Development of Quantum Dot Solar Cells

Okada Laboratory

Research Center for Advanced Science & TechnologyThe University of Tokyo

AM1.5

Wavelength

Efficient absorption by using multiple bandgaps

AM1.5

Wavelength

Efficient absorption by using multiple bandgaps

Efficient use of high-energysolar radiation

AM1.5

Wavelength

Efficient use of high-energysolar radiation

AM1.5

Wavelength

Hot carriers

3 V

Multiple junctions(Quantum size effect)

Eg

Intermediate bands(QD superlattice)

Multiple excitongeneration (MEG)

3 I

nanoscaleformats

Innovative PV : 50% Efficient Solar Cells

Intermediate Band Solar Cell：Principle

A. Luque and A. Martí, Phys. Rev. Lett. 78, 5014 (1997)

Conduction Band

Intermediate Band (IB)

p –type Host Semiconductor

n-type HostSemiconductor

IB 材料

Valence Band

Intermediate Band Solar Cell：Theoretical Efficiencies

η = 47% (1sun)η = 63% (Maximum concentration)

Maximum concentration1sun

5010Efficiency (%)

47%Eg=2.4 eVECI=0.9 eV

45

40

20

35

3025

15

1.5 2 2.5 30

0.5

1

E IV

(eV)

Eg (eV)

CB

-IB e

nerg

y ga

p(e

V)

CB-VB energy gap (eV)

Efficiency (%)

63%Eg=1.9 eVECI=0.7 eV

30

60

7010

5040

20

1.5 2 2.5 30

0.5

1

EIV

(eV)

Eg (eV)

CB

-IB e

nerg

y ga

p(e

V)

CB-VB energy gap (eV)

Y. Okada et al, Oyo-Buturi, 79, 206 (2010)

Intermediate Band Realized with 3D Quantum dot Superlattice

VB

CB

miniband

InAs QDGaAs

VB

CB

InAs QD

GaAs GaAs

GaAs

InAs QD

Single QDSingle QD 3D QD Superlattice3D QD Superlattice

Frank-van der Merwe Volmer-Weber Stranski-Krastanov (S-K)

2D Growth 3D Growth 2D→3D

Self-Assembled Growth of Quantum Dots

Molecular Beam Epitaxy (MBE)

Substrate

Source

Shutter

Heater

Growing layer

Chamber

VacuumPump

Heater

K-cell

Molecular beam

N2

H2

Plasma cell

H-cracking cell

Substrate

Source

Shutter

Heater

Growing layer

Chamber

VacuumPump

Heater

K-cell

Molecular beam

N2

H2

Plasma cell

H-cracking cell

InGaAs on (311)BInAs on (100)

K. Akahane et al, APL 73 (1998) 3411

Self-Organized InGaAs Quantum Dots on (311)B Substrate

QDs on (311)B substrate show;(1) Better size homogeneity(2) Higher in-plane density(3) Ordered periodic structure(4) Better heterointerface quality

Z.R. Wasilewski et al. JCG 201 (1999) 1131

Fabrication of 3D Quantum Dot Superlattice: Strain-Balancing

Accumulation of lattice strainin conventional approach

Strain-compensation growth: Strain/layer is balanced out

QDSpacer

Strainfield

InGaAlAs

III-V multijunction solar cells

GaAsP

Strain-Compensation Materials

Self-Organized Stacked InAs QDs on InP(311)B Substrates

Spacer thickness d = 20 nmNumber of stacked QDs = 30 layers

Average diameter = 63.2nmIn-plane dot density = 2.7×1010cm-2

Size uniformity ~ 12.3% Y. Okada et al, EU-PVSEC, Barcelona (2005)

on GaAs (311)B

InGaAs/GaNAs Quantum Dot Solar Cell : 1sun

ISC(mA/cm2)

VOC(V)

FF Efficiency(%)

QD solar cell on GaAs(311)B 24.26 0.791 0.840 16.12

Y. Okada, MRS Fall Meeting (Boston, 2008)

IB (3D QD Superlattice) Solar Cell：On-going Developments

A. Luque et al. Journal of Applied Physics 96, 903 (2004)

InAs/GaAs

IES-UPMUniv. of Glasgow

S. M. Hubbard et al. Applied Physics Letters 92, 123512 (2008)

Rochester Institute of TechnologyNASA

InAs/GaAsP InAs/Ga(N)As/GaAs

Univ. of Tokyo

Y. Okada et al. Journal of Applied Physics 106, 024306 (2009)

V. Popescu et al. Phys. Rev. B 78, 205321 (2008)

InAs/GaAsP InAs/AlGaAs

S. A. Blokhin et al. Physics of semiconductor Devices 43, 514 (2009)

St. Petersburg Russian AcademyIoffe Institute

NREL

η = 15.7% (100)16.1% (311)B

η = 18.32%

Intermediate Band (Quantum Dot Superlattice) Solar Cell

・ Efficiencies > 60% (under concentration) are possible with intermediate band solar cells. Concentrator cells are very cost-effective.

・ Fabrication of high-quality 3D quantum dot superlattice is required asa good intermediate band.

・Good size uniformity (< 10%) ・High density resulting in higher absorption・Close packing resulting in miniband formation

(dot spacing < 10nm)・Optimization of bandgap energies of materials

・ Top-down approaches are attractive for reducing cost.・ Photon management helps increase optical absorption in IB.

This work is supported by NEDO and METI, Japan.

Should be achieved soon