Conference Paper

NREL/CP-520-44886

August 2009

Photovoltaic-Reliability R&D 

Toward a Solar-Powered World 

Preprint

S. Kurtz

National Renewable Energy Laboratory

J. Granata and M. Quintana

Sandia National Laboratories

To be presented at the Society of Photographic Instrumentation 

Engineers (SPIE) Solar Energy + Technology Conference

San Diego, California

August 2-6, 2009
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1 

Photovoltaic-Reliability R&D toward a Solar-Powered World 

Sarah Kurtz*∗

a

National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO, USA 80401; 

b

Sandia National Laboratories, P.O. Box 5800, Albuquerque, NM USA 87185 

a

, Jennifer Granatab, and Michael Quintanab 

ABSTRACT

The continued exponential growth of photovoltaic technologies paves a path to a solar-powered world, but requires 

continued progress toward low-cost, high-reliability, and high-performance PV systems. High reliability is an essential 

element in achieving low-cost solar electricity by reducing operation and maintenance (O&M) costs and by extending 

system lifetime and availability, but these attributes are difficult to verify at the time of installation. Utilities, financiers, 

homeowners, and planners are demanding this information in order to evaluate their financial risk as a prerequisite to 

large investments. Reliability research and development (R&D) is needed to build market confidence by improving 

product reliability and by improving predictions of system availability, O&M cost, and system lifetime. Universities, 

industry, National Labs, and other research entities can be most effective by working together and in complementary 

ways. The Department of Energy supports a variety of research projects to improve PV-system reliability. These 

projects and current reliability issues for each PV technology are surveyed. 

Keywords: PV, reliability, failure analysis, accelerated testing, field-testing, lifetime prediction

1. INTRODUCTION

1.1 Growth of PV industry

The world PV industry grew exponentially from shipments of ~ 300 MW in 2000 to ~ 7 GW in 2008, as shown in Fig. 1. 

If this rate of growth can be continued, by 2016, the PV-installation rate will equal the average world new-electricitycapacity installation rate between 1996 and 2006, and, by 2018, the PV-installation rate would replace, annually, 5% of 

the current electricity generating capacity [1]. A solar-powered world will require even higher installation rates, but the 

possibility of reaching these significant milestones in < 10 years supports the vision of a solar-powered world.

1

10

100

GW of PV shipped

2000 2005 2010 2015 2020

Year

Annual new electricity capacity 1996-2006

Annual replacement of 5% of today's electricity capacity

Fig. 1. PV-industry growth curve showing exponential growth in recent years and how that growth might extrapolate into the future.

 

∗ sarah.kurtz@nrel.gov; phone 1 303 384-6475; www.nrel.gov
2 

Maintaining this exponential growth will require two key elements: achieving 1) acceptable costs and 2) low enough risk 

to facilitate adequate investment. Reliability plays a key role in both of these.

1.2 Importance of reliability

Improved system reliability will be essential toward maintaining the current exponential growth curve as well as to 

community acceptance of solar as a mainstream power source. The exponential growth shown in Fig. 1 could result in 

huge financial losses if systems fail to deliver as promised. Ultimately, a solar-powered world will define reliability in 

the context of “the lights go on when the switch is flipped.” This will require energy storage for when the sun isn’t 

shining – a topic that is outside of the scope of this paper. For this paper, we define reliability as “a PV system working 

as expected when the sun is shining with low O&M costs and long life.” Some of the ways in which reliability affects the 

cost and performance of PV are shown in Table 1.

The Solar Advisor Model [2] calculates the cost of solar electricity including factors such as the cost of money, incentive 

programs, etc. It’s also useful to consider the fundamental costs to society for deploying solar and how these are affected by reliability. Table 2 summarizes a simple analysis.

Table 1. Importance of reliability to reducing the cost of solar electricity.

Reliability factor Effect on cost of solar electricity Comments 

System lifetime Allows initial costs to be distributed over more 

kWh of production

Utility applications may benefit from ultra-long lifetimes, 

whereas rooftop products may optimally be designed to 

have lifetimes similar to roof lifetimes.

System 

availability

When system is not functioning because of failure 

or maintenance, fewer kWh are generated

System availability can significantly increase cost if the lack 

of system availability goes undetected or if replacement 

parts are unavailable, delaying repair.

Degradation in 

performance 

A 1% degradation per year over a 20 year lifetime 

will decrease the kWh generated by ~10%

Modules have shown < 1% degradation per year, but new 

products can show much higher degradation rates.

O&M costs All systems require maintenance, which translate 

directly into added cost of electricity

The O&M costs for the same product vary depending on

installation quality and the distance the repairman travels.

Predictability

Confident predictions of system performance, 

availability, and lifetime translate directly into 

lower interest and insurance rates

The interest and insurance rates are highly dependent on the 

application and financing scheme.

Table 2. Simplified cost analysis as a function of key assumptions.

Cost 

($/W) 

Generation rate 

(kWh/kW/yr)

Degradation 

rate (%/yr)

Average 

O&M cost 

($/kW/yr) 

Lifetime 

(yr)

Generation in 

lifetime 

(kWh/kW)

Cost in 

lifetime 

($/kW) 

Electricity cost

(cents/kWh)

5 1000 0 50 20 20,000 6000 30 

5 2000 0 50 20 40,000 6000 15 

5 2000 1 50 20 36,000 6000 17 

5 2000 0 500 20 40,000 15,000 38 

5 2000 0 50 50 100,000 7500 7.5 

2. QUALIFICATION-TEST DEVELOPMENT

2.1 History of Jet Propulsion Laboratory (JPL) Block Buys

In the late 1970s, JPL conducted a series of Block Buys of PV modules [3]. For each Buy, they required that the module 

design pass a set of tests. The modules were deployed and the resulting failures analyzed to revise the set of tests. A 

summary of the requirements is shown in Table 3 along with a subset of the requirements of today’s international 
3 

qualification test for silicon modules, IEC 61215 [4]. The IEC 61215 also includes a number of other tests, including 

application of damp heat for 1000 hrs. Qualification tests for thin-film (IEC 61646) and concentrator (IEC 62108) 

modules are similar. The use of these qualification tests to identify design flaws has been very helpful toward improving 

field performance. Whipple reported [5] that modules tested to JPL Block V had failure rates of < 0.1% compared with 

45% for pre-Block-V testing. A Block VI Buy was planned, but not implemented [6].

Table 3. Comparison of some of the primary features of JPL Block testing and today’s qualification test, IEC 61215.

Test Block I Block II Block III Block IV Block V IEC 61215

Year 1975 1976 1977 1978 1981 2009

Thermal Cycle (°C) 100 cycles

-40 to +90

50 cycles

-40 to +90

50 cycles

-40 to +90

50 cycles

-40 to +90

200 cycles

-40 to +90

200 cycles

-40 to +85 w current 

flow

Humidity or 

humidity/freeze (RH is 

relative humidity)

70 C, 

90%RH, 

68 hr

5 cycles

40 C, 

90%RH to 

23 C

5 cycles

40 C, 

90%RH to 

23 C

5 cycles

54 C, 

90%RH to 

23 C

10 cycles

85 C, 85%RH 

to -40 C

10 cycles

85 C, 85%RH to -40 C; 

1000 hr 85 C, 85%RH

Hot spots - - - - 3 cells, 100 

hrs

5 hr, worst case

Mechanical load - 100 cycles 

± 2400 Pa

100 cycles 

± 2400 Pa

10000 cyc. 

± 2400 P

10000 cyc. ± 

2400 Pa

3 cyc. 

2400 Pa

Hail - - - 9 impacts 

3/4” - 45 

mph

10 impacts 1” 

- 52 mph

11 impacts 25 mm - 23 

m/s 

High pot - < 15 μA 

1500 V

< 50 μA 

1500 V

< 50 μA 

1500 V

< 50 μA 

2*Vs+1000

1 min at 2*Vs+1000, 

then measure @ 500 

V: R X A > 40 MΩ⊕m2

2.2 History of additional qualification test development

In parallel with JPL’s work, the Joint Research Center of the European Commission, Ispra, Italy also developed 

qualification tests. Their early testing (CEC 201) [6] included hail, UV, wind, temperature cycling, “smog,” humidity 

cycling, and thermal degradation including shocks from cold-water spray. The CEC 201 test later evolved into CEC 

501 and 502 [6]. Clemson University subjected PV modules to the 85°C/85% relative humidity (damp-heat) test 

commonly used by the semiconductor industry [7]. The damp-heat test has been retained in today’s qualification tests 

and is often one of the most difficult tests to pass [8]. A review of the development of the qualification standards tests 

was published in 2008 [6].

3. DESIGN FOR RELIABILITY AT ALL DEVELOPMENT STAGES

3.1 Product development cycle

Consideration of reliability issues at every stage of development can save time and cost in the long term. Before a 

prototype is ever made, reliability should be systematically evaluated and anticipated issues addressed. Numerous tools 

are commonly used both in the design and design-review stages including Advanced Product Quality Planning, Design 

Failure Modes Effects and Analysis, Fault Tree Analysis, Design for Manufacturability, and Design Review Based on 

Failure Mode (originally developed by Toyota). Fig. 2 shows a simple depiction of the sort of cycle that is used to 

identify and remove design flaws at the prototyping stage. Design flaws can shorten product/system lifetime and reduce 

performance in the field, and, in some cases, can kill a development cycle before product realization even takes place. 

An outgrowth of this cycle is the development of a qualification test, as described in section 2, above, and, subsequently, 

tests/controls for use during manufacturing. The qualification test standard represents the cumulative knowledge of the 

community. However, such tests may not uncover failures for which tests have yet to be devised. The process in Fig. 2 
4 

or a related process must be reapplied every time a part of the design is changed. Thus, as long as the industry is 

attempting to reduce cost and improve performance, the product must be reevaluated for potential failures.

Fig. 2. Cycle for identifying and mitigating design flaws during product development.

3.2 Quality assurance (QA) during manufacturing

Careful control of manufacturing processes (a QA program) is an essential element of being able to deliver a consistently 

reliable product. After a product design is frozen and the product is put into production, the reliability of the product 

may falter if raw materials vary in purity, process conditions drift outside of acceptable ranges, new employees execute 

processes in a new way, etc. One simple strategy for QA is to periodically repeat the qualification test procedure on 

some fraction of the manufactured product. However, IEC 61215 can take weeks or months to complete, potentially 

allowing millions of dollars of flawed product to be shipped before a problem is identified. A preferred approach is to 

qualify each new raw material that comes into the process, identify acceptable ranges for each process control parameter, 

and use rapid tests to ensure that all aspects of the manufacturing are under control. Many companies use an ISO 9001 

process for ensuring control of the manufacturing process, but the ISO 9001 standard provides only the framework for 

tracking whether prescribed controls are in place; it does not attempt to define what controls are needed. Research is 

needed to identify the best (effective and low-cost) QA tests and controls to use. The results of these tests (e.g. silicon 

purity and adhesion strength) may be used as relevant information in a model that predicts long-term reliability.

3.3 Confident predictions

In section 1.2 we presented the importance of annual energy production, system degradation, O&M costs, and system 

lifetime on lowering the cost of solar electricity. Potential PV-system investments are evaluated using these and related 

metrics. Confident predictions are needed for both current and next-generation products. Ideally, industry can develop a 

new product/component and immediately predict the long-term performance of the modified system in the customer’s 

application/environment. Past history has shown that even small changes in product design can cause startling changes 

in reliability, sometimes in ways that were unexpected. Module and inverter qualification test sequences have been very 

successful in identifying design flaws that are likely to lead to early field failures; but these pass/fail tests provide the

basis for only qualitative, not quantitative, predictions. Careful control of manufacturing processes is an essential 

element of being able to deliver a consistently reliable product. In addition, confident predictions require statistically 

significant data from fielded systems with variable conditions as well as laboratory-generated accelerated-life-test data.

Reliability predictions can be made by 1) identifying a failure mechanism, 2) identifying the stresses that cause that 

failure, 3) determining the functional relationship (linear, quadratic, exponential, etc.) between the application of stress 

and the rate of failure, 4) applying accelerated stress and measuring the failure rates to quantify this functional 

relationship, 5) quantifying the stress expected in the field, and 6) putting all of this information together and applying 

probabilistic methods to predict the expected performance. Each of these steps is, ideally, repeated for each failure 

mechanism. Each measurement and model has an uncertainty. The challenge is to complete these steps with a prediction 

that has a high degree of confidence.

Fortunately, there are some PV products, primarily c-Si modules, which have been in the field for more than 20 years, 

allowing limited assessment of long-term performance from real-world experience, including identification of failure 

mechanisms, estimation of performance-degradation rates, and putting bounds on system lifetime. Based on such 

observations [9, 10], we may predict that “mature” Si modules will exhibit degradation rates between 0% and 1% per 
5 

year and will last 15-30 years. “Mature” thin-film modules typically exhibit degradation rates between 0% and 3% per 

year and last 10-20 years. By “mature” modules, we mean modules that have passed the qualification test sequence, 

have been tested in the field for at least 2-5 years, and have been manufactured long enough to have a strong QA 

program. Although these predictions do not come with 100% confidence for all “mature” modules, these predictions 

serve as a benchmark against which to compare new predictions for modules. 

There is less information available on Balance of System (BOS) components; inverters, disconnects, combiners, etc. 

Test standards for inverters have elevated the quality of the products entering the marketplace; historic field data reflect 

inferior service life compared with today’s improved inverters. Projected lifetimes based on warranties, have increased 

as well. Studies are being conducted to better define inverter performance degradation and develop acceleration factors. 

Still, the available information generated from system evaluations indicates that BOS components are significant 

contributors to reduced reliability/availability of fielded PV systems and are likely to be more important than the PV 

modules in determining system lifetime/performance [11]. 

Some failure mechanisms affect products in reproducible, quantifiable, and, therefore, predictable ways. For example, 

the life of a transistor may be quantified as a function of operating temperature, using measurements from thousands of 

samples to elucidate the variability and factors leading to that variability. In contrast, field failures of PV components 

are often traced to widely varied environment conditions, improper installation or local unexpected conditions (such as a 

critter that chews on wires). Thus, a prediction of the degradation or failure rate for one specific failure mechanism as a 

function of a specific stress becomes irrelevant if the life of a component is limited by a different failure mechanism or 

stress. 

Recognizing the challenges helps to direct efforts. It is the reliability of the PV system, rather than the individual PV 

component, that is typically most important to the customer. A systems-level approach to reliability predictions requires 

1) understanding of how each component affects system performance, 2) identification of interactions between components that increase the probability that reliability is impacted, 3) identification of the system components that dominate 

the system reliability, 4) quantification of the reliability functions for these most important components, and 5) use of 

these reliability functions to predict availability, reliability, O&M, and degradation rates of new systems. Literature 

studies have consistently shown that, at the system level, inverters dominate maintenance costs. For example, a study at 

Tucson Electric Power showed that 69% of unscheduled maintenance costs for PV-system operation were attributed to 

inverters [11], as shown in Fig. 3. This distribution depends on the age of the system [12]. 

Fig. 3. Unscheduled maintenance costs for PV system operation [11].

Sandia National Labs has been documenting best-practice reliability methodologies [13] and developing a systems-level 

reliability model called PVRAM (Photovoltaic Reliability and Availability Model). PVRAM is a comprehensive 

approach to estimating the reliability and availability of photovoltaic systems. A fully functioning stochastic model will 

incorporate aggregated field data from multiple systems in multiple environments and with multiple technologies. This 

data includes field failures, failure times, and repair times; a consistent time stamp is extremely important. Data is used 

to generate summaries of failures, failure distributions, and failure rates; and ultimately predict annual energy 

production, reliability, and availability. Fidelity is added to the model by generating information on specific failure 

modes and degradation rates through lab-based accelerated testing and controlled exposure tests, which aid in quickly 

defining the reliability functions used in the modeling. 
6 

4. SPECIFIC R&D TOPICS

The PV community has impressive experience with reliability testing, but substantial R&D is still needed. It is useful to 

distinguish the need for reliability testing from the need for R&D on reliability testing. The most common failures for 

PV modules were identified and largely addressed in the late 1970s and in the 1980s, as described above. These common failures are seen in new, poorly designed products today, but test methods for these failures are fairly well defined. 

In contrast, there are some types of failures for which appropriate test methods still need to be defined. For all of the 

areas described below, there is a need to 1) compile/understand failure modes, 2) develop diagnostics that assist in system failure mitigation, and 3) develop accelerated life tests and subsequent test standards. In general, much research is 

needed to understand how to improve the confidence in prediction of long-term performance. Confidence in long-term 

predictions can only be achieved through correlation of field data with the measurements that are done during application of the QA program and/or from accelerated tests of companion products. 

4.1 Balance of system

Despite improvement in the performance of many system components, these still tend to dominate PV-system maintenance costs [11]. Sandia National Labs is supporting an effort to develop an inverter qualification test. Reduction in 

cost and improvement in performance may be realized through new mounting systems (especially for building integration) and optimized system design to reduce losses from shading, etc. Key R&D topics currently include development 

of microinverters, improving arc-fault protection circuits, and extending the lifetime of the inverters by avoiding failures 

with electrolytic capacitors, fans, and other components. Table 4 summarizes some of the issues.

Table 4. Summary of reliability issues for system components.

Issue Description Tests References R&D need

Inverter malfunctionInverter may disconnect or 

stop functioning; electrolytic 

capacitors are especially 

problematic; the need to use 

a fan can limit lifetime. 

UL 1741; qualification test 

for inverters is being developed. Related standards 

include: IPC-9592; EN 

60730-1; GB/T 19064.

Mitigation, including allsolid-state (no capacitors) design; qualification test; early-failure 

test.

Arcing or other 

failure of system 

connections

Imperfect electrical contact at 

connectors can cause arcing 

leading to fires. 

Arc-fault detection & 

mitigation; qualification 

test.

Improper installationInstallation does not follow 

National Electric Code; modules are improperly mounted; 

many other possibilities

Training of installer [14, 15]

4.2 Common issues for all types of PV modules (including packaging)

Table 5 summarizes some of the most important reliability issues that are common to all types of PV modules. As the 

industry is looking for ways to reduce cost and improve the performance of PV modules, every element of the module is 

scrutinized. For many years cerium was added to solar glass to increase the absorption in the UV, thereby reducing the 

exposure of the encapsulant, e.g. ethylene vinyl acetate (EVA), to UV. Recently, some companies have begun using 

glass without cerium along with EVA that has been formulated to tolerate high UV doses. This change may be beneficial unless the EVA formulation is uncontrolled. Kempe, at NREL, has shown that modules without cerium lose adhesion 8X sooner, highlighting the importance of careful control of the EVA formulation for the UV-transmissive glass 

[16]. 

Replacement of EVA with other encapsulant materials could result in new failure modes. For example, some encapsulant materials soften at high temperatures, allowing module components to creep, potentially causing safety issues. A 

recent study by Kurtz, et al, documented the temperatures that are observed in the field and extrapolated these to the 

hottest places on earth to quantify expected thermal aging and the temperatures that might lead to a creep condition [32]. 

These and related results will be used as a basis for a standard method for testing the modules and materials.
7 

Table 5. Reliability issues to consider for most PV modules.

Issue Description Tests References R&D need

Loss of electrical 

connections (to cells, 

in junction box, or 

leads coming out of 

module)

Thermal cycling or other 

mechanical movement 

causes increase in series 

resistance through broken 

solder bonds or other 

failure.

Thermal cycling (with 

forward-bias current to 

increase heating of weak 

points); robustness of 

terminations test; outdoor 

exposure. 

[17, 18]

Delamination with 

subsequent moisture 

ingress

There can be many reasons for decreased adhesion and associated delamination; the subsequent 

moisture ingress can 

cause corrosion, safety 

issues, loss of mechanical 

integrity. 

Damp heat exposes sensitivity to moisture ingress; humidity-freeze 

testing can accelerate 

delamination when adhesion is poor; UV exposure 

may cause decrease in 

adhesion; the wet leakage 

current test detects failure; outdoor exposure. 

[19-28] EVA has been studied 

extensively, but new 

encapsulants are not 

as well understood.

Improper installation Can lead to mechanical 

damage of glass or frame, 

corrosion of frame and/or 

ground, and many other 

things.

Training of installer. [14, 15]

Glass fracture Fracture of the glass can 

lead to serious safety issues; may be a result of 

weak glass, improper handling of modules, hail, 

vandalism, etc.

Hail impact; mechanical 

load. 

[29, 30] Understanding of 

ramifications of using 

thinner glass or glass 

replacements; design 

of frame. 

Hot spots that are not 

adequately controlled 

by bypass diodes (hot 

spots can also be 

caused by loss of 

electrical connection, 

see above) or bypass 

diode failure

Inconsistency in manufacturing or nonuniform illumination (e.g. shading) 

can cause variation in 

photocurrent that leads to 

hot spots. If bypass diode 

protection is inadequate, 

the associated heating can 

cause numerous types of 

failures.

Hot-spot endurance. [3] Identification of tests

for new encapsulant 

materials that may 

soften or degrade at 

the high temperatures 

associated with shading or product nonuniformity. 

Junction-box failure Junction boxes can fail in 

many ways including mechanically because of poor 

mounting or because of 

thermal degradation since 

the junction box often operates above the module 

temperature. 

Robustness of terminations; temperature cycling; 

damp heat; mechanical 

load. 

[31] The mechanical and 

thermal properties of 

many new materials 

are not well understood; test criteria 

need to be developed.

4.3 Silicon modules

Some silicon modules have performed very well in the field, with less than 1 % degradation/yr for more than 20 years [9, 

10]. However, the demonstration that long life is possible is no guarantee that every module will perform well. Some of 

the key reliability issues for silicon modules are summarized in Table 6. Silicon module manufacturers are refining their 

product designs and processes to reduce cost and improve performance. R&D is needed to identify what effects these 
8 

changes will have on performance in the field and whether these will be associated with reliability issues. One issue that 

is specific to silicon modules is the effect of reduced silicon wafer thickness on cracking of wafers. 

A key desire of silicon-module manufacturers is to be able to do accelerated testing that will give quantitative assessments of service lifetimes or of expected reliability compared with other designs. The test-to-failure protocol described 

by Carl Osterwald [33], and following on previous work [34, 35], has been applied to a set of silicon modules, and 

preliminary results show the importance of voltage in driving corrosion and other degradation mechanisms [36]. 

Table 6. Key reliability issues for silicon modules. See also Table 5 for issues that are common to all module types.

Issue Description Tests References R&D need

Effects of 

use of new 

materials

As companies attempt to reduce the 

cost and improve the performance 

of Si modules, replacements for 

every component are being considered. 

All [32] The ramifications of replacing 

each component need to be 

carefully scrutinized and test 

procedures modified appropriately. 

Use of 

lower grade 

silicon

A common trend during the silicon 

shortage was to use silicon of lower 

or unknown purity. The lower purity 

can lead to increased light-induced 

degradation or other problems. 

Outdoor exposure; thermal 

cycling; etc.

How to classify cells, understand 

localized hot spots, and 

appropriately use bypass diodes 

for minimization of hot-spot 

degradation.

Cracked 

cells

Thinner wafers can lead to cracked 

cells, loss of electrical connection to 

parts of the cells, and hot spots.

Performance (IV curve); IR or 

EL imaging

[37-40] Understanding of how wafer 

thickness is related to cracking, 

how the cracking relates to performance, and how to mitigate 

the effects of cracking. 

4.4 Thin-film modules

The challenge of designing reliable thin-film products is reflected in the 70% failure rate for the damp-heat test reported 

recently [8]. Some of the key issues that are specific to thin-film modules are summarized in Table 7. The newer thinTable 7. Key reliability issues for thin-film modules. See also Table 5 for issues that are common to all module types.

Issue Description Tests References R&D need

Increase in series 

resistance after 

exposure to moisture

Moisture ingress is known to increase the series resistance of 

ZnO and some other materials 

commonly used in thin-film modules; this is a special issue for 

flexible products. 

Damp heat [27, 51]

[41]

Development of thin-film products that are not sensitive to 

moisture or development of a 

barrier that keeps moisture out 

of sensitive layers. 

Instability of thinfilm layers caused 

by delamination 

and/or diffusion

Thin-film products usually require 

many layers; if any of these has 

poor adhesion, delamination may 

occur, often breaking the electrical connection between active 

layers. Also, Cu and other elements are known to diffuse, 

causing changes in how the cells 

perform. 

Damp heat; 

temperature 

cycling

[18, 23, 52-

57]

Develop understanding of the 

diffusion/delamination mechanisms, what affects them, and 

how to test for instabilities. 

Nonuniformities in 

manufacturing, 

including scribeline issues

Nonuniformities can lead to “weak 

diodes” that may then run hotter 

than the rest of the module, providing a weak point for degradation. Similarly, many other 

manufacturing nonuniformities 

(especially at scribe lines) can 

lead to reliability problems.

Damp heat; 

temperature 

cycling

[23, 58] Much work is needed to understand what parameters 

must be carefully controlled 

during manufacturing and what 

nonuniformities can be tolerated. 
9 

film materials do not benefit from the large literature heritage that the semiconductor industry has provided silicon-PV 

manufacturers. The mechanisms for failure (including sensitivity to moisture, diffusion within the layers, problems at 

scribe lines, and the effects of non-uniform layer coating) are not thoroughly understood; so many years of research will 

be required to fully elucidate these. Kempe, at NREL, subjected copper indium gallium selenide (CIGS) minimodules to 

humid conditions and found an increase in series resistance that appeared to be associated with a reaction between the 

ZnO and water [41]. This sensitivity may be compounded by the sensitivity of other layers [42]. There are two 

strategies for addressing the sensitivity of thin-film modules to moisture: harden the cell or harden the packaging. Some 

transparent conductors are less sensitive than ZnO to moisture. Notably, indium tin oxide, fluorinated tin oxide, and 

amorphous indium zinc oxide are candidates [43, 44], but they must be grown in such a way that they enable good cell 

performance while still retaining their stable properties [45-47]. 

The study of the reliability of thin-film modules is complicated by metastabilities [48]. It has long been known that light 

causes amorphous silicon cells to degrade, with almost full recovery after annealing in the dark. However, 

metastabilities are also observed for CdTe and CIGS modules. In a recent study of CIGS modules, we observed a 

metastabilty of up to 30% in one module’s efficiency. Such metastabilities may be related to copper or other elemental 

diffusion, or may be related to the filling and emptying of trap states. It will be useful to define procedures for 

“stabilizing” the modules so as to separate reversible from irreversible changes [49, 50]. 

4.5 Concentrating PV (CPV) modules

The modularity of CPV designs increases the number of possible reliability issues, but also provides more flexibility in 

solving these. Table 8 summarizes some of the key issues that are specific to CPV. A lack of control of the temperature 

of the concentrator cell can lead to catastrophic failure. Methods for testing this bond have been established, but prototype development could proceed more quickly if faster tests could be applied, and some of the companies are questioning the current temperature cycling test because the flow of large currents in forward bias can cause cell failure. Bosco, 

et al, recently presented a thermal-cycling study that more than doubled the rate of cycling and showed a method for 

imaging voids with an infra-red camera [59]. Acceleration of the effects of high solar flux are particularly challenging 

because CPV optics are usually designed to deliver the highest concentration that is easily practical. However, results 

presented at this conference by Miller, et al, show that lens materials may absorb enough UV to avoid significant UV 

transmission to the cell [60]. 

Table 8. Key reliability issues for CPV modules. See also Table 5 for issues that are common to all module types.

Issue Description Tests References R&D need

Failure of thermal control for 

cell

In contrast to the case for 

flat plate, loss of thermal 

contact between a CPV cell 

and its heat sink can cause 

catastrophic failure. 

Thermal cycling 

(with current to 

increase heating 

of weak points)

[59, 61] Faster test to identify low-cost, reliability thermal management 

schemes. 

Effect of concentrated light 

(especially UV)

Some CPV systems experience hundreds of suns of 

UV exposure; even without 

UV light, the concentrated 

light can cause failures. 

UV exposure [60, 62] Identify when UV is important; identify the causes of other failures, 

including a quantitative understanding of the effect of heat and 

how to accelerate testing.

Degradation of 

optics

Abrasion and other environmental exposure can cause 

loss of optical throughput.

Hardness; UV 

exposure

[62] Different types of optical materials 

are sensitive to different types of 

stresses. The R&D must start with 

identifying which failure mechanisms are most important for the 

different designs. 
10

5. COMPLEMENTARY ROLES

The many R&D needs highlighted in the tables above represent a critical barrier to achieving a solar-powered world. 

Companies need to develop products quickly (within investors’ window) and the probability of success is dramatically 

increased if national laboratories and universities work alongside to explore a fundamental understanding of the science 

behind the failures and their solutions. Universities specialize in educating students; national labs can tackle problems 

that are long term or too difficult to be practical for universities. Fundamentally, industry exists to provide a return on 

investment, whereas universities and national labs strive to increase our knowledge and communicate that knowledge to 

the community. All three thrive through innovation. Companies have the best chance of success when working closely 

with universities and national labs to solve problems. Unfortunately, today’s venture capitalists often treat start-up PV 

companies similarly to the dot-com companies: they demand a novel idea and expect the product to be brought to market in secret within a short period of time. Success will be more likely if investors encourage sharing of information 

about reliability issues and agree to a time line that allows opportunity for adequate product development and reliability 

testing. 

Fig. 4. Schematic showing how the universities and national labs can work together to build a foundation upon which the 

companies can build a solar-powered world. Opportunities for innovation arise at every development stage.

6. SUMMARY

The phenomenal exponential growth of the PV industry positions the industry for reaching significant production volumes in less than ten years, but the creation of a solar-powered world requires that these systems operate reliably. Reliable products have been demonstrated in the field, but new products have unknown reliability. The failure mechanisms 

for PV systems are diverse and complicated. Reliability R&D is needed to identify and understand new failure modes, 

to develop new and better test methodologies, and to develop reliability models that allow confident predictions of system availability, performance, and lifetime. Industry’s efforts to develop reliable products are effectively supported by 

efforts at the universities and national labs to understand the fundamentals of PV-system reliability. 

7. ACKNOWLEDGMENTS

We thank Nick Bosco for help compiling the bibliography and Glenn Alers and Peter Hacke for useful comments on the 

manuscript. This work was supported by the U.S. Department of Energy under Contract No. DOE-AC36-08GO28308 

with the National Renewable Energy Laboratory. Sandia is a multiprogram laboratory operated by Sandia Corporation, a 

Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration 

under contract DE-AC04-94AL85000.
11

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12

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The continued exponential growth of photovoltaic technologies paves a path to a solar-powered world, but requires 

continued progress toward low-cost, high-reliability, and high-performance PV systems. High reliability is an 

essential element in achieving low-cost solar electricity by reducing operation and maintenance (O&M) costs and by 

extending system lifetime and availability, but these attributes are difficult to verify at the time of installation. Utilities, 

financiers, homeowners, and planners are demanding this information in order to evaluate their financial risk as a 

prerequisite to large investments. Reliability research and development (R&D) is needed to build market con¬fidence 

by improving product reliability and by improving predictions of system availability, O&M cost, and system lifetime. 

Universities, industry, National Labs, and other research entities can be most effective by working together and in 

complementary ways. The Department of Energy supports a variety of research projects to improve PV-system 

reli¬ability. These projects and current reliability issues for each PV technology are surveyed. 

15. SUBJECT TERMS

PV; reliability; failure analysis; accelerated testing; field-testing; lifetime prediction; 

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