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LBNL 9/15/06 Limiting factors in solar cell efficiency - how do they apply on the nano-scale ? D.G. Ast Cornell University

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Page 1: LBNL 9/15/06 Limiting factors in solar cell efficiency - how do they apply on the nano-scale ? D.G. Ast Cornell University

LBNL 9/15/06

Limiting factors in solar cell efficiency - how do they apply

on the nano-scale ?

D.G. Ast

Cornell University

Page 2: LBNL 9/15/06 Limiting factors in solar cell efficiency - how do they apply on the nano-scale ? D.G. Ast Cornell University

LBNL 9/15/06

Giant Single Molecule Cell (bulk collection)

record =24. 7 %

1-D “nano” cell

(depleted collection)

record = 13%

(triple - UniSolar)

3-D “nano” cell (dye or QM sensitized)

record= 12.6%

(Graetzel, wet)

From Macro to Nano

Page 3: LBNL 9/15/06 Limiting factors in solar cell efficiency - how do they apply on the nano-scale ? D.G. Ast Cornell University

LBNL 9/15/06

THE DEVICE VIEW

Jsc Voc FF

Jsc

Voc

Page 4: LBNL 9/15/06 Limiting factors in solar cell efficiency - how do they apply on the nano-scale ? D.G. Ast Cornell University

LBNL 9/15/06

Short Circuit Current

Jsc

Page 5: LBNL 9/15/06 Limiting factors in solar cell efficiency - how do they apply on the nano-scale ? D.G. Ast Cornell University

LBNL 9/15/06

BAND GAP

Black body: Thermal emission from cell via above bandgap photons.

Page 6: LBNL 9/15/06 Limiting factors in solar cell efficiency - how do they apply on the nano-scale ? D.G. Ast Cornell University

LBNL 9/15/06

Jsc

AM 1.5 Theoretical Upper Limit versus bandgap .

Page 7: LBNL 9/15/06 Limiting factors in solar cell efficiency - how do they apply on the nano-scale ? D.G. Ast Cornell University

LBNL 9/15/06

Jsc

1. Incident energy flux reduction

Reflection (Si ~ 10%)

Contact shading

Insufficient absorption ( d < )

Page 8: LBNL 9/15/06 Limiting factors in solar cell efficiency - how do they apply on the nano-scale ? D.G. Ast Cornell University

LBNL 9/15/06

Jsc 2. Failure to generate electron hole pairs

Sub bandgap photons (excluding two photon processes*)

Bulk free carrier absorption, “Auger”

Free carrier absorption due to V across

cell

Frank Keldysh effect

* investigated in Si for two photon processes

Page 9: LBNL 9/15/06 Limiting factors in solar cell efficiency - how do they apply on the nano-scale ? D.G. Ast Cornell University

LBNL 9/15/06

3. Failure of e-h to contribute to current

Bulk recombination (impurities, structural defects, )

Surface recombination

Non-contacted surface

Contacted surface

Jsc

Page 10: LBNL 9/15/06 Limiting factors in solar cell efficiency - how do they apply on the nano-scale ? D.G. Ast Cornell University

LBNL 9/15/06

Jsc 4. Remedies

Bulk Losses:

Clean Starting Materials

Gettering

Thin bulk ! => Light trapping design !

Collecting from fully depleted

layers with uniform (!) electric

fields. (No “hang through” !)

Page 11: LBNL 9/15/06 Limiting factors in solar cell efficiency - how do they apply on the nano-scale ? D.G. Ast Cornell University

LBNL 9/15/06

Surface Recombination

a. Non contacted area

Small Area ! (Implication for “nano” !)

Factors influencing S:

Capture cross section of surface states (in Si >>for

electrons than holes)

Hole and electron concentration at surface (Voutput

dependent as cminority increases with Vout)

Charge of passivating layer

Jsc

Page 12: LBNL 9/15/06 Limiting factors in solar cell efficiency - how do they apply on the nano-scale ? D.G. Ast Cornell University

LBNL 9/15/06

Jsc

Contact Areas:

Small area ! (Nano, again )

Heavy doping => “ Back surface field”

(Graetzel Patents on TiO2)

Page 13: LBNL 9/15/06 Limiting factors in solar cell efficiency - how do they apply on the nano-scale ? D.G. Ast Cornell University

LBNL 9/15/06

Open circuit voltage

Voc

Page 14: LBNL 9/15/06 Limiting factors in solar cell efficiency - how do they apply on the nano-scale ? D.G. Ast Cornell University

LBNL 9/15/06

Voc

1ln

o

loc I

I

q

kTV

Dh

ih

Ae

ie

NL

nqD

NL

nqDAI

22

0

Small ni , Long lifetimes (Diffusion distance)

1. “ Junction Leakage “

Page 15: LBNL 9/15/06 Limiting factors in solar cell efficiency - how do they apply on the nano-scale ? D.G. Ast Cornell University

LBNL 9/15/06

Green , semi-empirical

Kiss & Rehwald (thermodynamic)

Voc

Good a:Si-H (Roca, Meillaud et al.)

Page 16: LBNL 9/15/06 Limiting factors in solar cell efficiency - how do they apply on the nano-scale ? D.G. Ast Cornell University

LBNL 9/15/06

Additional contribution(s) to Jo

1. Generation current due to bulk “midgap” states

2. Generation current to due surface states.

Voc

n, diode ideality factor, function of operating conditions.

Sum over exponentials commonly expressed as

Page 17: LBNL 9/15/06 Limiting factors in solar cell efficiency - how do they apply on the nano-scale ? D.G. Ast Cornell University

LBNL 9/15/06

Efficiency tends to improve with illumination level.

Page 18: LBNL 9/15/06 Limiting factors in solar cell efficiency - how do they apply on the nano-scale ? D.G. Ast Cornell University

LBNL 9/15/06

2. Junction Defects (shorts, partial shorts, more general: Spatial inhomogeneity)

1. Scale with junction area (nano !)

2. Difficult to diagnose

3. The bane of the multicrystalline cell

Page 19: LBNL 9/15/06 Limiting factors in solar cell efficiency - how do they apply on the nano-scale ? D.G. Ast Cornell University

LBNL 9/15/06

FFEmpirical relation (Green)

1

72.0ln

oc

ococ

V

VVFF

Additional Effects

Series Resistance : Contacts & Leads

Parallel Resistance : Shunts !

Page 20: LBNL 9/15/06 Limiting factors in solar cell efficiency - how do they apply on the nano-scale ? D.G. Ast Cornell University

LBNL 9/15/06

Green:

Page 21: LBNL 9/15/06 Limiting factors in solar cell efficiency - how do they apply on the nano-scale ? D.G. Ast Cornell University

LBNL 9/15/06

Module EfficiencyCdTe modules are much less efficient than CdTe cells !

Cells must be uniform !

Cost

Not just materials but processing (CIS)

Stabilitya-Si:H, DSC ..

Page 22: LBNL 9/15/06 Limiting factors in solar cell efficiency - how do they apply on the nano-scale ? D.G. Ast Cornell University

LBNL 9/15/06

PERL cell

= 24 %

PERL => Passivated Emitter, Rear contact Locally diffused

GIANT SINGLE MOLECULE DEVICE

Page 23: LBNL 9/15/06 Limiting factors in solar cell efficiency - how do they apply on the nano-scale ? D.G. Ast Cornell University

LBNL 9/15/06

200 nm 100 nm

M. Schmidt, A. Schoepke, O. Milch, Th. Lussky, W. Fuhs

A-Si:H

1-D “NANO”

N(E) as d

Page 24: LBNL 9/15/06 Limiting factors in solar cell efficiency - how do they apply on the nano-scale ? D.G. Ast Cornell University

LBNL 9/15/06

A(luminum) Induced C(rystallization) I(on) A(ssisted)

D(eposition) poly-Si cell (M.Green) 9…12%

(Pinnacle of optical engineering)

Si reappears via M. Green

Page 25: LBNL 9/15/06 Limiting factors in solar cell efficiency - how do they apply on the nano-scale ? D.G. Ast Cornell University

LBNL 9/15/06

QM dots

3-D NANO

Page 26: LBNL 9/15/06 Limiting factors in solar cell efficiency - how do they apply on the nano-scale ? D.G. Ast Cornell University

LBNL 9/15/06

NANO CELLS:

Insufficient absorption => Multiple stacking

large surface area => junction leakage

increased lead length =>resistance, recombination

Contact shading => transparent electrodes e.g.TiO2 surface recombination, parasitic resistance,

E drops !

n mismatch => multiple bounce

Page 27: LBNL 9/15/06 Limiting factors in solar cell efficiency - how do they apply on the nano-scale ? D.G. Ast Cornell University

LBNL 9/15/06

Need to match 2 semiconductors + 1 Redox couple

Page 28: LBNL 9/15/06 Limiting factors in solar cell efficiency - how do they apply on the nano-scale ? D.G. Ast Cornell University

LBNL 9/15/06

QD dots sensitized

Higher absorption than dyes.

More corrosion resistant.

Layer 3 is the insulator (TiO2) between organic p conductor and layer 2, the F doped SnO contact layer. Potential large area problem.

Page 29: LBNL 9/15/06 Limiting factors in solar cell efficiency - how do they apply on the nano-scale ? D.G. Ast Cornell University

LBNL 9/15/06

Jsc

Absorption tuning by PbS QD growth

Page 30: LBNL 9/15/06 Limiting factors in solar cell efficiency - how do they apply on the nano-scale ? D.G. Ast Cornell University

LBNL 9/15/06

Jsc

Jsc at QD size below optimal optimum.

Monochromatic Light

max ~ 10%

Trade off between coverage (1x), QM size tuning, and transport loss.

Page 31: LBNL 9/15/06 Limiting factors in solar cell efficiency - how do they apply on the nano-scale ? D.G. Ast Cornell University

LBNL 9/15/06

Summary

Challenges:

Transition from single giant molecule cell to nano cell

1. Large increase in junction area .. junction defects.

2. Surface states

2. Transparent conductors…. bandgap matching.

3. Large contact area … shunts

Rewards :

1. Bandgap tuning

2. Easier implementation of “sub bandgap” and “hot carrier” . . . ; .. Conversion.

3. Large area for “in situ” chemical conversion !

Page 32: LBNL 9/15/06 Limiting factors in solar cell efficiency - how do they apply on the nano-scale ? D.G. Ast Cornell University

LBNL 9/15/06

Appendix

Page 33: LBNL 9/15/06 Limiting factors in solar cell efficiency - how do they apply on the nano-scale ? D.G. Ast Cornell University

LBNL 9/15/06

...

Page 34: LBNL 9/15/06 Limiting factors in solar cell efficiency - how do they apply on the nano-scale ? D.G. Ast Cornell University

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Page 35: LBNL 9/15/06 Limiting factors in solar cell efficiency - how do they apply on the nano-scale ? D.G. Ast Cornell University

LBNL 9/15/06


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