illumination within the electrode. The formula thus can be stated as:

Most often, another modified equation is used for estimating the STH efficiency as follows:

Here Jph is the photocurrent density and i]F is the faradaic efficiency. APPLIED BIAS PHOTON-TO-CURRENT EFFICIENCY (ABPE)

With the application of the external bias, the photocurrent density value increases and results in higher efficiency PEC processes. Several factors like cell configuration (two- or three-electrode), light source, CE, contact resistance, and the band structure of the semiconductor, etc., are known to influence the ABPE (Xiao et al., 2015; Zhou et al., 2013). This type of estimated efficiency is known as applied bias Photon-to-current efficiency (ABPE) and can be stated mathematically as:

The symbols Jph, Vapp, Ptotal are meant for photocurrent density, potential applied and intensity of light used for illumination. Taking into consideration that faradaic efficiency is less than unity, the applied bias STH conversion efficiency (AB-STH) can be expressed as:

And the equations are applicable for 2-electrode system. While, for a 3-electrode system, a REs is used, and the applied potential is thus converted into the reversible hydrogen electrode (RHE) by using the formula below:

For Ag/AgCl and SCE are silver and saturated calomel electrodes (SCE) respectively. Thus, the half-cell STH conversion efficiency (HC-STH) is considered to be the ability of the photoelectrode in converting the solar energy into chemical energy.

So for a photoanode:

While for a photocathode:

The dependence of the efficiency of a PEC cell on optical absorption, the width of space charge layer, sluggish reaction kinetics, and minority carrier diffusion necessitates the optimization of all the parameters for achieving high efficiency in a cost-effective manner. The physical properties can thus be optimized by varying the morphology (for e.g., in case of ID nanostructures). The high surface to volume ratio of ID structure facilitates better pathways for carrier transport. Thus, it became inevitably a matter of interest for several researchers.

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