Category Principles of Solar Cell Operation

TYPICAL SOLAR CELL STRUCTURES

1.3. The p-n Junction Solar Cell

The planar p-n junction solar cell under low injection is usually singled out for special analysis since realistic approximations exist that allow analytic solu­tions to be developed and used successfully for the description of practical devices. The success of this model is due, to a large extent, to the clear way the cell can be divided into three regions—emitter, junction region, and base—that serve a different purpose in solar cell operation.

The emitter and the base—which remain largely neutral during the cell operation—absorb the main part of the incident light and transport the pho­togenerated minority carriers to the junction...

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Light Trapping

In solar cells with a simple geometry, light rays enter the cell through the front surface and, if not absorbed, leave through the rear surface of the cell. More sophisticated arrangements exist that extend the path of light inside the cell, and they are usually referred to as optical confinement or light trapping. In crys­talline or amorphous silicon solar cells, light trapping is used to reduce the thickness of the cell without lowering the light absorption within the cell. Light trapping can also be used to enhance the open-circuit voltage [8,9].

The most common light-trapping features include a textured top surface combined with an optically reflecting back surface (Figure 9)...

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OPTICAL PROPERTIES

1.2. The Antireflection Coating

Most solar cells rely on a thin layer of a dielectric (an antireflection coating) to reduce the reflection of light from the front surface of the cell. This section gives a brief description of the reflection of light from a bare semiconductor, and from a semiconductor with a single-layer antireflection coating. The discussion is confined to the case of normal incidence of light onto a smooth planar surface.

Подпись: R Подпись: (n - 1)2 + k2 (n + 1)2 + k2 Подпись: (8)

The reflection coefficient from bare silicon for light incident from air is given by

where n and k are the refractive index and the extinction coefficient of the semiconductor, both in general functions of the wavelength 1 of light in vacuum. The extinction coefficient is related to the absorption coefficient a by

Подпись:k al

4pn

For single-layer ant...

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The Quantum Efficiency and Spectral Response

The quantum efficiency of a solar cell is defined as the ratio of the number of electrons in the external circuit produced by an incident photon of a given wavelength. Thus, one can define external and internal quantum efficiencies (denoted by EQE(l) and IQE(l), respectively). They differ in the treatment of photons reflected from the cell: all photons impinging on the cell surface are taken into account in the value of the EQE, but only photons that are not reflected are considered in the value of IQE.

If the internal quantum efficiency is known, the total photogenerated current is given by

Iph = q Ф(1){1 – R(1)}IQE(1)d1 (6)

(i)

where Ф(1) is the photon flux incident on the cell at wavelength l, R(l) is the reflection coefficient from the top surface (see Section 3...

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Solar Cell Characteristics in Practice

The I-V characteristic of a solar cell in practice usually differs to some extent from the ideal characteristic (1). A two-diode model is often used to fit an observed curve, with the second diode containing an ‘ideality factor’ of 2 in the

image011 Подпись: (4)

denominator of the argument of the exponential term. The solar cell (or circuit) may also contain series (Rs) and parallel (or shunt, Rp) resistances, leading to a characteristic of the form

where the light-generated current Iph may, in some instances, depend on the voltage, as we have already noted. These features are shown in the equivalent circuit of Figure 2 by the dotted lines...

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ELECTRICAL CHARACTERISTICS

1.1. 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 I-V characteristic is described by the Shockley solar cell equation

I = Iph – Io(eW – 1^ (1)

where kB is the Boltzmann constant, Tis 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

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...

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INTRODUCTION

Photovoltaic energy conversion in solar cells consists of two essential steps. First, absorption of light generates an electron-hole 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 electron – transport processes that contribute to the energy-conversion process. The same processes are shown on the band diagram of the semiconductor, or energy levels in the molecular devices.

Solar Cells. http://dx. doi. org/10.1016/B978-0-12-386964-7...

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