# Presentation: 107531

## 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*

## x-PV ‘standard’ interpretation

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

## Electronic processes in crystalline PV

- +

- +

- +

- +

- +

- +

- +

***E***

- electrons

- holes

- defects

*Cast:*

- field

- Defects cause uniform recombination and loss

## 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

## Conceptual Motivations

- Lack of crystallinity = disorder
- Small thickness

**It follows then...**

## Lack of transversal self-averaging:Lateral non-uniformities

- Voc1

**Random **
**micro-diodes**

**Local diode characteristics vary**

## Recombination pathways: weak diodes

- Low Voc micro-diode
- Runs exponentially strong current
- In the wrong direction

- Nonlinear shunt:
- No slope in J(V) @ V=0

- Signature effects:
- Low Voc, FF

*L ~ *1cm

## Microscopic nature of weak diodes

- Weak SPV

- Recombination path through CdS or depletion region

**Main junction **

**Back contact**

- Strong
- back barrier

- Poor doping
- Contamination
- Metal delamination

**Either front or back junction, or both**
**	****(more in detail described in our other talk)**

## Thin films tend to be insulators

- Depletion width

*	**L<<l,*
- 	Semiconductor
- High carrier concentration;
- Large thickness

*	**L>>l* ,
- 	Insulator
- Low carrier concentration;
- Small thickness

## 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

## Strong interfacial effects

*High equivalent doping*
*l*  - film thickness,
- Ns (cm-2) - surface state density

- CdS example:   *l*=10-5 cm,  * N**S*=1012 cm-2,    *N**D* = 1017 cm-3

**Interfacial states can act stronger than bulk doping**
**(Too few bulk states in thin films) **

## MIS model facts and venues

- Evidence:
- M-side having effect on VBI
- Highly resistive CdS, CdS:O
- 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

## Short drift time

- -- Typical uniform lifetime
-     in poor photoconductors

**Charge carriers do not have time to recombine**
**	****	****(possibly facilitated by GB)**

## ...In poly-crystalline PV

**...****In poly-crystalline PV**

***E***

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

- +

- +

- +

- +

- +

- +

- +

- +

- +

- +

- GB

- Weak
- diode

- +

- electrons

- holes

- field

*Cast:*

## 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)

## 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**