太阳能电池器件物理Solar Cell Device Physics（英文版）.pdf

Solar Cell Device PhysicsSolar Cell Device PhysicsSecond EditionStephen J.FonashAMSTERDAM BOSTON HEIDELBERG LONDONNEW YORK OXFORD PARIS SAN DIEGOSAN FRANCISCO SINGAPORE SYDNEY TOKYOAcademic Press is an imprint of ElsevierAcademic Press is an imprint of Elsevier30 Corporate Drive,Suite 400,Burlington,MA 01803,USAThe Boulevard,Langford Lane,Kidlington,Oxford,OX5 1GB,UK 2010 Elsevier Inc.All rights reserved.No part of this publication may be reproduced or transmitted in any form or by any means,electronic or mechanical,including photocopying,recording,or any information storage and retrieval system,without permission in writing from the publisher.Details on how to seek permission,further information about the Publishers permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency,can be found at our website: book and the individual contributions contained in it are protected under copyright by the Publisher(other than as may be noted herein).NoticesKnowledge and best practice in this field are constantly changing.As new research and experience broaden our understanding,changes in research methods,professional practices,or medical treatment may become necessary.Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information,methods,compounds,or experiments described herein.In using such information or methods they should be mindful of their own safety and the safety of others,including parties for whom they have a professional responsibility.To the fullest extent of the law,neither the Publisher nor the authors,contributors,or editors,assume any liability for any injury and/or damage to persons or property as a matter of products liability,negligence or otherwise,or from any use or operation of any methods,products,instructions,or ideas contained in the material herein.Library of Congress Cataloging-in-Publication DataFonash,S.J.Solar cell device physics/Stephen J.Fonash.2nd ed.p.cm.Includes bibliographical references and index.ISBN 978-0-12-374774-7(alk.paper)1.Solar cells.2.Solid state physics.I.Title.TK2960.F66 2010621.31244 dc22 2009045478British Library Cataloguing-in-Publication DataA catalogue record for this book is available from the British Library.For information on all Academic Press publications,visit our website:Printed in United States of America10 11 12 13 14 15 9 8 7 6 5 4 3 2 1To the memory of my parents,Margaret and Raymond,who showed me the path of intellectual pursuitsTo my wife Joyce for her continuing guidance and support along the wayTo my sons Steve and Dave,and their families,for making the journey so enjoyablePrefaceAs was the case with the first edition of Solar Cell Device Physics,this book is focused on the materials,structures,and device physics of photovoltaic devices.Since the first edition was published,much has happened in photovoltaics,such as the advent of excitonic cells and nanotechnology.Capturing the essence of these advances made writ-ing both fun and a challenge.The net result is that Solar Cell Device Physics has been almost entirely rewritten.A unifying approach to all the developments is used throughout the new edition.For example,this unifying approach stresses that all solar cells,whether based on absorp-tion that produces excitons or on absorption that directly produces free electronhole pairs,share the common requirement of needing a struc-ture that breaks symmetry for the free electrons and holes.The breaking of symmetry is ultimately what is required to enable a solar cell to pro-duce electric power.The book takes the perspective that this breaking of symmetry can occur due to built-in electrostatic fields or due to built-in effective fields arising from spatial changes in the density of states dis-tribution(changes in energy level positions,number,or both).The elec-trostatic-field approach is,of course,what is used in the classic silicon pn junction solar cell.The effective-fields approach is,for example,what is exploited in the dye-sensitized solar cell.This edition employs both analytical and numerical analyses of solar cell structures for understanding and exploring device physics.Many of the details of the analytical analyses are contained in the appendices,so that the development of ideas is not interrupted by the development of equations.The numerical analyses employ the computer code Analysis of Microelectronic and Photovoltaic Structures(AMPS),which came out of,and is heavily used by,the authors research group.AMPS is utilized in the introductory sections to augment the understanding of the origins of photovoltaic action.It is used in the chapters dedicated to different cell types to give a detailed examination of the full gamut of solar cell types,from inorganic pn junctions to organic heterojunctions xii Prefaceand dye-sensitized cells.The computer modeling provides the dark and light current voltage characteristics of cells but,more importantly,it is used to“pry open cells”to examine in detail the current components,the electric fields,and the recombination present during operation.The various examples discussed in the book are available on the AMPS Web site(www.ampsmodeling.org).The hope is that the reader will want to examine the numerical modeling cases in more detail and perhaps use them as a tool to further explore device physics.It should be noted that some of the authors specific ways of doing things have crept into the book.For example,many texts use q for the magnitude of the charge on an electron,but here the symbol e is used throughout for this quantity.Also kT,the measure of random thermal energy,is in electron volts(0.026 eV at room temperature)everywhere.This means that terms that may be written elsewhere as eqV/kT appear here as eV/kT with V in volts and kT in electron volts.It also means that expressions like the Einstein relation between diffusivity Dp and mobil-ity p for holes,for example,appear in this book as Dp kTp.Photovoltaics will continue to develop rapidly as alternative energy sources continue to gain in importance.This book is not designed to be a full review of where we have been or of where that development is now,although each is briefly mentioned in the device chapters.The intent of the book is to give the reader the fundamentals needed to keep up with,and contribute to,the growth of this exciting field.AcknowledgmentsAs with the first edition,this book has grown out of the graduate-level solar cell course that the author teaches at Penn State.It has profited considerably from the comments of the many students who have taken this course.All the students and post-docs who have worked in our research group have also contributed to varying degrees.Outstanding among these is Dr.Joseph Cuiffi who aided greatly in the numerical modeling used in this text.The efforts of Lisa Daub,Darlene Fink and Kristen Robinson are also gratefully acknowledged.They provided outstanding assistance with fig-ures and references.Dr.Travis Benanti,Dr.Wook Jun Nam,Amy Brunner,and Zac Gray contributed significantly in various ways,from proofread-ing to figure generation.The help of all these people,and others,made this book a possibility.The encouragement and understanding of my wife Joyce made it a reality.List of Symbols Element Description(Units)Absorption coeffi cient(nm 1,cm 1)1 Dimensionless quantity describing ratio of n-portion quasi-neutral region length to hole diffusion length 2 Dimensionless quantity describing ratio of n-portion quasi-neutral region length to the absorption length 3 Dimensionless quantity describing ratio of top-surface hole carrier recombination velocity to hole diffusion-recombination velocity in the n-portion 4 Dimensionless quantity describing ratio of the absorber thickness up to the beginning of the quasi-neutral region in the p-portion to absorption length 5 Dimensionless quantity describing ratio of p-portion quasi-neutral-region length to electron diffusion length 6 Dimensionless quantity describing ratio of the p-portion quasi-neutral-region length to absorption length 7 Dimensionless quantity describing ratio of back-surface electron carrier recombination velocity to the electron diffusion-recombination velocity Band-to-band recombination strength parameter(cm3s1)Magnitude of the energy shift caused by an interface dipole(eV)Thickness of dye monolayer in DSSC(nm)Grain size in polycrystalline materials(nm)C Conduction-band offset between two materials at a heterojunction(eV)xvi List of Symbols V Valence-band offset between two materials at a heterojunction(eV)0()Photon flux per bandwidth as a function of wavelength(m2s1 per bandwidth in nm)B Schottky barrier height of an M-S or M-I-S structure(eV)BI Energy difference between E C and E F for an n-type material or the energy difference between E F and E V for a p-type material at the semiconductor surface in an M-I-S structure(eV)C Photon flux corrected for reflection and absorption before entering a material(cm2s1per bandwidth in nm)W Workfunction of a material(eV)WM Workfunction of a metal(eV)Wn Workfunction of an n-type semiconductor(eV)Wp Workfunction of a p-type semiconductor(eV)Permittivity (F/cm)Device power conversion effi ciency Wavelength of a photon or phonon(nm)Gi Mobility of charge carriers in localized gap states(cm2/V-s)n Electron mobility(cm2/V-s)p Hole mobility(cm 2/V-s)Frequency of electromagnetic radiation(Hertz)Electric field strength(V/cm)0 Electric field present at thermodynamic equilibrium(V/cm)Electron effective force fi eld(V/cm)n Hole effective force fi eld(V/cm)p Charge density(C/cm 3)List of Symbols xvii n Cross-section of a localized state for capturing an electron(cm2)p Cross-section of a localized state for capturing a hole(cm2)E Exciton lifetime(s)RLn Electron lifetime(dictated by,n,or nA )for p-type n material(s)A Electron Auger lifetime for p-type material(s)n L Electron S-R-H recombination lifetime for p-type mate-n rial(s)R Electron radiative recombination lifetime for p-type n material(s)RLp Hole lifetime(dictated by,or A )for n-type pp p material(s)Ap Hole Auger lifetime for n-type material(s)L Hole S-R-H recombination lifetime for n-type p material(s)R Hole radiative recombination lifetime for n-type mate-p rial(s)Electron affi nity(eV)a Lattice constant(nm)Aabs Absorbance A*Effective Richardson constant(120 A/cm2/K2 for free electrons)(A/cm2/K2)AA1A Rate constant for the Auger recombination shown in Figure 2.18a(cm6/s)AA1B Rate constant for the Auger recombination shown in Figure 2.18b(cm6/s)AA1C Rate constant for the Auger transition shown in Figure 2.18c(cm6/s)AA1D Rate constant for the Auger transition shown in Figure 2.18d(cm6/s)c xviii List of SymbolsAA1E Rate constant for the Auger transition shown in Figure 2.18e(cm6/s)AA1F Rate constant for the Auger transition shown in Figure 2.18f(cm6/s)AA2A Rate constant for the Auger generation corresponding to Figure 2.18a(s1)AA2B Rate constant for the Auger generation corresponding to Figure 2.18b(s1)AC Solar cell area collecting photons in a concentrator cell(cm2 or m2)cAC Used in the density of states model g(E)e 1 2/3eV3 2AE Ec)(cm)(c AS Solar cell area generating current in a concentrator cell(cm2 or m2)vAV Used in the density of states model g(E)e 1 2/3eV3 2AEv E)(cm)(v Speed of light(2.998 1017 nm/s)d Distance or position in a device(cm,nm)DE Exciton diffusion coeffi cient(cm 2/s)Dn Electron diffusion coefficient or diffusivity(cm 2/s)TDn Electron thermal diffusion(Soret)coeffi cient(cm 2/K-s)Dp Hole diffusion coefficient or diffusivity(cm 2/s)TDp Hole thermal diffusion(Soret)coeffi cient(cm 2/K-s)e Charge on an electron(1.6 1019 C)E Energy of an electron,photon,or phonon(eV)EC Energy of the conduction-band edge,often called the LUMO for organic semiconductors(eV)EFn Spatially varying electron quasi-Fermi level(eV)EFp Spatially varying hole quasi-Fermi level(eV)Egm Mobility band gap(eV)List of Symbols xix EG Band gap(eV)Epn Energy of a phonon(eV)Ept Energy of a photon(eV)E0 Energy parameter in the model for the Franz-Keldysh effect defined by E 0 3 (m*)1/3(e )2/3 6.25 1018 2with m *,and expressed in MKS units(eV)EV Energy of the valence-band edge,often called the HOMO for organic semiconductors(eV)EVL Vacuum level energy(eV)Fe Total force experienced by an electron where F e e(d/dx)kT(dlnN/dx)Computed usingnC all terms in MKS units.Arises from the electric fi eld and the electron effective fi eld.(Newtons)Fh Total force experienced by a hole where Fh e(d(E)/dx)kT(dlnNV/dx)Computedp using all terms in MKS units.Arises from the electric field and the hole effective fi eld.(Newtons)gAA Carrier thermal generation rate for Auger process of Figure 2.18a(cm3-s1)g AB Carrier thermal generation rate for Auger process of Figure 2.18b(cm3-s1)g(E)Density of states in energy per volume(eV 1cm3)cg(E)Conduction-band density of states per volume e(eV1cm3)vg(E)Valence-band density of states per volume(eV 1cm3)e gpn(E)Phonon density of states(eV 1cm3)gthR Number thermally generated electrons in the conduction band and holes in the valence band per time per volume due to band-to-band transitions(cm 3-s1)G(,x)Number of Processes 3 5(see Fig.2.11)absorption events occurring per time per volume of material per 1bandwidth(cm3-s-nm1)xx List of Symbols G Gn Gp Gn ph(,x)Gp ph(,x)Gph(,x)h I()I I I(x)I0 J J0 JDK JFE JI Exciton generation rate(cm 3-s1)Represents any electron generation rate(cm 3-s1)Represents any hole generation rate(cm 3-s1)Free electron generation rate per time per volume of 1material per bandwidth(cm3-s-nm1)Free hole generation rate per time per volume of mate1rial per bandwidth(cm3-s-nm1)Free carrier generation rate per time per volume of material per bandwidth.Used when Gn ph(,x).ph(,x)Gp 1(cm3-s-nm1)Plancks constant(4.14 1015 eV-s)Plancks constant divided by 2 (1.32 1015 eV-s)Photon flux impinging on a device(cm 2-s1)Electrical current produced by a device(A)Exciton dissociation rate per area of interface(cm2-s1)Intensity(photons per area per bandwidth)of light as it 1travels through a material(cm 2-s-nm1)Intensity of incident light(photons per area per band1width)(cm2-s-nm1)Current density;terminal current density emerging from the device(A/cm 2)Pre-exponential term in the multistep tunneling model BTJMS J0e eAV(A/cm2)Dark current density(A/cm 2)Interface current density arising from field emission at a junction(A/cm2)Prefactor in the interface recombination current model V/n kTI(I)(A/cm2)J e 1 List of Symbols xxi JIR Interface current density arising from trap-assisted interface recombination.Also,specifi cally,current density lost to interface recombination at a heterojunction.(A/cm2)Jmp Current density at the maximum power point(A/cm 2)JMS Current density arising from multistep tunneling at a junction(A/cm2)Jn Conventional electron(conduction-band)current density(A/cm2)JOB Current density coming over an energy barrier at an interface(A/cm