Solar Cells

A solar cell or photovoltaic cell is a device that converts sunlight directly into electricity by the photovoltaic effect. Sometimes the term solar cell is reserved for devices intended specifically to capture energy from sunlight, while the term photovoltaic cell is used when the light source is unspecified.

photovoltaic device priciples
Consider a pn junction with a very narrow and more heavily dopend n-region.The illumination is through the thin n-side. The depletion region(W) or the space charge layer(SCL) extends primarily into the p-side. There is a built-in field Eo in this depletion layer. The electrodes attached to the n-side must allow illumination to enter the device and at the same time result in a small series resistance.They are deposited on the n-side to form an array of finger electrodes on the surface. A thin antireflection coating on the surface reduces reflections and allows more light to enter the device.

As the n-side is very narrow, most of the photons are absorbed within the depletion region(W) and within the neutral p-side and photogenerate EHPs in these regions. EHPs photogenerated in the depletion region are immediately separated by the built-in field Eo which drifts them apart. The electron drifts and reaches the neutral n+-side whereupon it makes this region negative by an amount of charge -e. Similarly, the hole drifts and reaches the neutral p-side and thereby makes this side positive. Consequently an open circuit voltage develops between the terminals of the device with the p-side positive with respect to the n-side. If an external load is connected, then the excess electron in the n-side can travel around the external circuit, do work, and reach the p-side to recombine withe the excess hole there. It is important to realize that without the internal field Eo it is not possible to drift apart the photogenerated EHPs and accumulate excess electrons on the n-side and excess holes on the p-side.

The EHPs photogenerated by long-wavelength photons that are absorbed in the neutral p-side diffuse around in this region as there is no electric field. Those electrons within a distance Le to the depletion region can readily diffuse and reach this region whereupon they become drifted by Eo to the n-side . Consequently only those EHPs photogenerated within the minority carrier diffusion length Le to the depletion layer can contribute to the photovoltaic effect.

Ohmic metal-semiconductor contacts are made to both the n-type and p-type sides of the solar cell, and the electrodes connected to an external load. Electrons that are created on the n-type side, or have been "collected" by the junction and swept onto the n-type side, may travel through the wire, power the load, and continue through the wire until they reach the p-type semiconductor-metal contact. Here, they recombine with a hole that was either created as an electron-hole pair on the p-type side of the solar cell, or are swept across the junction from the n-type side after being created there.
The voltage measured is equal to the difference in the quasi Fermi levels of the minority carriers ie. electrons in the p-type portion, and holes in the n-type portion.

From the equivalent circuit it is evident that the current produced by the solar cell is equal to that produced by the current source, minus that which flows through the diode, minus that which flows through the shunt resistor:

(1)
$$I = I _{L}- I_{D}-I_{SH}$$

where

* I = output current (amperes)
* IL = photogenerated current (amperes)
* ID = diode current (amperes)
* ISH = shunt current (amperes)

The current flowing through these elements governed by the voltage across them:

(2)
$$V_{j} = V + IR_{S}$$

where

* V = voltage across the output terminals (volts)
* I = output current (amperes)
* RS = series resistance (Ω)

By the Shockley diode equation, the current diverted through the diode is:

(3)
\begin{align} I_{D} = I_{0} \left\{\exp\left[\frac{qV_{j}}{nkT}\right] - 1\right\} \end{align}

where

* I0 = reverse saturation current (amperes)
* n = diode ideality factor (1 for an ideal diode)
* q = elementary charge
* k = Boltzmann's constant
* T = absolute temperature
By Ohm's law, the current diverted through the shunt resistor is:

(4)
\begin{align} I_{SH} = \frac{V_{j}}{R_{SH}} \end{align}

where

* RSH = shunt resistance (Ω)

Nanoparticle processing

Experimental non-silicon solar panels can be made of quantum heterostructures, eg. carbon nanotubes or quantum dots, embedded in conductive polymers or mesoporous metal oxides. In addition, thin films of many of these materials on conventional silicon solar cells can increase the optical coupling efficiency into the silicon cell, thus boosting the overall efficiency. By varying the size of the quantum dots, the cells can be tuned to absorb different wavelengths. Although the research is still in its infancy, quantum dot-modified photovoltaics may be able to achieve up to 42 percent energy conversion efficiency due to multiple exciton generation(MEG).

Quantum heterostructure

The combination of multiple heterojunctions together in a device is called a heterostructure. Quantum heterostructure is a heterostructure in a substrate (usually a semiconductor material), with size restricting the movements of the charge carriers and forcing them into a quantum confinement, leading to formation of a set of discrete energy levels at which the carriers can exist. Quantum heterostructures have sharper density of states than structures of more conventional sizes.
Examples of quantum heterostructures confining the carriers in quasi-two, -one and -zero dimensions are:

1.Quantum wells:
A quantum well is a potential well that confines particles, which were originally free to move in three dimensions, to two dimensions, forcing them to occupy a planar region. The effects of quantum confinement take place when the quantum well thickness becomes comparable at the de Broglie wavelength of the carriers (generally electrons and holes), leading to energy levels called "energy subbands", i.e., the carriers can only have discrete energy values.

2.Quantum wires:
A quantum wire is an electrically conducting wire, in which quantum effects are affecting transport properties.

3.Quantum dots:
A quantum dot is a semiconductor whose excitons are confined in all three spatial dimensions. As a result, they have properties that are between those of bulk semiconductors and those of discrete molecules

Exciton

An exciton is a bound state of an electron and an imaginary particle called an electron hole in an insulator or semiconductor.In current research, the bound electron and hole pairs (excitons) provide a means to transport energy without transporting net charge.Since an exciton is a bound state of an electron and a hole, the overall charge for this quasiparticle is zero. Hence it carries no electric current.

A vivid picture of exciton formation is as follows: a photon enters a semiconductor, exciting an electron from the valence band into the conduction band. The missing electron in the valence band leaves a hole (of opposite electric charge) behind, to which the electron is attracted by the Coulomb force. The exciton results from the binding of the electron with its hole. As a result, the exciton has slightly less energy than the unbound electron and hole. The wavefunction of the bound state is hydrogenic (an "exotic atom" state akin to that of a hydrogen atom). However, the binding energy is much smaller and the size much bigger than a hydrogen atom because of the effects and the effective mass of the constituents in the material.In a hydrogen atom the core and the electron can have parallel or antiparallel spin, the same is true for the exciton.

Two important classes of excitons exist depending on the extent of the periodic envelope function. When the envelope function is confined to just a few unit cells, the excitons are classified as Frenkel excitons. Due to their restricted spatial extent, the Heisenberg uncertainty principle indicates that their treatment necessitates dealing with the full bandstructure of the semiconductor. On the other hand, if the envelope function extends over several hundred Angstroms, near band edge electron and hole states can be used to describe them. Such exctions are called Mott excitons and are responsible for the excitonic physics in semiconductors.

References
1.http://kottan-labs.bgsu.edu/teaching/workshop2001/chapter6.htm
2.http://en.wikipedia.org/wiki/Exciton
3.http://en.wikipedia.org/wiki/Solar_cell
4.http://en.wikipedia.org/wiki/Quantum_heterostructure
5.http://en.wikipedia.org/wiki/Quantum_dot
6.S.O. Kasap “ Priciples of electronic materials and devices 3rd edition ” p. 551-553
7.http://www1.eere.energy.gov/solar/solar_cell_structures.html
8. Martin A. Green,Keith Emery,Yoshihiro Hisikawa and Wilhelm Warta, Solar Cell Efficiency Tables
(Version 30)