Su-Il Pyun

The present article deals with the working principles of regenerative wet photovoltaic cells compared to n-p-type junction dry photovoltaic cells and photo-electrochemical cathodic protection short-circuited cells from thermodynamic and electrokinetic aspects with a pedagogical motivation. Additionally, it introduces two hypothetical regenerative hydrogen/oxygen photovoltaic cells conceptually designed for the first time to our knowledge. Their working mechanisms are discussed in terms of the single shifts of the redox Fermi level to the flat band Fermi level, $$\left({E}_{\text{F},\text{ fb}}^{\text{n}}-{E}_{\text{F}}^{\text{redox}}\right)$$ E F , fb n - E F redox , in the negative potential direction and $$\left({E}_{\text{F},\text{ fb}}^{\text{p}}-{E}_{\text{F}}^{\text{redox}}\right)$$ E F , fb p - E F redox in the positive potential direction during illumination. These negative and positive single shifts are directly responsible for the generation of photo emf (electromotive force) and occurrence of both the negative inverse overvoltage, $${\eta }_{\text{h}}^{\text{n},\text{ l}}<0$$ η h n , l < 0 , for photosensitized anodic oxidation by the valence band (VB) minority holes in the n-type anode and the positive inverse overvoltage, $${\eta }_{\text{e}}^{\text{p},\text{ l}}>0$$ η e p , l > 0 , for photosensitized cathodic reduction by the conduction band (CB) minority electrons in the p-type cathode, respectively, on the energy band diagram as well as the photocurrent I vs voltage V polarization curve. The splitting of one unique equilibrium Fermi level $${E}_{\text{F}}$$ E F , at which chemical potentials of majority and minority carriers overlap in the dark, has been detailed between the two quasi-Fermi levels of majority and minority carriers, $$\left({nE}_{\text{F}}-{pE}_{\text{F}}\right)$$ nE F - pE F , which is a measure of the departure from thermodynamic equilibrium ( $$\left({nE}_{\text{F}}={pE}_{\text{F}}\right)$$ nE F = pE F at equilibrium), where the mass action law no longer applies. Both single shifts of the Fermi level during illumination are confirmed to be almost equal in value regarding the difference between the two quasi-Fermi levels, $$\left({nE}_{\text{F}}-{pE}_{\text{F}}\right)$$ nE F - pE F . This parameter (the diffusion [contact] potential multiplied by electronic charge at the dry photovoltaic cell) can be regarded as the light quanta energy gain stored in photoexcited minority carrier holes and electrons, which provides the thermodynamic affinity (driving force) necessary for the photo-sensitized anodic and cathodic transfer. This can never contradict the second law of thermodynamics. The negative single shift of the equilibrium potential, $${V}_{\text{eq}}^{\text{redox}}$$ V eq redox , of a redox couple to the flat band potential, $${V}_{\text{fb}}$$ V fb , driven by the photoexcited VB minority holes in the n-type anode, allows us, relative to the positive single shift of the $${V}_{\text{eq}}^{\text{redox}}$$ V eq redox to the $${V}_{\text{fb}}$$ V fb , caused by the photoexcited CB minority electrons in the p-type cathode, to qualitatively predict the photo- I vs - V polarization curves. These I – V curves are analogous to the electrochemical I – V curves of any self-driven galvanic cell in the dark. The expected photo- I – V curves are qualitatively justified by some experimental data published in other literature. The negative/positive inverse overvoltages, i.e., negative/positive single shifts of $${V}_{\text{eq}}^{\text{redox}}$$ V eq redox to $${V}_{\text{fb}}$$ V fb , are confirmed to be the limiting photo emfs in value. In short, the single shift of the Fermi level is divided by the electronic charge, $$\left\{-\frac{\left({E}_{\text{F},\text{ fb}}^{\text{n}}-{E}_{\text{F}}^{\text{redox}}\right)}{e}\right\}$$ - E F , fb n - E F redox e and $$\left\{-\frac{\left({E}_{\text{F},\text{ fb}}^{\text{p}}-{E}_{\text{F}}^{\text{redox}}\right)}{e}\right\}$$ - E F , fb p - E F redox e , which are named the negative and positive inverse overvoltages (zero inverse overvoltage at equilibrium in the dark), $${\eta }_{\text{h}}^{\text{n},\text{ l}}<0$$ η h n , l < 0 and $${\eta }_{\text{e}}^{\text{p},\text{ l}}>0$$ η e p , l > 0 , which again mean the limiting photo emfs, $$\left({V}_{\text{oc}}^{\text{n},\text{ l}}<0\right)$$ V oc n , l < 0 regarding the counter cathode for the n-type-based photovoltaic cell inclusive the photo-electrochemical cathodic protection cell and $$\left({V}_{\text{oc}}^{\text{p},\text{ l}}>0\right)$$ V oc p , l > 0 regarding the counter anode for the p-type-based photovoltaic cell, respectively. The formation of an electron-depleted space charge region of the n-type anode is confirmed to be adequate for migration of the majority electron and minority hole across the transition region into the surface prior to producing photosensitized reduction and oxidation currents as well as photovoltages, respectively, at both the regenerative wet photovoltaic cell and the photo-electrochemical cathodic protection cell. The same is true of a hole-depleted space charge region of the p-type cathode. By comparison, the presence of a depleted space charge region is well suited for the photoexcited minority EHP (electron-hole pair) to migrate (drift) across the transition region into the n-type and p-type regions, respectively, before recombining there and finally delivering a photocurrent as well as a photovoltage at the dry photovoltaic cell inclusive the n/p junction. The presence of a depletion region, as opposed to an enriched space charge region, is a key requirement for all three types of cells to function effectively.