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Physics of ultrathin photovoltaics. Ultrathin photovoltaics. Rare materials. Indium in CIGS solar cell. Tellurium in CdTe solar cell. To reduce material consumption. Ultrathin photovoltaics. Advantages Low cost Rapid process Material conservation. Submicron absorber thickness.

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Physics of ultrathin photovoltaics


Ultrathin photovoltaics

Rare materials

Indium

in CIGS solar cell

Tellurium

in CdTe solar cell

To reduce material consumption

Ultrathin photovoltaics

Advantages

Low cost

Rapid process

Material conservation

Submicron absorber thickness


Optical absorption

Light absorption

Thin absorption layer

Optical loss by insufficient light absorption

Light trapping

Texturing

Photonic crystal

Nanowire


Appl. Phys. Lett. 89, 163518 (2006)

Electrical characteristics

Depletion width vs. Film thickness

Ldep > l : Complete depletion

Film behaves as a dielectric

Bulk recombination is insignificant

Important features

Screening by electrodes

Leakage currents

Dielectric breakdown


Electrical characteristics

1. Electrode screening

Statistical fluctuation of random potential

In case of randomly distributed electric charge,

For typical gaussian distribution

And potential distribution

For n~1014-1016cm-3

For case of grain boundary charge,

Much less N~10 (assumption)


Physical Review B 69, 045325 (2004)

Electrical characteristics

1. Electrode screening

Screening by electrodes

Screening length

Physical meaning : φ fluctuation is balanced by potential drop

across the resistive electrode

Minimum Screening length

In case of , microdiodes interact strongly

When two diodes are in parrellel,

Low VOC diode under forward bias u and

Supplied by the diodes in the surrounding region within L


Physical Review B 69, 045325 (2004)

Electrical characteristics

1. Electrode screening

Particle size

Very small particle size : Large N → Insignificant potential fluctuation

Very large particle size : Columnar grain diameter exceeds l

→ Boundary potential vanishing towards the electrodes

Series resistance

Diodes in parallel : Most current through weak diode

→ Voltage drop

Series resistance : Equil out the differences in the diodes

(Low value)


Electrical characteristics

2. Leakage currents

Defects & Shunting pathways

Electron tunneling between defects

Hopping conduction through defect chain

Shunt resistance

Currents through diode and resistor add up

Reduce VOC & FF


Electrical characteristics

2. Leakage currents

Probability of finding an N-defect pathway

from ref. 9 - Mesoscopic Phenomena in Solids

Hopping resistance

Probabilistic distribution of pathway resistances


Electrical characteristics

2. Leakage currents

Geometrical consideration

When

radial distribution does not suppress tunneling

l/N

a

r

Characteristic distance


Electrical characteristics

2. Leakage currents

Probability of finding an N-defect pathway

Effective shunt resistance

To find at least one of effective leakage

Critical thickness

Lower bound estimate :

Non-ohmicity or correlated defect distribution increase


Electrical characteristics

3. Dielectric breakdown

Capacitive energy consideration

Driving force : Decrease in capacitive energy

by shunt formation and induced voltage decrease

Stored energy

By shunt resistance

Defect formation

W~10 GeV for typical

Defect generation energy w~10 eV

In reality, just several defects can form a shunt pathway


Electrical characteristics

3. Dielectric breakdown

Low illumination

Stored energy is a maximum at low light

Shunting & degradation


Summary

Efficiency loss mechanisms in ultra-thin solar cells

Screening by electrodes

Leakage currents

Dielectric breakdown

In standard device, above effects cannot be suppressed by technology improvement

Potential remedy : Properly designed interfacial layers

→ Increasing defect chan resistance

→ Mitigating leakage and breakdown vulerability


Mater. Res. Soc. Symp. Proc. 668 (2001), p. H6.4.

Case #1

Ultra-thin CdTe device

Device characteristic

Reduced photon collection in red

JSC loss is expected

In reality,

12% eff. in 3μm device

5% eff. In 0.5μm device

VOC drop is more responsible to efficiency loss


Solar Energy Materials & Solar Cells 90 (2006) 2263-2271

Case #1

Ultra-thin CdTe device

Device reoptimization

Reoptimized processes : CdCl2 treatment

Back contact diffusion

Chloride processing produces intermixed alloy layer of CdSTe more quickley

Shorter CdCl2 processing prevents overgrowth of alloy layer

Shorter back contact diffusion prevents Cu out-diffusion into the junction region


J. Appl. Phys. 98, 103703 (2005)

Case #2

Ultra-thin CIGS device

Optical absorption

At 0.5μm thickness,

Efficiency is not sensitive to Back contact reflectivity (RB)

There is no significant optical loss when t>0.5μm

Experimently, JSC losses in thin cells are higher than

calculation


J. Appl. Phys. 98, 103703 (2005)

Case #2

Ultra-thin CIGS device

Back contact

1. Depleted cells (d<0.3μm)

JSC is nearly independent

2. Medium thickness cells (d~0.4-1.0μm)

Larger back barrier

→ Larger back-contact depletion region

→ Attracts electrons and Increases JSC losses

3. Thick cells

Independent of back-contact barrier height


J. Appl. Phys. 98, 103703 (2005)

Case #2

Ultra-thin CIGS device

Bandgap grading

VOC increases in graded cell

(Reduced bulk recombination)

Grading in thick cells,

Improved current collection about 1-2 mA/cm2

In thin cells,

More significant current losses by bandgap increase

Almost no collection benefit in depleted cells


Nature 449, 885-889 (2007)

Case #3

Coaxial Si NW solar cell

Breakdown voltage

p-n diode : negative temperature dependence

→ Zener breakdown mechanism

p-i-n diode : little temperature dependence

→ Tunneling and avalanche breakdown mechanism

p-i-n diodes have higher breakdown voltage

Tunneling or leakage current is less significant in p-i-n

diodes


In our concept

We have to overcome following issues :

Optical light-trapping

Ohmic contact

Back contact barrier – Not required in fully depleted cells

p-i-n structure can be required

TCO(AZO)

CIGS thin layer

SiOx thin layer

CdS thin layer

Si nanowire

Si SUBSTRATE


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