# 'Lab to Large Scale' Transition for Non-Vacuum Thin Film CIGS Solar Cells 

## 

## Status Report No. 6

**November 1, 2003 -- January 31, 2004**

# NREL Contract \# XCQ-230630-30

# International Solar Electric Technology Inc. (ISET)

The goal of this contract is to identify the challenges faced by ISET in
scaling up its non-vacuum process from a laboratory to large scale
manufacturing and to develop workable solutions for these challenges
such that they can be readily implemented in large scale processing of
CIGS solar cells and modules. In phase II (2^nd^ year) of this 3-year
contract, ISET is focusing on i) further improvement in cell efficiency,
ii) evaluation of alternate deposition techniques for coating ISET's
inks uniformly on large area substrates, iii) developing monolithic
integration schemes for making CIGS modules on glass substrates and iv)
increasing material's usage and minimizing waste.

#### **I. Improving Solar Cell Efficiency**

Previously, we reported considerable cell voltage improvements by
surface modification with a novel gallium surface treatment, which
included spray deposition, film conversion, post-annealing and etching
steps. Our search for a selective dissolution procedure for removal of
unincorporated phases after modification showed us the effects of
various etchants on Ga-modified samples. We observed statistically
significant gains in Voc after modification but these results were
typically coupled with losses in current collection. Our initial
strategy to modify surface with gallium as the only group III cation was
a step in the right direction, however this route to gallium rich
adlayers posed fundamental challenges in stabilizing device currents due
to possible formation of defective binary and ternary phases at the
surface.

We got around this problem of deep current losses by treating the
surface with a slight twist on the normal scheme. Our gallium-only
surface treatment relies on extended high temperature selenization times
to fuse surface adlayers with the bulk chalcopyrite semiconductor. A few
problems arise with this method: (i) gallium cation in a semi-infinite
overlayer is a source of defective material at the surface (ii) high
temperature processing causes an eventual degradation of the Mo back
contact in a selenization ambient because of the formation of MoSe and
(iii) the surface modification process then becomes a costly
manufacturing alternative from the time and materials perspective.

We reasoned that copper diffusion into an In-Ga overlayer could form the
single-phase chalcopyrite we seek at the surface by maintaining a
favorable I/III ratio in the adlayer while minimizing the formation of
defective ternary phases at the near surface. In this modified approach
we began to modify the surface with both Group III chalcogenides in the
surface adlayer using our convenient spray-based technique. Rockett *et.
al.* previously reported that Ga-diffusivity in CIS films is more
dependent on film composition (in terms of the I/III ratio) than on
temperature, with higher rates of gallium diffusion on the indium rich
side \[1\]. In modifying the absorber, we are both driving in indium and
gallium while simultaneously diffusing copper into the adlayer.

Immediately after switching to the modified Group III chalcogenide
surface treatment the current collection shot up without the assistance
of post-etching while still maintaining improved cell Voc. With this
surface modification we were able to increase the efficiency of a 11+%
CIGS solar cell (fabricated by our non-vacuum method) to 13.7%(AM1.5).
Gains in the cell efficiency were primarily by an increase in the V~oc~
of up to 70 mV. We repeated our newly developed surface treatment on a
sister reference sample and increased the efficiency of the solar cell
once again primarily by increasing the V~oc~ all the while maintaining
device currents. The I-V characteristics of both cells are summarized in
Table I with respective curves below in Figure 1. All devices reported
here do not have anti-reflection coating.

**Table I.** Overview of CIGS Solar Cell Parameters with Surface
Modifications

  ----------------- ----------------- ----------------- -----------------
  Device Parameters Control           With Surface      Repeat
                                      Modification      Modification

  Voc (mV)          446               524               508

  Isc (mA)          3.12              3.00              3.19

  Jsc (mA/cm^2^)    37.1              35.7              37.9

  FF                0.69              0.73              0.68

  Pmax (mW)         0.96              1.15              1.10

  Eff (%)           11.4%             13.7%             13.0%

  Area (cm^2^)      0.084             0.084             0.084
  ----------------- ----------------- ----------------- -----------------

![](media/image1.wmf){width="4.2868055555555555in"
height="3.0708333333333333in"}

Figure 1. Current-voltage characteristics of champion solar cells made
by surface modification.

#### **II. Evaluating Alternate Techniques for Absorber Deposition**

Over the last period we demonstrated pigment-based printing of a 1.0-1.2
m thin film by inkjet onto a flexible foil substrate (Figure 2). Ink
chemistry plays an important role in determining the performance of the
pigment-based coatings. We are currently evaluating the rheological
characteristics of commercial pigmented printing inks and comparing
these with our own. The goal is to make formulation adjustments that
will mimic the physicochemical flow behavior of printing inks based on
adjusted formulations wherever necessary.

An important consideration in pigment-based inkjet printing is the
inherent disadvantage of dispersion stability commonly leading to a
clogging of the printer head after pigment particles flocculate in
suspension. The dispersion must have a high zeta potential value ()
ensuring dispersion stability and the average agglomerate size (AAS)
must be minimized to ensure the jetting potential of the material
through the nozzle. In this period, we have characterized our pigments
for particle size, measured the AAS value in suspension and performed
zeta potential measurements in order to characterize our dispersion
stability.

It is typically recognized in the industry that particles above 500 nm
size may cause plugging of ink jet nozzles compromising printing
quality. Our mean primary particle size, based on the synthesis
conditions is typically about 270 nm with an agglomerate size of 1-3
microns (see Figure 3). Our AAS value is well within the orifice size of
the inkjet which is in the tens of microns. Our ink can also be
pre-filtered prior to entering the jetting chamber leaving out the fines
poor component without changing the semiconductor composition which is
fixed nearly molecularly within the pigment. **[We have determined that
our pigments and standard formulation is already within the operating
domain of inkjet printing.]{.underline}**

![](media/image2.jpeg){width="3.877083333333333in"
height="2.7319444444444443in"}

**Figure 2.** Demonstration of pigment-based coating onto a flexible
foil substrate by ink jet printing.

![](media/image3.wmf){width="4.566666666666666in"
height="2.970138888888889in"}

**Figure 3.** Particle size distribution of nanoparticle oxide pigment
by Dynamic Light Scattering. Deconvolution by NNLS algorithm shows
primary particle size distribution centered at 270nm. The agglomerate
size in the suspension is 1143nm ± 300 nm.

Ink stability is a major issue for jetting fluids. To evaluate the
stability of our inks, we performed zeta potential measurement of our
pigment suspended in its mother liquor (at formulation pH). The general
cutoff for a stable colloid is normally taken as \> 30 mV, where a
greater magnitude of the zeta potential means greater stability against
flocculation and sedimentation. Figure 4 shows that the zeta potential
of our formulation which has been modified for inkjet printing has a
very high value of --78.59 mV, which indicates very high suspension
stability. These types of measurements allow us to predict the stability
of our nanoparticle oxide ink as we change the formulation to suit our
needs for printing precursor layers using inkjet printing.

![](media/image4.wmf)

**Figure 4.** Zeta potential measurement of ink jet compatible
formulation of ISET's nanoparticle oxide ink ( = -78.59mV) shows high
dispersion stability.

![](media/image5.wmf){width="5.986111111111111in"
height="2.966666666666667in"}

**Figure 5**. Control of Fluid Rheology in Nanoparticle Ink by
Formulation Adjustments. Coating viscosities converge with commercial
pigmented inks.

Rheology is yet another critical property governing flow characteristics
which determines coating performance. Our inks are typically
pseudoplastic (shear-thinning) in nature until higher shear rates where
we observe low Newtonian viscosity. At high shear, these low Newtonian
viscosities (1-10cps) give us a great deal of control over coating
performance. By adjusting the ink formulation, we converge the high
shear kinematic viscosities of our own nanoparticle ink with typical
pigmented printing inks (see Figure 5). This adjustment gets us closer
to obtaining specialty formulated inkjet printing inks for precursor
deposition.

**[Summarizing the results above, we have established that our
water-based pigmented inks and standard formulations are already within
the domain of inkjet printing. The particle size of our inks is within
the operating domain, the predicted stability of the formulation is
excellent and the rheology can be controlled by adjustments to the
formulation that are under our control.]{.underline}** With the right
printer, we will be commencing the ink-jet printing operations very
shortly.

#### **III. Monolithic Module Integration and Top Contact Development**

As of last report we have shown:

-   Exposure of the CIGS cells to temperatures around 170C, for curing
    of top contacts, degrades the performance of the cell underneath.
    All parameters, Efficiency, Voc, Jsc and FF degraded as a result of
    this exposure.

-   The degradation increases with increased exposure time at high
    temperature.

-   When inks are cured at 100C, the loss of cell performance is
    significantly reduced but not eliminated.

-   While most of the inks on hand have manufacturer specified curing
    temperatures well above 100C, several of them *can* be 'cured' to
    meet their specifications at 100C as well.

-   When these inks are screen printed onto CIGS cells, the performance
    of the cells with contacts is comparable -- and in some cases ever
    better, than the small area test cells that do not have the
    contacts.

-   The best large area cell (area = 0.95 cm^2^) with printed top
    contacts showed an efficiency of 12.3%, Voc = 0.49 V, Jsc = 40.1
    mA/cm^2^ and FF = 0.63.

-   Comparison with small area (0.084 cm^2^) test cells showed that the
    above performance was only limited by the cell underneath and not by
    the top contact.

The issues that remained were

###### Can the cell degradation be suppressed further by lowering the curing temperature without compromising contact quality,

-   Can the ZnO deposition temperature be lowered,

-   Does etching of ZnO and doping of silver ink improve the contacts,

-   How stable are the contacts.

Some of these issues were addressed this quarter.

###### Can inks be 'cured' at 80C

Since curing the silver inks at 100C still entailed a small loss in cell
performance we tested the 'curability' of inks at 80C. Test patterns of
some of the inks that were most promising from the last round of
experiments, were deposited and samples were cured on a open hot plate
at 80C. Table II shows the results of the test measurements.

**Table II.** Measured resistivity of silver inks cured at 80C.

  ------------------------ -------------- ------- ------- ------- ------- -------- -------- -------- -------- --------
  **Ink \#**               **1**          **2**   **3**   **8**   **9**   **12**   **14**   **15**   **19**   **20**

  Line thickness, microns  17.5           16.5    23      19      18      13.5     24       22       35       16.5

  Vendor Specified         \<51           35      \<200   25      \<38    50       116      51       7        25
  Resistivity,                                                                                                
  micro-ohm-cm                                                                                                

  Cure time, minutes       Measured                                                                           
                           Resistivity,                                                                       
                           micro-ohm-cm                                                                       

  30                       80             85      117     53      111     40       55       50       38488    30

  60                       78             82      112     52      106     39       49       47       993      29

  120                      76             78      108     51      103     38       47       46       210      29

  180                      75             77      105     51      101     37       47       46       103      28
  ------------------------ -------------- ------- ------- ------- ------- -------- -------- -------- -------- --------

As the numbers indicate, several of the inks exhibited resistivities
that were near or below their specified 'fully cured' values. This
observation was very encouraging because it indicated that, **[with the
inks we have available, the curing temperature can be lowered to 80C
without compromising the desired 'in-pattern'
conductivity]{.underline}**.

###### "Contact Resistance" of inks deposited on CIGS cells and cured at 80C

Four of the inks mentioned in Table II were then screen printed onto
CIGS cells made on Moly foil. I-V characteristics of small area cells on
each sample were measured before contact deposition on the sample and
again a couple of times in the next 2 hours during which the sample was
held at 80C. In parallel, on the same samples, I-V characteristics of
the larger area cells with printed contacts were also measured at
several intervals in the 2 hours. In all instances, the samples were
cooled to room temperature before I-V measurements were done. The
results of the best small and large area cells on each sample are given
in Table III.

**Table III.** Comparison of large and small area cells scribed on the
same sample. The large cells have printed contacts and the small cells
do not.

<table>
<colgroup>
<col style="width: 7%" />
<col style="width: 9%" />
<col style="width: 15%" />
<col style="width: 9%" />
<col style="width: 3%" />
<col style="width: 11%" />
<col style="width: 7%" />
<col style="width: 9%" />
<col style="width: 15%" />
<col style="width: 9%" />
</colgroup>
<tbody>
<tr class="odd">
<td colspan="4"><p>Best Small Cell</p>
<p>0.084 cm<sup>2</sup></p>
<p>(<strong>without</strong> Printed Contacts)</p></td>
<td rowspan="6"></td>
<td colspan="5"><p>Best Large Cell</p>
<p>0.95 cm<sup>2</sup></p>
<p>(with Printed Contacts)</p></td>
</tr>
<tr class="even">
<td><p><strong>Eff</strong></p>
<p><strong>%</strong></p></td>
<td><p><strong>Voc</strong></p>
<p><strong>V</strong></p></td>
<td><p><strong>Jsc</strong></p>
<p><strong>mA/cm<sup>2</sup></strong></p></td>
<td><strong>FF</strong></td>
<td><strong>Ink #</strong></td>
<td><p><strong>Eff</strong></p>
<p><strong>%</strong></p></td>
<td><p><strong>Voc</strong></p>
<p><strong>V</strong></p></td>
<td><p><strong>Jsc</strong></p>
<p><strong>mA/cm<sup>2</sup></strong></p></td>
<td>FF</td>
</tr>
<tr class="odd">
<td>8.6</td>
<td>0.42</td>
<td>31.8</td>
<td>0.65</td>
<td>3</td>
<td>8.6</td>
<td>0.42</td>
<td>35.3</td>
<td>0.58</td>
</tr>
<tr class="even">
<td>9.0</td>
<td>0.43</td>
<td>32.8</td>
<td>0.65</td>
<td>9</td>
<td>9.3</td>
<td>0.43</td>
<td>34.5</td>
<td>0.63</td>
</tr>
<tr class="odd">
<td>9.6</td>
<td>0.43</td>
<td>33.3</td>
<td>0.66</td>
<td>19</td>
<td>8.4</td>
<td>0.43</td>
<td>34.0</td>
<td>0.58</td>
</tr>
<tr class="even">
<td>9.6</td>
<td>0.43</td>
<td>35.2</td>
<td>0.63</td>
<td>20</td>
<td>9.3</td>
<td>0.42</td>
<td>34.7</td>
<td>0.64</td>
</tr>
</tbody>
</table>

Overall, the above results show that **[screen printed contacts cured at
80C do not change the cell efficiency significantly]{.underline}**. The
cell performance measured with the contacts, by far represents the
quality of the cells underneath the contact. A closer inspection of the
data reveals that while the printed contacts lower the fill factor by
introducing some series resistance, they increase the current density
due to improved current collection resulting in little or no change in
overall efficiency. Further experiments are planned to identify ways to
suppress the contacts related series resistance.

###### Long term stability of screen printed contacts

Long term stability of printed contacts is a critical factor in
determining the optimal ink for making contacts. To determine the long
term stability of the silver ink-ZnO junction, CIGS cells were evaluated
at several weeks' intervals on samples that had been cured at 100C. Data
was collected on small cells that did not have the contacts to get a
baseline measurement on how the *cell* was changing with time. Data was
collected on the large cells with the printed contacts to evaluate the
overall change in both the silver/ZnO interface as well as the
underlying cell. Difference between the two would yield information on
the evolution of silver/ZnO interface with time.

Evaluation of small cells on eleven different samples indicated that
there was typically a 0.5-1.5% loss in efficiency when the cells were
left under ambient conditions for almost three months. In contrast, the
cells with silver contacts lost 1.5% or more in efficiency in the same
time period. Looking at the difference between the two, it was clear
that there was some degradation of the contacts when the samples were
left in air. The 'best' inks were determined to be those that showed
least loss in efficiency compared to the underlying cell. Figure 6 shows
the observed change in efficiency of the best four samples.

![](media/image6.wmf){width="2.7006944444444443in"
height="2.2979166666666666in"}
![](media/image7.wmf){width="2.7006944444444443in"
height="2.2979166666666666in"}

![](media/image8.wmf){width="2.7in" height="2.3020833333333335in"}
![](media/image9.wmf){width="2.7006944444444443in"
height="2.2979166666666666in"}

**Figure 6.** Stability of CIGS cells with and without screen printed
silver contacts. Blue squares represent efficiencies measured on small
area (0.084 cm^2^) cells with probes placed on the ZnO. Black dots
represent efficiencies measured on large area (0.95 cm^2^) cells with
probes placed on the screen printed silver prints.

A caveat to this experiment is that these degradation results were
obtained with samples stored under ambient condition. Typically if the
samples were to be encapsulated soon after contact deposition, the
degradation results could be significantly different. In the coming
weeks we will be making modules and encapsulating them to find out how
the cells degrade when encapsulated.

######## **Future Work**

In the coming months we will be focusing on further optimizing the
surface modification conditions to improve the solar cell efficiency.
The ink-jet technique will be tested to deposit the absorber layers. We
will optimize the curing conditions for the silver inks and fabricate
modules with printed contacts.

######## **Reference**

1\. Rocket, A.A., Schroeder, D., Berry, G.D., *Gallium Diffusion and
Difussivity in CuInSe~2~ epitaxial layers.* Applied Physics Letters,
1996. **69**(26): p. 4068-4070.