© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-rapid.com pss Phys. Status Solidi RRL 6, No. 5, 190–192 (2012) / DOI 10.1002/pssr.201206068 Defect-band photoluminescence imaging on multi-crystalline silicon wafers Fei Yan *, 1 , Steve Johnston 1 , Katherine Zaunbrecher 1, 2 , Mowafak Al-Jassim 1 , Omar Sidelkheir 3 , and Kamel Ounadjela 3 1 National Renewable Energy Laboratory, Golden, CO 80401, USA 2 Colorado State University, Fort Collins, CO 80523, USA 3 Calisolar, Sunnyvale, CA 94085, USA Received 15 February 2012, revised 7 March 2012, accepted 7 March 2012 Published online 13 March 2012 Keywords silicon, solar cells, photoluminescence imaging, defects * Corresponding author: e-mail feiyan@gmail.com, * Current address: Applied Materials, Inc, Santa Clara, CA, USA © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 Introduction Different imaging techniques have been developed recently and turned into rapid and nonde- structive inline characterization tools. Electroluminescence (EL) imaging collects the light coming out from finished solar cells under forward bias [1, 2]. Spatially resolved in- formation about the recombination, resistance, and cracks can be obtained in seconds. However, because EL imaging requires electrical contacts, it is limited to use on the fin- ished solar cells and any steps thereafter. PL imaging uses optical excitation to generate electron–hole pairs and a camera detects the radiative recombination emission. It has a great advantage as a contactless technique and can be ap- plied to essentially all processing steps, from the silicon ingots and bricks to all processing steps on the wafer scale. Significant work has been done on the band-to-band PL emission around 1150 nm using silicon charge-coupled de- vice (CCD) cameras, which gives valuable information about the extended defects inside the silicon wafers [3–7]. Recently, imaging of the defect-band luminescence at around 1550 nm has been discussed as an alternative and/or complimentary way to study the material quality [8–10]. The band-to-band PL image and defect-band image have been compared on finished solar cells [9, 11]. For the defect band, four lines D1, D2, D3, and D4 have been re- ported at low temperature, while at room temperature only one broad emission peak can be observed [9–10, 12]. 2 Experiment In this Letter, we present our imaging work using an InGaAs camera, which collects defect-band emissions and allows us to study the defect-band PL im- ages on 156 × 156 mm mc-Si wafers that had gone through six processing steps. These steps include as-cut, texturing, emitter diffusion, phosphosilicate glass (PSG) removal, anti-reflection coating (ARC), and metallization. These wafers were selected from different bricks at different heights, including near the bottom, in the middle and near the top. We prepared 30 wafers from each step and exam- ined a total of 180 wafers, which give us enough data points for a statistical analysis. We used a FLIR SC2500 NIR InGaAs Camera (320 × 256 pixels) with a StingRay 25 mm wide angle lens for defect-band PL imaging. The detection sensitivity of the InGaAs camera is in the 0.9 to 1.7 μm range, which covers both the band-to-band emission at ~ 1.1 eV and the Defect-band emission photoluminescence (PL) imaging with an indium-gallium-arsenide (InGaAs) camera was applied to multi-crystalline silicon (mc-Si) wafers, which were taken from different heights of different Si bricks. Neighboring wa- fers were picked at six different processing steps, from as-cut to post-metallization. By using different cut-off filters, we were able to separate the band-to-band emission images from the defect-band emission images. On the defect-band emis- sion images, the bright regions that originate from extend- ed defects were extracted from the PL images. The area fraction percentage of these regions at various processing stages shows a correlation with the final cell electrical pa- rameters.