1
A Review of David
Photovoltaic
Cells
Toub
Department of Electrical and Computer Engineering, University of Rochester, Rochester New York 14627
DLToub@Gmail.com
Abstract— Photovoltaic cells provide an additional method of
acquiring energy, converting sunlight directly into electricity
through the use of semiconductors. Effective photovoltaic
implementation is reviewed, focusing on semiconductor
properties and overall photovoltaic system configuration.
Index Terms—energy conversion efficiency, photovoltaic, PV,
solar cell
I. INTRODUCTION
allowable energies of electrons which have received some
form of energy and are no longer bound to host atoms.
Semiconductors, characterized as being perfect insulators at
absolute zero, become increasingly conductive as temperature
is increased. As temperature becomes greater, sufficient
energy is transferred to a small fraction of electrons, causing
them to move from the valence band to the conduction band
and holes to move from the conduction band to the valence
band. The increase in temperature responsible for this entire
process is a direct result of external energy; in the case of PV
systems, it is incident photons due to illumination [1,2,4].
Energy policies have pushed for different technologies to
decrease pollutant emissions and reduce global climate
change. Photovoltaic technology (PV), which utilizes sunlight
to generate energy, is an attractive alternate energy source
because it is renewable, harmless, and domestically secure.
Because PV technology’s basic component is the PV cell
which produces less than three watts on average, cells must be
bundled in series/parallel configurations known as PV
modules or solar cells to achieve high powered tasks.
PV arrays produce power only when illuminated, and it is
therefore standard to employ a large energy storage
mechanism, most commonly a series of rechargeable batteries.
To prevent harmful battery overcharge and overdischarge
conditions and to drive AC loads, a charge controller and an
AC to DC converter must be implemented [1]. The primary
objective is to optimize PV cells and energy storage and to
increase overall system efficiency. In order to discuss
optimization, one must have a basic understanding of how PV
cells and storage mechanisms function.
II. FUNCTIONALITY
PV functionality relies upon the absorption of light within a
bulk or semiconductor material, most commonly a silicon pn
diode, providing a medium in which incident photons can be
converted to energy, usually in the form of heat. When
absorbed, a photon transfers energy to an electron in the
absorbing material and if the magnitude of incident photon
energy is greater than the electron’s work function, the photon
may raise an electron’s energy state or even liberate an
electron. Once liberated, the electrons are then free to move
around the semiconductor material influenced by present
phenomena of diffusion, temperature, and electric field [1, 2,
3].
The quantum theory of semiconductor devices states that all
semiconductors have a gap between their valence and
conduction bands. The valence band represents all allowable
energies of valence electrons that are bound covalently to
neighboring host atoms, and the conductive band represents all
Under the photoelectric effect, because photons incident
upon a pn diode can create electron-hole pairs at a cross
material junction, an electric potential difference across this
junction can be established. Under no illumination, electrons
and holes are separated at n and p regions respectively due to
the diode characteristic unidirectional current path. When
illuminated, PV cells are impacted by incident photons which
bombard cell electrons creating electron hole pairs. These
electron hole pairs then separate in response to the electric
2
field created by the cell junction, causing electrons to drift
back into the n region, and holes into the p region. A
bidirectional current path is created and energy can be
harnessed. With basic PV function understood, a solar cell
can now be designed [5,6]
III. PV CELL DESIGN
Because a PV cell is a simple pn diode, the well known
voltage transfer characteristic equations will be incorporated
into the design process.
As such, these characteristic
equations provide a means of determining ideal PV cell
performance limits. The VTC graph in figure 3 illustrates that
the cell has both a limiting voltage and current so open circuit
and short circuit operating conditions will not be detrimental
to its function. Under zero applied voltage, the short circuit
current simply becomes the photon induced current while the
open circuit voltage can be found by setting the cell current to
0, as shown by equations 3a and 3b respectfully [1,8].
It should be noted that open circuit voltage is only
logarithmically dependent on cell illumination while the short
circuit current is directly proportional. Because PV cells are
highly expensive, maximum power efficiency is desired. This
maximum point can be determined through differentiation or
by inputting open circuit and short circuit values into the
maximum power equation Pm = Vmax x Imax. Once VTC
conditions are found, the actual material composition and
layout of the cell must be determined [1,7].
module area and irradiance (power for electromagnetic
radiation at a surface), mainly =Pm/(E x Ac) [8].
When considering loss, semiconductor selection and contact
layout is of primary importance. Low band gap energy allows
photon energy to be more efficiently transferred to electrons
but results in a decreased electric field which in turn reduces
voltage. In the case of bulk devices, the optimal efficiencyvoltage trade off results when a semiconductor with a band
gap of 1.4 eV is used [1,6].
To
achieve
good
conduction
surrounding
the
semiconductor, the cell bottom is completely covered with
metal while top cell metal must be transparent or specially
arranged as not to block incident photons and further increase
loss. To achieve good conduction within the semiconductor, a
metallic cell grid reduces electron travel distance, thereby
decreasing resistance and ultimately loss. If this grid is too
large, it will block incident photons while if the grid is too
small, resistance will increase, yielding more loss [1,7].
When considering semiconductor variety, various groups of
crystalline silicon are used. These groups are separated by
crystallinity and crystal size in the ingot or wafer.
Monocrystalline or single crystalline silicon (c-Si) tends to be
expensive, and does not completely cover a square solar cell
module as it is cut from cylindrical ingots. Poly-C or
multicrystalline silicon (poly-Si or mc-Si) is formed from
large blocks of molten silicon carefully cooled and solidified.
These cells are less expensive and although they can cover a
full solar cell, they are less efficient. Ribbon silicon is formed
by drawing flat thin films from molten silicon but having a
multicrystalline structure. These cells have lower efficiencies
than poly-Si, but are extremely inexpensive due to reduced
silicon waste [5,4].
IV. FUTURE ADVANCEMENTS
To establish cell composition and layout, one must be
familiar with PV’s inherent problems and design
considerations. Photons, possessing a wide range of energies,
may or may not overcome band gap energy to knock an
electron loose. At the same time, if a photon possesses much
more energy than the required band gap energy, the extra
energy is lost. Because these phenomena hinder PV designs to
an average of 30% energy conversion efficiency, a designer’s
ultimate goal is to implement cell layout and composition to
meet efficiency specifications [6,7].
Energy conversion efficiency represents the percentage of
power converted and collected when a solar cell is connected
to an electrical circuit. Efficiency can be measured by
dividing the maximum power by the product of total solar
Although 86% of PV cells are designed with this first
generation semiconductor approach, second and third
generation cells consist of thin film deposits and electron
confined nanoparticle materials. Thin film technologies
reduce the required mass of light absorbing material, resulting
in reduced processing costs but also reduced energy
conversion efficiency. Because these thin films are nearly
mass-less, they can be stacked to form multiple layer film cells
which yield an average of 30% efficiency while standard
semiconductor efficiency is limited to 14% [9]. Utilizing the
same thin-film light absorbing materials, nanocrystalline solar
cells increase efficiency as they are covered with an extremely
thin coating of mesoporous metal oxide whose high surface
area helps to increase internal reflections and ultimately light
absorption probability and efficiency. This increase of
internal reflection helps to boost nanocrystalline PV cell
efficiency to over 40% [10,11].
V. EXTERNAL D ESIGN CONSIDERATIONS
One important design aspect often underestimated is the
structure used to support the PV array. A designer must
consider the load bearing weight of heavy modules, their
support structures, and snow and ice for certain climates.
Changes in wind complicate the job of a structural engineer,
resulting in unequal stress and tension dispersion throughout
3
the PV system. Often overlooked is pollution and midair
debris which may adhere to the photocells and hinder
illumination and photon interaction.
The PV module
orientation must be modified to reflect a specific angle of
incidence corresponding to the pollution altered index or
refraction [6].
When designing a storage system, one must consider how
much energy is required to perform the desired auxiliary
function. Given a simple PV-fan system example where a
solar module powers a fan, a designer must determine how
fast the fan must spin, if the speed is constant, and how often
the fan will be used without illumination. The amount of
power to run an appliance is normally provided in Ampere
Hours (Ah) obtained by dividing energy by voltage. A
designer can determine how long the fan is to function without
illumination, compute an Ah power requirement, and
ultimately decide on a battery provided this Ah rating. It is
also important to consider parasitic capacitance and resistance
effects due to storage mechanism-PV cell interconnects so a
corrected PV system load can be evaluated. This altered load
can impact the PV cell’s maximum power point and must be
considered [1,7].
VII. CONCLUSION
PV cells are a proven environmentally benign power source
whose attractive characteristics will continue to further
photovoltaic research. Because current PV systems are still
highly inefficient and uncommon, they are not yet cost
competitive with fossil fuel-based generators and are only
regularly used where there is no nearby power source.
Photovoltaic advancements in the fields of thin film and
nanocrystalline materials will continue to flourish and soon
increase PV efficiency to over 50%. As efficiency increases,
PV technology will attract a greater number of people,
resulting in reduced cost. Because the sun delivers ten
thousand times more energy than people currently consume,
photovoltaic improvements will one day replace
environmentally unfriendly power plants with a proven and
clean energy source [13].
VI. APPLICATIONS
With function and design in mind, one must inquire about
PV applications. PV cells are ideal energy candidates in areas
where electric-grid extensions are not offered, and where a
clean, environmentally friendly power source is desired.
Common examples of PV devices include roof-top
residential/commercial systems, remote water pumping
stations, telecommunications equipment, and traffic lights [6].
In the most popular application of a solar powered house,
PV cells absorb photons, send DC current through an inverter
which transforms the signal to 120 or 240-volt to utilize AC
appliances. The AC power enters the utility panel in the house
and is then distributed to appliances throughout the house.
Electricity that is not used will be recycled and reused in other
facilities [6,7].
Because PV cells represent an alternate energy source, its
applications are endless. Common examples include solar
fountain pumps, garden lights, water heaters, stand alone
battery chargers, automobiles, satellites, shuttles, and utility
grid sources [1].
Among the most impressive of PV improvements and
applications lies within nanotechnology, which allowed
scientists to create a plastic spray-on PV cell that can utilize
the sun’s infrared, invisible rays. Because the infrared
spectrum is utilized, solar cells can generate electricity even
on a cloudy day. Similar to paint, this composite can be
simply sprayed onto almost any material to serve as portable
electricity [13].
A piece of clothing coated with this composite could power
a cell phone or other wireless devices. A film coated hydrogen
powered automobile could continually recharge a car's battery.
Researchers envision futuristic "solar farms" where this plastic
material could be rolled across deserts to supply enough clean
energy for the entire planet's power needs [13].
REFERENCES
[1]
Ventre, Gerard. Messenger, Roger A. Ventre, Jerry. Photovoltaic
Systems Engineering. CRC Press Technology and Industrial. 2004
[2] Harmon, C. “Experience Curves of Photovoltaic Technology.” IIASA
Publications. 2000.
[3] “Photovoltaics.” Wikipeda, The Free Encyclopedia. Downloaded from
www.wikipedia.org on 12/02/06.
[4] American Journal of Physics -- Volume 61, Issue 3, pp. 286-287
American Association of Physics Teachers. March 1993
[5] “Two layer organic Photovoltaic Cell.” -- Volume 48, Issue 2, pp. 183185 Research Laboratories, Eastman Kodak Company, Rochester, New
York. Applied Physics Letters -- January 13, 1986
[6] Green, M. A. Solar cells: Operating principles, technology, and system
applications. Englewood Cliffs, NJ, Prentice-Hall, Inc., 1982
[7] F Lasnier. Photovoltaic Engineering Handbook TG Ang - A. Hilger
New York. 1990
[8] O’Regan, B. & Grätzel, M. “A low-cost, high efficiency solar cell based
on dye-sensitized colloidal TiO2 films.” Nature 353, 737–740 (1991).
[9] Shah, A. Torres, P., Tscharner, R. “Photovoltaic technology: the case
for thin-film solar cells.” Neuchatel, Switzerland. University of Applied
Science, Avenue de l'Hotel-de-Ville 7, CH-2400 Le Locle, Switzerland.
[10] Wohlre, Dieter. Meissner, Dieter. “Organic Solar Cells.” Advanced
Materials. Volume 3, Issue 3. Verlag GmbH & Co. KGaA, 1991.
[11] McCann,MichelleJ. Catchpole,KylieR. Weber, Klaus J. “A review of
thin film crystalline silicon for solar cell applications. Part 1 : native
substrates.” 2001
[12] Lovgren, Stefan.
“Spray-On Solar Power Cells Are True
Breakthrough.” National Geographic News.