# Presentation: 785415

## Physics of Large Area Thin-film Devices: Nonuniformities, Interfacial Layers, Reach-through Effects 

- V. G. Karpov
*Department of Physics and Astronomy,University of Toledo*

**There really is a physics of thin film PV**

## Contributors

- Diana Shvydka,
- Yann Roussillon,
- A. D. Compaan,
- D. M. Geolando,
- J. Drayton

## TF PV: characteristic features 

- Forgiving: different technologies do comparably
- Variability: cell to cell and module to module
- Sensitivity: weather, daytime, etc.
- Degradation kinetics: some are extremely stable
- Super-additive effects: ..acting on front and back..

## Physics specific to thin-film PV

***Small thickness and structural disorder do it***
- Lateral nonuniformities
- Surface state induced depletion/accumulation
- Reach-through effects (acting through the film)
- Shunting-like phenomena

## Lateral Nonuniformities at a glance

- both cells

- modules

- R. D. Duke *et. al*., www.socrates.berkley.edu/~rael/aSikenya.html

**Variability**

- in

- and

## Examples of nonuniformity observations

-  Other:** **OBIC; EBIC; PL (micro-PL); EL ; Thermography

- Electric potential variations
- (highly resistive electrode)

- Current statistics:
- large and small cells

## Random diodes

- Polycrystalline cell = random micro-diodes = random diode

- Weak (low Voc) diode under forward bias shunts the system,
- Create loss and variability, concentrate stresses...

- Current loss

## Modules, Random Diode Arrays, Losses

- TCO

- Scribe Resistance

- Random
- diodes

- Analytical estimates and numerical routine developed.
**Nonuniformity loss up to 30%**

## Nonuniformity self-healing

- Treatment conditions derived
- Composition designed

- SPV

- Electrochemistry at nonuniform **S**urface **P**hoto-**V**oltage
- Creates nonuniform IFL balancing the original nonuniformity

- Before
- treatment

- After
- treatment

## Self-healing examples

- Red wine

- Aniline based solution

- Self-healing alone increased CdTe PV efficiency
**From 2-3%**   (untreated) **To 10-11%**    (treated cells)
*Patent application filed*

## Interfacial states and effects in TF

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

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

**Surface treatment** (IFL,
- “buffer”, **deposition**)

**Similar changes by**

**Chemical doping** (Cu)

- ND

**or**

## MIS model of CdTe PV

**Model:**
**M** interacts with **S** (CdTe)
-  through **I** (CdS):
- Reach-through band bending.
**CdS Depletion (I): **
**through doping or surface**

**Observations:**
- Proper “Buffer” without Cu generates Voc=830mV.
- Adding Cu does not affect Voc

**Expectations:**
- Cu free
- Efficient cell
- Superior stability

## Verifying MIS model: examples

- No Buffer;
- Cu vs. no-Cu

- Buffer;
- Cu vs. no-Cu

- Buffer and Cu make equally strong junction.
- Adding Cu to “buffer” samples does not do anything.

## MIS model of CdTe PV (consequences)

**Deposition technique matters****: surface states.**
*Example 1: “same” CVD and sputtered HTR act differently.*
*Example 2: ****CdS deposition**** is crucial.*
**Interfacial treatments matter: ****IFL, “buffer”, etch...**
**Cu is not necessary**** when the deposition is “right”.**
**Major degradation not through Cu reactions in CdTe.**
**Suspected degradation modes:**
- (a) In Cu doped samples:
- 	Cu – related shunting through CdS;
- 	Back contact deterioration through Cu electromigration;
- (b) In Cu free samples:
- 	Other shunting; Interface degradation; Metal delamination;

## Back Diode (BD) vs. Reach-through (RT)

- Mystery resolved: BC treatments having strong effect on Voc.
**Rare (RT) spots of strong back barrier ruin Voc**.
- VRT instead of Voc.
- Explains microscopic nature of weak diodes.
- VRT depends on both back and front barriers: super-additive

**Physics **
**of**
**back contact (BC)**

## BD vs. RT (continued)

- Self-healing treatment
-   improves BC:
-   RTD spots healed
- Bad sample J-V steeper,
-   no rollover

- Bad BC :
- Abnormally high current
- Space charge  region J ~ V

## BD vs. RT: predictions - verified

- Low efficiency
- No rollover;
- High forward current;
- Repealing metal leaves surface with high Voc;
- No such for low Voc devices: Voc<“VRT”.

## Cu-free efficient stable CdTe cell

**Glass/TCO/HRT/CdS/CdTe/aniline/Au**
**13% efficient** (with 100 nm CdS),
**superior stability **(preliminary)
- 1cm2 CSS deposited,
- Sputtered HRT,
- Cold evaporated Au, no post-metal heat

## Conclusions

**Physical concepts specific to thin film PV are proposed:**
- Lateral nonuniformity;
- High surface/volume ratio: strong interfacial effects;
- Reach-through effects: electric field reaching through the film;
- Shunting-like phenomena (in progress)
**Practical implications:**
- Self-healing electrochemistry levels out the nonuniformity;
- Surface treatments instead of doping: Cu –free device;
- Cold evaporated Au back contact with Cu – free recipe;
- Efficient (preliminary) stable thin-film CdTe solar cell.

## MAJOR FY 2004 PUBLICATIONS 

- [1] V. G. Karpov, A. D. Compaan, and Diana Shvydka, *Random diode arrays and mesoscale physics of large-area  devices*, Phys. Rev B* ***69**, 045325, (2004).
- [2] V. G. Karpov, *Critical disorder and phase transitions in RDA*,  Phys. Rev. Lett., **91**, 226806  (2003).
- [3] Y. Roussillon, D. Giolando, Diana Shvydka, A. D. Compaan, and V. G. Karpov,* Blocking thin film nonuniformities: photovoltaic self-healing*,  Appl. Phys. Lett*. ***84***,* 616 (2004).
- [4] V. G. Karpov, Diana Shvydka,Yann Roussillon, and A. D. Compaan, *The mesoscale physics of large-area photovoltaics*, *Proceedings of 3**d** World Conference on Photovoltaic Energy Conversion*, Osaka, Japan, 2003
-  [5] Y. Roussillon, D. Giolando, Diana Shvydka, A. D. Compaan, and V. G. Karpov, *Reach-through band bending in semiconductor thin films*,  Appl. Phys. Lett*. *October 25 (2004).
- [ 6] D. Shvydka, J.P. Rakotoniaina,  O. Breitenstein, *Lock-in thermography and nonuniformity modeling of thin-film CdTe solar cells*, Appl. Phys. Lett*.* **84**, 729, (2004)
- [7] Y. Roussillon, V. G. Karpov, D. Shvydka, J. Drayton, and A. D. Compaan* Back contact and reach-through diode effects in thin-film photovoltaics*, J. Appl.  Phys. December 01 (2004)
- [8] V. G. Karpov, D. Shvydka, and Y. Roussillon *E**2 **phase transitions: thin-film breakdown and Schottky barrier suppression*,  Phys. Rev. B*, *October 15 (2004)