Measurements & Characterization • National Center for Photovoltaics

Electro-Optical Characterization

We use various electrical and optical experimental techniques to 

relate photovoltaic device performance to the methods and materials 

used to produce them. The types of information obtained by these 

techniques range from small-scale atomic-bonding information to 

large-scale macroscopic quantities such as optical constants and 

electron-transport properties. Accurate, timely measurements of 

electro-optical properties as a function of device processing provide 

the knowledge needed to troubleshoot problems and develop the 

knowledge base for reducing cost, maximizing efficiency, improving 

reliability, and enhancing manufacturability. We work collaboratively 

with you to solve materials- and device-related R&D problems. This 

sheet summarizes our primary techniques and capabilities. 

PHOTOLUMINESCENCE SPECTROSCOPY

Photoluminescence (PL) spectroscopy is a contactless, 

nondestructive method to probe the electronic structure of 

materials. Our capabilities include: various excitation wavelengths 

that allow for varying levels of volume excitation; a detection range 

extending from 0.4 to 2.7 μm; sample temperatures of 4 to 300 K; 

and mapping capabilities with 1- to 2-μm spatial resolution on the 

Fourier-transform-based system. The intensity and spectral content 

of the emitted photoluminescence is a direct measure of various 

important material properties, including: 

Bandgap determination. This is particularly useful when working 

with new compound semiconductors. 

Impurity levels and defect detection. The PL energy associated 

with these levels can be used to identify specific defects.

Recombination mechanisms. Analysis of PL helps to understand 

the underlying physics 

of the recombination 

process.

MINORITY-CARRIER LIFETIME SPECTROSCOPY

Minority-carrier lifetime spectroscopy is a contactless, 

nondestructive method to study the recombination processes 

of materials. This technique monitors—either optically or 

electrically—the return of photoexcited carriers back to equilibrium 

as a function of time, providing a measure of the “lifetime” of 

the excess carriers. Our capabilities allow for varying levels of 

volume excitation and sample temperatures of 4 to 300 K. Specific 

techniques include:

Time-Correlated Single-Photon Counting or Time-Resolved 

Photoluminescence—This optical detection technique provides 

exceptionally fast system response times of 20 ps (optical 

detection from 0.4 to 1.0 μm) and 100 fs (optical detection from 

0.5 to > 2 μm).

Microwave-Reflection Photoconductive Decay (μ-PCD) and 

Resonant-Coupled Photoconductive Decay (RCPCD)—These 

techniques monitor the change in conductivity and are suitable for 

both direct- and indirect-bandgap materials. They have a 5-ns 

time resolution for the μ-PCD systems (7 and 20 GHz) and ~50-ns 

resolution for RCPCD (~450 MHz) and are capable of one-dimensional 

and two-dimensional lifetime mapping with ~1-mm spatial resolution.

Applications

Minority-carrier lifetime. The strong sensitivity of minority-carrier 

lifetime to the presence of defects detrimental to device performance 

makes it a particularly sensitive measure of material quality.

Recombination processes. The dependence of lifetime on the 

level of photo-excitation and temperature is used to determine 

the underlying physics of the recombination process.

Surface/interface recombination velocity. The dependence 

of lifetime on the geometry of the device being tested or 

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• NREL researchers use scanning Fourier transform photoluminescence microspectroscopy to produce three-dimensional maps to determine the composition 

and quality of grown absorber materials.

Maps of the polysilicon lifetime at various excitation wavelengths provide 

information on bulk, surface, and grain-boundary recombinations.
excitation wavelength used can serve to distinguish between 

recombination processes occurring in the bulk of the device and 

those occurring at interfaces or boundaries.

FOURIER-TRANSFORM INFRARED AND 

RAMAN SPECTROSCOPY

Infrared spectroscopy is a nondestructive, highly sensitive 

technique that provides information about impurities, their chemical 

environment, and free-carrier properties. Performing both Fourier 

transform (FT)-Raman and Fourier transform infrared (FTIR) 

spectroscopy on a sample gives a complete picture of its bonding 

structure. Both techniques benefit from high FT sensitivity and can 

be used with database searches of our extensive libraries of FTIR 

and FT-Raman spectra to chemically identify samples and sample 

components.

Applications

Reflectance and transmittance spectroscopy. Includes large-area 

mapping of samples up to 8 inches in diameter. Useful in analyzing 

properties of transparent conducting oxides and silicon wafers.

Contaminant identification. Can be done over a large area, or 

in spots as small as 2 to 50 μm in diameter, depending on the 

technique used.

In-situ reaction analysis. Gas-solid reactions, relaxation 

in liquid crystals, solid-solid phase transformations, and 

aqueous fermentation reactions, on time scales ranging from 

microseconds to days.

Impurity and doping concentrations. Done either at room or 

cryogenic temperatures using FTIR spectroscopy. We can obtain 

sensitivities as high as parts per billion.

SPECTROSCOPIC ELLIPSOMETRY

Spectroscopic ellipsometry (SE) measures the near-IR/visible/

near-UV optical response of a sample. In bulk samples, SE can 

determine electronic transition energies, which provide knowledge 

of alloy composition, crystallinity, doping, temperature, and any 

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other variables that affect the electronic transitions in the material. 

In thin-film samples, including multilayer thin films common in 

solar cells, SE can determine layer thickness, surface roughness, 

interface roughness, and optical properties. SE operates in any 

transparent ambient, including vacuum, gases, liquids, and air. 

This versatility allows ellipsometry to be applied in-situ and/or 

in-line to monitor the deposition and processing of materials. 

NREL has pioneered the use of in-situ real-time SE (RTSE) to 

monitor the growth of extremely thin films of a-Si:H used in silicon 

heterojunction solar cells. Our current capabilities cover a spectral 

range from 0.2 to 1.7 μm, acquisition times less than 1s, and 

mapping capabilities for samples as large as 10 cm x 10 cm. 

Applications

Material parameters. Including the determination of 

composition, crystallinity, temperature, electronic structure, and 

other variables that affect optical properties.

Thickness of thin films and multilayer structures. Including 

the determination of thickness, interface roughness, and 

homogeneity in multilayer structures with thicknesses from submonolayer to tens of micrometers. 

In-line or in-situ monitor of the growth and processing. 

Monitoring of bulk or thin-film materials in real time inside the 

growth or processing chamber.

CAPACITANCE TECHNIQUES 

These techniques monitor the movement of electronic charge within 

a semiconductor device and provide a measure of free-carrier and 

electrically active defect-state properties. Measurements are taken 

of the charge storage capacity, or capacitance, across a rectifying 

junction. 

Capacitance-voltage (C-V) measurements determine the 

concentration of majority carriers in the bulk of the device, and/

or energy levels of interface states that often exist between the 

surfaces of dissimilar materials 

Deep-level transient spectroscopy (DLTS) examines the timedependent flow of charge into and out of localized energy states 

associated with defects in the semiconductor. DLTS can detect 

trap concentrations as low as 1012 cm-3 and as high as the 

doping level (constant-capacitance DLTS); distinguish between 

minority- and majority-carrier traps; determine activation energy 

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FTIR mapping capabilities are extremely useful for the quick, nondestructive 

characterization of heterogeneous free carrier properties of combinatorial growth 

research on transparent conducting oxides.

In-situ ellipsometry measurements of surface roughness and film thickness in the 

earliest stages of the deposition process shed light on the film nucleation process.

ds (Å)

db (Å)

Time (s)
and capture cross-section. Our capabilities allow the use of 

electrical or optical pulses to fill traps and a sample temperature 

range of ~20 to 475 K. DLTS is a nondestructive complement 

to secondary-ion mass spectrometry and Auger electron 

spectroscopy.

Admittance spectroscopy is used to characterize defect levels by 

measuring capacitance with varying frequency and temperature.

SCANNING DEFECT-MAPPING SYSTEM

Defect mapping uses optical scattering to quickly and accurately 

map defect distributions in silicon semiconductor wafers. To map 

defects, a wafer is first treated with an NREL-developed etch of 

hydrofluoric, acetic, and nitric acids to enhance the light scattered 

by the defects. The scanning defect-mapping system (SDMS) moves 

the treated wafer across a stationary laser beam and maps the 

defects for each location on the wafer. The amount of light reflected 

from an area is proportional to the dislocation density for that area 

and provides a direct statistical count of the number of dislocations.

Applications

Dislocation mapping. Quickly and accurately maps dislocation 

defects in silicon wafers.

Grain-boundary mapping. Quickly and accurately maps grain 

boundaries in silicon wafers.

Photovoltaic response mapping. The photocurrent maps may 

be compared with the dislocation and grain-boundary maps 

to show the effect of defects and crystalline discontinuities on 

photovoltaic response. 

Reflectance mapping. The SDMS can also map the 

reflectance of a completed cell (i.e., the amount of light that 

does not penetrate the cell). By mapping reflectance, the 

SDMS can calculate a cell’s performance based on the light 

actually absorbed by the cell, which is a better measure of the 

effectiveness of the cell material.

Minority-carrier diffusion length. The SDMS can measure the 

minority-carrier diffusion length—the average distance traveled 

by light-generated electrical carriers—in a completed solar cell.

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REFLECTANCE SPECTROSCOPY

The PV Reflectometer measures the reflectance spectrum of an 

entire wafer or cell, up to 6-in. x 6-in. The measured reflectance 

plots are deconvolved to derive physical parameters including 

surface roughness and texture, antireflective coating thickness, 

metallization area and height, and backside metallization properties. 

The PV Reflectometer makes the entire measurement in a fraction 

of a second.

COMPUTATIONAL MODELING

Solar cell models generally have been limited to one dimension and 

primarily have focused on current-voltage curves and quantumefficiency spectra. At NREL, we can simulate two-dimensional 

solar cells, as well as many electro-optical measurements such as 

time-resolved photoluminescence, electron-beam-induced current, 

near-field scanning optical microscopy, and photoconductive decay. 

In addition, PV Optics is a software package developed at NREL 

specifically for designing solar cells and modules. It uses a 

combination of wave optics and ray-tracing techniques to handle 

thin-film solar cells, wafer-based cells, and the entire module 

(including non-planar or textured interfaces). PV Optics can be used 

to optimize the optical performance of single- and multijunction 

cells and modules. We are also developing a new software package 

for a simplified electronic design of single- and multicrystalline 

silicon solar cells. This package will eventually be combined with 

PV Optics to yield a robust, accurate design package for all solar 

cells and modules.

TECHNIQUE DEVELOPMENT 

Drawing on the diverse expertise and deep experience of our team 

members, we are extending existing techniques and developing 

completely new ones to address particular issues related to 

materials or devices. Two current examples are the following:

Lifetime mapping of multicrystalline silicon wafers with ~1-mm 

resolution requires increased sensitivity beyond measuring a 

large sample area. As-grown, unprocessed multicrystalline 

silicon wafers tend to have microsecond lifetimes, so sensitivity 

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Antireflection film thickness is one of many parameters that can be mapped quickly 

and accurately.

Computational modeling sheds light on the effect of grain-boundary potential on 

short-circuit current.
improvements 

must still maintain 

adequate time 

response. We 

continue to research 

new sample coupling 

designs to improve 

measurement 

sensitivity and 

mapping speed of 

RCPCD. And we 

are also improving 

and adapting other 

lifetime measurement 

techniques such 

that lifetime can 

ultimately be used as 

a parameter for monitoring solar cell material in a manufacturing 

line and be useful for process control and performance 

feedback.

As part of developing manufacturing diagnostics, we are working 

on high-speed methods for monitoring solar cell fabrication 

processes. We recently developed a technique to predict wafer 

breakage during solar cell processing. This technique can be 

valuable in screening wafers that tend to break during solar cell 

fabrication by a known process schedule.

We developed PVSCAN, a high-speed optical scanning system 

that characterizes silicon solar cell materials and devices. Its 

output—spatial maps of dislocation density, grain distribution, 

reflectance, and photoresponses from near-junction and the 

bulk of a solar cell—helps crystal growers achieve high-quality 

material and process engineers develop processes for higher 

efficiency devices. PVSCAN was available from NREL for many 

years, but is now licensed for commercial manufacture to 

GTSolar (Nashua, NH).

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Technique/

Capability

Photoluminescence 

spectroscopy

Minority-carrier 

lifetime spectroscopy

Fourier-transform 

infrared (FTIR) and 

Raman spectroscopy

Scanning 

ellipsometry

Capacitance 

techniques 

Scanning defect 

mapping

Reflectance 

spectroscopy

Typical 

Applications

Determine bandgap, material 

quality. Identify defects

Measure minority-carrier lifetime, 

surface recombination. Determine 

dominant recombination mechanism

Identify contaminants. Analyze 

reaction in situ. Measure impurity 

concentration, inhomogeneity

Determine film thickness, 

crystallinity, composition, 

roughness, temperature, optical and 

electronic properties

Measure carrier concentration 

profiles, interface state density, 

deep-level properties

Map dislocation and grain-boundary 

distributions in silicon wafers

Determine numerous solar cell 

physical parameters, including 

surface roughness, film thickness, 

metallization properties

Detection

Range

0.4–2.7 μm

Optical: 0.4–1.0 μm with 2x10-11 s resolution

Optical: 0.4– >2.0 μm with 1x10-13 s resolution

PCD: 5x10-9 s resolution

1.3–100 μm

0.2–1.7 μm

Quasistatic to 100 MHz frequencies

103

 to 108 defects/cm2

Reflectance to 1% accuracy from 0.4–1.1 μm

NonDestructive?

Yes

Yes

Yes

Yes

Yes

No

Yes

Temperature 

Range

4–300 K

4–300 K

8–300 K

Room

temperature

77–360 K

Room

temperature

Room

temperature

Image/

Mapping?

Yes

Yes

Yes

Yes

Yes

Yes

Yes

ELECTRO-OPTICAL CHARACTERIZATION TECHNIQUES/CAPABILITIES

National Renewable Energy Laboratory

1617 Cole Blvd., Golden, CO 80401

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

Energy Efficiency and Renewable Energy,

operated by Midwest Research Institute • Battelle

Measurements and Characterization is within the

National Center for Photovoltaics at NREL

Web site: www.nrel.gov/pv/measurements

Phone: (303) 384-6675 • Fax: (303) 384-6604

Contract No. DE-AC36-99-GO10337

NREL/BR-520-40121 • June 2006 Printed on paper containing at least 50% wastepaper, including 20% postconsumer waste

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We have developed photoconductive decay 

techniques to measure lifetime on materials with 

indirect bandgaps, small bandgaps, and long lifetimes using low-pulse-rate lasers or inexpensive 

flashlamp and LED light sources.

Jim Yost/PIX109869