Deep Electronic States in CdTe Solar Cells Detected with Admittance
Techniques

F. H. Seymour, V. Kaydanov, T. R. Ohno

Colorado School of Mines, Physics Department

1523 Illinois Street, Golden, Colorado 80401 USA

<fseymour@mines.edu>

#### ABSTRACT

Deep electronic states (DES) in the absorber layer of thin film
polycrystalline CdTe solar cells are believed to be limiting their
energy conversion efficiency. Admittance spectroscopy and capacitance
transient techniques were applied with a custom-built temperature stage
to detect and characterize these DES. CdTe solar cells obtained from two
sources that had post-deposition back contact treatments with and
without Cu and CdCl~2~ were studied. DES were detected with activation
energies ranging from 0.13 eV to 0.83 eV. One hole trap signature with
E~a~\~0.15 eV was detected in all four cell categories in different
concentrations and was attributed to a combination of
$V_{\text{Cd}}^{-}$ and the chlorine A-center complex
$V_{\text{Cd}}^{2 -} - \text{Cl}_{\text{Te}}^{+}$defects. A second hole
trap signature with E~a~\~0.35 eV and detected only in Cu treated cells
was attributed to the copper substitutional $\text{Cu}_{\text{Cd}}^{-}$
defect. Analysis of current density voltage temperature data indicates
that the $V_{\text{Cd}}^{2 -} - \text{Cl}_{\text{Te}}^{+}$defect might
be responsible for the improved J~SC~ observed in the CdCl~2~ treated
cells.

**1. Objectives**

With respect to the Solar Program Multi-Year Technical Plan, this study
is in the category of fundamental R&D of flat plate thin-film CdTe solar
cells. The objective is to study micro-defects, their defect-related
electronic states, and their impact on cell performance and stability.

DES in CdTe polycrystalline thin film solar cells occur in
concentrations often exceeding the doping level and they influence the
electronic behavior of solar cell devices and are known to significantly
impact cell performance and stability.

Our goal is to detect and identify the defects that cause these DES and
relate their characteristics to solar cell performance and stability so
that higher efficiency CdTe solar cells can be produced.

**2. Technical Approach**

DES in CdTe/CdS cells were studied using admittance spectroscopy (AS)
and capacitance transient (CTr) measurements. With AS, the
characteristic time, , was determined from the position on the frequency
scale of the peak in the --dC~p~/dln() function where C~p~ is the
parallel equivalent circuit resistor capacitance and is the angular
frequency of the testing voltage. With CTr, the DLTS function,
dC~p~/dln(t), was analyzed. For the exponential time dependence C(t) =
$C_{t}^{0}$ exp(-t/), the peak of this DLTS function is located on the
time scale at t~peak~ = . The origin of the time scale, t=0, corresponds
to when the bias pulse is switched on or off. The DES signatures:
activation energy, E~a~, and apparent capture section, ~a~, were
estimated from the Arrhenius plot ln(T^2^) vs. 1000/T regression slope
and Y-intercept. The capacitance shift magnitude, $C_{t}^{0}$, was used
as a relative indicator of DES concentration for both techniques.

**3. Results and Accomplishments**

*3.1 Identification of DES*

Sets of CdTe solar cells fabricated by the National Renewable Energy
Laboratory (NREL) and the Institute for Energy Conversion (IEC) were
analyzed. Each set had post-deposition back contact treatments with and
without Cu as well as with and without CdCl~2~. Three distinct
signatures were detected with AS and these are listed in table 1.

Table 1. DES detected in CdTe solar cells with AS.

+---------+---------------+------------+---------------+---------------+
| DES     | Cu/CdCl~2~    | E~a~       | σ~a~          | $$C_{t}^{0}$$ |
|         |               |            |               |               |
|         |               | eV         | cm^2^         | nF cm^-2^     |
+---------+---------------+------------+---------------+---------------+
| *NREL   |               |            |               |               |
| Cell    |               |            |               |               |
| R       |               |            |               |               |
| esults* |               |            |               |               |
+---------+---------------+------------+---------------+---------------+
| H1      | yes/yes       | 0.133      | 4 X 10^-14^   | 2.1           |
+---------+---------------+------------+---------------+---------------+
|         | no/yes        | 0.128      | 8 X 10^-14^   | 3.0           |
+---------+---------------+------------+---------------+---------------+
|         | yes/no        | 0.128      | 2 X 10^-14^   | 1.1           |
+---------+---------------+------------+---------------+---------------+
|         | no/no         | 0.145      | 1 X 10^-12^   | 2.5           |
+---------+---------------+------------+---------------+---------------+
|         |               |            |               |               |
+---------+---------------+------------+---------------+---------------+
| H2      | yes/yes       | 0.305      | 9 X 10^-14^   | 0.9           |
+---------+---------------+------------+---------------+---------------+
|         | yes/no        | 0.292      | 6 X 10^-14^   | 0.5           |
+---------+---------------+------------+---------------+---------------+
|         |               |            |               |               |
+---------+---------------+------------+---------------+---------------+
| H3      | no/yes        | 0.475      | 7 X 10^-13^   | 0.6           |
+---------+---------------+------------+---------------+---------------+
|         | no/no         | 0.464      | 8 X 10^-13^   | 0.5           |
+---------+---------------+------------+---------------+---------------+
| *IEC    |               |            |               |               |
| Cell    |               |            |               |               |
| R       |               |            |               |               |
| esults* |               |            |               |               |
+---------+---------------+------------+---------------+---------------+
| H1      | yes/yes       | 0.160      | 5 X 10^-13^   | 1.1           |
+---------+---------------+------------+---------------+---------------+
|         | no/yes        | 0.157      | 8 X 10^-13^   | 3.1           |
+---------+---------------+------------+---------------+---------------+
|         |               |            |               |               |
+---------+---------------+------------+---------------+---------------+
| H2      | yes/yes       | 0.358      | 2 X 10^-13^   | 2.6           |
+---------+---------------+------------+---------------+---------------+
|         | yes/no        | 0.353      | 1 X 10^-13^   | 0.6           |
+---------+---------------+------------+---------------+---------------+
|         |               |            |               |               |
+---------+---------------+------------+---------------+---------------+
| H3      | no/yes        | 0.426      | 2 X 10^-13^   | 3.0           |
+---------+---------------+------------+---------------+---------------+
|         | no/no         | 0.475      | 7 X 10^-13^   | 0.6           |
+---------+---------------+------------+---------------+---------------+

The H1 signature is consistent with reported E~a~ values for both the
singly ionized cadmium vacancy acceptor defect $V_{\text{Cd}}^{-}$ (ref
1,2) and the chlorine A center complex
$V_{\text{Cd}}^{2 -} - \text{Cl}_{\text{Te}}^{+}$ (ref 2,3). Based on
$C_{t}^{0}$ values, the H1 concentration decreases with the Cu treatment
which is consistent with $\text{Cu}_{\text{Cd}}^{-}$ displacing
$V_{\text{Cd}}^{-}$ and the H1 concentration increases with the CdCl~2~
treatment which is consistent with the formation of
$V_{\text{Cd}}^{2 -} - \text{Cl}_{\text{Te}}^{+}$. H1 is below the
detection limit for the non-CdCl~2~ treated cells from IEC. The IEC
cells had a bromine methanol back contact etch treatment that leaves
behind a thin 1-2nm Te layer and is not a large source of
$V_{\text{Cd}}^{-}$. This contrasts with the NREL cells which had a more
aggressive nitric phosphoric acid etch that leaves behind a thicker
\~100nm Te layer and is a significant source for $V_{\text{Cd}}^{-}$
(ref 4).

H2 was only detected in the Cu treated cells. We found E~a~ values in
the range of 0.30 eV to 0.35 eV which is consistent with the
$\text{Cu}_{\text{Cd}}^{-}$ defect reported by others^5^ and we have
inferred that this is the $\text{Cu}_{\text{Cd}}^{-}$ defect.

H3 was only detected in the non-Cu treated cells. We found E~a~ values
in the range of 0.43 eV to 0.48 eV and it is possible that this is
either a doubly ionized cadmium vacancy $V_{\text{Cd}}^{2 -}$ (ref 1,6)
or a negative U-center tellurium interstitial $\text{Te}_{i}^{-}$ (ref
2). However, we remain cautious about assigning H3 to a specific defect.

Several minority and majority carrier DES were detected with CTr. The
most notable was a majority carrier trap, H5, detected during a +0.5 V
filling pulse. It was detected in both the NREL and IEC cells with
yes-Cu/yes-CdCl~2~ treatments. It is deep, with E~a~ in the range of 0.6
to 0.8 eV, it has a small capture cross section with σ~a~ in the range
of 10^-18^ to 10^-16^ cm^2^, and it has a large delta capacitance
amplitude with$C_{t}^{0}$\> 35 nF cm^-2^. It was detected in the
temperature range 300 K to 360K. There are indications that H5 also
occurs in the non-Cu and non-CdCl~2~ treated cells at higher
temperatures beyond the equipment range. This DES occurs near the
metallurgical junction and it is possible that it is caused by defects
resulting from the lattice mismatch or the sulfur intermixing near the
CdTe/CdS metallurgical junction. Because of its high concentration and
depth in the band gap, H5 could be a significant Shockley-Read-Hall
recombination center.

*3.2 DES correlation to performance*

Current density voltage (J-V) analyses with one sun illumination at
temperatures ranging from -190^o^C to 20^o^C were performed on the
differently processed cells. Of interest is the plot of short circuit
current J~SC~ versus T as shown in figure 1. The better J~SC~ that is
commonly observed with CdCl~2~ treated cells decreases gradually with
decreasing T and then abruptly falls off. This behavior was observed
with both NREL and IEC cells and contrasts with the non-CdCl~2~ treated
cells. The gradual decrease in J~SC~ can be attributed to decreasing
solar spectrum collection caused by an increasing band gap that is
dE~g~/dT\~ -1.7 meV/K The abrupt decrease in J~SC~ that begins in the
vicinity of -120^o^C to -140^o^C suggests that a single defect energy
level is freezing out and is responsible for the J~SC~ current
collection.

The energy level for the defect can be estimated from the Fermi energy
level given by E~f~=k~b~Tln(N~v~/N~a~) where the valence band effective
density of states is N~v~=2(2πm~p~k~b~T/h^2^)^1.5^. If we take the
doping level of N~a~\~10^14^ cm^-3^ and the effective mass for holes to
be m~p~=0.8m~o~, we get E~f~\~0.16 eV for this temperature range. This
is close to the reported E~a~ values for
$V_{\text{Cd}}^{2 -} - \text{Cl}_{\text{Te}}^{+}$ and indicates that it
might be responsible for the superior current collection observed with
CdCl~2~ treated cells.

![](media/image1.wmf){width="2.9895833333333335in" height="2.8125in"}

Fig 1. J~SC~ versus T for NREL CdTe solar cells

**4. Conclusions**

CdTe solar cells obtained from NREL and IEC that were treated with and
without Cu as well as with and without CdCl~2~ were analyzed with AS and
CTr. Three DES were identified with AS: $V_{\text{Cd}}^{-}$,
$V_{\text{Cd}}^{2 -} - \text{Cl}_{\text{Te}}^{+}$, and
$\text{Cu}_{\text{Cd}}^{-}$. Further DES were detected and characterized
with AS and CTr. J-V temperature analysis of J~SC~ indicates that
$V_{\text{Cd}}^{2 -} - \text{Cl}_{\text{Te}}^{+}$may be the primary
enabler for the enhanced current collection observed in CdCl~2~ treated
cells.

Further work is planned to enhance our understanding of the roles of
$V_{\text{Cd}}^{-}$, $V_{\text{Cd}}^{2 -} - \text{Cl}_{\text{Te}}^{+}$,
and $\text{Cu}_{\text{Cd}}^{-}$ in solar cell performance.

**ACKNOWLEDGEMENTS**

We thank Dave Albin from NREL and Brian McCandless from IEC for
supplying the solar cells used in these studies. This work was supported
by the Department of Energy under grant No. ADJ-2-30630-05.

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^3^D. M. Hoffmann et al, *Phys. Rev. B*, **45**, 6247 (1992)

^4^D. Albin et al, MRS proceedings, April 2002

^5^A. Balcioglu et al, *J. Appl. Phys*., **88**, 7175, (2000)

^6^P. Emanuelsson et al, *Phys. Rev. B*, **47**, 15578 (1993)

**MAJOR FY 2005 PUBLICATIONS**

F. H. Seymour, V. Kaydanov, T. R. Ohno, D. Albin, "Cu and CdCl~2~
influence on defects detected in CdTe solar cells with admittance
spectroscopy", *Appl. Phys. Lett.,* **87** 153507 (2005)