Barry R. Hansen

Photovoltaic module performance and durability following long-term field exposure

Our investigations of both new and field-aged photovoltaic modules have indicated that, in general, todays commercially available modules area highly reliable product. However, by using new test procedures, subtle failure mechanisms have also been identified that must be addressed in order to achieve 30-year module lifetimes. This paper summarizes diagnostic test procedures, results, and implications of in-depth investigations of the performance and durability characteristics of commercial modules after long-term field exposure. A collaborative effort with U.S. module manufacturers aimed at achieving 30-year module lifetimes is also described.

Dark current-voltage measurements on photovoltaic modules as a diagnostic or manufacturing tool

Dark current-voltage (dark I-V) measurements are commonly used to analyze the electrical characteristics of solar cells, providing an effective way to determine fundamental performance parameters without the need for a solar simulator. The dark I-V measurement procedure does not provide information regarding short-circuit current, but is more sensitive than light I-V measurements in determining the other parameters (series resistance, shunt resistance, diode factor and diode saturation currents) that dictate the electrical performance of a photovoltaic device. The work documented here extends the use of dark I-V measurements to photovoltaic modules, illustrates their use in diagnosing module performance losses and proposes their use for process monitoring during manufacturing.

Microwave-detected photoconductance decay

Microwave-detected photoconductance decay provides a contactless measurement of the recombination lifetime of free carriers in semiconductors following a pulse of optical excitation. Several complications in interpreting the results obtained by this method have prevented its widespread acceptance. Detailed models are proposed and verified experimentally using a commercially available apparatus. The model adequately predicts the behavior of the microwave reflectance as a function of wafer conductivity and system configuration. A light-biased variation of the technique which makes it possible to characterize lifetimes as a function of excess carrier density is described. This capability makes it possible to measure the emitter saturation current density for diffusion in high-resistivity wafers, a valuable process control tool.<<ETX>>

New methods for measuring performance of monolithic multi-junction solar cells

The commercialization of multi-junction solar cells for both space and terrestrial applications has increased the need to accurately determine cell performance using typical solar simulators and test equipment. This paper describes specific test methods recently applied in characterizing the performance of both tandem and triple-junction solar cells. Methods applied included: current-voltage measurements in forward and reverse bias using a xenon-arc solar simulator; absolute spectral response measurements of separate junctions using both light and voltage bias; a device simulation model; and a spectral mismatch calculation procedure tailored to multi-junction cells. Procedures are illustrated using measurements for GaInP-GaAs tandem cells, GaInP-GaAs-Ge triple-junction cells, and Ge cells supplied by different manufacturers.

Diagnostic analysis of silicon photovoltaic modules after 20-year field exposure

The objective of this study was to investigate the technology used by Spectrolab Inc. to manufacture photovoltaic modules that have provided twenty years of reliable service at Natural Bridges National Monument in southeastern Utah. A field survey, system performance tests, and a series of module and materials tests have confirmed the durability of the modules in the array. The combination of manufacturing processes, materials, and quality controls used by Spectrolab resulted in modules that have maintained a performance level close to the original specifications for twenty years. Specific contributors to the durability of the modules included polyvinyl-butyral (PVB) encapsulant, expanded metal interconnects, silicon oxide anti-reflective coating, and excellent solder/substrate solderability.

Measurement precautions for high-resistivity silicon solar cells

Several measurement characteristics have been identified that are unique to high-performance, high-resistivity silicon cells. These unique characteristics, which are due to features such as bulk carrier lifetimes on the order of 1 ms, excellent surface passivation, and light trapping, can lead to large performance measurement errors; however, they also offer insight into the physical mechanisms occurring in these devices. The characteristics that were found to have the largest influence on measured performance are described. These include a strong fill factor dependence on voltage sweep rate and direction, sublinear short-circuit current versus irradiance, light-soaking influence on open-circuit voltage and fill factor, and enhanced spectral response beyond 1100 nm due to optical light trapping. Cell measurements and analyses have included both small-area concentrator cells and large-area one-sun cells from various sources.<<ETX>>

A sensitivity analysis of the spectral mismatch correction procedure using wavelength-dependent error sources (solar cell testing)

A detailed investigation has been completed concerning the influence of wavelength-dependent random and bias measurement errors on the spectral mismatch correction procedure associated with solar cell performance measurements and reference cell calibration. It is shown that common distributions for random and bias measurement errors can result in an overall uncertainty of over +or-3% in short-circuit current using a solar simulator. This analysis has also identified a reference cell calibration procedure with one-half the uncertainty associated with the best outdoor calibration procedure.<<ETX>>

Development of a multi-purpose, pulsed-laser system for solar cell processing applications

The capabilities of a versatile Nd:YAG pulsed-laser system developed at Sandia National Laboratories for solar cell processing applications is described. The results of statistically based, multifactor experiments used to characterize the influence of laser-system process variables on patterns produced in silicon wafers and silicon-oxide layers are presented, and an initial assessment of laser-grooved solar cell processing conditions is given.<<ETX>>

Multiple junction cell characterization using the LBIC method: early results, issues, and pathways to improvement

A Light Beam Induced Current (LBIC) measurement is a non-destructive technique that produces a spatial graphical representation of current response in photovoltaic cells with respect to position when stimulated by a light beam. Generally, a laser beam is used for these measurements because the spot size can be made very small, on the order of microns, and very precise measurements can be made. Sandia National Laboratories Photovoltaic System Evaluation Laboratory (PSEL) uses its LBIC measurement technique to characterize single junction mono-crystalline and multi-crystalline solar cells ranging from miniature to conventional sizes. Sandia has modified the already valuable LBIC technique to enable multi-junction PV cells to be characterized.

Mfactor: A PC-based program for general spectral mismatch corrections (solar cell testing)

An IBM-PC-compatible computer program, called Mfactor, that uses a generalized procedure for making spectral mismatch corrections is described. The program operation is described, and several examples are given. A method to calibrate primary reference cells using a National Institute of Standards and Technology lamp in conjunction with Mfactor is presented.<<ETX>>