CHAPTER IA-1
Principles of Solar Cell Operation
Tom Markvarta and Luis Casta?erb
aSchool of Engineering Sciences, University of Southampton, UK
bUniversidad Politecnica de Catalunya, Barcelona, Spain
1. Introduction 7
2. Electrical Characteristics 10
2.1 The Ideal Solar Cell 10
2.2 Solar Cell Characteristics in Practice 13
2.3 The Quantum Efficiency and Spectral Response 15
3. Optical Properties 16
3.1 The Antireflection Coating 16
3.2 Light Trapping 17
4. Typical Solar Cell Structures 19
4.1 The pn Junction Solar Cell 19
4.1.1 The pn Junction 19
4.1.2 Uniform Emitter and Base 23
4.1.3 Diffused Emitter 23
4.2 Heterojunction Cells 25
4.3 The pin Structure 27
4.4 Series Resistance 29
References 30
1. INTRODUCTION
Photovoltaic energy conversion in solar cells consists of two essential
steps. First, absorption of light generates an electronhole pair. The electron
and hole are then separated by the structure of the device―electrons
to the negative terminal and holes to the positive terminal―thus generating
electrical power.
This process is illustrated in Figure 1, which shows the principal features of
the typical solar cells in use today. Each cell is depicted in two ways. One
diagram shows the physical structure of the device and the dominant electrontransport
processes that contribute to the energy-conversion process.
Figure 1 a The structure of crystalline silicon solar cell, the typical solar cell in use
today. The bulk of the cell is formed by a thick p-type base in which most of the incident
light is absorbed and most power is generated. After light absorption, the
minority carriers electrons diffuse to the junction where they are swept across by
the strong built-in electric field. The electrical power is collected by metal contacts
to the front and back of the cell Chapters Ib-2 and Ib-3. b The typical
galliumarsenide solar cell has what is sometimes called a heteroface structure, by
virtue of the thin passivating GaAlAs layer that covers the top surface. The GaAlAs
‘window’ layer prevents minority carriers from the emitter electrons to reach the surface
and recombine but transmits most of the incident light into the emitter layer where
most of the power is generated. The operation of this pn junction solar cell is similar in
many respects to the operation of the crystalline silicon solar cell in a, but the substantial
difference in thickness should be noted. Chapters Id-1 and Id-2. c The structure
of a typical single-junction amorphous silicon solar cells. Based on pin junction, this
Figure 1 Continued cell contains a layer of intrinsic semiconductor that separates
two heavily doped p and n regions near the contacts. Generation of electrons and
holes occurs principally within the space-charge region, with the advantage that
charge separation can be assisted by the built-in electric field, thus enhancing the collection
efficiency. The contacts are usually formed by a transparent conducting oxide
TCO at the top of the cell and a metal contact at the back. Light-trapping features in
TCO can help reduce the thickness and reduce degradation. The thickness of a-Si solar
cells ranges typically from a fraction of a micrometer to several micrometers.
Chapter Ic-1. d, e The typical structures of solar cells based on compound semiconductors
copper indiumgallium diselenide d and cadmium telluride e. The
front part of the junction is formed by a wide-band-gap material CdS ‘window’ that
The same processes are shown on the band diagram of the semiconductor, or
energy levels in the molecular devices.
The diagrams in Figure 1 are schematic in nature, and a word of
warning is in place regarding the differences in scale: whilst the thickness
of crystalline silicon cells shown in Figures 1a and 1f is of the order
of 100 micrometres or more, the thickness of the various devices in
Figures 1b1e thin-film and GaAs-based cells might be several
micrometres or less. The top surface of the semiconductor structures
shown in Figure 1 would normally be covered with antireflection coating.
The figure caption can also be used to locate the specific chapter in this
book where full details for each type of device can be found.
2. ELECTRICAL CHARACTERISTICS
2.1 The Ideal Solar Cell
An ideal solar cell can be represented by a current source connected in parallel
with a rectifying diode, as shown in the equivalent circuit of Figure 2.
The corresponding IV characteristic is described by the Shockley solar
cell equation
I 5Iph 2Io e
qV
kBT 21 e1T
Figure 1 Continued transmits most of the incident light to the absorber layer CuIn,
GaSe2 or CdTe where virtually all electronhole pairs are produced. The top contact is
formed by a transparent conducting oxide. These solar cells are typically a few micrometers
thick Chapters Ic-2 and Ic-3. f Contacts can be arranged on the same side of
the solar cell, as in this point contact solar cell. The electronhole pairs are generated in
the bulk of this crystalline silicon cell, which is near intrinsic, usually slightly n-type.
Because this cell is slightly thinner than the usual crystalline silicon solar cell, efficient
light absorption is aided here by light trapping: a textured top surface and a reflecting
back surface Chapter Ib-3. g, h The most recent types of solar cell are based on
molecular materials. In these cells, light is absorbed by a dye molecule, transferring an
electron from the ground state to an excited state rather than from the valence band to
the conduction band as in the semiconductor cells. The electron is subsequently
removed to an electron acceptor and the electron deficiency hole in the ground state
is replenished from an electron donor. A number of choices exist for the electron acceptor
and donor. In the dye-sensitised cell g, Chapter Ie-1, the electron donor is a redox
electrolyte and the role of electron acceptor is the conduction band of titanium dioxide.
In plastic solar cells h, Chapter Ie-2, both electron donor and electron acceptor are
molecular materials.
Figure 2 The equivalent circuit of an ideal solar cell full lines. Nonideal components
are shown by the dotted line.
where kB is the Boltzmann constant, T is the absolute temperature, q
.0 is the electron charge, and V is the voltage at the terminals of the
cell. Io is well known to electronic device engineers as the diode saturation
current see, for example, [1], serving as a reminder that a solar cell
in the dark is simply a semiconductor current rectifier, or diode. The
photogenerated current Iph is closely related to the photon flux incident
on the cell, and its dependence on the wavelength of light is frequently
discussed in terms of the quantum efficiency or spectral response see
Section 2.3. The photogenerated current is usually independent of the
applied voltage with possible exceptions in the case of a-Si and some
other thin-film materials [24].
Figure 3a shows the IV characteristic Equation 1. In the ideal
case, the short-circuit current Isc is equal to the photogenerated current
Iph, and the open-circuit voltage Voc is given by
Voc 5
kBT
q
ln 11
Iph
I0 e2T
The maximum theoretically achievable values of the short-circuit current
density Jph and of the open-circuit voltage for different materials are
discussed and compared with the best measured values in Chapter Ia-3.
The power P 5 IV produced by the cell is shown in Figure 3b. The
cell generates the maximum power Pmax at a voltage Vm and current Im,
and it is convenient to define the fill factor FF by
FF 5
ImVm
IscVoc
5
Pmax
IscVoc e3T
The fill factor FF of a solar cell with the ideal characteristic 1 will be
furnished by the subscript 0. It cannot be determined analytically, but it
Figure 3 The IV characteristic of an ideal solar cell a and the power produced by
the cell b. The power generated at the maximum power point is equal to the
shaded rectangle in a.
can be shown that FF0 depends only on the ratio voc5VockBT. FF0 is
determined, to an excellent accuracy, by the approximate expression [5]
FF0 5
voc 2lnevoc 10:72T
voc 11
The IV characteristics of an ideal solar cell complies with the superposition
principle: the functional dependence 1 can be obtained from the
corresponding characteristic of a diode in the dark by shifting the diode
characteristic along the current axis by Iph Figure 4.
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