Recent Progress on Intermediate-Band Solar Cells and Quantum Dot Technology for

High-Efficiency Photovoltaics

Yoshitaka OkadaYoshitaka OkadaResearch Center for Advanced Science & Technology

University of Tokyo

Intermediate Band Solar Cells (IBSCs): Concept

IV

V

P = I x V

I

2

Quantum Dot Intermediate Band Solar Cells (QD-IBSCs)

63%Eg=1.9 eVECI=0.7 eV

600.5

1

EIV

(eV

)IB

ene

rgy

gap

(eV

)

In(Ga)As/(Al)GaAsInAs/GaAsP

3VB

CB

miniband

InAs QDGaAs

Efficiency (%)

30

60

7010

5040

20

1.5 2 2.5 30

0.5E

Eg (eV)

CB

-IB

ene

rgy

gap

CB-VB energy gap (eV)

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

InAs/GaNAs Strain-Compensated QD-IBSC

50 stacks of InAs/GaNAs QDs

Cell size 3mm × 5mm

4

• Good size uniformity and no dislocations are achieved by strain compensation.

→ QD diameter: 24.6nmheight: 4.7nm

Size uniformity: 11.1%Sheet density: 5.0x1010cm-2

R. Oshima et al, Physica E 42 (2010) 2757

1.5 mA/cm2

for 50 stacks

InAs/GaNAs Strain-Compensated QD-IBSCIsc increases linearly up to ~ 50 layers. Drop in Voc saturates after ~ 30 stacks.

5

VOC(V)

ISC(mA/cm2)

FF (%)

η(%)

Diode factor

20 stacks

0.72 21.04 70.0 10.63 1.65

30 stacks

0.67 22.33 70.76 10.59 1.67

50 stacks

0.68 26.36 70.24 12.44 1.59

GaAs SC

0.94 20.26 77.74 14.80 2.00

VOC(V)

ISC(mA/cm2)

FF (%)

η(%)

Diode factor

20 stacks

0.72 21.04 70.0 10.63 1.65

30 stacks

0.67 22.33 70.76 10.59 1.67

50 stacks

0.68 26.36 70.24 12.44 1.59

GaAs SC

0.94 20.26 77.74 14.80 2.00

R. Oshima et al, Physica E 42 (2010) 2757

Conventional SCarea: (1-ra)A

Non-ideal QD-IBSC(low QD-IB absorption)

area: A

IBSC Performance on QD Absorption

Ideal QD-IBSCarea: raA

)(

)())(1(/

CICIaCVCV

CICICVCVaCVCVa

RGrRG

RGRGrRGrqJ

−+−=

−+−+−−=

　

Absorption ratio of IB

Detailed-balance model:

S. Yagi et al. 2nd Innovative PV Symp. (Tsukuba, 2009) 6

Single gap cell

1

1.1

1.2

Voc

(V

)

40

50

60

70J s

c/X

(m

A/c

m2 )

Concentration X x1 x100 x1000

IBSC Performance on QD Absorption

� Increase in ra leads to increase in JSC but decrease in VOC .

� Efficiency is below reference cell without QD-IBSC at 1sun, r < 0.5.

Detailed-balance analysis

Presentstatus

0 0.2 0.4 0.6 0.8 1

30

40

50

Eff

icie

ncy

(%)

ra

0.9

1V

QD-IB absorption ratio ra

without QD-IBSC at 1sun, ra< 0.5.

� High concentration drastically increases VOC and efficiency.

7S. Yagi et al. 2nd Innovative PV Symp. (Tsukuba, 2009)

Increase of efficiency in IB solar cells

Small absorption via IB states

Larger absorption via IB states

Under concentrationPhoton management

① ②

Pictorial Summary: Road to High Efficiencies

Maximize

ConstantConstant

8

� Doping (impurity doping or photo-filling) is important to maximize net generation rate via QD-IB.

� IBSCs work best under concentrated sunlight.

Effect of Doping and Sunlight Concentration

10

[×10+20]

−3s−

1 ) 1000 suns IBSC (w doping)1000 suns IBSC (w/o doping)

1 sun IBSC (w/o doping)1 sun IBSC (w doping)

Net carrier generation rate via IB

CB

IBGCI RCI

p-type Emitter (0.5µm)

IB region (1µm)

Case 1. Undoped (intrinsic) Case 2. n-type doped

9

0.5 1 1.50

5

Position, x (µm)

G* IB

/X (

cm−3

VB

GIV RIV

K. Yoshida and Y. Okada, NUSOD 2012, Shanghai (2012)

ND = NI/2

n-type Base (1µm)

Case 2. n-type doped

Short-circuit current Open-circuit voltage Conversion efficiency

1.1

1.2

1.3

Ope

n ci

rcui

t vo

ltage

(V

)

GaAs control

40

45

Sho

rt c

ircui

t cur

rent

den

sity

/X (

mA

/cm

2 )

GaAs controlIBSC w dopingIBSC w/o doping

30

40

Eff

icie

ncy

(%)

GaAs controlIBSC w doping

• IBSCs have non-linear dependence on concentration ratio.• Drop in open-circuit voltage of IBSC is reduced by high concentration ratio.• Photo-filling plays a important role to realize high efficiency.

To realize high conversion efficiency, IBSC should operate under high concentration ratio.

1 10 100 1000

1

Ope

n ci

rcui

t vo

ltage

(V

)

Concentration

GaAs controlIBSC w dopingIBSC w/o doping

1 10 100 1000Concentration

Sho

rt c

ircui

t cur

rent

den

sity

/X (

mA

/cm

IBSC w/o doping

1 10 100 1000Consentration

GaAs controlIBSC w dopingIBSC w/o doping

Concentration Ratio, X Concentration Ratio, X Concentration Ratio, X

10

Processing Developed at IES-UPM

11

QD-IBSC (50 layers)

50 nm, p+-GaAs (5e18)

150 nm, p+-GaAs (2e18) emitter

20 nm, i-GaNAsx50 QDs

(1000 nm)i-InAs, 2,0 ML

20 nm, i-GaNAs

1000 nm, n+-GaAs (1e17) base

250 nm, n+-GaAs (1e18) buffer

Substrate, n+-GaAs

1.35800K Blackbody

InAs/GaNAs Strain-Compensated QD-IBSC

20.3%21.2%

1 10 100 1000

1

1.1

1.2

1.3

Concentration

Ope

n−C

ircui

t V

olta

ge (

V)

GaAs Control

IBSC

Self-consistent device simulation

• In QD-IBSC, Voc and hence efficiency recover fast with concentration due to increased photo-generation rates from IB to CB.

12

15.7%

Long Carrier Lifetimes (~ 100ns) in InAs/GaAsSb QDs

Increasing Sb in GaAsSb

Calculated τradiative

K. Nishikawa et al, JAP 111 (2012) 044325 13

10%(4 ns) 0%

(1 ns)

18%(84 ns)

14% (25 ns)

Long Carrier Lifetimes in InAs/GaAsSb Type-II QDs

GaAsSbGaAsSb InAsQD

GaAs wall-inserted type-II InAs/GaAs QD recombination lifetime

Sb 18%Sb 18%1 ps

1 ns

Calcu

late

d Ho

le d

wel

l tim

e

D. Sato et al, JAP 112 (2012) 094305 14

GaAs wall

Hole dwell timeτdwell

Sb 18%Sb 18%

GaAs thickness (nm)0 1 2 3 4 5Ca

lcula

ted

Hole

dw

ell t

ime

Optimal values of GaAs thickness:2 ~ 3 nm

Γ= �

dwellτ

Long Carrier Lifetimes in Type-II QD System

Inserted GaAs walls2 nm and 12 nm

GaAs Sb 15 nm

GaAs0.82Sb0.18 15 nm

GaAs 50 nm

Type I InAs/GaAs ･･･ 4.6 ns (Delay time 10 ns)Sb 18% GaAs wall 0 nm ・・・ 94 ns (Delay time 90 ns)Sb 18% GaAs wall 2 nm ・・・ 220 ns (Delay time 90 ns)

15

InAs QD GaAs0.82Sb0.18 15 nm

InAs/GaAs(2 nm)/GaAs0.82Sb0.18

GaAs 50 nm

K. Nishikawa et al, MRS Fall Meeting, Boston 2012

マスタ タイトルの書式設定

p - GaAs emitter layer

AuZn/Au

p+ - AlGaAs window layer

GaAs 60 nm

8 QD stacks

1 period

--- In0.4Ga0.6As QDs layers ---

InGaAs QDs (8.7 MLs)GaAsSb (2 nm)GaAs (60 nm)

GaAsSb (7 nm)

InGaAs/GaAsSb Type-II QD-IBSC

n+ - GaAs (311)B substraten+ - GaAs buffer layer

n - GaAs base layer

In

8 QD stacks --- In0.4Ga0.6As QDs layers ---・Growth Temperature = 480 °C・Growth Rate = 1.0 µm/h

(high growth rate technique)・Growth interruption = 40 sec

--- GaAs & GaAsSb spacer layers ---・Growth Temperature = 480 °C・Growth Rate = 0.6 µm/h

--- Other layers ---・Growth Temperature = 580 °C・Growth Rate = 1.0 µm/h

16Y. Shoji et al, IEEE-PVSC, Austin (June 2012)

マスタ タイトルの書式設定

Sheet density: 3.2 × 1010 cm-2

Mean diameter: 54.1 nmMean height: 5.6 nm

InGaAs/GaAsSb Type-II QDs on GaAs (311)B

(2 µm x 2 µm)

Sheet density: 3.2 × 1010 cm-2

Y. Shoji et al, IEEE-PVSC, Austin (June 2012) 17

マスタ タイトルの書式設定

InGaAs/GaAsSb Type-II QD-IBSCWith anti-reflection coating

18Y. Shoji et al, IEEE-PVSC, Austin (June 2012)

19(June 2011 ~ March 2014)

Summary

� Strain-compensated (strain-balanced) growth gives high crystal quality of multiple stacks of In(Ga)As QD layers and good QD-IBSC characteristics.� Homogeneous, dense and ordered InGaAs QD 2D-arrays can be grown on GaAs (311)B.� Doping (impurity doping or photo-filling) and Concentration are important to maximize the

net generation rate via QD-IB:� Higher density QD arrays and longer lifetimes (IB) are required for good photo-absorption.

→ InGaAs QDs on GaAsSb ?

1. Higher current means more QDs and higher photon intensity (Concentration)

20

→ InGaAs QDs on GaAsSb ?

� Need for efficient Light Trapping.→ Rear surface texturing or scattering using metal nanoparticles.

2. Higher absorption means longer optical length

Thank you for your attention!