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

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

Contract No. DE-AC36-08GO28308

Process Development for High 

VOC CdTe Solar Cells

C.S. Ferekides and D.L. Morel 

University of South Florida

Tampa, Florida

Subcontract Report

NREL/SR-5200-51605

May 2011
NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

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

National Renewable Energy Laboratory

1617 Cole Boulevard

Golden, Colorado 80401

303-275-3000 • www.nrel.gov

Contract No. DE-AC36-08GO28308

Process Development for High 

VOC CdTe Solar Cells

C.S. Ferekides and D.L. Morel 

University of South Florida

Tampa, Florida

NREL Technical Monitor: Harin S. Ullal

Prepared under Subcontract No. XXL-5-44205-10

Subcontract Report

NREL/SR-5200-51605

May 2011
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

List of Figures ........................................................................................................................................... iv

List of Tables ............................................................................................................................................. vi

1 Introduction .......................................................................................................................................... 1

2 Open-Circuit Voltage (VOC) .................................................................................................................. 2

3 Summary of Fabrication Procedures – Experimental Methods ...................................................... 3

4 Front and Back Contact Materials ..................................................................................................... 5

4.1 Titanium Oxide (TiO2) ........................................................................................................5

4.1.1 Solar Cells with Titanium Oxide Buffer ....................................................................9

4.1.2 Solar Cells with Alloyed Titanium Oxide Buffers ..................................................11

4.2 Titanium Selenide (TiSe2) ................................................................................................. 12

4.2.1 Selenization of Ti Films ...........................................................................................12

4.3 Interfacial Dipole Layers ..................................................................................................13

4.3.1 Energy Band Diagram ..............................................................................................13

4.3.2 Interfacial Compounds & Their Application on CdTe Cells ...................................14

5 The Effect of CdTe Impurities on Solar Cell Performance ............................................................ 19

5.1 Phosphorous (P) Incorporation in the CdTe Source Material ...........................................19

5.2 The Effect of CSS(CdTe) Ambient ...................................................................................20

5.3 Antimony (Sb) as a Potential Dopant for CdTe ................................................................22

5.3.1The Need for the CdCl2 Heat Treatment (HT) ..........................................................23

5.3.2 The Effect of Excess “Surface-Sb” ..........................................................................24

5.3.3 The Effect of the Sb-diffusion Process Parameters .................................................25

6 Indium (In) Doped CdS ...................................................................................................................... 28

6.1 CBD-CdS Control Samples ..............................................................................................28

6.2 In Doping of CSS-CdS .....................................................................................................30

7 Measuring Collection in CdTe Cells ................................................................................................ 31

7.1 The Effect of Collection on Solar Cell Performance ........................................................32

Appendix – Polymers Used as IFLs in CdTe Cells ............................................................................... 35

References ................................................................................................................................................ 36
iv

List of Figures

Figure 1. SEM Images of TiO2 films deposited under different conditions: (a-top) reactive 

sputtering at room temperature; (b-middle) sputtering from TiO2 at 300°C; (c-bottom) 

reactive sputtering in O2/N2. NOTE: the left image magnification varies for the three

films; the right image magnification is the same (see scale at the bottom of each 

image). Credit: University of South Florida. ............................................................... 6

Figure 2. XRD spectra for TiO2 films prepared under different processing conditions. ... 7

Figure 3. Light J-V characteristics of a CdTe cell fabricated with TiO2 as a buffer layer 10

Figure 4. Monochromatic light J-V characteristics for a CdTe cell fabricated with a 

TiO2 buffer .................................................................................................................. 10

Figure 5. VOC of CdTe cells with Ti-Sn-O buffers. ......................................................... 11

Figure 6. Light J-V for CdTe cells fabricated with TiO2 buffers; (VOC’s of 880, 900 

and 950 mV; the 900 and 950 mV cells were fabricated with “alloyed” TiO2) ......... 12

Figure 7. XRD spectrum of a selenized Ti film, showing the presence of the TiSe2 

phase ........................................................................................................................... 13

Figure 8. Energy Band diagrams for metal/dipole/semiconductor interfaces showing 

a case where the orientation of the dipole results in an apparent increase in the 

metal work function (a), and a case where the dipole orientation results in a decreases 

in the metal work function (b). ................................................................................... 14

Figure 9. Increase in VOC vs. the type of interfacial polymer (SAMs). ........................... 16

Figure 10. Light I-V and SR data for CdTe cells contacted using polymers (SAMs) 

as interfacial contact layers; Mo/SAM/CdTe/CdS. .................................................... 18

Figure 11. VOC and FF for CdTe cells fabricated from a CdTe:P CSS source ................ 20

Figure 12. The effect of the CSS-CdTe deposition ambient on the VOC and FF of 

CdTe solar cells........................................................................................................... 20

Figure 13. Dark J-V characteristics (left: ln(J)-V; right: J-V) for representative cells 

fabricated under various N2/O2 mixtures (total pressure 10 torr) during the 

CSS-CdTe deposition.................................................................................................. 21

Figure 14. Light J-V characteristics of the CdTe cells of Fig. 11 .................................... 21

Figure 15. Sample arrangement used for the Sb diffusion experiments. Credit: University of 

South Florida ............................................................................................................... 23

Figure 16. Light J-V (left) and SR data (right) for representative CdTe;Sb cells from

table 9 .......................................................................................................................... 24
v 

Figure 17. Light (left) and dark (right) I-V for representative CdTe cells from 

table 10 ........................................................................................................................ 25

Figure 18. Light J-V for representative cells from table 11 ............................................. 26

Figure 19. Doping profiles (from C-V measurements) for the cells shown in Fig. 18 

(table 11) ..................................................................................................................... 26

Figure 20. The SR of CdTe;Sb cells fabricated with different Sb-diffusion 

temperatures ................................................................................................................ 26

Figure 21. Light J-V (left) and doping profile (right) of CdTe cells fabricated with and 

w/o Sb-diffusion.......................................................................................................... 27

Figure 22. Methods used to incorporate indium in CdS .................................................. 28

Figure 23. SR data for CdTe cells fabricated with CBD-CdS:In window layers (see 

table 12) ...................................................................................................................... 29

Figure 24. VOC and FF for cells with CSS-CdS deposited in the presence of oxygen .... 31

Figure 25. VOC and FF for cells with CSS-CdS deposited in inert ambient .................... 31

Figure 26. SR data for the cells of Figs 24 (left) and 25 (right) ...................................... 31

Figure 27. Monochromatic light J-V data for two “typical” CdTe solar cells. ................ 32

Figure 28. FF as a function of wavelength of the monochromatic light (data from 

Fig 27). ........................................................................................................................ 32

Figure 29. Collection functions for various wavelengths; the data for 640 and 700 nm 

are fitted with polynomials. ........................................................................................ 33

Figure 30. Collection functions for 640 and 700 nm and their effect on the light I-V 

of an otherwise “ideal” solar cell. ............................................................................... 34
vi 

List of Tables

Table 1. Summary of processes and materials utilized for the fabrication of CdTe solar 

cells ............................................................................................................................... 4

Table 2. XRD data for the TiO2 films shown in Fig. 2; “A” and “R” in the ID column 

identify the Anatase and Rutile phases of TiO2 respectively ........................................ 8

Table 3. The highest VOC (for the identified deposition conditions) obtained for 

CdTe/CdS cells fabricated on TiO2 buffer layers ......................................................... 9

Table 4. XRD data for the film with XRD spectrum shown below ................................. 13

Table 5. Polymers in Tetrahydrofuran as solvent ..................................................... 14

Table 6: Polymers in Ethanol as solvent ........................................................................... 15

Table 7: Procedures for polymer application and cell fabrication .................................... 15

Table 8. Net carrier concentration and depletion width for the cells shown in Figs 

13 &14 ........................................................................................................................ 22

Table 9. Process conditions and solar cell results for CdTe:Sb cells; the effect of the 

CdCl2 HT .................................................................................................................... 24

Table 10. Process conditions and performance data for CdTe:Sb cells; the effect of 

HCl etch ...................................................................................................................... 25

Table 11. Process conditions/solar cell results for CdTe:Sb cells; the effect of annealing 

time ............................................................................................................................. 26

Table 12. CdTe Solar Cells fabricated with CBD-CdS films as-deposited and doped 

with Indium ................................................................................................................. 29

Table 13. Effect of collection on the FF .......................................................................... 34
1 

1 Introduction

This is a cumulative and final report for Phases I, II and III of this NREL funded project 

(subcontract # XXL-5-44205-10)). The main activities of the project focused on the opencircuit voltage of the CdTe thin film solar cell

Quantifying the losses in the typical CdTe superstrate structure reveals that JSC has approached 

its practical limits, much more so than VOC and FF, which is the reason that recently the CdTe 

community has been turning its attention to VOC as the focus of efforts driven by higher 

efficiency goals [5].

. Although, CdTe continues to be one of the 

leading materials for large-scale cost-effective production of photovoltaics, the efficiency of the 

CdTe solar cell has been stagnant for the last few years. At the manufacturing front, the CdTe 

technology is fast paced and moving forward with US-based First Solar Inc. leading the world in 

CdTe module production, and poised to become the largest PV manufacturer in the world. In 

order to support industrial efforts and continue the advancement of this technology it will be 

necessary to continue improvements in solar cell efficiency. A closer look at the present stateof-the-art performance levels puts the three solar cell efficiency parameters, short-circuit current 

(JSC), open-circuit voltage (VOC), and fill factor (FF) in the 24-26 mA/cm2, 840-850 mV, and 74-

76% ranges respectively. During the late 90’s efforts to improve cell efficiency were primarily 

concerned with increasing JSC, simply by utilizing thinner CdS window layers in order to 

enhance the blue response (<510 nm) of the CdTe cell, which lead to the development of 

“buffers” (or high resistivity transparent films) [1-4]. The use of transparent bi-layers (lowρ/high-ρ), as the front contact is becoming a “standard” feature of the CdTe cell.

This report describes and summarizes the results from this 3-year project. 
2 

2 Open-Circuit Voltage (VOC) 

Improving VOC in the typical CdTe/CdS p-n heterojunction solar cell is not straightforward as a 

practical matter, due to several reasons. The greatest challenge faced by CdTe researchers is the 

fact that the CdTe cell components (and fabrication processes) have significant 

interdependencies and cannot be easily decoupled. Thus making both the fabrication and 

understanding of device operation for CdTe cells complex. For example, copper (Cu) was 

initially believed to be a key element for the formation of the back contact, until studies of 

impurity distribution in CdTe/CdS solar cells indicated that this element accumulates at the 

junction interface, where it also impacts the device performance/characteristics [6,7,8]. Work at 

USF that focused on incorporating Cu at the interface and not at the back contact, suggested that 

some Cu concentration at the junction interface (and in CdS) is required in order to achieve high 

performance [7,9]. Another example of the interdependence between cell components and 

processing is the effect of the CdCl2 heat treatment, or “activation” process (which is a standard 

fabrication step). This process is known to have three major effects on the CdTe solar cell: (a) 

CdTe grain enhancement; (b) enhanced interdiffusion between CdTe and CdS (that leads to the 

formation of a mixed Cd1-XSXTe crystal at the junction interface); (c) defect passivation/lifetime 

improvement via the formation of Cl-related complexes in CdTe. The extent to which these 

changes take place depends on the fabrication history/properties of the CdTe/CdS structure. 

Grain enhancement is predominant in small grain (<1 μm approx.) CdTe films, with large grain 

films (>1 μm approx.) undergoing little or no grain growth. Interdiffusion in CdTe/CdS 

junctions, during the activation process, is typically less when these are deposited at high 

temperatures (>500°C typ.). Therefore, the process of improving the VOC (and in general the 

performance) of CdTe cells is complicated by these interdependencies.

As already indicated above, the focus of this project is on the VOC of the CdTe solar cell. 

Improving this parameter will require new materials, device configurations, and possibly 

advanced fabrication techniques. Moving VOC beyond the present state-of-the-art 850 mV mark 

will require improvements/changes in one or more of the following: (a) recombination; it is 

believed that recombination is the dominant transport mechanism at the junction, and therefore 

reducing the recombination levels (i.e. reducing JO) could lead to higher VOC; (b) doping levels; 

the magnitude of VOC is related to the built-in potential of the junction, which is determined by 

the doping levels in the heterojunction partners. Therefore increasing the doping level (in 

particular in CdTe) could lead to higher VOC’s; (c) back contact barrier; it has been shown via 

several modeling efforts that the back contact energy can also affect VOC. Depending on the 

doping levels in CdTe, the band bending (and therefore VOC) in this layer can be determined by 

the energy of the back contact. Therefore large work function contact materials could also lead 

to improved VOC; (d) front contact/buffer: empirical evidence suggests that the front contact can 

also have a significant influence VOC, and therefore alternative front contact materials (buffers) 

could also play a key role in improving this device parameter. 

The main objective of this project was to develop materials/processes that will lead to the 

advancement of the open-circuit voltage of CdTe solar cells; the main areas investigated were:

1. impurities in the CdTe absorber: the focus in this case was on controlling the net hole 

concentration in CdTe, the ultimate objective being to increase the doping in this layer 

and therefore the built-in voltage (and VOC); 

2. high-work function back contact materials; 
3 

3. improved front contact: in this case the focus was on investigating new buffer layers, as 

well as studying the effect of impurities in CdS; 

4. back contact development; a new approach to improve the back contact of the CdTe cell 

by using interfacial dipoles was explored. 

3 Summary of Fabrication Procedures – Experimental Methods

The solar cell fabrication processes utilized during this project are summarized in Table 1. The 

close-spaced sublimation (CSS) was used for the deposition of all CdTe films. Although this 

process is well known for its high throughput (deposition rates over 1 μm/min are easily 

achievable), it is not very suitable for dopant incorporation; for this project (as it will be 

discussed later) an attempt was made to incorporate the dopant in the source material. Cadmium 

sulfide window layers were deposited by two methods: (a) chemical bath deposition (CBD), and 

(b) CSS. To-date no attempt has been made to incorporate a dopant during the CBD process 

(this is an option being considered for future activities); Indium was used as as a dopant for CSSCdS films.

As already discussed above, Cu is an important element used for the fabrication of CdTe solar 

cells. For certain tasks of this project Cu was eliminated from all solar cell fabrication steps, in 

order to decouple the effects of the various impurities being studied.

It should also be noted that during the early stages of this project the baseline TCO material (i.e. 

CVD-SnO2) was temporarily unavailable, due to the prohibitively high costs of the fluorine (F) 

doping source (Halocarbon 13B1 or bromotrifluormethane CBrF3). Instead sputtered indium-tin 

oxide (ITO) and SnO2 were used as the front contact bi-layer (ITO/SnO2). No significant effects 

on device performance were identified as a result of this change.

Solar cells were characterized using standard techniques for light and dark current-voltage (J-V), 

spectral response (SR), and capacitance-voltage (C-V) characteristics. Materials characterization 

including x-ray diffraction spectroscopy (XRD), energy dispersive x-ray spectroscopy (EDS), 

and scanning electron microscopy (SEM) were also used on an as-needed basis. Secondary Ion 

Mass Spectroscopy (SIMS) and Auger analysis are being carried out in collaboration with 

NREL.
4 

Table 1. Summary of processes and materials utilized for the fabrication of CdTe solar cells

Materials COMMENTS

Substr.

7059 

Borosilicate 

Glass

Borosilicate glass cleaned in dilute HF solution (1:10) and rinsed 

with DI water

Transparent 

Contact

SnO2:F by MOCVD (Tetramethyltin, O2, F-source: Halocarbon 13B1)

ITO by sputtering of In2O3:Sn @ TSUB=300°C, or co-sputtering of In2O3

and SnO2

Buffer Layer

(high-ρ)

SnO2 by MOCVD (as above; undoped)

SnO2 by sputtering of Sn (reactive) or SnO2 targets

TiO2 and Alloys by sputtering; from TiO2 or Ti (reactive sputtering)

CdS 

by: (a) Chemical Bath Deposition (CBD) and

(b) Close-spaced sublimation (CSS); (5N)

CdTe

CdTe (5N) by CSS

CdCl2

HT

CdCl2 (4N) Direct application of CdCl2 onto CdTe by evaporation followed by 

heat treatment

Back Contact

Polymers Used as interfacial layers (IFL) between CdTe and the metallic 

electrode

Graphite (a) doped with HgTe:Cu; (b) undoped (used as received)

Mo by RF sputtering
5 

4 Front and Back Contact Materials

Among the main tasks of this project is the study of new materials for both the back and front 

contacts of the CdTe cell. Titanium dioxide (TiO2) – a material previously considered by the 

CdTe national Team – was studied and incorporated as a buffer in CdTe cells; titanium selenide 

(TiSe2), a large work function material [10,11], was investigated as a back contact candidate. 

This section provides a description of the processing and material properties of these two 

materials.

4.1 Titanium Oxide (TiO2) 

Sputtering of a ceramic TiO2 target in argon (Ar), and reactive sputtering of a metallic Ti target 

(in Ar/O2 ambient) were used for the deposition of TiO2 films. The substrate temperature was 

varied from RT to 300°C. In some cases N2 was also added to the gas mixture. The electrical 

resistivity for all TiO2 films was too high to measure with conventional (4-point probe) means. 

The film structural properties were studied using XRD and SEM. Figure 1 shows representative 

SEM images for films deposited using: 

a) reactive sputtering from a Ti target (top);

b) sputtering from a TiO2 target (middle); 

c) sputtering in the presence of N2 (bottom).

In all instances the grain size ranges from 100-200 nm; however, the “grains” in films (a) and (b) 

appear to be clusters of smaller particles, unlike film (c) where the grains appear to be better 

developed; also film (c) appears to be relatively more porous. 

X-ray diffraction analysis of TiO2 films revealed that reactively-sputtered as-deposited at room 

temperature films (similar to film (a) in Fig.1), exhibited poor crystallinity – dark blue data in 

Fig. 2; the same was found to be true for films deposited at higher substrate temperatures from 

TiO2 (similar to film (b) in Fig. 1) not shown in Fig. 2. The cystallinity of these films appears to 

improve when they are heat-treated at high temperatures (over 600°C) in air – green and purple 

data in Fig. 2. The remaining film in Fig. 2 (orange data) was deposited with N2 in the sputtering 

ambient at a substrate temperature of 250°C. Table 2 lists the peaks identified for the various 

films in Fig. 2. In all cases the films appeared to contain both TiO2 phases (i.e. anatase and 

routile).
6 

(b) by sputtering from TiO2 (in Ar) at 300oC

(c) by reactive sputtering (O2/N2) at 250oC

(a) by reactive sputtering (Ti in O2) at room temperature

Figure 1. SEM Images of TiO2 films deposited under different conditions: 

(a-top) reactive sputtering at room temperature; (b-middle) sputtering 

from TiO2 at 300°C; (c-bottom) reactive sputtering in O2/N2. NOTE: the left 

image magnification varies for the three films; the right image 

magnification is the same (see scale at the bottom of each image). 

Credit: University of South Florida.
7 

0

50

100

150

200

250

20 30 40 50 60 70 80

2 Theta

Intensity 

GLASS RT/650C Air RT/650C Air

250C in N2 RT

Figure 2. XRD spectra for TiO2 films prepared under different processing 

conditions. 
8 

Table 2. XRD data for the TiO2 films shown in Fig. 2; “A” and “R” in the ID column identify the 

Anatase and Rutile phases of TiO2 respectively

Sample 2 Theta [°] d-spacing [Å] FWHM Rel. Int. [%] ID

TDEP=RT 25.329 3.5164 0.2362 89.65 (101) A

Ann. 650°C/Air 27.422 3.2526 0.1181 100 (110) R

(green data) 48.179 1.8888 0.3149 38.26 (200) A

54.593 1.6797 2.304 6.49 (211) A

TDEP=RT 25.346 3.5141 0.1574 100 (101) A

Ann. 650°C/Air 27.343 3.2617 0.4723 45.44 (110) R

(purple data) 48.105 1.8915 0.2362 43.45 (200) A

55.18 1.6632 0.576 24.81 (211) A

TDEP=250°C (O2&N2) 25.626 3.4763 0.1181 100 (101) A

(orange data) 38.908 2.3148 0.4723 33.66 (200) R
9 

4.1.1 Solar Cells with Titanium Oxide Buffer

Titanium oxide films were incorporated in solar cell structures as buffers:

SnO2:F/TiO2/CdS/CdTe/Back contact. 

The cell processing characteristics for the devices fabricated with TiO2 buffers were:

• Front contact: 

o CVD - SnO2:F

o

• CdS: by Chemical bath deposition (thickness: 90-100 nm) 

TiO2 by sputtering (process variations listed in table below)

• CdTe: by the close-spaced sublimation (thickness: 5-6 μm) 

• CdCl2-heat treatment @ 390°C 

• Back Contact:

o Doped graphite annealed @ 250°C†

Table 3 lists the highest VOC values obtained for cells fabricated using TiO2 as a buffer. In one of 

these instances where the cells were fabricated on TiO2 sputtered in the presence of N2 a VOC of 

880 mV was measured. Nevertheless, the overall cell behavior for these devices was very poor, 

and results are notably noisy with significant data scattering in particular of the VOC. 

Table 3. The highest VOC (for the identified deposition conditions) obtained for CdTe/CdS cells 

fabricated on TiO2 buffer layers

TiO2 Deposition Conditions VOC

[mV] 

FF Range

Ar [mT] N [%] 2 [mT] O2 [mT] T [°C] Thickness 

[nm] 

4.5 1.5 - 180 30 880 5-25

4.5 1.5 - 180 10 840 8-27

4.5 1.5 - 180 20 790 13-22

1 - 4 250 30 780 10-12

- - 5 250 30 820 7-9 

 

† These contacts contain Cu
10 

The light J-V characteristics of all TiO2-

based solar cells were very similar, with 

FF’s always being less than 25%; an 

example is shown in Fig. 3. This J-V 

behavior suggests strong collection in 

reverse bias (JL is in the range of 22-23 

mA/cm2

); however, JL collapses near zero

volts leading to the poor FF and poor 

overall cell performance. This is believed 

to be due to the high resistivty of TiO2. If 

the cause for this collapse in collection is 

better understood it is possible that TiO2

buffers can provide a viable option for 

fabricating higher VOC CdTe solar cells. 

Carrier collection in TiO2-based solar 

cells was evaluated using 

“monochromatic” J-V measurements, 

where the complete J-V characteristics of 

solar cells were measured using 

interference filters (with 20 nm 

bandwidth), with the intensity of the light 

source adjusted to correspond to AM1.5 

(within the specific wavelength range)=

Although, based on the overall solar cell 

results measured to date, some general 

trends were observed (i.e. as a function of 

TiO2 processing conditions), the spread of 

the results (both VOC and FF) was in most cases very large and no conclusions could be drawn on 

how variations in the properties of the TiO2 films influence cell performance. This behavior 

points to the possibility that (non-uniform) micro-diode effects play a critical role in the observed 

solar cell behavior. Another common behavior observed during this study is that within the 

same substrate (i.e. identically processed devices) increases in VOC were accompanied with 

decreases in the FF.

. 

Results from this measurement, for one of 

the samples that exhibited VOC near 900 

mV, are shown in Fig. 4. Qualitatively, 

there appear to be no significant changes 

in the overall shape of the light J-V, 

suggesting that collection within CdTe is 

to first order independent of bias and 

wavelength. 

 

= A discussion on using monochromatic J-V measurements to estimate efficiency losses due to inefficient collection 

is included later in this report.

-0.040

-0.020

0.000

0.020

0.040

-2.00 -1.50 -1.00 -0.50 0.00 0.50 1.00 1.50

Voltage [Volts]

Current Density [A/cm2]

12-12a-14

Figure 3. Light J-V characteristics of a CdTe cell 

fabricated with TiO2 as a buffer layer

-1.4E-03

-1.2E-03

-1.0E-03

-8.0E-04

-6.0E-04

-4.0E-04

-2.0E-04

0.0E+00

2.0E-04

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0

Voltage [Volts]

Current Density [A/cm2] 

460 520

700 800

Figure 4. Monochromatic light J-V characteristics 

for a CdTe cell fabricated with a TiO2 buffer 
11 

4.1.2 Solar Cells with Alloyed Titanium Oxide Buffers

As indicated in the previous section the high resistivity of TiO2 is suspected to be one of the 

reasons for the poor J-V characteristics associated with devices with TiO2 buffers. During the 

latter parts of Phase II, TiO2 films were revisited in order to determine whether their properties 

(specifically resistivity) could be controlled with alloying. To this date, TiO2 films have been 

alloyed with other commonly used 

transparent oxides (ZnO and SnO2).

Film resistivity has not been affected by 

alloying (films remain too resistive to 

measure with a 4-point probe); nevertheless, 

alloying appears to have an effect on the VOC

of the device, with the overall J-V shape 

remaining unchanged (same as Fig. 3). The 

data shown in Fig. 5 show the VOC as a 

function of the amount of Sn in the Ti-Sn-O 

alloys; the values corresponding to 

Sn/(Sn+Ti) of 20% have exhibited VOC’s in 

the 900-950 mV range. It has been 

mentioned previously that there is significant 

scattering in the VOC of cells with TiO2

buffers; this remains the case for the results 

shown in Fig. 5; however, it should be noted 

that the data is from 3 different experiments, 

and in all instances the high VOC substrates 

(i.e. samples with Sn/(Sn+Ti)=20%) yielded 

cells with VOC of at least 900 mV, suggesting that the alloying effects can be reasonably 

reproduced. Figure 6 compares J-V characteristics from high VOC cells fabricated with TiO2-

based buffers (includes TiO2 alloyed with SnO2). The arrows in the figure mark the shift in the 

J-V curves as VOC increases. As mentioned previously one of the observed trends in these 

devices was a decrease in the FF with increasing VOC. Alloying TiO2 with ZnO has resulted in 

lower VOCs independent of the Zn/(Zn+Ti) ratio. 

Although, much remains to be understood about the role of buffers, in particular TiO2-based films, it 

appears that they do “influence” the VOC of CdTe cells; therefore buffers should remain as one of the 

viable methods to increase this solar cell parameter. The challenge still remains identifying the important 

buffer properties, and “tuning” them for optimum solar cell (VOC) performance.

600

650

700

750

800

850

900

950

1000

20% 40% 60% 90%

Sn/(Sn+Ti)

VOC [mV]

Figure 5. VOC of CdTe cells with Ti-Sn-O buffers. 
12 

4.2 Titanium Selenide (TiSe2) 

As part of the task that addresses the characteristics of the back contact of the CdTe solar cell, 

this project focused on the investigation of a promising class of materials, layered compound 

metals, which combine high chemical inertness with high work function and metallic 

conductivity. These properties make these materials good candidates for the back contact 

electrode. Chemical inertness is desired to limit chemical reactions at the interface with CdTe, 

and high work function is required to enable the formation of ohmic contacts (the CdTe layers 

are typically p-type, i.e. a hole injecting contact is needed). Potential layered compound 

candidates are the selenides TiSe2, VSe2, NbSe2 and TaSe2, as well as their corresponding 

sulfides. 

4.2.1 Selenization of Ti Films

Based on the existing deposition facilities the decision was made to investigate TiSe2 films; early 

efforts focused on preparing this compound by selenizing Ti. Metallic Ti films were sputterdeposited on glass slides to a thickness of 200-400 Å, and subsequently exposed to a Se flux 

under high vacuum conditions; the selenization process was carried out in a CIGS chamber 

which contained excess Se. It was found that the formation of TiSe2 by this method depends on 

the substrate temperature. For temperatures below 400°C no TiSe2 was detected. The Ti films 

appeared (visual inspection) to change color at temperatures around 400°C. At selenization 

temperatures of 425°C the TiSe2 phase was detected; this can be seen in Fig. 7 where the xrd 

spectrum of a Ti selenized film clearly shows diffraction peaks that have been found to 

correspond to TiSe2. Table 4 lists the identified peaks and compares them to pdf data. 

While Ti selenization resulted in TiSe2 films it was not possible to prepare this compound on 

CdTe with this approach. Efforts to selenize Ti films deposited directly on CdTe have to-date 

failed due to the fact that the selenization conditions (elevated substrate temperature/high 

vacuum) cause the partial evaporation of CdTe. 

-2.5E-02

-2.0E-02

-1.5E-02

-1.0E-02

-5.0E-03

0.0E+00

5.0E-03

-2.0 -1.0 0.0 1.0 2.0

Voltage [V]

Current [A]

880 mV

900 mV

950 mV

-2.0E-05

-1.5E-05

-1.0E-05

-5.0E-06

0.0E+00

5.0E-06

1.0E-05

0.6 0.7 0.8 0.9 1.0 1.1

Voltage [V]

Current [A]

880 mV

900 mV

950 mV

Figure 6. Light J-V for CdTe cells fabricated with TiO2 buffers; (VOC’s of 880, 900 and 950 

mV; the 900 and 950 mV cells were fabricated with “alloyed” TiO2) 
13 

4.3 Interfacial Dipole Layers

During the last phase of this project a novel method of forming back contacts to CdTe solar cells 

was investigated. The approach has been used in organic devices and has shown promise in 

modifying the work function of metals and transparent contacts. Using self-assembled monolayers (SAMs) or layers of dipole polymers, the work function of silver was modified from 3.8 to 

5.5 eV using alkanethiols[16]. For this project we collaborated with Dr. Peter Zhang of the 

Department of Chemistry at USF; Dr. Peter Zhang is an expert in the synthesis of organic 

compounds, and his group synthesized all compounds used for this part of the project. 

4.3.1 Energy Band Diagram

The use of an interfacial dipole can be used to improve the contact characteristics by modifying 

the band alignment. Figure 8 shows the two scenarios where a metal comes in contact with a ptype semiconductor via an interfacial dipole. For the case depicted on the left, the polarity of the 

dipole is in a direction that results in an “effective metal work function” higher than what the 

actual work function of the metal, and on the right the situation is reversed (i.e. the “effective 

work function” is lower). For CdTe solar cells the scenario depicted on the left would be 

beneficial since it would help improve the p-type contact by effectively increasing the metal 

work function.

Table 4. XRD data for the film with XRD spectrum shown below

This work PDF 01-083-0980 – 

TiSe2

2θ

[o]

d-spacing

[Å]

2 θ

[o]

d-spacing

[Å]

%

Difference

14.646 6.0483 14.742 6.004 0.7%

29.6473 3.0133 23.138 3.06227 1.6%

Figure 7. XRD spectrum of a selenized Ti film, showing the 

presence of the TiSe2 phase
14 

4.3.2 Interfacial Compounds & Their Application on CdTe Cells

Several organic compounds were synthesized and dissolved in two different solvents 

(tetrahydrofuran and some in ethanol). The intent was to compare compounds that would result 

in different polarity (i.e. would tend to favor one of the two cases considered in Fig. 8) and also 

have different dipole strengths. The table below shows lists the various organic compounds 

synthesized for this study. 

Table 5. Polymers in Tetrahydrofuran as solvent

Sample# Polymer 

name

Chemical 

Formula

Concentration

[mol/L]

Polymer 

Structure

Relative 

dipole 

strength

1 IV-283-1B C69H78N4O2 0.005 Electronegative

2 V-100-1 C38H22Br4N4O2 0.005 Electronegative

Figure 8. Energy Band diagrams for metal/dipole/semiconductor interfaces showing a case where 

the orientation of the dipole results in an apparent increase in the metal work function (a), and a 

case where the dipole orientation results in a decreases in the metal work function (b).
15 

Table 6: Polymers in Ethanol as solvent

Sample# Polymer 

name

Polymer 

chemical 

formula

Concentration

[mol/L]

Polymer 

structure

Relative 

dipole 

strength

1 IV-284-1A

IV-284-1B

IV-284-1C

C8H5NO2 200

100

50

Electropositive

2 IV-212A C7H5ClO2 0.2 Electropositive

3 p-Anisic 

acid

C8H8O3 0.05 Electronegative

The above-described compounds were spin-coated onto the surface of CdTe (which had been 

previously CdCl2 treated, rinsed and etched to remove surface impurities and oxides) prior to 

depositing Mo by sputtering. The application process was developed in-house empirically using 

guidance from literature. Speed, acceleration, total time, a 2-step vs. 1-step spin coating process, 

and the amount of material dispensed were optimized to result in uniform coatings. A total time 

of 50-60 seconds and a speed of 1500 rpm, were found to be the most critical spin coating 

parameters to produce uniformly spread 1 drop of solution and dry the solvent. Following the 

application of the polymers, the samples were loaded into the vacuum chamber for the deposition 

of Mo; prior to the Mo-deposition the samples were heated at 100°C for 15 minutes to remove all 

remaining solvent. The table listed below includes key process paramters for cell fabrication and 

polymer application: 

Table 7: Procedures for polymer application and cell fabrication 

Cell Fabrication Polymer Spin Coating

1 SnO2; MOCVD bi-layer rpm=1500

2 CdS; CBD Acceleration=3

3 CdTe; CSS (4-5 μm) Time=1 min

4 CdCl2 HT Solution volume=1 drop

5 Rinse and etch (in Br2/methanol soln)

The first series of experiments were performed on samples fabricated as described in table 7 (no 

Cu intentionally added at any step of the cell fabrication process). Figure 9 shows the change in 

VOC from a reference sample (i.e. no polymer used); it should be noted that all VOCs for these 

cells were very poor (300-550 mV). The two reasons for the low VOCs are believed to be the 

absence of Cu, and the potential presence of pinholes in the CBD CdS films; the CdS-CBD 

 

 Spin coater setting (1-5)
16 

process is being re-evaluated as it appears to be generating a large amount of powder in solution, 

something that was not present in the past. The result of this is thinner films and increased 

pinhole concentration. 

Figure 9. Increase in VOC vs. the type of interfacial polymer (SAMs).
17 

Based on the results shown in Fig. 9 some general trends have been observed; note that the 

second to last number in the sample IDs shown in Fig. 9 designate the number of drops 

dispensed (i.e. ...2d, ...4d, ...8d) during the application and is therefore expected to produce a 

thicker polymer film.

1. Compounds 283-1B and V-100-1 showed consistent and the largest increase in terms of 

VOC (approx. a 52% increased for sample 283-1B 8d_1). Qualitatively, these were 

expected to yield higher VOCs based on the potential strength and alignment of the dipole 

formed. 

2. The use of a larger number of drops produced higher VOCs. This behavior may be due to 

more complete coverage of the surface of CdTe with the polymer films, or an increased 

thickness of the polymer. 

3. Increasing the amount of solution used resulted in more uniform cell-to-cell results on the 

same substrate; this may also suggest complete and uniform coverage as mentioned in 

step 2. 

Light I-V and SR data for a set of samples are shown in Fig. 10 below; the VOC as indicated 

above is much lower than the 850 mV state-of-the-art values (in the 500 mV range); a roll over 

in the first quadrant suggests a barrier is present at the back contact; no significant shunting is 

present, and the series resistance is relatively high due to the fact that these are not isolated dot 

cells and therefore the TCO adds a substantial component to the series resistance. The current 

densities from the light I-V data suggest JSC’s well above 21-23 mA/cm2 (cell areas are 0.1 cm2); 

the SR data, which is taken at low light intensities, shows a substantial loss in JSC and this is an 

issue that must be resolved in the future.
18 

Although, some interesting trends were observed with the use of polymers to modify the work 

function of Mo, this work remains for all practical purposes very preliminary; many process 

parameters appear to be critical in studying the effect of polymer interfacial layers; the most 

critical ones are believed to be the condition of the CdTe surface, the method of application of 

the polymers, and the polymer properties. 

Subsequent experiments included Cu during the cell fabrication process (CuTeX was deposited 

by sputtering on the CdTe surface following the Br-methanol etch step); although VOCs increased 

substantially (low 700’s mV) the use of Cu appeared to dominate cell performance “masking” 

the effect of the polymers. It is believed that additional work on this approach will be valuable 

and could produce improvements in the contact formation process. Such an effort should be 

accompanied with surface/interface analysis techniques to characterize the CdTe/polymer/metal 

interfaces.

Figure 10. Light I-V and SR data for CdTe cells contacted using polymers (SAMs) as interfacial contact 

layers; Mo/SAM/CdTe/CdS. 
19 

5 The Effect of CdTe Impurities on Solar Cell Performance 

A considerable effort of this project is dedicated to incorporating impurities in CdTe in order to 

increase its net doping (hole) concentration and therefore the built-in junction potential and VOC

of the solar cells. Several approaches have been considered in order to affect the doping 

characteristics of CdTe; these included: (a) incorporating phosphorous (P) in the CSS CdTe 

source material; (b) varying the gas ambient during the CSS-CdTe deposition (relative amounts 

of O2 and N2); (c) diffusion of antimony (Sb) into CdTe following the CdTe deposition by CSS. 

The following sections summarize the results obtained for each of these cases.

5.1 Phosphorous (P) Incorporation in the CdTe Source Material

This series of experiments was one of the first attempts (at USF) to dope CdTe during the CSS 

deposition. It must be noted that the CdTe films used for the fabrication of high efficiency cells 

(at USF) are deposited by CSS in the presence of oxygen (mixture of He and O2); one of the 

effects of the use of O2 during the CSS deposition is more compact grains due to an apparent 

increase in the nucleation sites; in addition, O2 is often found to enhance the p-type 

characteristics of CdTe.

In order to add P to the CdTe source material, powders of CdTe and Cd2P3 were mixed and 

sealed in an evacuated quartz ampoule. The relative amounts of the two powders corresponded 

to approximately 5% at. of P. The quartz ampoule was subsequently heated to temperatures up 

to 700°C. After approx. 48 hrs at this temperature the mixed powder was removed from the 

ampoule, pulverized and remixed. This powder (to be referred to as P-doped from this point 

forward) was then used to deposit “CdTe:P” films by CSS (all CSS depositions described in this 

section were carried out in an inert ambient - 100% He). The first few depositions resulted in 

unusually non-uniform films; the reason for the observed non-uniformities is believed to be due 

to inhomogenieties in the P-doped powder. In order to improve film uniformity the P-doped 

powder was used to deposit a thick CdTe film (approx. 300 μm) onto a glass substrate. This 

CdTe:P coated glass slide was then used as the source material for the CSS process; prior to 

using this source material for cell fabrication it was analyzed using EDS. The analysis indicated 

that 3% at. P was present, which was in relatively good agreement with the amount of P initially 

introduced in the CdTe powder (5% at.). 

A series of CdTe films were deposited onto CdS/SnO2(bilayer)/glass substrates for solar cell 

fabrication (CSS conditions: He-ambient; TSUB=550°C; TSRC=630°C). In order to decouple the 

effect of the impurities incorporated in the CdTe:P films, some cells were completed without the 

use of the CdCl2 heat treatment, and all cells were fabricated with Mo back contacts (i.e. no Cu 

was used for the fabrication of the back contact). 

The characteristics of the completed devices were in general very poor bringing into question the 

quality of the mixed powders; Figure 11 shows the VOC and FF as a function of the processing 

conditions. The “best” devices from this group of cells were the ones fabricated using optimum 

(390°C) CdCl2 conditions clearly indicating that the most critical processing step in this case is 

the CdCl2 treatment, and therefore at this time it appears that “P-doping” is not having a 

beneficial effect on the device characteristics. EDS analysis of one of the CdTe:P films indicated 

that no P was present. This result does not necessarily suggest that P was completely absent in 

these films; it is possible that the amount of P incorporated in the film was well below the 

detection limits of this technique. The tentative conclusion at this time is that P does not 
20 

transport at the same rate as CdTe during the CSS deposition process, and the CdTe source may 

be depleted of P after the first few depositions (possibly by the second or third deposition). 

Resolving this issue and determining whether P can be incorporated in CSS-CdTe films used for 

cell fabrication will require further investigation. A series of CdTe:P films has been forwarded 

to NREL for SIMS analysis. 

5.2 The Effect of CSS(CdTe) Ambient

As indicated previously, the CdTe films utilized for the fabrication of high efficiency cells are 

deposited in an O2-containing ambient (He/O2). During the first year of this project the effect of 

a N2-O2 CSS ambient on the characteristics of CdTe cells was studied. All devices discussed 

here were fabricated using baseline fabrication conditions (CBD-CdS; Cu-doped graphite back 

contact; CdCl2 heat treatment). The only intentional process variation was the composition of 

the gas mixture during the CSS-CdTe 

deposition: The total pressure was fixed at 

10 torr, and the relative amounts of N2 and 

O2 were adjusted to result in the following 

N2/O2 ratios: 9/1, 7/3, 5/5, and 1/9. The 

performance of these devices (VOC and FF) 

is shown in Fig. 12 (minimum of 3 cells per 

condition). These results show a clear trend 

of improving VOC’s and FF with increasing 

O2 (decreasing N2) partial pressure.

Dark J-V characteristics for representative 

devices are shown in Fig. 13. The Ln(J)-V 

shown on the left suggest that the device 

fabricated with CdTe with the lowest O2

partial pressure has a higher dark current 

(JO), which partially accounts for the lower 

VOC and FF for the cells fabricated with a 

N2/O2 ratio of 9/1. However, the other three 

0

100

200

300

400

500

600

700

No CdCl2 CdCl2 @

390 ̊C

CdCl2 @

410 ̊C

Annealing Conditions

VOC [mV] .

20

25

30

35

40

45

50

No CdCl2 CdCl2 @

390 ̊C

CdCl2 @

410 ̊C

Annealing Conditions

FF

Figure 11. VOC and FF for CdTe cells fabricated from a CdTe:P CSS source

750

775

800

825

850

875

900

9/1 7/3 5/5 1/9

N2/O2

VOC [mV]

0.50

0.60

0.70

0.80

FF

VOC FF

Figure 12. The effect of the CSS-CdTe deposition 

ambient on the VOC and FF of CdTe solar cells
21 

cells seem to have identical dark currents especially in the range of 0.5-0.75 volts (i.e. near VOC). 

The dotted line in the same graph marks the JSC magnitude, and assuming superposition it should 

cross the dark J-V at VOC. Although series resistance effects interfere with the dark J-V 

characteristics around VOC, all four J-V characteristics seem to be converging around the same 

value of VOC. 

The linear dark J-V shown in Fig. 13 (right) suggest that increasing amounts of O2 during the 

CdTe growth lead to a considerable decrease in the forward dark currents beyond VOC (the turnon voltage of the J-V curve “shifts” to higher voltages). This behavior does not transfer to the 

light characteristics (i.e. no crossover), and it 

has been previously attributed to the 

properties of the CdS films 

(photoconductivity) and not the properties of 

CdTe. It is therefore possible that the O2

used during the deposition of CdTe has an 

effect on the CdS, since the CdS is exposed 

to this ambient for approximately 2 minutes 

prior to the start of the CdTe deposition. 

The best performers in this group are cells 

with the largest shift in their dark J-V (i.e. 

lower forward current at voltages above 1.0 

volt). This result seems to point that the 

observed device performance variations are 

due to the influence of the CdS and not 

CdTe, and seems to support a device model 

where CdS has insulating properties and the 

device behaves as an MIS structure [14].

The light J-V characteristics for the same 

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

0.50 0.75 1.00

Voltage [V]

Current Density [A/cm2] 

9/1 3/7 5/5 1/9

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

-1.00 -0.50 0.00 0.50 1.00 1.50

Voltage [V]

Current Density [A/cm2] 

9/1 3/7 5/5 1/9

Figure 13. Dark J-V characteristics (left: ln(J)-V; right: J-V) for representative cells 

fabricated under various N2/O2 mixtures (total pressure 10 torr) during the CSS-CdTe 

deposition

-0.025

-0.020

-0.015

-0.010

-0.005

0.000

0.005

0.010

-0.20 0.05 0.30 0.55 0.80

Voltage [Volts]

Current Density [A/cm2] 

9/1 7/3 5/5 1/9

Figure 14. Light J-V characteristics of the CdTe 

cells of Fig. 11
22 

cells are shown in Fig. 14. These suggest that the lower FF (and VOC), for the device fabricated 

with a N2/O2 ratio of 9/1, is due to softening of the J-V curve around the maximum power point 

as a result of collection losses, and not due to shunting (shunt resistances calculated at a reverse 

bias of 1.5-2.0 Volts were very similar for all devices). This suggests that the use of O2 during 

the CSS-deposition of CdTe affects the strength of the field in this layer (and therefore 

collection); at this time it is not clear whether the collection in CdTe is affected by Oincorporation in CdTe, or by the properties of the window layer (CdS), which as indicated above 

appears to be affected by the use of O2 during the CdTe deposition. 

The same devices (shown in Figs. 13 & 14) were characterized using C-V measurements. The 

values for the net hole concentration (NA-ND) and the depletion width were calculated from the 

linear portion of the [1/C2 vs. V] graph and the capacitance at 0V bias respectively, and are listed 

in Table 8. All values for carrier concentration are well within the range typically observed for 

CdTe devices (i.e. low 1014

cm

-2). Nevertheless, there appears to be a slight (but consistent) 

increase in the net hole concentration as the O2 partial pressure. As previously described (see 

Fig. 12) the VOC also increased with O2 partial pressure, by approximately 40-50 mV. To first 

order it appears that there is a correlation between the net hole concentration and VOC; this 

evidence supports the claim that oxygen acts as a p-type dopant in CdTe. However, the increase 

in doping level is also accompanied by a decrease in the depletion width, which would lead to 

poorer collection (and therefore lower FF), which is not the case for these cells.

Even though a small increase in carrier concentration is observed, it seems that a “saturation” 

level is reached for this device property. This is most likely associated with the nature of CdTe, 

being a II-VI semiconductor, and the fact that self-compensation is often the limiting factor for 

achieving high doping levels in these materials. It is also evident that the effect of the CSS 

ambient (oxygen in particular) is rather complex, and based on these results it could be 

concluded that both the absorber (CdTe) and window layer (CdS) are affected by the presence of 

O2 during the CdTe growth. The results clearly suggest that O2 is beneficial to the CdTe cell, but 

fully understanding the role of O2 will require additional studies.

Table 8. Net carrier concentration and depletion width for the cells shown in Figs 13 &14 

N2/O2

9/1 7/3 5/5 1/9

NA-ND [cm-3] 1.98 x 1014 2.77 x 1014 3.46 x 1014 3.4 x 1014

WD @ 0 Volts [μm] 3.03 1.82 1.98 2.03

5.3 Antimony (Sb) as a Potential Dopant for CdTe

Antimony (Sb) is another group V impurity, which was considered as a potential p-type dopant 

for CdTe. At this time it should also be noted that Sb2Te3 has been used as a back contact 

material to CdTe solar cells, and therefore it may play a dual role in CdTe i.e. as a dopant and 

also as a back contact electrode, if the processing conditions lead to the formation of the Sb2Te3

compound on the surface of CdTe. Following the results obtained with P, it was decided to 

attempt to incorporate Sb in CdTe via a post-deposition method, rather than during the CSS 

deposition process. Following the CSS deposition of CdTe, a thin film of Sb (20-30 nm) was 

deposited onto CdTe by sputtering. The structures were subsequently heat-treated in inert 

ambient, in order to cause Sb to diffuse into the CdTe film; annealing temperatures varied from 

300 to 525°C, and annealing times from 20 to 160 mins. Some of the early experiments 
23 

suggested that Sb was evaporating from the CdTe surface; in order to limit Sb loss during the 

heat treatment, an Sb-coated glass slide (with 200-300 nm of Sb) was placed on top of the CdTe 

for the remainder of the experiments (see Fig. 15); all cells exposed to this process will be 

referred to as CdTe:Sb (or Sb-doped). In some instances the CdTe surface was etched in dilute 

HCl solution in order to remove any Sb remaining on the CdTe surface and eliminate any 

influence this may have on the formation of the back contact. It is well known that the 

fabrication process of high efficiency CdTe cells intentionally introduces several impurities in 

the solar cell, which include copper, chlorine and oxygen. The work described in this section 

was intended to investigate and understand the role of Sb as a potential p-type dopant. All cells 

discussed in this section were fabricated with CSS-CdTe films deposited in the presence of O2

(see O2 effect in a previous section), and the back contacts were fabricated using molybdenum 

(Mo) sputter-deposited at room temperature (no Cu was intentionally used during the cell 

fabrication process).

5.3.1 The Need for the CdCl2 Heat Treatment (HT)

The improvements in the performance of CdTe solar cells as a result of the CdCl2 HT are well 

known. Initial experiments with CdTe:Sb focused on fabricating solar cells with and without the 

CdCl2 HT, in order to determine whether this step could be excluded from the cell fabrication 

process and therefore be able to decouple the effects of Sb from other impurities.

Table 9 lists processing conditions and solar cell performance for CdTe:Sb solar cells. These 

results are among many that clearly underscore the great importance of the CdCl2 HT. The 

improvements in both the VOC and the FF are substantial and in this case translate to an 

approximate increase in efficiency from a range of 6.5-7.2 to 11.0-11.3%; an increase of 4-5%. 

At this time it was decided that going forward with the Sb-doping work, the CdCl2 HT should be 

included as part of the cell fabrication sequence, simply because its impact on solar cell 

performance could not be accomplished by other means. However, a substantial number of cells 

were fabricated without the CdCl2 HT, in order to try to better understand the role of the various 

impurities and processes (see table 9 results).

Sb

Cadmium Telluride

Cadmium Sulfide

Tin Oxide (bi-layer)

Glass Superstrate

Glass Slide

Sb

Sb

Cadmium Telluride

Cadmium Sulfide

Tin Oxide (bi-layer)

Glass Superstrate

Glass Slide

Sb

Figure 15. Sample arrangement used for the Sb diffusion 

experiments. Credit: University of South Florida
24 

Table 9. Process conditions and solar cell results for CdTe:Sb cells; the effect of the CdCl2 HT

Sample ID “A” “B” “C” “D”

CdTe In O2 ambient; thickness 5-6 μm

Sb Thickness on CdTe [nm] 20

Heat Treatment (Sb Diffusion)

T[°C]/time[min] 400/25 450/25 430/25 430/25

CdCl2 HT None None Yes Yes

Contact Sputtered Mo;

VOC [mV] 710-730 740-750 800-810 810-830

FF [%] 37-41 41-44 61 61-62

The improvements in performance (due to the CdCl2 HT) are evident from the light J-V and 

spectral response (SR) data, for a representative set of cells, shown in Fig. 16. Although, the 

roll-over (slope around VOC) behavior, indicative of the presence of a back contact barrier, is 

evident in all cells, it is more severe in the cells not exposed to the CdCl2 HT. Large enough 

barriers at the back contact could also result in losses in JSC, which is also the case for the cells 

fabricated without the CdCl2 HT. 

5.3.2 The Effect of Excess “Surface-Sb”

The next set of cells listed in Table 10 are among those fabricated without the CdCl2 HT, and in 

this particular case the objective was to determine whether excess Sb, left on the surface of CdTe 

after the diffusion-heat treatment, had an effect on the characteristics of the solar cells. In two of 

the cases listed in Table 10 the solar cells were etched in HCl following the Sb-diffusion process 

in order to remove excess Sb. 

Solar cell performance is poor in all instances (compared to state-of-the-art cells); the reason for 

the poor performance is as indicated in the results presented above (table 9 & Fig 16) the 

exclusion of the CdCl2 heat treatment. Figure 17 shows the light and dark I-V characteristics 

from representative devices. It is clear, based on the dark and light I-V rollover, that the limiting 

mechanism in all cells is a large barrier at the back contact. However, cells etched with HCl (to 

remove excess Sb from the CdTe surface) exhibit consistently lower VOC’s and JSC’s, and their 

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

400 500 600 700 800 900

Wavelength [nm]

QE

A - w/o CdCl2

B - w/o CdCl2

C - w CdCl2

D - w CdCl2

-0.03

-0.02

-0.01

0.00

0.01

0.02

0.03

-0.50 -0.25 0.00 0.25 0.50 0.75 1.00

Voltage [Volts]

Current [A] 

A - w/o CdCl2

B - w/o CdCl2

C - w CdCl2

D - w CdCl2

Figure 16. Light J-V (left) and SR data (right) for representative CdTe;Sb cells from table 9
25 

forward dark currents (beyond the junction turn-on voltage) are suppressed more severely. All 

these characteristics seem to suggest that the back contact barrier is larger in the case where 

excess Sb was removed (i.e. cells etched with HCl). It is therefore tentatively concluded that the 

presence of Sb at the surface of CdTe leads to a reduction in the back contact barrier. Antimony 

telluride (Sb2Te3) has been used by others as an effective back contact material for CdTe [15].

Table 10. Process conditions and performance data for CdTe:Sb cells; the effect of HCl etch

Sample ID “400/HCl” “400/NO HCl” “450/HCl” “450/NO HCl”

CdTe In O2 ambient; thickness 5-6 μm

Sb Thickness on CdTe [nm] 20

Heat Treatment (Sb Diffusion)

T[°C]/time[min] 400/25 400/25 450/25 450/25

HCl Etch Yes No Yes No

CdCl2 HT None

Contact Sputtered Mo;

VOC [mV] 680-690 710-730 630-680 740-750

FF [%] 37-38 37-41 32-37 41-44

5.3.3 The Effect of the Sb-diffusion Process Parameters

The optimum conditions for the Sb-diffusion process have not been established; the temperature 

has been varied from 400-525°C, the time from 20-180 mins, and the thickness of Sb deposited 

onto the CdTe surface from 20-50 nm; as indicated earlier an Sb-coated glass slide was also 

placed on the CdTe surface during the Sb-diffusion step. 

Table 11 lists the performance data for CdTe:Sb cells for which the annealing time was varied 

from 40-160 mins. While the annealing time of 40 mins yields the lowest performance cells in 

this set, annealing times of 80-160 mins result in essentially identical VOC’s and FF’s.

-5.0E-05

5.0E-05

1.5E-04

2.5E-04

3.5E-04

-0.25 0.00 0.25 0.50 0.75 1.00

Voltage [Volts]

Current [A] 

400/NO HCl 450/NO HCl

400/HCl 450/HCl

-0.02

-0.01

0.00

0.01

-0.50 -0.25 0.00 0.25 0.50 0.75 1.00

Voltage [Volts]

Current [A] 

400/NO HCl 450/NO HCl

400/HCl 450/HCl

Figure 17. Light (left) and dark (right) I-V for representative CdTe cells from table 10
26 

Table 11. Process conditions/solar cell results for CdTe:Sb cells; the effect of annealing time

Sample ID “40” “80” “120” “160”

CdTe In O2 ambient; thickness 5-6 μm

Sb Thickness on CdTe [nm] 30

Heat Treatment (Sb Diffusion)

T[°C]/time[min] 400/40 400/80 450/120 450/160

CdCl2 HT YES

Contact Sputtered Mo;

VOC [mV] 700-730 750-770 730-770 740-770

FF [%] 58-63 60-62 61-63 61-64

The light J-V for representative cells are 

shown in Fig. 18. The lowest performer in 

the group, the cell with the shortest Sbannealing time, also seems to exhibit the 

“least” rollover. It is not clear at this time 

what mechanism causes this behavior, 

however, it is possible that increased carrier 

concentration in the CdTe near the back 

contact for the cells annealed for long times 

(80-16 mins), results in increased band 

bending at that interface which will result in 

a larger barrier for holes (moving from CdTe 

to the contact). Resolving this issue will 

require numerical modeling that can 

consistently capture these differences in 

these devices. The doping profiles for the 

same cells are shown in Fig. 19, and they 

-0.03

-0.02

-0.01

0.00

0.01

0.02

-0.50 -0.25 0.00 0.25 0.50 0.75 1.00

Voltage [Volts]

Current [A] 

40 mins

80 mins

120 mins

160 mins

Figure 18. Light J-V for representative cells from 

table 11

1.0E+13

1.0E+14

1.0E+15

1.0E+16

1.0E-04 2.0E-04 3.0E-04 4.0E-04

Depletion Width [μm]

NA [cm-3] .

40 min 80 min

120 min 160 min

Figure 19. Doping profiles (from C-V 

measurements) for the cells shown in Fig. 18 

(table 11) 

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

400 500 600 700 800 900

Wavelength [nm]

Q.E

400C 450C

500C 525C

Figure 20. The SR of CdTe;Sb cells fabricated 

with different Sb-diffusion temperatures
27 

appear to partially support the increased doping claim; the lowest doping levels are observed in 

the device with the shortest annealing time.

Figure 20 shows the SR of several cells for which the Sb-diffusion time was fixed at 25 mins and 

the temperature was varied from 400-525°C. These data clearly demonstrate that at high 

annealing temperatures the collection of carriers decreases dramatically independent of the 

illumination wavelength. This type of behavior could be explained with increased interfacial 

recombination. Solar cell performance initially increased (up to 450°C); however, above 450°C 

the performance became “noisy” (i.e. increased scattering in performance data) and clearly 

decreased above 500°C. Based on these results the annealing temperatures were limited in the 

400-450°C range for most experiments. 

Based on the to-date results with Sb-diffusion, it appears that effective doping of CdTe (i.e. 

>1015cm-3) is yet to be accomplished with this approach. Even though in several experiments 

there is consistent increase in the hole concentration, the doping levels measured were always 

within the general range typical of CdTe devices (fabricated w/o Sb) i.e. 1014

cm-3. In general Sb 

may present a unique opportunity for CdTe devices, since it appears to potentially benefit the 

back contact region of the device (i.e. CdTe surface) as well as the bulk doping of CdTe. From a 

stability point of view, it may lead to the elimination of Cu from device processing, an element 

that is often suspected to be responsible for device degradation. Figure 21 shows light J-V 

characteristics and the CdTe doping profile for cells fabricated on the same substrate, in order to 

eliminate experimental variations/errors. The only processing difference between the two is that 

one of the devices was subjected to the Sb-diffusion process. It is clear that both the VOC and the 

doping levels in the CdTe:Sb device are higher (the VOC by 50 mV), suggesting that increasing 

the doping in CdTe further could potentially yield even higher VOC’s as expected.

1.0E+12

1.0E+13

1.0E+14

1.0E+15

1.0E+16

2.0E-04 3.0E-04 4.0E-04 5.0E-04

Depth [μm]

NA [cm-3] .

10-3A-5-x2(Sb)

10-3A-5-2(No Sb)

-0.03

-0.02

-0.01

0.00

0.01

0.02

0.03

-0.50 -0.25 0.00 0.25 0.50 0.75 1.00

Voltage [Volts]

Current [A] 

10-3A-5-x2(Sb)

10-3A-5-2(No Sb)

Figure 21. Light J-V (left) and doping profile (right) of CdTe cells fabricated with and w/o 

Sb-diffusion 
28 

6 Indium (In) Doped CdS

Another activity undertaken under this project dealt with using intentionally doped CdS films for 

the fabrication of CdTe/CdS solar cells, in order to determine whether increasing the n-type 

conductivity of CdS could be beneficial to solar cell performance. In theory, one would expect 

that an increase in the net doping in either heterojunction partner to lead to an increase in the 

built-in potential and therefore VOC. However, it has been previously found that a compensating 

impurity in this material (CdS), such as Cu, leads to improved performance [9]; others have also 

proposed a device operation model to explain how device performance can benefit from a 

compensated or “insulating” CdS[14]. 

For this project In was introduced in CdS films in two ways as shown in Fig 22: (a) by 

depositing In onto the SnO2/glass substrate before the CdS deposition by CSS; by depositing In 

onto (b) CSS-CdS or (c) CBD-CdS; and (d) by depositing CSS-CdS from an In-doped CdS 

source (purchased with In). In the first 3 cases, the substrates were annealed prior to the 

deposition of CdTe. 

6.1 CBD-CdS Control Samples

The first few experiments to dope CdS with Indium, utilized CBD-deposited CdS films, in order 

to establish a baseline for device performance. Indium was sputtered at room temperature onto 

CBD-CdS at two thicknesses 5 and 10 Å. The films were subsequently processed into solar cells 

using “baseline” processes.

Glass

SnO2

In – few Å

CSS-CdS

Glass

SnO2

In – few Å

CSS-CdS

Substrate

CdS:In

Glass

SnO2

In – few Å

CBD-CdS

(a) (b)

(c) (d)

Glass

SnO2

In – few Å

CSS-CdS

Glass

SnO2

In – few Å

CSS-CdS

Glass

SnO2

In – few Å

CSS-CdS

Glass

SnO2

In – few Å

CSS-CdS

Substrate

CdS:In

Glass

SnO2

In – few Å

CBD-CdS

Glass

SnO2

In – few Å

CBD-CdS

(a) (b)

(c) (d)

Figure 22. Methods used to incorporate 

indium in CdS
29 

Table 12. CdTe Solar Cells fabricated with CBD-CdS films as-deposited and doped with Indium

CBD-CdS:In Cells Baseline Cells

Cell ID VOC

[mV]

FF 

[%]

Indium 

Thickness 

[Å]

Anneal Cell ID VOC

[mV]

FF 

[%]

5-17a-6-1 770 45

10 Y 

5-17a-6-x1 820 71

# 2 750 46 # x2 810 72

# 3 760 47 # x3 810 72.

5-17a-7-1 530 48

10 N 

5-17a-7-x1 820 68

# 2 530 45 # x2 810 69

# 3 530 42 # x3 810 72

6-6a-4-1 780 67

5 Y 

6-6a-4-x1 830 71

# 2 760 67 # x2 830 71

# 3 760 67 # x3 830 72

6-6a-5-1 780 69

5 N 

6-6a-5-x1 820 73

# 2 760 69 # x2 820 73

# 3 750 67 # x3 820 73

Annealing, following the deposition of In 

onto CdS (for In in-diffusion), appears to 

be beneficial, although at the small In 

thickness (of 5 Å), there is no apparent 

benefit to device performance. This may 

suggest that the excess In in the devices 

(with 10 Å of In) which were not annealed, 

may have resulted in increased interface 

recombination, or may have diffused into 

the CdTe side of the junction resulting in a 

buried homojunction; SR measurements 

seem to support the former (i.e. increased 

interfacial recombination), as shown in 

Fig. 21; the sample with 10 Å of In (not 

annealed) exhibits a wavelength 

independent reduction in its SR. 

Even though, the “control” experiment 

suggested that the incorporation of indium 

in CdS resulted in decreased performance, similar experiments were carried out with CSS-CdS 

films. It should be noted that under a previous project, the effect of Cu incorporation in CdS was 

investigated; in that case the best approach to controllably introduce small amounts of Cu in CdS 

was by dipping the samples in a CuCl solution; Cu sputtering (similar to the approach taken here 

to introduce In in CdS) appeared to introduced excessive amounts of Cu and was ineffective. 

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

400 500 600 700 800 900

Wavelength [nm]

Q.E.

6-6A-4-1 6-6A-5-1

5-17A-6-1 5-17A-7-1

Figure 23. SR data for CdTe cells fabricated with 

CBD-CdS:In window layers (see table 12) 
30 

Introducing In in CdS using a similar approach (i.e. solution of In salts) has not produced 

noteworthy results.

6.2 In Doping of CSS-CdS

The deposition of CdS by the close-spaced sublimation has been studied under past projects. A 

key finding was the beneficial effect of deposition CdS in the presence of oxygen. For the 

CdS:In experiments described in this report CSS-CdS films were prepared in either inert or O2-

containing ambient. Indium was deposited by sputtering at room temperature to thicknesses of 1, 

2.5, 4, and 10 Ǻ; even at these small thicknesses the amount of indium in CdS is on the order of 

0.2-2% which is relatively high for doping purposes. 

Figures 24 and 25 show VOC and FF data (2-3 cells per condition) for cells for which the CSSCdS was deposited in O2-ambient (Fig. 24) and in inert ambient (Fig. 25). It is rather difficult to 

draw any conclusions on the effect of the relative amount of In based on these results, although it 

appears that the best cell (VOC/FF 810mV/72%) was fabricated with CSS-CdS(O) and the least 

amount of In (i.e. 1 Ǻ), suggesting that In-incorporation in CdS is actually detrimental to device 

performance (at least at this high levels). Clearly the most important process parameter is the 

CSS-CdS ambient; the results confirm past findings where it was demonstrated that cells 

fabricated with CSS-CdS(O) are better performers than cells fabricated with CSS-CdS deposited 

in inert ambient.

Spectral response measurements for the cells in Figs 24 and 25 are shown in Fig. 26. In this case 

there is clear evidence of photocurrent loss with increasing In. Initially the loss appears to be 

wavelength independent, but eventually exhibits some wavelength dependency suggesting 

increased deep losses; the photo-current losses observed are not optical, and variations in the 

thickness of CdS (varying blue QE) are not intentional; CSS CdS films exhibit both spatial 

thickness variations within the same substrate (on the order of 10-20%), as well as some run to 

run variations. 

All solar cell results obtained to-date suggest that CdS doped with In does not yield 

improvements in device performance due to potential In diffusion into CdTe or inefficient 

doping of CdS; the former is very likely due to the high processing temperatures used for solar 

cell fabrication, and it would lead to additional compensation in CdTe. These issues should be 

addressed by analyzing CdS:In films and junctions with methods that will provide information 

on the location and amount of In incorporated in the films and devices; both films and junctions 

have been forwarded to NREL for SIMS analysis. 
31 

7 Measuring Collection in CdTe Cells

During this project a series of solar cells were evaluated using monochromatic light J-V 

measurements. We used these data to perform simple analysis on the J-V characteristics of the 

device and measure the impact of inefficient collection on diode and solar cell parameters.

Figure 27 shows two sets of monochromatic light J-V data for typical CdTe solar cells (FF=69-

72%; VOC>810 mV). The interference filters used for these measurements had a bandwidth of 

20 nm; and the light intensity was adjusted to correspond to AM1.5 within that bandwidth (i.e. if 

the entire spectrum was covered the sum of the monochromatic JSCs would add up to the AM1.5 

JSC). Figure 28 is a plot of the FF for these two cells (Fig. 27) as a function of wavelength; the 

dotted lines mark the “white” FF of the devices (i.e. for 100 mW/cm2 AM1.5 intensity). 

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

400 500 600 700 800 900

Wavelength [nm]

QE [%]

1 A 4 A

2.5 A 10 A

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

400 500 600 700 800 900

Wavelength [nm]

QE [%]

1 A 4 A

2.5 A 10 A

Figure 26. SR data for the cells of Figs 24 (left) and 25 (right) 

250

350

450

550

650

750

850

1 2.5 4 10

In Thickness [Å]

VOC [mV]

50

55

60

65

70

75

80

85

90

FF [%]

VOC FF

Figure 25. VOC and FF for cells with CSS-CdS 

deposited in inert ambient

500

550

600

650

700

750

800

850

1 2.5 4 10

In Thickness [Å]

VOC [mV]

50

55

60

65

70

75

80

85

90

FF [%]

VOC FF

Figure 24. VOC and FF for cells with CSS-CdS 

deposited in the presence of oxygen
32 

It is clear that the FF decreases as the wavelength is increased, suggesting that carriers generated 

by deeply absorbed photons are not effectively collected. The AM1.5 FF lies between the two 

extreme values measured with monochromatic light; 65 and 75% for 800 and 460 nm 

respectively. These results clearly suggest that the FF (and VOC) are hampered by inefficient 

collection. It is therefore suggested that improved transport in CdTe would also benefit the VOC

of the CdTe cell.

7.1 The Effect of Collection on Solar Cell Performance

Assuming that collection at 520 nm is 100%, the data in Fig. 27 were normalized, and a 

“collection” function vs. voltage was obtained by dividing all other J-V data by the 460 nm set. 

These “collection” functions are shown in Fig. 29; collection for 520 nm is 100% (this was the 

original assumption), and collection for all other wavelengths decreases with the wavelength. 

Figure 27. Monochromatic light J-V data for two “typical” CdTe solar cells.

0.64

0.66

0.68

0.7

0.72

0.74

0.76

450 500 550 600 650 700 750 800 850

Wavelength [nm]

FF

Cell #1

Cell # 12

w FF #1

w FF # 12

Figure 28. FF as a function of wavelength 

of the monochromatic light (data from Fig 

27).
33 

Two of the collection functions (for 640 and 700 nm) were also fitted with polynomial 

expressions (as shown in Fig. 29). 

The two fitted collection functions (H(640) and H(700)) are shown in Fig. 30 along with light JV data for three cells; the dark blue data points represent an ideal device, and the green and red 

data represent non-ideal cells with collection losses corresponding to functions H(640) and 

H(700) respectively. The FF for the ideal device is 83% and decreases to 81 and 75% for 

collection functions H(640) and H(700) respectively. This simple analysis clearly demonstrated 

the potential advances in efficiency for the CdTe cell if collection (i.e. transport) in CdTe could 

be further improved.

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

1.00

0 0.2 0.4 0.6 0.8

Voltage [Volts]

"Collection Function"

800 775 700

640 520 Poly. (700)

Poly. (640)

Figure 29. Collection functions for various 

wavelengths; the data for 640 and 700 nm are 

fitted with polynomials.
34 

Table 13. Effect of collection on the FF

Ideal Solar Cell With H(640) With H(700)

83.3% 80.8% 74.9%

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

1.00

0 0.2 0.4 0.6 0.8

Voltage [Volts]

"Collection Function"

H(700)

H(640)

-0.03

-0.025

-0.02

-0.015

-0.01

-0.005

0

0.005

-0.25 0 0.25 0.5 0.75 1

Voltage [Volts]

Current [A]

Figure 30. Collection functions for 640 and 700 nm and their effect on the light I-V of an 

otherwise “ideal” solar cell.
35 

Appendix – Polymers Used as IFLs in CdTe Cells

N HN

NNH

COOH

Chemical Formula: C69H78N4O2

Exact Mass: 994.6125

Molecular Weight: 995.3832

IV-283-1B 5mg/mL (0.005mol/L), 

Tetrahydrofuran as solvent

Cl

COOH

Chemical Formula: C7H5ClO2

Exact Mass: 155.9978

Molecular Weight: 156.5664

IV-212A 0.2mol/L in ethanol

p-Anisic acid 0.05mol/L in ethanol

O

COOH

Chemical Formula: C8H8O3

Exact Mass: 152.0473

Molecular Weight: 152.1473

Sample 2

Sample 3

Sample 4

Ethanol: CH3CH2OH

Tetrahydrofuran: O

Sample 1(reference)

CN

COOH

Chemical Formula: C8H5NO2

Exact Mass: 147.032

Molecular Weight: 147.1308

Samples: IV-284-1A 0.2mol/ml 5ml(Ethanol)

 IV-284-1B 0.1mol/ml 5ml(Ethanol)

 IV-284-1C 0.05mol/ml 5ml(Ethanol)

Other sample:

Electronegative

Electron positive

Electron positive

N HN

HNNH

COOH

Br

Br Br

Br

Chemical Formula: C38H22Br4N4O2

Exact Mass: 881.8476

Molecular Weight: 886.2229

V-100 4.4mg/mL (0.005mol/L), 

Tetrahydrofuran as solvent

new sample:
36 

References 

1. X. Wu, J. C. Keane, R. G. Dhere, C. DeHart, D. S. Albin, A. Duda, T. A. Gessert, S. Asher, 

D. H. Levi, P. Sheldon, “16.5%-Efficient CdS/CdTe Polycrystalline Thin-Film Solar Cell”, 

Proc of 17th European PVSEC, (2001) pp. 531-534

2. X. Wu, S. Asher, D. H. Levi, D. E. King, Y. Yan, T. A. Gessert, and P. Sheldon, 

“Interdiffusion of CdS and Zn2SnO4 layers and its application in CdS/CdTe polycrystalline 

thin-film solar cells”, Journal of Applied Physics, 89, 8, (2001), pp. 4564-4569 

3. C. S. Ferekides, R. Mamazza, U. Balasubramanian, and D. L. Morel, “Transparent 

Conductors and Buffer Layers for CdTe Solar Cells”, Thin Solid Films, 480-481, (2005) pp. 

224-229 

4. S. Gayam, S. Bapanapalli, H. Zhao, L. Nemani, D. L. Morel and C. S. Ferekides, “The 

Structural and Electrical Properties of Zn-Sn-O Buffer Layers and their Effect on CdTe Solar 

Cell Performance”, in print Journal of Thin Solid Films (2007)

5. Gloeckler, M. Fahrenbruch, A.L. Sites, J.R., “Numerical modeling of CIGS and CdTe 

solar cells: setting the baseline”, proc. 3rd WCPEC, (2003), pp. 491-494 

6. K. Durose, S. Asher, W. Jaegermann, D. Levi, B. McCandless, W. Metzger, H. Moutihno, P, 

Paulson, C. Perkins, J. Sites, G. Teeter, and M. Terheggen, “Physical Characterization of thin 

film solar cells”, Progress in Photovoltaics, 12, (2004), pp. 177-217 

7. K. Barri, M. Jayabal, H. Zhao, D. L. Morel, S. Asher, J. W. Pankow, M. R. Young, and C. S. 

Ferekides, “Introduction of Cu in CdS and Its Effect on CdTe Solar Cells”, Proc. of the 31st

IEEE PVSC, (2005), pp. 287-290 

8. B. Tetali, V. Viswanathan, D. L. Morel and C. S. Ferekides, “Effects of Thermal Stressing on 

CdTe/CdS Solar Cells”, presented at the 29th IEEE PVSC, New Orleans, LA, May 19-24, 

2002. 

9. S. Erra, C. Shivakumar, H. Zhao, K. Barri, D. L. Morel and C. S. Ferekides, “An Effective 

Method of Cu Incorporation in CdTe Solar Cells for Improved Stability”, in print Journal of 

Thin Solid Films (2007)

10. D. Kraft, U. Weiler, Y. Tomm, A. Thissen, A. Klein and W. Jaegermann: "Alternative back 

contacts for CdTe solar cells: a photoemission study of the VSe2/CdTe and TiSe2/CdTe 

interface formation", Thin Solid Films 431 pp. 382-386 (2003) 

11. R. H. Friend, S. S. P. Parkin and D. Jerome: "The Transport-Properties of Vanadium-Doped 

Tise2 under Pressure", Journal of Physics C-Solid State Physics 15 (25), pp.L871-L874 

(1982). 

12. Frey, G. L., K. J. Reynolds, and R. H. Friend, Adv. Mater. 14, 4, (2002) p 265 

13. K. J. Reynolds, G. L. Frey, and R. H. Friend, Applied Physics Letters, 82, 7, (2003), p 1123

14. V. G. Karpov, Diana Shvydka, and Yann Roussillon, “Physics of CdTe Photovoltaics: from 

Front to Back”, MRS Spring Meeting 2005, March 28 – April 1, San Francisco, CA 

15. D. L. Bätzner, A. Romeo, H. Zogg, R. Wendt, A.N. Tiwari “Development of efficient and 

stable back contacts on CdTe/CdS solar cells” Thin Solid Films, 387, 1-2, (2001) pp. 151-

154 

16. Dert de Boer, A. Hadipour, M. M. Mandoc, T. van Woundenbergh, and P. W. M. Blom, 

“Tuning of Metal Work Functions with Self-Assembled Monolayers”, Adv. Mater., 17, 5, 

(2005), 621