• This study is motivated by reports of immediate on‐sun

degradation or failure in concentrator photovoltaic cell

assemblies for ~1% parts.

• Over 500 bare III‐V multi‐junction cells were initially

characterized, put on‐sun, and evaluated.  

• Correlations between initial characterization and imme‐

diate degradation are discussed.

Introduction Procedures         

• Light and dark current‐voltage traces (IV) and EL images were

recorded for over 500 bare cells before packaging. High and

low pass filters are used to image GaInP and GaAs junctions.

• IR images were taken of cells lacking adequate EL emission (at

50mA).

• Bare cells were then packaged into receiver assemblies and           

the voltage response at 10mA was recorded (Figure 1).

Approximately 500 bare cells are shown in a current point plot (Figure

3). Areas of the graph are highlighted for further review. Highlighted

colors correspond to graphs and images throughout this poster.

The majority of all cells showed an initial response of less than 1mA at

1.5V (highlighted in green, Figure 3). These cells experience minimal

change due to solar exposure, with a voltage change between ‐0.04V

and 0.04V at 10mA, if any.  

Cells with pre‐exposure IV tests above 100mA (at 1.5V) are severely

shunted. These cells, highlighted in red, had no detectable electrolu‐

minescence emission.  

The region between 1mA and ~10mA includes a large number of cells

that experience positive change where Vf>Vi, highlighted in orange, or

negative change Vf<Vi, highlighted in blue (Figure 3).

Analysis of light IV curves suggest that the change in cell response is

primarily due to a change in shunt resistance, while negative or posi‐

tive change corresponds to a decrease or increase, respectively. It is

suspected that all cells in the orange highlighted region experienced

shunt improvement due to solar exposure.

Shunts primarily affecting

the GaAs layer may appear

as a bright spot in the GaInP

layer (Figure 5b) with a cor‐

responding dark area in GaAs

layer (Figure 5c). The bright

spot is due to current crowd‐

ing in the GaInP area adja‐

cent to the GaAs shunt.

Greater shunting in the GaAs

compared to GaInP layer is

confirmed by analysis of fil‐

tered light IV curves.  

Conclusions

Correlations in Characteristic Data of   Concentrator Photovoltaics Cassi Sweet, Nick Bosco, Sarah Kurtz

Presented at the 2011 Photovoltaic Module Reliability Workshop, 16‐17 February 2011,  Golden, Colorado

This poster does not contain any proprietary or confidential information. NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC. NREL/PO‐5200‐51246 • February 2011

All cells in the red region of the current point plot (Figure 3) are shunted and did not suffi‐

ciently emit at 50mA for EL imaging. An IR image of a severely shunted cell shows the

shunt location where high current density produces excess heat (Figure 4a). The shunt lo‐

cation varies from cell to cell in this region.

Samples in the orange highlighted region have two distant dark IV shapes (shown in or‐

ange, Figure 4) with some variation in the EL images artifacts. An example of such is

shown in Figure 4b.  

All samples in the blue region of the current point plot (Figure 3) have a shunt that pro‐

duces an EL image with bright spots on the edge of the cell near the bus bar (Figure 4c).

This is due to manufacture probe damage for all cells in this region. Additionally, all cells

share a similar IV shape as in the blue dark IV curve (Figure 4).  

Cells in the lower current point IV range experience minimal change due to solar exposure

(green region, Figure 3) and produce clean initial bare cell EL images (Figure 4d).  

Figure 4. Initial bare cell dark IV trace curves are plotted on log scale for various cells that reside in highlighted re‐

gions of Figure 3. Each unfiltered EL image is representative of a typical cell in each region. Colors correspond accord‐

Representative bare cell dark IV curves are presented in Figure 2. Current data at 1.5V bias is plotted against the change in voltage

(recorded at 10mA) between pre and post on‐sun exposure (Figure 3). This produces a distribution of points comparing the cells’ initial

IV condition to the change after solar stress.

Figure 2. Initial Bare cell dark IV plots of several samples tested.

Current information at 1.5V is recorded for comparison.

A large majority of cells  that experienced change due to im‐

mediate solar exposure, tested above 1mA at 1.5V in current‐

voltage bare cell analysis before packaging and solar exposure.

Functional cells experiencing change due to exposure pre‐

dominantly show ~10mA response at 1.5V in bare cell IV tests.  

Correlations exist between the initial dark IV characteristic and

artifacts found in the  bare cell EL images. Results suggest that

artifacts observed in the bare cell may serve as an indicator

for early on‐sun degradation. It is suspected cells experiencing

short term change due to solar stress may be correlated to a

change in shunt resistance.   

Figure 3. Current (mA) data from bare cell dark IV traces at 1.5V is

plotted with the change in packaged cell voltage response to

10mA due to solar exposure.    

Figure 5. Initial bare cell images of one cell unfiltered (a) that display two types of artifacts seen in

multiple cells in the orange highlighted region of Figure 3. Low and high pass filters are used to ob‐

tain images of the GaInP (b) and GaAs (c) junctions.  

(a) (b) (c)

• Cell assemblies were fit into modules and exposed to outdoor

solar stress of at least 750DNI for a minimum of 4 hours.

• Voltage response at 10mA was also recorded after exposure.

Results ‐  Initial IV and cell change due to on‐sun exposure

Current (mA)

at 1.5V

Results ‐  Filtered EL Images

(b)

(a) (c)

(d)

Results ‐  Initial IV, EL and cell change correlations

CPV Cell

Die Adhesive

Substrate Copper Plating

Figure 1. Cross sectional diagram of CPV receiver