CdTe Solar Cells: Basic Model and Common Deviations · With Input from Alan Fahrenbruch and Victor...

May 5, 2005 Device-Physics Subteam

CdTe Solar Cells:Basic Model and Common DeviationsJim Sites, Jun Pan, and Markus Gloeckler

Colorado State UniversityWith Input from Alan Fahrenbruch and Victor Karpov

CdTe Team Meeting

Goals:(1) Show the basic band structure.(2) Identify mechanisms that detract

from performance.(3) Clarify the language used.

Outline


“Standard” CdS/CdTe Band PictureTypical band diagram and J-VVariations with carrier densities and

thicknesses (p-n vs. p-i-n vs. MIS)

Deviations from “Standard” J-V BehaviorBack-contact barrierCdS/CdTe interfacial effectsCollection efficiency Comment on non-uniformities

Approaches


Analytic (classical physicists)Aesthetically pleasing and intellectually satisfyingComparisons among researchers straightforwardDirect comparison of basic principles with J-V curvesComplications require math approximations that can distort results

Circuit (electrical engineers)Usually shows dominant featuresAssumes general analytic solution is knownNo distance scale, hence caution requiredMay oversimplify results

Numerical Simulation (computational scientists)Handles mathematical complicationsCan reveal unsuspected featuresGraphical presentation follows naturallyHard to compare with others or convince skeptics

Basic Model


Current - Voltage Band Diagram (Φbc = 0.4 eV)

Dark

Light

η = 16.2 %

p = 2×1014

EC

EF

EV

CdTe (4 µm)

SnO2 (0.5 µm)

CdS (80 nm)

Light, V = 0Dark Very Similar

Intermediate thicknesses, small back barrier, high-efficiency conditions

Small Cliff

Variation with CdTe Carrier Density


Current - Voltage

2×1013

2×1013

p = 2×1014

2×1017

2×1017

Band Diagram (Φbc = 0.4 eV)

CdSSnO2

CdTe

Voltage goes up, current down. CdS thick enough, doped enough that SnO2 not part of junction.

Quantum Efficiency


Current - Voltage

2×1013

p = 2×1014

2×1017

Quantum Efficiency

inc. pdec. LDin CdTe

10 nm CdS

80

200

(1) Red collection lower for higher p (less depletion); similar effect with shorter diffusion length (2) Blue collection lower for thicker CdS

Current-Voltage Parameters


Current - Voltage

2×1013

p = 2×1014

2×1017

J-V Parameters

Hole Density [cm-3]

General, but not dramatic, increase in efficiency with hole density

p-n vs. p-i-n (CdTe driven)


Band Diagram (Φbc = 0.4 eV)

No abrupt transition. Either p-n or p-i-n possible.

Division of p-d plane

2×1013

2×1017

CdTe p- or i

CdS n (80 nm)

SnO2 n+ (M)

p-n vs. MIS (CdS driven)


Position [µm]

0.4 0.5 0.6 0.7C

ondu

ctio

n B

and

Ene

rgy

[eV

]0.0

0.1

0.2

0.3

0.4

SnO2 CdS

CdTe (2×1014) (4 µm)

1015

3×1016

n = 1017

M I (or n) S (or p)

Band Diagram (Φbc = 0.4 eV)Division of n-d Plane

Again, no abrupt transition; cells can be either p-n or MIS

Larger Back Barrier (Φbc = 0.6 eV)


Case 1: Back-contact depletion does not overlap with primary-junction depletion

Forward Bias (V ~ VOC)Zero Bias

Little hole impedance

Hole impedance in both directions

Impact of back barrier changes with voltage

“Rollover” Effect


Current near VOC impeded in both directions (reduced collection in forward bias)

Φbc = 0.4

0.5

0.6

Requires separation of the two depletion regions

(next slide)

For 4-µm CdTe

Overlapping Back Barrier


Case 2: Partial overlap of back-contact and primary-junction depletion (key parameter is conduction-band/quasi-Fermi-level difference)

Φbc = 0.4

Primary effect is lower voltage. Secondary effect: partial reversal of photocurrent.

Φbc = 0.4

0.6

Φbc = 0.4

0.6

V ~ VOC

EC

EFn

Φbc = 0.4

0.5

0.6

Interfacial Recombination


ECnCdS = 1014

nCdS = 1017EFn

EV

SnO2CdS CdTe

ΦIR (1014) ΦIR (1017)

Varies with CdS carrier density: higher n in CdS keeps holes away (modest voltage decrease at lower density)

Voltage also varies if interface trap density changes (could be the larger effect)

Collection Efficiency (CE)


Current - Voltage Collection Efficiency

p = 2×1014

Φbc = 0.4 eV

less CdTebulk lifetime

9%12% 16%

True CE at modest V

Artificial CE near VOC and above

Reduced photocurrent; larger effect at higher voltage

Summary of J-V Distortions


Simple ShuntRoll-over

(current reduction)

Collection Efficiency

(photocurrent reduction)

Overlapping Barriers orInterfacial Recombination (forward-current increase,

or voltage reduction)

Typical signatures. Possible to have more than one present. Possible, in fact likely, that problems are not spatially uniform.

Conclusions


(1) Several effects can degrade CdTe performance, and proper separation can be a challenge.

(2) Cells work with p-n, p-i-n, or MIS structures, but choice can affect seriousness of other effects.

(3) Back-barrier by itself restricts current (reduced collection and “rollover”).

(4) Back-barrier and primary-junction depletion overlap leads to (a) enhanced forward current (reduced voltage) and (b) partial reversal of photocurrent (reduced collection).

(5) Reduced CdS carrier density, or increased trap density, enhances interfacial recombination (reduced voltage).

(6) Collection efficiency effects likely in lower efficiency cells.(7) All effects are likely to be bigger problem when stress drives

copper from back-barrier region.(8) All effects discussed unlikely to occur uniformly over solar cell.

CdTe Solar Cells: Basic Model and Common Deviations · With Input from Alan Fahrenbruch and Victor...

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