IEEE JOURNAL OF PHOTOVOLTAICS, VOL. 3, NO. 1, JANUARY 2013 387 Correlations Between Mapping Spectroscopic Ellipsometry Results and Solar Cell Performance for Evaluations of Nonuniformity in Thin-Film Silicon Photovoltaics Lila R. Dahal, Zhiquan Huang, Dinesh Attygalle, Carl Salupo, Sylvain Marsillac, Nikolas J. Podraza, and Robert W. Collins Abstract—An understanding of the relationship between mate- rials property and thin-film solar cell performance variations over large areas is of interest for evaluating the impact of macroscopic nonuniformities in scale-up from laboratory cells to production modules. In this study, we have spatially correlated the properties of the hydrogenated silicon (Si:H) i- and p-layers—as mapped over a 13 cm × 13 cm substrate area—with device performance param- eters from an array of a-Si:H based n-i-p dot cells. To evaluate ma- terials and device nonuniformities, a 16 × 16 array of dot cells has been fabricated over the substrate area, and this same area has been mapped by spectroscopic ellipsometry (SE). Analysis of the SE data over the full area provides maps of i-layer thickness and band gap, p-layer thickness and band gap, and p-layer surface roughness thickness for the n-i-p solar cell structure. The mapped values ad- jacent to the devices have been correlated with photovoltaic (PV) device performance parameters. When sufficient nonuniformity exists, these correlations enable optimization based on specific val- ues of the fundamental properties. Alternatively, if the optimum set of properties has been identified, the impact of deviations due to macroscopic uniformities can be evaluated. Index Terms—Amorphous semiconductors, ellipsometry, hydro- gen, nanocrystals, photovoltaic (PV) cells, silicon. I. INTRODUCTION AND OVERVIEW I N thin-film photovoltaics (PV) technologies, it is critical to achieve optimum device performance on the laboratory scale and then to reproduce this optimum in larger area configurations Manuscript received June 2, 2012; revised August 12, 2012 and September 13, 2012; accepted September 17, 2012. Date of publication October 22, 2012; date of current version December 19, 2012. This work was supported by Air Force Research Laboratory, Space Vehicles Directorate under Contract FA9453- 08-C-0172 and by the Ohio Department of Development’s Wright Centers of Innovation Program under Contract TECH 07-026. L. R. Dahal was with the University of Toledo, Toledo, OH 43606 USA. He is now with NSG Pilkington North America, Inc., Northwood, OH 43619 USA (e-mail: Lila.Dahal@nsg.com). Z. Huang, D. Attygalle, N. J. Podraza, and R. W. Collins are with the De- partment of Physics and Astronomy and the Center for Photovoltaics Innova- tion and Commercialization, University of Toledo, Toledo, OH 43606 USA (e-mail: Zhiquan.Huang@rockets.utoledo.edu; Dinesh.Attygalle2@utoledo. edu; Nikolas.Podraza@utoledo.edu; Robert.Collins@utoledo.edu). C. Salupo is with the Center for Photovoltaics Innovation and Com- mercialization, University of Toledo, Toledo, OH 43606 USA (e-mail: Carl.Salupo@utoledo.edu). S. Marsillac is with the Department of Electrical and Computer Engineering, Old Dominion University, Norfolk, VA 23529 USA (e-mail: smarsill@odu.edu). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JPHOTOV.2012.2221081 for industrial-scale application. In thin-film PV technologies— in particular for hydrogenated amorphous and nanocrystalline silicon (a-Si:H and nc-Si:H), as well as for Cu(In,Ga)Se 2 (CIGS)—significant differences exist between champion cells and modules [1]. One component of this difference relates to large-area deposition uniformity. For example, in the research laboratory, optimum a-Si:H and nc-Si:H for the i-layers of solar cells are prepared within narrow regions of deposition parame- ter space [2]–[4]. The a-Si:H i-layer [2], [3] and the p-layer [5] for the a-Si:H solar cell are prepared under maximum H 2 di- lution conditions while avoiding nanocrystallite nucleation and coalescence, respectively, based on the concepts of protocrys- tallinity. In contrast, steady-state growth of nc-Si:H requires minimal H 2 dilution while avoiding conversion to the amor- phous phase [4], [6]. It is a challenge to maintain such conditions over large areas, and this can account in part for the performance difference between laboratory cells and production modules. As a result, methods that map fundamental properties of PV materials over large areas, enabling local correlations between these properties and device performance, can be useful for un- derstanding the uniformity issues that limit the device perfor- mance. We have applied high-speed spectroscopic ellipsometry (SE) based on rotating compensator and multichannel detec- tion principles as a mapping probe [7] of property-performance correlations for dot cells fabricated in a large-area array. High- speed acquisition of SE data has become increasingly advanced through the ability of the rotating compensator to identify and account for the unpolarized component of the reflected beam. Thus, even very rough surfaces that scatter light, such as those in actual PV devices, can be probed, as was demonstrated in [8]. In addition, SE analysis has advanced due to the development of analytical expressions that reduce the number of parameters required in data analysis; in the case of a-Si:H-based alloys, for example, only a single band gap parameter is needed in some cases [9], [10]. SE can provide not only the starting point information of thicknesses, band gaps, and spectroscopic op- tical properties in the form of the complex dielectric function (ε = ε 1 + i ε 2 ) of solar cell component layers but can access grain size, defect density, disorder, and stress through the param- eters that describe ε as well. We have performed this method- ology for the first time on a-Si:H solar cells in order to demon- strate the concept. We find expected and unexpected correlations between local material properties and solar cell performance. 2156-3381/$31.00 © 2012 IEEE