Si solar cells

The properties of the band gap in semiconductors often control the applicability of these materials in practical applications. To give just one example, Si is of great importance as a material for solar cells. The basic phenomenon that allows Si to be used in this way is that a photon can excite an electron in Si from the valence band into the conduction band. The unoccupied state created in the valence band is known as a hole, so this process has created an electron-hole pair. If the electron and hole can be physically separated, then they can create net electrical current. If, on the other hand, the electron and hole recombine before they are separated, no current will flow. One effect that can increase this recombination rate is the presence of metal impurities within a Si solar cell. This effect is illustrated in Fig. 8.4, which compares the DOS of bulk Si with the DOS of a large supercell of Si containing a single Au atom impurity. In the latter supercell, one Si atom in the pure material was replaced with a Au atom,... 

Ohmori T, Go H, Yamaguchi N, Nakayama A, Mametsuka H, Suzuki E (2001) Photovoltaic water electrolysis using the sputter-deposited a-Si/c-Si solar cells, Int J Hydrogen Energy 26 661-664... 

Figure 13 shows typical results of the degradation of crystalline Si solar cells having a back surface field and reflector structure (Si-BSFR), which were qualified by National Space Development Agency of Japan (NASDA) for space usage, when irradiated by 10-MeV protons and 1 -MeV electrons. The pn junction of the cell samples, with a size of 2 cm x 2 cm X 50 pm, was fabricated by phosphorus (P) doping to a depth of 0.15 pm into boron... 

Figure 14 Prediction (solid line) and actual flight data (squares) of the degradation of Si solar cells equipped in ETS-6.

Handbook of Si Solar Cells for Space Application. Private communication by JAERI and NAS-DA, 1999. 

Solar energy is difficult to be utilized due to its diluteness. In order to use it in a large scale, therefore, the irradiation must be collected. Especially when the conversion module is expensive (e.g., the Si solar cell), the collection and concentration of the solar energy is economically advantageous. 

High solar-energy conversion efficiency. A high efficiency equal to that of the amorphous Si solar cell has been obtained as a laboratory development and efficiencies greater than 10% might be possible. 

Experiments show that in high quality Si solar cells the superposition principle is valid to a good approximation. In CdS/Cu2S, amorphous Si [139] and in polymer solar cells some of these approximations are grossly violated. As an example consider the effect of series resistance. In the presence of the series resistance Rs the dark current is given by,... 

A dark forward current 7d flows in the cell if a voltage is applied to the junction. To avoid confusion and for the sake of clarity we will call this current a junction current 7j measured in the dark, i.e. 7d = 7j(dark). This dark current continues to flow even if the cell is illuminated. The only requirement for the current to flow is that there should be a voltage at the junction. We designate this current measured under illumination as 7j(illumination). We need to introduce this notation because unlike in Si solar cells, in organic solar cells 7d = Tj(dark) is not equal to 7j(illumination). The junction current... 

Fig. 5.25. Plots of (a). /sc(l7oc) and (b) dark current J([(V) at different temperatures. In good quality single crystal Si solar cells /sc and / j are equal if Voc = V. In amorphous Si solar cells /c >> /(I proving that dark current increases on illumination [146].

IMEC is conducting research on thin-film crystalline Si solar cells, Gas solar cells, and on new materials and technologies such as plastic solar cells to improve the efficiency and cost of solar cells [135]. 

These devices are usually encapsulated in plastic. Si diode photocells have an expected lifetime of 40 years. In late 2001, with batteries to provide power at night, desert climates can get solar power for about 0.08 per kWh (kilowatt-hour) using Si solar cells, batteries, and electronic inverters. By contrast, nuclear and hydroelectric power plants can provide power at 0,015 to 0.03 per kWh. Solar power is already cheaper than internal combustion... 

The key ingredient for new ways to harness sunlight is the solar cell, or photovoltaic (PV) cell, a device that absorbs photons from sunlight (or artificial sunlight) and generates an electrical current [9]. Figure 10.6 shows the evolution over time of photovoltaic cells since 1975 [4]. The first photovoltaic cell was a pn junction based on Si and had an efficiency of 5% to 6% [3] at present, Si pn junctions have overall power efficiencies of about 22% (maybe 10% in commerce) at the moment, Si solar cells are expensive but durable. 

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