Category Physics of Solar Cells

Prospects for the Future

In the first chapter we saw that our present energy economy cannot continue in its current form, because we are in the process of changing the environment in which we live. Mankind and all other forms of life have developed over millions of years by adapting to the conditions around us in a continuous process of evolution. We have good reason to describe changes to these conditions, which are too rapid for life to adapt to, as natural disasters. Life on earth is an extremely complex system, and even today we do not fully understand the interrelations within this system. If we do not wish to risk life as a whole or parts thereof by significant departures from the present equilibrium, we should tolerate only minor changes in the prevail­ing conditions...

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Up – and down-conversion of photons

A considerable loss of energy in a solar cell is due to the photons with energy hCO < 8g which are not absorbed. It would be very convenient if two or more of these useless photons could be converted into one photon with energy h(& > 8g, which could then be absorbed by the solar cell. In the following discussion, Єо defines the band gap of the solar cell for which small-energy photons will be up-converted. That such an up-conversion of the photon energy is not forbidden by thermodynamics is demonstrated by Figure 8.14, which shows a device consisting of a tandem of two small band gap solar cells connected to an LED...

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Impurity photovoltaic effect

In Section 3.6.2 we have discussed non-radiative transitions between the bands and an impu – rity level. Impurities with energies for electrons in the middle of the energy gap were found to greatly enhance recombination, which is detrimental for the efficiency. In the analysis, gener­ation of electron-hole pairs by optical transitions was neglected. Now we do just the opposite. Our model now permits only radiative transitions between the bands and to and from an im­purity level. Thermalization of free charge carriers is considered, but not impact ionization or non-radiative recombination.[32]

The model, as shown in Figure 8...

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Two-step excitation in three-level systems

If interaction with phonons cannot be prevented, thermalization losses can be reduced by dividing the incident spectrum over more than one transition as we have seen with tandem cells. In a three-level system, where the levels can be bands as well, three different transitions may occur in a single material: directly from the lower level to the upper level and in addition by a two-step process from the lower level to the intermediate level and from there to the upper level. In both ways electrons are generated at the upper level and holes at the lower level.

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Energy conversion with hot electrons and holes

For conventional solar cells based on thermalization of electrons and holes in the absorber, complete conversion of chemical energy into electrical energy was achieved by membranes, n-type for the transport of electrons to one contact and p-type for the transport of holes to the other contact. This type of membrane is not sufficient for hot carriers. In addition to the selec­tive transport of electrons and holes, the membranes must now also serve the thermodynamic function of producing chemical energy from the heat of the electrons and holes by cooling them down to the temperature of the environment.

We will discuss this problem for the electrons, the solution can then easily be applied to the holes as well...

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Hot electrons from impact ionization

The temperature 7 of the electrons and holes in the absorber can be found very easily from the magnitude of the emitted energy current. In contrast to the interaction with phonons, where at open-circuit the emitted photon current is equal to the absorbed photon current, for impact ionization at open-circuit the emitted energy current must be equal to the absorbed energy current, since by impact ionization/Auger recombination the electron-hole system does not lose energy. The chemical potential of the emitted photons is then py = t~[c + T]h = 0, and we can calculate the temperature 7д of the electrons and holes by using Planck’s Law in Eq. (2.32) for the emitted energy current. For maximum concentration of the incident solar radiation we find, of course, 7a = 7s at open circuit.

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Impact ionization

Electrons and holes possessing large kinetic energies as a result of generation by high-energy photons can dissipate their kinetic energy in two ways. One is by elastic collisions with the lattice atoms, in which energy is transferred in small portions to the lattice atoms until thermal equilibrium with the lattice is established. The other is by inelastic collisions with the lattice atoms in which, by impact ionization, another electron is knocked off its chemical bond or, in other words, in which a free electron and a free hole are produced, as shown in Figure 8.8. Both processes take place in parallel and compete with each other...

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Thermo-photovoltaic energy conversion

The solar-thermal conversion method of Section 2.1.1 can be modified to be applicable to solar cells. Figure 8.7 illustrates the principle.

A focussing optical system is used to concentrate the solar radiation onto an intermediate absorber which, as a result, is heated to the temperature 7. Solar cells with an energy gap Eg are placed concentrically around the intermediate absorber. They have an interference filter on their surface, which transmits all photons with £q < Йш < Eg + de without loss and reflects all other photons, which cannot be used optimally, back to the intermediate absorber. These photons, together with the photons emitted by the solar cell and transmitted by the filter, help to maintain the temperature 7д of the intermediate absorber...

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