Development of reliable interconnect systems for ......Abhijit Namjoshi, Dakai Ren, Lindsey Clark, Marty DeGroot, Rebekah Feist, Leonardo López NREL PV Module Reliability Workshop

Development of reliable interconnect systems for photovoltaic modules

Abhijit Namjoshi, Dakai Ren, Lindsey Clark, Marty

DeGroot, Rebekah Feist, Leonardo López NREL PV Module Reliability Workshop

Feb 25 – 26, 2014 Golden, CO

1

2

Introduction

Problem statement

Power degradation in reliability testing

Solution Approach

Isolating the problem

Understanding physics of failure

Component level testing for process development

Applying solution and validating improvements

Conclusions

Outline

3

Dow POWERHOUSE™ shingle – Reinventing the roof with unique BIPV product

Safe, Durable and Reliable

Ease of Installation, aesthetically appealing

Empowering home owners with clean energy

Introduction

Body Copy

4

Interaction between design and reliability performance

Glass

Encapsulant

Encapsulant

Cell string

Backsheet

Design

Considerations

Environmental Chambers -Thermal Cycling -Damp Heat -Dry Heat etc.

Reliability

Testing

5

Location of Rs increase necessary to understand the problem

Observed power degradation driven by Rs increase

Driven by

6

• Cell

• Junction

• TCO

• Interconnect

• ECA

• Ink

• Interfaces

• Front Bus – ECA

• ECA – Ink

• Ink – TCO

• Within Cell

• Cell back – ECA

• ECA – Back Bus

Several failure modes can contribute to Rs increase

Component level testing helped identify back contact degradation as root cause

ECA

PV Cell

Ink

ECA

Front Bus Bar

Back Bus Bar

7

Electrically Conductive Adhesive (ECA)

SS

Ag Ag

Ag

Ag

Ag epoxy

epoxy

epoxy

• Epoxy resin with more than 70 wt% silver flakes

• Provides mechanical bonding and conductive path

• Properties readily tailored

ECA selection considerations: product and process attributes, such as substrate surfaces, curing conditions, stresses, reliability, ease of dispense…

8

• Material Properties

• ECA selection

• Surface treatment

• Process Parameters

• ECA coverage

• ECA cure

• Bondline thickness

• Performance Measurement

• Power & resistance measurement

• Failure Analysis

• Mechanical Testing

• Environment aging

Used multi-pronged approach for issue resolution

9

Established technique for in situ cure estimation 10

7

108

109

1010

-100 -50 0 50 100 1500.0

0.1

0.2

0.3

0.4

0.5

5 min 6 min

8 min 10 min

14 min 18 min

90 min

Sto

rag

e M

od

ulu

s G

' (P

a)

Ta

n

Temperature (C)

Torsional Dynamic Mechanical Analysis

Developed DMA based techniques to understand evolution of ECA curing vs. process conditions

“Glass transition temperature as an in situ cure index of electrically conductive adhesives in solar photovoltaic

module interconnect assemblies”, Dakai Ren et al., Solar Energy Materials & Solar Cells 107 (2012) 403 - 406

10

ECA can be optimized differently for top / bottom

Need to optimize cure conditions

Constant Temperature

0 5 10 15 20 25 300

20

40

60

80

100

ECM 1541S (180 C)

Henkel CE3103WLV (150 C)

Co

nv

ers

ion

(%

)

Cure Time (min)

ECA 1ECA 2

Increasing curing time

Challenge: balance the cure temperature profile for even cure performance

ECA

PV Cell

Ink

ECA

Front Bus Bar

Back Bus Bar

11

Added “layer of protection” to minimize back contact degradation

Conductive patch added on back of the back ribbon

Back of Cell Back of Cell

Back contact

bus bar

Conductive

patch

12

Stable power performance Pmax reflected in stable series resistance Rs

Conductive fixes improve power performance

Driven by

Before

After Before

After

13

Technical:

Selection of appropriate ECA for performance is important

ECA cure important for performance… especially reliability

ECA adhesion important for reliability performance

Conductive fixes improved reliability performance

Problem solving:

Reliability demonstration takes time… start early

Proper isolation of problem location critical to narrow scope of investigation

Component level testing can accelerate mechanism identification

Understanding physics of failure key to improve reliability performance

Broadened thinking needed for timely “out of the box” solutions

Conclusions