Piezoeffect in CdSCdTe solar cells

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Piezo-effect in CdS/CdTe solar cells

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

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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
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Historical background

• Strong piezo-effect in CdS known since 1950s
• 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).

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Nature of piezo-electricity crystals without
inversion symmetry
Quartz
Uncompressed
Compressed along X1
Compressed perpendicular to X1
polarized
unpolarized
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Base-line voltage estimate
P 107 N/m2 for CdS d31 10-11 C/N C0 10-4
F/m2
P pressure along c axis d31 piezoelectric
constant (relates stress ? to c axis to
piezoelectric polarization ? to c axis) C0
geometrical capacitance
dV 1V
Measured by XRD tensile strain in CdS film (d
10-3) translates into dV 0.5 V
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Resistivity

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

Mobile charges neutralize piezoelectric effect
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Experimental set-up
Reliable 18th century
Demo available here
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Results large-area pressure point
Uniform pressure, tip area 4.5 mm2
Light intensity low light 0.02 Sun high light
0.04 Sun

• Consistent with estimate
• dVoc 0.5V
• Fully reversible
• Unexpectedly non-linear
• Hysteresis

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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
CSU cell, UT Cu-Au contacts
UT cell, Au contacts
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Plausible explanation suppression of MIS
structure
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
I
S
S
M
M
I
CdTe
TCO
CdS
V
z
Dipole layers
-


-
x

-
z
Hysteresis due to trap state recharging
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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)
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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