III-V multijunction solar cells for concentrating photovoltaics

Concerns about the changing environment and fossil fuel depletion have prompted much controversy and scrutiny. One way to address these issues is to use concentrating photovoltaics (CPV) as an alternate source for energy production. Multijunction solar cells built from III-V semiconductors are being evaluated globally in CPV systems designed to supplement electricity generation for utility companies. The high efficiency of III-V multijunction concentrator cells, with demonstrated efficiency over 40% since 2006, strongly reduces the cost of CPV systems, and makes III-V multijunction cells the technology of choice for most concentrator systems today. In designing multijunction cells, consideration must be given to the epitaxial growth of structures so that the lattice parameter between material systems is compatible for enhancing device performance. Low resistance metal contacts are crucial for attaining high performance. Optimization of the front metal grid pattern is required to maximize light absorption and minimize I²R losses in the gridlines and the semiconductor sheet. Understanding how a multijunction device works is important for the design of next-generation high efficiency solar cells, which need to operate in the 45%-50% range for a CPV system to make better economical sense. However, the survivability of solar cells in the field is of chief concern, and accelerated tests must be conducted to assess the reliability of devices during operation in CPV systems. These topics are the focus of this review.