Recent Progress on Intermediate-Band Solar Cells and Quantum Dot - - PDF document

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

Yoshitaka Okada Yoshitaka Okada

Research Center for Advanced Science & Technology University of Tokyo

Intermediate Band Solar Cells (IBSCs): Concept

I V V P = I x V I

2

Quantum Dot Intermediate Band Solar Cells (QD-IBSCs)

63% Eg=1.9 eV ECI=0.7 eV

60

0.5 1 EIV (eV)

energy gap (eV)

In(Ga)As/(Al)GaAs InAs/GaAsP

3

VB CB miniband InAs QD GaAs Efficiency (%)

30 60

70 10

50 40 20

1.5 2 2.5 3 0.5 E Eg (eV)

CB-IB ener 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: 24.6nm height: 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-IBSC

Isc 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 SC area: (1-ra)A Non-ideal QD-IBSC (low QD-IB absorption) area: A

IBSC Performance on QD Absorption

Ideal QD-IBSC area: raA

) ( ) ( ) )( 1 ( /

CI CI a CV CV CI CI CV CV a CV CV a

R G r R G R G R G r R G r q J − + − = − + − + − − =

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 70

Jsc/X (mA/cm2)

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

Present status

0.2 0.4 0.6 0.8 1 30 40 50

Efficiency (%) ra

0.9 1

V

QD-IB absorption ratio ra

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

High concentration drastically

increases VOC and efficiency.

7

• S. 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 concentration Photon management

① ②

Pictorial Summary: Road to High Efficiencies

Maximize

Constant Constant 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 IB

GCI RCI

p-type Emitter (0.5µm) IB region (1µm) Case 1. Undoped (intrinsic) Case 2. n-type doped 9

0.5 1 1.5 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 Open circuit voltage (V) GaAs control 40 45

rt circuit current density/X (mA/cm

2)

GaAs control IBSC w doping IBSC w/o doping 30 40 Efficiency (%) GaAs control IBSC 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 O Concentration GaAs control IBSC w doping IBSC w/o doping 1 10 100 1000 Concentration

Short ci

IBSC w/o doping 1 10 100 1000 Consentration GaAs control IBSC w doping IBSC 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-GaNAs

x50 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

1.3

5800K Blackbody

InAs/GaNAs Strain-Compensated QD-IBSC

20.3% 21.2%

1 10 100 1000 1 1.1 1.2 1.3

Concentration Open−Circuit Voltage (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

GaAsSb GaAsSb InAs QD GaAs wall-inserted type-II

InAs/GaAs QD recombination lifetime

Sb 18% Sb 18% 1 ps 1 ns Calculated Hole dwell time

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

14

GaAs wall

Hole dwell time

τdwell

Sb 18% Sb 18% GaAs thickness (nm) 1 2 3 4 5 Calculated Optimal values of GaAs thickness: 2 ~ 3 nm

Γ = ℏ

dwell

τ

Long Carrier Lifetimes in Type-II QD System

Inserted GaAs walls 2 nm and 12 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)

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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

• -- In

Ga As 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 substrate n+ - 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

16

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

マスタ タイトルの書式設定

Mean diameter: 54.1 nm Mean 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-IBSC

With anti-reflection coating

18

• Y. 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!