Solar Cell Device Physics

By Stephen Fonash

There has been an enormous infusion of new ideas in the field of solar cells over the last 15 years; discourse on energy transfer has gotten much richer, and nanostructures and nanomaterials have revolutionized the possibilities for new technological developments. However, solar energy cannot become ubiquitous in the world's power markets unless it can become economically competitive with legacy generation methods such as fossil fuels.

The new edition of Dr. Stephen Fonash's definitive text points the way toward greater efficiency and cheaper production by adding coverage of cutting-edge topics in plasmonics, multi-exiton generation processes, nanostructures and nanomaterials such as quantum dots. The book's new structure improves readability by shifting many detailed equations to appendices, and balances the first edition's semiconductor coverage with an emphasis on thin-films. Further, it now demonstrates physical principles with simulations in the well-known AMPS computer code developed by the author.

• Classic text now updated with new advances in nanomaterials and thin films that point the way to cheaper, more efficient solar energy production
• Many of the detailed equations from the first edition have been shifted to appendices in order to improve readability
• Important theoretical points are now accompanied by concrete demonstrations via included simulations created with the well-known AMPS computer code

• Language: English
• Publisher: Academic Press
• Release date: Jun 17, 2010
• ISBN: 9780080912271

Stephen Fonash

Dr. Stephen Fonash is Bayard D. Kunkle Chair Professor Emeritus of Engineering Sciences at Penn State University and Chief Technology Officer of Solarity LCCM, LLC. His activities at Penn State include serving as the director of Penn State’s Center for Nanotechnology Education and Utilization (CNEU), director of the National Science Foundation Advanced Technology Education Center, and director of the Pennsylvania Nanofabrication Manufacturing Technology Partnership. Prof. Fonash’s education contributions focus on nanotechnology post-secondary education and workforce development. His research activities encompass the processing and device physics of micro- and nanostructures including solar cells, sensors, and transistors. He has published over 300 refereed papers in the areas of education, nanotechnology, photovoltaics, microelectronics devices and processing, sensors, and thin film transistors. His book “Solar Cell Device Physics” has been termed the “bible of solar cell physics” and his solar cell computer modeling code AMPS is used by almost 800 groups around the world. Dr. Fonash holds 29 patents in his research areas, many of which are licensed to industry. He is on multiple journal boards, serves as an advisor to university and government groups, has consulted for a variety of firms, and has co-founded two companies. Prof. Fonash received his Ph.D. from the University of Pennsylvania. He is a Fellow of the Institute of Electrical and Electronics Engineers and a Fellow of the Electrochemical Society

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Material

Chapter One

Introduction

1.1 Photovoltaic Energy Conversion

Photovoltaic energy conversion is the direct production of electrical energy in the form of current and voltage from electromagnetic (i.e., light, including infrared, visible, and ultraviolet) energy. The basic four steps needed for photovoltaic energy conversion are:

1. a light absorption process which causes a transition in a material (the absorber) from a ground state to an excited state,

2. the conversion of the excited state into (at least) a free negative- and a free positive-charge carrier pair, and

3. a discriminating transport mechanism, which causes the resulting free negative-charge carriers to move in one direction (to a contact that we will call the cathode) and the resulting free positive-charge carriers to move in another direction (to a contact that we will call the anode).

The energetic, photogenerated negative-charge carriers arriving at the cathode result in electrons which travel through an external path (an electric circuit). While traveling this path, they lose their energy doing something useful at an electrical load, and finally they return to the anode of the cell. At the anode, every one of the returning electrons completes the fourth step of photovoltaic energy conversion, which is closing the circle by

4. combining with an arriving positive-charge carrier, thereby returning the absorber to the ground state.

In some materials, the excited state may be a photogenerated free electron–free hole pair. In such a situation, step 1 and step 2 coalesce. In some materials, the excited state may be an exciton, in which case steps 1 and 2 are distinct.

A study of the various man-made photovoltaic devices that carry out these four steps is the subject of this text. Our main interest is photovoltaic devices that can efficiently convert the energy in sunlight into usable electrical energy. Such devices are termed solar cells or solar photovoltaic devices. Photovoltaic devices can be designed to be effective for electromagnetic spectra other than sunlight. For example, devices can be designed to convert radiated heat (infrared light) into usable electrical energy. These are termed thermal photovoltaic devices. There are also devices which directly convert light into chemical energy. In these, the photogenerated excited state is used to drive chemical reactions rather than to drive electrons through an electric circuit. One example is the class of devices used for photolysis. While our emphasis is on solar cells for producing electrical energy, photolysis is briefly discussed later in the book.

1.2 Solar Cells and Solar Energy Conversion

The energy supply for a solar cell is photons coming from the sun. This input is distributed, in ways that depend on variables like latitude, time of day, and atmospheric conditions, over different wavelengths. The various distributions that are possible are called solar spectra. The product of this light energy input, in the case of a solar cell, is usable electrical energy in the form of current and voltage. Some common standard energy supplies from the sun, which are available at or on the earth, are plotted against wavelength (λ) in W/m²/nm spectra in Figure 1.1A. An alternative photons/m²-s/nm spectrum is seen in Figure 1.1B. The spectra in Figure 1.1A give the power impinging per area (m²) in a band of wavelengths 1 nm wide (the bandwidth Δλ) centered on each wavelength λ. In this figure, the AM0 spectrum is based on ASTM standard E 490 and is used for satellite applications.1 The AM1.5G spectrum, based on ASTM standard G173, is for terrestrial applications and includes direct and diffuse light. It integrates to 1000 W/m². The AM1.5D spectrum, also based on G173, is for terrestrial applications but includes direct light only. It integrates to 888 W/m².2 The spectrum in Figure 1.1B has been obtained from the AM1.5G spectrum of Figure 1.1A by converting power to photons per second per cm² and by using a bandwidth of 20 nm. Photon spectra Φ0(λ), exemplified by that in Figure 1.1B, are more convenient for solar cell assessments, because optimally one photon translates into one free electron–free hole pair via steps 1 and 2 of the four steps needed for photovoltaic energy conversion.

Figure 1.1 Solar energy spectra. (a): Data expressed in watts per m ² per 1 nm bandwidth for AMO (from Ref. 1 , with permission) and for AM1.5G, and AM1.5D spectra (from Ref. 2 , with permission). (b): The AM1.5G data expressed in terms of impinging photons per second per cm ² per 20 nm bandwidth.

Standard spectra are needed in solar cell research, development, and marketing because the actual spectrum impinging on a cell in operation can vary due to weather, season, time of day, and location. Having standard spectra allows the experimental solar cell performance of one device to be compared to that of other devices and to be judged fairly, since the cells can be exposed to the same agreed-upon spectrum. The comparisons can be done even in the laboratory since standard distributions can be duplicated using solar simulators.

The total power PIN per area impinging on a cell for a given photon spectrum Φ0(λ) is the integral of the incoming energy per time per area per bandwidth over the entire photon spectrum; i.e.,

(1.1)

where an example Φ0(λ), expressed as photons/time/area/bandwidth, is plotted in Figure 1.1B. In Equation 1.1 the quantity h is Planck's constant and c is the speed of light. The electrical power POUT per area produced by the cell of Figure 1.2 operating at the voltage V and delivering the current I as a result of this incoming solar power is the product of the current I times V divided by the cell area.

Figure 1.2 Cross-section of a typical solar cell. The area of photon impingement and the area of current production are the same. The anti-reflection (AR) coating has the function of reducing reflection losses. The collecting electrodes (cathode and anode) are shown with the top electrode being transparent.

Introducing the current density J defined as I divided by the cell area allows POUT to be written as

(1.2)

A plot of the possible J-V operating points (called the light J-V characteristics) of the cell of Figure 1.2 is seen in Figure 1.3. The points labeled Jsc and Voc represent, respectively, the extreme cases of no voltage produced between the anode and cathode (i.e., the illuminated solar cell is short-circuited) and of no current flowing between the anode and cathode (i.e., the illuminated solar cell is open-circuited). At any of the operating points seen in Figure 1.3, POUT is given by the JV