The working principle of a solar cell

Referencing: Solar Energy - the Physics and Engineering of Photovoltaic Conversion, Technologies and Systems, Chapter 3.

All the basics of a solar cell are described in this post.  Firstly, the photovoltaic (PV) effect is the generation of a potential difference at the junction or interface between 2 different materials due to electromagnetic (EM) radiation.  This effect is closely related to the photoelectric effect, where the light absorbed by a material causes electrons to be emitted.  This is due to the frequency of light, which should be above the material-dependent threshold frequency.

Light, which is part of EM radiation, is understood to consist of energy quanta called photons.  A photon's energy is:

E = hv

where h is Planck's constant, and v is the frequency of the incident light.

Next, the PV effect can be described by 3 processes in sequence (refer to the diagram below):
- Photons are absorbed at the junction of materials, thereby generating charge carriers.
- Due to the junction, the photo-generated charge carriers are separated.
- Charge carriers are collected at the terminals of the junction.

In the first process, we consider ideal semi-conductors as absorbers, where there are 2 energy levels: the lower valence band edge (E_V) and the higher conduction band edge (E_C).  No allowed energy states exist between these bands for electrons.  This energy difference is known as the bandgap (E_G).  A photon is absorbed if it has an energy greater than E_G.  Upon absorption, an electron is excited from E_V to E_C.  Hence:

E_G = E_C - E_V = hv

After excitation, an empty space is created at E_V where the electron once was.  This space is called a hole and is considered a positive charge and behaves like a particle that moves.  Thus, the absorption of a photon creates an electron-hole pair.  The radiative energy of the photon is converted into the chemical energy of the electron-hole pair.

In the second process, the junction of the materials act like semi-permeable membranes, dividing the electron and hole to prevent them from recombining.  This allows the energy stored in the electron-hole pair to be used in an external circuit.  In most solar cells, n and p type materials form the membranes.  They have to be designed to allow the charge carriers to reach the membranes before recombining, which means that the absorber must not be too thick.  The absorber thickness forms part of the distance electrons and holes must travel to the membrane.

In the final process, the charge carriers are extracted from the solar cells using electrical contacts as part of an electric circuit.  The chemical energy of the electron-hole pairs is converted to electricity, where the electrons will go through the external electric circuit, and recombine with the holes at a metal-absorber interface.

Status and prospects of PV technology

Referencing: Solar Energy - the Physics and Engineering of Photovoltaic Conversion, Technologies and Systems, Chapter 2.

As of 2013, the largest cumulative installed PV power capacity is installed in Europe.  The next largest share of installed power is in Asia Pacific, of which the largest share is in Japan.  There's also a large increase in installed power in China from 2010 to 2013.  The fraction of PV installations in different countries can be seen in this diagram, courtesy of the European Photovoltaic Industry Association EPIA.

There are 2 components in the price of a PV system - module price, and the price of non-modular components of a PV system, also known as the cost of the balance of system.  As of 2011, the average retail price of PV modules has reduced to below 1 USD per Watt-peak (W_p), where W_p is the peak or maximum power a PV module produces when it is illuminated with the standardised AM1.5 solar spectrum.

However, the costs of the balance of system (racking, wiring, inverter, batteries, maintenance, etc) has not reduced as much.  Hence, PV technologies with high energy conversion efficiencies cost less to produce and deliver the same PV power.  This is because less area and non-modular components are needed for higher energy conversion efficiency systems.

A comparison can be made between the non-fossil fuel electricity generation technologies, where the total installed power of each technology is multiplied by a Capacity Factor C_F to derive the average power generated.  The final results after applying the C_F is shown in the diagram below.  It can be seen that solar power will become the most important non-fossil fuel electricity generation technology by 2020.

This forecast is due to 2 factors.  Firstly, solar energy as a renewable energy is available in abundance.  Solar energy reaching Earth is 10,000 times larger than the total energy consumption of mankind.  Secondly, solar PV systems can be installed in a decentralised manner, on the rooftops of the homes of electricity consumers.  Due to the falling costs of PV systems (below grid parity), the costs of PV generated electricity is cheaper than electricity from the power grid.

Energy

Referencing: Solar Energy - the Physics and Engineering of Photovoltaic Conversion, Technologies and Systems, Chapter 1.

The unit of energy is Joule (J) or Watt-Second(Ws), but in energy markets, a very large unit of kWh is used instead because the quantities are very large.

1 kWh = 3,600,000 Ws

The quantities are however very small in solid state physics, where we evaluate how a solar cell works.  The unit used is the electron volt (eV).

1 eV = 1.6 x 10^-19 J

Energy is never produced, but is converted from one form to another.  The largest energy consumer in a human body is the brain, and it depends on the heart to pump blood throughout the body to reach the brain.  The energy for that comes from food consumption, and the energy from pumping blood is lost in the friction and resistance in blood vessels throughout the body.  This is one example of energy conversion.

Energy is also used outside of the human body.  In modern society, these energy usage includes heat, transport, and electricity.  The demand for energy is growing due to the increase in world population and living standard.  The source or supply of our energy mostly comes from fossil fuels, which are rapidly depleted, becoming difficult to extract, and produces greenhouse gases.

There are various methods of energy conversion.  The basic process flows from thermal heat engines, produced by nuclear and chemical sources of energy, to electric generators.  Heat energy can also be used directly in society.  Electric generators can also be directly supplied with gravitational or wind energy without heat engines.  Finally, there are chemical and solar sources of energy to produce electricity directly.

The most common source of energy is solar energy, directly or indirectly extracted from the Sun.  The Sun causes temperature differences in the atmosphere to create winds.  Waves are created by winds.  Clouds and rain providing gravitational hydropower is also due to evaporation of water by sunlight.  Solar cells directly extracts solar energy to produce electricity.