Organic solar cells have been invented which are cheaper and easier to make than thin film and which have great promise in small, personal applications. The best of these are made of polymers (a general name for plastics) with long chemical names abbreviated as P3HT and PCBM. They have different bandgaps and different affini­ties for electrons and holes. Rather than separating them into layers as in CdTe cells, these two polymers are mixed completely together to form what is called a bulk heterojunction material. The mixture melts at a temperature below 100°C and, in liquid form, is easily coated onto a substrate, where it solidifies. The substrate can be a piece of cloth! By cooling the mixture at a particular rate, it self-organizes into connected clumps where the P3HT and PCBM are separated. A cartoon of this is shown in Fig. 3.45.

When a photon strikes a P3HT region (A), it creates an electron-hole pair. The electron then follows the A path to the top transparent electrode. (Electrode is defined in footnote 45.) The hole is attracted to the PCBM (B) region because of the natural electric field that arises between the two materials, and the hole fol­lows the B path to the metal electrode. Similarly, when a photon strikes a B region, the electron jumps into the A region, the hole stays in B, and both charges move to their respective electrodes following the strands of A and B. When the two elec­trodes are connected through a load, the electron current provides the solar power. The fortuitous way these polymers organize themselves avoids all the complicated layers of silicon or CdTe in conventional cells, but the trick is to get the right self­organization by slowly cooling the mixture with careful temperature control.46

The first experiments used a polymer layer less than a quarter of a micron (1/4000th of a millimeter) thick and less than a tenth the size of a postage stamp. A sunlight-to-electricity conversion efficiency of 4.4% was achieved [18], together with a high filling factor (defined above) of 67%. Many efficiency claims are decep­tively high because small samples collect sunlight from the edges as well as the top, but in this case a proper test was done at the National Renewable Energy Laboratory to avoid this. Further improvement was made in 2009 using a polymer called PBDTTT, whose chemical name would take up two lines. The partner material was not a polymer but carbon in the form of fullerene, commonly known as buckyballs, the familiar spherical carbon lattices made of triangles and named after Buckminster Fuller. This organic solar cell was 6.77% efficient, had high output voltage, and captured more of the infrared energy than the previous model [19]. The current was also reasonable in spite of the crooked paths that the electrons have to follow.

With efficiencies comparable to those of amorphous silicon cells, organic solar cells have great possibilities because they are inexpensive and can be put into almost anything, such as hand-held electronic devices and fabrics. They have already been built into backpacks to charge iPods and cell phones. They are not suitable for large installations, however, because the polymers are attacked by oxygen and last only one or two years. However, they will last almost indefinitely in an oxygen-free environment such as the inside of double-glazed windows.46

Further in the future are such inventions as dye-sensitized and quantum-dot solar cells. Dye-sensitized cells, also called Gratzel cells, consist of nanoparticles of titanium dioxide (TiO2), each only about 20 nm in diameter, coated with a layer of dye, as depicted in Fig. 3.46. (The prefix nano indicates sizes measured in billionths of a meter or millionths of a millimeter.) TiO2 is a large bandgap semiconductor, so by itself it would absorb only ultraviolet light. The dye, however, is excited by sunlight of any desired color and can inject an electron into the nanoparticles. The electron then hops from one particle to another to get to one electrode. This leaves the dye with an electron missing, so it has to grab one from the electrolyte (a con­ducting liquid containing iodine) in which the particles are immersed. Efficiencies of 11-12% have been observed in the laboratory, but what it would be in production is unknown. Since a part of the cell is liquid, it has to be sealed, which is rather inconvenient. Solid or gel electrolytes have been tried, but their efficiencies are very low, 4% or so [17].

Since the electrons have to jump numerous times to get to the positive electrode, the motion can be speeded up by using nanowires or nanotubes instead of nanopar­ticles. Figure 3.47 shows how this would work. The nanowires are heavily coated with dye, and electrons can readily flow along them right to the electrode at the bottom. In this case, the wires are made of zinc oxide (ZnO) instead of TiO2. Carbon nanotubes have also been used. The tubes, 360 nm long, have a surface area 3,000 times that of a flat surface [21], but of course no amount of surface area can collect more sunlight than falls on the surface facing the sun. Efficiencies of 12% have been observed in the laboratory.

A further improvement can be obtained by replacing the dye with quantum dots (QDs), which are nanocrystals of InP (indium phosphide) or CdSe (cadmium sele — nide). These are really small, only about 3 nm in diameter. They can be coated onto TiO2 or ZnO nanowires to replace the dye coating in Fig. 3.46 or 3.47a. By varying the size of the dots, different colors of the solar spectrum can be absorbed. When a photon hits a QD, an electron-hole pair is created, and the electron falls into the nanowire and is carried straight to an electrode, as in a dye cell. QD cells

can have higher efficiency than dye cells because they can violate the theoretical limit shown in Fig. 3.44. They can give both higher voltage and higher current [22]. Normally, when a photon has more than enough energy to push an electron across the bandgap into the conduction band (Fig. 3.32), the extra energy goes into the electron. These “hot electrons” then cool and drop down to the bottom of the conduction band, so the output voltage is only the bandgap voltage. In QDs, the hot electrons cool much more slowly and can get into the circuit before losing all their energy, so the cell’s output voltage can be higher than assumed by the simple theory. Furthermore, the hot electrons can have enough energy to create more electron — hole pairs by themselves, without photons. This increases the cell’s current over the theoretical limit.

Though quantum-dot solar cells are still in the experimental stage, the way to make nanowires [23] and QDs [24] is well documented. They share all the advan­tages of organic solar cells in small applications and have the prospect of much better efficiencies. They have not been proved to be suitable for solar farms.

Heat can drive electric currents directly by the Seebeck effect, giving rise to thermoelectric power, which is illustrated in Fig. 3.48. If we apply heat to one side of a thermoelectric material, the hot particles at the top move faster than the cold particles at the bottom, so particles tend to drift from top to bottom. Now if on the right side, we have an electron-rich (n-type) material, the electrons will be driven from the top electrode to the bottom electrode. To close the circuit, we put an electron-deficient material (p-type) on the left, where the holes will drift down­wards, and we connect the two bottom electrodes to a load. The electrons will then flow through the wire from right to left to fill the holes. Since the electrons are negative, the electrical current goes from left to right. A working arrangement might look like that in Fig. 3.49. Solar concentrators are used to increase the heat applied to the thermo-photovoltaic (TPV) cell, and the bottom of the cell has to be kept cool by water or air flow.

Emitter (tungsten

(T >2000 °С)

TPV ce

Back surface

reflector

Water ( or forced-air ) heat exchanger

Fig. 3.49 Illustration of thermo-photovoltaic solar cell (Basic Research Needs for Solar Energy Utilization, US Department of Energy Office of Science workshop, April 2005)

This idea is still in the initial stages of testing the thermoelectric efficiencies of compounds like PbTe, Bi2Te3, AgSbTe2, and AgBiSe2 and formulating new ones. Note that the latter two are type I-V-VI semiconductors [25]. Research is also proceeding on using nanowires and quantum well structures for this purpose [26, 27].