Spectroscopic Ellipsometry (SE) in Photovoltaic Applications Jianing Sun, James Hilfiker, Greg Pribil, and John Woollam c-si PVMC Metrology Workshop July 2012, San Francisco
PV key issues Material selection and Module design Processing and Optimization Efficiency, Cost, Reliability, Lifetime etc. 2
E p-plane Ellipsometry s-plane plane of incidence p-plane E ~ R i p ρ tan( Y) e ~ Rs s-plane Measures polarization change (Y and Δ) when light reflects from a surface. Non-destructive technique Can determine refractive index and thickness High sensitivity to thin films Suitable for in situ 3
Data Analysis Optical Model Experimental data Film Thickness Optical Constants 4
Composition and Band Gap Sqrt(e2)*E Composition: Si 1-x Ge x Imag(Dielectric Constant), 2 50 40 30 20 10 a-si 70% Poly-Si 85% Poly-Si c-si Photon Energy (ev) Crystallinity 0 1.0 2.0 3.0 4.0 5.0 6.0 5 2.5 2.0 1.5 1.0 0.5 0.0 CdMgTe (PV) CdMgTe (CdCl2 dip and anneal, PC) CdMgTe (N2 depo) Band Gap 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 Photon Energy (ev)
Extinction Coefficient (k) Index of Refraction (n) From Research to Production 2.4 2.1 1.8 1.5 1.2 0.9 0.6 1.0 0.8 0.6 0.4 0.2 Middle of Deposition After Deposition After Annealing 0.0 300 600 900 1200 1500 1800 Wavelength (nm) 6
Examples of SE in PV Applications Anti-reflection coatings (ARC) Transparent Conductive Oxides (TCOs) Amorphous, microcrystalline, and poly-si films CdTe and CdS Organic PV layers Back Contact. 7
ARC - SiNx on Textured c-si Texturing and ARC increase light flux and greatly reduce reflection Very challenging for optical characterization SE has shown success in such samples Requires advanced instrumentation and special geometry 8
Single layer TCO TCO TiO 2 Not good model NIR absorption used to monitor conductivity Index/conductivity often vary with film depth Graded TCO 2 Good Model 9
Multi-layer TCOs Complex TCO stacks: Multi-Layer structure Rough surface SnO 2 :F SiO 2 SnO 2 Substrate 10
a-si:h Thin films Very thin layers (10s or 100s nm) Small optical contrast among layers Multi-sample analysis P-layer p-layer i-layer n p-layer i-layer k n-layer 11
CdTe / CdS Real(Dielectric Constant), 1 Real(Dielectric Constant), 1 Thin CdS layer to reduce absorption 7.0 6.0 5.0 4.0 3.0 2.0 CdS Optical Constants 1.0 0.0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 Photon Energy (ev) 10 8 6 4 2 0 1 2 CdTe Optical Constants -2 0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 Photon Energy (ev) 1 2 6.0 5.0 4.0 3.0 2.0 1.0 10 8 6 4 2 Imag(Dielectric Constant), 2 Imag(Dielectric Constant), 2 12 < 1 > < 2 > 15 12 9 6 3 0 Model Fit Exp Eb 60 Exp Eb 75-3 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 9 Photon Energy (ev) 6 3 0-3 -6 CdTe CdS 1458nm 210nm TEC Model Fit Exp Eb 60 Exp Eb 75-9 0.0 1.0 2.0 3.0 4.0 5.0 12 6.0 7.0 Photon Energy (ev)
Transmission Organic PV Index of Refraction Extinction Coefficient Index of Refraction Extinction Coefficient Anisotropic Films 0.9 0.8 0.7 PEDOT:PSS 125nm in degrees Y in degrees 0.6 0.5 60 40 20 0 24 21 18 15 12 Model Fit 50 nm 75 nm 125 nm PEDOT:PSS 125nm Model Fit 50 nm 75 nm 125 nm PEDOT:PSS 9 0 1 2 3 4 Photon Energy PEDOT:PSS (ev) 75nm 50nm PEDOT:PSS 0.4 PEDOT:PSS 13 2.2 2.0 1.8 1.6 1.4 1.0 0.8 0.6 0.2 n xy n z 75nm 50nm 0.0 0 1 2 3 4 J.A. Woollam Co., Photon PVMC Energy c-si (ev) Metrology Workshop, July 2012 2.2 2.0 1.8 1.6 1.4 1.0 0.8 0.6 0.4 0.2 n xy n z 0.0 0 1 2 k xy k z Photon Energy (
Structure and Anisotropy D. Yokoyama, A. Sakaguchi, M. Suzuki, C. Adachi, Organic Electronics, 10 (2009) 127-137. 14
Back Contact Thin c-si leads to potential back contact absorption and reflection SE measurement provides optical information for back contact layer. V.A. Popovich, M. Janssen, I.M. Richardson, T. van Amstel, I.J. Bennett, 2th European Photovoltaic Solar Energy Conference, 21-25 Sept 2009, Hamburg, Germany 15
Summary- SE in PV Optical constants Optical constants Film thickness Band Gap Material selection and module design Processing and optimization Thickness and uniformity Composition Crystallinity Roughness Anisotropy Band gap Efficiency, cost, reliability, lifetime etc. Sheet Resistance Degradation and mechanism 16
Short Term Challenges in c-si PV Challenge #1: Understanding AR behavior on polished and textured substrate Challenge #2: Charactering thinner and multilayer ARCs Challenge #3: In-line monitoring 17
Long Term Challenges in c-si PV Dimensional challenge: Challenge #1: Understanding the interface between back contact and Si as Si thickness is further reduced. Challenge #2: Feasibility of SE in c-si PV if finger line space and width shrink. 18
Thank you! 19