Subcontract Report

NREL/SR-520-45214

March 2009

Manufacturing Process 

Optimization to Improve Stability, 

Yield and Efficiency of CdS/CdTe 

PV Devices 

Final Report,

December 2004 – January 2009

W.S. Sampath, A. Enzenroth, and K. Barth

Colorado State University

Fort Collins, Colorado
National Renewable Energy Laboratory

1617 Cole Boulevard, Golden, Colorado 80401-3393

303-275-3000 • www.nrel.gov

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

Office of Energy Efficiency and Renewable Energy

Operated by the Alliance for Sustainable Energy, LLC

Contract No. DE-AC36-08-GO28308

Subcontract Report

NREL/SR-520-45214

March 2009

Manufacturing Process 

Optimization to Improve 

Stability, Yield and Efficiency of 

CdS/CdTe PV Devices

Final Report,

December 2004 – January 2009

W.S. Sampath, A. Enzenroth, and K. Barth

Colorado State University

Fort Collins, Colorado

NREL Technical Monitor: Harin S. Ullal

Prepared under Subcontract No. XXL-5-44205-02
This publication was reproduced from the best available copy

Submitted by the subcontractor and received no editorial review at NREL

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iii 

Table of Contents

1. Summary.............................................................................................................. 1

2. Introduction ...........................................................................................................2

3. Research Results................................................................................................. 2

3.1 Improving Performance and Manufacturing .................................................. 2

3.2 Studies on Interconnection and Module Packaging ...................................... 4

3.3 Detailed Device Analysis, Steady State Photocapacitance........................... 7

3.4 Modification of the Pilot System .................................................................... 9

3.5 Cost Estimates for Large Scale Manufacturing with Our Technology.......... 14

3.6 Modification of the Pilot System .................................................................. 15

3.7 Higher Efficiency ......................................................................................... 17

3.8 On-Line Process Control............................................................................. 29

3.9 Numerical Modeling..................................................................................... 20

3.10 Miscellaneous Studies............................................................................... 22

4. References......................................................................................................... 24

5. Appendices ........................................................................................................ 24
1 

1. SUMMARY

Under the Thin Film Partnership Program, we have made significant progress. The 

reproducibility of device efficiency and stability has been demonstrated during long duration 

operation of the system with a single charge loading for the CdS, CdTe and CdCl2 process. In 

order to advance back contact processing, a significant upgrade to increase the processing space 

in our pilot scale system has been designed, fabricated and has been installed. A series of 

detailed studies on the effect of oxygen during the CdCl2 treatment has been performed. Oxygen 

has been shown to improve the effectiveness of the CdCl2 treatment. Mathematical modeling of a 

package design has yielded results that show that moisture can be kept from our device for 22 

years at 65C temperature with saturated humidity. Hardware to perform steady state 

photocapacitance (PHCAP) has been designed, built and tested in our laboratory. The effect of 

processing on the deep states in CdTe devices can be seen quantitatively. Optimum process 

conditions lead to lower photocapacitance signals. The feasibility of cooling the substrate in 

vacuum for optimum processing of the back contact has been demonstrated. A large number of 

experiments have been performed on laser scribing of CdS/CdTe films. Mechanical scribing has 

yielded the lowest contact resistance between the scribed region and the metallization, to date. 

Researchers at National Starch and Chemicals (NSC) have performed many experiments to 

screen print carbon, nickel and silver films in patterns. The adhesion of these films was tested 

according to ASTM 3359B and achieved a 5B or "excellent" rating. 

A fixture for automated cooling of the substrate in vacuum has been designed, fabricated, 

installed and tested. This is needed for optimum processing of the back contact. Large area 

devices ( ~ 4 sq. cm.) in the form of strips have been fabricated and tested at NREL. Efficiency 

of 10.9% has been measured on these strips. The air-to-vacuum-to-air seals in the pilot system 

have a glass filled teflon liner. These liners have experienced wear and this results in scraping of 

the glass substrates in the seal. Improved liners have been designed and fabricated, installed and 

tested in the system. Desiccant containing materials for the edge seal and sealing of the holes in 

the back glass have been tested in damp heat tests at NREL with very promising results. 

Hardware modifications to the pilot system for automated cooling of the substrate in vacuum 

have been completed and device efficiencies of 11.83 % have been made in "all forward" 

process.

Significant advances to the modeling of large area vapor sources and modeling of gas 

transport in the system have been made. Significant advances to the modeling of the substrate 

transport, temperature distribution and deflection in the glass substrate, large area vapor sources, 

diffusion of gases in the vapor source and modeling of gas transport in the system have been 

made. Significant progress has been made in characterizing films with ellipsometry and x-ray 

fluroscence (XRF) and the use of these methods for on-line process control is being investigated. 

In additio©n the use of reflection intensities for quality control is promising.

Efforts to improve device efficiency and advance device characterization are underway. 

Devices of efficiency 12.8% (NREL verified) have been fabricated on low iron glass with 

SnOx:F TCO. Devices of efficiency 13.7% (tested at CSU photovoltaics lab.) have been 

fabricated on low iron glass with SnOx:F TCO without AR coating. 

A process for removing the coated films to reduce the amount of solid waste has been 

developed. Significant technical assistance was provided to AVA Solar.
2 

2. INTRODUCTION 

The aim of this Thin Film PV Partnership Program (TFPP) effort is to advance the 

understanding of device stability, efficiency and process yield for CdTe PV devices. 

Methodologies for reliably processing larger areas and increasing device efficiencies are being 

developed. We have actively participated in the National CdTe R&D Team activities. Our 

ongoing collaborations with the PV Testing Lab at CSU, NREL, Pacific Northwest Natl. Lab and 

other CdTe Team Members are resulting in further advances. The efforts in this project will be a 

significant step towards realizing the full potential of CdTe PV. 

3. RESEARCH RESULTS 

3.1.1 Process Reproducibility: 

3.1. Improving Performance and Manufacturability

Reproducibility of efficiency and stability has been demonstrated during long duration 

operation of the system with a single charge loading for the CdS, CdTe and CdCl2 process 

[Sampath 2004]. Currently efforts are underway to advance the back contact processing to the 

same level. An average efficiency of 11.4% with a standard deviation of 0.54% was 

demonstrated during nine hour processing with a single charge loading for the CdS, CdTe and 

CdCl2 process [Sampath 2004]. The stability of these devices is shown in Figure 1.

0

2

4

6

8

10

12

14

0 2500 5000 7500

Time [hrs]

Average Efficiency [%]

14843

15242 and 44

15642,43,44

fit

Figure 1: Average efficiency vs accelerated stress time for three groups of cells (12 cells total). 

Inset shows substrate IDs. The stress conditions are 65C, OC, 5 hours ON/ 3 hours OFF.

As shown in Figure 1, groups of cells fabricated with the same process conditions in three 

separate fabrication runs exhibit nominally the same stability performance during accelerated 

stress testing. Devices processed identically to the ones in Figure 1, but without the Cu back 

contact had Voc’s ≥ 700 mV. We have shown earlier that devices with Voc greater than 700 mV 

without a Cu back contact have good stability [Barth 2005, Sampath 2004].

3.1.2. Oxygen and the CdCl2 Treatment: 

The presence of oxygen during the CdCl2 treatment leads to better devices. In order to 

study the effect of oxygen during the CdCl2 treatment, a system for introducing controlled 

amounts of oxygen directly into the CdCl2 vapor source and the CdCl2 annealing station was 
3 

designed, built and installed. The system incorporated a mass flow controller for precise control 

and modification to the graphite sources. A series of studies were under taken which involved 

nearly 225 substrates. 

These studies have shown that oxygen during the CdCl2 treatment improves the device 

performance. The JV performance of two example cells is shown in Table 1 and Figure 2. A Cu 

containing back contact was not applied to either of the cells in Table 1 or Figure 2. However, 

the JV performance of devices with out intentional copper is predicative of the stability of fully 

processed devices [Barth 2005]. 

Table 1

Cell Voc Jsc ff Eff[

[mV] [mA/cm^2] [%] [%]

17638-3: 0.6% 

O2

685 19.68 40 5.55

17650-3: 2% O2 745 20.34 48 7.32

Figure 2: Light JV traces for devices with varied oxygen during the CdCl2 treatment. Red trace: 

0.6% O2 ; blue trace: 2% O2.

3.1.3. Process Optimization: 

The process has been optimized with CdS from ASARCO. The earlier supplier 

(Balzers/Unaxis) discontinued the manufacturing of CdS pellets. In addition, the CdS from 

Unaxis was more expensive. The efficiency and stability of devices with CdS from ASARCO is 

comparable to device efficiency and stability with CdS from Unaxis. In addition, the process has 

been optimized with purer material for the back contact processing from a different supplier. The 

earlier supplier had lower purity. The efficiency and stability results with the newer material are 

comparable to the results with the older material. These optimization studies involved nearly 400 

substrates. Efforts to obtain uniformity of back contact processing over long duration are 

currently underway. X-Ray Photoelectron Spectroscopy (XPS) at CSU has been used in these 

process optimization studies.

-0.4 -0.3 -0.2 -0.1 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

-0.0100

-0.0080

-0.0060

-0.0040

-0.0020

0.0020

0.0040

0.0060

0.0080

0.0100

0.0120

0.0140

0.0160

0.0180

0.0200

0.0220

0.0250
4 

3.2.1 Studies on Interconnection: 

3. 2. Studies on Interconnection and Module Packaging

Processes for creating a series interconnection for our modules is being developed. This 

work includes film scribing and the application of electrode materials for the "rollover" contact. 

A significant number of experiments on laser scribing of many of our films for interconnection 

has been performed at US Laser and Spectra Physics. As we reported earlier, for scribing the 

TO:F, the Nd:YAG laser has been found to be suitable. For scribing the CdS/CdTe films, 

experiments have been performed with both a Nd:YAG laser and a vanadate laser. The scribing 

experiments have been performed at 122 conditions. We have observed that scribing from the 

glass side results in better scribe quality and lower contact resistance for both flash lamp pumped 

Nd YAG and the diode pumped Nd vanadate laser. The CdS/CdTe scribes are characterized 

with optical microscopy, scanning electron microscopy and energy dispersive spectrometry. 

Though significant improvements have been demonstrated over previous experiments, the scribe 

parameters have not yielded a sufficiently low contact resistance (target 1 ohm for 1 cm) using 

the current spray applied electrode material. These experiments are continuing. Some of the 

experiments at Spectra Physics are being performed with "galvanometer scanning" system and 

this may lead to very high productivity. 

Our group continues to collaborate with Dr. David Miles of NSC on optimizing the back 

electrode (metallization) and rollover interconnection process. The replacing of the current spray 

process for metallization with a screen print process is being investigated. This has advantages 

for process throughput, material utilization and lower volatile organic compound emissions. 

Since screen printing allows the metallization to be performed in patterns, a separate process for 

isolating the metallization in not needed. NSC is developing optimum formulations for screen 

printing for our CdTe PV application. It is envisioned that the back electrode will consist of 

screen printing of carbon and nickel in patterns. Many substrates have been provided to NSC for 

these studies. 

3.2.2. Modeling Moisture Transmission and Encapsulation: 

Significant progress has been made in the conceptual design of a package that keeps 

moisture away from our devices. Our 16.5 x 16.5 inch module is designed to have a 1" border 

around the edge. In this region, it is planned to use the Truseal solar edge tape which contains 

desiccant. Calculations based on the finite difference method have been performed. The aim of 

the calculations is to see how long a 1" edge seal will keep moisture away from our devices. For 

this, we calculated how long a 1" thick disc of the Truseal solar edge material will take to 

saturate with water if one side was exposed to saturated water at 65C and the disc was at 65 C 

was determined. The data for water transmission rate was obtained from Truseal. The finite 

difference calculations gave 22.3 years for the 1" thickness to saturate. These calculations were 

repeated by researchers at Truseal and similar results were obtained by them.

This is a conservative model because PV modules rarely reach 65C for a few hours in a 

year and this calculation is done assuming the side is exposed to saturated water on one side. 

Experiments are currently underway at Truseal to refine the data on water transmission. After the 

updated water transmission is obtained, the calculations will be refined, the model will be 

experimentally validated and results published.

Preliminary studies on encapsulation with EVA have been performed. A vacuum bag 

with an accumulator was used in these studies. In this approach, the glass/EVA/glass is placed in 
5 

the vacuum bag and the bag with the accumulator is evacuated and placed in the furnace. The 

purpose of the accumulator is to provide a vacuum reservoir for the gases that are evolved during 

the encapsulation to accumulate. These experiments have resulted in bonding between the glass 

and the EVA. However the bubbles could not be fully removed. In addition when the 

temperature of the EVA was measured with a thermocouple, a high degree of variability was 

observed. 

A fixture to quantitatively measure the moisture ingress through the encapsulation has 

been developed. The fixture consists of a closed glass container placed in a temperature 

controlled furnace. The container has a small amount of water to saturate the container to 100% 

relative humidity (RH). This fixture is capable of testing encapsulated 3" X 3" specimens at 

different temperatures and 100% RH. A measured quantity of indicating desiccant is placed in 

the specimen. The indicating desiccant with CoCl2 and CaSO4 changes color when a certain 

quantity of moisture has penetrated through the encapsulation. By estimating moisture ingress at 

different temperatures and using well known mathematical approaches, the ability of 

encapsulants to protect the devices from moisture under field conditions can be predicted. 

3.2.3 Studies on Interconnection:

Processes for creating a series interconnection for our modules is being developed. This 

work includes film scribing and the application of electrode materials for the "rollover" contact. 

Many experiments on laser scribing of many of our films for interconnection have been 

performed at US Laser and Spectra Physics. For scribing the TO:F, the Nd:YAG laser has been 

found to be suitable. For scribing the CdS/CdTe films, experiments have been performed with 

both a Nd:YAG laser and a vanadate laser. Recent scribing experiments have been performed at 

Spectra Physics application lab. Over 115 different conditions have been explored. The results 

of the laser scribing conditions on the contact resistance to the spray applied electrode is shown 

below in Figure 3.From the figure, it can be seen that the scribe parameters have not yielded a 

sufficiently low contact resistance (target 1 ohm for 1 cm) using the current spray applied 

electrode material. The laser wavelength was 1064 nm. Experiments are continuing with laser

light wavelength of 532 nm. Initial results show improved contact resistance values to 

approximately 5 ohm -cm of contact. 

As we reported earlier, we have demonstrated the feasibility of mechanical scribing to 

produce contact resistance of 1 ohm for 1 cm. Recently mechanical scribing at high speeds has

been performed and the measured contact resistance between the mechanical scribe and the 

sprayed electrode material was measured at 1 to 2 ohms for 1 cm of scribe length.

Researchers at NSC under the guidance of Dr. David Miles are optimizing the back 

electrode (metallization) and rollover interconnection process. Replacing of current spray 

process for metallization with a screen print process is being investigated. This has advantages 

for process throughput, material utilization and lower volatile organic compound emissions. 

Since screen printing allows the metallization to be performed in patterns, a separate process for 

isolating the metallization in not needed. NSC is developing optimum formulations for screen 

printing for our CdTe PV application. NSC has performed many experiments to screen print 

carbon, nickel and silver films in patterns. Initial samples have recently been received from NSC. 
6 

Figure 3: Contact resistance as a function of laser pulse energy density at 1064 nm wavelength. The different 

symbols denote different laser power, repetition rate and scan speed

The samples show that the screen printing technique can provide adequate line width resolution 

and accuracy. Adhesion was measured for all the materials on Pilkington Tec 15 glass and of the 

carbon material on CdS/CdTe following ASTM 3359B. The results were "5B" or excellent 

adhesion. The resistance values for interconnection of the silver based ink are excellent; 

however the resistance of the nickel and possibly the carbon need further optimization. This is 

currently under investigation at NSC. 

3.2.4 Hail Impact Studies:

Many glass/EVA/glass specimens of size 16.5"X16.5" were fabricated and tested at ASU-PTL 

with the hail impact test. The hail impact tests did not cause any mechanical damage to the 

specimens. The top glass was tempered and passed thru the system. Many 16.5 x 16.5 inch 

tempered substrates have been transported with the load lock system and heated to 500 oC in 2 

minutes. Vendors with expertise in tempering TEC15 substrates with minimal distortions have 

been identified. A residual stress of 2700 psi. remained after processing. In addition, the 16.5" X 

16.5" laminates withstood 80 lbs. of load (static load for passing IEC 61646). Some specimens 

were tested to failure in the static test. The failure load was 400 lbs. The studies on hail impact 

tests and static loading will be part of MS thesis for Mark Armijo.

3.2.5 Encapsulation

Initially the design of the packaging for our modules will utilize glass/EVA/glass packaging. 

In order to reduce moisture ingress into the device, polymers with desiccants will be used for 

sealing the edges and also the holes in the back glass. Fixtures simulating the package design 

with the desiccant containing materials have been fabricated and are being tested at NREL under 

damp heat (85/85). A small quantity of moisture indicating material placed inside the fixture has 

not changed color after 3000+ hours of testing for the fixtures simulating the edge seal. The 

fixtures without the desiccant containing materials have failed (the moisture indicating material 

:

1064 nm laser scribing results

0

1

10

100

1000

10000

100000

6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0

laser pulse energy density (J/cm2/pulse)

resistance for 1 cm of scribe (ohm-cm)

2.1-2.5

2.6-2.10

3.6-3.10

4.1-4.5

4.6-4.10

Round 3

round 3 not through

films

about a 2x difference suggested 

undamaged tin oxide
7 

inside the fixture has changed color). The tests are continuing. This result is very promising. An 

advanced method for encapsulation is also being developed by National Starch and Chemicals 

(NSC) with our input. Based on our input and their past expertise, NSC has developed a 

promising approach for encapsulation. The new method involves screen printing a polymer 

based material (developed by NSC for OLED applications) at the edges on the back side of the 

front glass and then pressing the back glass to the front glass. Subsequently, the polymer is cured 

by UV exposure for fraction of a second. One glass/glass package with 0.25" wide encapsulant 

has been tested in the damp heat test (85/85) for 1300 hours. A small quantity of indicating 

desiccant placed in the middle between the two glass plates has not changed color. In addition to 

studies on packaging, the feasibility of scribing our films with automated systems has been 

demonstrated.

Hardware to do steady state photocapacitance (PHCAP) has been designed, built and 

tested in our laboratory. The photocapacitance technique measures changes in device capacitance 

under the illumination of sub-bandgap monochromatic illumination. The measurements are done 

at 77K. The relative magnitudes of the capacitance changes in the PHCAP spectrums are 

proportional to the defect densities. The PHCAP spectrums for our devices processed at various 

conditions are shown in Figure 4. The yellow curve with X markers in Figure 4(a) is a device 

that received no CdCl2 treatment. The blue and pink curves are devices that received poor and 

optimum CdCl2 treatment respectively. The poor CdCl2 treatment shows greater defect density 

than those with the optimum CdCl2 treatment. As shown in Figure 4(b) the devices with Cu back 

contact have lower photocapacitance than the devices without copper. A correlation between 

initial device efficiency and defect densities has also been shown. The photocapacitance has 

been normalized to the intensity of illumination at different wavelengths.

3.3 Detailed Device Analysis, Steady State Photocapacitance: 

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

0.73 0.77 0.83 0.89 0.95 1.03 1.13 1.24 1.38

Incident Photon Energy (eV)

normalized photocapacitance 

poor Cd chloride

no Cd chloride

optimum Cd

chloride

 

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

0.73 0.77 0.83 0.89 0.95 1.03 1.13 1.24 1.38

Incident Photon Energy (eV)

normalized photocapacitance 

no Cu

with Cu

(a) (b)

Figure 4: Normalized PHCAP spectrum of deep states with (a) varied CdCl2 treatment, (b) with and without the Cu 

back contact.
8 

The capacitance increases with incident light in Figure 4 are related to the emission of 

holes from deep states and the decreases in capacitance are related to the emission of electrons 

from deep states. However due to the complex interactions of multiple deep levels (both majority 

and minority traps) with possible overlap of optical cross sections it is difficult to assign exact 

emission threshold energies. Further work is underway to understand the PHCAP spectrums. 

Taru Takamia has completed a MS thesis on PHCAP and Jim Sites served in his committee. 

Some of the thesis work is presented here. Two cells processed identically to cells from the 

PHCAP study were sent to Steve Johnston at NREL for DLTS characterization. 

3.3.1 Device Characterization Studies

A summary of the preliminary work on the transient ion drift (TID) method of 

characterizing mobile Cu ions was presented in the poster session on May 8th at the 2006 

WCPEC4 in Kona Hawaii, (paper enclosed). The WCPEC presentation was an an extension of 

the results presented in the talk, “Observations of Cu Diffusion in CdTe PV Devices” at the 2006 

CdTe R&D Team at NREL. The TID method is well known as a means of characterizing the 

drift of Cu ions in c-Si material. Using TID it is possible to obtain the density and the diffusion 

coefficient of Cu ions. The work in our lab is an attempt to adapt the TID method to thin (2 μm) 

polycrystalline CdS/CdTe devices. An initial study of CdS/CdTe devices using TID indicates 

that increasing the quantity of Cu applied during the back contact formation process increases the 

Cui

+ density. However, for devices with an optimum CdCl2 treatment the Cui+ density is on the 

order of 2E13 cm-3. Also a preliminary estimate of the Cu ion diffusion coefficient is D(Cui) = 

1.3E-6 [cm2

/sec] * exp(-0.29 eV/KB*T) which closely matches the value found by others for Cui

diffusion in c-CdTe. The WCPEC4 paper qualitatively discusses the possible effects of grain 

boundaries, a thin neutral region and a non-uniform doping profile on TID measurements. In 

order to use TID as a practical method for characterizing Cu diffusion in CdTe PV devices more 

work is needed to quantitatively address these issues. Al Enzenroth met with Xuanzhi Wu and 

Su-Huai Wei at NREL on June 29th to discuss Cu diffusion mechanisms in CdTe. 

:

Some of the preliminary work on the photocapacitance (PHCAP) method of 

characterizing deep states or traps was presented at the 2006 EMRS in Nice France (paper 

enclosed). The EMRS presentation was an extension of the results presented at the 2005 CdTe 

R&D Team at NREL,in the talk, “Steady State Photocapacitance (PHCAP) Study of CdS/CdTe 

PV Devices”.

The introduction to the EMRS paper discusses problems in using deep level transient 

spectroscopy (DLTS) and thermal admittance spectroscopy (TAS) to characterize thin CdTe PV 

cells that have nearly depleted absorbers. Photocapacitance and optical DLTS with sub-bandgap 

light are shown to be promising techniques for characterization of deep levels in thin CdTe 

devices. Both steady state and transient PHCAP were used to characterize deep states related to 

the CdCl2 treatment in thin CdTe PV devices. In the study it was found that a non-optimum 

CdCl2 treatment increased the density of deep states while an optimum treatment decreased the 

density of deep states over an untreated sample. Three distinct deep levels were found the upper 

half of the band gap in all devices surveyed. A discussion of possible assignments of defects to 

these deep levels is presented. 

A collaboration has been initiated with Anura Samantilleke of the Department of 

Chemistry at the University of Bath in the UK to characterize our CdTe devices using modulated 

electroreflectance (ER). The ER technique was described in a presentation by Ramesh Dhere 
9 

entitled ”Study of CdS/CdTe Junction by Modulated Reflectance”, at the 2006 CdTe R&D Team 

meeting. Initial ER results from the University of Bath are promising. An example ER spectrum 

is shown in Figure 5 below. There are two critical points for CdTe in Figure 5 at 1.482 eV and 

1.525 eV. These values would indicate that there is little or no intermixing of S into the CdTe 

layer. The fit to the ER data also indicates that the crystalline quality of the CdTe is very good. 

Also there is a “lift off” of the ER spectrum at ~ 2.4 eV near the band edge energy of CdS. Since 

the spectrum continues to increase towards higher energies this is an indication of intermixing of 

the SnO:F TCO and the CdS layer. 

EER of CdTe 

Energy(eV)

1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0

∆R/R -4e-5

-2e-5

0

2e-5

4e-5

6e-5

8e-5

1e-4

Energy(eV) vs Corrected eR/R 

Col 11 vs Col 18 

Col 11 vs Col 28 

EER of CdTe 

Energy(eV)

1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0

∆R/R -4e-5

-2e-5

0

2e-5

4e-5

6e-5

8e-5

1e-4

Energy(eV) vs Corrected eR/R 

Col 11 vs Col 18 

Col 11 vs Col 28 

Figure 5: Electroreflectance spectrum of fully processed CdS/CdTe PV cell. Colored traces are fits to the 

ER data (black)

Fred Seymor of Colorado School of Mines has performed admittance spectroscopy 

measurements on several cells fabricated at the CSU MEL. These cells included poor CdCl2

without Cu and good CdCl2 with and without Cu. A brief summary of the results follows. For 

poor CdCl2 without Cu there was a uniform spatial distribution of deep states with an activation 

energy of 0.320 eV. For a cell with good CdCl2 treatment there was a uniform spatial distribution 

of deep states with an activation energy of 0.130 eV. This deep state may be assigned to the Cl 

A-center complex or other states related to Cl. The addition of Cu to a cell with a good CdCl2

treatment introduces a deep state at 0.350 eV. This deep state has a relatively high density and a 

non-uniform spatial distribution. The decrease in capacitance signal with increasing temperature 

indicates a steep concentration gradient near the back contact. This deep state may be assigned to 

CuCd. In all three cells there was evidence of deep states with a continuous energy band with 

activation energies deep in the band gap. The concentration of this distributed band was lowest 

for the cell with good CdCl2 with Cu.

The pilot system for processing 3.6 x 3.1 inch substrates is being modified to more 

closely simulate the back contact processing for actual industrial manufacturing. With the 

current configuration of the pilot system, Cu is introduced immediately after the CdCl2 treatment 

steps. The substrate enters the copper process at too high a temperature resulting in devices with 

3.4 Modification of the Pilot System.
10 

high Rs. To avoid this, a pause is introduced after the CdCl2 treatment step to allow the 

substrates and process sources to cool. After a nominal 3 hour wait, the substrate motion 

direction is reversed and the now cool samples are exposed to a vapor flux of the Cu compound 

followed by annealing. This completes the formation of the Cu containing back contact. 

To simulate the Cu back contact process for actual production the pilot system is being 

lengthened to allow fabrication of devices without the pause and change in substrate motion ("all 

forward" process). The modified pilot system would also allow processing of devices in a 

manner which emulates a continuous production system. In addition, new process 

methodologies can be investigated which can not be performed on the unmodified system. 

These include longer CdCl2 treatment times and copper processing. The current system is not 

long enough to apply the back contact with the longer CdCl2 processing. The longer CdCl2

processing times have resulted in devices with higher Voc and Jsc without the Cu back contact 

processing. This milestone will enable precise definition of conditions for all the semiconductor 

processing steps (including the back contact) needed to fabricate high efficiency, stable CdTe 

devices. This process definition will be implemented in the 2 MW/yr. production prototype 

system for 16.5 x 16.5 inch substrates.

A large number of experiments have been performed to explore the feasibility of 

reducing the time needed to cool the substrate before the copper processing. This will reduce the 

length of the "all forward" system. Active contact conduction cooling of the substrate (rather 

than radiation cooling) has been investigated. This would lead to a reduction in the length of the 

vacuum system and an increase in process control. Many thermal interface materials were 

explored including silicone polymers with fillers. These materials were not suitable for repeated 

contact with the substrate. A structure made from water cooled copper plate and multiple foils 

extending for contacting the substrate was also tested. Preliminary heat transfer analysis and 

experiments suggest that contacting the substrate with a copper plate with a flat, smooth surface 

should be adequate for cooling the substrate. 

Cooled plate Transport belt

Substrate spacing

IR detectors

Thermocouple

Heating station

Cooled plate Transport belt

Substrate spacing

IR detectors

Thermocouple

Heating station

Figure 6: Schematic of contact cooling test apparatus in vacuum chamber

A series of measurements of the cooling rate of glass substrates were performed with the 

test apparatus shown schematically in figure 6. The apparatus was installed in our pilot system. 

This allowed for the transport the glass substrates through the heating station and then under the 

water cooled plate. During measurements, the pressure was varied between 1 Torr and 40 mTorr 

and one measurement was done at atmospheric pressure as a control. The spacing between the 

water cooled plate and the glass substrate indicated in Figure 6 was a close contact in most of the 

cases. In one case a space of 0.010 inch was maintained. The substrate was heated to 170 C in 

the heating station and then indexed under the contact cooling plate. Measurements of the 
11 

substrate temperature were performed with either a thermocouple or with two infrared (IR) 

sensors. The water cooled plate was maintained at approximately 20 C.

The results of the temperature measurements are shown in Figure 7. As shown in Figure 

7 there is an increase in cooling rate with and increase in background pressure. The fastest 

cooling rate was at atmospheric pressure. In this case convection from the bottom of the substrate 

can not be neglected. However other tests at atmospheric pressure where the bottom of the 

substrate was insulated to prevent convection (not shown) closely match the result shown in 

Figure 6 for atmospheric pressure. Also shown is the predicted cooling rate for radiation alone. 

The contact cooling rate in all cases was faster than simple radiation. At a background pressure 

of 1 Torr the substrate cooled to 37 C in two minutes and 30 C after 4 min. Also shown is a case 

at 1 Torr pressure where the cooling plate was spaced away from the substrate by 0.010 inches. 

In this case the cooling rate closely matches the rate of a substrate in direct contact to the cooling 

plate at a pressure of 300 ~ 400 mTorr. 

Variation in cooling rate

0

20

40

60

80

100

120

140

160

180

0 50 100 150 200 250 300

time [sec]

temperature [C]

 predicted for radiation

40~77 mTorr

40~77 mTorr

300~400 mTorr

1 Torr

1 Torr

1 Torr

1 Torr 0.010 in. space

fit

1 atomosphere

Figure 7: Plot of substrate temperature with time for different background pressure and cooling plate spacing. Open 

symbols indicate thermocouple measurements; closed symbols indicate IR sensor measurements. All substrates used 

for the IR measurements had a CdTe film.

The results indicate that an adequate cooling rate can be obtained with a minimum 

pressure of 1 Torr of nitrogen gas. The cooling plate is best maintained in close contact to the top 

surface of the substrate. No glass cracking was observed during any of the testing. A reasonable 

fit to most of the data could be obtained by using a temperature difference model dT = K*(T1-

T2)*dt where the T2 is the cooling plate temperature and T1 is the substrate temperature, dT is 

the change in substrate temperature in time dt. The constant K is dependant on the ambient 

pressure. A comparison of the substrate temperature measured by the two IR sensors indicates 

that the cooling of the substrate is uniform within approximately 4 C. Based on the above 

experiments a fixture to contact cool the substrate that allows the contact cooled regions to be 

raised to 1 Torr ~ 10 Torr pressure is being designed and will be fabricated and implemented in 

the pilot system. Since the operating pressure is typically 40 mTorr, the fixture will be sealed 

against the substrate using O ring seals.

3.4. Large Area Processing:

A production prototype system has been developed to process larger substrates. This 

activity is supported by DOE-EERE and cost shared by National Starch and Chemicals (NSC). 
12 

This is an Inventions and Innovation Program award to AVA Technologies LLC and is the only 

Category 3 award ever granted by this program. The results described in this Section 3.4 are 

partially supported by this program and performed by AVA Technologies in collaboration with 

CSU's Materials Engineering Lab. 

 

3.4.1. Processing Larger Substrates

The semiconductor deposition system for the 2MW/Yr production line was designed to 

be fabricated in modular sections. One section of the vacuum chamber including the substrate 

transport and four deposition stations has been fabricated and two stations have been tested. 

Load locks to bring substrates into and out of the vacuum space have also been fabricated and 

substrate motion has been tested. The new modular section includes all pump ports, 

feedthroughs, access doors, water cooling plates and a support frame is currently being 

fabricated. Many 16.5 X 16.5 inch substrates have been heated from 25C to ~500C in two 

minutes with no glass cracking. 

3.4.2 Thermal Uniformity Measurements in the large area substrates

In order to obtain consistent device performance, a high degree of thermal uniformity is 

desired. Steady state temperature measurements of the graphite blocks in our deposition sources 

showed a ± 1.5° C variation on a 400o C base across the 14.5 x 14.5 in. pocket. In order to 

develop a model of the temperature distribution in the substrate, finite element analysis (FEA) 

simulation of substrate temperatures have been done at CU Boulder. The preliminary results 

predict that the substrate temperature difference would be larger during transient motion than 

during steady state conditions. A series of temperature measurements were undertaken to provide 

feedback to the FEA model and to give an initial map of thermal uniformity during transient 

substrate motion.

The temperature measurements were performed by using thermocouples mounted in the 

glass substrates. A diamond coated drill bit was used to drill 0.040 in. diameter holes 0.125 in. ~ 

0.25 in. deep into the glass substrates and the thermocouples were inserted into the holes. The 

measurements were performed at pressures and source temperatures that would be used during 

prototype production. The experimental setup is shown in Figure 8. It was not possible to pass 

the thermocouples through the load locks so the substrate was placed in position 1) in Figure 8

during warm-up of the heater. 

Figure 8: Schematic of temperature measurement setup Figure 9: Measured substrate temperatures

 at selected points. 

Heater CdS

(1) (2)

TC

Chamber

Heater CdS

(1) (2)

TC

Chamber

Substrate temperature

0

100

200

300

400

500

600

0 50 100 150

time [sec]

temperature [C]

(1) west edge

(2) lead edge

(3) trailing edge

(4) center
13 

After the heater and CdS source had reached operating temperature the substrate was moved 

from position 1) to position 2) in 6 seconds. The substrate was held in position 2) for 140 seconds to 

acquire the temperature data for the substrate heating sequence. At the end of 140 seconds the 

substrate was moved back to position 1). Figure 9 shows temperature vs. time data for selected points 

on a substrate during the heating sequence. The initial temperatures at t = 0 are for the substrate in 

position 1) of Figure 8. The heating sequence begins at t = 10 sec when the substrate is in position 2) 

of Figure 8. As can be seen in Figure 9 there is a large spread (~ 125o C) in initial temperatures. This 

is due to the fact that for the sources to reach operating temperature requires about 2 hours. This 

initial temperature difference will be much smaller during actual process operation when the pause 

time will be 2 minutes. 

Figure10: Predicted substrate temperature [oC] exiting 

 heater, entering CdS station.

The most pertinent temperature difference in the raw data shown in Figure 8 is between 

the center and the west edge since these points start at t = 0 within 1o C of each other. The final 

temperature difference is 3o C after 120 seconds. This is an acceptable amount of variation. 

As mentioned above the initial temperature difference between the leading edge and the 

other points is large. One other factor that affects the raw data shown in Figure 8 is that the 

leading edge has been heated for six seconds more than the trailing edge. This slow indexing 

time was used to eliminate damage to the thermocouple probes. In actual operation the substrate 

index time to move from station to station will be faster. In order to use the raw data to predict 

actual substrate temperatures during a process run the raw data was “normalized”. This was done 

by using a data fit to the measured raw data. The data fit was a bulk Euler integration model 

using glass parameters from the manufacturer. 

The initial substrate temperatures were predicted using calculations based on radiation 

heat transfer. The time of heating the glass was normalized to 120 seconds of heating time. This 

is valid since all points on the substrate will have received 120 seconds of heating as the 

substrate passes into the CdS station. The bulk model was then used with the estimated initial 

starting temperatures and a uniform heating time to predict substrate temperatures as the 

substrate enters the CdS station. The center to edge temperature difference of 3o C was 

preserved. The resulting substrate temperature uniformity map is shown in Figure 9. As can be 

458

467

458

448 451 448

448

448 448

Leading edge

458

467

458

448 451 448

448

448 448

458

467

458

448 451 448

448

448 448

Leading edge
14 

seen in Figure 9 the predicted temperature for the center of the leading edge is 16o C hotter than 

the center of the substrate. This is the greatest predicted variation. 

The FEA model predicts that substrate uniformity will only increase as the substrate 

passes through the CdS and CdTe stations. So the thermal variation shown in Figure 9 should 

decrease as the substrate continues to be processed. However, it is desirable to decrease the 

thermal variation of the substrate in the heating station. Efforts are currently underway to reduce 

the temperature variation. The radiation shielding in the heater is being modified to reduce the 

heating of the leading edge of the substrates before they enter the heater. The real time measured 

data is also being used to refine the FEA model.

A detailed cost analysis was conducted to determine the manufacturing and facility costs 

for manufacturing CdTe PV by our technology. In addition the development costs to advance 

the technology from the current pilot stage to a manufacturing stage was estimated. These 

analysis were performed by AVA Technologies LLC of Fort Collins. The methodology and 

results are summarized here. 

3.5. Cost Estimates for Large Scale Manufacturing with our Technology 

3.5.1 Manufacturing Cost Analysis Background: 

A detailed cost analysis was performed for a 20 MW/yr. PV manufacturing facility using 

our technology. This analysis uses knowledge developed from the operation of the pilot scale 

manufacturing facility since 1997 and the development of a production prototype system for 

processing 16.5x16.5 inch substrates. A manufacturing plant with an annual throughput of 

nominally 20 MW/yr. processing 120x60 cm frameless modules with a 63 watt rating is the basis 

for the analysis. This plant throughput was chosen because it represents an incremental step 

towards large scale manufacturing and is comparable to other facilities. Please note that in this 

analysis no building or land capital costs are included. However, equipment costs and rent are 

included. 

• Nominally 20 MW/yr. facility

• Operating 2 shifts/day, 7 days/week, 49 weeks/yr.

• 120x60 cm thin film modules 

• Module rating of 63 watts

• Glass/glass laminate package without frame (typical for CdTe PV)

3.5.2 Methodology: Significant effort was taken to ensure the accuracy of the data and the 

validity of the estimations used to generate this analysis. With almost no exceptions, all the 

equipment and materials costs and labor requirements used in this analysis were obtained from 

direct quotes or consultation with reputed vendors. This was augmented with the combined 35+ 

years experience of CSU's Materials Engineering Lab in thin film PV and vacuum equipment 

design. The analysis was performed using a spreadsheet which was provided to Ken Zweibel of 

NREL. 

• The equipment and materials costs and labor requirements used in this analysis were 

obtained from direct quote or consultation with reputed vendors. 
15 

• Detailed, automated spreadsheet was utilized to generate costs. 

• There are 25 processes to produce modules with this advanced technology. Most of the 

processes utilize commercially available standard industrial equipment optimized for 

high throughput. 

• Thin film modules are monolithically integrated and have no "cell" stage of processing. 

There are many significant differences in both the semiconductor and packaging 

processing between c-Si and thin film modules. 

The results of the analysis demonstrates that this CdTe thin film PV manufacturing technology 

has significantly lower manufacturing and equipment costs than other PV technologies.

• The manufacturing costs for modules are less than $1.00/Watt.

• These manufacturing costs are significantly lower (approximately 3-4 times) than other 

technologies according to data from another study [NREL 2005]. 

• The one time equipment costs are less than $10 million for 20 MW/yr. facility. 

• These one time equipment costs are at least 5 times lower than the published costs for 

other technologies. 

The equipment cost for the 10 MW Antec CdTe thin film facility in Germany was 

estimated at $27 million [Bonnet 1992]. This facility has only half the throughput of the facility 

described here. A 30 MW Q-cells factory (crystalline silicon cells and modules) is 

approximately $75 million [Solarbuzz 2005]. Wurth Solar intends to invest approximately $66 

million in a 15 MW thin film CIS facility [Solarbuzz2 2005]. 

Facility cost

CdTe thin film reported here approx. $10 

Technology ($ million, normalized to 20 MW/yr.)

Antec (CdTe) $54

Q-Cell (silicon) $50 (May include building, not all modules, some cells)

Wurth Solar (CIS) $88 (May include building)

The module packaging and the substrate preparation processes are quite similar between 

the different technologies. The difference is primarily in the semiconductor processing. Since 

1991, the focus of our efforts has been to reduce the cost of the thin film semiconductor PV 

processing. The cost advantages of our in-line, continuous, lean manufacturing technology can 

be clearly seen from the above numbers. 

3.6.1. Cooling Substrates in Vacuum

3.6 Modification of the Pilot System

The pilot system for processing 3.6 x 3.1 inch substrates has been modified to more 

closely simulate the back contact processing suitable for actual industrial manufacturing. With 

the old configuration of the pilot system, the Cu back contact process head is located 

immediately after the CdCl2 treatment heads. If the substrate enters the copper process at too 

high a temperature, the resulting devices have a high series resistance. To avoid this, a pause to 

cool the substrate was introduced in the old configuration of the pilot system. After a nominal 3 

.
16 

hour wait, the substrate motion direction is reversed and the now cool samples are exposed to a 

vapor flux of the Cu compound followed by annealing. This completed the formation of the Cu 

containing back contact in the old configuration. The old configuration is not suitable for large 

volume manufacturing. 

To simulate the Cu back contact process for actual production the pilot system has been 

lengthened to allow fabrication of devices without the pause and change in substrate motion ("all 

forward" process). The modified pilot system would also allow processing of devices in a 

manner which emulates a continuous production system. An automated system for cooling 

substrates in vacuum has been designed, fabricated, installed and tested. The cooling fixture was 

tested with automated substrate movement. The substrates were heated to 200 C and after 4 

minutes of radiation cooling were cooled with the fixture to 26 C in two minutes. The results 

indicate that an adequate cooling rate can be obtained to maintain a cycle time of 2 minutes. The 

measurements of substrate temperatures were performed with infrared sensors. The cooling of 

the substrates during processing was measured by IR detectors. A journal article has been drafted 

on the in-situ cooling approach and has been published by the Journal of vacuum Science and 

Technology. A patent application has also been filed. Device efficiencies of 11.83% have been 

obtained with an "all forward" process. It is noted that the total time for processing the 

11.83% device is only 22 minutes. This includes glass heating, CdS deposition, CdTe 

deposition, CdCl2 treatment and back contact processing. 

The modified pilot system will enable precise definition of conditions for all the 

semiconductor processing steps (including the back contact) needed to fabricate high efficiency, 

stable CdTe devices. This process definition will be implemented in the 2 MW/yr. production 

prototype system for 16.5 x 16.5 inch substrates. 

3.6.2. Modification of the Seals:

The pilot system for processing 3.1 X 3.6 inch substrates has a differentially pumped airto-vacuum-to-air seal that has a glass filled teflon liner for supporting the belt. This system has 

been operational since 1997. This system has been used to process nearly 10,000 substrates. The 

wear in the liner is causing the substrate to contact the seal and this leads to pinholes. In 

addition, the adhesive bond between the teflon liner and the aluminum support plate had also 

degraded. A new teflon liner with improved attachment to the aluminum support plate has been 

designed, fabricated and tested .

3.6.3. Strip Cells

Many large area devices in the form of narrow strips of ~0.7 cm. wide and 4 sq. cm. total area 

were processed to study the performance of these devices. The widths are representative of cells 

in modules. Devices were tested at NREL and the efficiency of 10.88% was measured and 

efficiency of 11.1% was also measured independently at IEC. These measurements indicate that 

module efficiency of 10% based on active area is feasible with our technology. The 10% module 

efficiency was utilized in the cost calculations developed for our technology. These calculations 

were sent earlier and summarized in the recent AVA Tech. team presentation.

:
17 

3.7 Higher Efficiency

We have demonstrated NREL verified efficiencies of 12.44% on unmodified Pilkington 

TEC15 glass. In order to obtain higher efficiencies, low iron sodalime (white glass) glass plates 

have been obtained and cut to size suitable for our pilot system. Fifty substrates have been sent to 

Sierratherm for APCVD (atmospheric pressure CVD) coating with TO:F. The coated substrates 

have been received and have been given to Professor Sites' group for optical studies. Coated 

samples have also been received from Solaronix. Deposition experiments with these substrates 

will be performed soon. The APCVD process is suitable for low cost high volume processing. 

After the current batch of samples, the use of barrier layer deposited by APCVD and other 

methods will be investigated. Discussions have been held with Pillkington and PPG and they 

have indicated that they may produce white glass with "TEC" type tin oxide coating in the 

future.

The coated substrates have been studied and the results are: 

A. The optical transmission of low iron glass in the visible spectrum is comparable to 7059 

borosilicate glass (nearly 92% without AR coating). 7059 glass has been used in many 

high efficiency devices.

B. MgF2 AR coating has been used in many high efficiency devices. MgF2 is slightly soluble in 

water and may not be suitable for outdoor applications. The MgF2 could be replaced with 

an oxide AR coating made by a pyrolytic process. Oxide AR coated substrates were 

obtained from Denglass and these substrates have very similar transmission to MgF2

coated glass.

C. Significant decrease in the transmission (~ 3 mA/sq. cm.) has been observed due to the TO:F 

coating. Efforts to reduce the transmission losses in the transparent conducting oxide are 

currently underway. 

Deposition experiments with substrates from Sierratherm have been performed. In one 

run the average efficiency of devices with the Pilkington TEC 15 substrates was 10.8%. The 

efficiency with white glass substrates was 12%. Cross-sectional SEM studies showed that the 

white glass had a higher thickness for CdS (nearly 1800 angstroms). This indicates that 

optimization of CdS thickness would result in higher efficiency. These efforts are currently 

underway. After the current batch of samples, the use of barrier layer will be investigated. A few 

substrates with intrinsic tin oxide barrier coating have been obtained from Chris Ferekides. The 

coated substrates have been studied and the results are shown in Figure 11 below. 

Three different substrates have been investigated for higher efficiency. These are: 

TEC15, Low Fe, and Low Fe with i-SnOx coating. The i-SnOx coatings were provided by Chris 

Ferekides of USF. Our current standard process produces Jsc of 20 mA/sq. cm. at ~800mV.(open 

blue squares have slightly smaller CdS thickness compared to the standard). When the CdS 

thickness was reduced, an increase in Jsc was observed, but the Voc decreased. Nearly 100 

substrates and 24 devices were part of the thin CdS study. Six devices have been provided to 

Prof. Sites' group for LBIC and QE studies.
18 

Figure 11: Jsc Voc scatter plot of devices with thinner CdS on three different substrates

Further optimization of the TCO and CdS will be needed for obtaining higher 

efficiencies. Four 3 X 3 inch. Low Fe substrates with ITO/i-SnOx coating are being provided by 

Chris Ferekides of USF. Xuanzhi Wu of NREL has supplied four 3 X 3 inch LOF TEC 15 

substrates with a ZTO buffer layer. These substrates will be used for future efforts to increase 

device efficiency.

In order to obtain higher efficiencies, more transparent glass plates with ITO applied by 

planar magnetron sputtering and ZnO:B by LPCVD have been obtained. These substrates have 

transmission greater than 90% between wavelengths of 500 to 800 nm as compared to ~74% for 

TEC15. These substrates did not produce devices with higher efficiencies.

Devices that have 25 mA/sq. cm. current density with Voc of 800 mV. (JV plot shown in 

Fig. 12) have been made in our laboratory. The values were also got from the JV tester at AVA. 

Both testers are calibrated with devices from NREL. These use ITO coated glass that is more 

transparent and commercially available. This current density is nearly 25% higher than the 

normal 20 mA/sq. cm. These do not have the AR coating and with AR coating we can get to 26 

mA/sq. cm. The glass is low iron white glass and back contact processing was not applied to the 

device. With optimization, we should be able to make 14 to 15% small area devices and 11 to 

12% modules and these efforts are currently underway.

Devices of efficiency 12.8% (NREL verified) have been fabricated on low iron glass with 

SnOx:F TCO. Devices of efficiency 13.7% (tested at CSU photovoltaics lab.) have been 

fabricated on low iron glass with SnOx:F TCO without AR coating.

Jsc Voc scatter plot

18

20

22

24

0 200 400 600 800

Voc [volts]

Jsc [mA/cm^2]

20259 Tech 15 20260 low Fe 20261 low Fe/ i-SnO

20863 Tech 15 20864 low Fe 20865 low Fe
19 

Figure 12. J_V curve for devices with more transparent ITO.

Spectroscopic ellipsometry is being investigated as a tool for on-line process control. It is 

hoped that ellipsometry would provide a nondestructive means of measuring thickness 

uniformity of CdS/CdTe films on large areas. It is also possible that by tracking optical constants 

measured by ellipsometry it would be possible to correlate this data to process variation. Efforts 

are underway to qualify the technique and verify its usefulness. 

3.8. On-Line Process Control:

In order for ellipsometry to predict layer thicknesses and optical constants for a multifilm stack an analytical model must be developed. Sandeep Kohli of the CSU central 

instrumentation facility has used a J. A. Woollam Vase ellipsometer to collect experimental 

spectrums from several samples processed in our laboratory. Working in collaboration with the 

Woollam Co. Sandeep constructed an analytical model to predict the optical constants and layer 

thicknesses for the measured samples. The model fit to the experimental data for a representative 

sample is shown in Figure 13. The model has 14 layers and the mean squared error for this fit is 

24.22, which is considered to be a good fit.

The most difficult part of developing the analytical model was to obtain a good fit to the 

glass/SnOx substrate. Sandeep eventually used 6 layers including one layer with graded optical 

constants to fit the measured data. Initial profilometry measurements of the CdS/CdTe layer 

thickness match the CdS/CdTe thickness predicted by the model. Cross sectional SEM 

characterization of the samples is also underway to verify the ellipsometry data.
20 

Figure 13: Spectroscopic ellipsometry data and model fit for a LOF Tec 15 substrate with 

SnO:F/CdS/CdTe:Cu which had received CdCl2 treatment. 

At this point a reasonable model has been obtained and now the predictive capability of 

the model must be further verified. Optimization and further development of the model will also 

be pursued. Al Enzenroth of the CSU Materials Engineering Laboratory attended a four day 

spectroscopic ellipsometry short course on data analysis presented by the Woolam Co. at

Arizona State University. In order to exercise the model more samples with varied film 

thicknesses will be fabricated. Ellipsometry data will be obtained from these samples and the 

model predictions will be checked. Multiple ellipsometry measurements on 3 in. X 3 in. samples 

will also be performed to measure uniformity. 

It has been shown that the refractive index n and the extinction coefficient k change with 

the processing and n and k values approach single crystal values after the chloride treatment. In

addition to ellipsometry where changes in polarization are measured, the use of total intensity for 

on-line process monitoring is being investigated and the results are promising. Two journal 

articles have been written on the ellipsometry results and both have been accepted for 

publication.

In addition to the use of ellipsometry, the use of x-ray floroscence (XRF) for process 

monitoring and control has been demonstrated. Quantitative relationship between the film 

thicknesses and the x-ray signal in XRF has been developed. The feasibility of measuring the 

amount of copper in the samples used for the back contact has also been shown. The studies with 

XRF was performed by Nathan Schuh, a graduate student in our laboratory.

3.9.1. Thermal Modeling: Many designs of the large area (2' X 4') vapor sources have been

analyzed with the finite element method (FEM) for thermal uniformity. These models included 

non-linear radiation boundary conditions and body to body radiation. The designs were analyzed 

with the ANSYS finite element software. More than 18 different cases were analyzed. A typical 

result of the analysis is given below. The temperatures are in degrees C. The results shown below 

are for the bottom plate of the CdTe source perpendicular to the substrate motion in the center. 

3.9 Numerical Modeling:

Generated and Experimental

Wavelength (nm)

200 400 600 800 1000 1200

Ψ in degrees

0

5

10

15

20

25

30

Model Fit 

Exp E 55°

Exp E 65°

Exp E 75°
21 

The results are for a design with adequate radiation shielding that results in good thermal 

uniformity. The results indicate that thermal uniformity of ± 2 C can be obtained on a horizontal 

plane.

3.9.2. Modeling Gas Flow: A detailed modeling of the gas flow in the 3" X 3" pilot system 

using computational fluid dynamics (CFD) using the software FLUENT has been completed. 

The modeling was performed by a senior Dan Sweeney in our department with the help of his 

advisor Prof. Hiroshi Sakurai. The modeling showed that the pressure distribution within the 

pilot system was very uniform. There was a concern that the numerous vapor sources and other 

structures combined with the gas flow could lead to non-uniformities in pressure and the 

modeling showed that this was not the case. Currently nitrogen and oxygen are injected into two 

separate ports in the system. Based on the modeling it was decided to mix the gases prior to the 

injection to obtain more uniform distribution of the composition. A gas mixing unit has been 

designed with CFD modeling. CFD modeling of the larger system has been completed. An 

example of the flow patterns from the CFD modeling is shown below. The modeling showed that 

the use of commercially available gas diffusers significantly reduce the exit velocities at gas 

injection. 
22 

 

In addition to the above, many modeling studies have been completed. These include:

a. Calculation of the sticking coefficient show that only one in 50 condense. This is for the CdTe 

source. This explains why the film uniformity is better than the temperature uniformity.

b. The vapor pressure of CdS in the CdTe source is likely to incorporate 0.7% CdS in the CdTe 

film and this will lower the bandgap of the film slightly. The vapor pressure of CdTe in the 

CdCl2 source is likely to incorporate 0.017% CdTe in the CdCl2 film. This is unlikely to hurt the 

Cdcl2 treatment.

3.10.1 Outdoor testing

3.10 Miscellaneous Studies:

Eleven optimally processed devices are being tested outdoors under open circuit 

conditions. These devices were recently measured and the results are shown below in Figure 14. 

It is noted that most of the devices have maintained an efficiency of 10% after nearly 5 years 

outdoors at open circuit conditions. Accelerations factors of 7 to 100 have been reported in the 

literature for CdTe devices between open circuit and max. power condition. 

: 
23 

Figure 14: Average efficiency versus time for groups of cells in outdoor test conditions

3.10.2. New Vendor for CdS and CdTe: 

Until now, we have obtained CdS and CdTe from ASARCO. Since ASARCO no longer 

produces CdS and CdTe, we are obtaining these materials from 5N Plus Inc. The new materials 

have been tested on the pilot system and preliminary results indicate that the optimum process 

conditions with the new materials are close to that of the old materials. 

3.10.3.Cleaning films: We actively looked into removing the coating from waste coated plates 

prior to diposal. This will substantially reduce the solid waste to be to blast the surface and the 

CdS and CdTe can be filtered. Five 16 X 16 plates processed by AVA were cleaned by a service 

provider using abrasive blasting with water. They have constructed a setup to do the cleaning. It 

takes 30 sec. to remove the coating and overall 2 mins. to place, clean and remove the samples. 

The cleaned glass plates, water and sand have been tested by the federal TCLP test and found to 

be non-hazardous. The hazardous material collects at the bottom of the waste water and can be 

collected for recycling if needed. This provides an attractive means for removing the films before 

disposal. It is pointed out that the coated films pass the TCLP test and the procedure was 

developed to keep Cd from landfills. These results have been reproduced in our laboratory using 

a traditional sand blaster with abrasive slurry. There was no detectable change in the thickness 

implying that only the coating has been removed.

3.10.4. Thermal Gradient Testing of Glass: A fixture for testing glass under thermal gradients 

(colder at the edges and vice-versa) have been constructed. Glasses of size 16.5” X 16.5” have 

been tested. The surface condition of the edge has a significant effect on the glass breakage due 

to thermal gradients. The #2 ground samples have been found to withstand a gradient of 70C 

(colder at the edges). No failure was observed when 10 substrates were tested.

Outdoor Performance, Desiccated, OC bias

0

2

4

6

8

10

12

14

0 200 400 600 800 1000 1200 1400 1600 1800 2000

total time [days]

average efficiency [%]

6650-3,8,2B 6552-9,2B and 6649-1B 7236-4,5B and 7233-1B
24 

4. REFERENCES

Barth, K. L., R. A. Enzenroth, and W. S. Sampath, "Consistent Processing and Long Term 

Stability of CdTe Devices", presented at the 31 IEEE PVSC, January 3-7, 2005, Lake Buena 

Vista, Florida. 

Bonnet: Bonnet, D. Int. J. of Solar Energy 1992, Vol. 12. pp 1-14.

NREL: Study of PV manufacturing costs available at: 

 http://www.nrel.gov/ncpv/pv_manufacturing/cost_capacity.html, accessed July 2005.

Sampath, W. S., K. L. Barth, R. A. Enzenroth, "Progress in Continuous In-Line Processing of 

CdS/CdTe Devices, Including Large Area (16” X 16”) Deposition" presentation to the Solar 

Program Review Meeting, Denver CO, November 2005. available at 

:http://www.nrel.gov/ncpv/thin_film/docs/sampath2005solarreview.ppt

Solarbuzz: "Evergreen Solar and Q-cells AG in Manufacturing Plant Joint Venture", Solarbuzz 

1/14/05. Available www.solarzuzz.com/News/NewsEUMA58.htm, accessed July 2005. 

Solarbuzz2: "Würth Solar to Invest €55 million in Thin Film Manufacturing Plant", Solarbuzz, 

5/23/05, http://www.solarbuzz.com/News/NewsEUMA68.htm , accessed July 2005. 

Sampath, W. S., Kurt Barth and Al Enzenroth, “STABILITY, YIELD AND EFFICIENCY OF 

CdS/CdTe DEVICES”, Thin Film Partnership Program, Subcontract ADJ-1-30630-07, 

National Renewable Energy Laboratory Final Report. 2004. Available at

http://www.nrel.gov/ncpv/thin_film/pdfs/csu_sampath_final_report2004.pdf 

Barth, K. L., R. A. Enzenroth, and W. S. Sampath, "Consistent Processing and Long Term 

Stability of CdTe Devices", presented at the 31 IEEE PVSC, January 3-7, 2005, Lake Buena 

Vista, Florida.

5. APPENDICES 

5.1. Description of Activities with the National CdTe Team

Our group supported the basic research efforts of the CdTe Team. We continue to supply 

devices, films and substrates as well as experimental data to the Team members. A selection of 

the research efforts supported are listed below: 

1) Nine cells and 24 samples were sent to Larry Olsen at PNNL for studies with the 

moisture barrier

2) Two samples and two devices to CS Jiang of NREL for SKPM

3) Two substrates to C Corwine of CSU PV group for PL

4) Two cells to T. McMahon for Rsh characterization

5) Four cells to A. Fahrenbruch for transient current characterization

6) Twenty cells to A. Davies of CSU PV group for micro LBIC characterization

7) One substrate to Tim Gessert

8) 12 cells with an outdoor fixture to N. Dehre of FSEC for outdoor condition testing

9) Eight full 3.1 x 3.5 in. substrates to CSM for various studies
25 

10) 19 cells were provided to Alan Davies (Prof. Sites' group) and these cells were stress 

tested and it was shown that at optimum processing, good stability can be obtained. This 

was presented at the CdTe Team Meeting.

11) Many interactions with Larry Olsen of PNNL regarding the application of CdTe devices 

for another application have occurred. 

12) Graduate student Tushar Shimpi graduated with a MS degree and Jim Sites served on his 

committee.

13) Two presentations (one by CSU and one by AVA Tech.) were given at the CdTe Team 

meeting.

14) Twenty slides on our research were provided to Ramesh Dhere of NREL for his tutorial 

at the IEEE PV Conference.

15) Marc Landry and Ramesh Dhere (both of NREL) and visitors from Ziyax Corp. were 

given a tour of our laboratory and possibilities of collaboration were discussed. Other 

visitors include: Steve Johnson, researchers from PPG, etc.

16) Our group has developed a technique for defining CdTe cell areas on glass substrates. 

The technique consists of applying a contact mask and removal of unmasked areas with 

an abrasive blast. Xuanzhi Wu supplied four CdTe substrates for initial testing of the 

abrasive blast technique on NREL devices. Circular areas of ~ 0.69 cm2 were defined 

and the cells were returned to NREL for evaluation.

17) Many interactions with Larry Olsen of PNNL regarding the application of CdTe devices 

for another application have occurred. 

18) Al Enzenroth from our group presented papers at the IEEE meeting and E-MRS meeting 

(discussed above). He also attended the CdTe workshop in France.

19) Collaborations continued with CSM, X. Wu of NREL and a new collaboration with 

Anura Samantilleke of the Department of Chemistry at the University of Bath in the UK 

has been initiated. 

20) . Five substrates processed upto the chloride treatment were provided to Tim Gessert of 

NREL for his studies with ZnTe:Cu with a summer student.

21) 2. Alan Davies from the CSU Photovoltaics laboratory has been participating in 

processing devices in the laboratory. 

5.2. Description of Interactions with Industry 

Specialty electronic coatings are used as an electrode for our PV devices. These 

materials are supplied by Acheson Industries which is a part of National Starch and Chemical 

(NSC is the US subsidiary of the approximately $10 Billion/yr, ICI Group). NSC manufactures 

critical products for the semiconductor, electronics and photonic industries. Our group has a 

continuing involvement with NSC to advance CdTe PV under the I and I program. During this 

last year, 126 substrates to NSC for metallization studies.

A large corporation capable of very large scale manufacturing had expressed interest in 

pursuing a potential partnership for PV manufacturing with our technology. Numerous 

teleconferences and site visits have been conducted with high level representatives of the 

company. Documentation was also provided which includes:

A. A detailed review of any environmental and occupational safety aspects with our 

technology
26 

B. A detailed break down of the cost to manufacturing large volumes of PV with our 

technology.

C. A non-disclosure agreement. 

D. Description of the advantages of our technology over competing technologies.

E. Details on the intellectual property ownership. 

These requests are in support of developing a potential partnership relationship. The 

environmental analysis was prepared with the assistance of an industrial hygienist and 

demonstrated that only simple, industry standard environmental health and safety practices are 

needed for to protect workers in a large scale manufacturing facility using our technology. This 

effort was enabled by the materials available at http://www.nrel.gov/cdte/.

Peter Meyers and Roger Green of First Solar visited our facility and were given a full 

tour and presentation on our activities. This was followed with a conference call from Nelson 

Devoe for possible consulting. 

Southwall Technologies and an investor group represented by Jim Frantz were given 

detailed presentations on the advantages of CdTe PV manufacturing. 

Based on their request, Solar Fields has been provided details of our metallization and 

they were put in touch with NSC.

Five cells were provided to Trace Photonics for Beta radiation studies.

John Bleem from the local utility, Platte River Power Authority, was given a facility tour. 

The deputy director and two program managers of the Colorado Office of Energy 

Management and Conservation (OEMC) visited our laboratory and were given a demonstration 

and brief presentation. They have offered to assist with the technology transfer efforts.

5.3. Additional Activities

In November 2005, the well known CdTe PV researcher, Dr. D. Bonnet, visited CSU. In 

addition to visiting J. Sites lab, Dr. Bonnet was given a full tour of the Materials Engineering 

Lab. A demonstration of our pilot scale CdTe manufacturing system was conducted. The 

current status of the 2 MW/yr. prototype manufacturing line being developed by AVA Tech. and 

CSU demonstrated. Dr. Bonnet gave a presentation to our group on CdTe manufacturing 

facilities of ANTEC of Germany.

We are collaborating with Prof. Roop Mahajan of University of Colorado, Boulder. He is 

an internationally renowned authority in the field of heat transfer in processing semiconductors. 

This work through the International Center for Science, a UN agency, in collaboration with our 

group has been selected for funding. The funding will support two students and demonstration 

projects in Mexico and India. This will provide more opportunity for studying stability under 

field conditions.

A series of measurements of the residual stress in the glass substrates after processing 

through our continuous pilot scale system have been completed as per ASTM C1279. Glass 

substrates with different levels of residual stress have been sent to the ASU Photovoltaic Testing 

Lab for hail impact tests. These studies will establish the stress levels needed to pass the hail 

impact test for modules as per IEC 61646. 

Prof. Wilbur's group at Colorado State University was supported with 11 devices and 

characterization for high altitude balloon studies and the devices performed well in these studies. 
27 

These studies were part of the Colorado Space Grant program and efficiencies of 15% were 

measured at high altitudes at temperatures of -23 degrees C. The results were presented at Jet 

Propulsion Laboratory.

Graduate students, Tushar Shimpi, Mark Armijo and Rick Lueb were supported through 

this project. They have assisted in the activities described in this report. Tushar Shimpi and Toru 

Takamiya have performed their graduate research on this project and have graduated with 

Masters degree. The objective of Tushar Shimpi's thesis was to develop an understanding of the 

hail impact test and Toru Takamiya conducted research on the Photocapacitance method 

described earlier. Hershel Shelly is conducting his Ph. D. research on this project. His research is 

to study the pilot scale process from an industrial engineering perspective. Chetan Malhothra of 

CU Boulder has completed his Ph.D. through thermal and deposition modeling of the process.

Marh Armijo has made significant progress towards completing his MS thesis and his study is to 

understand the mechanics of the composite module. 

5.4. Student Activities

5.5 Collaborative Studies and Other Activities:

During this phase, the following collaborative activities were conducted:

1.Brian Murphy, Fred Seymour and Russell Black of Primestar Solar were given a detailed 

tour of the lab. and many insights on the effect of processing on device stability were 

discussed.

2. Our laboratory was part of an organized self guided tour of the American Solar Energy 

Society. Nearly 100 visitors visited the laboratory over a six hour period.

3. Mani Manivannan from the Global Research Center of GE has joined our department and 

is continuing his research on CdTe photovoltaics. 

4. Many substrates and devices were processed for a large glass manufacturer with 

significant interest in developing TCOs for the CdTe PV industry.

5. Three seminars were arranged by Jim Sites, Tim Gessert and Dean Levi. These were well 

attended by CSU and AVA personnel. These were well received and tapes were sent to 

researchers in the glass industry.

6. Many substrates and devices were processed for a large glass manufacturer with 

significant interest in developing TCOs for the CdTe PV industry.

7. Alan Davies from the CSU Photovoltaics laboratory has been participating in processing 

devices in the laboratory and has accepted a position with AVA Solar. Many devices have 

been provided for the graduate studies of Alan Davies and Lei Chen from the photovoltaics 

laboratory under Jim Sites and Sampath served on the committee. Alan has completed his Ph. 

D. Lei Chen has also accepted a position with AVA Solar.

8 Many substrates with different processing conditions have been processed for Prof. 

Manivannan for his collaborative studies with NREL on the effect of processing conditions 

on the films. In his studies with NREL researchers, the lifetimes increased with longer times 

in the CdCl2 Treatment but the efficiency had an optimum CdCl2 treatment. His article on the 

diffusion of copper in the solar energy journal has been accepted for publication.
28 

9. Many devices have been provided to Dr. Sanjeev for radiation studies.

10. Many small area devices and modules from AVA Solar are being tested under 

accelerated testing in our laboratory.

11. Collaborative research on the use of evaporated films for metallization have been 

conducted in collaboration with Prof. Amy Prieto on chemistry at CSU. Dr. Srinivas, a newly 

hired post-doctoral fellow in the laboratory conducted the experiments.

12. Collaborative research on the use of ion beam sputtered films for metallization have been 

conducted with Prof. John Williams of the mechanical Engineering Dept. of CSU .

13. Nearly 40 representatives from the Denver South Chamber were given a lecture and tour 

of the laboratory.

5.6 Presentation to Senator Salazar:

A detailed presentation was made to senator Salazar and the presentation was well 

received. The presentation was attended by Harin Ullal and Bolko Vonrodern of NREL and 

many senior administrators from CSU, City and the County.

Sampath, W. S., K. L. Barth, R. A. Enzenroth, "Progress in Continuous In-Line Processing of 

CdS/CdTe Devices, Including Large Area (16” X 16”) Deposition" presentation to the Solar 

Program Review Meeting, Denver CO, November 2005. available at

5.7. Publications and Presentations from this project:

http://www.nrel.gov/ncpv/thin_film/docs/sampath2005solarreview.ppt

Barth, K. L, R. A. Enzenroth, and W. S. Sampath, "Progress in Continuous In-Line Processing of 

CdS/CdTe Devices Including LargeArea (16 x 16 inch) Deposition" paper for the Solar 

Program Review Meeting, Denver CO, November 2005. Published in proceedings.

Malhotra, C. P., Mahajan, R. L., Sampath, W. S., “Use of a Commercial Finite Element Software 

for Predicting Deposition Rates and Uniformity in High Knudsen number PVD”, accepted 

to IMECE 2005: ASME International Mechanical Engineering Congress Orlando, FL, 

November 5–11, 2005

"Mass Production of CdTe Solar Photovoltaics" Presentation and facility tour for the Fort 

Collins, Colorado Chamber of Commerce Chamber Environmental Committee, Oct 12, 

2005. 

Malhotra, C. P., Mahajan, R. L., Sampath, W. S., Barth, K. L., Enzenroth, R. A., “Prediction of 

Spatial Evolution of Temperature across Substrates during the Deposition Process for 

Manufacturing large area CdTe Photovoltaic panels,” submitted to IHTC-13: 13th 

International Heat Transfer Conference Sydney, August 13-18, 2006

Barth, Kurt, W. S. Sampath, Robert "Al" Enzenroth, "Project Update Facility Tour", 

Presentation, Discussion and Facility Tour with the with representatives of US DOE, and 

Industry, April 5, 2005.
29 

Enzenroth, R. Albert, K. L. Barth, and W. S. Sampath, "Correlation of Stability to Varied CdCl2

Treatment and Related Defects in CdS/CdTe PV Devices as Measured by Thermal 

Admittance Spectroscopy", Journal of Physics and Chemistry of Solids, pp 1883-1886, 66 

(2005).

Barth, Kurt, "Mass Production of Thin Film Solar Photovoltaics", Invited seminar For US DOE 

Golden Field Office March 17, 2005. 

Malhotra, C. P., Mahajan, R. L., Sampath, W. S., Barth, K. L., Enzenroth, R. A., “Control of 

Temperature Uniformity during the Manufacture of Stable Thin-Film Photovoltaic 

Devices”, submitted to International Journal of Heat and Mass Transfer in March 2005

Sampath, W. S., Robert. A. Enzenroth and Kurt L. Barth, "Mass Production of Thin Film Solar 

Photovoltaics", Presentation, Discussion and Facility Tour with the Fort Collins Utilities, 

and CSU Facilities, March 3, 2005.

Enzenroth, R. A., K. L. Barth and W. S. Sampath, "Thermal Admittance Spectroscopy Study: 

Preliminary Observations of a Meyer-Neldel Relationship in CdTe Devices", Invited talk to 

senior Researchers and managers, NREL Golden CO, February, 16, 2005.

Barth, K. L., R. A. Enzenroth, and W. S. Sampath, "Consistent Processing and Long Term 

Stability of CdTe Devices", presented at the 31 IEEE PVSC, January 3-7, 2005, Lake 

Buena Vista, Florida. 

Enzenroth, R. A., K. L. Barth and W. S. Sampath, "Thermal Admittance Spectroscopy Study: 

Preliminary Observations of a Meyer-Neldel Relationship in CdTe Devices", presented at the 

31 IEEE PVSC, January 3-7, 2005, Lake Buena Vista, Florida. 

Barth, K. L., R. A. Enzenroth, and W. S. Sampath, "Advances in Continuous In-Line Processing of 

CdS/CdTe Devices: Stability and Scale-Up", DOE Solar Energy Technologies Program Review 

Meeting, October 25-28, 2004, Denver, Colorado. 

Sampath, W. S. K. L. Barth, "Mass Production of CdTe Solar Photovoltaics", American Society of 

Mechanical Engineers (ASME), Centennial Section Laboratory tour and presentation, Nov. 18, 

2004.

Malhotra, C. P, R. L. Mahajan, W.S. Sampath, K. L. Barth, R. A. Enzenroth, "Control of 

Temperature Uniformity during the Manufacture of Stable Thin-film Photovoltaic Devices", 

ASME International Mechanical Engineering Congress & Exposition, (IMECE 2004), 

November 13-19, 2004, Anaheim, California.

Barth, K. L., "Mass Production of CdTe Solar Photovoltaics", Mechanical Engineering Seminar 

Series, Colorado State University, October 15, 2004. 

Enzenroth, R. Albert, K. L. Barth, W. S. Sampath, "Correlation of Stability to Varied CdCl2

Treatment and Related Defects in CdS/CdTe PV Devices as Measured by Thermal 

Admittance Spectroscopy", 14th International Conference on Ternary and Multinary 

Compounds, September 27-October 1, 2004, Denver, Colorado, USA. 
30 

5.7.1. National CdTe R&D Team Meeting Presentations

Sampath, W. S, R. Albert Enzenroth, and Kurt L. Barth, "Advances in Continuous In-Line 

Processing of CdTe PV Devices" National CdTe Team Meeting, Sponsored by the DOE 

through the National Renewable Energy Laboratory, Golden, CO, May 5 and 6, 2005. 

Available at NREL website: www.nrel.gov/ncpv/thin_film/docs/Sp-top-Sampath.ppt

Barth, Kurt L., R. Albert Enzenroth and W. S. Sampath, "Progress at AVA Technologies LLC" 

National CdTe Team Meeting, Sponsored by the DOE through the National Renewable 

Energy Laboratory, Golden, CO, May 5 and 6, 2005. Available at NREL website: 

/www.nrel.gov/ncpv/thin_film/docs/Ind-AVA%20Tech.ppt#366,1,Progres

Enzenroth, R. A., K. L. Barth, and W. S. Sampath "Estimation of Acceleration Factors by 

Comparison of Performance During Accelerated Stress and in Outdoor Conditions", 

National CdTe Team Meeting, Sponsored by the DOE through the National Renewable 

Energy Laboratory, Golden, CO, May 5 and 6, 2005. Available at NREL website: 

/www.nrel.gov/ncpv/thin_film/docs/Stab-Al%20Enzenroth.ppt

Enzenroth, R. A. T. Takamiya, K.L. Barth, and W.S. Sampath, "Steady State Photocapacitance 

(PHCAP) Study of CdS/CdTe PV Devices", National CdTe Team Meeting, Sponsored by the 

DOE through the National Renewable Energy Laboratory, Golden, CO, May 5 and 6, 2005. 

Available at NREL website: www.nrel.gov/ncpv/thin_film/docs/Mat-chemAl%20Enzenroth.ppt

Enzenroth, R. Albert, K. L. Barth, W. S. Sampath, "Stability Sub-Team Meeting" October 25, 2004, 

Denver, Colorado, USA. 

Barth, K. L., R. A. Enzenroth, and W. S. Sampath, "Advances in Continuous In-Line Processing 

of CdS/CdTe Devices: Stability and Scale-Up", DOE Solar Energy Technologies Program 

Review Meeting, October 25-28, 2004, Denver, Colorado.

5.7.2 Refereed Journals:

Enzenroth, R. A.T. Takamiya, K. L. Barth, and W. S. Sampath, "Photocapacitance Study of Deep 

Levels in Thin CdTe PV Devices", accepted in Thin Solid Films (2006) 

Robert A. Enzenroth, Kurt L. Barth, W. S. Sampath and V. Manivannan, "Measurement of 

cooling rates of a superstrate cooling apparatus for an integrated in-line manufacturing process for 

thin-film photovoltaic devices", Accepted to the Journal of Vacuum Science and Technology, 

2006 

R.A. Enzenroth, K.L. Barth, W.S. Sampath and V. Manivannan, " In-line manufacturing of CdTe 

thin film photovoltaic devices", Submitted to Journal of Manufacturing Science and Engineering, 

under review.

R.A. Enzenroth, K.L. Barth, W.S. Sampath and V. Manivannan, " Performance of inlinemanufactured CdTe Thin Film Photovoltaic devices", Journal of Solar Energy Engineering, 

vol 129, (2007) 327. 
31 

R. A. Enzenroth, Kurt L. Barth, W.S. Sampath and V. Manivannan, " Measurement of cooling 

rates of a superstrate cooling apparatus for an integrated in-line manufacturing process for thin 

film photovoltaic devices". J. Vac. Sci. Technol. A 25(2)(2007);0734-2101/2007/25(2)L9/3.

R. A. Enzenroth, Kurt L. Barth, W.S. Sampath and V. Manivannan "Thermal model for a

superstrate cooling apparatus for an integrated in-line manufacturing process for thin film 

photovoltaic devices". J. Vac. Sci. Technol. B 25(6) 2007.

.

V. Manivannan, Robert A. Enzenroth, Kurt L. Barth, Sandeep Kohli, Patrick McCurdy and W. S. 

Sampath, " Microstructural features of CdTe Photovoltaic thin film devices", Thin Solid Films, 

516 (2008) 1209-1213.

Tushar M. Shimpi, Kurt L. Barth, Robert Al. Enzenroth, W.S. Sampath and V. Manivannan; 

"Effect of Hail Impact on Thermally Tempered Glass Substrates Used for Processing CdTe PV 

Modules". J. of Testing and evaluation, vol.36 No.2 paper ID JTE101036

R. A. Enzenroth, Kurt L. Barth, W.S. Sampath and V. Manivannan "Thermal model for a 

superstrate cooling apparatus for an integrated in-line manufacturing process for thin film 

photovoltaic devices" J. Vac. Sci. Technol. B 25(6) 2007.

5.7.3 Refereed Conferences:

Enzenroth, R. A., K. L. Barth, and W. S. Sampath, "Transient Ion Drift Measurements of 

Polycrystalline CdTe PV Devices", presented 2006 IEEE 4th World Conference on Photovoltaic 

Energy Conversion-4, (2006).

Enzenroth, R. A.T. Takamiya, K. L. Barth, and W. S. Sampath, "Photocapacitance Study of Deep 

Levels in Thin CdTe PV Devices", presented European Materials Research Society (EMRS) 

Spring meeting, Strasbourg France (2006) 

Barth, Kurt L., R. Albert Enzenroth and W. S. Sampath, "Brief update of progress at AVA 

Technologies LLC" National CdTe Team Meeting, Sponsored by the DOE through the National 

Renewable Energy Laboratory, Golden, CO, March 2006. Can be got from: 

http://www.nrel.gov/pv/thin_film/docs/cdte_team_march_2006.pdf

Enzenroth, R. A., K. L. Barth, and W. S. Sampath, " Observation of Copper Diffusion in CdTe 

PV Devices", National CdTe Team Meeting, Sponsored by the DOE through the National 

Renewable Energy Laboratory, Golden, CO, March 2006. Can be got from: 

http://www.nrel.gov/pv/thin_film/docs/cdte_team_march_2006.pdf

Sampath, et. al; "Advances in Continuous In-Line Processing of Stable CdS/CdTe Devices", 

33rd IEEE PV Specialists Conference, May 2008.

Sampath, et. al; "Continuous In-Line Processing of CdS/CdTe Solar Cells", Invited Talk at 

American Vacuum Society (AVS), Boston, October 2008.
32 

5.7.4 Other Presentations:

Barth, K. L, R. A. Enzenroth, and W. S. Sampath, "Advancing Thin Film CdTe PV 

Manufacturing", Thin Film PV Kickoff Meeting, GE Global Research Center, April 4, 2006.

W. S. Sampath, "Mass Production of CdTe Solar Photovoltaics" Presentation for the Northern 

Colorado chapter of IEEE January 17, 2007.

Sampath, W. S., Robert. A. Enzenroth and Kurt L. Barth, "Mass Production of Thin Film Solar 

Photovoltaics", Presentation, Discussion and Facility Tour for numerous groups including GE, 

Reliance Industries, NEA, Sequel, Murphy and Durier, Blackstone, Barings, local investment 

group, Primestar Solar, Nanosolar and many others. 
F1146-E(10/2008)

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1. REPORT DATE (DD-MM-YYYY)

March 2009

2. REPORT TYPE

Subcontract Report

3. DATES COVERED (From - To)

December 2004 - January 2009

4. TITLE AND SUBTITLE

Manufacturing Process Optimization to Improve Stability, Yield and 

Efficiency of CdS/CdTe PV Devices: Final Report, December 2004 -

January 2009

5a. CONTRACT NUMBER

DE-AC36-08-GO28308

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

6. AUTHOR(S)

W.S. Sampath, A. Enzenroth, and K. Barth

5d. PROJECT NUMBER

NREL/SR-520-45214

5e. TASK NUMBER

PVB91210

5f. WORK UNIT NUMBER

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

Colorado State University

Dept. of Mechanical Engineering

Fort Collins, Colorado 80523

8. PERFORMING ORGANIZATION

REPORT NUMBER

XXL-5-44205-02

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Renewable Energy Laboratory

1617 Cole Blvd.

Golden, CO 80401-3393

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NREL

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NREL/SR-520-45214

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13. SUPPLEMENTARY NOTES

NREL Technical Monitor: Harin S. Ullal

14. ABSTRACT (Maximum 200 Words)

The reproducibility of CdTe device efficiency and stability has been demonstrated during long-duration operation of 

the system with a single charge loading for the CdS, CdTe, and CdCl2 process. A fixture for automated cooling of 

the substrate in vacuum has been designed, fabricated, installed, and tested. Significant advances to the modeling 

of large-area vapor sources and modeling of gas transport in the system have been made. Efforts to improve device 

efficiency and advance device characterization are under way. A process for removing the coated films to reduce the 

amount of solid waste has been developed. 

15. SUBJECT TERMS

PV; CdTe; optimization; stability; yield; efficiency; devices; reproduciibility; modeling; cooling; characterization;

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