We have already seen that multijunction solar cells use thin films made of III-V or II-VI materials. The problem with crystalline silicon is that it is what is called an indirect bandgap material. We need not go into the physics of this. What it means is that a palpable thickness of silicon (about 0.1 mm) is needed to absorb photons, and we saw in Box 3.5 how hard it is to make pure silicon. Thin-film materials, on the other hand, have direct bandgaps. The absorption is so good that thicknesses are measured in microns,39 typically 1 Mm, which is a thousandth of a millimeter. By comparison, the thickness of an ordinary piece of paper is about 100 Mm (0.1 mm or 0.004 in.), the same as a human hair. Thin films that can absorb 98% of sunlight are only 1% of that thickness. No wonder that even a thin layer of sunscreen spread on the skin can protect against sunburn. Since crystalline silicon in a solar cell has to be over 100 Mm thick, thin-film solar uses 100 times less semiconductor material than silicon.

However, the small amount of material required for thin-film solar cells is not the main reason for their success. It is because manufacturing techniques developed by First Solar, Inc. of the USA and other companies have reduced the cost so that solar power is commercially viable. Development advances much more rapidly when support moves from the government to private industry, where the monetary incentive is strong. First Solar became dominant in its field by optimizing the use of CdTe (cadmium telluride). This material, with a bandgap of 1.45 eV, combines the best combination of voltage and current for the higher power output from a single layer. First Solar started with a plant in Ohio with 90 MW/year of production capability, then added a 120-MW/year plant in Germany and a 240-MW/year plant in Malaysia. It has contracted with China to produce 30 MW in 2010, then 100 and 870 MW by 2014, and finally a total of 1,000 MW by 2019. The entire production process, from deposition of all the layers to assembly and to testing, takes only 2.5 h on their automated production line. Benefiting from economy of scale, First Solar has lowered the cell cost to below $1/W and the module cost to $110/m2. The goal is to bring this down to $0.50/W or $1.50/W including balance-of-system. The cost of electricity would be 6-8 0/kWh.40 Producing 1 GW/year in solar cells would give the company one-sixth of the world’s share.

The layers of a CdTe solar cell are shown in Fig. 3.41. The layers are deposited on a 60 cm x 120 cm glass superstrate 5 mm thick. This is about the size of a quarter-sheet of 4 x 8-feet plywood and will yield many cells. Below that is a thin SiO2 layer for insulation, followed by a transparent conducting layer of SnO2, which is the top electrical contact. A thin layer of CdS (cadmium sulfide) follows. Only about 0.1 Mm thick, it serves as the n-doped layer in Fig. 3.33. It must be thin to allow the light to reach the absorbing layer of CdTe. Sulfur is a Column VI element, which has been left out of Fig. 3.34 to avoid clutter, so CdS is a II-VI compound. It turns out that CdS is naturally slightly n-doped in production, and CdTe is slightly p-doped [8], so the other layers in Fig. 3.38 are not necessary to separate the electrons from the holes, greatly simplifying the device. The main CdTe layer

Incident sunlight

Glass superstrate

Silicon dioxide

Fluorine doped

tin oxide

Cadmium sulfide

Cadmium tellunde

Nickel

Aluminum

Ethyl vinyl acetate

Glass laminate

Fig. 3.41 Schematic of a CdS/CdTe solar cell (IEEE Spectrum, August 2008) is 1-5 мт thick; Gupta et al. [9] have shown that the performance does not improve much beyond 0.75 Mm. At the bottom is the other electrode, made of gold, nickel, or aluminum, followed by a plastic binder and a glass protector. Laser scribing is used between the deposition of the various layers to divide the cell into smaller cells and to connect them in a series to raise the voltage to 70 V. After all this, the whole sheet is annealed between 400 and 500°C in CdCl2 gas to improve the efficiency by as much as a factor of 2.41 The reason for this is not well understood. Such a cell puts out about an ampere of current and up to 75 W of power at 10.6% efficiency.41 Improvement to 12% may be possible.

The record efficiency achieved in the laboratory is 16.5%. To do this, the trans­parent conductor at the top, usually tin oxide, was replaced by cadmium stannate, which has higher conductivity and is more transparent. A buffer layer of zinc stan­nate was then added below it.42 As current flows through the cell, its internal resistance causes energy to be lost as heat. This loss is measured by the filling factor, which is the percentage of the ideal power that is actually usable. The best that can be achieved is 77%.42 Although the general production process is well known41 (see ref. [8]), the know-how details are closely guarded secrets. For instance, the bottom contact tends to be unstable, and adhesion is affected by the annealing step.

Thin-film materials competing with CdTe are amorphous silicon (a-Si:H) and copper indium gallium diselenide (CIGS). Amorphous silicon has a low efficiency of 6-7%, but it has had a head start because the manufacturing equipment had been developed in the semiconductor industry. This material loses the red part of the
solar spectrum, and there are attempts to add a 2-pm layer of microcrystalline silicon to add the blue part. The efficiency might then go up to 11% to compete with CdTe. CIGS has a laboratory efficiency of 19.5% vs. 16.5% for CdTe. In modules, the efficiencies are 13 and 11%, respectively; and in production they are 11.5 and 9% [8]. CIGS is harder to make, but it is being pursued because of the possibility of 25% efficiency. Currently, it has only a 1% market share, compared with 30% for CdTe and 60% for a Si.43