Solar cells

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Solar cells

"The Sun, with all the planets revolving around it, and depending on it, can still ripen a bunch of grapes as though it had nothing else in the Universe to do."
(Galileo Galilei)

Thin film solar cell Solar cell surface CIS solar cell

Solar cells
(source/copyright: Hahn-Meitner-Institut Berlin)

Solar cells are in fact large area semiconductor diodes. Due to photovoltaic effect energy of light (energy of photons) converts into electrical current. At p-n junction, an electric field is built up which leads to the separation of the charge carriers (electrons and holes). At incidence of photon stream onto semiconductor material the electrons are released, if the energy of photons is sufficient. Contact to a solar cell is realised due to metal contacts. If the circuit is closed, meaning an electrical load is connected, then direct current flows. The energy of photons comes in "packages" which are called quants. The energy of each quantum depends on the wavelength of the visible light or electromagnetic waves. The electrons are released, however, the electric current flows only if the energy of each quantum is greater than WL - WV (boundaries of valence and conductive bands). The relation between frequency and incident photon energy is as follows:

Where there is: h - Planck constant (6.626·10^-34 Ws²), v - frequency (Hz)

Solar cell features

Crystalline Silicon Solar Cells

Among all kinds of solar cells we describe silicon solar cells only, for they are the most widely used. Their efficiency is limited due to several factors. The energy of photons decreases at higher wavelengths. The highest wavelength when the energy of photon is still big enough to produce free electrons is 1.15 μm (valid for silicon only). Radiation with higher wavelength causes only heating up of solar cell and does not produce any electrical current. Each photon can cause only production of one electron-hole pair. So even at lower wavelengths many photons do not produce any electron-hole pairs, yet they effect on increasing solar cell temperature. The highest efficiency of silicon solar cell is around 23 %, by some other semi-conductor materials up to 30 %, which is dependent on wavelength and semiconductor material. Self loses are caused by metal contacts on the upper side of a solar cell, solar cell resistance and due to solar radiation reflectance on the upper side (glass) of a solar cell. Crystalline solar cells are usually wafers, about 0.3 mm thick, sawn from Si ingot with diameter of 10 to 15 cm. They generate approximately 35 mA of current per cm² area (together up to 2 A/cell) at voltage of 550 mV at full illumination. Lab solar cells have the efficiency of up to 20 %, and classically produced solar cells up to 15 %.

Monocrystalline solar cells Polycrystalline solar cells

Monocrystaline solar cells - left (Photo: Denis Lenardic), Polycrystaline solar cells - right (Source/Copyright Solar-fabrik).

Amorphous Silicon Solar Cells

The efficiency of amorphous solar cells is typically between 6 and 8%. The Lifetime of amorphous cells is shorter than the lifetime of crystalline cells. Amorphous cells have current density of up to 15 mA/cm2, and the voltage of the cell without connected load of 0.8 V, which is more compared to crystalline cells. Their spectral response reaches maximum at the wavelengths of blue light therefore, the ideal light source for amorphous solar cells is fluorescent lamp.

Solar cell I-U characteristics

Solar cell power

Solar cell characteristics.

Solar cell testing

Test flow and test criteria for solar cells can you find on Qcells web site, if you follow the link.

Photovoltaic modules

A photovoltaic module is the basic element of each photovoltaic array. It consists of many jointly connected solar cells. According to the solar cell technology we distinguish monocrystalline, polycrystalline and amorphous solar modules. More about photovoltaic modules can you read in module section.

Solar cell models

The simplest solar cell model consists of diode and current source connected parallelly. Current source current is directly proportional to the solar radiation. Diode represents PN junction of a solar cell. Equation of ideal solar cell, which represents the ideal solar cell model, is:

Ideal solar cell equation

Where is: I_Ph - photocurrent (A), I_S - reverse saturation current (A) (aproximately range 10^-8/m²), V - diode voltage (V), V_T - thermal voltage (see equation below), V_T = 25.7 mV at 25°C, m - diode ideality factor = 1...5 x V_T (-) (m = 1 for ideal diode)

Ideal solar cell model

Thermal voltage / V_T / ( V ) can be calculated with the following equation:

Where is: k - Boltzmann constant = 1.38 x 10^-23 J/K, T - temperature ( K ), q - charge of electron = 1.6 x 10^-19 As

Real Solar cell model with serial and parallel resistance R_s and R_p,
the consequences of resistances are voltage drop and parasitic currents

The working point of the solar cell depends on load and solar insolation. In the picture, I-U characteristics at short circuit and open circuit conditions can be seen. Very important point in I-U characteristics is Maximal Power Point - MPP. In practice we can seldom reach this point, because at higher solar insolation even the cell temperature increases, and consequently decreasing the output power. As a measure for solar cell quality fill-factor - FF is used. It can be calculated with the following equation:

Fill Factor

Where is: I_mpp - MPP current ( A ), V_mpp - MPP voltage ( V ), I_sc - short cirquit current ( A ), V_oc - open cirquit voltage ( V )

In the case of ideal solar cell fill-factor is a function of open cirquit parameters and can be calculated as follows (Stone, see literature below):

Where is: v_oc - voltage calculated with equation below ( V )

Where is: k - Boltzmann constant = 1.38 x 10^-23 J/K, T - temperature ( K ), q - charge of electron = 1.6 x 10^-19 As, m - diode ideality factor ( - ), V_oc - open cirquit voltage ( V )

For additional explanations and further solar cell models description please see literature below (Quaschning, Stone, Wagner for example).

Solar cells related web sites

Q Cells - Production of solar cells explained in a short movie "How is a solar cell produced?"

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Webmaster's choice - interesting video about solar cell production.

ProjectSol - On these pages you can take a interesting journey into a photovoltaic cell. You will learn how the silicon atoms convert sunlight into electricity.

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Webmaster's choice - interesting Flash animation about solar cells and photovoltaic conversion.

New technologies - spherical solar cells

Spheral Solar Power - Spheral Solar Power cells produce electricity at considerably lower cost than conventional solar technology, and on a cost-par with fossil-fuel based electricity in many regions of the world.

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Webmaster's choice - spherical solar cells, principles and applications.

Kyosemi Corporation - provides high reliability products of opto-electronics to be used for optical communications, ATM, auto-vending machines, OA/FA equipments, precision optical instruments and hazard prevention equipments.

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New technologies - organic photovoltaic cells and photoelectrolysis

Global Photonic Energy Corporation - Incorporated in 1994 Global Photonic Energy Corporation, Inc. is a renewable energy technology development company. GPEC is harnessing photonic energy (Sunlight) using small-molecule organic materials to produce electricity and hydrogen - or - "Photo Fuel™".

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Literature and more information

Wagner, A.: Photovoltaik Engineering; Die Methode der Effektiven Solarzellen-Kennlinie; Springer, 1999.
Quaschning, V.: Simulation der Abschattungsverluste bei solarelektrischen Systemen; Verlag Dr. Köster Berlin, 1. Auflage September 1996.
Photovoltaische Anlagen; Leitfaden für Elektriker, Dachdecker, Fachplaner, Architekten und Bauherren.
Solarserver: Photovoltaics, solar electricity and solar cells in theory and practice.
Grunow, P., Lust, S., Sauter, D., Hoffmann, V., Beneking, C., Litzenburger, B., Podlowski, L.: Weak light performance and annual yields of PV modules and systems as a result of the basic parameter set of industrial solar cells; Proc. of the 19th PVSEC, Paris, 2004, p. 2190.
Grunow, P., Clemens, P., Hoffmann, V., Litzenburger, B., Podlowski, L.: Influence of micro cracks in multi-crystalline silicon solar cells on the reliability of PV modules; Proc. of the 20th PVSEC, Barcelona, 2005, 5BV.4.26.
Grunow, P., Sauter, D., Hoffmann, V., Huljic, D., Litzenburger, B., Podlowski, L.: The influence of textured surfaces of solar cells and modules on the energy rating of PV systems; Proc. of the 20th PVSEC, Barcelona, 2005, 5BV.4.27.
Measuring Photovoltaic Cell I-V Characteristics with the Model 2420 3A SourceMeter® Instrument. (27 kB)
National Centre for Photovoltaics - NCPV - The Basic Physics and Design of III-V Multijunction Solar Cells - Part 1. (630 kB)
Brabec, C.J., Sariciftci, N.S., Hummelen, J.: Plastic Solar Cells, Adv. Funct. Mater. 2001, 11, No. 1, February. (637 kB)
Petritsch, K.: Organic Solar Cell Architectures.