CPV Cell Infant Mortality Study 

Preprint 

Nick Bosco, Cassi Sweet, Timothy Silverman, 

and Sarah Kurtz 

Presented at the 7th International Conference on Concentrating 

Photovoltaic Systems (CPV-7) 

Las Vegas, Nevada 

April 4-6, 2011 

NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC. 

Conference Paper 

NREL/CP-5200-51337 

May 2011 

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CPV Cell Infant Mortality Study 

Nick Bosco, Cassi Sweet, Timothy J. Silverman and Sarah Kurtz 

National Renewable Energy Laboratory
 

Golden, CO USA 

Abstract. Six hundred and fifty CPV cells were characterized before packaging and then after a four-hour concentrated 

on-sun exposure. An observed fielded infant mortality failure rate was reproduced and attributed to epoxy die-attach 

voiding at the corners of the cells. These voids increase the local thermal resistance allowing thermal runaway to occur 

under normal operation conditions in otherwise defect-free cells. FEM simulations and experiments support this 

hypothesis. X-ray transmission imaging of the affected assemblies was not found capable of detecting all suspect voids 

and therefore cannot be considered a reliable screening technique in the case of epoxy die-attach. 

Keywords: multi-junction, photovoltaic, thermal runaway, CPV, die-attach, void. 

PACS: 88.40.jp 

INTRODUCTION 

A 0.5–1% infant mortality failure rate has been 

observed for some III-V multi-junction cell 

assemblies [1]. Considering the series connection of 

cells within a module, a 1% failure rate will have a 

much larger consequence in terms of power production 

for the entire module and system. The failures 

observed for these cell assemblies are characterized by 

their open circuit voltage dropping instantaneously to 

zero within the first hours of on-sun exposure and are 

suspected to be a manifestation of thermal runaway. 

The goal of the present study is to identify the root 

cause of this characteristic failure in an effort to define 

an appropriate screening procedure such that this 

population of cells may be removed from production. 

In a previous set of experiments, it was found that a 

~4 area % void located in the solder die-attach at the 

corner of the cell would precipitate a thermal runaway 

failure through both a prescribed current ramp once 

~3.5 A forward bias is achieved, and similarly open 

circuit conditions while on-sun under high 

concentration [2]. These failures are similarly 

characterized by the open circuit voltage dropping 

instantaneously to zero. 

In the current study, 650 bare cells were 

characterized before assembly and then again 

following packaging and an on-sun exposure intended 

to precipitate the infant mortality failure. In an 

additional effort, a finite element model was created to 

confirm that voids alone are capable of precipitating 

thermal runaway. 

MATERIALS AND METHODS 

Cell Characterization and Exposure 

Six hundred and fifty III-V triple-junction (1-cm2) 

solar cells were acquired for this study. This 

population was not previously screened and selected, 

as would be the case prior to next level assembly. The 

cells were initially characterized electrically by dark 

JV response and their electroluminescence images 

recorded. The cells were then packaged into receivers 

while maintaining cell traceability. At the receiver 

level, the cell assemblies consist of an epoxy dieattach and direct-bonded copper (DBC) substrate. 

Once packaged, a screening of each cell was 

performed. This screen was to ascertain if any damage 

had occurred through packaging or shipment and 

consisted of an application of 10 mA forward bias 

where EL emission was visually checked and the 

resulting voltage response recorded. Each receiver 

was then placed on-sun under 500x concentration for 

at least four hours with a DNI of 750 W/m2

 or greater. 

Following exposure, the same 10 mA forward bias 

screen was performed. Those cells that did not emit 

and/or demonstrated a significant drop in voltage were 

considered failures and tagged for subsequent 

characterization. Each cell suspected of failure was 

imaged via IR emission at 100 mA forward bias and 

X-ray transmission. The dark JV response was also 

repeated and the cell assemblies finally cross-sectioned 

to optically image the die and die-attach in the 

suspected area of failure. 

1 
Finite Element Modeling 

A finite element model of a CPV cell assembly was 

constructed using COMSOL to simulate the coupled 

electrical and heat transfer problem of a CPV cell in 

forward bias. The cell is modeled as a laminate 

composed of a silver busbar, a germanium cell and a 

gold back contact, attached with solder or epoxy to a 

direct-bond copper (DBC) substrate. For simplicity, 

the busbar covers the entire top surface of the cell. It is 

shown that this gives a reasonable approximation for 

the busbar’s behavior in forward-bias simulations. The 

top surface of the busbar is held at electrical ground 

and a uniform inward current density is applied to the 

bottom surface of the solder. At the interface between 

the busbar and the cell, a temperature- and currentdependent voltage drop according to the constitutive 

equation for a triple-junction cell is enforced: 

 3 

V(J,T ) = 3kT ln  + ( ) (1) n  J 

q  CT 3  Eg,i T

 i=1 

where n=1 is the diode ideality factor, q is the charge 

of an electron, k Boltzmann’s constant, C =15 is a 

fitting constant and Eg is the temperature-dependent 

bandgap energy expressed in terms of the Varshni 

Formula for each of the three junctions [2-5]. The 

remaining surfaces are electrically insulated. The back 

surface of the substrate is held at a constant 

temperature, simulating a heat sink. In a typical 

simulation, the entire assembly starts at the same 

temperature as the heat sink and current through the 

cell is ramped up from approximately zero at 

0.1 A/cm2

/s. Several sizes of voids are simulated in the 

die-attach under one corner of the cell and the 

resulting temperature and local current density at this 

critical corner is monitored. 

RESULTS 

The initial dark JV response of a representative 

sampling of the entire population is presented in Fig. 

1. A detailed treatment of all data is beyond the scope 

of this paper and therefore addressed elsewhere [6, 7]. 

Of the initial 650 cells three were identified to exhibit 

the characteristic infant mortality failure: ~2.5 V and 0 

V @ 10mA pre- and post-exposure screen, 

respectively. The initial dark JV responses of these 

three samples are highlighted in Fig 1 by the bold 

green (sample (a)), red (sample (b)) and blue (sample 

(c)) curves. The corresponding initial 

electroluminescence imagines are presented in Fig 2. 

Samples (b) and (c) dark JV and EL response are well 

within the majority of the population which were 

unaffected by the on-sun exposure. 

FIGURE 1. Representative sampling of dark JV response of 

the initially characterized bare cells, with the three infant 

mortalities highlighted. 

FIGURE 2. EL images of the three infant mortality cells, 

pre-packaging and exposure. 

While sample (a) does exhibit a higher leakage current 

than the other two failures, it also resides in a portion 

of the overall population that was otherwise unaffected 

through exposure. 

2 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The IR image of one of the three identified infant 

mortalities, sample (c), is presented in Fig. 3. The 

other two identified infant mortalities demonstrate 

similar IR images that are characteristic of a current 

shunt that has developed under the corner of the cell. 

In this case, the hot spot in the IR image is due to Joule 

heating of the device since all applied current travels 

through the shunt. X-ray transmission images of two 

of the three failed cells are presented in Fig. 4. 

FIGURE 3. Representative IR image of a failed cell in the 

infant mortality population. 

FIGURE 4. X-ray transmission image of two failed cells in 

the infant mortality population. The locations of epoxy void 

and cell failure are circled. 

A common screening technique, the image of sample 

(c) shows evidence of a large void in the epoxy dieattach under the lower right corner of the cell while the 

image of sample (b) does not demonstrate such an 

obvious artifact in the location of cell failure (circled 

regions). The corresponding cross-sectional images 

taken of these samples are presented in Fig. 5 (The 

cracked and pitted appearance of the cell is an artifact 

of the polishing process). The lower left of sample (c) 

(c-L) contains epoxy to the corner of the cell while the 

lower right corner (c-R), and location of the failure, 

contains a large void as depicted in the X-ray image, ~ 

4.5 area %. Contrary to the negative indication for 

voids of X-ray image of sample (b), however, a 

significant epoxy void is also present in its affected 

corner, ~3.7 area %. Similar to sample (b), sample (a, 

not presented) did not show evidence of voiding in its 

X-ray image, though one was present at the failed 

corner and approximated to be ~3.6 area % through 

detailed examination of the X-ray and cross-section 

images. 

FIGURE 5. Optical micrographs of cross-sectioned infant 

mortality samples in the corner of failure. 

ANALYSIS 

From the original population of 650 cells, three 

characteristic infant mortality failures were identified. 

This represents less than the 0.5–1% failure rate 

observed in the field, however, the original population 

was not screened, as would be the case in production. 

If a limit of acceptable leakage current were applied, 

approximately 50 cells would have been excluded 

thereby increasing the experimental failure rate to field 

levels. 

The characteristic infant mortality failures appear 

to be a result of a packaging failure (epoxy die-attach 

void) rather than a pre-existing defect at the bare cell 

level. It is speculated that the increased thermal 

resistance due to the epoxy void precipitates thermal 

runaway in that corner of the cell: a feedback loop of 

Joule heating and current crowding that ultimately 

results in the formation of a current shunt rendering 

the device as an overall short. In order to further 

confirm this hypothesis, a finite element model was 

used to simulate thermal runaway in an otherwise 

defect free CPV cell. 

3 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The FEM is first validated by simulating the local 

current density at the corner of the cell over a solder 

void of 3, 4 and 5 area % against the applied current, 

Fig. 6. It is presumed that when the rate of change of 

the local current density becomes very large, thermal 

runaway has occurred. This is the case for the 4 area 

% void at approximately 3.5 A as in good agreement 

with previous experiments [2]. 

35
 

3 %
 

) local current density (A/cm2 

with the three infant mortalities identified in the 

present study and therefore suggests the voids alone 

were sufficient to cause failure. 

CONCLUSIONS 

An infant mortality failure rate observed for some 

fielded III-V multi-junction cell assemblies has been 

reproduced for a characterized 650-piece sample set. 

These characteristic failures could not be correlated 

with cell defects, but can be explained by thermal 

runaway at epoxy die-attach voids located at a corner 

of the CPV cell. These voids create areas of higher 

thermal resistance that promote thermal runaway in 

otherwise defect-free cells. This explanation is 

consistent with both FEM simulation and on-sun 

experiment. While X-ray imaging of the packaged 

cells is commonly used to detect these voids, only one 

of the three packages showed an artifact in its image 

consistent with a die-attach void. Reduced contrast in 

imaging the less dense epoxy die-attach, as opposed to 

solder, may reduce the effectiveness of this screening 

technique for these assemblies. In light of the FEM 

simulations, a forward bias current proof test could be 

applied as a method for evaluating the adequacy of the 

4 % 

30
 5 %

25
 

20
 

15
 

10
 

5
 

0
 

0 1 2 3 4 5 

applied current (A) 

FIGURE 6. FEM simulation of the local current density 

above a solder void with increasing applied current for three packaging. Effort should also be aimed toward directly 

void sizes. addressing the packaging issue. 

35 ACKNOWLEDGMENTS 0.5 % 1 % 

30 1.5 % 

2 % This work was supported by the U.S. Department 2.5 % 

3 % 25 of Energy under Contract No. DE-AC36-08GO28308 3.5% 

4 % with the National Renewable Energy Laboratory. 4.5 % 20 

15 REFERENCES 

10 [1] N. Bosco and S. Kurtz, "Quantifying the Thermal 

Fatigue of CPV Modules," AIP Conference 5 Proceedings, vol. 1277, pp. 225-228, 2010. 

[2] N. Bosco and S. Kurtz, "Accelerated Testing and 0 On-Sun Failure of CPV Die-Attach," in IEEE 0 2 4 6 8 10 ASTR, Denver, CO, 2010. applied current (A) [3] J. C. C. Fan, "Theoretical temperature dependence 

of solar cell parameters," Solar Cells, vol. 17, pp. FIGURE 7. FEM simulation of the local current density 

) local current density (A/cm2 

above an epoxy void with increasing applied current for nine 

void sizes. 

A similar family of curves is produced to simulate 

the present case of infant mortality failures, Fig. 7. 

309-315. 

[4] B. V. Zeghbroeck, Principles of Semiconductor 

Devices: Available at 

http://ecee.colorado.edu/~bart/book/, 1997. 

[5] S. K. Geoffrey, et al., "Concentrator multijunction 

The local current density in the cell above the voided 

corner is plotted against applied current for a 1-cm2 

cell. If the experimentally determined threshold of an 

applied current of 3.5 A to precipitate thermal runaway [6] 

is applied, all voids greater than ~2.5 area % will result 

in on-sun failure. This result is in good agreement [7] 

4 

solar cell characteristics under variable intensity 

and temperature," Progress in Photovoltaics: 

Research and Applications, vol. 16, pp. 503-508, 

2008. 

C. Sweet, et al., "Correlations in Characterisitic 

Data of Concentrator Photovoltaics," in NREL PV 

Module Reliability Workshop, Golden, CO, 2011. 

C. Sweet, et al., to be submitted.