## STUDIES OF BASIC ELECTRONIC PROPERTIES OF CdTe-BASED SOLAR CELLS & THEIR EVOLUTION DURING PROCESSING & STRESS

###### **Quarterly Technical Status Report for the Quarter ended April 30, 2003**

Under Prime Contract No. DE-AC36-99-GO10337; Subcontract No.
ADJ-2-30630-05

#### **Subcontractor: Colorado School of Mines**

### **Principal Investigators: V. Kaydanov and T. R. Ohno**

### **Contributors:** 

**Professors R.T Collins, P. Sutter, and E. Sutter;**

#####  Grad. Research Associates A.S. Gilmore, S. Feldman, and F. Seymour 

####### Contract technical monitor: Kenneth Zweibel

In the past quarter we were continuing development and improvement of
approaches and techniques for studying electronic properties of defects
in polycrystalline thin film solar cells. In particular, we were
focusing on admittance spectroscopy (AS) for detecting and studying
defect levels in the bandgap, high resolution imaging
electroluminescence (EL) as a method for studying micro-nonuniformities,
and scanning tunneling microscopy (STM) and spectroscopy (STS) which are
aimed at mapping and studying nonuniformities on the subgrain scale. All
these methods were applied to CdTe/CdS cells. In this report we describe
and analyze changes in AS data and EL patterns caused by cell stressing
(cells were fabricated at First Solar). Also the STM images of
CdS/CdTe/ZnTe:Cu structures are presented and discussed.

**Admittance Spectroscopy (AS)**

We show (Figs. 1-3) and discuss here the data for two cells stressed for
7 days that present the major features and the differences supposedly
caused by different stress conditions: open circuit (OC) -- cell \#
E4-31, and reverse bias (RB) - cell \# E4-34. The features at the
extremely high frequencies are not discussed because their origin is
ambiguous. Only the data for f ≤1MHz are analyzed below. Reducing
capacitance and rising conductance toward higher frequency are due to
oscillations of the trapped charge under applied AC testing voltage.

The C(f) dependence for E4-34(RB) in Fig.1 obviously tends to saturation
in the frequency range of \~1MHz. If we assume the testing frequency to
be much higher than the characteristic frequency for the traps of all
types, then the trap contribution to capacitance is negligible, hence

measured value is equal to the "depletion" (geometrical) one. The
estimated depletion width is ≈2.7μ. Measured capacitance decreases by a
factor of 1.8 between 20Hz and 1MHz. It means that the total
concentration of traps with the characteristic times shorter than \~10ms
is of the same order, maybe even higher, than the doping level. A
gradual decrease in capacitance indicates presence of more than one trap
level, maybe continuous distribution of states over energy. The C(f)
dependence for E4-31(OC) saturates in the same frequency range and at
almost the same level. However the low frequency value (C~LF~) is only
30% higher, which means that concentration of traps with τ\>10ms in this
cell is significantly lower than in E4-34(RB).

![](media/image1.wmf) **Figure 1.** Capacitance versus frequency
**Figure 2.** Conductance versus equency

> ![](media/image3.wmf)
>
> **Figure 3.** Conductance divided by frequency as a function of
> frequency

More dramatic is the difference in the G(f) dependencies for the two
cells (Fig. 2). The G value for E4-34(RB), which is only of 10^-7^Ω^-1^
cm^-2^ at f=20 Hz, increases up to 10^-4^ Ω^-1^ cm^-2^ at 100KHz. The
sublinear dependence (G ∝f ^0.65^) indicates the distributed trap
density of states in agreement with the shapes of C vs.f and (G/f) vs.f
plots in Figs. 1 and 3. Quite different is the G(f) dependence for
E4-31(OC): G~LF~ ≈10^-4^ Ω^-1^ cm^-2^ (compare with 10^-7^ for E4-34)
and stays practically the same up to 100KHz. To explain this, we assume
that the measured conductance has two components: G(f)=G~d~+G~t~(f).
G~d~ is the frequency independent DC conductance of a diode. Since we
measure AS in dark and at zero bias voltage, it is reasonable to assume
that G~d~=G~zb,~ zero bias conductance determined from the slope of dark
J(V) curve. The trap related component G~t~ is provided by trap
recharging hence varies with frequency. The G(f) dependence cannot be
observed in some frequency range if G~t~(f)/G~d~\<\<1 in this range.
Table 1 shows that G~LF~ \> G~zb~=G~d~ for both cells, hence the
conductance growth with frequency should be expected.

## Table 2. Low frequency (LF) conductance and dark "zero bias" conductance

  ----------------- -------------------------- --------------------------
                    G~LF~(f=20Hz), Ω^-1^       G~zb~, Ω^-1^ cm^-2^
                    cm^-2^                     

  E4-31 (OC)        1.2x10^-4^                 1.75x10^-6^

  E4-34 (RB)        2x10^-7^                   6.7x10^-8^
  ----------------- -------------------------- --------------------------

However for the cell E4-31(OC), G(f) ≈const. in the frequency range of
20Hz to 100KHz.The only explanation we can propose now is that there is
a great amount of slow traps (τ\>10ms) in this cell whose contribution
to the measured conductance saturates at f\<20Hz, hence it does not
increase with the further frequency increase. This suggestion does not
contradict the observed C(f) dependence for this cell (Fig.1). Indeed
the contribution of traps to capacitance is negligible when their
characteristic frequencies are much lower than the testing voltage
frequency. Just this was suggested to explain the $G(f)$dependence in
this cell.

The conclusions based on the AS measurements are as follows:

-   There is a large concentration of traps both in RB and OC stressed
    cells, comparable to doping level or even higher.

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-   Concentration of "fast" traps ($\tau < \text{10}\text{ms}$) is
    several times greater in the RB cell than in the OC one. The trap
    levels are distributed in some energy range. Their density decreases
    for the shallower levels ("faster" traps) as follows from the shape
    of the graph for E4-34 in Fig. 3.

-   In the OC cell the amount of "slow" traps
    ($\tau\text{>>}\text{10}\text{ms}$) is much (at least three orders
    of magnitude) greater than for the RB cell.

Electroluminescence (EL)

In general, the response of EL to stress has been a significant drop in
mean EL intensity.

![](media/image4.wmf){width="1.1666666666666667in" height="1.05in"}The
EL pattern for FS cells changed dramatically after stressing (Fig. 4).
While the as-prepared cell demonstrated a mottled, but relatively
uniform EL emission pattern, in stressed cells it comes only from
isolated spots. In particular, in the 7 day-stressed cells the bright EL
region concentrates at the cell periphery (RB stress) or close to it (OC
stress). In this region the EL intensity is so high that pixels are
saturated which means that intensity here is at least 200 times higher
than in the dark inner parts of a cell.

![](media/image6.wmf){width="2.942361111111111in"
height="1.2972222222222223in"} Figure 4. EL images for FS cells. From
left to right: as-prepared; 56 days OC stress;

> 7days RB stress; 7 days OC stress. Stress in light.

Observed changes in EL mean intensity and spatial distribution can be
caused by degradation of p-n junction region where recombination of
injected carriers occurs, but also by the processes in other parts of a
cell, particularly in the back contact region. The following discussion
is focused on the latter. EL is caused by forward current, and we study
it in dark environment. The analysis of dark J-V characteristics shows a
significant increase in dark forward bias resistance, especially strong
in the RB stressed cells. The voltage across the cell is uniform over
the cell area, because high conductance TCO front contact and back
contact metallization both are surfaces of constant electric potentials.
Hence the current density is inversely proportional to the back contact
resistance per unit area:$j = \frac{V}{\rho_{\text{bc}}}$. If the latter
is strongly non-uniform, i.e., degraded severely over major part of the
back contact area, but much less at the edge region and in some small
interior spots, then the current density can vary significantly. EL
intensity increases with current density (linearly or more rapidly),
thus it could be very bright at the areas in the low
$\rho_{\text{bc}}$regions and rather low in the areas of enhanced
$\rho_{\text{bc}}$.

Our preliminary experiments verified these expectations. We have
separated (cut) some cell interior fragments that looked rather dark in
the whole cell images and measured EL intensity for the same forward
current/area ratio. First, forward bias dark resistance per unit area
was found considerably higher for the cut than for the whole cell. This
fits the idea that the regions of bright EL provide shunting effect.
Second, the EL intensity in the separated fragment was much higher than
before separation, as expected. Now the pattern of brighter spots was
well seen on the dark background. Thus the non-uniform degradation of
the back contact specific resistance provides a significant contribution
in the nonuniformity in EL distribution. Vice versa, studying EL is a
convenient (nondestructive) test of the back contact resistance
non-uniformity and its evolution under stressing.

# **Scanning Tunneling Microscopy on CdTe/CdS Solar Cells**

We have previously demonstrated that reproducible scanning tunneling
microscopy (STM) and spectroscopy (STS) are feasible on CdTe/CdS solar
cells without the back contact metallization. While the uncoated CdTe
surface is too resistive for stable tunneling, evaporation of a thin
(\~50 nm) p^+^-doped ZnTe back contact layer leads to a stable tunneling
contact to the cell.

Current images, obtained from current-voltage (I-V) curves measured as a
function of tip position over the cell (Fig. 5), clearly show
non-uniform current injection into the cell. Areas of material with
relatively uniform injected current coexist with high-current pathways,
localized mostly but not exclusively at grain boundaries. We have
performed model calculations to understand the nature of the information
obtained by STM current imaging. Clearly, the conducting ZnTe layer
plays a key role in enabling stable tunneling. We have evidence that it
also creates a distinct and unconventional contrast mechanism. The
polycrystalline structure of a cell makes it likely that vertical
current transport (in the direction across the junction) is not uniform,
but consists of a network of 'current channels' carrying a large
fraction of the vertical current (Fig. 6). If the STM tip is placed
directly on the top of a current channel (e.g., a grain boundary), the
total resistance to current flow consists of the tunneling resistance,
R~t~, in series with the resistance of the current channel, R~c~,
beneath the tip. If, however, the tip is moved over low-conductance CdTe
(e.g., within a grain) an additional series resistance R~s~ is
encountered, which hinders the spreading of the injected current in the
ZnTe contact to the nearest current channel. The position dependent
resistance of current spreading in the ZnTe, R~s~(x,y), provides
contrast in STM current maps between regions of low and high conductance
in vertical transport through the cell. A comparison of model
calculations of the injected current as a function of injector tip
position with experimental results shows excellent agreement, strongly
supporting the proposed STM contrast mechanism. Probably a continuous
rather thin back contact metallization will likely *not* provide such a
contrast, since the resistance R~s~ of current spreading in a metal can
be expected to be much smaller than both R~c~ and R~t~. The contrast
mechanism used here emphasizes the transport through the cell, and
essentially *ignores* the information contained in the characteristics
of electron tunneling across the vacuum gap between tip and ZnTe layer.
We argue, however, that the tunneling characteristics relate mostly to
the electronic structure of the ZnTe layer, and can be omitted if we aim
at characterizing transport through the underlying polycrystalline CdTe.

![](media/image7.png){width="2.8722222222222222in"
height="2.6041666666666665in"}

**Figure 6.** 6 μm × 6 μm composite STM image, consisting of an overlay
of a 3D topographic

scan with a current image obtained at a tip sample bias of 1.5 V. Color
scale: dark -- low current; bright -- high current. Zones of higher
injected current are often localized at grain boundaries, but occur also
within some of the grains.

![](media/image8.png){width="4.677777777777778in"
height="1.7465277777777777in"}

**Figure 2**. Schematic representation of the contrast mechanism in STM
current imaging on ZnTe-terminated CdTe/CdS solar cells.

We believe that this contrast mechanism will allow evaluating
non-uniform transport through complete polycrystalline solar cells in
general, i.e., not limited to the system CdTe/CdS. We also foresee
applications to other non-uniform structures, such as a-Si:H, in which
microscopic voids coexist with denser material so that non-uniform
current transport can be anticipated \[1\]. Near-surface transport
channels have been mapped by us with spatial resolution of the order of
10 nm, i.e., about two orders of magnitude below the grain size.
High-resolution current maps with resolutions down to 1 nm should be
feasible in principle. Such maps will provide detailed information on
current collection and transport near grain boundaries, which has been
the subject of intense debate recently \[2, 3\]. Furthermore, our unique
ability to obtain high-resolution maps of vertical transport may prove
particularly powerful if used in the context of controlled cell
processing. The effect of processing steps such as CdCl treatment or Cu
deposition beneath the back contact can thus be studied in detail and
with very high spatial resolution.

Our first STM data demonstrate that current transport in the direction
toward the junction of a CdTe/CdS cell is highly non-uniform. Under
operating conditions carriers will likely be generated within grains,
but will be collected and transported across the junction along a
network of discrete conduction paths. A large fraction of the total
junction current thus flows in low-resistance paths that cover only a
very small fraction of the cell area. This finding is consistent with
previous models of grain boundary potentials \[4\], and opens up new
opportunities for understanding and improving current collection in
CdTe/CdS solar cells. Importantly, the current paths are not always
localized at grain boundaries, but are often found within grains. This
fact points to a conduction mechanism involving specific defects,
possibly decorated by impurities such as Cu. The nature of these defects
is still unknown. However, we believe that the statistical correlation
of the microstructure and defect distribution (mapped by TEM) with maps
of current paths obtained by STM will ultimately lead to the
identification of the microscopic nature of the current transport
mechanism in CdTe/CdS cells.

**References**

\[1\] D. L. Williamson, D. W. M. Marr, J. Yang, B. Yan, and S. Guha,
Phys. Rev. B **67**, 075314

(2003).

\[2\] M.K. Herndon, A. Gupta, V. Kaydanov, and R.T. Collins, Appl. Phys.
Lett. 75, 3503 (1999).

\[3\] I. Visoly-Fisher, S.R. Cohen, and D. Cahen, Appl. Phys. Lett.
**82**, 556 (2003).

\[4\] L.M. Woods, D.H. Levi, V. Kaydanov, G.Y. Robinson, and R.K.
Ahrenkiel, in Proc. 2^nd^ World Conf. on PV Solar Energy Conversion, J.
Schmid et al., eds., pp. 1043-1046.