Regenerative solar cells

The state of the art regenerative solar cell consists of the sensitizer cis-Ru(dcb)2(NCS)2 anchored to colloidal anatase Ti02 films with an Lil-l2 organic electrolyte. An i-V curve recorded with 1000 W m of air mass 1.5 illumination at 25 °C is shown in Figure 25 [154]. From these data, a Eqc of 0.79 V, an 4c of 4.83 mA, a fill factor of 0.71 and an of 0.11 were obtained. The active area of the electrode was kept low, 0.249 cm, to minimize resisitive losses associated with the tin oxide glass and the nanocrystalline semiconductor film. The power efficiency was not significantly changed when the temperature was raised from 20 to 60 °C. 

Figure 25. A typical i-V curve recorded with 1000 W/m of air mass 1.5 illumination at 25°C for a regenerative solar cell based on ci5-Ru(dcb)2(NCS)2/Ti02.

Dye sensitization of electrodes is an old area of science with a rich history. The field has experienced renewed interest owing to the development of high surface area colloidal semiconductor electrodes. These materials yield impressive solar conversion efficiencies when employed in regenerative solar cells that have already found niche applications and have the real possibility of replacing traditional solid-state photovoltaics. Thus for the first time in history a solar cell designed to operate on a molecular level is useful from a practical point of view. It is also likely that other applications in the growing areas of molecular photonic materials will arise. 

In this chapter, I give an account of the historical development of semiconductor photoelectrochemistry and nanostructured photovoltaic devices in Section 1.2, and then Sections 1.3-1.6 provide a brief introduction to the major cell types discussed in the remainder of the book the ETA (extremely thin absorber) cell, organic and hybrid cells, dye-sensitised solar cells (Gratzel cells) and regenerative solar cells. 

Regenerative solar cells (RSCs), also known as semiconductor/liquid photoelectro-chemical cells, electrochemical photovoltaic solar cells, wet photovoltaic cells or liquid-... 

Figure 1.8 Cell schematics for a regenerative solar cell based on (a) an n-type photoelectrode (b) ap-type photoelectrode. The top diagrams show the cell reactions under illumination, the middle diagrams the electronic energy levels and band bending, and the bottom diagrams the cell current-voltage (I-U) characteristics with the photoelectrode and counter electrode (CE) currents shown in the same quadrant. The maximum power point is located at the point on the current-voltage curve at which the rectangle of maximum area may be inscribed in this quadrant. The photovoltage V, the electron and hole quasi-Fermi levels E and fip and the solution Fermi level f o.R, the open-circuit potential Ugc of the photoelectrode and the standard redox potential 17 ° of the 0,R redox couple are also shown.

Table 1.1 Maximum-power conversion efficiency of selected regenerative solar cells...

In principle, electrochemical photovoltaic cells (also known as regenerative solar cells) can be constructed very simply. They consist of a semiconductor electrode, an electrolyte containing a redox system and a counter electrode, as illustrated in the lower part of Fig. 2.30. The energy scheme for a cell with an -type semiconductor and a redox system with a standard potential close to the valence band is shown in the upper part of Fig. 2.30. On illumination of such a cell, holes are created and transferred to the reduced species of the redox couple. The electrons reach the ohmic contact on the backside of the semiconductor, traverse the external circuit, do useful work, and reach the counter electrode, at which they reduce the oxidised component... 

The competition between redox reaction and anodic dissolution became very important in the development of stable regenerative solar cells on the basis of semiconductor-liquid junctions. As shown in the previous section, it is determined by the thermodynamic and kinetic properties of the processes involved. Information on the competitions between these reactions cannot be obtained entirely from current-potential curves, because in many cases they do not look very different upon addition of a redox system, especially if the current is controlled by the light intensity. Therefore, a rotating ring disc electrode (RRDE) assembly consisting of a semiconductor disc and a Pt ring is usually applied, i.e. a technique which makes it possible to determine separately the current corresponding to the oxidation of a redox system [62, 63]. 

The fact that they produce large photovoltages (Fig. 8) and intensive photocurrents at sufficiently negative electrode potentials (Fig. 13) make the systems n-MoSe2 1 /IJ and n-WSe2 1 /IJ most attractive for regenerative solar cells. The first long term experiment... 

Fig. 13. Photocurrents across n and p-type WSe2 in contact with 1 M H2SO4 and after addition of r/I2 and Fe2+,3+ respectively. The mode of operation of a regenerative solar cell based on the I /IJ redox couple is visualized as well as that of a p-n solar cell decomposing HI into /5H2 + 4I2 (inserted pictures)...

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