Kamel Ounadjela

Comparison of photoluminescence imaging on starting multi-crystalline silicon wafers to finished cell performance

Photoluminescence (PL) imaging techniques can be applied to multicrystalline silicon wafers throughout the manufacturing process. Both band-to-band PL and defect-band emissions, which are longer-wavelength emissions from sub-bandgap transitions, are used to characterize wafer quality and defect content on starting multicrystalline silicon wafers and neighboring wafers processed at each step through completion of finished cells. Both PL imaging techniques spatially highlight defect regions that represent dislocations and defect clusters. The relative intensities of these imaged defect regions change with processing. Band-to-band PL on wafers in the later steps of processing shows good correlation to cell quality and performance. The defect band images show regions that change relative intensity through processing, and better correlation to cell efficiency and reverse-bias breakdown is more evident at the starting wafer stage as opposed to later process steps. We show that thermal processing in the 200°-400°C range causes impurities to diffuse to different defect regions, changing their relative defect band emissions.

Light-induced degradation in upgraded metallurgical-grade silicon solar cells

We report on light-induced degradation (LID) of multicrystalline solar cells made of upgraded metallurgical-grade (UMG) silicon. Cells made of wafers from various locations in an UMG ingot were subjected to LID study. We investigated the kinetics of the LID by measuring changes in the solar cell performance parameters with illumination time and under various illumination intensities; the LID occurred in a logarithmic scale with time. The amount of LID changes with the wafer location, depending on boron and oxygen concentration profiles in the ingot. Further, by mapping the diffusion length using light beam-induced current, we found that the degradation occurred in the bulk of grains in the multicrystalline cells but not at extended defects, such as grain boundaries and dislocations. The LID of UMG-Si is essentially consistent with the literature-reported LID of electronic-grade (EG) Si when it contains high levels of boron and oxygen concentration. Despite the significantly higher levels of many impurities in UMG-Si than in EG-Si, the LID of UMG-Si is dominated by the formation of the boron and oxygen defect complex, which is the mechanism of LID for EG-Si.

Quality characterization of silicon bricks using photoluminescence imaging and photoconductive decay

Imaging techniques can be applied to multicrystalline silicon solar cells throughout the production process, which includes as early as when the bricks are cut from the cast ingot. Photoluminescence (PL) imaging of the band-to-band radiative recombination is used to characterize silicon quality and defects regions within the brick. PL images of the brick surfaces are compared to minority-carrier lifetimes measured by resonant-coupled photoconductive decay (RCPCD). RCPCD is a transient photoconductive decay technique that monitors the recombination of excess carriers using a frequency of about 420 MHz. Carriers are excited by nanosecond laser pulses of long-wavelength light in the range of 1150 nm. The low frequency and long penetration depth of light promote measurement of carriers away from the surface such that lifetimes of up to 100 μs are measured in upgraded-metallurgical-grade silicon, and up to 200 μs in electronic-grade silicon bricks. PL intensity shows correlation to lifetime in addition to the valuable spatial information from top to bottom of the brick and defect regions throughout the brick.

Superior low-light-level performance of upgraded metallurgical-grade silicon modules

The industry is becoming critically sensitive to solar energy delivered in kilowatt-hours rather than in kilowatt at illumination peak intensity. This is because when comparing systems or modules, it is more relevant to compare the energy delivered during an entire day than the energy delivered during peak illumination, which happens for very few hours on the same day. For that reason, low-light-level performance is an important parameter that greatly influences the total energy yield of a PV system. This is especially important in low annual insolation regions such as northern Europe or the northeast United States. Low-light-level performance can vary significantly even within a particular PV technology. In this contribution, results of low-light performance of three modules are presented. The first module uses Calisolar upgraded metallurgical-grade (UMG) Si solar cells, the second module uses standard monocrystalline Si cells, and the third module uses standard electronic-grade (EG) Si cells. The modules were first tested at NRELs Outdoor Testing Facility. The low-light-level performance of the three modules indicated a markedly higher module output for the module with Calisolar UMG cells. Because angle of incidence, temperature, and spectral variations can significantly influence these data, these modules were also measured using a Spire indoor solar simulator. Measurements at 200, 400, 600, 800, and 1000 W/m2 corroborated our outdoor tests and the superior performance of the UMG-based modules. We observed that the shunt resistance of the module with Calisolar UMG cells is higher than that of the other two modules, which can explain the higher module output. Thus, as the light intensity decreases, the light IV curve moves toward the lower part of the diode characteristics where Rshunt and J02 (which describes recombination in the space charge region) dominate. In this low-light regime, a decrease of Rshunt drastically reduces the VOC and fill factor. Cells with higher Rshunt are less affected. Large-scale system outputs have also confirmed higher performance module output in low-light conditions for modules using Calisolar cells.