C.R. Osterwald

Commonly observed degradation in field-aged photovoltaic modules

Degradation leading to failure in photovoltaic modules follows a progression that is dependent on multiple factors, some of which interact causing degradation that is difficult to simulate in the lab. This paper defines observed degradation in field-aged modules, including degradation of packaging materials, adhesional loss, degradation of interconnects, degradation due to moisture intrusion, and semiconductor device degradation. Additionally, this paper suggests that the onset and progression of degradation need to be studied to gain a more comprehensive understanding of module degradation rates and module failures.

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>>

Electrochemical corrosion of SnO2:F transparent conducting layers in thin-film photovoltaic modules

We report on a degradation mechanism in thin-film photovoltaic (PV) modules activated by damp heat and voltages similar in magnitude to those generated by PV modules in power generation systems. This mechanism, which appears to be an electrochemical process involving the soda-lime glass superstrate with its conductive SnO2:F layer, can be greatly accelerated by subjecting modules to elevated temperatures and humidity, both of which increase the leakage currents between the frame and the active PV layers. Water vapor can affect the module damage in two ways: (1) by enhancing leakage currents, and (2) by entering through the module edges, it appears to promote the chemical reaction responsible for the SnO2 corrosion. Damage has been found to occur in both a-Si and CdTe modules.

Degradation analysis of weathered crystalline-silicon PV modules

We present an analysis of the results of a solar weathering program that found a linear relationship between maximum power degradation and the total UV exposure dose for four different types of commercial crystalline Si modules. The average degradation rate for the four modules types was 0.71 % per year. The analysis showed that losses of short-circuit current were responsible for the maximum power degradation. Judging by the appearance of the nondegraded control modules, it is very doubtful that the short-circuit current losses were caused by encapsulation browning or obscuration. When we compared the quantum efficiency of a single cell in a degraded module to one from an unexposed control module, it appears that most of the degradation has occurred in the 800-1100 nm wavelength region, and not the short wavelength region.

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

Quadruple-Junction Inverted Metamorphic Concentrator Devices

We present results for quadruple-junction inverted metamorphic (4J-IMM) devices under the concentrated direct spectrum and analyze the present limitations to performance. The devices integrate lattice-matched subcells with rear heterojunctions, as well as lattice-mismatched subcells with low threading dislocation density. To interconnect the subcells, thermally stable lattice-matched tunnel junctions are used, as well as a metamorphic GaAsSb/GaInAs tunnel junction between the lattice-mismatched subcells. A broadband antireflection coating is used, as well as a front metal grid designed for high concentration operation. The best device has a peak efficiency of (43.8 ± 2.2)% at 327-sun concentration, as measured with a spectrally adjustable flash simulator, and maintains an efficiency of (42.9 ± 2.1)% at 869 suns, which is the highest concentration measured. The Voc increases from 3.445 V at 1-sun to 4.10 V at 327-sun concentration, which indicates high material quality in all of the subcells. The subcell voltages are analyzed using optical modeling, and the present device limitations and pathways to improvement are discussed. Although further improvements are possible, the 4J-IMM structure is clearly capable of very high efficiency at concentration, despite the complications arising from utilizing lattice-mismatched subcells.

Comparison of Degradation Rates of Individual Modules Held at Maximum Power

In this paper, we present a comparison of maximum power degradation rates of individual modules under outdoor conditions in Golden, Colorado. Test modules include single- and polycrystalline-Si (x-Si, poly-Si), amorphous Si (a-Si, single, dual, and triple junction), CdTe, Cu-In-Ga-Se-S (CIS), and c-Si/a-Si heterostructure, from nine difference manufacturers. From monthly blocks of output power data, ratings were determined using multiple regressions to Performance Test Conditions (PTC). Plotting the power ratings versus time allowed degradation rates to be calculated from linear regressions. We also include a summary of module degradation rates obtained from the open literature over the past five years. Compared with the common rule-of-thumb value of 1% per year, many modules are seen to have significantly smaller degradation rates. A few modules, however, degrade significantly faster

Outdoor PV degradation comparison

As photovoltaic (PV) penetration of the power grid increases, it becomes vital to know how decreased power output may affect cost over time. In order to predict power delivery, the decline or degradation rates must be determined accurately. At the Performance and Energy Rating Testbed (PERT) at the Outdoor Test Facility (OTF) at the National Renewable Energy Laboratory (NREL) more than 40 modules from more than 10 different manufacturers were compared for their long-term outdoor stability. Because it can accommodate a large variety of modules in a limited footprint the PERT system is ideally suited to compare modules side-by-side under the same conditions.