Peter Hebert

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.

Status of C3MJ+ and C4MJ Production Concentrator Solar Cells at Spectrolab

Multijunction solar cells based on III-V semiconductors, having recently demonstrated 43.5%, remain the worlds most efficient solar cells, and the preferred technology in point-focus and dense-array concentrator photovoltaic (CPV) system architectures. The year 2011 proved to be a pivotal year for CPV technology, with multiple power plant installations in the megawatt to tens of megawatt scale. Spectrolab is working closely with CPV system manufacturers to provide a reliable and well-characterized cell technology, in volumes commensurate with this increasing demand. The evolutionary C3MJ+ and the C4MJ cell technologies are the latest in a sequence of CPV solar cell designs, with conversion efficiencies approaching or greater than 40%. Both technologies have completed detailed characterization and qualification programs, including accelerated laboratory and (for C4MJ) on-sun reliability testing, and have entered into high volume production at Spectrolabs manufacturing facility in Sylmar, CA. The metamorphic C4MJ technology affords new opportunities to optimize cell designs, taking into consideration both the spectral optical transmittance of a particular CPV system and the installation sites average solar resource over a typical meteorological year.

Triple-junction GaInP/GaAs/Ge solar cells-production status, qualification results and operational benefits

In 1999 Spectrolab completed design and qualification, and began production on the next generation of multijunction solar cells-a triple-junction GaInP/GaAs/Ge. With over 20% AM0 conversion efficiency at an operating temperature of 60/spl deg/C, this cell provides 8-11% more power than competing dual-junction designs in GEO orbit after 15 years (6/spl times/10/sup 14/ 1-MeV electron equivalence). Spectrolab is currently qualifying an improved triple-junction cell capable of delivering over 22% AM0 conversion efficiency under these same conditions, with a beginning-of-life operating efficiency of 27%.

Advanced III-V Multijunction Cells for Space

III-V solar cells have become the dominant power generation technology in space, due to their unparalleled high efficiency, reliability in the space environment, and ability to be integrated into very lightweight panels. As remarkable as these attributes are, new types of space III-V solar cells are continually reaching new heights in performance. Commercially-available multijunction solar cells with 30% conversion efficiency under the AM0 space spectrum are just around the corner. Understanding of radiation resistance and thermal cycling reliability has reached levels never before attained, and is resulting in new standards of reliability. A flurry of research activity has resulted in very-thin, flexible, and extremely lightweight space solar cells and panels in several groups around the world, capable of being folded or rolled into a smaller stowage volume for launch than has been possible to date. This approach combines the very high efficiency and reliability of III-V multijunction cells with the thin, flexible PV blanket functionality normally associated only with thin-film polycrystalline or amorphous PV technology. This paper discusses the latest developments in III-V space solar cell technology, and explores opportunities for still higher performance in the future

Minority carrier lifetime and radiation damage coefficients of germanium

We report on the measurement of minority carrier lifetime and on the radiation damage resistance of bulk Ge. Lifetime measurements are performed using the resonance-coupled photoconductive decay (RCPCD) method. Specifically, we examine the dependence of the lifetime as a function of the Ge resistivity and various 1 MeV electron radiation fluences. We measure hole lifetimes ranging from /spl sim/0.9-34 /spl mu/s for n-type Ge samples, corresponding to diffusion lengths of /spl sim/30-400 /spl mu/m. Electron lifetimes in p-type Ge range from /spl sim/0.6-19 /spl mu/s, corresponding to diffusion lengths of /spl sim/30-420 /spl mu/m. Lifetime measurements are also made after exposure to 1 MeV electron fluences ranging from 10/sup 13/ to 10/sup 15/ cm/sup -2/ and these results are used to estimate the minority carrier lifetime and diffusion length damage coefficients K/sub /spl tau// and K/sub L/.

Advancements in GaInP/sub 2//GaAs/Ge solar cells - production status, qualification results and operational benefits

In 2001 Spectrolab completed design and qualification, and began production on the third-generation multijunction solar cell - the improved triple-junction GaInP/sub 2//GaAs/Ge. With over 21% AMO conversion efficiency at an operating temperature of 60/spl deg/C at end-of-life, this cell has 16% more power than competing dual-junction designs in GEO orbit after 15 years (7/spl times/10/sup 14/ 1-MeV electron equivalence). Spectrolab is currently qualifying the fourth-generation triple-junction solar cell capable of delivering over 22% AMO conversion efficiency under these same conditions, with a beginning-of-life operating efficiency of 28%.

Status of 40% production efficiency concentrator cells at Spectrolab

Multijunction solar cells based on III–V semiconductors are the most efficient solar cells in the world, and of the established photovoltaic technologies, have the greatest potential for future growth in efficiency. Champion cells with efficiency greater than 40% have been demonstrated by several groups since 2006, and in that same period, the efficiency of cells in mass production has also increased steadily. These devices offer the promise of very competitive solar power systems exploiting the high efficiency under high optical concentration. To this end, Spectrolab is conducting a multi-year program to develop solar cells with still higher efficiency and substantial cost reductions and to fully characterize and qualify them for reliable performance in the field. Development of the fourth production generation with 40% average production efficiency is nearing completion. Cell design and performance will be presented, with a summary of qualification and field test status. Progress in ongoing efforts to automate cell production for cost reduction and increased manufacturing capacity will be discussed. Development of these high-performance multijunction CPV cells is key to the emergence of CPV technology as the lowest cost solar power solution in high DNI areas.

Towards commercialization of concentrator multijunction photovoltaic modules

Concentrating photovoltaic (CPV) modules occupy the middle ground between conventional, flat-plate photovoltaics and concentrating solar power (CSP) technologies. CPV promises to deliver the best of both worlds: the highest efficiency of any photovoltaic system combined with the higher capacity factor and scale-up potential of dual-axis tracking systems characteristic of CSP power towers. Having demonstrated cell efficiencies over 40%, the CPV industry has now embarked on an aggressive program to demonstrate cost-effective, highly-reliable CPV modules in the field.

ARE ELECTRO‐LUMINESCENCE DEFECTS IN CONCENTRATOR III‐V CELLS RESPONSIBLE TO THERMAL RUNAWAY AND SUDDEN DEATH?

Two types of failure of III‐V cells in CPV system by Daido Steel have been observed. One is thermal runaway and another is what we call a electrical shock. This paper will discuss on the frequency of the cell failure seen in a field and on experiments to determine the root cause of thermal runaway. Failures by the electrical shock were not related to thermal runaway, and a packaging solution to the failure by the electrical shock was found which will be published at another time. A detailed investigation of 30 kW field was undertaken to identify failed cells. After the other failure mechanism has been removed, experiments can be conducted on thermal runaway. Thermal runaway can occur due to loss of thermal conduction, such as voids or discontinuities in the thermal interchange material bonding cell to heat sink. It has been hypothesized that thermal runaway can also occur at location of cell defects as identified by electroluminescence. So far it we have not been able to induce thermal runaway at location...

Qualification testing of 40% metamorphic CPV solar cells

Spectrolab is qualifying its fourth generation of terrestrial concentrator multijunction cells (C4MJ). Prototypes of this product have been tested with an average efficiency of 40% at 50 W/cm2 illumination. This new generation is a departure from previous production technology in that, for the first time, it employs metamorphic rather than lattice-matched technology.