The direct conversion of sunlight into electricity uses thephotovoltaic effect [54–56]. The focus of the present work is on the characterization of silicon-based devices designed to enable photovoltaic energy conversion of sunlight energy into electricity. For more infor- mation on photovoltaic energy conversion, solar cell physics, and design architectures, the reader is referred to the literature [28, 31, 57–65].
Solar electric energy generation using the photovoltaic effect may be enabled using
Figure 1.1.3: The photovoltaic effect at a pn junction showing the formation of an electron and hole due to the absorption of a photon of energy ¯hω. The bulk is p- type, while the emitter is n-type. The majority of the base region is quasi-neutral with respect to the electric potential, and diffusion of the photo-generated minority carriers dominates. Some minority carriers (electrons in the case of a p-type base) diffuse towards the depletion region and are influenced by the electric field close to the pn junction. The electric field formed at the junction region drives the excess charge carriers so that the electrons will flow in only one direction. The generation of an electric field at the depletion region establishes a voltage on the device, while the flow of charges establishes a current. The current and voltage together establish the output power of the device.
an electronic device called a solar cell [66–68]. Figure 1.1.3 shows a cross-section of a solar cell. The photovoltaic effect was discovered in 1839 by Alexandre Edmond Becquerel [54]. Becquerel noticed that certain metal/electrolyte systems provided electrical currents upon incidence of light [54, 55]. This effect is due to the liberation of electrons upon absorption (annihilation) of photons. A combination of the diffusion of liberated electrons (holes) and forces on these charge carriers by the electric fields in the material causes charge carriers to flow, producing a net current and a voltage at the device terminals [56, 62, 69]. Modern solar cells use semiconductors, not electrolyte solutions6, to harness the photovoltaic effect. Silicon-based technologies dominate the market [58, 70].
When sunlight is incident on a solar cell, the energy is transferred to electrons bound in the material [71, Chapter 8, p. 195]. Electrons will go from a lower energy state called the valence band, into a higher energy state called the conduction band, leaving behind a positive charge in the valence band, known as a hole. The minimum photon energy
6With the exception of organic solar cells.
Figure 1.1.4: The I-V curve (a) and simple diode model of a solar cell (b), demonstrate the operation of a solar cell. The power curve in blue calculated by P = V I shows a maximum for a particular current and voltage. In (b) the simple diode is based on parametersIL, the current generated at the source when the device is illuminated, and I0, the saturation current.
required to excite the electron into the conduction band is known as the bandgap energy.
The liberated electrons and holes are able to move around the material [72, Chapter 2, p. 27]. The result of the annihilation of light energy is the creation of two charges in the solar cell, one negative and another positive.
Inside the solar cell different layers of material are created through a process called doping or diffusion [73]. The solar cell shown in Figure 1.1.3 has two regions, a base fabricated with an excess of free positive charge carriers, and an emitter with an excess of free negative charge carriers. The boundary of these regions forms a pn junction [68, 69, 74] resulting in an electric field that causes charge carriers to drift. The physics of the pn junction is described in detail in sources such as the book bySze [65, Chapter 2,p. 79- 133] or Luque, et al. [28, Section 3.2.8,p. 102]. Due to drift and diffusion, a net current flow is achieved in the device upon illumination. The result is that light energy is converted into a flow of electrons which may power a connected load.
The photovoltaic conversion efficiency ηP V can be written as the division of the electrical energy by the sunlight energy as
ηP V = VmppImpp
R IAM(λ)dλ (1.1.10)
where Vmpp is the voltage and Impp is the current at the maximum power point of the
solar cell. IAM represents the optical power incident at ground level (see Figure 1.1.2).
The denominator integrates to a constant value which depends on the location of the photovoltaic system, as well as meteorological conditions, or the module inclination. A standard solar cell I-V curve is shown in Figure 1.1.4, together with the power-voltage curve. The solar cell must operate at a particular point on the curve. The output power peaks at the maximum power point. Optimization of this electrical energy generation process in an economical manner is the key aspect of silicon wafer-based photovoltaic research and engineering [75, 76]. For example, selective emitters, back surface fields, and specific chemical processing procedures for passivation and texturing of silicon wafer solar cells have been developed [58]. More details on the fabrication of silicon wafer solar cells may be found in theHandbook of Photovoltaic Science and Engineering by Luque, et al[28], and other books [59, 77].