Keith Emery

A polymer tandem solar cell with 10.6% power conversion efficiency

An effective way to improve polymer solar cell efficiency is to use a tandem structure, as a broader part of the spectrum of solar radiation is used and the thermalization loss of photon energy is minimized. In the past, the lack of high-performance low-bandgap polymers was the major limiting factor for achieving high-performance tandem solar cell. Here we report the development of a high-performance low bandgap polymer (bandgap <1.4 eV), poly[2,7-(5,5-bis-(3,7-dimethyloctyl)-5H-dithieno[3,2-b:2′,3′-d]pyran)-alt-4,7-(5,6-difluoro-2,1,3-benzothia diazole)] with a bandgap of 1.38 eV, high mobility, deep highest occupied molecular orbital. As a result, a single-junction device shows high external quantum efficiency of >60% and spectral response that extends to 900 nm, with a power conversion efficiency of 7.9%. The polymer enables a solution processed tandem solar cell with certified 10.6% power conversion efficiency under standard reporting conditions (25 °C, 1,000 Wm−2, IEC 60904-3 global), which is the first certified polymer solar cell efficiency over 10%.

Temperature dependence of photovoltaic cells, modules and systems

Photovoltaic (PV) cells and modules are often rated in terms of a set of standard reporting conditions defined by a temperature, spectral irradiance and total irradiance. Because PV devices operate over a wide range of temperatures and irradiances, the temperature and irradiance-related behavior must be known. This paper surveys the temperature dependence of crystalline and thin-film, state-of-the-art, research-size cells, modules and systems measured by a variety of methods. The various error sources and measurement methods that contribute to cause differences in the temperature coefficient for a given cell or module measured with various methods are discussed.

Advanced high-efficiency concentrator tandem solar cells

Computer modeling studies of two-junction concentrator tandem solar cells show that infrared (IR)-responsive bottom cells are essential to achieve the highest performance levels in both terrestrial and space applications. These studies also show that medium-bandgap/low-bandgap tandem pairs hold a clear performance advantage under concentration when compared to high-bandgap/medium-bandgap pairs, even at high operating temperatures (up to 100 degrees C). Consequently, two novel concentrator tandem designs that utilize low-bandgap bottom cells have been investigated. These include mechanically stacked, four-terminal GaAs-0.95-eV-GaInAsP tandem, and monolithic, lattice-matched. three-terminal InP-0.75-eV-GaInAs tandem. In preliminary experiments, terrestrial concentrator efficiencies exceeding 30% have been achieved with each of these designs. Methods for improving the efficiency of each tandem are discussed.<<ETX>>

50% Efficient Solar Cell Architectures and Designs

Very high efficiency solar cells (VHESC) for portable applications that operate at greater than 55 percent efficiency in the laboratory and 50 percent in production are being created. We are integrating the optical design with the solar cell design, and have entered previously unoccupied design space that leads to a new paradigm. This project requires us to invent, develop and transfer to production these new solar cells. Our approach is driven by proven quantitative models for the solar cell design, the optical design and the integration of these designs. We start with a very high performance crystalline silicon solar cell platform. Examples will be presented. Initial solar cell device results are shown for devices fabricated in geometries designed for this VHESC program

Hot spot susceptibility and testing of PV modules

To probe the sensitivity for localized heating of commercial amorphous silicon and crystalline modules, several intrusive and nonintrusive experiments were performed. In the intrusive experiments, each cell in several commercial amorphous silicon modules was evaluated separately and in groups for localized heating effects. Damage in amorphous silicon modules occurred under reverse-bias conditions in the dark above a 5-20 mAcm/sup -2/ cell current density at the interconnection between cells. Shading can cause a larger temperature rise than current mismatch. For the monolithic amorphous silicon modules investigated, the current mismatch between each cell was substantial, but the temperature rise was negligible because of the rather low shunt resistance.<<ETX>>

Practical considerations in tandem cell modeling

Abstract Computer modeling of two-junction, tandem solar cells is performed with an emphasis on exploring the sensitivity of cell design and performance to important, practical parameters such as subcell connectivity, incident spectrum, junction temperature, concentration ratio and top cell quantum efficiency. The accuracy of the model is verified by comparing calculated bandgap-dependent, normalized conversion efficiency temperature coefficients with those measured experimentally for state-of-the-art, single-junction cells. Examples of the effects of operational parameter variations are presented. Tandem designs based on independently connected subcells are shown to have several advantages. Based on the modeling work, novel, low-bandgap, InP-based devices have been developed which appear promising for bottom cell applications in two-junction tandems. In particular, epitaxially grown, high-performance p/n homojunctions in Ga0.47In0.53As layers lattice matched to InP substrates have been fabricated. The results of performance testing the Ga0.47In0.53As cells under mild concentration ratios suggest that a practical efficiency of at least 35% is possible for a GaAs/Ga0.47In0.53As mechanically stacked, two-junction tandem cell which is independently connected and operated under a concentration ratio of 500 suns (ASTM E891-87 direct spectrum, 25°C).

High-performance concentrator tandem solar cells based on IR-sensitive bottom cells

Abstract Computer simulations of two-junction, concentrator tandem solar cell performance show that IR-sensitive bottom cells are required to achieve high efficiencies. Based on this conclusion, two novel concentrator tandem designs are under investigation: (1) a mechanically stacked, four-terminal GaAs/GaInAsP (0.95 eV) tandem, and (2) a monolithic, lattice-matched, three-terminal InP/GaInAs tandem. In preliminary experiments, terrestrial concentrator efficiencies exceeding 30% have been achieved with each of the above tandem designs. Methods for improving the efficiency of each tandem type are discussed.

The world photovoltaic scale: an international reference cell calibration program

This paper describes the World Photovoltaic Scale (WPVS) international reference cell calibration program. The WPVS provides a scale for PV performance measurements that has been established through round-robin calibration of a group of primary monocrystalline Si reference cells and is traceable to Systeme International (SI) units. Procedures for recalibration of the reference cell group have been devised, along with procedures for admittance and calibration of new reference cells. A reference cell package has been designed that meets the unique needs of the WPVS. It is hoped that the existing WPVS group will eventually be replaced with cells of the new design that have passed a comprehensive acceptance-test procedure. Copyright

Procedures for evaluating multijunction concentrators

Procedures at NREL and Fraunhofer ISE for evaluating multijunction cells are detailed with a triple-junction GaInP/GaAs/Ge concentrator cell designed and grown at Spectrolab and processed at NREL, and a tandem Fraunhofer ISE Ga/sub 0.35/In/sub 0.85/P/Ga/sub 0.83/In/sub 0.17/As cell as examples. The one-sun efficiency and I/sub sc/ for the triple-junction device measured at Fraunhofer ISE and NREL agreed within 0.2%, well below the /spl plusmn/6% uncertainty estimated by NREL. The procedures for determining the one-sun characteristics involve determining the quantum efficiency and using it for spectral correction during the I-V characterization. The characteristics under concentration are evaluated with a flash simulator.

A comparison of photovoltaic module performance evaluation methodologies for energy ratings

The rating of photovoltaic (PV) modules has always been a controversial topic in the PV community. There is no industry standard methodology to evaluate PV modules for energy production. This issue must be discussed and resolved for the benefit of system planners, utilities, and other consumers. Several methodologies are available to rate a modules peak power, but do any accurately predict energy output for flat-plate modules? This paper analyzes the energy performance of PV modules using six different energy calculation techniques and compares the results to the measured amount of energy produced. The results indicate which methods are the most effective for predicting energy output in Golden, Colorado, under prevailing meteorological conditions.