J. A. del Cueto

Combinatorial studies of Zn-Al-O and Zn-Sn-O transparent conducting oxide thin films

Abstract In this work, we discuss the development of combinatorial deposition and analysis tools for the investigation of and the optimization of transparent conducting oxides. Library deposition by co-sputtering followed by optical analysis is shown to be a facile way to achieve these goals. Initial work focused on Zn-Al-O libraries with low Al contents as a test case. Subsequent work has focused on the ZnO-SnO2 tie line. Local maxima in the composition dependence of the conductivity were found for Zn/Sn ≈2:1 (Zn2SnO4) and Zn/Sn ≈1:1 (ZnSnO3). For these two representative stoichiometries, constant composition films have also been grown by pulsed laser deposition.

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.

Comparison of energy production and performance from flat-plate photovoltaic module technologies deployed at fixed tilt

Performance data for 14 photovoltaic modules deployed at fixed latitude tilt in the field are presented and compared. Module performance is monitored continuously for optimum power characteristics. Flat-plate module technologies representative of crystalline, amorphous, and polycrystalline silicon, and cadmium telluride and copper indium diselenide, are scrutinized for energy production, effective efficiency and performance ratio (PR)-ratio of effective to reference efficiency. Most performance ratios exhibit seasonal fluctuations largely correlated to air or module temperatures, varying between 80% and 100%. These ratios tend toward larger values during winter and vise versa, except for amorphous silicon and cadmium telluride modules. In a-Si modules, the situation appears reversed: better PRs are obtained during late summer. The effective efficiency, PR, and average daily and yearly energy production are analyzed and quantified.Performance data for 14 photovoltaic modules deployed at fixed latitude tilt in the field are presented and compared. Module performance is monitored continuously for optimum power characteristics. Flat-plate module technologies representative of crystalline, amorphous, and polycrystalline silicon, and cadmium telluride and copper indium diselenide, are scrutinized for energy production, effective efficiency and performance ratio (PR)-ratio of effective to reference efficiency. Most performance ratios exhibit seasonal fluctuations largely correlated to air or module temperatures, varying between 80% and 100%. These ratios tend toward larger values during winter and vise versa, except for amorphous silicon and cadmium telluride modules. In a-Si modules, the situation appears reversed: better PRs are obtained during late summer. The effective efficiency, PR, and average daily and yearly energy production are analyzed and quantified.

Performance test of amorphous silicon modules in different climates - year four: Progress in understanding exposure history stabilization effects

In a round robin outdoor exposure experiment carried out in three different climates, we have previously demonstrated that amorphous silicon (a-Si) PV modules reach higher stabilized performance levels in warmer climates. The four-year experiment involved three identical sets of thin-film a-Si modules from various manufacturers deployed outdoors simultaneously in three sites with distinct climates. Each PV module set spent a one-year period at each site before a final period at the original site where it was first deployed. The experiment aimed to determine the light-induced degradation and stabilization characteristics of a-Si regarding specific history of exposure, and to compare degradation rates in different climates. We propose that after the initial sharp degradation associated with the Stabler-Wronski effect (SWE) has passed, the subsequent stabilized performance levels attained will depend largely on light exposure and a characteristic temperature associated within a coherent time-scale. PV modules which were first deployed at the lowest-temperature site for one year, reaching a stabilized state, and were then further deployed at higher temperature sites for two more years, experienced considerable recovery in output parameters (Pmax and FF). However, when further deployed back at the original, lowest-temperature site, performance degraded back to the first year, original level.

Striving for a standard protocol for preconditioning or stabilization of polycrystalline thin film photovoltaic modules

Polycrystalline photovoltaic (PV) modules containing cadmium telluride (CdTe) or copper indium gallium diselenide (CIGS) thin film materials can exhibit substantial transient or metastable current-voltage (I-V) characteristics depending on prior exposure history. Transient I-V phenomena confound the accurate determination of module performance, their reliability, and their measured temperature coefficients, which can introduce error in energy ratings models or servicelifetime predictions. Indeed, for either of these two technologies, a unique performance metric may be illusory without first specifying recent exposure or state—even at standard test conditions. The current standard preconditioning procedure for thin-film PV modules was designed for amorphous silicon (a-Si), and is likely inadequate for CdTe and CIGS. For a-Si, the Staebler-Wronski effect is known to result from defects, created via breaking of weak silicon bonds or light-activated trapping at the device junction, occurring rapidly upon light-exposure. For CdTe and CIGS devices, there is less agreement on the causes of metastable behavior. The data suggests that either deep-trapping of charge carriers, or the migration and/or electronic activation of copper may be responsible. Because these are quite disparate mechanisms, we suspect that there may be a more practical preconditioning procedure that can be employed prior to accurate performance testing for CdTe and CIGS modules. We devise a test plan to examine and compare the effects of light soaking versus forward-biased dark exposure at elevated temperatures, as parallel strategies to determine a feasible standard protocol for preconditioning and stabilizing these polycrystalline PV technologies, and report on the results of our tests.

Method for analyzing series resistance and diode quality factors from field data of photovoltaic modules

Abstract A procedure is detailed in which the effects of series resistance and diode quality factors are separately analyzed and quantified using current–voltage (IV) data gathered in situ from three photovoltaic (PV) modules. Series resistance losses are implicated to be largely responsible for reduction in the fill factor values for intensities of 60% of one sun or greater. The data presented are from one cadmium telluride (CdTe) and two amorphous silicon (a-Si) modules and were taken in field deployment with a data acquisition system.

Stability of CIS/CIGS modules at the outdoor test facility over two decades

We examine the status and question of long-term stability of copper indium diselenide (CIS) photovoltaic (PV) module performance for numerous modules that are deployed in the array field, or on the roof of, the outdoor test facility (OTF) at NREL, acquired from two manufacturers. Performance is characterized with current-voltage (I–V) measurements obtained either at standard test conditions (STC) or under real-time monitoring conditions, taken over the course of many years. We present and scrutinize I–V characteristics for degradation modes. Analysis yields that CIS PV modules can exhibit either moderate (2% to 4% per year) to negligible or small (less than 1% per year) degradation rates, and that the predominant loss mode appears to be fill factor diminution, often associated with increases in the series resistance in some of the modules. A secondary mode of degradation observed comprises metastable changes to the open-circuit voltage. The featured modules are deployed on three separate testbeds.

Degradation and capacitance: voltage hysteresis in CdTe devices

CdS/CdTe photovoltaic solar cells were made on two different transparent conducting oxide (TCO) structures in order to identify differences in fabrication, performance, and reliability. In one set of cells, chemical vapor deposition (CVD) was used to deposit a bi-layer TCO on Corning 7059 borosilicate glass consisting of a F-doped, conductive tin-oxide (cSnO2) layer capped by an insulating (undoped), buffer (iSnO2) layer. In the other set, a more advanced bi-layer structure consisting of sputtered cadmium stannate (Cd2SnO4; CTO) as the conducting layer and zinc stannate (Zn2SnO4; ZTO) as the buffer layer was used. CTO/ZTO substrates yielded higher performance devices however performance uniformity was worse due to possible strain effects associated with TCO layer fabrication. Cells using the SnO2-based structure were only slightly lower in performance, but exhibited considerably greater performance uniformity. When subjected to accelerated lifetime testing (ALT) at 85 - 100 °C under 1-sun illumination and open-circuit bias, more degradation was observed in CdTe cells deposited on the CTO/ZTO substrates. Considerable C-V hysteresis, defined as the depletion width difference between reverse and forward direction scans, was observed in all Cu-doped CdTe cells. These same effects can also be observed in thin-film modules. Hysteresis was observed to increase with increasing stress and degradation. The mechanism for hysteresis is discussed in terms of both an ionic-drift model and one involving majority carrier emission in the space-charge region (SCR). The increased generation of hysteresis observed in CdTe cells deposited on CTO/ZTO substrates suggests potential decomposition of these latter oxides when subjected to stress testing.

Seasonal performance of a-Si single- and multijunction modules in two locations

Module performance data collected at two sites are analyzed in order to identify the respective magnitudes of seasonal annealing and degradation in comparison to spectral effects. It is demonstrated in this paper that at one site (Loughborough, UK) the spectrum dominates and very little seasonal annealing is observed. In contrast, at the other site (Golden, US), half of the seasonal variation can be attributed to spectral changes while the other half must be attributed to thermal annealing of defects. Differences between multi-junction categories are investigated and it is shown that single-junction devices exhibit a greater seasonal annealing than multi-junctions, while the latter tend to be more influenced by spectral effects.

Performance of single-junction a-Si modules under varying conditions in the field

We report on the actual performance of large-area photovoltaic (PV) modules deployed outdoors, which are representative of single-junction amorphous silicon (a-Si) thin-film technology. They are part of the performance and energy-ratings testbed (PERT) at the Outdoor Test Facility (OTF) and are under continuous measurement and load conditions, as executed by a data acquisition system (DAS). The goals are to analyze and compare energy production and performance for various PV technologies deployed in actual field conditions and to validate the PV energy-ratings methodology being developed for modules. For single-junction a-Si, we found the effective efficiency is very similar to that measured at standard conditions.