Development of high efficiency solar absorbers

229 Pages·2014·25.42 MB·English

ANABSTRACTOFTHETHESISOF
RamRavichandran forthedegreeofDoctorofPhilosophyin
ElectricalandComputerEngineeringpresentedonMay22,2014.
Title: DevelopmentofHighEfficiencySolarAbsorbers.
Abstractapproved:
JohnF.Wager
Current cadmium telluride and copper indium gallium diselenide thin-film solar cells
(TFSCs) utilize thick absorbers (2 - 4 µm). For efficient carrier extraction in these TFSCs,
theabsorberlayerrequireshighcarriermobilitiesandalongminoritycarrierlifetime,which
necessitates the use of a high purity, defect-free thin film. Developing new materials with
absorption strengths stronger than those of current materials allows an ultra-thin (<1 µm)
absorbertobeincorporatedinadrift-basedTFSC.Devicesimulationindicatesthatabuilt-in
driftfieldaidscarrierextraction,reducingmobilityandlifetimerequirements.
Iron- and copper-based materials are investigated within the context of ultra-thin ab-
sorbers. FeS is unstable due to the formation of deleterious, low band-gap phases while
2
Fe GeS and CuSbS exhibit a sluggish, non-abrupt onset of absorption, limiting their ap-
2 4 2
plicationinanultra-thindrift-basedTFSC.Cu SbS exhibitsdesirableopticalandelectrical
3 4
properties with a simulated TSFC efficiency of 19% for a 750 nm thick absorber layer. A
newtetrahedrite-basedabsorber,Cu Zn Sb Se demonstratesexceptionallystrongabsorp-
10 2 4 13
tion, with a simulated TSFC efficiency of 21% for a 250 nm thick absorber, indicating that
Cu SbS and Cu Zn Sb Se have potential for high efficiency drift-based TFSC applica-
3 4 10 2 4 13
tions.
(cid:13)cCopyrightbyRamRavichandran
May22,2014
Allrightsreserved
DevelopmentofHighEfficiencySolarAbsorbers
by
RamRavichandran
ATHESIS
submittedto
OregonStateUniversity
inpartialfulfillmentof
therequirementsforthe
degreeof
DoctorofPhilosophy
PresentedMay22,2014
CommencementJune2014
DoctorofPhilosophythesisofRamRavichandranpresentedonMay22,2014
APPROVED:
MajorProfessor,representingElectricalandComputerEngineering
DirectoroftheSchoolofElectricalEngineeringandComputerScience
DeanoftheGraduateSchool
I understand that my thesis will become part of the permanent collection of Oregon State
University libraries. My signature below authorizes release of my thesis to any reader upon
request.
RamRavichandran,Author
ACKNOWLEDGMENTS
Theworkpresentedhereinwouldnothavebeenpossiblewithoutthesupportandmo-
tivationofmyadvisors,Dr. JohnWagerandDr. DougKeszler. IowethankstoDr. TomPlant
for guiding me towards my graduate career, and my committee members, Dr. Ted Brekken
andDr. BillWarnes.
I cannot thank my family enough for all the help and support over the years. Thank
youforbeingthere,ateverystepofallmyendeavors,sharingmysuccessesandhelpingme
overcomemyfearsandfailures.
I am very thankful to Chris Tasker, Rick Presley, and Manfred Dittirich who help
maintain the cleanroom, and provide a learning environment even for those who are not
mechanically inclined! The absorber group, including Dr. Robert Kokenyesi, Dr. Jaeseok
Heo,Dr. BrianPelatt,andGregAngelos,hasbeencrucialtotheworkpresentedherein. I’m
lookingforwardtoourcontinuedfriendship.
I owe thanks to many people in Corvallis who have helped me make OSU home,
especially Dr. William Cowell, Ira Jewell, John McGlone, and Pratim Chowdhury. Thanks
forbeingpartnersincrimeandgivingmereasonstostepawayfromthelab. Dr. JaeseokHeo
andDr. LipingYucontributedtothetheoreticalcalculationspresentedinthiswork.
ThisworkwasfundedbytheU.S.DepartmentofEnergy,OfficeofScience,Officeof
BasicEnergySciencesunderContractNo. DE-AC36-08GO28308toNREL.
TABLEOFCONTENTS
Page
1. MOTIVATION-APATHWAYTOWARDSSUSTAINABLESOLARENERGY 1
2. SUNLIGHT,SOLARABSORBERS,ANDSOLARCELLS .................. 3
2.1 SunlightandSemiconductors......................................... 3
2.1.1 SolarSpectrum ................................................. 3
2.1.2 AbsorptioninSemiconductors................................... 5
2.1.3 Recombination.................................................. 15
2.2 SolarCellDevicePrimer............................................. 20
2.2.1 SolarCellDeviceConfigurations ................................ 21
2.2.2 Current-VoltageCharacteristicsofTFSCs ........................ 24
2.2.3 EfficiencyLimitsforaSingle-JunctionSolarCell................. 32
2.3 Thin-FilmAbsorberMaterials........................................ 35
2.3.1 CadmiumTelluride-CdTe ...................................... 35
2.3.2 Cu(In,Ga)Se -CIGS............................................ 40
2
2.3.3 AmorphousSilicon.............................................. 45
2.3.4 IronPyrite-FeS ............................................... 49
2
2.4 Summary........................................................... 52
3. EXPERIMENTALTECHNIQUES ......................................... 53
3.1 ThinFilmFabrication................................................ 53
3.1.1 RFMagnetronSputtering........................................ 53
3.1.2 ElectronBeamEvaporation...................................... 57
3.1.3 Post-DepositionAnneal ......................................... 61
3.2 MaterialCharacterization............................................ 62
3.2.1 SpectroscopicEllipsometry...................................... 63
3.2.2 OpticalSpectroscopy............................................ 68
3.2.3 HallEffectMeasurements ....................................... 70
3.2.4 SeebeckMeasurements.......................................... 71
3.3 DeviceSimulations.................................................. 74
3.4 Conclusions ........................................................ 76
TABLEOFCONTENTS(Continued)
Page
4. TFSCDEVICESIMULATIONOFHIGHABSORPTIONMATERIALS ....... 80
4.1 Primaryquestionsaddressedbydevicesimulations..................... 80
4.2 TheAbsorber....................................................... 81
4.3 TFSCDeviceConfiguration.......................................... 83
4.4 Carrierconcentrationvariation........................................ 86
4.5 Absorberlayerthicknessvariation .................................... 88
4.5.1 Drift-basedTFSCs .............................................. 88
4.5.2 Diffusion-basedTFSCs.......................................... 93
4.6 Minoritycarrierlifetime ............................................. 98
4.6.1 Drift-basedTFSCs .............................................. 100
4.6.2 Diffusion-basedTFSCs.......................................... 105
4.7 Conclusions ........................................................ 111
5. IRONBASEDSOLARABSORBERS ...................................... 113
5.1 FeS -Pyrite........................................................ 113
2
5.2 Fe GeS -BasedAbsorbers ........................................... 121
2 4
5.2.1 FGSThinFilmSynthesisandCharacterization ................... 122
5.2.2 DeviceSimulationsofFGS-basedTFSCs ........................ 132
5.3 Conclusions ........................................................ 142
6. COPPER-BASEDSOLARABSORBERS ................................... 144
6.1 Cu-V-VI(V=Sb,Bi;VI=S,Se)familyofmaterials................... 144
6.1.1 DesignparadigmsforCu-V-VImaterials ......................... 145
6.1.2 Thin-filmdepositionandcharacterization ........................ 147
6.1.3 TFSCdevicesimulations ........................................ 154
6.1.3.1 CuSbS ................................................. 155
2
TABLEOFCONTENTS(Continued)
Page
6.1.3.2 Cu SbS ................................................ 165
3 4
6.2 Tetrahedrites-Cu M Sb Ch (M=Cu,Mn,Zn,Al,In;Ch=S,Se).... 170
10 2 4 13
6.2.1 FabricationandcharacterizationofCu Sb S thinfilms ......... 175
12 4 13
6.2.2 ReducingthecarrierconcentrationinCu Sb S ................. 178
12 4 13
6.2.3 BandgaptuninginCu Zn Sb S .............................. 182
10 2 4 13
6.2.4 Cu Zn Sb Se asaTFSCabsorberlayer ....................... 185
10 2 4 13
6.3 Conclusions ........................................................ 193
7. CONCLUSIONSANDRECOMMENDATIONSFORFUTUREWORK ....... 195
7.1 Conclusions ........................................................ 195
7.1.1 TFSCdevicesimulations ........................................ 195
7.1.2 Iron-basedabsorbers ............................................ 196
7.1.3 Copper-basedabsorbers ......................................... 197
7.2 Recommendationsforfuturework.................................... 199
7.2.1 TFSCDeviceSimulation ........................................ 199
7.2.2 Fe-basedabsorbers.............................................. 199
7.2.3 Cu-V-VIbasedabsorbers ........................................ 199
7.2.4 Tetrahedrite-basedabsorbers..................................... 200
7.2.5 V estimationfromphotoluminescencemeasurements........... 200
OC
BIBLIOGRAPHY ............................................................ 202
LISTOFFIGURES
Figure Page
2.1 The spectral power density of sunlight outside the atmosphere (AM0), at
theEarth’ssurface(AM1.5),andablackbodyradiationat5800K.......... 4
2.2 (a)Absorptionofabove-bandgaplightinasemiconductorresultsinapho-
togeneratedelectron-holepairduetointerbandtransitions. (b)Relaxation
oftheexcitedelectronresultsintheemissionoflightwithenergyequalto
thebandgap............................................................. 5
2.3 Density of states trends for s-, p-, and d-orbital derived bands, specifying
themaximumelectronicoccupancyandexpectedeffectivemasstrendsfor
eachband. .............................................................. 8
2.4 (a) k-space representation of an energy band diagram for a direct band
gapmaterial,and(b)absorptioncoefficientofCdTe. Adirectbandgapis
characterizedbyastrongonsetofabsorptionjustabovethebandgap....... 11
2.5 (a) Reciprocal space representation of an energy band diagram for an in-
direct band gap material, and (b) absorption coefficient of silicon. An
indirectbandgapmaterialischaracterizedbyanon-abrupt,gradualonset
ofabsorptionabovethebandgap......................................... 12
2.6 Estimationofthe(a)indirectbandgap,and(b)directbandgapofsilicon.
An indirect band gap of Eindr ∼ 1.1 eV can be extracted, while a direct
G
bandgapEdr ∼3.3eVisalsoobserved................................... 13
G
2.7 Absorption coefficient versus photon energy for a Cu Mn Sb S and a
10 2 4 13
Cu Sb S thin film which have a band gap, E ∼ 1.8 eV. However, an
12 4 13 G
additionalabsorptionfeaturecanbeseenintheCu Mn Sb S thin-film
10 2 4 13
at E ∼ 1.6 eV, indicating the presence of an additional sub-band gap
sb
phase in the thin-film. Cu Sb S , on the other hand does not exhibit
12 4 13
sub-band gap phases, but has a characteristic feature below the band gap
indicatingfree-carrierabsorption. ........................................ 14
2.8 Recombination mechanisms in a semiconductor. (a) Radiative recombi-
nation, accompanied with the emission of a photon with energy equal to
the band-gap. (b) SRH, or trap-mediated recombination, and (c) Auger
recombination........................................................... 16
2.9 (a) Electron capture by an ionized donor trap in a p-type semiconductor,
and(b)holecapturebyanionizedacceptortrapinann-typesemiconductor. 19
2.10 DeviceconfigurationsforaTFSC.P-njunction(diffusioncell)at(a)equi-
librium, (b) under illumination, (c) and illumination with a forward bias.
P-i-njunction(driftcell)at(d)equilibrium,(e)underillumination,and(f)
illuminationwithaforwardbias.......................................... 22