Response of Cds/CdTe Devices to Te Exposure of Back Contact: Preprint Page: 4 of 7
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performance is attained when WD (controlled by the formation
of VCd+OTe, VCa+CTe, and CUd) is narrow enough to produce
a drift field in the CdTe absorber that is strong enough to
overcome the relatively poor lifetime of the minority-carriers
(controlled by the presence or formation of VTe, Tei, TeCa, and
Cui), but still wide enough to limit the effects of voltage-
dependent collection (i.e., photocarriers should be generated
primarily within the depletion region when the device is
biased near the maximum power point [MPP]). [1] To further
complicate matters, other studies suggest that for some process
conditions, Cu diffusion from the contact reduces both
minority-carrier lifetime in the CdTe and doping in the CdS
(both effects reducing device performance). However, for
other process conditions, Cu diffusion can increase minority-
carrier lifetime (thus increasing device performance!). [2]
In contrast to the relatively complicated description above,
we believe an alternative pathway to produce CdTe layers
with superior material quality may be to control the formation
of intrinsic CdTe defects by modifying deposition and/or post-
deposition processes. In this study, we report on one post-
deposition process in which the CdTe back surface is exposed
to a small partial pressure of Te before the application of a
ZnTe:Cu/Ti contact. These initial results provide a captivating
suggestion that the range of optimum stoichiometry between
the formation of VTe (insufficient Te) and Tei and/or Teca
(excess Te) may be more narrowly bounded than previously
appreciated.
II. EXPERIMENTAL
The superstrate devices used for this study had the structure
glass/SnO2:F/SnO2/CdS:O/CdTe/ZnTe:Cu/Ti. The glass was
1-mm aluminosilicate; TCO layers were deposited by CVD by
reaction of tetramethyltin + oxygen (+ bromotrifluromethane -
if doped); the CdS:O layers were deposited by chemical bath
deposition; the CdTe was deposited by close-space
sublimation at 600 C; and treated in CdCl2 vapor at 400 C for
5 min.
The ZnTe:Cu/Ti contact that did not include Te exposure
was produced as follows: Samples were placed into a
multisource vacuum-processing chamber and preheated for
120 min to a contact-deposition temperature of 340 C. Prior to
ZnTe:Cu deposition, approximately 100 nm of material was
removed using Ar ion-beam milling with a 3-cm Kaufman-
type ion gun operating at a beam energy and current of 500 eV
and 6 mA, respectively. ZnTe:Cu layers (2 wt.% Cu) were
deposited by radio-frequency (r.f.) sputtering to a thickness of
0.4 pm followed by 0.5 pm of Ti was deposited using direct
current (d.c.) magnetron sputtering. The contacted samples
were allowed to cool in the vacuum chamber for at least 2 hrs
after Ti deposition. For samples that included Te exposure, the
CdCl2-treated back surface was exposed to Te as the sample
temperature was raised to 340 C in the same chamber that is
used for contacting (i.e., before the ion-beam milling step). At
this time the amount of Te added to this surface is uncertain,but it is believed to be a relatively small - on the order of a
few nm if it were allowed to condense into a Te film.
Following the contact formation processes, a pattern of
individual 0.25-cm2 cells was defined photolithographically on
all samples. Cell definition was done by two-step chemical
etching, first using TFT Ti Etchant (Transene Co. Inc.,
Rowley, MA) to remove the Ti, followed by an aqueous
solution of 39% FeCl3 to remove the ZnTe:Cu and CdTe. A
perimeter contact onto the SnO2 layer was formed with
soldered indium.
Analysis of the resulting materials and devices included
light/dark current voltage (LIV/DIV), capacitance voltage
(CV) at 100 kHz, room-temperature spectroscopic
photoluminescence (RTPL), low-temperature spectroscopic
photoluminescence (LTPL), time-resolved photoluminescence
(TRPL), and secondary ion mass spectrometry (SIMS)
measurements. RTPL, LTPL, and TRPL measurements are
taken from the glass side of the superstrate devices.
Additional experimental details are provided in ref. 2.
III. RESULTS
Figure 1 shows a sequence of LIV curves detailing the
change in LIV performance of CdS/CdTe devices before and
after exposure of the back contact to Te. The figure shows
that, immediately before Te exposure, the devices demonstrate
a baseline efficiency of ~14% (device 14480 in Fig. 1). This is
typical for CdS/CdTe devices made at NREL using the
processes described in the Experimental section.20 -
E
Ea 0-
E
H0 -
-10-
U
-20 -- 14160_P Before Te Adpitionl (14 3%)
- 11I01 _M At -r Ie A-diton (1 1%)
- 14 0'2A_- R-crverV 1 r58%)
- 1450_P D RecoVery (11 016)
- - 140O5_P Fill Pecorarv (13 6%)I I I 1I
-0.2 0.0 0.2 0.4 0.6
Voltage (V). 1
0.8 1.0Fig. 1. LIV characteristics of CdS/CdTe devices produced before
and after Te exposure prior to the ZnTe:Cu/Ti contacting process.
When the back surface of the devices is exposed to Te, the
LIV efficiency drops to ~1% (device 14501 in Fig. 1).
Furthermore, after the vacuum system is used for Te exposure,
the next several devices contacted in the same vacuum system
demonstrate poor performance (see representative devices2
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Gessert, T. A.; Burst, J. M.; Ma, J.; Wei, S. H.; Kuciauskas, D.; Barnes, T. M. et al. Response of Cds/CdTe Devices to Te Exposure of Back Contact: Preprint, article, June 1, 2012; Golden, Colorado. (https://digital.library.unt.edu/ark:/67531/metadc829032/m1/4/: accessed July 16, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.