Mirrors at the cell surface and mirrors on the rear also cannot remove all the sub-bandgap light, since parasitic absorption also occurs in the cover glass, encapsulant, and front contacts, depending on the module configuration. Mirrors at the Si cell/encapsulant interface are limited by the multiple reflections needed to remove sub-bandgap light. We found definitively that spectrally-selective mirrors on the outside of the module cover glass offer the best performance, both under ideal conditions and for realistic, low- complexity mirrors. We also experimentally fabricated mirrors, incorporated them into mini-modules, and performed outdoor testing of performance at NREL. We connect these optical properties to energy yield under realistic, outdoor conditions. Here we build from our prior accomplishments in developing mirror optimization techniques, and make use of simulation methods that accurately capture the optical properties of the modules throughout the visible and near-infrared spectrum. We also studied the performance of these reflectors in different types of modules, including Al BSF, PERC, and bifacial. We studied two complementary concepts: the performance of ideal structures to determine the upper limits to performance, and low-complexity structures made from standard materials as cost-effective implementations. The goal of this project was to quantitatively assess optical strategies for improving the energy yield via a combination of improved anti-reflection and sub-bandgap light rejection. Prior to this project, relatively little was known about the optical strategies for temperature reduction in bifacial modules. The issue of heating can be even more critical in bifacial modules: high fractions of rear irradiance can lead to higher operating temperatures. Furthermore, many of these approaches are complex and expensive to fabricate.
However, there are many competing strategies to reject this light: selective reflectors could be integrated into the outer module glass, at the textured more » Si-encapsulant interface, or on the rear. Since much of the elevated temperature of the modules derives from parasitic absorption of sub-bandgap sunlight by the contacts, encapsulants, or other materials, reflectors that remove the sub-bandgap radiation from the module before it is absorbed promise to reduce the operating temperature. These elevated operating temperatures lead to diminished performance, reducing efficiency by ~0.4%/K for crystalline Si cells and decreasing module lifetime. When installed outdoors, Si solar cells typically operate 20 – 30 K above ambient conditions. Solar Energy Technologies Office OSTI Identifier: 1184501 Report Number(s): SAND-2014-19059J Journal ID: ISSN 2156-3381 540684 Grant/Contract Number: AC04-94AL85000 Resource Type: Accepted Manuscript Journal Name: IEEE Journal of Photovoltaics Additional Journal Information: Journal Volume: 5 Journal Issue: 1 Journal ID: ISSN 2156-3381 Publisher: IEEE Country of Publication: United States Language: English Subject: 14 SOLAR ENERGY current-voltage characteristics mathematical model minimization photovoltaic cells photovoltaic systems silicon solar power = , (SNL-NM), Albuquerque, NM (United States) Sponsoring Org.: USDOE Office of Energy Efficiency and Renewable Energy (EERE), Renewable Power Office. Publication Date: Research Org.: Sandia National Lab. (SNL-NM), Albuquerque, NM (United States) Yingli Green Energy International, Zurich (Switzerland).Yingli Green Energy Americas, Inc., San Francisco, CA (United States).