A national laboratory of the U.S. Department of Energy

Office of Energy Efficiency & Renewable Energy

National Renewable Energy Laboratory 

Innovation for Our Energy Future 

Manufacturing Process 

Optimization to Improve 

Stability, Yield, and Efficiency 

of CdS/CdTe PV Devices 

Phase II, Annual Technical Report 

January 2006 – February 2007 

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

Colorado State University 

Fort Collins, Colorado 

Subcontract Report 

NREL/SR-520-41664 

May 2007 

NREL is operated by Midwest Research Institute ● Battelle Contract No. DE-AC36-99-GO10337 
National Renewable Energy Laboratory

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Operated for the U.S. Department of Energy 

Office of Energy Efficiency and Renewable Energy 

by Midwest Research Institute • Battelle 

Contract No. DE-AC36-99-GO10337

Subcontract Report 

NREL/SR-520-41664 

May 2007 

Manufacturing Process 

Optimization to Improve 

Stability, Yield, and Efficiency 

of CdS/CdTe PV Devices 

Phase II, Annual Technical Report 

January 2006 – February 2007 

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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Table of Contents 

1. SUMMARY........................................................................................................................ 1 

2. INTRODUCTION............................................................................................................... 1

3. RESEARCH RESULTS .................................................................................................... 1 

 3.1 Cooling Substrates in Vacuum................................................................................. 1 

 3.2 Modification of the Seals.......................................................................................... 2 

 3.3 Large Area Devices ................................................................................................. 2 

 3.4 Advanced Encapsulation Method............................................................................. 2 

 3.5 Higher Efficiency...................................................................................................... 3 

 3.6 Device Characterization Studies:............................................................................. 4 

 3.7 Ellipsometry:............................................................................................................ 6

 3.8 Production Prototype Development: ....................................................................... 7 

 3.9 Design of Module:.................................................................................................... 7 

 3.10 Update on Outdoor Testing:..................................................................................... 7 

4. PHASE III PLANS ............................................................................................................. 8 

5. REFERENCES.................................................................................................................. 8 

6. APPENDICES ................................................................................................................... 9 

 6.1 Collaborative Studies and Other Activities: .............................................................. 9 

 6.2 Description of Interactions With Industry ................................................................10 

 6.3 Student Activities: ...................................................................................................10 

 6.4 Presentation to Senator Salazar: ............................................................................10 

 6.5 The Refereed Journal Publications that were Completed in this Phase 

 are Listed Below: ....................................................................................................11 

 6.6 The Conference Publications that were Completed in This Phase are 

 Listed Below: ..........................................................................................................11 

 6.7 Other Publications ..................................................................................................11 

List of Figures 

Figure 1: Jsc Voc scatter plot of devices with thinner CdS on three different substrates......... 4

Figure 2: Electroreflectance spectrum of fully processed CdS/CdTe PV cell. Colored 

 traces are fits to the ER data (black)........................................................................ 5 

Figure 3: Spectroscopic ellipsometry data and model fit for a LOF Tec 15 substrate with 

SnO:F/CdS/CdTe:Cu which had received CdCl2 treatment...................................... 6

Figure 4: Average efficiency versus time for groups of cells in outdoor test conditions ........... 8

iii
1. SUMMARY 

 Under Phase II of the Thin Film Partnership Program, we have made significant progress. 

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. Device 

efficiencies of 11.83 % have been made in "all forward" process. 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 large area devices. 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. Efforts to improve device efficiency and advance device characterization 

are underway. Desiccant containing materials for the edge seal and sealing materials for the 

holes in the back glass have subjected to damp heat tests at NREL with very promising results. 

Significant progress has been made in characterizing films with ellipsometry and the use of 

ellipsometry for quality control is being investigated. An additional patent was issued on this 

technology during the last phase. 

 A prototype production system to process 16.5x16.5 inch substrates continues to be 

advanced. This activity is supported by another program from DOE-EERE and cost shared by 

National Starch and Chemicals (NSC). An advanced encapsulation method has been tested by 

NSC with promising results. 

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. 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. 

Successful completion of the three phases of this project will be a significant step towards 

realizing the full potential of CdTe PV. 

3. RESEARCH RESULTS 

3.1. Cooling Substrates in Vacuum: 

 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 

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. 

1
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 accepted for publication to 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. 

3.2. 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 leading to pinholes in the films. 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. 

3.3. Large Area Devices: 

 Many large area devices in the form of 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 

show module manufacturing costs less than $1/Watt and have been presented at CdTe R&D 

Team Meetings. 

3.4 Advanced Encapsulation Method: 

 During the course of our interactions with NSC, we have expressed the need for simpler 

and more effective encapsulation methods to lower manufacturing costs. Based on our input and 

their past expertise, NSC has developed a promising approach for encapsulation. The new 

2
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 seal has been tested in the damp heat test (85/85) for 1300 

hours. A small quantity of moisture indicating material placed in the middle between the two 

glass plates has not changed color. This result is very promising and NSC has applied for a 

patent on this technology. 

3.5 Higher Efficiency: 

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

TEC 15 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 atmospheric pressure CVD coating with TO:F. 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. The use of barrier layer 

deposited by APCVD and other methods will be investigated. A number of studies on improved 

substrates have been performed. 

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. 

 Devices have been fabricated on the substrates from Sierratherm. 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. The results on 

devices processed with these substrates are shown in Figure 1 below. 

3
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

Figure 1: Jsc Voc scatter plot of devices with thinner CdS on three different substrates 

 To date, three different substrates have been investigated for higher efficiency. These are: 

TEC 15, Low Fe, and Low Fe with i-SnOx coating. A few substrates with intrinsic tin oxide 

barrier coating have been obtained from Chris Ferekides. Our current standard process produces 

Jsc of 20 mA/sq. cm. at ~800 mV. Figure 1 shows the current and voltage performance of 

devices fabricated on the different substrates. When the CdS thickness was reduced, an increase 

in Jsc was observed, but the Voc decreased. The open blue squares represent devices on the 

standard Pilkington glass which have slightly smaller CdS thickness compared to the standard. 

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. 

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

efficiencies. To aid in this effort, Xuanzhi Wu of NREL has supplied four 3.6x3.1 inch LOF 

TEC 15 substrates with a ZTO buffer layer. These substrates will be used for future efforts to 

increase device efficiency. Additionally, PPG Industries is providing advanced substrates and 

film characterization. Discussions have been held with Pillkington and PPG and both have 

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

3.6 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 

WCPEC 4 in Kona Hawaii. The WCPEC presentation was 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

4
diffusion in c-CdTe. The WCPEC 4 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. 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 in the 

upper half of the band gap in all devices surveyed. 

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 

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 2 where two critical points are shown 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

ΔR/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

ΔR/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 2: Electroreflectance spectrum of fully processed CdS/CdTe PV cell. Colored traces are fits 

to the ER data (black) 

5
3.7. Ellipsometry: 

Spectroscopic ellipsometry is being investigated as a quality control tool. 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. 

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. Kohli 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 3. The model has 14 layers and the mean squared error for this fit is 

24.22, which is considered to be a good fit. 

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°

Figure 3: Spectroscopic ellipsometry data and model fit for a LOF Tec 15 substrate with 

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

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. 

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 x 3 in. samples will 

also be performed to measure uniformity. 

6
Fred Seymour (currently with Primestar Solar) performed admittance spectroscopy 

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

CdCl2 treatment without Cu and with good CdCl2 treatment both with and without Cu. For poor 

CdCl2 treatment 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. 

3.8 Production Prototype Development: 

 A system for processing 16.5 x 16.5 inch substrates is been developed by our group. 

When completed and run in production mode, the throughput of this semiconductor processing 

system will be 2 MW/year. Recent advancements include the completion of the vacuum system 

and testing of the load lock substrate transport system and the installation and operation of two 

complete process stations. The design of the other process stations have been completed and are 

being fabricated. Thermal uniformity of +/- 1.5 o

C over the deposition area within the vapor 

source was measured for a temperature of 400 °C. The process stations are designed for 

continuous running for 40 hours or more without reloading. The system has been tested for a 

continuous 78 hour thermal run (without source charge). Many 16.5 x 16.5 inch substrates 

including tempered substrates have been transported with the load lock system and heated to 500 

oC in 2 minutes. 

3.9 Design of Module: 

 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 other specialized materials will be used for 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 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. 

3.10 Update on Outdoor Testing: 

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 4. It 

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

7
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. 

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

Figure 4: Average efficiency versus time for groups of cells in outdoor test conditions 

4. PHASE III PLANS 

 As in Phase I and II, we will advance the tasks outlined in the statement of work during 

Phase III. The focus of the efforts will be to further develop the understanding of stability, yield 

and efficiency of CdS/CdTe devices. We anticipate that further progress will be made towards 

processing 16.5x16.5 inch substrates into modules. We will continue our collaborative activities 

with other research groups and provide assistance to industries as in Phase I and Phase II. 

5. 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. 

8
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. 

6. APPENDICES 

6.1 Collaborative Studies and Other Activities: 

During this phase, the following collaborative activities were conducted: 

1) 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. 

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

for another application have occurred. 

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

his committee. 

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

meeting. 

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

at the IEEE PV Conference. 

6) 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 and researchers from PPG Industries. John Bleem from 

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

7) 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. 

8) 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. 

9) As described above, 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. 

10) 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. 

9
11) 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. 

12) Mani Manivannan from the Global Research Center of GE has joined our department 

and is continuing his research on CdTe photovoltaics. He visited NREL with Al 

Enzenroth from our group to discuss collaboration. 

6.2. Description of Interactions with Industry 

 Our CSU organization has a ongoing relationship with AVA Technologies LLC of Fort 

Collins to advance CdTe manufacturing. The members of AVA Technologies are also faculty at 

Colorado State University. AVA Technolgies has been awarded a DOE Inventions and 

Innovation (I&I) grant. A subcontract to CSU was part of this I & I grant. The close 

relationship that AVA Tech has with a major research university is very beneficial for both 

parties. This I & I grant then dovetails with the CSU activities. Recently AVA Technologies 

has been reorganized into AVA Solar Inc. 

 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, many samples were provided to NSC for metallization studies. 

 During the last year, a large corporation capable of very large scale manufacturing has 

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. 

6.3. Student Activities: 

 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 

completed his MS thesis and the objective of Tushar Shimpi's thesis was to develop an 

understanding of the hail impact test. Hershel Shelly is conducting his Ph. D. research on this 

project. His research is to study our process from an industrial engineering perspective. Chetan 

Malhothra of CU Boulder completed his Ph.D. His Ph.D dissertation was thermal and deposition 

modeling of our process. Marh Armijo has made significant progress towards completing his MS 

thesis and his study is to understand the mechanics of the composite module. 

6.4 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 von Roedern of NREL and 

many senior administrators from CSU, City of Fort Collins and Larimer County. 

10
6.5. The Refereed Journal Publications that were Completed in this Phase are 

Listed Below: 

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 in-line 

manufactured CdTe Thin Film Photovoltaic devices", Submitted to Journal of Solar Energy 

Engineering, Under review. 

V. Manivannan, Robert A. Enzenroth, Kurt L. Barth, Sandeep Kohli, Patrick McCurdy and W. S. 

Sampath, " Microstructural features of CdTe Photovoltaic thin film devices", Submitted to Thin 

Solid Films, Under review. 

6.6. The Conference Publications that were Completed in This Phase are Listed 

Below: 

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. 

http://www.nrel.gov/pv/thin_film/docs/cdte_team_march_2006.pdf 

6.7 Other Publications 

Barth, K, R. Enzenroth, and W. S. Sampath, "Advancing Thin Film CdTe PV Manufacturing", 

Thin Film PV Kickoff Meeting, Given to reputed members of industry, April 4, 2006. 

W. S. Sampath, "Mass Production of CdTe Solar Photovoltaics" Presentation for the Northern 

Colorado chapter of IEEE January 17, 2007. 

11
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 

Reliance Industries, NEA, Sequel, Murphy and Durier, Blackstone, Barings, local investment 

group, Primestar Solar, Nanosolar and many others. 

Barth, K. L, R. A. Enzenroth, and W. S. Sampath, US patent application, 20050158891 

Apparatus and processes for the mass production of photovoltaic modules. 

12
F1146-E(12/2004) 

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1. REPORT DATE (DD-MM-YYYY)

May 2007 

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Subcontractor Report 

3. DATES COVERED (From - To)

January 2006 – February 2007 

5a. CONTRACT NUMBER

DE-AC36-99-GO10337 

5b. GRANT NUMBER

4. TITLE AND SUBTITLE

Manufacturing Process Optimization to Improve Stability, Yield, and 

Efficiency of CdS/CdTe PV Devices: Phase II, Annual Technical 

Report, January 2006 – February 2007 

5c. PROGRAM ELEMENT NUMBER

5d. PROJECT NUMBER

NREL/SR-520-41664 

5e. TASK NUMBER

PVB75201 

6. AUTHOR(S)

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

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Colorado State University 

Materials Engineering Laboratory 

Fort Collins, Colorado 80523 

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NREL 

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National Renewable Energy Laboratory 

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14. ABSTRACT (Maximum 200 Words): This report describes a fixture for automated cooling of the substrate in vacuum that 

has been designed, fabricated, installed, and tested. This is needed for optimum processing of the back contact. Device 

efficiencies of 11.83% have been made in an "all forward" process. Large-area devices (~4 cm2

) in the form of strips 

were fabricated and tested at NREL. Efficiency of 10.9% were measured on these large-area devices. The air-tovacuum-to-air seals in the pilot system have a glass-filled Teflon liner. These liners have experienced wear, resulting in 

scraping the glass substrates in the seal. Improved liners were designed, fabricated, installed, and tested in the system. 

Efforts to improve device efficiency and advance device characterization are under way. Desiccant-containing materials 

for the edge seal and sealing materials for the holes in the back glass were subjected to damp-heat tests at NREL with 

very promising results. Significant progress was made in characterizing films with ellipsometry, and the use of 

ellipsometry for quality control is being investigated. An additional patent was issued on this technology during the last 

phase. 

15. SUBJECT TERMS

PV; optimum process; large-area; devices; back contact; glass substrate; ellipsometry; automated cooling; liner; dampheat testing; characterization; 

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