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Contract No. DE-AC36-08GO28308

CPV Cell Characterization 

Following One-Year Exposure 

in Golden, Colorado

Preprint

Nick Bosco and Sarah Kurtz

Presented at the 10th International Conference on Concentrator 

Photovoltaics (CPV-10)

Albuquerque, New Mexico

April 7–9, 2014

Conference Paper

NREL/CP-5J00-62484

August 2014
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CPV Cell Characterization Following One-Year Exposure 

In Golden Colorado 

Nick Bosco and Sarah Kurtz

National Renewable Energy Laboratory, 16253 Denver West Parkway

Golden, CO 80401

Abstract. A CPV module containing 30 III-V multijunction cells was operated on–sun for one year in Golden, 

Colorado. Each cell was characterized prior to and following exposure. A module power degradation of 10 % was 

observed and found to be a result as an overall decrease in cell short circuit current and the presence of at least one 

shunted cell. A positive correlation between initial shunt current and an increase in shunt current following exposure 

was also found. Cell exfoliation was also observed and found to be coincident with the presence of water and/or 

charring of the cell package due to an off-sun event. 

Keywords: Concentrator Photovoltaic, Exfoliation, Exposure, Reliability

PACS: 88.40.hj, 88.40.jp

INTRODUCTION

High concentration photovoltaic (PV) modules are 

typically a complex mechanical structure of optics, PV 

cells, interconnects and heat sinks, especially when 

compared to their one-sun flat-plate analogue. It is 

therefore not surprising that most studies of CPV 

module outdoor degradation primarily find failures 

associated with the module, such as optics degradation 

and failure of thermal connections [1-3]. Specific 

degradation studies of the III-V multijunction cells that 

now commonly populate CPV modules primarily tout 

excellent reliability [4]. In this paper we detail the 

one-year results of an experiment whose aim is to 

specifically monitor the degradation of III-V 

multijunction PV cells placed in-service within a CPV 

module. 

MATERIALS AND METHODS

A ~500x geometric concentration CPV module 

containing thirty III-V multijunction photovoltaic cells 

was used in this study. The packaging included a 

coverslip of glass attached to the cells’ active surface 

with a silicone-based adhesive. This scheme left the 

edges of the cells exposed. The module was modified 

to allow ex-situ testing of each cell assembly. This 

modification required access to the interior of the 

module that disrupted its original sealing scheme. 

Each cell was initially evaluated for its electrical 

performance via a dark and one-sun illuminated 

current-voltage (I-V) measurements. The module was 

then placed on a two-axis tracker for one-year of 

outdoor exposure in Golden, Colorado. The tracker 

has an angular accuracy of ± 0.1 ̊ and limits of azimuth 

and zenith travel of 106 ̊ to 212 ̊ and 68 ̊ to 0 ̊, 

respectively. The module was connected to a Daystar 

Multi-Tracer, which took I-V curves at 15-minute 

intervals, and otherwise kept the module at its 

maximum power point during its exposure. Following 

the one-year of service, the module was disassembled 

and the measurement of each cell’s individual 

electrical characteristics repeated. Each cell was also 

photographed and its electroluminescence imaged at 

this time. Following cell level characterization, the 

module was reassembled and placed back on-sun. 

Therefore this paper reports the one-year observations 

of an on-going experiment.

Cell exfoliation was found following the year’s 

exposure; therefore an exposure test was performed on 

pristine, similarly packaged cells. Two cell assemblies 

were imaged for their electroluminescence emission 

then submerged in tap water. One cell was exposed to 

room light and one placed in the dark. Each cell’s 

electroluminescence emission was re-imaged 

following 24 hours of exposure. Secondary ion mass 

spectrometry (SIMS) was performed following 

exposure to determine the layer in which exfoliation 

occurred. 

RESULTS AND ANALYSIS

On-sun module level I-V curves taken shortly after 

commissioning and just prior to the one-year analysis 

are presented in Fig. 1. The conditions during these 

measurements were an irradiance of 925 W/m2 and 

ambient temperature of 24.5 C. The module’s short 
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circuit current had dropped 5.3 %, fill factor 5.2 % and 

power 10.2 %. The presence of at least one lowcurrent cell is also evident. 

Fig. 1 On-sun module level I-V curves taken upon 

commissioning and shortly prior to the one-year analysis.

A comparison of each cell’s electrical performance 

before and after the one-year of service is presented in 

Figs. 2-4. Each cell’s shunt current is evaluated as its 

dark current response at 1.5 V forward bias, Fig. 2. Of 

the thirty cells evaluated, cells 5, 8, 10 and 14 showed 

the largest changes in electrical performance. A 

positive correlation of initial shunt current with an 

increase in shunt current is apparent in Fig. 2 for the 

majority of the samples. There appears to be no 

similar correlation for changes in short circuit current 

though its average decrease is 4 %, Fig. 3. The cell’s 

open circuit voltage was largely unchanged from their 

initial values except for two cells (5 & 8) that appear 

to be severely shunted, Fig. 4. 

Fig. 2 Change in shunt current. Black symbols represent 

exfoliated cells, and open symbols shunted cells.

The electroluminescence (EL) images of the four 

cells showing the largest changes are presented in Fig 

5. Cells 5 and 8 show evidence of a large shunt in the 

top sub-cell under the top bus bar. This is evident as a 

bright spot in these un-filtered images. These cells; 

however, showed no anomaly in the initial I-V curve 

that would have predicted the formation of this shunt. 

Cells 10 and 14 contain a shunt within their interior 

likely present prior to exposure and the cause of their 

initially high shunt current. 

Fig. 3 Change in ISC. Black symbols represent those cells 

that exhibited exfoliation.

Fig. 4 Change in VOC

Exfoliation, characterized by a loss of EL 

emission at the cell’s perimeter, was observed for six 

of the cells evaluated, cells 1, 6, 15, 21, 26 and 30. 

These cells are represented by black symbols in Figs. 2 

and 3. In three cases exfoliation was adjacent to 

charring of the package, cells 1, 6 and 15, presumably 

from an off-sun event, Fig. 6. Cell 6 exhibited the 

largest area of exfoliation of the cells with charring, 

and had the largest drop in short circuit current, 

besides the severely shunted cell 5. Exfoliation was 

also observed in three cells, cells 21, 26 and 30, that 

exhibited evidence of the presence of standing water, 

such as stains on the cover glass and aggregation of 

dirt and sand, Fig. 7. Each of these cells was located 

at the bottom of the module further corroborating 
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likelihood for contact with water. Cell 26 was 

observed to have the largest extent of exfoliation 

associated with water and also was one of the cells 

with the largest drop in short circuit current. The 

exfoliated cells exhibit a large change in shunt current 

that was not predicated by their initial shunt current 

and therefore standout from the positive correlation in 

Fig. 2.

Fig 5. EL images of the cells that showed the largest 

changes in electrical performance. 

Fig 6. Optical (left) and EL image of cell 1 highlighting 

exfoliation coincident with charring at the cell’s edge

Fig 7. Optical (left) and EL image of cell 26, highlighting 

cell exfoliation coincident with the presence of standing 

water.

Fig 8. EL image before (left) and following 24-hour room 

light exposure in water.

Fig 9. Optical images of cell (left) and removed coverslip 

following 24-hour room light exposure in water.

The preceding observations prompted a quick 

investigation into the susceptibility of similarly 

packaged cells to exfoliate in the presence of water. 

The cell submerged in water and exposed to room light 

exhibited extensive exfoliation as evident in its EL 

emission, Fig. 7. The glass coverslip was 

subsequently removed from this cell for further 

analysis and resulted in the exfoliated portion 

remaining on the coverslip, Fig. 8. SIMS analysis of 

the exfoliated cell indicates that the top sub–cell was 

separated. The cell exposed in darkness showed no 

signs of exfoliation, suggesting the mechanism of 

exfoliation is an electrochemical reaction. 

DISCUSSION

Following one-year’s on-sun service in Golden,

Colorado the module in this investigation suffered a 

10 % loss in power. From its I-V curves, this loss of 

power may be resolved as an overall drop in short 

circuit current combined with the effect of at least one 

shunted cell. Once disassembled, individual 

characterization of the PV cells supported the module 

level measurement as both physical and electrical 

changes were observed that help to explain this 

degradation. The average decrease in the cells short 

circuit current was roughly 4% and two cells (5 & 8) 

were found to be severely shunted, likely the cause of 

the step observed in the module level IV curve. At the 

cell level, an initially high shunt current was found to 

correspond with an increase in shunt current following 
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the exposure. Two cells (10 &14) that exhibited the 

largest combination of initial and final shunt current 

contained shunts within the field of the cell that were 

easily observed in their EL image. The two cells 

(5 & 8) that had become severely shunted following 

exposure showed no anomaly in their initial I-V curve 

that would have predicted the formation of their 

shunts. These shunts, however, are located at the 

perimeter of the cell along the bus bar. The edges of 

the cells investigated were not protected in any way, 

which may have allowed loose debris in the module to 

short the edge of these cells. Infiltration of the module 

by water and debris may have been exacerbated by its 

modification for the purpose of this experiment. This 

debris, dirt and sand, was especially prominent on the 

cells located at the bottom of the module along with 

stains on the glass coverslips consistent with standing 

water. This evidence of standing water was coincident 

with cell exfoliation, as was adjacent charring of the 

package presumably from an off-sun event. As 

demonstrated in a parallel experiment, the cells 

exposed in this study are susceptible to exfoliation, 

which acts to separate the top sub–cell, by exposure to 

water and light. It follows that the degradation of 

these cells cannot be predicted by their initial electrical 

performance but are rather a result of susceptible cells, 

insufficient edge protection and exposure to a harmful 

environment.

REFERENCES

[1] J. R. González, M. Vázquez, C. Algora, and N. 

Núñez, "Real-time reliability test for a CPV 

module based on a power degradation model," 

Progress in Photovoltaics: Research and 

Applications, vol. 19, pp. 113-122, 2011.

[2] K. Araki, H. Uozumi, M. Kondo, T. Takamoto, T. 

Agui, M. Kaneiwa, et al., "Development of a new 

550× concentrator module with 3J cells -

performance and reliability," in Photovoltaic 

Specialists Conference, 2005. Conference Record 

of the Thirty-first IEEE, 2005, pp. 631-634.

[3] N. Bosco, C. Sweet, M. Ludowise, and S. Kurtz, 

"An Infant Mortality study of III--‐ V 

Multijunction Concentrator Cells," Journal of 

Photovoltaics, Accepted, April 2012.

[4] C. Algora, "Reliability of III–V concentrator solar 

cells," Microelectronics Reliability, vol. 50, pp. 

1193-1198, 9// 2010.