Electrons are shared in covalent bonds between atoms of Si. A bound electron has the lowest energy state.

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1 Photovoltaics Basic Steps the generation of light-generated carriers; the collection of the light-generated carriers to generate a current; the generation of a large voltage across the solar cell; and the dissipation of power in the load and in parasitic resistances.

2 Electrons are shared in covalent bonds between atoms of Si. A bound electron has the lowest energy state. If the electron has enough thermal energy to break free of its bond, then it becomes free and has some minimum energy. This minimum energy is called the "band gap" of a semiconductor. The space left behind by a free electrons allows a covalent bond to move from one electron to another, thus appearing to be a positive charge moving through the crystal lattice. This empty space is commonly called a "hole", and is similar to an electron, but with a positive charge.

3 Band Gap An electron-volt is equal to the energy gained by an electron when it passes through a potential of 1 volt in a vacuum. The band gap of a semiconductor is the minimum energy required to move an electron from its bound state to a free state. The lower energy level of a semiconductor is called the valence band. The energy level at which an electron is free is called the conduction band. Photons with less energy than the band gap pass through the material. Photons with more energy, heat the material.

4 Sunlight and PV The solar spectrum covers a range of about 0.5 ev to about 2.9 ev. Each photon carries Most PV cells cannot use about 55% of the energy of sunlight, because this energy is either below the band gap of the material or carry excess energy.

5 Doping Both the electron and hole can participate in conduction and are called "carriers". The concentration of these carriers is called the intrinsic carrier concentration. Semiconductor material which has not had impurities added to it in order to change the carrier concentrations is called intrinsic material. n-type semiconductors are doped with atoms (Phosphorous) with one more valence electron than the silicon. The majority carriers are Negatively charged electrons. The minority carriers are holes. p-type semiconductors are doped with atoms (boron) with one less valence electron. The majority carriers are Positively charged holes.

6 PV Cell Sandwiching the p- and n-type materials creates a p/n junction at their interface. Excess electrons move from the n-layer toward the p-layer side. Holes flow in the opposite direction. The p-n junction, prevents recombination by separating the electrons and the holes. If the light-generated minority carrier reaches the p-n junction, it is swept across the junction by the electric field, where it is now a majority carrier. Because of the flow of electrons and holes, the two semiconductors behave like a battery, creating an electric field at the surface where they meet the p/n junction.

7 Amorphous Silicon A typical amorphous silicon cell uses a p-i-n design, in which an intrinsic layer (i-layer) is sandwiched between a p- layer and an n-layer. 3-layer sandwich sets up an electric field between the p- and n-type layers. Photons are absorbed in the p-layer. Light generates free electrons and holes in the intrinsic region if the photons are more energetic than the band gap. Current flows because the free electrons and holes are generated within the electric field.

8 Multiple Junction Cells Individual cells with different bandgaps are stacked on top of one another. The sunlight falls first on the material having the largest bandgap. Photons not absorbed in the first cell are transmitted to the second cell, which then absorbs the higherenergy portion of the remaining solar radiation while remaining transparent to the lower-energy photons. These selective absorption processes continue through to the final cell, which has the smallest bandgap. The top cell is gallium indium phosphide, followed by a "tunnel junction" to allow the flow of electrons between the top and bottom gallium arsenide cells.

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10 Quantum Efficiency The "quantum efficiency" is the ratio of the number of carriers collected by the solar cell to the number of photons of a given energy incident on the solar cell.

11 Spectral Response Band gap for silicon The spectral response is the ratio of the current generated by the solar cell to the power incident on the solar cell.

12 I-V curve Maximum power & efficiency The short-circuit current (I sc ) is the current through the solar cell when V=0. Commercial cells: I sc = 28 to 35 ma/cm 2 The open-circuit voltage, V oc, is the maximum voltage available from a solar cell, and this occurs at I=0. V oc is then a measure of the amount of recombination in the device. Commercial devices on multicrystalline silicon typically have opencircuit voltages around 600 mv.

13 Efficiency Temperature Effect G! PV =! o + " PV (TPV # 25 C) + $ log( ) 1000 For crystalline and polycrystalline silicon modules (c-si and p-si) β is between %/K and -0.1 %/K (Mattei et al., 2006). The temperature coefficient of amorphous silicon modules (a-si) is lower, %/K.

14 Efficiency Efficiency is defined as the ratio of energy output from the solar cell to input energy from the sun. The maximum efficiency measured for a silicon solar cell is currently 24.7% Recombination of holes and electrons must be minimized to maximize solar to electric efficiency. Optical losses: light is reflected from the front surface, or not absorbed in the solar cell. Minimize parasitic resistive losses. Increases in temperature reduce the band gap of a semiconductor

15 PV Modules A PV module consists of individual solar cells electrically connected together to increase their power output. They are packaged so that they are protected from the environment and so that the user is protected from electrical shock. Most modules contain 36 solar cells in series. This gives an open-circuit voltage of about 21V under standard test conditions, and an operating voltage at maximum power and operating temperature of about 17 or 18V.

16 Resources Excellent online tutorial: DOE site: