Tandem cells

Designing tandem cells is complex. For example, each cell must transmit efficiently the insufficiently energetic photons so that the contacts on the backs of the upper cells are transparent to these photons and therefore caimot be made of the usual bulk metal layers. Unless the cells in a stack can be fabricated monolithically, ie, together on the same substrate, different external load circuits must be provided for each cell. The thicknesses and band gaps of individual cells in the stack must be adjusted so that the photocurrents in all cells are equal. Such an optimal adjustment is especially difficult because the power in different parts of the solar spectmm varies under ambient conditions. Despite these difficulties, there is potential for improvement in cell conversion efficiency from tandem cells. 

Water Splitting by Visible Light using a Tandem cell 749... 

Figure 21 Circuit diagram of a tandem cell for water photolysis.

Figure 22 The energy level diagram (Z-scheme) for photocatalytic water decomposition by a tandem cell.

PEC Setups with Multijunction and Multiple PV Cells 7.6.1 PEC Setup with Tandem Cells... 

PV current-voltage and power-voltage characteristics of a standard GaInP2/GaAs tandem cell were measured under 100 mW/cm2 illumination. The PV parameters under illumination were an open-circuit voltage Vroc = 2.32 V, a short-circuit current density Jsc = 13.4 mA/cm2, a fill factor FF = 0.88, and a PV conversion efficiency of 28.5%. Under the... 

Fan, J.C.C., Tsau, B.Y., and Palm, B.J., Optimal design of high-efficiency tandem cells, Proc. 16th IEEE Photovoltaic Specialists Conf, 692, 1982. 

Kocha, S.S., Montgomery, D., Peterson, M.W., and Turner, J.A., Photoelectrochemical decomposition of water utilizing monolithic tandem cells, Solar Energ. Mater. Solar Cells, 52,389,1998. 

This cell involves the absorption of light by dye molecules spread on the surface of the semiconductor, which upon light absorption will inject electrons into the conduction band of the n-type semiconductor from their excited state. The photo-oxidized dye can be used to oxidize water and the complementary redox process can take place at the counter electrode [46,47]. Tandem cells such as these are discussed in Chapter 8. 

Multi-junction PV Tandem Cells for Hydrogen Production 501... 

DSSC-based Tandem Cell for Solar Hydrogen Production... 

Fig. 8.13 Z-scheme approach to photocatalytic water splitting using a DSSC based tandem cell.

Gao X, Kocha S, Frank AJ, Turner JA (1999) Photoelectrochemical decomposition of water using modified monolithic tandem cells, Int J Hydrogen Energy 24 319-325... 

Production of 100 mm x 100 mm demonstrator PEC water-splitting tandem cells (WOj/TiOj Fe203Ai02), with scale-up program toward 300 mm x 300 mm devices - Switzerland and the UK. 

Solar cells devices based on CuPc and C6o fullerene have reached power conversion efficiencies of up to 5% [266,271], the highest efficiency for a Pc-containing solar cell being represented by a tandem cell in which two cells, each of them composed of a mixed donor/acceptor CuPc/C6o layer sandwiched between homogeneous donor and acceptor layers, are coupled (efficiency = 5.7%) [272],... 

Fig. 6.48. A micromorph tandem cell, for which a ZnO layer can be used as front contact (i.e., window layer), back contact, and/or intermediate reflector [58]...

Results obtained with individual a-Si H and pc-Si H solar cells are now being used to develop micromorph solar cells. Stabilized efficiency of 10.8% has so far been obtained for a micromorph tandem cell deposited on LP-CVD ZnO B front TCO, with an initial efficiency of 12.3% (see Fig. 6.57). As neither fine optimization of front ZnO surface roughness especially adapted to micromorph solar cell, nor intermediate reflector have been used to obtain this result, there is still a promising room for improvement in this field. 

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