gessert talk

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Advances in the In-House CdTe Research Activities at NREL T. Gessert, X. Wu, R.G. Dhere, H. Moutinho, S. Smith M. Romero, C. Corwine*, J. Zhou, and A. Duda National Renewable Energy Laboratory *Colorado State University Acknowledgements First Solar, LLC U.S. DOE Contract No. DE-AC36-99GO10337

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Areas Highlighted High Efficiency and Manufacturability Advanced Nano-Probe Techniques Back Contact Investigations Radiative Recombination Studies

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High Efficiency and Manufacturability Compare Substrates: Commercial Soda-lime glass/SnO2 Borosilicate glass/CTO Integrate Novel Layers: Zinc-Tin-Oxide (ZTO) buffer layer Oxygenated Nanocrystalline CdS (CdS:O) layer Optimize Device at Lower Temp: ~625°C (NREL World Record Cell) <570ºC (Soda-Lime Glass) Reduce CdTe thickness Typical NREL = 8-12 µm Target = 5 µm Use of NREL Processes with Soda-Lime Glass (X. Wu and J. Zhou, 19th European PVSEC, June 2004)

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High Efficiency and Manufacturability Incorporate “One heat-up step” Process

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High Efficiency and Manufacturability * NREL confirmed total-area efficiency ** Cell# 1-5 on Tek15; Cell# 6 on Asahi textured SnO2/SL glass substrate Device Results (Commercial Soda-Lime Glass/SnO2 Substrates)

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High Efficiency and Manufacturability * NREL confirmed total-area efficiency Uniformity Results (Commercial Soda-Lime Glass/SnO2 Substrates)

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High Efficiency and Manufacturability Demonstrate a process to produce high-efficiency, thin-film CdTe solar cells on commercial SL- glass /SnO2 substrates Incorporated ZTO and CdS:O into commercial SL-glass devices. NREL-confirmed total-area efficiency of more than 14%. Transferred related technologies for preparing NREL-developed materials to industrial partner: Recipes, Quality Criteria, Characterization Procedures Soda-Lime Glass Devices Summary

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Advanced Nano-probe Techniques Conductive AFM (C-AFM) Analysis of Br/Meth Pre-contact Etch (H. Moutinho, R. Dhere et. al., 19th PVSC, June 2004) AFM Simultaneously Acquired C-AFM

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Advanced Nano-probe Techniques Near-Field Scanning Optical Microscopy (NSOM) Analysis (S. Smith, et. al., Appl. Phys. Lett. 85 (17) 2004) AFM Simultaneously-Acquired Near Field Optical Beam Induced Current n-OBEC Image Intensity Photocurrent Collected near Grain Boundaries up to ~5 Times Higher than Photocurrent Collected on Grain!!

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Back Contact Investigations ZnTe:Cu Deposition Temperature ZnTe:Cu Thickness Controlled Cu Incorporation During ZnTe:Cu Contacting

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Back Contact Investigations CV study of ZnTe:Cu thickness CV study of effect of CdTe thickness Controlled Cu Incorporation During ZnTe:Cu Contacting

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Back Contact Investigations Contact deposition Temperatures less than optimum Contact Deposition Temperatures greater than optimum Very Thick ZnTe:Cu Contacts (i.e., Lots and Lots of Cu!) (To Be Pub. 31 IEEE PVSC)

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Back Contact Investigations Contact with ZnTe:Cu that is too thick (too much Cu into CdTe) CdS/CdTe Evolution During Back Contacting (To Be Pub. 31 IEEE PVSC)

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CdTe Radiative Recombination Studies 4 mm CdS TCO CdTe 4 mm 200 meV ZnTe:Cu/Ti contacting NREL First Solar CL Temperature = 77K Cross-Sectional Cathodoluminescence (T. Gessert. M. Romero, et. al., 3rd WCPEC, Osaka, 2003) Junction Region Back Contact Region

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CdTe Radiative Recombination Studies Temperature-Dependent PL of Crystalline and CdTe Films Low-Temperature CL (19.7 K) Controlled Diffusion of Cu into CdTe (C. Corwine*, T. Gessert, et. al., Submitted Appl. Phys. Lett,) Cu Deposited Surface *Colorado State University

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Conclusions Conclusions Significant progress has been made in understanding many aspects of polycrystalline CdTe device technology. Reproducible devices using ZTO and CdS:O on SL glass >14%. Nano-probe analysis indicates some insight for high collection. ZnTe:Cu contact analysis reveals insight to junction evolution One key to significant future improvements in performance and/or stability will be understanding and controlling defect formation during various process steps.