# Presentation: 143739

## CdTe Stability Subteam SessionCdTe National Team MeetingMay 5-6, 2005

- Subteam Leaders: **Dave Albin**, Sally Asher

- _Participating Team Members __(presenters in __**bold**__)_:
- IEC  (**S. Hegedus**, B. McCandless, K. Dobson)
- CSU (**A. Enzenroth**, **S. Demtsu**, J. Sites, A. Davies, K. Barth, SW Sampath)
- UT (D. Shvydka, **A. Compaan**, V. Karpov, Z. Liu)
- PNNL (**L. Olsen**, S. Kunda, G. Graff)
- USF (C. Ferekides)
- CSM (F. Seymour, S. Feldman)
- FSEC (N. Dhere)
- NREL (T. McMahon, J. Pankow, J. del Cueto, M. Kempe, G. Jorgensen)

## Subteam Objectives and Work Scope(defined Feb. 2004)

- Objective: Establish stress testing protocols that quantitatively assess the stability of polycrystalline thin film CdTe PV cells and modules and serve as useful indicators of module stability in the field.
- Work Scope:
- Identification and measurement of degradation _Modes_
- Identification and Quantification of degradation _Mechanisms_ *(oxidation, diffusion, defect mutation, electrochemical corrosion, reaction, electromigration, etc.)*
- Identification and Quantification of _Stress Factors_ and their individual and collective impact on changes in device performance.  *(Stress factors include electrical bias, illumination, ambient humidity, ambient oxygen, temperatures, interactions with encapsulation materials, etc.)*
- Development of Stress Testing _Protocols_ and standards based upon those protocols
- Quantitative determination of those changes in cell and module performance that are attributable to mechanisms inherent to the internal structure of the device and those mechanisms that are due to features or limitations of encapsulation/packaging

## Where are we and how did we get here?

- Pre-2000
- Stability as an important topic identified during late 1990’s
- Initial work focused around general use of Cu in contact recipes, collection of general “stress” data (JV and CV vs. illumination soak-time at ~100C, different bias conditions) – (Asher, Sites)
- “Alternative” backcontact and structure designs with and without Cu (Gessert, Rotlevi, Albin, Hodes, Viswanathan, etc.)
- 2000 - 2003,
- Organization-specific characterization of Cu use and JV-stress data, beginning to see similarities in Cu distribution, JV changes (everyone)
- 1-Dim modeling approaches implemented (Fahrenbruch)
- Solidifying Modes of Degradation: Voc decrease separated from backcontact behavior, dark Rs degradation mode (McCandless, Hegedus)
- Temperature Effects: Changing degradation modes with Temperature (Ferekides, Tetali, Meyers)
- Sporadic injection of critical Module Performance and Stability Data, primarily 1st level metrics, % (Cunningham, Emery, Rose, Meyers, del Cueto)– other possible modes (Rs, Rsh)?
- Empirical and Theoretical basis for presence and importance of -nonuniformities and the application of 2-Dim modeling (Karpov, Shvydka, Rose)
- Extension of 1-Dim characterization tools to 2-Dim mechanisms (Sites, Shvydka, Feldman, Hiltner, Pudov,Glockler, Nagle, Rakotoniaina, Breitenstein, others)
- 2003-present
- Introduction of Humidity as a Stress Variable (Dobson, Olsen, Kempe)
- Distinguishing between -nonuniformities at the junction and backcontact (Feldman, Shvydka)
- Application of IR-thermography to study localized device shunting (Breitenstein, McMahon, Shvydka)
- Junction and Shunting identified as distinctly different degradation modes.  Relative impact of each identified (McMahon, Albin, Berniard)

## On Modes and Mechanisms

- From Webster: modes are the “*manner, way, or method*” of doing; degradation modes are the “manner, way, or method” of how something degrades
- Mode identification requires a definition for degradation
- no worse than 10% drop in performance in 10 years.  Of course, better is better (Rose, Jan 2000 Team notes)
- Degradation modes for PV result in decreased performance
- Reliability studies focus on the early identification and removal of failure modes, thus improving reliability
- Degradation modes have underlying mechanisms (increased recombination, weak-diode formation, metal diffusion)
- Degradation modes can occur simultaneously
- Degradation modes have different activation energies,  therefore, it is likely that _*in use*_, one condition will dominate or be rate-limiting.
- Understanding the physical nature (electrical, chemical, structural, optical) nature of that dominating *mechanism* will suggest methods for eliminating that *mode* and thus improve reliability.

## On Identifying the Right Mode

- Understanding the physical nature (electrical, chemical, structural, optical) nature of the dominating *mechanism* (*in the module*) will suggest methods for eliminating that *mode* (in the module) and thus improve (module) reliability.

- the
- same?

- indoor – ALT (65C)

- Field Monitoring

- indoor – ALT (65C, 85C, 100C)

- outdoor monitoring

## Some Observations are not Degradation Modes

- SIMS profiles of Cu during before and after stress indicate that there are no significant changes in the Cu distribution at the backcontact and junction interfaces

- “we see a 2X increase in Cu in the CdTe bulk after 24000 hrs of stress, but no change in CdS.  If the device is optimally processed: Cu is not necessarily a 1st order stability problem”
- (e.g., the level of Cu present in the CdS or at the backcontact is not a factor).

_*Al Enzenroth:*_

## Mode #1 –decrease in Voc 
uniform degradation mechanism – junction recombination
non-uniform degradation mechanism – spatial formation of weak-diodes, “holes”, -nonuniformities, “reach-thru” diodes – occur without affecting Rsh
Mode #2 – Roll-over in 1st quadrant
is this really a degradation mode, or is it a signature of the mechanism responsible for Mode #1?
mechanistically we model this as a uniform degradation associated with either increased back barrier potential or increased series resistance
Mode #3 – Degraded Resistive Terms, Rsh & Rs
primarily treated as non-uniform
simple metallic shunting or contact delamination
more complex, e.g. “holes” – combination of low Rs and weak-diodes (Shvydka, Feb 2004)
Rsh seen predominantly at higher temperatures, Rs at lower (Tetali, Ferekides, Jan 2000)

- uniform degradation mechanism – junction recombination
- non-uniform degradation mechanism – spatial formation of weak-diodes, “holes”, -nonuniformities, “reach-thru” diodes – occur without affecting Rsh
- Mode #2 – Roll-over in 1st quadrant
- is this really a degradation mode, or is it a signature of the mechanism responsible for Mode #1?
- mechanistically we model this as a uniform degradation associated with either increased back barrier potential or increased series resistance
- Mode #3 – Degraded Resistive Terms, Rsh & Rs
- primarily treated as non-uniform
- simple metallic shunting or contact delamination
- more complex, e.g. “holes” – combination of low Rs and weak-diodes (Shvydka, Feb 2004)
- Rsh seen predominantly at higher temperatures, Rs at lower (Tetali, Ferekides, Jan 2000)

- On Modes and Mechanisms— current knowledge —

## Mode #1 – Voc decrease, uniform mechanism

- Results from over the past 5 years which include FS with FS contacts, FS with IEC contacts, UT with UT contact, and BP with BP contact

- Junction recombination increases 5-500X after stress

- Based on 1-D models and dark JV curves

- (Hegedus, McCandless, Team Notes May 1999, Jan 2000, Jan 2001

- Mode #1 – Voc decrease, uniform mechanism

## ~ 1 cm (1 sun, TCO)

- V

- I

- L

- Voc

- Jsc

- Jw

- Jw

*l*

- V

- I

**WD**

**Integral**

**–****sheet resistance,  *****j*****- s.c. current density**

- One weak micro-diode affects

*Numerical 2D modeling: **Low Voc, FF, no Rsh*

**WD**

*WD *
*shunting*
*mechanism*

*Integral *
*J/V:*
*Low Voc,*
*Low FF,*
*no Rsh*

- Karpov, Compaan, Shvydka,  Appl. Phys. Lett. vol 80, No. 22  (2002)
- Karpov, Harju, Doer, 28th IEEE PVSC (2000)

- Mode #1 – Voc decrease, non-uniform mechanism

## Mode #2 – Roll-Over (uniform mechanism)

- Mode #2 – Roll-Over (uniform mechanism)

- Roll-over commonly reported for NREL, IEC, FS devices, FS modules, CSM devices
- Mechanistically, it was shown to be a separate mechanism from degradation mode #1. (e.g., FS with IEC contact stressed and recontacted. Jo increases from 3E-8 to 8E-7 mA/cm2 with stress but is unchanged with recontacting. (Hegedus, McCandless, Birkmire “Analysis of stress induced degradation in CdS/CdTe solar cell” Proc. 28th IEEE, 2000)
- The absence of roll-over does not improve Jo, or Voc,..thus is it really a degradation mode, or rather, a signature of the mechanism associated with Mode #1?

## Mode #3 – resistive (non-uniform mechanism)

- Mode #3 – resistive (non-uniform mechanism)

- Again, mechanistically we now show separation of shunt and junction related degradation modes.  Shunt mode = 78% total degradation; junction mode = 11%

- McMahon, et al., J. Appl. Phys. 97 (2005)
- Albin, et al., 31st IEEE PVSC (2005)

## are used “to obtain timely information on the reliability of simple product components, materials, and to provide early identification (and removal) of failure modes, thus improving reliability.” (Juran’s Quality Handbook – 5th Edition, McGraw-Hill (1999), p. 48.16.)
Methods of acceleration:
increase use rate 
cyclic testing between evening-day load and thermal conditions (CSU 5/3 test)
increase aging rate
temperature & humidity accelerate chemical and electronic processes
increase level of stress
if cells fail at Voc bias, then accelerate mode at 2 * Voc.
measure degradation vs time under some combination of ALT parameters
routinely search for (t=0) observations which correlate to (t>0) stability
high Voc prior to contact application = stability (Sampath, Barth, Enzenroth)
low-light Voc dispersion = non-uniformities = instability (Shvydka, Karpov)

- Methods of acceleration:
- increase use rate
- cyclic testing between evening-day load and thermal conditions (CSU 5/3 test)
- increase aging rate
_**temperature**_ & humidity accelerate chemical and electronic processes
- increase level of stress
- if cells fail at Voc bias, then accelerate mode at 2 * Voc.
- measure degradation vs time under some combination of ALT parameters
- routinely search for (t=0) observations which correlate to (t>0) stability
- high Voc prior to contact application = stability (Sampath, Barth, Enzenroth)
- low-light Voc dispersion = non-uniformities = instability (Shvydka, Karpov)

- On Accelerated Life Testing

**Notes:**
Note:
It’s okay to combine different acceleration methods.  For example, Al combines increasing the use rate (light on/off) and aging rate(65C) methods when he does his ALTs.  Performing aging rate studies (elevated temperatures) with different bias (stress level) is another example.

One type of test that we may not be doing enough of is using higher bias’s than what nature would suggest.  Rather than bias at Voc (J=0), should we consider V= 2Voc (with J=0 again).  Another test would involve passing more current through the device than Jsc (to accelerate degradation associated with joule heating).

## On Degradation Curves

- P(t) = P(0)[1-exp(kt) ] (in general)
- P(t) = P(0) + kT[ln (1- t/)] (specifically ref#1)

- Type B – 1st-order “growth”; convex curves, e.g. fatigue cracks, crack propogation, dendritic growth processes

- Performance Parameter, P

- P(0)

- A

- C

- B

- P(0) x 0.9

- Stress Time

- ref 1:  Karpov, et al., Appl Phys. Lett. 80(22) (2002)
- ref 2:  Enzenroth, et al., Proc. 29th European PV Solar Energy Conf., (2004)

**Notes:**
linear processes – tire wear

convex – fatigue cracks – crack tip propogation – dendritic nucleation and tip growth

concave – depletion processes (something runs out)

## On Degradation Curves

| T(C) | AF |
| --- | --- |
| 50 | 1 |
| 65 | 4.9 |
| 85 | 33.4 |
| 100 | 123 |
| 200 | 88,000 |

- Performance Parameter, P

- P(0)

- A

- C

- B

- P(0) x 0.9

- 0                            5                           10 yrs     T=  50 C

- 0                         8940                  17,880 hrs    T=  65 C

- 0                         1311                     2623 hrs    T=  85 C

- 0                          356                        712 hrs    T= 100 C

- 0                          0.5                             1 hr      T= 200 C

- Temperature Acceleration Factors based on 1 eV mechanism and “use” temp = 50 C
- (1eV mechanism proposed for 65-100 C ALT (Sites, Jan 2000, Team Notes)

- “Different modes have different activation energies.  For example, C may dominate at lower stress temperatures, B may dominate at higher ones.”

**Notes:**
linear processes – tire wear

convex – fatigue cracks – crack tip propogation – dendritic nucleation and tip growth

concave – depletion processes (something runs out)

## On Degradation Curves

- 10%

- 20%

- 30%
- (1000 A CdS)

**non-stabilized**
- (800 A CdS)

**stabilized**

- Mode #1 - Voc, FF loss attributed to increased Jo uniformly or non-uniformly)
- Uniform mechanism – incremental decreases in “**stabilized**” performance level
- Non-uniform mechanism – incremental decreases in “**stabilized**” performance, ramifications to module performance
- Mode #3  - Resistive,  shunt, “hole” (weak-diode, Rs), or backcontact Rs effect
- responsible for **non-stabilized** performance (kills small cells; greatly drops performance/stability of modules

**T=100 C**

## Summary Statements

- 2-3 strong degradation modes identified to date:
- Junction Degradation shown by decreased Voc, increased recombination (10% effect)
- Backcontact Degradation shown by roll-over (possibly mode #1, contact problem?)
- Resistive Degradation
- a “non-uniform” phenomena
- can “kill” small devices
- simple metallic shunts
- weak-diodes + Rs components (holes)
- backcontact effects
- Mode #3 shows a strong temperature dependence.  Resistive degradation (shunting) identified as problematic, but mostly at higher stress temperatures (T  80 C)  Series resistive terms may dominate at lower temperatures.
- Mode 1 – If “stabilized” level of performance exceeds definition of “degradation” = 1%/year, then is it “degradation”,..or is it simply “process optimization”
- Do we blissfully assume that the “56-day” test (Voc bias, T, illum) reveals degradation modes of use to PV module reliability.
- need better integration of module stability (beyond 1st level metrics) data into team activities (e.g. FSEC)

- vs.

- or