I. Electroluminescence characterization of recombination, optical and resistive losses in one and two dimensional solar cell devices, and II. A detailed assessment of the solar generation potential of rooftops using lidar: case study of Newark DE
Date
2019
Authors
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Journal ISSN
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Publisher
University of Delaware
Abstract
Electroluminescence (EL) is a fast, simple, high spatial resolution and non-destructive imaging characterization technique that has been used extensively for identification of loss mechanisms in solar cells and modules. Since it is the reciprocal action of the photovoltaic effect taking place in solar cells, many of the processes that influence solar cell characteristics and performance such as recombination, resistive and optical effects also influence the EL emission spectra. Highly relevant and critical parameters such as the local junction voltage (VJ), effective diffusion length (Leff), ideality factor (n), series resistance (RS) and the dark saturation current density (J0) have been spatially mapped and derived from electroluminescence images. Most of the EL discussed in literature focuses on one-dimensional (1D) diffused junction Si solar cells and to a lesser extent on diffused junction interdigitated back contacts (IBC) cells. Little research has been extended to heterojunction devices especially IBC silicon heterojunction (IBC-SHJ) 2D devices with complex lateral current transport. In thin film solar technology, EL studies mostly focus on the electronic non-uniformities such as bandgap fluctuations. ☐ In this work, we apply EL techniques to different device structures and geometries to isolate and quantify the underlying loss mechanisms detrimental to the device efficiency by relying on the detailed balance theory between emission and absorption. 1) For studying the influence of optical effects, we focus on 1D front junction (FJ) and bifacial (BF) devices and also 2D front junction silicon test structures with IBC patterns (FJ-IBC). We implement and extend EL methods to identify the plasmonic optical losses at the rear metal contact and quantify the role of Indium Tin Oxide (ITO) in alleviating these losses. 2) For characterizing resistive effects, we focus on 1D FJ and 2D IBC-SHJ devices and derive spatial maps of VJ, RS and J0 using an iterative approach to analyze EL at multiple injection current densities and compare EL derived parameters to those obtained from standard global dark diode analysis methods. We also confirm the fundamental relationship between EL emission and the minority carrier concentration and derive ideality factors from EL by varying the injection bias current density in 1D FJ and 2D BJ-IBC devices and test structures. 3) For recombination effects, we derive Leff from EL images for 2D IBC-SHJ devices. Leff corresponds to the distance minority carriers’ travel from their injection point to regions of lower concentration before they are lost to non-radiative recombination. 4) We introduce intentional non-radiative recombination centers in the form of highly defective laser defect spots (LDS) in FJ devices and BJ silicon test structures and analyze the spatial profile of minority carriers as they recombine with these localized defects that act as non-radiative recombination centers with high surface recombination velocity (SRV). 5) Finally, we apply EL to 1D polycrystalline thin film direct bandgap Cu (In, Ga) Se2 (CIGS) solar devices and correlate EL emission intensity with the open circuit voltage (VOC) and validate Rau’s second reciprocity theorem using data from ACIGS devices. Rau’s second theorem relates the VOC to the EL quantum efficiency. Future work will focus on spatial profiling of minority carriers as they recombine with LDS in 1D BJ devices at different spacing , EL characterization of different device geometries and CdTe thin film devices, temperature dependent EL and combining spectral and spatial EL. ☐ Light Detection and Ranging (LiDAR) is a high resolution tool for remote sensing and mapping of morphological data using lasers and is widely available. We applied Geographic Information System (GIS) tools and methods to publically available LiDAR 3D data sets as a robust analytical tool to derive the available suitable rooftop space for solar installation based on shading, slope, orientation and a minimum of 10 m2 contiguous area. Using a series of well established guidelines, the solar generation capacity and cost is quantified through the System Advisory Model (SAM) software, based on module efficiency, inverter efficiency, DC to AC ratio, degradation as well as other well established parameters. Comparison with other methods used in the literature and this NREL based method is discussed and the city of Newark is shown as a case study for solar generation. The focus will be on flat roofs in government/UD owned buildings that pave the way for future city-wide solar deployment. Future work will focus on implementing bifacial modules, dual tilt-east west orientation modules and analyzing opportunity costs.