The Technology Of Solar Cells

Solar Cell Overview


Introduction

Solar cells are devices which use the photoelectric effect to convert electromagnetic radiation that falls on them into electricity1. They are constructed from a semiconductor material, like silicon, linked to a metal conductor and covered by a layer of anti-reflective glass that protects the semiconductor wafers from damage whilst still allowing light through.
Individual cells, otherwise known as photovoltaic (PV) cells, are assembled together as panels or modules which are then further grouped together as arrays2. Cells can be connected in series, to boost voltage, or parallel, to for maximum current.

Operation

When photons of electromagnetic radiation strike a semiconductor material they either: reflect off the surface, pass through without striking anything or are absorbed by electrons in the material’s crystal lattice. These electrons are in the valence band which means they are tightly bound in covalent bonds between atoms and unable to move. Absorbing energy from a photon, however, can excite them into the so called ‘conduction band’ where they are free to move throughout the material. The excited electron leaves behind a free space in the lattice which can be filled by an electron from a neighbouring atom. This leaves another space which is filled by an electron from the next atom and so on. In this way positive ‘holes’ can then be said to move through the material in a similar way to negative electrons.
These electron-hole pairs then flow through the cell, either by an electrostatic field established across it (as in p-n junction cells) or by a gradient in the electrochemical composition (as in newer thin film cells), and are ‘collected’ at metal contacts as a current which can then power an external load.
Photovoltaic cells are limited by the ‘band gap’; or energy difference between the valence and conduction bands. Photons with exactly this energy create one electron-hole pair, in which case all their energy goes to electrical output. However photons of lower energies are absorbed as heat and do not contribute at all to usable energy. Similarly photons which higher energies are partly wasted as the extra energy again goes to heating up the cell via lattice vibrations (phonons). As the sun emits radiation across the whole range of photon energies (or equivalently; wavelengths of light) large parts of the spectrum will not contribute to electrical power, limiting cell efficiency.
Other limitations on efficiency include optical losses such as reflection, electrical resistance, within the cell and connectors, and impurities and defects in the crystal structure.

Types of cell

Most common solar cells are of the silicon p-n junction type so these will be discussed here. Other types of cells such as thin film cells, light absorbing dyes and organic/polymer cells do exist but these are seldom used for spacecraft panels so will be bypassed for now.
P-n junction cells consist of two types of silicon with slightly different electrical properties, placed in contact. Excited electrons diffuse across this junction to recombine with holes but can only flow one way due to the electric field created by the imbalance of charge (due to the different way the two types of silicon react to incident radiation). This diode like effect leads to a current flow in one direction which can then proceed down any connected wire to the load. These cells can then be single junction or multijunction.
Single junction cells have just two types of silicon and are typical of older, first generation panels. Multijunction cells consist of many different semiconductor materials (for example gallium arsenide GaAs, germanium Ge and gallium indium phosphide GaInP) sandwiched together in layers. This creates a cell with several different band gaps, allowing photons of many different energies to excite electrons and therefore allowing larger parts of the electromagnetic spectrum to be covered. Modern commercial, multijunction cells can reach efficiencies of up to 29%4, while experimental models under laboratory conditions have recorded over 40% efficiency5 at converting light into electrical power.

Clyde Space

Clyde Space are a Glasgow based company that purchases solar cells from Spectrolab and EMCORE and assembles them in various panels. Clyde Space’s products include ‘plug and play’ modules developed specially for CubeSat sizes 1U, 2U and 3U, as well as customisable panels up to 500 by 500mm6. The cells used are high efficiency triple junction cells.
Clyde Space are experts in the field of solar power for small satellites and can deliver custom assembled cells in eight weeks. They also sell corresponding batteries and other power subsystems6.
For data on specific cells including types, dimensions, masses and efficiencies see solar cell spreadsheet.

Solar Cell Thermal Properties

According to Spectrolab the emissivity (or emittance ε) and absorptance (α) of their Triangular Advanced Solar Cell is7:

ε=0.85
α=0.92

EMCORE have so far not replied to a query for values for their Advanced Triple Junction Cell but for now a good approximation will be their Dual Junction Solar cell which is made of the same material in a slightly different construction. The values for this are8:

ε=0.87
α=0.90

These values are dependent of wavelength, emission/absorption angle and temperature, but a typical engineering approximation is to say that emissivity does not depend on wavelength and so they are constants. This is the so called ‘grey body’ approximation.
Grey bodies radiate heat with a power given by the Stefan-Boltzmann law and absorb heat dependent on their area9:

P_r=εσAT^4

where Pr is total power radiated, ε is emissivity as above, σ=5.6703×〖10〗^(-8) W/(m^2 K^4) is Stefan’s constant, A is the object’s total area and T its temperature.

P_a=α A_p

where Pa is total power absorbed from the Sun, α is absorptance as above, is solar flux (taken as air mass zero = 1366.1 Wm-1, see power production) and Ap is the area the object projects to the Sun. This value will change depending on the exact orientation of the satellite’s spin axis (again, see power production).
If the object can be assumed to be in thermal equilibrium, such as our satellite, then the absorbed power will be the same as the radiated power and these two equations can be equated to find the equilibrium temperature:

P_a=α A_p=P_r=εσAT^4
T=∜((α A_p)/εσA)

The peak wavelength of the radiation emitted by the object will depend on this temperature and is given by Wien’s displacement law9:

λ_max=2.898mmK/T

where T is the object’s temperature in kelvin.

References

1. http://www.electronicsweekly.com/Articles/2007/06/29/41717/printed-electronics-is-the-sky-the-imit.htm
2. http://www.spectrolab.com/prd/prd.asp
3. The Basic Physics and Design of III-V Multijunction Solar Cells
4. http://www.spectrolab.com/DataSheets/NeXtTJ/nxttj.pdf
5. http://www.nrel.gov/news/press/2008/625.html
6. http://www.clyde-space.com/what_we_do/tailored_systems/solar_panels
7. http://www.spectrolab.com/DataSheets/TNJCell/tnj.pdf
8. http://www.emcore.com/assets/photovoltaics/SPRAT_Paper_Final.pdf
9. Physics for scientists and Engineers (6th edition) Paul A Tipler and Gene Mosca


Information on the Solar Cells Available

Company Product Name/No Mass (kg) Type (Silicon, Gallium, Arsenide etc.) Min. Efficiency (%) Dimensions Source
EMCORE Triple-Junction Satellite Solar Cell 84 mg/cm2 Germanium Triple Junction 28.5% 26.6 cm2 x 140 μm (also custom available) http://www.emcore.com/assets/photovoltaics/Emcore_BTJ_DS_May_07.pdf
EMCORE Triple-Junction w/ Monolithic Diode 84 mg/cm2 Germanium Triple Junction 28.0% 26.6 cm2 x 140 μm (also custom available) http://www.emcore.com/assets/photovoltaics/Emcore+BTJM+Solar+Cell+Data+Sheet_May-07.pdf
EMCORE Adv. Triple Junction (ATJ) 84 mg/cm2 Germanium Triple Junction 27.5% 26.6 cm2 x 140 μm (also custom available) http://www.emcore.com/assets/photovoltaics/PhotoV_ATJ_Prod_Sheet.pdf
EMCORE Adv. Triple-Junction w/ Monolithic Diode 84 mg/cm2 Germanium Triple Junction 27.0% 27.5cm2 x 140 μm (also custom available) http://www.emcore.com/assets/photovoltaics/ATJM_Solar_Cell_DS.pdf
Spectrolab Next Triple Junction (XTJ) Solar Cells 84 mg/cm2 Germanium Triple Junction 29.9% Up To 60 cm2 x 140 μm http://www.spectrolab.com/DataSheets/NeXtTJ/nxttj.pdf
Spectrolab Ultra Triple Junction (UTJ) Solar Cells 84 mg/cm2 Germanium Triple Junction 28.3% Up to 32 cm2 x 140 μm http://www.spectrolab.com/DataSheets/TNJCell/utj3.pdf
Spectrolab Improved Triple Junction (ITJ) Solar Cells 84 mg/cm2 Germanium Triple Junction 26.8% Up to 31 cm2 x 175 μm http://www.spectrolab.com/DataSheets/TNJCell/tnj.pdf
Spectrolab GaInP2/GaAs/Ge Triple Junction Solar Cells 84 mg/cm2 Germanium Triple Junction 25.1% Up to 30 cm2 x 175 μm http://www.spectrolab.com/DataSheets/TJCell/tj.pdf
Spectrolab GaInP2/GaAs/Ge Dual Junction Solar Cells 84 mg/cm2 Germanium Dual Junction 21.5% Up to 30 cm2 x 175 μm http://www.spectrolab.com/DataSheets/DJCell/dj.pdf
Spectrolab GaAs/Ge Single Junction Solar Cell 100 mg/cm2 Germanium Single Junction 19.0% Up to 49 cm2 x 175 μm or 140 μm http://www.spectrolab.com/DataSheets/SJCell/sj.pdf
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