How thin film PV can be different

1
How thin film PV can be different

• Presented by Victor Karpov
• Contributions by
• D. Shvydka, Y. Roussillon,
• A. D. Compaan, D. M. Giolando
• University of Toledo

2
x-PV standard interpretation

• 1-Dimensional p-n junction
• Defect dominated loss
• Material degradation due to defect generation

3
Electronic processes in crystalline PV
Defects cause uniform recombination and loss
Cast



electrons
holes
E


defects


field
4
Some motivations for revisiting

• So imperfect, how come it works?
• Optically inactive back contact hits Voc
• Strong buffer layer effects
• Super-additive front and back
• Variations between nominally identical devices

5
Conceptual Motivations

• Lack of crystallinity disorder
• Small thickness

It follows then
6
Lack of transversal self-averagingLateral
non-uniformities
Random micro-diodes
Voc1
Local diode characteristics vary
7
Recombination pathways weak diodes
Low Voc micro-diode Runs exponentially strong
current In the wrong direction
Nonlinear shunt No slope in J(V) _at_ V0
Signature effects Low Voc, FF
L 1cm
8
Microscopic nature of weak diodes
Main junction
Back contact
Strong back barrier
Weak SPV
Recombination path through CdS or depletion region
Poor doping Contamination Metal delamination
Either front or back junction, or both (more in
detail described in our other talk)
9
Thin films tend to be insulators
Depletion width

• Lltltl,
• Semiconductor
• High carrier concentration
• Large thickness

• Lgtgtl ,
• Insulator
• Low carrier concentration
• Small thickness

10
MIS rather than P-N junction

• P-N junction
• Higher N beneficial
• M does not affect VBI

• MIS junction
• N does not matter (I)
• M affects VBI

11
Strong interfacial effects
High equivalent doping l - film thickness,
Ns (cm-2) - surface state density
CdS example l10-5 cm, NS1012 cm-2, ND
1017 cm-3
Interfacial states can act stronger than bulk
doping (Too few bulk states in thin films)
12
MIS model facts and venues

• Evidence
• M-side having effect on VBI
• Highly resistive CdS, CdSO
• Photoconductive CdS
• Piezo-effect in CdS
• Cu depleting CdS

• Venues
• Work on CdS and interfaces
• CdS/TCO(buffer) and CdS/CdTe stresses and states
  matter
• Cu is not necessary for SPV when deposition is
  right

13
Short drift time
-- Typical uniform lifetime in poor
photoconductors
Charge carriers do not have time to
recombine (possibly facilitated by GB)
14
In poly-crystalline PV

• Almost no uniform recombination
• Nonuniform recombination in weak diodes
• Nonuniformities and contacts matter

Cast


GB



E
Weak diode


electrons

holes



field
15
Degradation mechanisms

• Increase in nonuniformity
• Metal shunting
• Cu path shunting through CdS
• Diode weakening under localized stress
• Back barrier deterioration
• Cu diffusion from back surface
• Adhesion failure and metal delamination
• In all cases - structure related degradation
  rather than material (CdTe is quite stable)

16
Conclusions

• MIS type of device
• Lateral nonuniformities
• Nonlinear shunts (weak diodes, etc.)
• Strong interfacial effects
• Uniform recombination irrelevant
• Degradation - nonuniformity and back contact
  failure
• Qualitatively consistent with the observations