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 

Process Development for High 

Voc CdTe Solar Cells 

Phase I Annual Technical Report 

October 2005 – September 2006 

C.S. Ferekides and D.L. Morel 

University of South Florida 

Tampa, Florida 

Subcontract Report 

NREL/SR-520-41525 

April 2007 

NREL is operated by Midwest Research Institute ● Battelle Contract No. DE-AC36-99-GO10337 
Process Development for High 

Voc CdTe Solar Cells 

Phase I Annual Technical Report 

October 2005 – September 2006 

C.S. Ferekides and D.L. Morel 

University of South Florida 

Tampa, Florida 

NREL Technical Monitor: Harin S. Ullal 

Prepared under Subcontract No. NDJ-2-30630-18 

Subcontract Report 

NREL/SR-520-41525 

April 2007 

National Renewable Energy Laboratory

1617 Cole Boulevard, Golden, Colorado 80401-3393 

303-275-3000 • www.nrel.gov 

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
 

This publication was reproduced from the best available copy 

submitted by the subcontractor and received no editorial review at NREL

NOTICE

This report was prepared as an account of work sponsored by an agency of the United States government. 

Neither the United States government nor any agency thereof, nor any of their employees, makes any 

warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or 

usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not 

infringe privately owned rights. Reference herein to any specific commercial product, process, or service by 

trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, 

recommendation, or favoring by the United States government or any agency thereof. The views and 

opinions of authors expressed herein do not necessarily state or reflect those of the United States 

government or any agency thereof. 

Available electronically at http://www.osti.gov/bridge

Available for a processing fee to U.S. Department of Energy 

and its contractors, in paper, from: 

U.S. Department of Energy 

Office of Scientific and Technical Information 

P.O. Box 62 

Oak Ridge, TN 37831-0062 

phone: 865.576.8401 

fax: 865.576.5728 

email: mailto:reports@adonis.osti.gov

Available for sale to the public, in paper, from: 

U.S. Department of Commerce 

National Technical Information Service 

5285 Port Royal Road 

Springfield, VA 22161 

phone: 800.553.6847 

fax: 703.605.6900 

email: orders@ntis.fedworld.gov

online ordering: http://www.ntis.gov/ordering.htm

Printed on paper containing at least 50% wastepaper, including 20% postconsumer waste
TABLE OF CONTENTS 

LIST OF FIGURES.......................................................................................................................................iv 

LIST OF TABLES.........................................................................................................................................iv 

1.0 INTRODUCTION............................................................................................................................. 1 

2.0 OPEN CIRCUIT VOLTAGE (VOC)................................................................................................... 1 

3.0 SUMMARY OF FABRICATION PROCEDUDES – EXPERIMENTAL METHODS......................... 2 

4.0 FRONT AND BACK CONTACT MATERIALS ................................................................................ 3 

 4.1 Titanium Oxide ...................................................................................................................... 3 

 4.1.1 Solar Cells with Titanium Oxide Buffer....................................................................... 6 

 4.2 Titanium Selenide (TiSe2) ..................................................................................................... 7 

5.0 THE EFFECT OF CdTe IMPURITIES ON SOLAR CELL PERFORMANCE ................................. 8 

 5.1 Phosphorous Incorporation in the CdTe Source Material..................................................... 8 

 5.2 The Effect of CSS(CdTe) Ambient ...................................................................................... 10 

 5.3 Antimony (Sb) as a Potential Dopant for CdTe................................................................... 12 

 5.3.1 The Need for the CdCl2 Heat Treatment.................................................................. 13 

 5.3.2 The Effect of Excess “Surface-Sb”........................................................................... 14 

 5.3.3 The Effect of the Sb-diffusion Process Parameters................................................. 15 

6.0 INDIUM (In) DOPED CdS ............................................................................................................. 17 

7.0 SIMS SAMPLES ........................................................................................................................... 18 

REFERENCES........................................................................................................................................... 19 

iii
LIST OF FIGURES 

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

at room temperature; (ii-middle) sputtering from TiO2 at 300°C; (iii-bottom) reactive 

sputtering in O2/N2 ..................................................................................................................... 4 

Figure 2. XRD spectra for TiO2 films prepared under different processing conditions............................. 5 

Figure 3. J-V characteristics of a CdTe cell fabricated with TiO2 as a buffer layer .................................. 7 

Figure 4. XRD spectrum of a selenized Ti film, showing the presence of the TiSe2 phase ..................... 8 

Figure 5. VOC and FF for CdTe cells fabricated from a CdTe:P CSS source ........................................... 9 

Figure 6. The effect of the CSS-CdTe deposition ambient on the VOC and FF of CdTe solar cells ....... 10 

Figure 7. 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 .................. 11 

Figure 8. Light J-V characteristics of the CdTe cells of Fig. 6 ................................................................ 11 

Figure 9. Sample arrangement used for the Sb diffusion experiments .................................................. 13 

Figure 10. Light J-V (left) and SR data (right) for representative CdTe:Sb cells from table 6.................. 14 

Figure 11. Light (left) and dark (right) I-V for representative CdTe cells from table 7 .............................. 15 

Figure 12. Light J-V for representative cells from table 8......................................................................... 16 

Figure 13. Doping profiles (from C-V measurements) for the cells shown in Fig. 12 (table 8)................. 16 

Figure 14. The SR of CdTe;Sb cells fabricated with different Sb-diffusion temperatures ........................ 16 

Figure 15. Light J-V (left) and doping profile (right) of CdTe cells fabricated with and 

w/o Sb-diffusion ....................................................................................................................... 17

LIST OF TABLES 

Table 1. Summary of processes and materials utilized for the fabrication of CdTe solar cells............... 3 

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 ....................................................................... 6 

Table 3. The highest VOC (for each set of deposition conditions) obtained for CdTe/CdS cells 

fabricated on TiO2 buffer layers................................................................................................. 6 

Table 4. XRD analysis for the film shown in Fig. 4.................................................................................. 8 

Table 5. Net Carrier concentration and depletion width for cells shown in Figs 6 & 7 .......................... 12 

Table 6. Process conditions and solar cell results for CdTe:Sb cells; the effect of the CdCl2 HT ........ 13 

Table 7. Process conditions and performance data for CdTe:Sb cells; the effect of HCl etch ............. 14 

Table 8. Process conditions and solar cell results for CdTe:Sb cells; the effect of annealing time ...... 15 

Table 9. Samples prepared at USF and being evaluated by SIMS at NREL........................................ 18 

iv
1.0 INTRODUCTION 

This is the Phase I report for this NREL funded project (subcontract # NDJ-2-30630-18). The 

focus of the project is on the open-circuit voltage of the CdTe thin film solar cell. 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 LLC leading the world in CdTe module production. In order to support the industry 

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 state of 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. These efforts lead to the underscoring of the important 

role “buffers” (or high resistivity transparent films) play in CdTe cells [1-4]. The use of 

transparent bi-layers (low-ρ/high-ρ), as the front contact is becoming a “standard” feature of the 

CdTe 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]. 

2.0 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 decoupled. 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 

1
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 is to develop materials/processes that will lead to the 

advancement of the open-circuit voltage of CdTe solar cells; the main areas to be investigated 

are: (a) impurities in the CdTe absorber: the focus in this case will be 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); (b) high-work function back contact materials; and (c) 

improved front contact: in this case the focus will be on investigating new buffer layers, as well 

as studying the effect of impurities in CdS. 

3.0 SUMMARY OF FABRICATION PROCEDURES – EXPERIMENTAL METHODS 

The main solar cell fabrication processes being utilized for this project are summarized in Table 

1. The close-spaced sublimation (CSS) is being 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 are being deposited buy 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 is being 

used as as a dopant for CSS-CdS 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 role 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 (undoped) were used as the front contact bi-layer (ITO/SnO2). 

Solar cells are 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, energy dispersive x-ray spectroscopy 

(EDS), and scanning electron microscopy (SEM) are also being carried out at USF. Secondary 

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

NREL. 

2
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 

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

Transparent Contact 

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

and SnO2

SnO2 by MOCVD (as above; undoped) 

Buffer Layer (hi

ghρ) SnO2 by sputtering of Sn (reactive) or SnO2 targets 

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 

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

Back Contact 

Mo by RF sputtering 

4.0 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. During the first phase of this project 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 under the three main process variations: (i) reactive sputtering 

from a Ti target (top), (ii) sputtering from a TiO2 target (middle), and (iii) sputtering in the 

presence of N2 (bottom) In all instances the apparent grain size ranges from 100-200 nm; 

however, the “grains” in films (i) and (ii) appear to be clusters of smaller particles, unlike film (iii) 

where the grains appear to be better developed; also film (iii) appears to be relatively more 

porous. 

3
(ii) by sputtering from TiO2 (in Ar) at 300o

C

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

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

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

(i-top) reactive sputtering at room temperature; (ii-middle) sputtering from 

TiO2 at 300°C; (iii-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).

4
X-ray diffraction analysis of TiO2 films revealed that reactively-sputtered as-deposited at room 

temperature films (similar to film (i) 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 (ii) 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 contained both TiO2 phases (i.e. anatase and routile). 

100

150

200

250

Intensity

GLASS RT/650C Air RT/650C Air

250C in N2 RT

0

50

20 30 40 50 60 70 80

2 Theta

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

conditions. 

5
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 

4.1.1 Solar Cells with Titanium Oxide Buffer 

The ultimate goal is to incorporate buffers such as the TiO2 films discussed above, in solar cell 

structures and study their impact on solar cell performance. 

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

Front contact: 

CVD - SnO2:F 

TiO2 by sputtering (process variations listed in table below)

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

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

CdCl2-heat treatment @ 390°C 

Back Contact: 

Bromine/methanol (0.01% vol.) etch for7-10 secs 

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 with N2 in the ambient a 

VOC of 880 mV was measured (which is the highest VOC measured during this project). 

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

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

cells fabricated on TiO2 buffer layers 

TiO2 Deposition Conditions 

Ar [mT] N2 [mT] O2 [mT] T [°C] Thickness 

[nm] 

VOC

[mV] 

FF Range 

[%] 

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

6
The light J-V characteristics of all TiO2-based 

solar cells were very similar in all instances, 

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; 

however, near the zero volt bias point there is a 

collapse in JSC (i.e. collection) which leads to 

the poor FF and poor overall cell performance. 

This could be caused by the high resistivty of 

TiO2. If the cause for this collapse in collection 

is better understood and identified it is possible 

that TiO2 buffers can provide a viable option for 

fabricating higher VOC CdTe solar cells. 

Although, 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 can be drawn at this time; only a 

few substrates exhibited uniform performance. 

This behavior points to the possibility that (non-uniform) micro-diodes are affecting the overall 

device performance A 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. 

-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. J-V characteristics of a CdTe cell 

fabricated with TiO2 as a buffer layer 

Work on TiO2 buffers has been suspended at the time this report was written. For future 

activities the possibility of doping TiO2 in order to decrease its resistivity may have to be 

considered. 

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 focuses 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 promising 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. 

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

early efforts focused on preparing this compound by selenizing Ti. Metallic Ti films were 

sputtered deposited on glass slides to a thickness of 200-400 Å, and subsequently exposed to a 

Se flux under high vacuum conditions; the selenization process is carried out in a CIGS 

chamber which contains 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 (to the naked eye) 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. 4 

were 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. 

7
Table 4. XRD analysis for the film shown in Fig. 4

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 4. XRD spectrum of a selenized Ti film, showing the 

presence of the TiSe2 phase 

A major challenge faced with the use of TiSe2 as a back contact material has been its formation 

onto the CdTe surface. 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. Although selenization of Ti is still being 

pursued, currently other material candidates and the possibility of depositing TiSe2 by coevaporation at lower temperatures are being considered. 

5.0 THE EFFECT OF CdTe IMPURITIES ON SOLAR CELL PERFOMANCE 

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. During the first phase of this project 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 

8
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, re-pulverized and remixed. This powder (to be referred to as Pdoped 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 nonuniformities is not clear at this time, however, it is presumed 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 

is 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 5 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 is not present 

at doping levels in CdTe (i.e. ppm), which is well below the detection limits of these technique. 

The tentative conclusion at this time is that P does not transport at the same rate as CdTe 

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

9
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 any P has been incorporated in the 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. 6 

(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. 

750

775

800

825

850

875

900

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

N2/O2

VOC [m

0.50

0.60

0.70

0.80

F

V]

F

VOC FF

Figure 6. The effect of the CSS-CdTe deposition 

ambient on the VOC and FF of CdTe solar cells 

Dark J-V characteristics for representative devices are shown in Fig. 7. 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 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. 7 (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 [12]. 

10
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]

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

The light J-V characteristics for the same cells are shown in Fig. 7. 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 O-incorporation 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. 

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

3/7 5/5 1/9

Figure 7. 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 8. Light J-V characteristics of the CdTe 

cells of Fig. 6 

11
The same devices (shown in Figs. 7 & 8) were characterized using C-V measurements. The 

ble 5. Net carrier concentration and depletion width for the cells shown in Figs 6 & 7 

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 5. 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. 6) 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. 

 

Ta

 N2/O2

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

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

WD @ 0 Volts [μm] 3.03 1.82 1.98 2.03 

ven though a small increase in carrier concentration is observed, it seems that there a 

.3 Antimony (Sb) as a Potential Dopant for CdTe 

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

dopant for CdTe. At has been used as a back 

contact material to Cd ole in CdTe i.e. as a 

dopant and also as a ba lead to the formation 

of the Sb2Te3 com ained with P, it was 

decided to attempt to thod, rather than during 

the CSS deposition. of Sb (20-30 nm) was 

deposited onto CdTe heat treated in inert 

ambient, in order to cau peratures varied from 

300 to 525°C, and ann the early experiments 

suggested that Sb wa mit Sb loss during the 

heat treatment, an Sb s used on top of the CdTe 

for the remainder of o this process will be 

referred to as CdTe:S e was etched in dilute 

HCl solution in order ce and eliminate any 

influence this may well known that the 

fabrication process of everal impurities in the 

solar cell, which include ibed in this section was 

intended to investigat type dopant. All cells 

fabricated to-date and di CdTe films deposited 

in the presence of O back contacts were 

fabricated using mo erature (no Cu was 

intentionally used dur

E

“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. 

5

 this time it should also be noted that Sb2Te3

Te solar cells, and therefore it may play a dual r

ck contact electrode, if the processing conditions

pound on the surface of CdTe. Following the results obt

 incorporate Sb in CdTe via a post-deposition me

 Following the CSS deposition of CdTe, a thin film 

 by sputtering. The structures were subsequently

se Sb to diffuse into the CdTe film; annealing tem

ealing times from 20 to 160 mins. Some of

s evaporating from the CdTe surface; in order to li

-coated glass slide (with 200-300 nm of Sb) wa

the experiments (see Fig. 9); all cells exposed t

b (or Sb-doped). In some instances the CdTe surfac

 to remove any Sb remaining on the CdTe surfa

 have on the formation of the back contact. It is

 high efficiency CdTe cells intentionally introduces s

copper, chlorine and oxygen. The work descr

e and understand the role of Sb as a potential pscussed in this section were fabricated with CSS2 (see O2 effect in a previous section), and the

lybdenum (Mo) sputter-deposited at room temp

ing the cell fabrication process). 

12
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 going forward this step could be excluded from the cell 

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

Table 6 lists processing conditions and solar cell performance for CdTe:Sb solar cells. These 

results are among several 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, the intend is to include 

solar cells fabricated without the CdCl2 HT in this study whenever necessary, in order to better 

understand the role of the various impurities and processes (see table 7 results). 

Table 6. 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 CdCl HT) are evident from the light J-V and 2

spectral response (SR) data, for a representative set of cells, shown in Fig. 10. 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 no t 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. 

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

Figure 9. Sample arrangement used for the Sb diffusion 

experiments

13
5.3.2 The Effect of Excess “Surface-Sb” 

The next set of cells listed in Table 7 are among those fabricated without the CdCl2 HT, and in 

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

t ses listed in Table 7 t lar cells etched in ollowing t b-diffusion 

p der to remove excess S

S ll compared to state-of-the-art cells); the reason for 

t ate result ted ab le 6 & he 

exclusion t treatment re 11 show ht and d rk I-V char ristics 

from represe ices. It is clear on the d light I-V over, that t iting 

mechanism i is a large barrier at the back contac lls etched with HCl (to 

remove exce the CdTe su t c ly low nd JS

forward dark (beyond the jun rn-on v e suppr ore se All 

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

erformance data for CdTe:Sb cells; the effect of HCl etch 

Sample ID “400/HCl” “400/NO 

HCl” 

“450/HCl” “450/NO 

HCl” 

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

CdTe after

wo of the ca

 in or

he so were HCl f he S

rocess b. 

olar cell performance is poor in a

he poor performance is as indic

instances (

d in the s presen ove (tab Fig 10) t

 of the CdCl2 hea . Figu

, based

s the lig

 dark an

a

 rollacte

ntative dev he lim

n all cells t. However, ce

ss Sb from rface) exhibi onsistent er VOC’s a C’s, and their

currents ction tu oltage) ar essed m verely.

these cha

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 [13]. 

Table 7. Process conditions and p

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 

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

400 500 600 700 800 900

Wavelength [nm]

QE

-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

A - w/o CdCl2

B - w/o CdCl2

C - w CdCl2

D - w CdCl2

Figure 10. Light J-V (left) and SR data (right) for representative CdTe;Sb cells from table 6

14
-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] 

5.3.3 The Effect of the Sb-diffusion Process Parameters 

A set of specific optimum conditions for the Sb-diffusion process has not been established yet; 

to-date 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 Sbcoated glass slide is also placed on the CdTe surface during the Sb-diffusion step. 

Table 8 lists the performance data for CdTe:Sb cells for which the annealing time was varied 

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

Table 8. Process conditions and sol l r r CdT ls; th

annealing tim

“40” “160” 

fr

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

a cel esults fo e:Sb cel e effect of

e 

Sample ID “80” “120” 

CdTe In O2 ambient; thick 6 ness 5- μm 

Sb Thickness on CdTe [nm] 30 

Heat Treat iffusion) 

T[° ] 4 40 5 45 0 ment (Sb D

C]/time[min 00/40 0/80 4 0/120 0/16

CdCl2 HT YES 

Contact Sputtered Mo; 

VOC [mV] 7 7 7 7 00-730 50-770 30-770 40-770

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

The light J-V for representative cells are shown in Fig. 12. The lowest performer in the group, 

the cell with the shortest Sb-annealing time, also seems to exhibit the “least” roll-over. 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-

6 mins), results in increased band bending at that interface which will result in a larger barrier 

ed 

oping claim; the lowest doping levels are observed in the device with the shortest annealing 

time. 

1

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. 13, and they appear to partially support the increas

d

-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/NO HCl 450/NO HCl

400/HCl 450/HCl 400/HCl 450/HCl

Figure 11. Light (left) and dark (right) I-V for representative CdTe cells from table 7 

15
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] .

-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

40 min 80 min 80 mins

120 mins

Figure 12 s of several cells f 

wh fixed at 25 

m ried from 

4

demonstr high annealing 

temperature llection of carriers 

decreases d independent of the 

illumination h. This t

behavior coul ined with incr

terfacial recombination. Solar cell 

ho SR ws the or

ich the Sb-diffusion time was 

ins and the temperature was va

00-525°C. These data 

ates that at 

clearly 

s the co

ramatically

wavelengt ype of 

d be expla eased 

in

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. 

160 mins

Figure 12. Light J-V for representative cells from

table 8 

120 min 160 min

Figure 13. Doping profiles (from C-V measurements) 

for the cells shown in Fig. 12 (table 8) 

30%

60%

70%

90%

Q.E

40%

50%

80%

100%

400C 450C

0%

10%

20%

400 500 600 700 800 900

Wavelength [nm]

500C 525C

Figure 14. The SR of CdTe;Sb cells fabricated 

with different Sb-diffusion temperatures 

16
Based on the to-date results with Sb-diffusion

>10

, it appears that effective doping of CdTe (i.e. 

15cm-3) is yet to be accomplished with this

there is consistent increase in the hole concen

within the general range typical of CdTe device

Sb may present a unique opportunity for CdT

the back contact region of the device (i.e. Cd

From a stability point of view, it may lead to t

element that is often suspected to be responsible

 approach. Even though in several experiments 

tration, the doping levels measured are always 

s (fabricated w/o Sb) i.e. 1014 cm-3. In general 

e devices, since it appears to potentially benefit 

Te surface) as well as the bulk doping of CdTe. 

he elimination of Cu from device processing, an 

 for device degradation. Figure 15 shows light 

-V characteristics and the CdTe doping profile for cells fabricated on the same substrate, in 

rder to eliminate experimental variations/errors. The only processing difference between the 

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

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

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

 

6.0 INDIUM (In) DOPED CdS 

Another activity initiated during the first year of this project deals with using indium-doped CdS 

films for the fabrication of CdTe/CdS solar cells in order to determine whether increasing the ntype conductivity of CdS could be beneficial to solar cell performance. In theory, one would 

expect that an increase in the 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, such as Cu, leads to improved performance. For this 

project In is being introduced in CdS in two ways: (a) by evaporating In onto CdS before or after 

the CdS deposition by CSS, and annealing it, and (b) by using an In-doped CSS-CdS source 

(purchased with In). At this time results are preliminary, but they seem to suggest that In-doping 

of CdS results in lower performance. 

J

o

tw

the V

that incr

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] .

-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)

10-3A-5-x2(Sb)

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

Figure15. Light J-V (left) and doping profile (right) of CdTe cells fabricated with and w/o

Sb-diffusion 

17
7.0 SIMS SAMPLES 

In order to improve on our understanding of the role of the various impurities discussed in this 

report, a series of films have been forwarded to NREL for SIMS analysis (listed in table 9). 

These measurements could provide information on the concentration levels of the “dopants” in 

the various films, and whether these accumulate at any of the device interfaces, similar to what 

has been shown with Cu and Cl. Following the results of this set, it may be necessary to follow 

up with films that have been processed with the CdCl2 heat treatment (following the introduction 

of the impurities listed below). 

Table 9. Samples prepared at USF and being evaluated by SIMS at NREL 

Sample # Thicknesses Comments 

8-9A-2x CdTe: 9.5 μm Sb: 2 Å; CdTe(O) sample 

8-9B-2x CdTe: 7.9 μm Sb: 5 Å, CdTe(O) sample 

8-9B-3x CdTe: 8.4 μm Sb: 20 Å, CdTe(O) sample 

2-24A-6x CdTe: 4.2 μm Sb: 2 Å, CdTe sample 

2-24A-7x CdTe: 4.3 μm Sb: 5 Å, CdTe sample 

2-24A-8x CdTe: 4.2 μm Sb: 20 Å, CdTe sample 

8-10-1 CdTe: 9.6 μm P doped CdTe film without contact 

8-10-1x CdTe: 9.6 μm P doped CdTe, contacted with Mo 

6-6b-12x CdTe:7.08 μm, 

CdS : >1000 Å (approx.) CdS:In (10 Å before CdS deposition) 

6-6a-11 CdTe:4.8 μm, 

CdS : >1000 Å (approx.) CdS:In (10 Å after CdS deposition) 

8-13-1A CdS Source material CdS:In

18
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. 

4 am alli, H. Zhao D. L. Morel and C. S. Ferekides, “The 

ra l Properties Buffer Layers and their Effect on CdTe 

 Ce in print Jou 7)

5 ckler ruch, A.L. of CIGS and CdTe 

 cell seline”, pro 91-494 

6 os . Jaegerman ss, W. Metzger, H. Moutihno, 

lso J. Sites, G. en, “Physical Characterization 

 film rogress in P p. 177-217 

7 ri, hao, D. L. M kow, M. R. Young, and C. S. 

kide of Cu in CdS ”, Proc. of the 31st

 PV 287-290 

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

Te te s, LA, May 19-

24, 2002

9 ra, K. des, “An Effective 

Method o e Solar Cells for Improved Stability”, in print Journal of 

Soli s (2007)

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

L874 (1982). 

12. 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 

13. 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 

224-229 

. S. Gay

Structu

, S. Bapanap

l and Electrica

, L. Nemani,

of Zn-Sn-O 

Solar ll Performance”, rnal of Thin Solid Films (200

. Gloe , M. Fahrenb Sites, J.R., “Numerical modeling

solar s: setting the ba c. 3rd WCPEC, (2003), pp. 4

. K. Dur e, S. Asher, W n, D. Levi, B. McCandle

P, Pau n, C. Perkins, 

 solar cells”, P

Teeter, and M. Terhegg

of thin

. K. Bar

hotovoltaics, 12, (2004), p

M. Jayabal, H. Z orel, S. Asher, J. W. Pan

Fere s, “Introduction and Its Effect on CdTe Solar Cells

IEEE SC, (2005), pp.

 V. Viswanathan

on Cd /CdS Solar Cells”, presen d at the 29th IEEE PVSC, New Orlean

. 

. S. Er C. Shivakumar, H. Zhao, 

f Cu Incorporation in CdT

 Barri, D. L. Morel and C. S. Fereki

Thin d Film

1

19
LIST OF PUBLICATIONS (’06-‘07) 

olid Films (2007)

7)

06)

NAT

“Stab

Marc

“Fabr

Meet 2006, Golden, CO, Brian McCandless and Chris Ferekides 

1. “Surface science studies of Cu containing back contacts for CdTe solar cells”, b. Spath, K. 

Lakus-Wollny, J. Fritsche, C. S. Ferekides, A. Klein, W. Jaegermann, in print in Thin Solid 

Films (2007) 

2. “Photoluminescence studies of CdTe films and Junctions”, S. Vatavu, H. Zhao, V. Padma, 

R. Rudaraju, D. L. Morel, P. Gaşin*, Iu. Caraman**and C. S. Ferekides, in print Journal of 

Thin Solid Films (2007)

3. “An Effective Method of Cu Incorporation in CdTe Solar Cells for Improved Stability”, S. 

Erra, C. Shivakumar, H. Zhao, K. Barri, D. L. Morel and C. S. Ferekides, in print Journal 

of Thin S

4. “The Structural and Electrical Properties of Zn-Sn-O Buffer Layers and their Effect on 

CdTe Solar Cell Performance”, S. Gayam, S. Bapanapalli, H. Zhao, L. Nemani, D. L. 

Morel and C. S. Ferekides, in print Journal of Thin Solid Films (200

5. “Development of ZnSexTe1-x p-type contacts for high efficiency tandem structures”, S. 

Vakkalanka, C. S. Ferekides, and D. L. Morel, in print Thin Solid Films (20

IONAL TEAM MEETING PRESENTATIONS 

ility Studies of CdTe Devices: Cu in Back Contact vs. Cu in CdS”, CdTe Team Meeting, 

h, 2006, Golden, CO 

ication perspective on open-circuit voltage in CdTe/CdS thin film solar cells”, CdTe Team 

ing, March,

20
F1146-E(12/2004) 

REPORT DOCUMENTATION PAGE Form Approved 

OMB No. 0704-0188

The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, 

gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this 

collection of information, including suggestions for reducing the burden, to Department of Defense, Executive Services and Communications Directorate (0704-0188). Respondents 

should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a 

currently valid OMB control number. 

PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ORGANIZATION.

1. REPORT DATE (DD-MM-YYYY)

APRIL 2007 

2. REPORT TYPE

Subcontractor Report 

3. DATES COVERED (From - To)

October 2005 - September 2006 

5a. CONTRACT NUMBER

DE-AC36-99-GO10337 

5b. GRANT NUMBER

4. TITLE AND SUBTITLE

Process Development for High Voc CdTe Solar Cells: Phase I Annual 

Technical Report, October 2005 - September 2006 

5c. PROGRAM ELEMENT NUMBER

5d. PROJECT NUMBER

NREL/SR-520-41525 

5e. TASK NUMBER

PVB75201 

6. AUTHOR(S)

C.S. Ferekides and D.L. Morel 

5f. WORK UNIT NUMBER

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

University of South Florida 

4202 E. Fowler Avenue 

Tampa, Florida 33620 

8. PERFORMING ORGANIZATION 

REPORT NUMBER

NDJ-2-30630-18 

10. SPONSOR/MONITOR'S ACRONYM(S)

NREL 

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Renewable Energy Laboratory 

1617 Cole Blvd. 

Golden, CO 80401-3393 11. SPONSORING/MONITORING 

AGENCY REPORT NUMBER

NREL/SR-520-41525 

12. DISTRIBUTION AVAILABILITY STATEMENT

National Technical Information Service 

U.S. Department of Commerce 

5285 Port Royal Road 

Springfield, VA 22161 

13. SUPPLEMENTARY NOTES

NREL Technical Monitor: Harin S. Ullal 

14. ABSTRACT (Maximum 200 Words): The focus of this project is the open-circuit voltage of the CdTe thin-film solar cell. 

CdTe continues to be one of the leading materials for large-scale cost-effective production of photovoltaics, but 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 U.S.-based First Solar LLC leading the world in CdTe module 

production. To support the industry efforts and continue the advancement of this technology, it will be necessary to 

continue improvements in solar cell efficiency. A closer look at the state-of-the-art performance levels puts the three 

solar cell efficiency parameters of short-circuit current density (JSC), open-circuit voltage (VOC), and fill factor (FF) in the 

24–26 mA/cm2

, 844–850 mV, and 74%–76% ranges respectively. During the late 1090s, efforts to improve cell 

efficiency were primarily concerned with increasing JSC, simply by using thinner CdS window layers to enhance the blue 

response (<510 nm) of the CdTe cell. These efforts led to underscoring the important role “buffers” (or high-resistivity 

transparent films) play in CdTe cells. The use of transparent bi-layers (low-ρ/high-ρ) as the front contact is becoming a 

“standard” feature of the CdTe cell. 

15. SUBJECT TERMS

PV; open-circuit voltage; thin film; solar cells; manufacturer; short-circuit current density; fill factor; high resistivity 

transparent films; cadmium telluride (CdTe); conversion efficiency; buffers; performance; 

16. SECURITY CLASSIFICATION OF: 19a. NAME OF RESPONSIBLE PERSON 

a. REPORT 

Unclassified 

b. ABSTRACT 

Unclassified

c. THIS PAGE 

Unclassified

17. LIMITATION 

OF ABSTRACT

UL 

18. NUMBER 

OF PAGES 

19b. TELEPHONE NUMBER (Include area code)

Standard Form 298 (Rev. 8/98) 

Prescribed by ANSI Std. Z39.18