Optimization of Graded CIGS Solar Cells Using TCAD Simulations
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Optimization of Graded CIGS Solar Cells Using TCAD Simulations Mankoo Lee, Dipankar Pramanik, Haifan Liang, Ed Korczynski, and Jeroen van Duren Intermolecular, Inc., 3011 North First Street, San Jose, CA 95134 United States ABSTRACT To understand paths towards higher efficiency (η) for copper-indium-gallium-(sulfur)selenide [CIG(S)Se] solar cells, we investigated a variety of absorber composition grading schemes for various back-side gallium (Ga), front-side sulfur (S), and double-graded Ga composition depth profiles in TCAD 1D/2D simulations. We fitted experimental results of a Back-Side Graded (BSG) solar cell with our TCAD models, prior to investigating other grading and interface schemes. The BSG solar cell was fabricated on a High Productivity Combinatorial (HPC™) platform based on sputtering Cu(In,Ga) followed by selenization. Our TCAD simulation methodology for optimizing CIG(S)Se solar cells started with a sensitivity analysis using 1D Solar-cell CAPacitance Simulator (SCAPS) [1] by selecting a typical range of key model parameters and analyzing the impact on η. We then used a 2D commercially-available Sentaurus simulation tool [2] to incorporate wavelength-dependent optical characteristics. As a result, we provide insight in the impact of grading schemes on efficiency for a fixed ‘material quality’ equal to an in-house BSG solar cell. We also quantify the effects of interface layers like MoSe2 at the Mo/CIG(S)Se interface, and an inverted surface layer at the CIG(S)Se/CdS interface. INTRODUCTION Thin film CIG(S)Se solar cells should theoretically have the highest η at band-gap (Eg) of 1.4-1.5 eV assuming a flat Ga depth profile of ~0.6. However, the best reported η today is 20.3% at Ga/(Ga+In)~0.32 composition and Cu/(Ga+In) ratio from 0.80 to 0.92 for a graded absorber corresponding to an Eg of ~1.15 eV [3]. Higher Eg results in lower efficiencies due to recombination [4]. Compared to an ungraded (flat) case, BSG can create an extra electrical field for better carrier collection, while Front-Side Grading (FSG) can potentially reduce the carrier recombination. BSG of Ga alters the conduction band (Ec) to improve both Jsc and Voc, while FSG of S alters the valence band (Ev) to reduce recombination [5]. Previous TCAD simulations did not provide a conclusive answer as to the best possible grading shape, nor whether Double-Side Grading (DSG) by Ga alone is better than double grading with both Ga & S [4-7]. We also must understand the impact of MoSe2 at the Mo/CIG(S)Se interface, and various CIG(S)Se/CdS interface layer scenarios, since studies have shown various levels of inter-diffusion at these interfaces. Here we present TCAD simulations to increase our insight on the impact of the shape and nature (Ga, and/or S) of the single- and double-grading to optimize both charge generation and collection for Eg of less than ~1.3 eV. After an accurate fitting for an in-house BSG CIGSe cell having η = 16.2% as measured at Standard Test Conditions, we examined the benefits of double grading with Ga alone compared to the
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