**Thin CdTe Solar Cells with High Throughput Processing**

B. E. McCandless, K. D. Dobson, S. S. Hegedus, W. A. Buchanan, D. Desai,
R.W. Birkmire

Institute of Energy Conversion, University of Delaware, Newark, DE 19716

<bem@udel.edu>

**ABSTRACT**

Progress towards reducing CdTe film thickness and increasing performance
and processing throughput for vapor transport (VT) deposited CdTe/CdS
solar cells is highlighted. VT devices with 5-7 μm thick CdTe deposited
on moving substrates at 9 μm/min using a 20 min vapor CdCl~2~ treatment
at 415ºC achieved Voc up to 840 mV and η \> 13%. Devices with η = 11%
were obtained with 2 min CdCl~2~ treatments at 465ºC. For cells with
CdTe thickness \~1.5 μm, η \>10% was obtained. V~oc~ and FF decreased
for cells with CdTe deposited at \>80 μm/min and for cells with \<1.5 μm
CdTe. Devices using transparent ZnTe contacts have similar performance
to those with opaque contacts and are being used to separate back
contact and primary junction effects.

**1. Objectives**

Program objectives are to evaluate processing throughput limits and
their effects on device performance and stability in CdTe/CdS thin-film
solar cells and determine limits to junction performance and develop
pathways towards V~oc~ \~ 900 mV.

**2. Technical Approach**

Meeting the objectives and maintaining relevance for industrial
application requires processing flexibility with quantifiable and
scalable processes, similar to those found in existing commercial
efforts. The following R&D paths are thus being explored: 1) high rate,
high T CdTe VT deposition in low vacuum onto moving substrates; 2) vapor
post-deposition processing capable of short treatment times; 3)
correlation of CdTe surface chemistry with bulk properties and device
performance and stability; 4) bifacial device measurements using
transparent contacts.

**3. Results and Accomplishments**

*3.1 Thin CdTe with High Throughput Processing*

The IEC VT system allows CdTe deposition onto a 10 x 10 cm substrates
translating at linear velocities up to 100 cm/min and is fully
controllable with respect to source and substrate temperature, pressure,
flow rate, ambient composition and translation speed \[1\]. Source
effusion matches modeled predictions and static equivalent growth rates
\>80 μm/min with \>50% utilization of CdTe source have been achieved.
For 100% utilization, achievable by adding baffles, a source T = 840ºC
would yield \>100 μm/min growth rate and a 2 μm film could be deposited
over a 1 m length (module size) in 2 min.

Solar cells are routinely made using LOF TEC-15 SnO~2~/soda-lime glass
coated with \~50 nm thick Ga~2~O~3~. Cells are fabricated by: 1)
chemical surface deposition (CSD) of CdS films for \>80% utilization of
Cd species during growth\[2\]; 2) VT deposition of CdTe; 3) vapor
CdCl~2~ treatment at 400ºC for 15 min; and 4) surface etch and Cu/Ni
contact deposition. The performance of cells with CdTe from 0.75 − 7 μm
deposited at \~9 μm/min is shown in Table 1; junction quality is
maintained over a thickness range from 1.4 to 7 μm. Increasing CdTe
growth rate to 81 μm/min (t = 5 μm) dramatically reduced grain size and
gave: V~oc~ = 736 mV, J~sc~ = 24.8 mA/cm^2^, FF = 53.4% and η = 11%.
Table 2 lists device results for cells with 5 μm CdTe processed with
reduced vapor CdCl~2~ treatment time at pCdCl~2~ = 3 mTorr and shows
that the path to reducing time is increasing CdTe temperature during the
treatment.

Table 1. CdTe/CdS cell results: CdTe thickness

+--------------+----------+---------------+------------+------------+
| CdTe         | Voc      | Jsc           | FF         | Eff        |
|              |          |               |            |            |
| thk          | (mV)     | (mA/          | (%)        | (%)        |
|              |          |               |            |            |
| (μm)         |          | cm2)          |            |            |
+--------------+----------+---------------+------------+------------+
| 7            | 842      | 24.1          | 64         | 13.0       |
+--------------+----------+---------------+------------+------------+
| 6            | 808      | 23.8          | 69         | 13.3       |
+--------------+----------+---------------+------------+------------+
| 5            | 792      | 23.4          | 67         | 12.3       |
+--------------+----------+---------------+------------+------------+
| 2            | 780      | 21.5          | 64         | 10.7       |
+--------------+----------+---------------+------------+------------+
| 1.4          | 807      | 23.0          | 57         | 10.5       |
+--------------+----------+---------------+------------+------------+
| 1.1          | 743      | 22.0          | 52         | 8.5        |
+--------------+----------+---------------+------------+------------+
| 0.75         | 723      | 21.0          | 50         | 7.5        |
+--------------+----------+---------------+------------+------------+

Table 2. CdTe/CdS cell results: vapor CdCl~2~

+-----------------+------------+-----------+----------+--------------+
| Vapor CdCl2     | Voc        | Jsc       | FF       | Eff          |
|                 |            |           |          |              |
| (ºC/min)        | (mV)       | (mA/      | (%)      | (%)          |
|                 |            |           |          |              |
|                 |            | cm2)      |          |              |
+-----------------+------------+-----------+----------+--------------+
| None            | 570        | 15.9      | 46.9     | 4.2          |
+-----------------+------------+-----------+----------+--------------+
| 415/25          | 801        | 23.0      | 71.7     | 13.2         |
+-----------------+------------+-----------+----------+--------------+
| 410/20          | 808        | 23.8      | 69.2     | 13.3         |
+-----------------+------------+-----------+----------+--------------+
| 440/2           | 634        | 20.4      | 60.2     | 7.8          |
+-----------------+------------+-----------+----------+--------------+
| 465/2           | 755        | 24.2      | 60.2     | 11.0         |
+-----------------+------------+-----------+----------+--------------+

*3.2 Film Chemistry*

CdTe oxidation impedes contact formation and influences junction
operation and cell stability. XPS, GIXRD and surface energy measurements
have shown the primary oxides to be CdO and CdTeO~3~ \[3\], while
CdTe~2~O~5~ has been found in samples treated or stored in humid
ambient. The specific oxides formed affects film defect chemistry and
grain boundary composition. Surface treatment of CdTe in aniline after
CdCl~2~ treatment has been implemented to remove oxides and form a
uniform and stable crystalline Te-rich surface, via a photocatalytic
process, with minimal over-etching of the film or grain boundary
penetration \[4\]. Treatment is carried out in solutions containing 0.1
M aniline, 0.01 M p-toluenesulfonic acid and 1 M NaCl, with illumination
either directly on the CdTe film or on the glass side of the sample.
Devices exceeding 12% conversion efficiency have been obtained when
processed with the aniline treatment and completed with Cu-based back
contacts.

*3.3 Device Analysis*

Semi-transparent ZnTe:Cu contacts have been developed for bifacial J(V)
and QE measurements of transport properties and separation of front and
back junction effects and are important for tandem cell applications.
Devices with ZnTe:Cu contacts have very similar performance to those
with opaque Cu/Ni or graphite contacts. A previously developed model for
bifacial QE analysis of CIS based devices as a function of absorber
thickness (t), depletion width (W) and diffusion length (L) has been
applied to CdTe cell analysis \[5\]. The back QE, (illumination through
the ZnTe) is more sensitive to fitting L and W than front QE and has
been fit in over 10 devices from different VT runs and surface
treatments. The value of L increases with thickness: L = 0.3-0.4 for t =
2.5 µm, L = 0.6-0.8 for t = 5 µm, L = 1.0 for t = 10 µm. Bifacial J(V)
measurements as a function of voltage bias and temperature are in
progress.

The treatment of the CdTe surface prior to ZnTe contact is crucial to
obtain a low resistance non-blocking contact. Br~2~-CH~3~OH consistently
produces a partially blocking contact with ZnTe:Cu. Br-Dichrol-Hydrazine
is effective but is too penetrating and causes shorting for t \< 5 µm.
Aniline treatment has given very good ZnTe:Cu contacts with no
light-dark crossover, blocking curvature or shunting even for t = 1.5
µm. Fig. 1 shows the J(V) curves for a 5 µm CdTe device with
Br~2~-CH~3~OH surface treatment having FF = 68% and V~oc~ = 810mV.
Roll-over in forward bias indicating a blocking contact only occurs for
front light or in the dark. When illuminated through ZnTe, there is no
forward bias curvature; this has been observed on many samples, suggests
the back contact is photo-active and is consistent with recontacting
studies \[7\].

**4. Conclusions**

High throughput processing of high efficiency CdTe/CdS cells is
demonstrated. Cells with V~oc~ approaching 900 mV were fabricated.
Maintaining high efficiency in cells with CdTe thickness \<1 μm will
require overcoming losses in junction quality and understanding the
influence of the back contact.

![](media/image1.wmf){width="2.636111111111111in"
height="2.4368055555555554in"}

Fig. 1. J(V) curve for illumination through front (CdS) or back
(ZnTe:Cu) contacts.

**ACKNOWLEDGEMENTS**

This work is supported by NREL subcontract ADJ-130630-12.

**REFERENCES/MAJOR FY 2005 PUBLICATIONS**

\[1\] B. E. McCandless, W. A. Buchanan, R. W. Birkmire, "High Throughput
Processing of CdTe/CdS Solar Cells," Conf. Rec. 31^st^ IEEE (2005).

\[2\] B. E. McCandless and K. D. Dobson, "Processing options for CdTe
thin film solar cells," Solar Energy 77 (2004) 839-856.

\[3\] B. E. McCandless. "Glancing Incidence X-Ray Diffraction of
Polycrystalline Thin Films," Proc. Mat. Res. Soc. (2005).

\[4\] K. D. Dobson, S. A. Einstein, D. D. Sadowsky, B. E. McCandless, R.
W. Birkmire,"Understanding Aniline Surface Treatment of CdTe," Proc.
Mat. Res. Soc. (2005).

\[5\] D. Desai, S. Hegedus, B. McCandless, D. Ryan, "Transparent ZnTe:Cu
contacts for bifacial characterization of CdTe solar cells," Proc. Mat.
Res. Soc. (2005).

\[6\] S. Hegedus, D. Desai, D. Ryan, B McCandless, "Transient
Degradation and Recovery of CdS/CdTe Solar Cells," Conf. Rec. 31^st^
IEEE (2005).

\[7\] S. S. Hegedus and B. E. McCandless, "CdTe contacts for CdTe/CdS
solar cells: effect of Cu thickness, surface preparation and
recontacting on device performance and stability," Solar Energy
Materials and Solar Cells 88 (2005) 75-95.

\[8\] K. Durose, S. Asher, W. Jaegerman, D. Levi, B McCandless, W.
Metzger, H. Moutinho, P. D. Paulson, C. Perkins, J. Sites, G. Teeter, M.
Terheggen, "Physical characterization of Thin-Film Solar Cells," Prog.
in Photovolt: Res. Appl 2004: 12: 177-217.