Distinguishing morphological and electrical defects in polycrystalline silicon solar cells using scanning electron acoustic microscopy and electron beam induced current L. Meng a,n , D. Nagalingam a , C.S. Bhatia a , A.G. Street b,c , J.C.H. Phang a,c a Centre for Integrated Circuit Failure Analysis and Reliability (CICFAR), Department of Electrical and Computer Engineering, National University of Singapore, 4 Engineering Drive 3, Singapore 117576, Singapore b Qualcomm CDMA Technologies, 5775 Morehouse Drive, San Diego, CA 92121, USA c Inscope Labs Pte Ltd., 28 Ayer Rajah Crescent # 03-01, Singapore 139959, Singapore article info Article history: Received 21 December 2010 Accepted 12 May 2011 Keywords: Morphological defect Electrical defect Solar cell Scanning Electron Acoustic Microscopy Electron Beam Induced Current abstract Morphological and electrical defects in polycrystalline silicon solar cells are distinguished by scanning electron acoustic microscopy (SEAM) and electron beam induced current (EBIC) techniques, respec- tively. It was found that while some defects are both morphologically and electrically detectable, some are predominantly only either electrical or morphological in nature. Combining both SEAM and EBIC is therefore an ideal approach as the two techniques can provide complementary information on both the morphological and electrical manifestation of the defects. & 2011 Elsevier B.V. All rights reserved. 1. Introduction Polycrystalline silicon (poly-Si) is a promising candidate for low cost and large area solar cells. However, one major limitation of poly-Si solar cells, as compared to their counterparts made of single crystal Si, is their low efficiencies. The poor performance of these cells is attributed to the existence of morphological and electrical defects [1]. For the investigation of morphological defects, scanning electron acoustic microscopy (SEAM) is a preferred tool due to its non-destructive nature, its ability of subsurface imaging and depth profiling, as well as the ease in sample preparation [2,3]. SEAM, however, is not sensitive to non- morphological electrical defects, i.e. electrical defects without morphological features. In addition, even within a single mor- phological defect, the electrical properties may vary at different locations [4]. On the other hand, electron beam induced current (EBIC) is an ideal technique for characterization of electrical defects such as recombination sites, doping level inhomogeneities [5] and electrical irregularities in solar cells [6]. However, EBIC requires a pn junction and the detection range is limited to a few diffusion lengths from the junction. In other words, electrical defects that are further away from the junction cannot be revealed by the EBIC technique. In this paper, we have utilized both SEAM and EBIC as complementary methods for characterization of defects in poly-Si solar cells. Morphological and electrical defects in a poly-Si solar cell were first distinguished by SEAM and EBIC, respectively. We would subsequently show that a defect could be predominantly morphological in nature (as revealed by SEAM) and yet electrically undetectable (by EBIC), and vice versa. This implies that neither SEAM nor EBIC alone is enough for the defect investigation; combining both SEAM and EBIC is a better approach instead as the two techniques can provide complementary information on both the electrical and morphological manifestation of the defects. 2. Experimental setup A Hitachi S2700 scanning electron microscope (SEM) is mod- ified for both SEAM and EBIC imaging. Fig. 1 shows the block diagram of the SEAM and EBIC setups. For the SEAM setup, an intensity-modulated electron beam is achieved with the beam blanking system that comprises a function generator, amplifier and blanking plates [7]. When the modulated electron beam irradiates the sample surface, localized heating induces acoustic waves that propagate through the sample and are detected by Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/solmat Solar Energy Materials & Solar Cells 0927-0248/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2011.05.022 n Corresponding author. Tel.: þ65 6516 1568; fax: þ65 6516 7912. E-mail address: lei.meng@nus.edu.sg (L. Meng). Solar Energy Materials & Solar Cells 95 (2011) 2632–2637