Fonash Introduction to Light Trapping in Solar Cell and Photo-detector Devices

1. Auflage 2014
ISBN: 978-0-12-416637-0
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 76 Seiten

New Approaches to Light Trapping in Solar Cell Devices discusses in detail the use of photonic and plasmonic effects for light trapping in solar cells. It compares and contrasts texturing, the current method of light-trapping design in solar cells, with emerging approaches employing photonic and plasmonic phenomena. These new light trapping methods reduce the amount of absorber required in a solar cell, promising significant cost reduction and efficiency. This book highlights potential advantages of photonics and plasmonics and describes design optimization using computer modeling of these approaches. Its discussion of ultimate efficiency possibilities in solar cells is grounded in a review of the Shockley-Queisser analysis; this includes an in-depth examination of recent analyses building on that seminal work.

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

Autoren/Hrsg.

Fonash, Stephen J.

Leseproben

Chapter 1

A Brief Overview of Phenomena Involved in Light Trapping

Abstract

Light trapping is the capturing of photons for use in applications such as solar cells, sensing, photo-electrochemistry, and thermal photovoltaics. Light enters the structures involved in these applications through refraction, scattering, or the systematic scattering provided by diffraction. The chapter examines these entry paths and the various optical processes that can be present in a light trapping structure.

Keywords

light absorption

light trapping

solar cells

photovoltaics

photodetectors

photons

radiation modes

trapped traveling modes

trapped localized modes

Light trapping is the capturing of as many photons as possible from an impinging electro-magnetic (E-M) wave with the objective of generating heat or charge carriers, excitons, or both [1]. For our purposes, the “light” being trapped may lie anywhere in that part of the E-M spectrum extending from the infrared to the ultraviolet. This range encompasses interesting frequencies, such as the photon-rich part of the solar spectrum, the emission and absorption frequencies of living things, and astronomically useful frequencies. Trapping of this light is vital to many applications, including sensing [2,3], photovoltaics [1], photo-electrochemistry [1,4], solar fuel production [5], and thermal photovoltaics [6]. We limit ourselves in this text to light trapping for production of charge carriers, excitons, or both. Some discussions break light trapping into light capture and light trapping. With the thin devices and, in particular, the thin photon absorbers now used in high-speed detectors and advanced solar cells today, it is rather artificial to separate light capture and light trapping. In this text, they are treated as one light trapping process, as seen in Figure 1.1.

Fig. 1.1 A control region showing the definition of light trapping used in this text. Light trapping endeavors to minimize the light energy propagating away at the top control surface. Normal impingement onto a general structure is assumed in this figure.

When light enters a structure, we will find that it can reside in a number of different modes: radiation modes, trapped traveling modes (guided modes1 and Bloch modes2), and trapped localized modes (Mie modes3 and plasmonic modes) as well as hybridizations4 of these. We will develop familiarity with all of this terminology in Chapter 2. For now, we can note that light trapping may be thought of as the task of populating some or all of these various modes possible in a structure with photons.

The optical phenomena that provide the key tools for light trapping are listed in Table 1.1 along with their definitions, as utilized in this monograph. After reviewing these tools in the remainder of this chapter, we will move in Chapter 2 to examining those modes in which photons may be stored in a structure. In Chapter 3, we will look at various light-trapping structures. As may be seen from Figure 1.2a–d, these structures can range from devices with simple ¼ wavelength antireflection coatings (ARCs) to devices with honeycomb gratings. When needed, we will consider polarization effects. Chapter 4 provides final comments on light-trapping structures and approaches.

Table 1.1

Light-Trapping Phenomena and Definitions

Interference

The phenomenon whereby two or more E-M waves existing at a point constructively or destructively add together to some degree at that point.

Scattering

The result of impinging E-M waves bouncing off of objects by being absorbed and emitted. Material property dependent. In general, can be elastic or inelastic.6

Reflection

The result of some portion of an impinging E-M wave being scattered backwards. Material property dependent. Generally taken as elastic.6

Diffraction

The result of impinging E-M waves bouncing off of objects by being absorbed, effectively instantaneously emitted, and constructively interfering in certain specific directions. Taken as elastic.6

Plasmonics

The result of an impinging E-M wave being absorbed by the extremely numerous electrons of a metal thereby exciting an oscillating plasma. This plasma dissipates energy through electron collisions and also reradiates an E-M scattered wave. Material property dependent. Over all, inelastic.6

Refraction

The result of an impinging E-M wave changing direction and wavelength due to a change in the transmission medium through which it is passing. Material property dependent. Taken as elastic.6

Fig. 1.2 Some examples of light-trapping structures: (a) A “traditional” planar solar cell with an ARC; (b) a solar cell with a periodically arranged pyramidal TCO array; (c) a solar cell with a periodic array of TCO domes; (d) a solar cell with a randomly textured TCO layer; and (e) a solar cell with a honeycomb nanoscale grating structure.

In our brief discussion of the phenomena of Table 1.1, we occasionally will use the approximation of geometrical optics.5 However, our approach will generally be to employ physical optics and, when needed, to implement it with full numerical solutions of Maxwell’s equations.

1.1. Interference

Interference is listed first in Table 1.1 because it is the defining trait of waves. This basic behavior can be considered by watching two electric field waves coming from two plane wave sources (source 1 and source 2). If we examine these waves at some point which is 1? from source 1 of the first wave and 2? from source 2 of the second wave, then wave 1 at this point is of the form xiˆ+Ayjˆ+Azkˆeik1?r1?-?1t with k-vector 1? and angular frequency ?1, whereas, wave 2 is of the form xiˆ+Byjˆ+Bzkˆeik2?r2?-?2t with k-vector 2? and angular frequency ?2. As we know, these waves simply add up at our point of interest to

?=Axei[k1?r1?-?1t]+Bxei[k2?r2?-?2t]iˆ+Ayei[k1?r1?-?1t]+Byei[k2?r2?-?2t]jˆ+Azei[k1?r1?-?1t]+Bzei[k2?r2?-?2t]kˆ

(1.1)

where ? is the total electric field. The total E-M energy density U present at our arbitrary point is proportional to the square of the magnitude of the electric field [8,9]; i.e.,

=???·??*

(1.2)

where ?*is the complex conjugate of? and the permittivity at the point in question. The total photon density is therefore proportional to this U. Using Eq. 1.2 it follows that,

=eAx2+Bx2+AxBxe-i[k1?r1?-?1t]ei[k2?r2?-?2t]?????????+ AxBxei[k1?r1?-?1t]e-i[k2?r2?-?2t]+Ay2+By2+AyBye-i[k1?r1?-?1t]ei[k2?r2?-?2t]?????????+ AyByei[k1?r1?-?1t]e-i[k2?r2?-?2t]+Az2+Bz2+AzBze-i[k1?r1?-?1t]ei[k2?r2?-?2t]?????????+ AzBzei[k1?r1?-?1t]e-i[k2?r2?-?2t]

(1.3a)

This expression can be somewhat simplified to

=?Ax2+Bx2+Ay2+By2+Az2+Bz2+2AxBx+2AyBy+2AzBzcosk1?·r1?-k2·?r2?+?2t-?1t

(1.3b)

Equation 1.3b is interesting because it shows that the quantity ¯, energy density averaged over time and space, has the value that we would probably expect; i.e., it is

¯=?Ax2+Bx2+Ay2+By2+Az2+Bz2

(1.4)

The cosine term in Eq. 1.3b also shows that interference between these waves can increase or decrease the expected average energy density ¯ at different places and times by as much as 2AxBx +...