Evaluation von laserbearbeiteten Si-Nanopartikeldnnfilmen fr denEinsatz in der Photovoltaik
Presentation to the Master Thesisby Levon Altunyan
OutlineIntroduction and MotivationExperiments and ResultsType I CellsType II CellsOutlook
Problem Description and Solution*Fig: Schematic drawing of a solar cell withBSF
Classical solutions - negative impact on cells:Suggested Solution:Different expansion coefficients of Al and SiSpin-coated Si nanoparticlesWarping of the cell observedControlled, brief, local heating Difficulties in subsequent productionSintered with the Silicon layer Increased probability of breakageCreate highly doped p+-type regionBenefit in cost per watt reduction
Particle Size DeterminationLiquids of Si-nanoparticles:HWR.p-doped (boron).5%wt and 10%wt.
*Fig: Determination of the Si-nanoparticle size via DLS measurement
Conclusions:Particles keep their size even after three weeks time.Graph fit Gaussian distribution: Mean diameter value = 100 d.nm Standard diameter deviation = 9 d.nm.
Layer Thickness Determination*Fig: Si-Layer Thickness vs. Position on Substrate; Back Surface Top View.
Conclusions: Average height hSiNp = 650nm (25nm).Inhomogeneous thickness due to substrate size.Peak in middle due to deposition method/speed.
Crystallization using an IR laser:Wavelength = 808 nm;Pulse length = continuous;Pulse profile: 13mm 50m;Power (max) ~ 452 W;Process Chamber:Volume V chamber=(12)l;0. no visible laser illumination;visible laser illumination/no change of the surface;optimal = change to silver like color of the surface;slightly scratched layer;ablation of cell's layer;layer is totally removed;Safe Regions Determination*Fig: IR laser systemFig: Layer Thickness vs. Spin Speed, One Spin Phase
Fill factor, FF = 59,21%.Cell efficiency, = 12, 93%.Low series (Rs) and high shunt (Rsh) resistancesReference Cell Type I with BSF*Antireflex Coating (SiN)n-layerp-layerAl Paste (BSF)AgAg InkAg Ink Type I with BSFFig: IV-Characteristic of Reference Cell Type I Type II Type IFig: Cell Types
*Initial Parameters Type I Cells
NameScan Parameters Fill FactorEfficiencya4Laser Intensity, I = 1 15%;Scan Velocity, V =100 mm/min;FF = 41 % = 6,38 %
*Sample Treatment ProcedureProcedures Applied on Type I Cells
*Final IV-Characterisations Type Ia.) Open Circuit Voltage and Short Circuit Current;b.) Fill Factor and Cell Efficiency;Random distribution of data points;Difficult extraction of pronounced trend;Further investigations using different cell structure needed.
* Possible diffusion of front Ag contacts into n-layer. Probability that front contacts get even further - to the p-layer.NSi 1 mEDX ConciderationsAgFig: EDX on the Front Surface Side of the SampleTmeltAg = 961, 93 Cn-type layer d=(0,30,4)mTmeltSi = 1414 CD = 3, 557 m2/sTcritical = (1111 1141) C
*SEM InvestigationsHighly reflectiveNon-reflectiveDifference in colour! 10 m
*IV-Characterisations Type II CellsFig: On-Off Ratio, Comparison of Cells With and Without Si-nanoparticlesFig: Efficiency of Type II samples with Si-nanoparticlesLower laser intensities (low heating):no particles - build-in defects removed;with particles high resistivity -> low on-off ratios;Higher laser intensities (increased heating):decremental effect on the cell structure -> low on-off ratios;
Cell efficiency = 2,95% (Type II) observed.
->high on-off ratios; higher efficiency
->low on-off ratios; lower efficiency
*Conductivity MeasurementsTotal conductivity, total 2, 57 10-3 S/cm;Conductivity for not laser treated particles, total 3, 52 10-3 S/cm;Fig: Four Point Measurement Schematic PictureFig: Conductivityof Si-nanoparticles Spin-coated on Intrinsic Si-wafers Irradiated for Different Laser Intensities
Where:U23 is the potential difference b/n the inner probes;I1 is a known current passing through the outer probes;A is the area through which current flows;dtotal is the total thickness of the measured wafer;s is the common contact length between the contact stripes;L is the distance between the inner contact stripes;total is the total conductivity of the material under test;
SummarySize and stability of the particles inside the dispersion was determined.The characteristic curves of different treated samples were examined.Fill Factor of FF = 41%; cell efficiency = 6,38% (Type I) was obtained.Fill Factor of FF = 27%; cell efficiency = 2,95% (Type II) was observed.Estimated doping depth to at least hBSF = 5 m (SEM).An initial work with thin-film Kapton foils was carried out.
OutlookMore thorough studies of the regions characterized by a highly reflective surface.Further investigations of the correlation between crystallinity and diode behavior.Remove native silicon surface oxide with hydrouoric acid before laser treatment.Use more scans at higher intensity.Pulsed UV-Laser treatment on Kapton foils.*
*AcknowledgementsTHANK YOU to:Prof. Dr. Roland Schmechel for giving me the opportunity to work on this exciting topic.Dr. Niels Benson and Dipl.Ing. Martin Meseth for their time and guidance during the development of this work. Their advices contributed to the pleasant and fruitful experience that I obtained during this time.The whole team of the NST department for their support concerning my work in the laboratory.
Thank you for your attention!!!*
*weight percent (wt%)**Semi-ready Solar Cell with Anti-reflex Coating and Pre-deposited Front Side AgContacts 6 pre-heating, Ipreheat = 50% @ Vpreheat = 10m/min; 1 sinter Scan, Isintern = 30% @ Vsintern = 0,2m/minIn case of a sample (10 10mm2) which is scanned with velocity of Vsintern = 1m/min a total time of tscan = 0, 6s is needed.When introducing more energy inside the Si-nanoparticles layer by slower speeds (e.g. Vsintern = 0,2m=min) the needed time is as well increased to tscan = 3s.
*Highly vs. Low Reflective Area Comparison; Semi-ready Solar Cell with Anti-reflexCoating and Pre-deposited Front Side Ag Contacts (Sample No 10, 01.09.2011: 6 preheating,Ipreheat = 50%@Vpreheat = 10m=min; 1 sinter scan, Isintern = 30%@Vsintern = 0; 2m=min)
*The electrical resistivity, , can be derived as:total =(U23/I1)*(A/L)=(U23/I1)*(dtotal*s/L)=1/total where,A is the area through which current flows.dtotal is the total thickness of the measured wafers is the common contact length between the contact stripesL is the distance between the inner contact stripes;total is the total conductivity of the material under test;*