# Presentation: 202735

## Piezo-effect in CdS/CdTe solar cells

- Presented by Diana Shvydka
- Contributions by
- J. Drayton, V.G. Karpov, A.D. Compaan
- University of Toledo

## Introduction: unexpected observation

- Observation during measurement of CdS electrical properties -  dependant on pogo-pin pressure
- On solar cell: Voc reversibly decreases with pressure

- solar cell

## Historical background

- Strong piezo-effect in CdS known since 1950’s
- Shows both in crystals and thin films
- Was expected to be important in CdS PV
- Was not observed in Cu2S-CdS cells

- SOME REFERENCES:
- J. D. Zook and S. T. Liu, “Pyroelectric effects in thin film”, J. Appl. Phys. 49, 4604 (1978).
- F. A. Pizzarello, “The effect of metal contact on acoustic generation in thin film CdS”, J. Appl. Phys. 38, 1752 (1967).
- D. Berlincourt, H. Jaffe, L. R. Shiozawa, “Electroelastic Properties of the Sulfides, Selenides, and Tellurides of Zink and Cadmium”, Phys. Rev. 129, 1009 (1963).
- M.S. Bennett, J. J. Kramer, “The effect of piezoelectrically coupled stress on the junction characteristics of CdS-Cu2S solar cells”, J. Appl. Phys. 54, 7159 (1983).

## Nature of piezo-electricity crystals without inversion symmetry

- Uncompressed

- Compressed
- along X1

- Compressed
- perpendicular to X1

- polarized

- unpolarized

- Quartz

## Base-line voltage estimate

- P ~ 107 N/m2
- for CdS:
- d31 ~ 10-11 C/N
- C0 ~ 10-4 F/m2

***V *~ 1V

- Measured by XRD tensile strain in CdS film (~ 10-3) translates into ***V* ~ 0.5 V

- P – pressure along *c* axis
- d31 – piezoelectric constant (relates stress  to *c* axis to piezoelectric polarization  to *c* axis)
- C0 – geometrical capacitance

## Resistivity

- Piezo-effect is observed in highly resistive films only (CdS:Cu, CdS:Ag)
- Observable within the depletion width of a Schottky barrier
- Shunting and surface imperfections can suppress piezo effect

- Mobile charges neutralize piezoelectric effect

## Experimental set-up

- Reliable 18th century

- Demo available here

## Results: large-area pressure point

-  Consistent with estimate
- Voc ~ 0.5V
-  Fully reversible
-  Unexpectedly non-linear
-  Hysteresis

- Uniform pressure, tip area = 4.5 mm2

- Light intensity:
- low light ~ 0.02 Sun
- high light ~ 0.04 Sun

## Results: small-area pressure point

- Pogo-pin, hard to estimate area of the pressure point
- Applied force ~ 10-20 times lower than for large-area tip

- UT cell, Au contacts

- CSU cell, UT Cu-Au contacts

## Plausible explanation: suppression of MIS structure 

- z

- x

- z

*V*

- Dipole layers

- M

- I

- S

- M

- I

- S

**+**

**+**

**+**

**-**

**-**

**-**

- 1

- 2

- 3

- 1 – original structure
- 2 – moderate pressure: change in insulator insignificant
- as long as MIS holds
- 3 – strong pressure: MIS structure converted into weak p-n junction

- Hysteresis due to trap state recharging

- TCO

- CdS

- CdTe

## Implications

- Deposition-related strain and piezo-voltage from
- TCO/buffer/substrate combination
- Subsequent CdTe deposition
- CdS compensation
- Shunting and interfacial states reduce piezo-effect
- Cu-doping related strain and piezo-voltage*
- Correlation between Cu and deposition
- Cu may not be necessary
- Part of Voc may come from piezo-voltage in CdS

- * M.Paulraj et al., “Characterizations of undoped and Cu doped CdS thin films using photothermal and other techniques”, Phys. Stat. Sol. (a) **202**, 425 (2005)

## Summary

- Strong reversible change in Voc under pressure
- Absolute value consistent with piezo-parameters of CdS
- Non-linearity plausibly explained by piezo-effect in MIS structure
- Numerous implications
- Verifications are called upon