NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Contract No. DE-AC36-08GO28308

Identification and 

Characterization of Performance 

Limiting Regions in Poly-Si 

Wafers Used for PV Cells

Preprint

Harvey Guthrey

Colorado School of Mines and National Renewable 

Energy Laboratory

Brian Gorman 

Colorado School of Mines

Mowafak Al-Jassim

National Renewable Energy Laboratory

Presented at the 37th IEEE Photovoltaic Specialists Conference 

(PVSC 37)

Seattle, Washington

June 19-24, 2011

Conference Paper

NREL/CP-5200-50645

July 2011
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1 

IDENTIFICATION AND CHARACTERIZATION OF PERFORMANCE LIMITING REGIONS 

IN POLY-SI WAFERS USED FOR PV CELLS

Harvey Guthrey1,2, Brian Gorman1, Mowafak Al-Jassim2 

1

Colorado School of Mines, Golden, Colorado, United States 2

National Renewable Energy Laboratory, Golden, Colorado, United States

ABSTRACT

As demand for silicon photovoltaic (PV) material 

increases, so does the need for cost-effective feedstock 

and production methods that will allow enhanced 

penetration of silicon PV into the total energy market. The 

focus on cost minimization for production of polycrystalline 

silicon (poly-Si) PV has led to relaxed feedstock purity 

requirements, which has also introduced undesirable 

characteristics into cast poly-Si PV wafers. To produce 

cells with the highest possible conversion efficiencies, it is 

crucial to understand how reduced purity requirements 

and defects that are introduced through the casting 

process can impair minority carrier properties in poly-Si 

PV cells. This is only possible by using multiple 

characterization techniques that give macro-scale 

information (such as the spatial distribution of 

performance-limiting regions), as well as micro and nanoscale information about the structural and chemical nature 

of such performance-limiting regions. This study 

demonstrates the usefulness of combining multiple 

techniques to analyze performance-limiting regions in the 

poly-Si wafers that are used for PV cells. This is done by 

first identifying performance-limiting regions using macroscale techniques including photoluminescence (PL) 

imaging, microwave photoconductive decay (μPCD), and 

reflectometry), then using smaller-scale techniques such 

as scanning electron microscopy (SEM), electron 

backscattered diffraction (EBSD), laser ablation inductively 

coupled mass spectrometry (LA-ICP-MS), 

cathodoluminescence (CL), and transmission electron 

microscopy (TEM) to understand the nature of such 

regions. This analysis shows that structural defects as 

well as metallic impurities are present in performancelimiting regions, which together act to decrease conversion 

efficiencies in poly-Si PV cells.

BACKGROUND

Crystalline silicon has been studied extensively over 

several decades. Despite a large body of knowledge on 

crystalline silicon photovoltaics, the research has not 

shown a clear correlation between processing conditions 

and the spatial distribution and character of defects in 

directionally solidified silicon. In addition, researchers do 

not completely understand the exact mechanisms by 

which conversion efficiencies are limited. Because the 

conversion efficiencies are directly related to the 

properties of minority charge carriers, this is an important 

characteristic of poly-Si PV cells to study. Studies on 

minority carrier properties in poly-Si are typically based on 

the analysis of a transient associated with carrier 

excitation or some form of luminescence [2]. Information 

about the structure of different regions within poly-Si 

materials is gained through research techniques such as 

transmission electron microscopy(TEM) [3, 4] and electron 

backscatter diffraction (EBSD) [5]. Methods such as 

electron beam induced current (EBIC), 

cathodoluminescence (CL) and photoluminescence are 

used for the identification and study of defects [6, 7]. 

However, the most useful studies employ multiple 

techniques where each technique complements another 

and, thus, presents a more complete understanding of 

factors related to conversion efficiencies. The studies that

use multiple techniques typically focus on one aspect of 

the poly-Si such as distribution of carrier lifetimes, 

distribution of impurities, defect structure, or grain 

morphology. Because of this approach, no definitive study 

has been done to relate the macro-scale phenomena 

associated with processing to atomic-scale phenomena 

that are responsible for the limitations of poly-Si PV cellconversion efficiencies.

EXPERIMENT 

In this study, a series of poly-Si wafers were removed from 

a brick toward the center of a directionally solidified ingot. 

This allows multiple techniques to be performed on 

essentially the same region, even if they are of a 

destructive nature. The wafer was as-cut and received no 

preparation for the PL imaging or μ-PCD mapping. PL 

imaging was performed with a Princeton 

Instruments/Acton PIXIS 1024BR camera cooled to -70 ̊C. 

The optical excitation source is four 30W laser diodes with 

808nm wavelength. μ-PCD mapping was performed on a 

SEMILAB tool with 904nm excitation wavelength. To 

create the reflectometry map, the sample was polished 

and then etched to reveal the defects and allow them to 

scatter incident light. The other techniques including CL, 

EBSD, TEM and LA-ICP-MS required polishing; and, in 

the case of CL, passivation for the measurements. The 

passivation was achieved by an HF dip and subsequent 

storage in an iodine/methanol solution. CL was done on a 

JEOL 5800 SEM equipped with a Ge detector for imaging. 

EBSD was performed on a JEOL 4300 SEM with Oxford 

Instruments EBSD software. A FEI T30 TEM was used to 

image individual defects. The dimensions of the wafers 

used are 156mm x 156mm and 0.18mm thick.
2 

RESULTS

The photoluminescence image (Figure 1) reveals the 

presence of non-radiative recombination centers, which 

are represented by the dark regions in the image. The 

light regions are where radiative recombination is 

dominant and photon emission is more likely. These dark 

regions include grain boundaries as well as defect 

clusters. Certain grains also appear darker than 

neighboring grains, which indicates a difference in 

luminescence from grain to grain.

Figure 1 Photoluminescense image of the entire wafer.

Figure 2 Reflectometry map of the entire wafer.

The results of the reflectometry mapping (Figure 2) allow 

for identification of regions of the wafer with dislocation 

densities that are higher than the average for the wafer. 

The blue regions indicate a low dislocation density, and 

the red and yellow regions represent a high dislocation 

density. The right side of Figure 2 has the highest 

dislocation density where the red regions indicate a defect 

density of approximately 106 cm-2.

The microwave photoconductive decay technique was 

used to create a map (Figure 3) of the minority carrier 

lifetimes across the wafer. Because the wafer was not 

passivated prior to the measurement, surface 

recombination was dominant as indicated by the 

measured lifetimes of less than 2 μs. The right side of the 

wafer, as well as a region toward the bottom of the wafer, 

exhibit very poor minority carrier lifetimes.

A region identified as having poor minority carrier 

properties was isolated, and an EBSD orientation map 

was constructed for the region (Figure 4a) using the 

inverse pole figure color scheme (Figure 4b). This 

technique identified the presence of twin boundaries as 

well as other randomly oriented boundaries.

Figure 3 μ-PCD map of the entire wafer.

a) b)

Figure 4 a) EBSD orientation maps of the region of 

interest. b) Inverse pole figure color scheme.
3 

The cathodoluminescence image in Figure 5 shows 

multiple non-radiative recombination centers (dark areas)

contained in the same region isolated to create the EBSD 

map (Figure 4). The dark line extending from the top to the 

bottom of the image indicates a grain boundary, while the 

smaller scattered features represent individual dislocations 

or clusters of dislocations. The contrast associated with 

the non-radiative recombination centers varies and 

indicates the degree to which minority carriers are 

hindered in a particular region.

LA-ICP-MS identified 19 impurity elements within this 

performance-limiting region. Of the 19 detected, the 

elements Al, Cu, Cr, Fe, and Ti are known to introduce 

energy levels deep in the band gap of silicon. Because 

the type of impurities was not known prior to analysis, no 

standard was used and the data is purely qualitative. 

The TEM image (Figure 6) confirms the presence of 

dislocations within this region. The contrast associated 

with dislocations originates from distortions of the lattice 

surrounding the dislocation.

Figure 5 CL image from performance-limiting region.

DISCUSSION

The results of mapping the entire wafer identify the 

performance-limiting regions. The PL image is 

representative of the intensity of luminescence associated 

with band-to-band recombination. Luminescence is 

decreased in regions that harbor non-radiative 

recombination centers associated with energy levels deep 

in the band gap. Thus, the dark regions in the PL image 

indicate where minority carriers will most likely encounter 

non-radiative recombination centers, which will limit

conversion efficiencies. The μ-PCD technique extracts the 

minority carrier lifetimes at various positions on the wafer 

through analysis of the excitation transient.

Figure 6 Bright-field TEM image of dislocations in a 

performance-limiting region.

Because the wafer was not passivated prior to the 

measurement, the measured lifetimes are not 

representative of the bulk lifetime for the wafer. Even with 

this consideration, the analysis still shows the variation of 

minority carrier lifetimes across the wafer. This is useful in 

correlating poor minority carrier lifetimes with non-radiative 

recombination centers. The dislocation density maps 

produced by the reflectometry technique generally 

reinforce the assumption that high dislocation densities 

result in poor minority carrier properties. The combination 

of these three mapping techniques used on the same 

region of a poly-Si wafer reveal the relationship among 

defects, non-radiative recombination centers, and poor 

minority carrier lifetimes. In general, regions with high 

concentrations of defects and non-radiative recombination 

centers also exhibit poor minority carrier properties.

Based on the results of the mapping techniques, the 

researchers selected a region that exhibited non-radiative 

recombination, high dislocation density, and poor minority 

carrier lifetimes. The electron backscatter diffraction 

(EBSD) technique provided an orientation map (Figure 4a) 

of this region showing the presence of grain boundaries of 

various characters. The cathodoluminescence (CL) image 

(Figure 5) shows that grain boundaries as well as 

individual dislocations are present within this region and 

act as non-radiative recombination centers. The different 

features in the CL image have varying degrees of contrast, 

and it is not clear from the data what the origin of this 

variation is. The bright-field TEM image (Figure 6) 

confirmed dislocations in which one is visible due to the 

contrast produced by lattice distortions around the 

dislocation.

Because of the lack of prior information about the impurity 

content within the wafer, it was not possible to use a 

standard for the LA-ICP-MS analysis. This makes the 

data purely qualitative. However, LA-ICP-MS did identify 
4 

19 impurity elements. Of these, Al, Cr, Cu, Fe, and Ti are 

known to introduce energy levels that lie deep in the band 

gap of silicon. The presence of these elements even at 

the ppb level is known to have detrimental effects on PV 

device performance.

SUMMARY

This study investigates how researchers can combine 

multiple characterization techniques to produce a greater 

understanding of the performance limitations associated 

with directionally solidified poly-Si for PV cells. Largescale mapping techniques (PL imaging, μ-PCD, 

reflectometry) are shown to be effective in identifying 

spatial variations in performance-limiting regions. The 

results of the mapping techniques are then used to select 

appropriate areas where more information about the 

performance-limiting factors can be extracted. The 

performance-limiting regions were found to contain both 

structural features (EBSD, CL, TEM), as well as impurities 

(LA-ICP-MS) that contribute to the overall performance 

limitations of poly-Si PV cells.

ACKNOWLEDGMENTS 

Thanks to Steve Johnston for providing the PL images and 

Bhushan Sopori for providing the reflectometry maps. This 

work was supported by the U.S. Department of Energy 

under Contract No. DE-AC36-08-GO28308 with the 

National Renewable Energy Laboratory.

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