Crystalline solar cells

Single-jet crystal growth method, 19 179 Single-junction crystalline solar cells, 23 41-42... 

Fig. 8.3 Generic design of single-crystalline solar cell.

Crystalline solar cells are heavy and expensive to manufacture. However, their efficiency in converting sunlight has historically been superior to thin-film. Crystalline cells are constructed with silicon semiconducting materials. 

Table I shows dut measurable gains in efficiency are possible using a high performance spectrum shifting dye (high quantum yield and low emission loss) for a dual-junction GaAs based crystalline solar cell. Since die two junctions were likely well current-matched, die additional photons were assumed to be emitted at two different wavelengths, with each wavelengdi being absorbed by a different junction. This was necessary because the two junctions are connected in series and therefore an increase in current production by one junction will be limited by the current production of the other junction. Therefore, the photons where divided such that the addition current added to each junction was equivalent. Quantum efficiency measurements showed that the top junction was active with minimum bottom junction absorption at 490 nm and that the bottom junction was active at 800 run widi minimal top junction absorptioiL The shifted photons were therefore split between 490 nm and 800 nia The external quantum efficiency of die device was 0.8 and 0.88 at 490 nm and 800 nm, respectively.

Professor Fleischmann queried the energy payback period for photovoltaic devices. Professor Bard commented that considerations of both the energy and the economic payback period dictated the use of inexpensive thin-film devices containing only small amounts of semiconductors. But even now, crystalline solar cells of conventional design had a practicable payback period in some circumstances. The Solarex Corporation, for example, was able to meet all the energy required by their manufacture of silicon solar cells from roof-mounted panels of such cells. 

A typical a-Si H single-junction solar cell along with its operating characteristics are shown schematically in Figure 8.15. In a crystalline solar cell the optimal energy gap for maximum efficiency is between 1.4 and 1.5 eV. In a-Si H devices the optimal value may be somewhat higher, closer to 1.8 eV, if the device does not collect current well. Fortrmately, it is easy to produce a-Si H with a mobility gap energy near the optimal value. 

Germane is used, along with silane, SiH, to make amorphous or crystalline siUcon solar cells having an extended solar energy absorption range to increase conversion efficiency. 

There ate three basic technology options for making solar cells with do2ens of variations on each. These approaches ate conveniently grouped as follows thick (- 300 fiTo) crystalline materials, concentrator cells, and thin (- 1 fiva) semiconductor films. 

Small-area thin-film CdTe solar cells have been fabricated with sunlight-to-electricity conversion efficiencies near 16%, comparable to crystalline siUcon solar cells in large-scale manufacturing. Large-area monolithic integrated CdTe modules have been fabricated with efficiencies of ca 10%, comparable to crystalline siUcon modules commercially available. 

Single-Crystal Silicon. Silicon is still the dominant material in photovoltaic. It has good efficiency, which is 25% in theory and 15% in actual practice. Silicon photovoltaic devices are made from wafers sliced from single crystal silicon ingots, produced in part by CVD (see Ch. 8, Sec. 5.1). However, silicon wafers are still costly, their size is limited, and they cannot be sliced to thicknesses less than 150 im. One crystalline silicon wafer yields only one solar cell, which has an output of only one watt. This means that such cells will always be expensive and can only be used where their high efficiency is essential and cost is not a major factor such as in a spacecraft applications. 

The optical properties of a-Si H are of considerable importance, especially for solar-cell applications. Because of the absence of long-range order, the momentum k is not conserved in electronic transitions. Therefore, in contrast to crystalline silicon, a-Si H behaves as though it had a direct bandgap. Its absorption coefficient for visible light is about an order of magnitude higher than that of c-Si [74]. Consequently, the typical thickness (sub-micrometer) of an a-Si H solar cell is only a fraction of that of a c-Si cell. 

The many modifications to the conventional RF PECVD method show that one still is trying to find methods that will in the end lead to improved material properties. This is especially the case for the intrinsic metastability of a-Si H. In this respect, the stable material that is obtained at discharge conditions at the edge of crystallinity is very promising. Also, the quest for higher deposition rates while at least maintaining device quality material properties shows the industrial drive behind the research. Faster deposition allows for more solar cells to be produced in the same time. 

Conversion and Storage of Solar Energy using Dye-sensitized Nano crystalline Ti02 Cells 745... 

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